Petrology and Geochemistry of the Dos Cabezas Mountains, Cochise County, Arizona

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Petrology and geochemistry of the Dos Cabezas Mountains, Cochise County, Arizona

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Authors Erickson, Rolfe Craig, 1936-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/565194 PETROLOGY AND GEOCHEMISTRY

OF THE DOS CABEZAS MOUNTAINS

COCHISE COUNTY, ARIZONA

Rolfe Craig Erickson

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF GEOLOGY

In Partial Fulfillment of the Requirements For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 6 9 THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

I hereby recommend that this dissertation prepared under my direction by ______Rolfe Craig Erickson______

entitled Petrology and Geochemistry of the Dos_____

Cabezas Mountains, Cochise County, Arizona

be accepted as fulfilling the dissertation requirement of the

degree of ______Doctor of Philosophy______

Dissertation Director Date

After inspection of the final copy of the dissertation, the

following members of the Final Examination Committee concur in

its approval and recommend its acceptance:*

This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination. STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the Univer sity Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

/ SIGNED: v

This dissertation was done while the author was employed by the geochemistry division of the Geochronology

Laboratories of The University of Arizona, Tucson, Arizona.

For three years the author was supported by a National

Science Foundation Cooperative Graduate Fellowship, and for the remainder of the time was supported as an employee of the laboratory by National Science Foundation Grant

GP-3738, under the guidance of Dr. Paul Damon as principal investigator.

In any investigation as long and complex as this one, the advice, support, and counsel of very many people must be recieved, and any list of formal acknowledgements such as this must be partial. My main thanks go to Dr.

Damon, for.the great support, stimulation, and encourage ment he has given me over the past several years. Dr.

Evans Mayo trained the writer in techniques of volcanology and granite tectonics and stimulated him to use them.

Dr. Robert DuBois furnished work space and equipment for much of the petrologic part of this study, and put up with a good deal of clutter in its final stages.

Numerous stimulating conversations, especially with Donald Livingston, Richard Mauger, and A. William

iii Iv

Laughlin helped refine and deepen the author’s geologic

common sense. Michael Bikerman, A. William Laughlin, and

Ahmad Hashed helped in running the argon isotopic analyses.

Judith Porcious and Richmond Bennett did the potassium analy

ses.’ Raymond Eastwood did the X-ray fluoresence analyses

for Rb and Sr, while Judith Percious, Richard Mauger, and

Donald Livingston ran and prepared the strontium isotope

samples. Crushing and grinding equipment was made avail

able through the courtesy of the Arizona Bureau of Mines.

The writer would also like to thank the members of

his committee in both the Geology and Chemistry departments

for their careful reading of the rough draft of the manu

script of this dissertation. He would also like to mention

his great appreciation of the generally fine treatment given

him by the many ranchers in the Dos Cabezas mountains and

their surroundings. Special thanks are due to Mr. and Mrs.

Roy Holland of Rancho Sacatal near Dos Cabezas village, who

housed the writer during much of his field.work. The

writer would also like to thank Mrs. Bea Grove of Willcox,

in whose home the writer was a guest for some months.

Finally, the writer would like to thank those many

fellow students in the Geology Department of The University

of Arizona who accompanied him on field trips and helped

refine his ideas. Special thanks are due to Tom Dirks

for identifying several fossil assemblages from the Paleo

zoic sequence in the Dos Cabezas mountains. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS...... xii

LIST OF TABLES ...... XV

ABSTRACT...... : ...... xvi

1. INTRODUCTION...... 1

Techniques ...... 1 Previous W o r k ...... ?

2. THE OLDER PRECAMBRIAN PINAL SCHIST ...... 8

Terminology ...... 8 The Western Pinal Schist Terrain ...... 10 Structure ...... ••... 10 Metamorphic Rank ...... Ik Original Character of the Schist Units • . 1? The Eastern Pinal Schist Terrain ...... 18 Structure of the Eastern Pinal Schist T e r r a i n ...... 1’9 Metamorphic R a n k ...... 21 Original Character of the Rocks in the Eastern Pinal Schist Terrain • • . • • 23 Precambrian Dacite Porphyry Stock .... 29 The Southern Pinal Schist Terrain ...... 30 Structure ...... 32 Metamorphic Rank ...... •• 33 Original Character of the Pre-Metamorphism Rocks in the Southern Pinal Schist Terrain ...... • • . 3^ Overall Character of the Pinal Schist in the Dos Cabezas Mountains ...... 35 Age of the Pinal Schist in the Dos Cabezas Mountains ...... 37

3* NON-RAPAKIVT PRECAMBRIAN GRANITOID ROCKS IN THE DOS CABEZAS M O U N T A I N S ...... 4u The Eaton Quartz Monzonite Gneiss ...... 4l Structure ...... 42 Age of the Eaton G n e i s s ...... 42 Second Body of Presumed Eaton Gneiss ..... 44 v vi

TABLE OF CONTENTS-Continued

Page

The Sommer Quartz Horizon!te Gneiss ...... # Structure ...... •••• 47 Age of the Sommer Gneiss ...... 49 Rough Mountain Quartz Monzonite Gneiss .... 50 S t r u c t u r e ...... 51 A g e ...... 52 Correlation of the Rough Mountain Gneiss With Other Precambrian Gneissic Intrusives ...... Sheep Canyon Quartz Monzonite Gneiss ..... Structure ...... A g e ...... •...... Correlation of the Sheep Canyon Gneiss With Other Precambrian Gneisses ...... - The Cienaga Quartz Monzonite Gneiss . . . . ; S t r u c t u r e ...... Correlation With Other Precambrian Gneisses of the Dos Cabezas Mountains . . . . * 58 Age of the Cienaga Gneisses • ...... The Polecat Quartz Monzonite ...... 61 Structure ...... 61 A g e ...... 62 Plutons Similar to the Polecat Quartz Monzonite ...... 62 Precambrian Aplite Dikes ...... 63

4. DOS CABEZAS RAPAKIVI QUARTZ MONZONITE ...... 66

Characteristics of the Classic Rapakivi • • • 6? The Dos Cabezas Rapakivi Pluton ...... 68 Field Characteristics...... 68 Rapakivi Petrography ...... 78 Model Rapakivi Magma ...... 91 Conclusions ...... 99 Studies on Rapakivis Previous to Present Work. 102 Solid-State Granitization Hypotheses ... 103 Magmatic Rapakivi Genesis Hypotheses .... 107

5. YOUNGER PRECAMBRIAN AND POST-PRECAMBRIAN SEDIHENTATIONAL HISTORY ...... 113

Younger Precambrian Sediments ...... 113 Nature of the Contact Between the Rapakivi Quartz Monzonite and the Overlying Paleozoic Sequence ...... 115 vii

TABLE OF CONTENTS-Continued

Page

Paleozoic Sedimentational History ...... 124 Bolsa Quartzite ...... 125 El Paso Formation ...... 12? Portal Formation ...... 128 Escabrosa Limestone ...... 129 Horquilla Limestone ...... 130 Concha Limestone ...... 131 Mesozoic Sedimentational History ...... 133 Terminology ...... 133 Glance Conglomerate .••» ...... 134 Bisbee Formation ...... 136 Cenozoic Sedimentational History ...... 138 Total Post-Precambrian Sedimentation ...... 139

6 . FAULT STRUCTURES OF THE DOS CABEZAS MOUNTAINS . . l4l

West-Northwest Trending Faults ...... l4l The Apache Pass Fault ...... 142 Other West-Northwest Trending Faults . . . 152 Fault A ...... 152 Fault B ...... 153 Faults C and D ...... 154 Age of the West-Northwest Trending Faults. 155 The North-Northeast Trending Faults . . . 157 Mid-Tertiary Structural Development . . . l6l Indicators of Depth of Crystallization of Various Parts of the Dos Cabezas Mountains ...... 163

?. CRETACEOUS INTRUSIVE VOLCANIC BRECCIA TERRAIN . . l66

General Characteristics of Breccia Units . . . 166 Separate Breccia U n i t s ...... 176 Green Volcanic Breccias ...... 177 Petrography ...... 177 Contact Relations ...... ••••• 178 Purple Volcanic Breccia ...... 180 Main Purple Breccia ...... 180 Other Purple Breccias ...... I81 White Volcanic Breccias ...... I85 Genesis of the Volcanic Breccias ...... 186 Megascopic Fragments ...... * I87 Ground Mass Fragments and Crystals .... 188 Breccia Formation Mechanism ...... I89 viii

TABLE OF CONTENTS-Continued

Page

Fluidization ...... 199 Foliation ...... 206 Solidification of the Breccias . . « . . 210 Intrusive Magmatic Bodies Related to the Intrusive Breccias ...... 214 Other Examples of Intrusive Volcanic Breccias 216

8 . ' EARLY CEN0Z0IC (LARAMIDE) STOCKS AND DIKES . . . 224

Diabasic and Ophitic Intrusives ...... 224 Diabases ...... 224 Government Peak Ophite ...... 228 Age of the Diabase and Ophite Intrusives 228 Quartz Diorite Stocks ...... 230 Cowboy Stock ...... 230 Silver Camp S t o c k ...... •••• 232 Mascot Stock ...... 233 4503 Quartz Diorite ...... 235 Buckeye Dike Group ...... "...... 237 Maverick Stock ...... 237 Basalt Dikes ...... 239

9. MID-CENOZOIC STOCKS AND D I K E S ...... 243

Mid-Tertiary Dikes ...... 243 Turkey Track Dike Group ...... 243 Hornblende Andesite Dike Group • . . • • 244 Dacite Porphyry Dike Group ...... 247 Ninemile Granodiorite Stock ...... 249 Miscellaneous Granitoid Intrusives of Possible Mid-Tertiary Age ...... 252 317 Quartz Monzonite ...... • . • 252 Quartz Monzonite Near the Alma Mine . . 254 Quartz Dikes ...... 254

10. GEOCHRONOLOGY OF THE DOS CABEZAS MOUNTAINS . . . 257

Precambrian Geochronology ...... ' 257 Precambrlan Pinal Schist ...... 257 Precambrian Plutons ...... 262 Dos Cabezas Rapakivi Quartz Monzonite 262 Eaton Gneissic Quartz Monzonite • • 263 Polecat Quartz Monzonite ...... 268 Rough Mountain, Sheep Canyon, and Cienaga Quartz Monzonite Gneisses . 275 ix

TABLE OF CONTENTS-Continued

Page

Sommer Quartz Monzonite Gneiss • • • 276 Conclusions , ...... 276 K-Ar Age D a t a ...... 276 Tertiary Reheating of PreCambrian Terrains . 279 Late Cretaceous-Eocene (Laramide) Geo- chronologic R e s u l t s ...... * 288 Mid-Tertiary Oligocene-KiocenePlutonism . . 292 Overall Chronology of the Dos Ca.bezas M o u n t a i n s ...... 296

11. THE DOS CABEZAS MOUNTAINS IN THE FRAMEWORK OF THEIR IMMEDIATE SURROUNDINGS ...... 302 PreCambrian Deposition ...... 303 PreCambrian Metamorphism and Structural D e v e l o p m e n t ...... 305 PreCambrian Acidic Intrusions ...... 307 Precambrian Thermal Perturbation ...... 311 Paleozoic Sedimentation • * ...... 311 Post-Permian Pre-Comanchean Deformation and Volcanism ...... 312 Deposition of the Bisbee Formation ...... 315 Post-Comanchean Pre-Paleocene Deformation 316 Post-Bisbee Volcanism ...... 320 Post-Volcanism Intrusion ...... 320 Basin and Range Uplift and Faulting . . . • 321

APPENDIX A: DESCRIPTIONS OF PETROGRAPHICALLY ANALYSED SAMP L E S ...... 324-

Petrography of Pinal Schist Samples r'. . . . 324- Samples From the Western Pinal Schist Terrain ...... 325 Phyllites ...... 325 Feldspathic Argillites and Felds- pathic Phyllites ...... 326 Phyllites Containing Tuff Crystal Debris ...... 326 Metamorphosed Lava Flows (Meta- volcanics)...... 330 Metamorphosed Limestone ...... 333 Metamorphosed Oolitic Ferruginous Shale ...... 334 Metamorphosed Basalt Flow . . . . . 335 Amphibolites ...... 336 X

TABLE OF CONTENTS-Continued

Page

Samples From the Eastern Pinal Schist T e r r a i n ...... 339 P h y l l i t e s ...... 339 Feldspathic Argillites and Felds- pathic Phyllites ...... 339 Metaconglomerate ...... 340 Metamorphosed Lava Flows ...... 341 ' Amphibolites ...... 342 Metamorphosed Dacite Porphyry Stock 344 Samples From the Southern Pinal Schist T e r r a i n ...... * ...... 345 Quartzite ...... 345 P h y l l i t e s ...... 346 Petrography of Non-Rapakivi Precambrian Granitoid Intrusive Rocks ...... 346 Eaton Quartz Monzonite Gneiss . . . . . 346 Sample of Second Body of Presumed Eaton . ■ Gneiss ...... 349 Sommer Quartz Monzonite Gneiss . . . . . 350 ■ Rough Mountain Quartz Monzonite Gneiss . 353 Sheep Canyon Quartz Monzonite Gneiss . . 355 Cienaga Quartz Monzonite Gneiss . . . . 357 Polecat Quartz Monzonite ...... 36l Petrography of Rapakivi Quartz Monzonite Samples ...... 364 Petrography of Volcanic Breccia Units , . . 364 Green Volcanic Breccia Units ...... 364 Purple Volcanic Breccia Units ...... 372 White Volcanic B r e c c i a ...... 381 Intrusive Magmatic Rocks Associated With the Breccias ...... Petrography of Early Cenozoic (Laramide) Intrusives ...... • . 385 Diabasic Intrusives ♦ ...... 386 Quartz Diorite Stocks ...... 390 Cowboy Stock ...... 390 Silver Camp Stock ...... 393 Mascot Stock ...... 394 4503 Quartz Diorite ...... ♦ 395 Buckeye Dike Group ...... 396 Maverick Stock ...... 398 Basalt Dikes ...... 400 xi

TABLE OP CONTENTS-Continued

Page

Petrography of Md-Cenozoic Intrusives . . . 403 "Turkey Track" Pla'gioclase Andesite Porphyry ...... • 404 Hornblende Andesite Dikes ...... 405 Dacite Porphyry Dike Group ...... 406 Ninemile Granodiorite Stock ...... 407 31? Quartz Monzonite ...... 409 Minor Granitoid Intrusives Along the Range Border ...... 410

APPENDIX B: GEOCHRONOLOGIC EXPERIMENTAL TECHNIQUES 4l3

Sampling ...... 413 K-Ar Analyses ...... 4l7 Potassium Analyses ...... 4l7 Argon Analysis ...... 4l8 Rb-Sr Analysis...... 425 Rb-Sr Ratio Determination ...... 425 Strontium Isotope Analysis ...... 427 Isochron Calculation for Rb-Sr Suites . 431

REFERENCES ...... 436 LIST OF ILLUSTRATIONS

Figure Page

1. Dos Cabezas Mountains ...» ...... pocket

2* General View of the Central Dos Cabezas Mountains • • • ...... 3

3 . Dos Cabezas Mountains Pinal Schist ...... 11

4-. Primary Depositional Features in the Pinal Schist ...... 26

5* Primary Cross-Bedding in the Pinal Schist • . 2?

6 . General View of Eastern Pinal Schist Terrain ...... 28

?. Location of Precambrian Granitoid Intrusives and Intrusive Samples ...... 43

8 . Two Gneissic Precambrian Intrusives ...... 45

9. Two Precambrian Plutonic Units From the Northern Dos Cabezas Mountains ...... 46

10. Primary Structures in the Dos Cabezas Rapakivi Quartz Monzonite ...... 71

11. Contrast Between the Rare Gneissic and Common Little-Foliated Rapakivi Quartz Monzonite ...... 72

12. Microscopic Views of Wiborgite and Pyterlite Rapakivi Minerals ...... 73

13* Map of Distribution of Mantled Microcline Phenocrysts in Dos Cabezas Rapakivi Pluton ...... 74

xii xiii

LIST OP ILLUSTRATIONS-Continued Figure Page

14. Rose Diagram of Strike Orientations of 148 Potassium Feldspars Showing Oblong Outlines, Within a 10 Square Foot Area in the Dos Cabezas Rapakivi Quartz Monzonite .... 75

15* Primary Structures in the PreCambrian and Cambrian Rocks Near the Unconformity Surface Cut on the Rapakivi Quartz Monzonite ...... 117

16. Rock Structures and Textures Along and Near the Unconformity Between the Bolsa and the Rapakivi ...... 118

17* Primary Depositional Structures in the Basal Conglomerate of the Bolsa Quartzite-1 . . 119

18. Primary Depositional Structures in the Basal Conglomerate of the Bolsa Quartzite-2 . . 120

19* Major PreCambrian (?) and Post-Paleozoic Faults 1^3

20. General Features of the Apache Pa6s Fault Zone in the Central Dos Cabezas Mountains . . l45

21. Plan and Location of Dos Cabezas Welded Intrusive Volcanic Breccia ...... 168

22. Some General Characteristics of the Volcanic Breccia Terrain and Breccia Rock .... 170

23* Internal Foliation in the Green Volcanic Breccia Along the Northern Boundary of the Breccia Terrain ...... 171

24. Breccia Bodies in the Northern Part of the Volcanic Breccia Terrain ...... 172

25* Textures in the Purple Breccia Dike Cutting the Bisbee Formation near the Southeastern Margin of the Breccia Terrain ...... 173

26. Formation of Volcanic Breccia Textures . . . 198

27* Geology of the Piedmont!te Hills, Tucson Mtns., Arizona ...... 218 xiv

LIST OF ILLUSTRATIONS-Contijiued

Figure ' Page

28. Laramide Magmatic Intrusives; Plan and Location of Samples ...... 226 .

29. Mid-Tertiary Magmatic Intrusives; Plan and Sample Locations ...... 24$

30. Pictures of Typical Mid-Tertiary Units . . . 251

31• Dos Cabezas Rapakivi Quartz Monzonite Whole Rock Isochron ...... 264

32. Eaton Gneissic Quartz Monzonite Whole Rock Isochron ...... 265

33• Polecat Quartz Monzonite Mineral-Whole Rock I s o c h r o n ...... 266

34. Matching of General Basin and Range Province Post-90 m.y. K-Ar Histogram with Dos Cabezas Data ...... 293

35• Overall Chronology of the Main Events in the Dos Cabezas ...... 298

3 6 . Apparent K-Ar and Rb-Sr Ages in the Dos Cabezas Mountains ...... 299

37. Metasediments and a Dacite Porphyry Stock From the Pinal Schist ...... 328

3 8 . Nonsedimentary Units in the Pinal Schsit . ♦ 329

39* Examples of the Main Phases of the Volcanic Breccia ...... 375

40. Special Features of the Volcanic Breccia . • 376

41. Estimated Precision for Determining Radio genic Ar^O and the Ar^vK^® Ratio for Typical Analyses ...... 426 LIST OP TABLES

Table’ Page

1. Table of Laramide and Mid-Tertiary Biased K-Ar Ages------280

2 , Modal Data on PreCambrian Bocks— ---- -— --- 365

3* Modal Analyses of Typical Members of Laramide Stock and Dike Group------A01

4. ' Modal Data on Hid-Tertiary Rock Units— --- — 411

5. Argon-3 8 SpikeComposition-— ------419

6 . Potassium-Argon Data---- —— ------:-- 421

7. Rubidium-Strontium Data------— — -— --- — - 433

xv ABSTRACT

The Dos Cabezas Mountains lie in the northeastern- most part of Arizona, in Cochise county. They are a medium-sized range of some 150 square miles area, and are

almost wholly surrounded by unconsolidated basin-fill

material. Host of the range is composed of a number of

Precambrian igneous and netamorphic rock masses. Its core

is composed of a large, complex terrain of Cretaceous intru

sive volcanic breccias and magmatic aphanites. A large

number of Laramide and mid-Tertiary intrusive plutons and

dikes cut the range.'

The Precambrian rocks consist of eight granitoid

plutons and three areas of phyllitic and argillitic meta

morphosed sediments and volcanics. The metamorphic rocks

display a primary greenschist facies dynamothermal meta-

morphic fabric and a later superimposed biotite-forming

hornfelsic thermal metamorpliic fabric.' The metasediments

are mostly phyllites and argillites, but contain over

1 0 ,0 0 0 feet of metaconglomerate showing marked primary

cross-bedding. Many of the metamorpliic units are weakly

metamorphosed volcanic flows or tuff-contaminated fluvial

clastic sediments. These rocks are all classified as

Pinal Schist, although some may be equivalent to the

xvi xvil

Mazatzal.Quartzite,

The pinto ns consist of a pre"Pinal~metamorphlsm dacite porphyry stock, one quartz monzonite gneiss syn- kineiaatic with the Pinal dynamo thermal metamorphism, and four gneissic quartz monzonite plutons which appear to postdate the Pinal metamorphism and imply a mild tectonic event at about 14^0 million years ago, and tiro large post- kinematic quartz monzonite stocks which are of circa 1400 million years age. One of these latter stocks displays prominent rapaklvi texture; this is considered to be the result of normal magmatic crystallization. The texture is caused by reaction breakdown of hornblende to form biotite among crystals floating in the magma, thereby extracting potassium from the magma and temporarily halting potash feldspar crystallization while allowing plagioclase crystal lization. Rb-Sr dating of the plutons reveals that one of * the older post-Pinal gneisses is 1470 - 30 m.y. old, while the rapakivi is 1380 - 30 m.y. old and the other large stock is 1425 m.y.’ old and has undergone a marked Sr redistribu tion at 1000 m.y. ago; this thermal event has biased all the Precambrian K-Ar ages in the northwestern part of the range toward 1000 m.y., also.'

A large complex assemblage of Cretaceous welded in trusive volcanic breccias underlies 17 square miles of the xviii core of the range. They are largely composed of small angular fragments torn from foundering large fragments of surficial andesite flows, sinking in a fluidized bed. The gas source was a crystallizing magma at depth; entrained quartz and plagioclase crystals from this magma appear in the breccia ground mass. The breccias are cut by a large number of small mafic magmatic intrusives.

Several large diabasic and quartz dioritic plutons of Cretaceous or Paleocene age appear in the range and mark

Laramide plutonism,’ IC-Ar data from all but the northwestern most part of the Precambrian rocks in the range display a remarkably uniform Paleocene age which reflects a Paleocene thermal metamorphism.

Mid-Tertiary plutonism is recorded by several mafic dike sets, including one of 11 Turkey Track” andesite por phyry, a granodiorite stock, and numerous quartz veins,

Basin and Range block faulting is not obvious in the range, but may account for its present high-standing nature, especially along the northern range margin.

Dynamothermal metamorphism is recorded strongly in the Pinal Schist, and dynamic tectonism, at circa 1^50 million years,'- Thermal metamorphism is recorded at 1000 million years, circa 55 million years, and circa 35 million years. Plutonism is recorded before Pinal metamorphism, during Pinal metamorphism, then over the 1470-1380 million xix year interval, in the Cretaceous-Paleocene Laramide interval, and in the mid~Terti£try Oligocene-Hiooene interval. CHAPTER 1

INTRODUCTION

This study resulted from the writer1s curiosity about the large rapakivi granitoid pluton in the southern Dos Cabezas mountains. Preliminary investiga tion showed that the entire range was the site of an unusually complex assemblage of Precambrian metasediments and plutons." The range also included a very unusual volcanic breccia suite and a number of probable Laramide and younger stocks and dikes. The decision was made to study the entire Dos Cabezas mountains using modern field, petrologic, and geochronologic techniques, and to develop the history of the range in as detailed a manner as a two or three year study would allow. Figure 2 shows two pictures of the central part of the range.

Techniques

The initial phase of the work was largely field oriented. Perhaps some 300 days were spent in the field in all. The range was napped by the writer using a semi reconnaissance technique in which the more diversified and complex areas were mapped in considerably more detail than the simple and homogenous ones. In homogenous terrains,

1 2

especially in the large granitoid plutons in the northern part of the range, traverses about a mile to a half a mile

apart were run across the area and anything considered

significant (see below) was mapped.* In areas of moderate

complexity, such as the Pinal series terrain in the

western part of the range, an area mapping system was used,

and the area was criss-crossed until all bodies of mappable

character had been located (see below). In very complex

terrains, as in large parts of the volcanic breccia, a

return to the traverse system was made and the area tra

versed was characterized by its general structure and

rock type.

The level of discrimination between mappable and

unmappable features was as follows. In parts of the range

where the various units were quite distinct from each

other (e.g., Precambrian schist, Precambrian quartz

monzonite, Laramide stocks, mid-Tertiary dikes) an effort

was made to map each unit and its relationships to the

others.1 Intrusives less than about 50 feet long were

not mapped, nor were local textural zones in large plutons

if the zones were less than a few hundred square feet in

size.

Within the complex Precambrian metasediments, no

attempt was made on the map to delineate different meta

sediment types, such as phyllite, conglomerate, and ^ Q_ Figure 2: General View of the Central Dos Cabezas Mountains

(a): Panoramic shot of the central Dos Cabezas mountains as seen from the southwest. The lenob on the skyline is the Dos Cabezas (two heads) themselves, seen as one in this view. The white masses marked pi are Pinal (?) quartzite of older Precambrian age. The line marked A. P. fault is the trace of the Apache Pass fault zone, the major structure of the range. Volcanic breccia (Vb) of several types makes up most of the hills and main ridge behind the fault and the Pinal outcrops/ The dark ridges just above the Apache Pass fault on the left are green basalt, which has invaded the breccias.

(b): Panoramic shot of the central Dos Cabezas mountains from the north. The sloping peal: near the right- hand margin of the picture is Simmons Peak (7^29 feet), which is volcanic breccia (3v) in its upper parts. The broad arch of the ridge going left to the Dos Cabezas and other distant skyline peaks are also volcanic breccia. The lower masses of ridges and uplands in the foreground are composed of Rough Mountain gneiss (Rngn). The Maverick stock, which cuts the Rough Mountain gneiss, cannot be seen in this view as its exposure is almost wholly on the upland of Maverick mountain and cannot be seen from the road. 3

figure 2 : General vlew of the Central Dos Cabezas Mountains feldspathic argillitev Some separation on this basis was done in the petrographic study later (see Appendix A)/

Metamorphosed basic sills and dikes in the metasediments were mapped and distinguished from the metasediments un less the basic bodies were less than a foot or so thick or were shorter than about 50 feet.

Within the complex volcanic breccia terrain, no real attempt was made to separate the large number of small individual dikelike magmatic and breccia bodies cutting the major breccia masses, but rather the areas were characterized by the main rock type present. Thus, within a given area labelled purple volcanic breccia on the map (Figure 1), the majority of the rocks exposed in the area will be purple breccia, but small breccia bodies of green and white breccia may be present together with numerous small dikelike masses of purple or greenish hued aphanites; most of the latter have evidently crystal lized from a liquid.4 Xenolithic blocks in the breccia were mapped.if more than about 10 feet in length.

The writer made no real attempt to detail the stratigraphy of the Paleozoic and Mesozoic sediments in the Dos Cabezas mountains, as it was believed this would take up too much time and was not essential to the main interest of the study. The base of the Cambrian Bolsa

•Qiartzite was mapped in some detail as it provided a 5 good marker horizon for structural study. Paleozoic lime stone assemblages were not generally broken down into’ separate formations, but, in a few areas where good fossil assemblages were found, names were applied. Much of the

Paleozoic limestone outcrop areas are labelled undifferen tiated limestone on the map (Figure 1). Mesozoic units were mapped as Bisbee Formation and Glance Conglomerate, the term Bisbee formation covering everything strati- graphically above the last massive conglomerate horizon; see Chapter 5 on sedimentational history of the area for a complete discussion of the Bisbee and Glance relation ships."

For the petrographic part of the study, some 950 samples of the various units were collected; of these perhaps 300 were thin sectioned and analysed by microscopic means and another 100 were slabbed and stained for modal analysis. An attempt was made to determine the true character of each major rock unit found in the field work; as an example, the petrographic breakdown of the Pinal schist shows the existence of three distinct areas of

Pinal schist in the range, and within each of these areas some ten to twenty samples were collected," sectioned, and examined to get some idea of the rock types exposed there-’ and their relative proportions. Thus, the eastern Pinal 6

schist terrain is characterized toy a thick conglomerate

sequence overlain toy a sequence of cross-bedded arkoses,

which in turn are overlain toy a phyllite and metavolcanic

sequence,’. As another example, note the several modal

analyses of the Polecat granite pluton in Appendix A.;

No tight statistical analysis was made of the variations

in these modes, tout the variation gives some idea of the

internal variability of the pluton and its overall

character.'

With respect to the structural part of the study,

only major visible faults were mapped."’ Joints were not

mapped, nor were inferred faults generally located.

Foliation and/or lineation within any unit was mapped as

well as the pattern of coverage of the area would allow

(see above), and an effort was made to detail the internal

structure of the various plutons, dikes, metasediment

bodies, etc.. At least a few structural attitudes were

obtained in every section of land underlain toy Pre-Cambrian

metasediments or toy post Pre-c amtorian sediments. The

writer was more Interested in the structure of the Paleozoic

and Mesozoic sediments than in their stratigraphy, and.

the structure of these units is consequently detailed more

than their stratigraphy on the main map (Figure 1).

Samples of the radiometrically dated rock units

were collected after field study of the units in question

L was complete, or nearly so.' Great effort, culminating in partial success, was made to avoid collecting samples for radioactive dating which had been perturbed by thermal events younger than the event to be dated.’

Previous Work

The most recent previous work in the Dos Cabezas mountains was the reconnaissance geologic map done by

Cooper (i9 6 0 ); this map outlined many of the major units and structures in the range and was quite helpful to the writer in his general field work, Sabins (1957a, 1957b) has studied the general geology and stratigraphy of the northern Chirlchahua mountains and the easternmost Dos

Cabezas mountains; the writer used Sabins * map of the stratigraphic units in the eastern Dos Cabezas range as part of Figure 1. In addition, Sabins* terms for the

Paleozoic stratigraphic units were used,’ ' Jones and

Batcheller (1953) give some measured sections in the westernmost Dos Cabezas mountains west and south of Dos

Cabezas village. Shields (19^0) mapped a small area in the Dos Cabezas mountains where a large quartz dikes cut the Bisbee Formation between Dos Cabezas village and the old Mascot mine. CHAPTER 2

THE OLDER PRECAMBRIAN PINAL SCHIST

The Pinal Schist in the Dos Cabezas mountains is represented by several tens of thousands of feet of weakly to moderately metamorphosed politic and tuffaceous politic sediments, and numerous types of volcanic flows.

The Pinal Schist was invaded in many places by basaltic sills and dikes before and during metamorphism. Three separate areas totaling approximately 28 square miles in size make up the main Pinal Schist exposures. The three areas are all somewhat distinct in terms of lithology and structure, and will be discussed separately.

Terminology

The name "Pinal Schist" is used to refer to all

Older Precambrian metarnorphic rocks in the Dos.Cabezas mountains, although as later discussed this designation may not be wholly accurate.

The name was first used by Ransome (190^) for the Precambrian (now Older Precambrian) metamorphic rocks in the Pinal mountains. The exposures of the Older

Precambrian metamorphic rocks in the Dos Cabezas moun tains are lithologically similar', in part, to the ones

8 in the Pinal mountains; also, the Older Precambrian metamorphic rocks of the tvio areas have the same general chronologic and structural position in the Pre- c ambrian sequenced Additionally, this name has been used earlier by other authors (Gilluly, 1956; Cooper and Silver, 1964) for the same general types of Older Pre-c ambrian rocks in Cochise county.': It is felt that there is sufficient justification for the use of the term in the Dos Cabezas mountains.’

The term schist, as applied to the Pinal rocks in the Dos Cabezas range, is something of a misnomer, as a very small percentage of the outcrops show that high grade a metamorphic development. The greatest part of the Pinal units in the Dos Cabezas mountains are ar gillites and feldspathic argillites, with considerable exposures of phyllite and conglomerate, making up most of the rest of the outcrops. True schist, with individually distinguishable crystals of biotite, muscovite, and other minerals arranged in a marked planar foliation, is only found commonly in parts of large xenolithic blocks of

Pinal metamorphic units enclosed in Pre-b'ambrian granitoid rocks lu the northern Dos Cabezas mountains. Because of widespread use of the term, however, it will be retained in describing the Older Pre-c'-ambrian metamorphic rocks in the Dos Cabezas mountains 10

The Western‘Pinal Schist Terrain

Exposures of Pinal Schist underlie some 12 square miles of the westernmost part of the Dos Cabezas mountains

(see Figure 1 and Figure 3)• This area will henceforth be referred to as the western Pinal Schist terrain. Fol lowing the primary Older Pre-c'ambrian metamorphism, the

Pinal Schist metamorphic units were intruded by Prec ambrian

Sommer quartz monzonite gneiss and Polecat quartz monzonite, and.then by Mesozoic volcanic breccia and Tertiary stocks and dikes,'

Structure

The main map (Figure 1) shows the present structural

character of the area.' The pattern within it is essentially

simple, with east-northeast to east-west foliation strikes prevailing in the westernmost part of the area, and north

east strikes on foliation prevailing throughout most of

the rest of the area. The one structurally anomalous area

is found where the Sommer gneiss intrudes the Pinal Schist

in the northwesternmost part of the western schist terrain;

here the metasediments are bent into a broad anticlinal

structure whose axis plunges some 50 to 70 degrees to the

east.' In this structure strikes of foliation are generally

northwest or north-south in the northern limb of the fold, while those of the south limb are northeast in trend;- the F/gure 3. /Dos Cobezos A4ounro/ns P/no/

south limb merges with the general mass of Pinal Schist

in the rest of the area south of the fold,' This fold is

thought to be due to intrusion of the Sommer quartz monzonite gneiss into the Pinal Schist during late or waning metamorphism of the Pinal sediments and volcanics,'

Local deflection of the foliation from the regional pattern

takes place along some faults, as in Sec, 1?, T13S, R26E, where the northeast striking Pinal Schist has been de

flected to the west-northwest by apparent drag along a

fault.'

Foliation dips are shallow in the westernmost

part of the exposure, averaging perhaps 35 degrees, but

the dips of the foliation grow steeper to the east.

Foliation in the central part of the western Pinal Schist

terrain averages about 60 degrees, while much of the

foliation in the eastern part of the terrain is vertical.

A basalt flow in the westernmost part of this terrain is

upright, and shows the bedding in the schist to be upright

at least throughout the area of shallowly dipping layers.6

Marked lineation is present in many outcrops of

the Pinal Schist, especially in the western and central

parts of the western Pinal Schist terrain, Lineation is

always in the foliation plane, and is usually seen as

myriads of minute parallel streaked out crystals of the

various minerals present in the rock. Occasionally a 13 minute wrinkling or coarser folding of small anticlines and synclines occurs, with the axes of the little folds parallel to the general lineation trend. The former type of lineation is common to all types of Pinal metamorphic rocks except the phyllites, where the latter type is common."

Foliation in the sedimentary and flow units is, so far as the writer has been able to determine, everywhere parallel to primary bedding and flow structure in the various units now composing the schist.' These original textures and structures of the various units in the schist have only partially been destroyed by later shearing and recrystallization, and reasonable reconstruction of the original character of the various units can be made/

There is no definite evidence that any part of the metamorphosed stratigraphic sequence in the western Pinal

Schist terrain of the Dos Cabezas mountains has been repeated by faulting or folding, at least to any major degree.

No large scale lithologic repetitions have been noted, nor have a large number of faults of large size been observed in the area/ Only one, perhaps two, large faults cut the

Pinal Schist in this western terrain, and the displacement along the single certainly known one is less than a mile/

There are no obvious isoclinal folds in the western Pinal

Schist terrain, nor any indication of repetition of units due to large scale folding, aside from the one previously 14 described anticline in the northwestern part of the area.

The writer believes that the western Pinal Schist terrain is essentially a single large block whose structure can best be represented as part of the axial region and

southeastern limb of a very large anticline whose axis

trends northeast without discernible plunge just to the west of the westernmost Pinal Schist exposures in the Dos

Cabezas mountainsThis large block has been secondarily

folded locally (as by the Sommer quartz monzonite gneiss

intrusive) and broken by a few large faults and, no doubt, many small ones / The above conclusions imply a very great

stratigraphic thickness, on the order of perhaps five to

seven miles of original vertical thickness,

Metamorphic Rank

The metamorphic rank of the western Pinal Schist

terrain is low to moderate. Figure 3 shows the position

of the different samples examined within this terrain, and

shows the marked isograd boundary which separates biotite-

sericito-epidote and biotite-sericite bearing mineral as

semblages from chlorite-sericite-epidote and chlorite-

sericite bearing ones. Mote that on Figure 3 the southwestern

and north central parts of the western Pinal Schist

terrain lie outside the zone of biotite development.

The overall distribution of the biotite bearing rocks

is neither in zones paralleling the foliation of the 15 schist, nor in zones confined to intrusive contacts of post-Pinal Schist intrusive plutons

The features by which the two general metamorphic rock suites in this schist terrain may be distinguished from each other show up in the microscopic study of the units, as follows:

1. - Sections taken from rocks in the chlorite-sericite-

epidote or chlorite-sericite bearing metamorphic

rocks are weakly foliated unless they are phyllites,

but the foliation can always be seen, Sericite

and/or chlorite and crystals of other minerals

lie in parallel elongate clusters in the foliation

plane, and within the clusters the individual

crystals of chlorite or sericite have their

cleavages aligned in a subparallel arrangement/

This implies that the chlorite and sericite

crystallized during the dynamo thermal meta

morphism of the Pinal Schist where the texture

is found,* Development of chlorite is mostly

confined to the amphibolites interlayered with

the metasediments

2. In biotite bearing foliated Pinal rocks, the

individual aggregates of biotite and other

minerals still have a platy aspect, just like

mineral aggregates in the chlorite bearing metamorphics; Within these crystal aggregates,

however, individual hiotite crystals lie at

random orientations, as if they had formed after

conclusion of the primary folding and shearing

which was evidently still going on as the chlorite

and sericite hearing mineral assemblages were

being formed. Biotite is found in the phyllites

as well as in the amphibolites and is often as

sociated with magnetite.

There is no evidence that the formation of bio tite in the Pinal Schist postdates the Intrusion of the

Pre Cambrian granitoid plutons that cut the Pinal Schist

in this western terrain, but, as mentioned above, the biotite bearing rocks do not lie in neat contact zones near plutons on the present erosion surface.1 However,

it is thought by the writer that the present distribution of metamorphic biotite in the Pinal Schist may be due to

thermal metamorphism around still buried extensions of

Pre-cambrian plutons; this would account for the lack of

conformity of the biotite bearing schist zones to the

outlines of the Pre-c ambrian plutons on the present sur

face." A buried extension of the Polecat quartz monzonite running north-northwest toward the Sommer gneiss at a

shallow depth beneath the present surface could, for 17 example, account for the present distribution of blotlte in the Pinal Schist in this terrain as a simple contact metamorphic aureole mineral development

I-Ietamorphic rank in the chlorite bearing rocks is equivalent to the chlorite zone of Barrow or the greenschist facies of Eskola; according to Turner and Verhoogen

(1 9 6 5 ) the assemblage lies in the quartz-albite-muscovite- chlorite subfacies of the greenschist faciesThis facies designation is confirmed by the presence of tremolite in a small metamorphosed limestone mass in the Pinal Schist outside the biotite bearing zone in this terrain.

The metamorphic rank of the biotite bearing rocks

corresponds to the biotite zone of Barrow or the green-

schist facies of Eskola, and lies in the quartz-albite-

epidote-biotite subfacies of Turner and Verhoogen* s greenschist facies.1

Original Character of the Schist Units

The original gross petrographic character of the different stratigraphic units in the Pinal Schist can be reasonably reconstructed in many cases. In the western

Pinal Schist terrain these original units are quite

diversified in type/ There are politic sediments, often with an added apparently tuffaceous volcanic component;

there are arkoses; there are a large number of true igneous

flow rocks of several types; there is at least one small 18

"body of limestone and a small body of oolitic iron formation.

All evidence suggests that these units were laid down as

a layered stratigraphic sequence.- These units were then

invaded by a large number of basaltic sills and dikes, and

the entire assemblage of strata plus later intrusives was

then deformed and metamorphosed.’ Some of the basaltic

intrusives seem to have appeared late in this metamorphism.

Detailed discussion of the petrographic character

. of the various analysed samples is contained in Appendix A."

The Eastern Pinal Schist Terrain

The eastern and northeastern Dos Cabezas mountains

north of the Apache Pass fault zone contain large exposures

of Pinal schist. These exposures are continuous and under

lie some 13 square miles of ground. These exposures will

henceforth be referred to as the eastern Pinal Schist

terrain. A general view of the terrain is given in Figure 6 V The Pinal Schist in this area is composed primarily

of metasedimentary units, but in the eastern part it con

tains some tuff contaminated sediments and one known true

lava flow.’ Several large amphibolite plugs and dikes are

present in this schist terrain, and a large dacite porphyry

stock of presumed Pre cambrian age cuts it. The entire

foregoing assemblage has been intruded by Pre-c"ambrian

Sheep Canyon quartz monzonite gneiss, Eaton granodiorite 19 gneiss, Rough Mountain quartz monzonite gneiss, and Polecat quartz monzonite, as well'as "by Mesozoic volcanic breccia and Tertiary stocks and dikes/

Structure of the Eastern Pinal Schist Terrain

The structural character of the eastern Pinal Schist terrain can be seen most completely on Figure 1.' This area can, for structural purposes, be divided roughly in half by a line trending north-south through Government Peak (see

Figure 1 ) The western half, roughly, is characterized by north-northeast trending foliation strikes and southeasterly dips ranging from vertical to near-vertical in its western part to an average of about 55 degrees in its eastern por tion. In the area immediately around Government Peak

(Sec.' 20, Tl^S, R28E) the foliation strike common to this western half changes to west-northwest or east-west in orientation; this change defines a large open synclinal fold with a northeasterly trending axis plunging some 50 to 60 degrees to the northeast (see Figure 1). These west- northwest to east-west strikes are the common ones over all this eastern half of the eastern Pinal Schist terrain.

The dips associated with them are either to the north or south and usually very steep to vertical and locally variable. A dual foliation is present in the phyllites in the southern half of Sec. 28, Tl^S, R28E; this type 20 of feature is not found anywhere else in the Dos Cahezas mountains.3 Lineation is only present in the eastern

Pinal Schist terrain in the southern part of the exposure

east of Government Peak, and there it generally plunges

east or southeast in the foliation plane/ Cross-bedding

in the coarse elastics, which make up most of the western half of the Pinal Schist terrain (which essentially cor responds with the western limb of the major syncline described above) enables tops to be determined.' In all

exposures in the western limb of the syncline, the beds

show tops up to the southeast on southeast dips/ Although

somewhat wrinkled, the entire western half of the eastern

Pinal Schist terrain is a rather uniform homoclinal block;

this also defines the structure of the western limb of

the major syncline described above/ Cross-bedding

vanishes east of Government Peak and no top determina

tions were made.

There is no indication of major folds in the

eastern half of the eastern Pinal Schist terrain, in the

area corresponding to the eastern limb of the major

syncline/ The limb dips more steeply than the western

one, but shows only a little more wrinkling and minor

folding/ It is assumed by the writer that the major

syncline is the only large fold in the eastern Pinal

Schist terrain. 21

At least one major north-northeast trending fault breaks•across this Pinal terrain (see Figure 1) and the writer suspects there is another buried by the alluvium which fills the lower levels of the major canyon trending northeast just west of the Ninemile stock (see Figure i).!

Depending on the direction from which one approaches these faults on the ground, one will encounter either repetition or omission of beds near the fault; the amount is, however,

small, and in any case is easily accounted for. It does not seem likely that significant fault repetition or omission of beds has affected the eastern Pinal Schist

terrain, and the writer feels that the folded and wrinkled pattern shown on Figure 1 allows the reconstruction of the original stratigraphic thickness in the exposures to a reasonable degree.1 In this respect the eastern and western Pinal Schist terrains are alike.

Hetamorphic Rank

The metamorphic pattern in the eastern Pinal

Schist terrain is somewhat more complex than in the western

one.' A later hornfelsing contact metamorphic development

presumably due to one or more buried Mesozoic plutons has

affected earlier textures and mineralogy, as noted in the

discussion on amphibolites in Appendix A. This event is

not recorded in the amphibolites in the western Pinal

terrain. 22

The bio-bite isograd boundary has a rather erratic character, and is only approximated by Figure 3»’ The biotite-carrying rocks essentially occupy the western, northern, and eastern parts of the area, with some local exceptions, as can be seen in Figure 3.; As in the western part of the range, the biotite bearing metamorphites belong to the quartz-albite-epidote-biotite subfacies of the greenschist facies of Turner and Verhoogen (1 9 6 5 ) while the lower-grade rocks occupy the quartz-albite-epidote- chlorite subfacies of their greenschist facies. As in the western Pinal Schist terrain, the grade pattern shows no relationship to foliation-paralleling zones, or to the contacts of post-Pinal metamorphism intrusives as now ex posed on the present erosion surface/ As in the western

Pinal Schist terrain, the observed pattern of biotite distribution is ascribed to contact metamorphism due to buried Pre Cambrian intrusives. In this case the most likely such buried intrusive would be a buried connection between the Rough Mountain quartz monzonite gneiss and the Sheep Canyon quartz monzonite gneiss.

In addition to the Pinal Schist in the eastern

Pinal Schist terrain itself, there are a number of nearby large masses of Pinal Schist of varied sizes within the

Preo-ambrian intrusives in the central and south-eastern

Dos Cabezas mountains. These masses are essentially 23 xenoliths, so far as can be told. All these xenolithic blocks have been metamorphosed to a very high degree; microscopically they show rounded, quartz grains in an orthoclase or orthoclase-sericlte or orthoclase-muscovite

crystal aggregate.' They contain little or no biotite or magnetite^ In the field these units look like fine grained granitoid gneisses or foliated fine-grained dikes

or plugs of fine-grained granitoid intrusives,” and they were so mapped in some places until their true character was seen under the microscope.'

Original Character of the Rocks in the Eastern Pinal

Schist Terrain

As in the western Pinal Schist terrain, meta morphism in this area has been weak enough so that the

original textures and structures of the Pinal Schist rocks

before their metamorphism can be determined or strongly

inferred from their present compositions and textures.

Unlike the western Pinal Schist terrain, this

eastern one is composed primarily of metamorphosed sedi

ments, and the great majority of the non-amphibolite

samples reveal solely clastic detrital sedimentary

characteristics, although two igneous flow units were

observed.' The premetamorphism sedimentary sequence is,

grossly, composed of coarser and coarser units as one

goes from cast to west across the area.' The easternmost 24 exposures of the Pinal Schist in the eastern Pinal Schist terrain are characterized by phyllites with a very fine grained - 0.1 mm. assemblage of quartz, sericite, biotite, and magnetite; these rocks show very well-developed folia tion and very fine compositional lamination (see Appendix

A). They are interpreted by the writer as former detrital clastic shales, although, as there is some volcanisin associated with them, there may be some small tuffaceous proportion in the original material. As one moves west toward the central part of the area, in the vicinity of

Government Peak, coarser feldspathic and quartzese argil lites and meta-arkoses (see Appendix A) become the dominant type of unit, and shallow-angle cross-bedding begins to appear (see Figure 4b). •

Perhaps the most striking feature of all the Pinal

Schist areas in the Dos Cabezas mountains shows up to the west of Government Peak in the eastern Pinal Schist terrain.

As one goes to the west, the average size of the detritus in the very weakly metamorphosed sediments gets coarser and coarser, and shallow-angle cross-bedding becomes quite pronounced. Strings of pebbles and cobbles' appear, and as one continues to the west conglomerate zones begin to appear in the Pinal rocks. See Figures 4 and 5» Cross- bedding is especially marked on the slopes leading up to the western boundary of the area, and large arcuate festoon- 25 type cross-beds several feet thick have been seen (see

Figure 5&)• See Appendix A for petrographic descriptions.

Somewhat more than 10,000 feet of Pinal sediments in the eastern Pinal Schist terrain are of this type." The writer had hoped that this sequence of conglomerates would represent the base of the Pinal Schist in the Dos Cabezas mountains, but this is not the case/ Near the western

end of the eastern Pinal Schist terrain, at the original

stratigraphic bottom of the conglomerate sequence, the

coarse elastics give way to intensely metamorphosed phyllites and mica schists, and these latter units are

intruded by the Rough Mountain quartz monzonite gneiss

and the Polecat quartz monzonite, all along the western boundary of the eastern Pinal Schist terrain/ See Figure

Two flow units, one a dacite porphyry and the

other a rhyolite porphyry, were found in the easternmost

part of the eastern Pinal Schist terrain/ These units are

described in detail in Appendix A;

There are several amphibolite bodies of rather

stocklike or dikelike character in the eastern Pinal schist

terrain.'• They show a two-stage metamorphic development, as

described in detail in Appendix A. Briefly, these bodies

show a marked foliated texture in hand specimen, marked by Figure 4: Primary Depositional Structures in the Pinal Schist

(a): Conglomeratic Pinal Schist exposure on the west side of Happy Camp Canyon, The pen in the lower part of the picture is five inches long. Mote the single large cobble of dark aphanite and the numerous pebbles of dark and light materials above and around the area where the pen is.

(b): Typical shallow cross-bedding in the Pinal Schist just west of Government Peak in the eastern Pinal Schist terrain. At least 5000 feet of strata on the cast side of Happy Camp Canyon are characterized by this type of bedding; it represents shallow channel cut-and-fill" patterns of deposition. Tops are to left. Generally rocks showing this bedding are very weakly metamorphosed arkoses. Figure 4s Primary Depositions! Structures in the Pinal

Schist Figure 5: Primary Cross-Bedding in the Pinal Schist

(a): Large-scale channel-cutting cross-bedding in vrealdy metamorphosed arkosc in the Pinal Schist along Happy Camp canyon. The exposure as pictured is about four feet across.- The beds are dipping nearly vertically, with tops to the left. A few small pebbles can be seen along some bedding surfaces in the left part of the picture.

(b): This shows a slabbed block of Pinal Schist from Happy Camp canyon, showing small-scale cross-bedding offset by miniature faults running from top to bottom of the slab, at right angles to the bedding," The eraser is about two inches long. Figure 5: Primary Cross-Bedding in the Pinal Schist 2 3

Figure 6: General View of Eastern Pinal Schist Terrain

A typical Pinal Schist terrain north of the

Apache Pass fault zone• This is from the eastern Pinal

Schist terrain, looking into Sec. 27, Tl4s R28S. Area marked ap is probable Precanbrian apllte. Zone marked oph is a small dike of Government Peak ophite paralleling foliation. The Pinal (pi) is mostly sericite phyllite.

For a view of the Southern Pinal terrain see Figure 2(a), oriented dark mineral "bands and bands of light-colored crystals.- Hie primary foliation presumably dates from the time of metamorphism and deformation of the enclosing

Pinal Schist." Development of radiating hornblende crystal groups in the dark-mineral bands of the amphibolites in dicates a second thermal metamorphism of probable

Cretaceous age, caused by buried Cretaceous plutons,

Pre--Cambrian Dacite Porphyry Stock

A large dacite porphyry stock of probable Older

Precambrian age cuts the Pinal raetasediments in the south western part of the eastern Pinal Schist terrain. It underlies some three square miles of exposure there (see

Figure 1)

Good primary flow structure is present in the unit, and shearing is quite weak. The internal flow foliation trends northwest, at right angles to the strike of the enclosing metasediments.

The age assignment of the dacite porphyry is tentative, but is regarded by the writer as reasonable, based on the following evidence.' First, the stock in vades the Pinal Schist but is itself invaded by Cretaceous volcanic breccia; this puts maximum limits on its age.

Second, there is no present evidence in the Dos Cabezas mountains for any intrusive events between circa 1400 30 million years ago and the later part of the Cretaceous period, when volcanic breccia invaded the core of the range/ There is Jurassic plutonism in the Mule mountains about 100 miles to the southwest, however, so that, on this evidence alone some consideration must be given to the possibility that the stock was emplaced within the

1400 circa ?0 million year time interval in which no known plutonism has yet been recorded in the Dos Cabezas mountains.

The strongest evidence for the age of the dacite porphyry comes from the observation that the stock has a metamorphic biotite development exactly like that in the Pinal Schist metamorphic units in both the eastern and western Pinal Schist terrains. This strongly implies to the writer that the stock is pre-biotite development in age, and, since this biotite development is thought to be due to partially buried plutons about 1400 million years in age, as discussed above, this puts the stock into an

Older Pre-Cambrian age bracket. No present data forbid this interpretation.

Southern Pinal Schist Terrain

The southern Pinal Schist terrain is composed almost wholly of quartzite of- two main varieties, which make up about four square miles in the western part of

the southern Dos Cabezas mountains, south of the Apache

Pass fault zone (see Figure 1). The character of the 31 quartzites is quite similar to that of the quartzite masses south of the Apache Pass fault in the northern Chirichahua mountains as described by Sabins (1957b).' The quartzites in the Dos Cabezas mountains are present in one large mountainous mass and a large number of small xenolithic blocks in the Pre c ambrian intrusives of the southern

Dos Cabezas mountains♦' There is also a small area of the southeastern part of the range which is underlain by highly metamorphosed nonquartzitic Pinal Schist; this is Included in the southern Pinal Schist terrain.* See Figure 1 for locations, especially Sec. 35, T14S, R27E, and Sec.5 2 6 3,

T15S R27E, and Sec. 33, .T14S R28E.

Although the quartzite units here are tentatively identified as Pinal Schist units, it is possible that they actually correlate with the younger Hazatzal quartzite of central Arizona described by "Wilson (1939). Age data on the intrusives which cut the quarzites allow this inter pretation, but do not compel it/ Because the other large metasedimentary masses in the range can be assigned to the

Pinal on more certain grounds of at least partial lithologic similarity to Pinal exposures in the type area and in . surrounding mountains, the writer will assign this mass to the Pinal as well. Also, there is some phyllite in this sequence, interlaycred with the quartzites, and it is quite identical in character to that in the other Pinal 32

Schist terrains. The same metamorphic pattern prevails in all three terrains, too, as will soon he descrihed.

Shields (1940) assigned a Cretaceous age to these quartzite sediments, relating them to the Bisbee group of

Ransome (1904). The intrusion of the quartzites in the main western exposure by Fre e ambrian quartz monzonite forbids such an interpretation.

Structure

Structurally, the main mass of quartzite is a vertically-plunging, hook-shaped fold opening to the west, with a southern limb trending generally northeast and a sharply bent northern limb trending west-northwest.

Foliation dips throughout are steep to vertical, as is the fold axis (see Figure 1). Local wrinkling and squeezing modifies the overall fold pattern in detail.

Foliation in the quartzites is parallel.to obvious bedding in many places and is presumed to be so throughout the fold. In many places the quartzite seems massive, with no trace of original bedding left. Bedding, where visible, is marked out by stringers of magnetite in parallel layers.

In places rehealed shear joints with dark stains along them can be mistaken for bedding if one is not careful; in a few places it was impossible to tell if bedding was being observed or not. 33 The xenolithic "blocks of Pinal Schist in the rapakivi pluton have west-northwest foliation trends for the most part, which suggests to the writer that they were part of some larger mass of Pinal Schist during the time the rapakivi was intruding them. They appear to be classic roof pendants.- These xenolithic blocks are composed of the two quartzite varieties. One such xenolithic block is responsible for an interesting stratigraphic structure at the base of the Cambrian

Bolsa Quartzite, which will be discussed in Chapter

Metamorphic Rank

Petrographically, the Pinal Schist in the southern Pinal Schist terrain is quite simple. Most of the quartzite in the main mass and the xenolithic blocks is a white, massive unit, thick-bedded, with thin dark bands of magnetite parallel to bedding, where bedding could be seen, Interbedded with the white variety are layers of red-brown quartzite frequently containing large

2-3 mm. randomly oriented crystals of muscovite. The quartzite, contains local zones of phyllite and very fine grained quartzite which were preferentially sampled for metamorphic grade determinations.

As can be seen in Figure 3» a biotite-bearing

zone crosses the center of the main mass of the southern Pinal Schist terrain*' It has a quartz-biotite-sericite- magnetite assemblage, the same as that in the metamorphosed biotite-bearing rocks of the other Pinal Schist terrains.

Within this southern Pinal Schist terrain, rocks north and south of the biotite-bearing ones show quartz-chlorite- sericite-magnetite and quartz-sericite-magnetite assem blages.1 These assemblages are the same as those in the metamorphosed rocks in the western and eastern Pinal

Schist terrains, and have the same rank.

Original Character of the Pre-lletamorphic Rocks in the

Southern Pinal Schist Terrain

These quartzites" seem to have originally been nearly pure, very thick-bedded quartz sands, with local interlaminated magnetite-bearing sands and a minor amount of admixed argillaceous material and fine quartz clay.

Those quartzites now red-brown in color seem to have had a higher iron content and more argillaceous material in them than the white quartzites did. See Appendix A for a full discussion of these rocks.

No metamorphosed volcanic rocks or amphibolites were found in the southern Pinal Schist terrain.

In addition to the typical quartzites of this terrain and their related phyllites, there is an exposure of about one-half square mile of highly metamorphosed

Pinal Schist in the southeasternmost Dos Cabezas mountains. • 35 south of the Apache Pass fault. This body is invaded by a small body of coarse-grained quartz monzonite gneiss simi lar to the Eaton gneiss in the northern Dos Cabezas range, and is also intruded by the eastern part of the rapakivi quartz monzonite pluton (see Figure 1).

This mass of Pinal Schist is very highly meta morphosed, and was mapped originally as "aplitic granitoid gneiss"; later investigation showed that under the micro scope this rock showed the same texture as exists in the * • t Pinal xenoliths in the Pre Cambrian intrusives in the northern Dos Cabezas range. Small circa 1-2 mm. rounded quartz grains lie in an interlocking mass of potash feldspar crystals of very small size. These minute potash feldspar crystals are markedly sericitized. Vlhere- ever found in the Dos Cabezas mountains, these highly metamorphosed masses of Pinal Schist are in contact with or surrounded by Prec ambrian granitoid intrusives.

Overall Character of the Pinal

Schist in the Dos Cabezas Mountains

The massive quartzite terrain in the Pinal Schist

lies' south of the Apache Pass fault. The Pinal Schist

terrains composed of flow rocks and normal clastic

sediments of generally fine grain (now metamorphosed)

lie north of it. The lithologic contrast between the

two terrains is very marked, as the typical units of the 36 pre-metamorphic rock suite in either terrain are either absent or very rare in the other.' The structural con trast between the rather right, vertically plunging folds in the southern quartzite terrain, seen south of the

Apache Pass fault in both the Dos'Cabezas mountains and the northern Chirichahua mountains, and the rather broad, open, moderately plunging folds in the Pinal Schist terrains north of the fault is also noteworthy. The writer believes that this structural and lithologic contrast between.the Pre c'ambrian metamorphic terrains on the two sides of the Apache Pass fault indicate a great deal of movement along the fault, or more precisely, along its Pre cambrian equivalent.

It is most likely that these terrains were origi nally separated primarily in a lateral direction; the folding in the quartzites south of the fault can be interpreted as drag folding showing left-lateral movement ' along the fault in Fre e ambrian time.

The Pinal Schist terrains in the Dos Cabezas mountains represent moderately metamorphosed stratigraphic sequences of great thickness,' Neglecting possible removal or repetition of stratigraphic intervals by unnoticed folds or by faulting, which the writer assumes to be a reason able thing to do, the western and eastern Pinal Schist terrains together record a minimum of perhaps 45,000 feet 37 of strata.' The southern terrain, while directly displaying perhaps 7,000 feet of strata, implies a great deal more, as marked by the apparent roof pendants of quartzite in the rapakivi quartz monzonite, which are several miles from the main mass of quartzite.' The two areas above record a total of more than 5 2 ,0 0 0 feet of thickness for their overall stratigraphic accumulation.

The overall stratigraphic sequence displays a great variety of li thologic types as diversified as anything in the post Pre-c'ambrian sequence in Arizona and records far greater thicknesses of total strata than are recorded for the total of post Pre-cambrian deposition in any part of southern Arizona. For example, the total thickness of Cambrian through Cretaceous strata in the

Chirichahua-Dos Cabezas mountains area is perhaps 11 to

12,000 feet (Wilson, 1962).' This is only one-fifth the amount of strata, at a minimum, visible in the same area in the Pinal Schist. The great majority of the depositional history of the Dos Cabezas mountains is contained in its

Pre-Cambrian metasediments

Age of the Pinal Schist in the Dos Cabezas Mountains

The absolute age of the Pinal Schist exposures in the Dos Cabezas mountains is essentially unknown in 4 any strict sense and must "be decided in some measure 38

largely by inference. Geochronologic results discussed

here will be discussed in much more detail in Chapter 9»

The writer did not determine any dates in the

Dos Cabezas mountains which could fix. the absolute age

of the Pinal Schist to within better than a few hundered

million years. The oldest age obtained on the rocks of

the range is 14?0 ± 30 million years, obtained on a

whole-rock Rb-Sr isochron on the Eaton gneiss!c quartz

monzonite. This gneiss is intrusive into the Pinal Schist

in the northern Dos Cabezas and forms, at least in that

'place, a minimum age boundary for the Pinal Schist. The

large quartzite masses south of the Apache Pass fault

are cut by the Dos Cabezas rapakivi quartz monzonite,

which gives a whole-rock Rb-Sr isochron age of I3 8 O -

30 million years, and forms a minimum age boundary for

these latter metasediments.

It should be noted that the age data so far quoted

also allow the interpretation that the quartzite terrain

south of the Apache Pass fault is Mazatzal quartzite; work

by Livingston and Damon (1 9 6 7 ) in the Bronco Ledges area

along the Salt river in central Arizona shows an age of

1600 - 100 million years for a rhyodacite ignimbrite below

a Mazatzal-like quartzite sequence, which sequence is in

turn intruded by a 1420 million year old pluton. 39

The age of deposition of the sediments in the

Pinal Schist in the Johnny Lyon Hills, 30 miles west of the Dos Cahezas mountains, has been determined by Silver and Deutch (I9 6 I ). Rhyolites interbedded with the sedi ments while they were being laid down give an age of

1720 million years by the U-Pb method on zirconsThe granodiorite which cut the sediments after metamorphism gives an age of 1660 million years by the same method,'

The metamorphic fabrics and rock types developed in the Johnny Lyon Hills and in the Dos Cabezas mountains in the Older Pre-Cambrian metasediments are types characteristic of regional dynamo thermal metamorphism.

In both the above areas there is evidence for only one such metamorphism; if they are the same metamorphic event," then the Pinal in the Dos Cabezas mountains must have been metamorphosed before 1660 million years. Mo Pre- cambrian sediments more than 1800 million years old are presently known in southern Arizona, and it is reasonable to assume as a hypothesis awaiting further examination that the metamorphosed sediments and volcanics in the

Pinal in the Dos Cabezas mountains were laid down between

1660 and 1800 million years ago. CHAPTER 3

NON-RAPAKIVI PRECAIiBRIAN GRANITOID

ROCKS IN THE DOS CABEZAS MOUNTAINS

A complex assemblage of Precambrian Igneous rocks

of post-Pinal Schist metamorphism age is present in the

Dos Cabezas mountains. These units vary greatly in area

of exposure and texture, but are rather grossly similar

in mineralogy and composition; all are rather coarse-

.grained quartz monzonites. Perhaps one-third of the total area of these rocks is underlain by units of predominatly

gneissic aspect; these commonly show marked planar

orientation of biotite and/or feldspar crystals. The

other, non-gneissic units in this assemblage are intrusive

porphyritic bodies showing a pronounced dual foliation

orientation of their contained potash feldspar phenocrysts,

but no throughgoing foliation affecting all phases.

These various units will next be discussed indivi

dually and then collectively by main type. All the units

mentioned in the following discussion have been given

provisional names for the purpose of clarity in discussion.

The gneissic units are as follows; first, the

Sommer quartz, monzonite gneiss, named after a triangulation

tower in the center of its outcrop area; second, the

40 41

Rough Mountain quartz monzonlte gneiss, named after the

excellent exposures of it on that mountain mass; third,

the Eaton quartz monzonite gneiss, named after its ex

cellent exposures in the Eaton canyons on the north side

of the range; fourth, the Sheep Canyon quartz monzonite

gneiss, named by Sabins (1957b) the Sheep Canyon Granite

after the main canyon which cuts its limited outcrop area;

and, last, the Cienaga gneiss, occurring in two different

types, named after a spring in the northern part of its

outcrop area.

There are two large essentially non-foliated

plutons of quartz raonzonitic composition in the Dos Cabezas

mountainsThe first is the Polecat quartz monzonite,

named after the Polecat canyons cutting the center of

its outcrop area; the second is the distinctive rapakivi-

textured quartz monzonite pluton in the southern part of

the range. This latter unit is henceforth called the Dos

Cabezas rapakivi quartz monzonite, after the village of

Dos Cabezas, Arizona, which is built on the pluton.

The Eaton Quartz Monzonite Gneiss

This unit underlies about four square miles of

the north central part of the Dos Cabezas mountains (see

Figure 1 and Figure 7)• The Eaton gneiss intrudes the

Pinal Schist, and is in turn intruded by the Polecat

quartz monzonite and the Rough Mountain gneissic quartz 42 monzonltev In the field,- the Eaton gneiss is a coarse

grained biotite-rich leucocratic rock, which is, however, darker than the above younger units which cut it.

As discussed in more detail in Appendix A, the

Eaton gneiss is a calcic quartz monzonite, showing a

strong primary mineral-orientation foliation with a super

imposed subparallel weak shear foliation.'

Structure

The field relations of the Eaton gneiss are rather

simple. It displays a quite uniform character, containing

a marked steeply dipping generally northward-trending

foliation marked in outcrop by sub-parallel elongated masses of biotite. The gneiss includes numerous blocks

and xenoliths of the Pinal Schist; some of these are of

considerable size (see Figure 1),

Age of the Eaton Gneiss

The Eaton quartz monzonite gneiss is the oldest

of the Precambrian bodies in the Dos Cabezas mountains whose age has been accurately determined, and which is

intrusive into the Pinal Schist. It gives an age of

14?0 - 30 million years on a whole-rock Rb-Sr, isochron.

See Chapter 10 for a full discussion of the dating results

on this unit *oy Mx/e So/np/e

Q0?. /Jo re Sct/rjp/m

T/5S

f/gurc. 7 Zocor/on of Precomtor/on Gron/ro/ct Snrrxjs/u'es o/xJ Intrus/ve Samp/es A S 7 E £pr 2}4

Second Body of Presumed Eaton Gneiss

A small body of quartz monzonite gneiss lies in the southeastern part of Sec, 32, T14S, R28E (see Figure

1 and Figure 7). This gneiss is separated by about six miles of intervening rock units and the Apache Pass fault zone from the Eaton gneiss outcrops, but is very much like the Eaton in all its observable characteristics. It intrudes the Pinal Schist on three sides and is covered by younger alluvium on the fourth. In the field, it shows a marked northerly-trending, steeply dipping foliation, and its mode (see Table2)gives a calcic quartz monzonite composition for the unit. Its gneissic texture is the

same as that in the main exposure of Eaton gneiss; it is defined by short discontinuous streaks of biotite crystals.

This is the only case where a Precambrian intrusive rock having outcroppings on one side of the Apache Pass fault in the Dos Cabezas mountains has any outcroppings at all on the other side of the fault. Presumably the

Eaton gneiss terrain was once far more widespread at this

level than it presently is.

The Sommer Quartz Monzonite Gneiss

This small Precambrian pluton appears in the westernmost part of the Dos Cabezas mountains, where

it intrudes the Pinal Schist. It is in turn intruded Figure 8: Two Gneiss!c Pr'ecanbrian Intrusives.

(a): A slab of Eaton quartz nonzonite gneiss showing an internal foliation which rises from lower right to upper left across the face .of the specimen. The eraser is about two inches long.

(b): A slab of Rough Mountain gneiss!c quartz monzonite. It shows a faint foliation climbing steeply to the right of the picture from the base of the specimen. The eraser is about two inches long. Mote the finer grain size relative to the Eaton gneiss; this is a general field re lationship and aids in distinguishing the two. The Eaton is also much more biotite-rich. Figure 8 : Two Gnelssic Precambrlan Intrusives Figure 9 : Tiro Precambrian Plutonic Units From the Northern Dos Cabezas Mountains.

(a): A slab of the Sommer gneissic quartz non-' zonite. It has an internal foliation rising vertically along the slab face. Note that the grain size is larger than that in the Rough Mountain gneiss, while it is more biotite-poor than the Eaton gneiss.

(b): A slab of Polecat quartz monzonite, showing poor alignment of large potassium feldspars along a folia tion trend rising gradually from right to left going across the specimen. Note that the foliation is much weaker than that in any of the gneisses; the potassium feldspars are also more euhedral." 4 6

Figure 9: Two PreCambrian Plutonic Units from the North ern Dos Cabezas Mountains 4?

by the Polecat quartz nonzonlte.1 Figure 1 shows the de

tails of its contacts» It has about two square miles of

total outcrop, although the exposures of the unit are

scattered over several sections of land. The total ex

tent of the unit is probably not much greater than its

present surface exposure; the gneiss is partially buried

by Tertiary alluvium, however, and this forbids any ■v-. . definite statement about extent.

In field exposure the rock is a reddish-brown

color due to rather intense weathering; the weathering

' is quite deep and strong in most of the outcroppings.

The unweathered rock is a pale gray color, as can be

seen in a few places in stream bottoms.’

As discussed in Appendix A, the Sommer gneiss

has a quartz monzonitic composition. It has a marked

foliation, due to subparallel arrangement of potassium

feldspars and later shear fractures. It is quite hetero

geneous conposltionally.

Structure

In most, but not all exposures of the Sommer

gneiss, a weak to strong foliation is observable, marked

by orientation of platy feldspars and minute shears

paralleling this orientation. See figure 9(a). Thin-

section analysis indicates that the foliation is probably

a primary crystal orientation structure, with the shearing 48 imposed after the rock had become at least fairly rigid.

With respect to its overall outcrop pattern, the rock is quite uniform in grain size but not so uniform in mode.

In one dikelike zone, along the southeastern, part of the main body of gneiss, there lies a mass of material with distinctly finer grain than the main Sommer gneiss.

This body is presumed to be a late part of the Sommer magma.'

The Sommer gneiss-Pinal Schist contact shows local discordant character, and several dikes of the

Sommer gneiss cut the Pinal Schist beyond the main contact of the two bodies. All evidence indicates that the Sommer gneiss is intrusive into the Pinal Schist,

The Sommer gneiss occupies the core of a broad open-folded east-northeast plunging anticlinal structure in the Pinal Schist. The internal foliation of the Sommer gneiss crudely parallels the foliation in the metamorphic terrain, and the field relations would indicate that the

Sommer gneiss is probably a syntectonic or late-tectonic intrusive into the Pinal Schist and dates from the time of original folding and metamorphism of the Pinal.

Geochronologic evidence is perhaps at variance with this interpretation, as will be seen.

The Sommer gneiss contains a large number of small, generally foliated amphibolite bodies of several textures 49 and grain sizes. The orientation of the foliation in these amphibolites is generally parallel to the foliation in the surrounding gneiss (which is, in turn, parallel to the foliation in the enveloping schist) and is seldom parallel to the long axis of the amphibolite unit (see

Figure 1). It is felt that these small amphibolites were intruded into the Sommer gneiss after it became rigid enough to fracture, and were then deformed along with it in the closing stages of what may loosely be

called Pinal Schist age tectonism. The great similarity

of the mineralogy, structure, and textures of these amphi bolites to those in the Pinal Schist terrain surrounding

the Sommer gneiss almost unavoidably suggests that the

amphibolites in the two rock types have the same origin, with respect to time of intrusion and time of metamorphism.

Geochronologic evidence allows but does not compel this

interpretation. .

Age of the Sommer Gneiss

K-Ar dating of hornblende from an amphibolite

which may or may not cut the Pinal Schist and the Sommer

gneiss gives an age of 1175 ~ 30 million years; another

K-Ar date on biotite from the Sommer gneiss itself gave •J-* 1100 - 30 million years. These ages are certainly minimal,

and the author tends to regard them as representing ages

more directly related to the tine of perturbation of 50 the nearby Polecat quartz monzonite at 1000 million years ago than to time of original crystallization of either the amphibolite or the Sommer gneiss,1 The two main lines of thought which support this conclusion are as follows,

First, the Sommer gneiss and the dated amphibolite are very strongly correlated, on the basis of a common dynamo thermal metamorphic event, with the metamorphism of the Pinal Schist in the western Dos Cabezas, As al ready described, the author thinks there is some reason to believe that the metamorphism of the Pinal Schist in the

Dos Cabezas mountains took place about 1650-1750 million years ago,1 Secondly, were the actual ages of the Sommer gneiss and the amphibolite to be as shown, it would compel the interpretation that there had been a dynamo thermal metamorphic event of considerable vigor in tills part of

Arizona about 1100-1200 million years ago. No trace of such a metamorphic event has been found anywhere else in

Arizona. The writer believes that the Sommer quartz monzonite gneiss has a true age probably quite close to that of the metamorphism of the original Pinal sediments; that is, about 1700 - 100 million years.

Rough Mountain Quartz Monzonite Gneiss

This unit makes up large parts of the north central

Dos Cabezas range, and outcrops especially conspicuously 51 on the mountain mass after which it is named (see Figure

1 and Figure 2). It intrudes the Pinal Schist and the

Eaton quartz monzonite gneiss, and is in turn intruded by the Polecat quartz monzonite, as well as by much younger Laramide and mid-Tertiary small intrusive bodies.

The Rough Mountain gneiss is of quartz monzonitic composition (see Appendix A), a^d shows a marked foliation due to subparallel arrangement of potassium feldspar crystals. The rock is quite homogenous compositionally.

Structure

The Rough Mountain quartz monzonite gneiss is a medium-grained leucocratic rock, showing in general a rather weak foliation (see Figure 8 (b) and Figure 1) which is usually of northerly.strike and vertical dip; the dip, however, tends to flatten out and depart from the vertical in places, especially near the northernmost margin of the . range in the easternmost part of the gneisses1 exposure.

A second foliation, normal to the first, is occasionally seen. In some areas of exposure no foliation is visible.

Where visible, the foliation is made visible on the outcrop by the subparallel orientation of small potassium feldspar crystals of tabular form; in thin section, biotite and quartz have the same gross orientation as the feldspar, but this cannot be observed in the outcrops due to rather 52 intense weathering,' Microscopic observation indicates that the foliation is a primary flow structure. In samples along the northern margin of the exposures the foliation is perhaps most marked, and here some shearing parallels the feldspar-marked foliation in outcrop. In general, however, shearing is of a much lower intensity than in the Sommer quartz monzonite gneiss earlier described.

Age

The Rough Mountain quartz monzonite gneiss invades the Pinal Schist and the Eaton gneiss, and is in turn invaded by the Polecat quartz monzonite. Its age is thus bracketed and lies between the 1470 - 30 million year age of the Eaton and the 1425 million year age of the Polecat.

A figure of 1440 - 3° million years should be a reasonable estimate of the age of the Bough Mountain quartz monzonite gneiss.

Correlation of the Rough Mountain Gneiss with Other Pre- cambrian Gneissic Intrusives

The mineralogic data which forbid exact correlation of the Rough Mountain and Sommer quartz monzonite gneisses as parts of the same' parent magma, in spite of their similar appearance in the field, are as follows, First, the potassium feldspar in the Rough Mountain gneiss is microcline and that of the Sommer gneiss is orthclase.

Secondly, the dark-mineral assemblage in the Rough Mountain 53 gneiss is markedly simpler than that in the Sommer; the

former has no chlorite or sericite in it while the latter

has abundant crystals of both as part of its fundamental

mineralogy. Third, the Rough Mountain gneiss shows almost

no shear foliation, while the Sommer gneiss shows a great

deal.

In addition, field study shows that, as previously

mentioned, the age of the Rough Mountain must be very close

to 1440 - 30 million years. If the writer’s belief, that

.the Sommer gneiss corresponds in age to the time of the

last stages of Pinal Schist metamorphism is correct, then

the age data on the two units positively forbid any cor

relation between the Sommer and Rough Mountain gneisses.

The Rough Mountain gneiss obviously intrudes the

Eaton gneiss, as well as having a markedly different field

appearance and mineralogy, and there is no possibility of

correlation of the two, except perhaps in the broadest

sense of common parentage in some less differentiated

magma at depth.

Another indication that the Sommer and Rough

Mountain gneisses are not part of the same gneissic terrain

lies in the presence of an extensive amphibolite suite

in the Sommer gneiss, which suite has no correlative

group of dikes in the Rough Mountain exposures. Sheep Canyon Quartz Monzonite Gneiss

The third of the gneissic Precambrian units in the northern part of the Dos Cabezas mountains is the Sheep

Canyon quartz monzonite gneiss (see Figure l ) . 1 This is

the same unit referred to as the Sheep Canyon Granite by

Sabins (1957^) but as the author is using a more detailed

classification system than Sabins used for the acidic plutons of the range, he: prefers the name given hero.

This unit underlies about three square miles of the north

eastern part of the Dos Cabezas mountains, where it intrudes

the Pinal Schist and is in turn intruded by the mid-Tertiary

Ninemile granodiorite stock.

The Sheep Canyon gneiss is quartz monzonitic in

composition (see Appendix A) and "is rather heterogeneous

compositionally. It shows a marked primary flow foliation

seen in feldspar orientation.

Structure

This rock mass is a leucocratic unit of medium

grain size, showing a pronounced foliation in most out

crops. It closely resembles the Rough Mountain gneiss.

The foliation has a general northwestern trend, but

foliation strikes swing to the east in the easternmost

outcrops.' Dips vary widely, from vertical to horizontal,

and appear to define a number of broad open folds or

domes in the gneiss. A probable outlying exposure of 55 this gneiss is found in the NE1/4 of Sec. 35» T14S R28Z, and another in Sec. 5» Tl^S R28E.

Age

Mo age data, are available on the Sheep Canyon quartz monzonite. It is only definable as post-Pinal

Schist and pre-mid-Tertiary in age. In structure, mineral

ogy, and appearance, however, it closely resembles the

other medium-grained gneisses in the northern Dos Cabezas mountains, and is almost certainly a Precambrian intrusive.

It is not cut by any Precambrian intrusive, so no limiting

age data are available from such relations. It has many

similarities to the Sommer gneiss in field and .thin sec

tion characteristics, and may be as old as that unit;

that is, some 1650-1750 million years. All the dated

Precambrian intrusives in the Dos Cabezas mountains are

1380 million years old or more, and a likely minimal age

for the Sheep Canyon quartz monzonite gneiss is 1400 million

years.

Correlation of the Sheep Canyon Gneiss Uith Other Pre

cambrian Gneisses

The Sheep Canyon gneiss is not likely to be cor

related with the Eaton gneiss by anyone who has seen the

two units in the field. The Eaton gneiss is the more 56 coarse-grained of the two, and its foliation in outcrop is largely marked by subparallel clots of biotite crystals; the foliation of the Sheep Canyon gneiss is marked by sub parallel feldspar crystals. .

The Sheep Canyon gneiss is similar in some respects to the Rough Mountain and Sommer gneisses. All three are porphyritic quartz monzonite gneisses whose foliation is marked by planar parallelism of their enclosed feldspars.

They all have similar grain sizes and similar appearance in the field.

On more detailed examination, however, distinctions among these units appear. The potassium feldspar in the

Sheep Canyon gneiss is a little different from the feldspars in the other two gneisses, and in addition, the low modal abundance of biotite and the presence of muscovite in the rock help distinguish the Sheep Canyon gneiss from its neighbors.

Like the Rough Mountain gneiss, the Sheep Canyon gneiss has no amphibolite suite contained within it, and in this respect is unlike the Sommer gneiss. The Sheep

Canyon gneiss, however, is more intensely sheared in many of its outcrops than the Rough Mountain gneiss, and in this respect looks more like the Sommer gneiss. Correla tion of these three gneisses is, in short, ambiguous. More geochronologic data are needed before a final resolution 57 can toe made.'

For the present, the writer adopts the following tentative correlation. The Sheep Canyon and Rough Moun tain gneisses are considered to toe essentially contempo raneous on the basis of their geographic association and the absence of foliated amphibolite suites from tooth rock

types. These two gneisses are both thought to toe much younger than the Sommer gneiss; the age difference between

the Sommer gneiss and the other two is probably on the order of 200 -2 5 0 million years.

The Clenaga Quartz Honzonite Gneiss

All the aforementioned units, with the exception

of the small mass of presumed Eaton gneiss described

earlier lie north of the Apache Pass fault zone in their

entirety. The main gneissic unit south of the fault, the

Cienaga gneiss, lies in between the two parts of the Dos

Cabezas rapakivi quartz monzonite pluton, and underlies

-about three square miles of exposure in the southeastern

corner of the Dos Cabezas mountains. Modal data (Table 2 )

reveal a quartz monzonitic composition for both facies.

It is post-Pinal Schist and pre-rapakivi in age. The

unit shows a major and a minor phase, labelled as Cgn^

and Cgn2 respectively on Figure 1. The main unit (Cgn^)

is very rich in large tabular potash feldspar phenocrysts, 58 while the minor unit is quite poor in them. Both phases, however, have nearly Identical modes (see Table 2). The two phases of the Cienaga gneiss are presumed to be equivalent; indeed, in some parts of the outcrop area the two types are Intermixed and essentially inseparable. In the field the two types may have either gradational or sharp contacts with each other.

Structure

The gneiss shows a characteristic structure in both phases/ The potash feldspar phenocrysts commonly

show marked preferred orientation which defines a folia

tion plane in the gneiss. Frequently a weaker, second foliation crosses the first at near right angles. The

foliation planes always dip very steeply,' The strong

foliation usually trends northwest and the weaker north

east in the observed outcrops. The rock is low in biotite,

and quite leucocratic.

Correlation With Other Procambrian Gneisses of the Dos

Cabezas Mountains

The Cienaga gneiss.has the following characteris

tics, which may be related to the other Precambrian

gneisses in the Dos Cabezas range for purposes of correlation.

First, it lacks amphibolite intrusXons. Second, it is

markedly porphyritic in most outcrops. Third, shearing 59 Is TJeak to absent. Fourth, thin sections show muscovite and biotite. Fifth, thin sections also show that there are two plagioclases in at least some of the Cienaga gneiss and that these plagioclases are generally weakly twinned and zoned. Characteristics one and three show no cor relation with the Sheep Canyon gneiss, perhaps, but the plagioclases are different in the two gneisses. Charac teristics two and five imply strong correlation with the

Rough Mountain gneiss; the plagioclase correlation is • quite striking. The writer feels that the best tentative correlation would be to relate the Rough Mountain and

Cienaga gneisses in a quite tentative way as part of the same general magmatic body. Since the Rough Mountain and Sheep Canyon gneisses have already been correlated in a previous section, the wisest correlation is perhaps to relate the Cienaga, Rough Mountain, and Sheep Canyon gneisses together as part of the same restricted period of plutonism in the central Dos Cabozas mountains; and leave open the whole question of magmatic connection between the units concerned.

Age of the Cienaga Gneisses

The above correlation implies an age of 14^0-

1400 million years for the Cienaga gneiss.; Data from the area of the Dos Cabezas south of the Apache Pass fault reveal a similar picture. These Cienaga gneiss 60 outcrops, along their northeastern contact, lie very close to, but not in contact with, the snail body of Eaton gneiss south of the fault (see Figure 1), and the field relations make it seem very likely that the Cienaga gneiss intrudes the Eaton in that area. If so this puts an older age limit of 1470 « 3° million years on the Cienaga body.

The unfoliated rapalcivi quartz monzonite pluton gives an age of 1380 - 30 million years, and, since it . intrudes the Cienaga gneiss, this date establishes the minimum age of the gneiss. The Cienaga gneiss, like the

Eaton and Rough Mountain gneisses, is part of a group of

gneissically foliated plutons emplaced within the period

1400-1470 million years in the area of the present Dos

Cabezas mountains/ This is an important observation,

since markedly foliated plutons of this age have not, so

far as the writer is aware, been described before in

this part of the southwest. The existence of strongly

foliated plutons implies a certain amount of dynamic

tectonic activity in the Dos Cabezas area in that period

of time. The Dos Cabezas mountains are quite close to

the Texas Lineament (see Chapter 6 ) and it may be that

minor movement along this major fracture during this 1400-

1470 million year period of time caused local tectonism

and, perhaps, metamorphism, not seen in other more 61 quiescent areas of Arizona in the same time interval.

The Polecat Quartz Honzonlte

Perhaps half of the PreCambrian terrain in the northern part of the Dos Cabezas mountains is composed of the Polecat quartz monzonite,1 This large stock under lies about 18 square miles of the visible outcrop of the range, and comprises the larges single unit in it. The extent of the pluton may be far more than its outcrops in the Dos Cabezas mountains imply, as trill be seen.

In the Dos Cabezas range, the pluton invades the Pinal

Schist, the Rough Mountain gneiss, and the Eaton gneiss.

The Polecat pluton has a rather characteristic field appearance throughout its outcrop area. It is a coarse grained, markedly porphyritic rock, gray on fresh surfaces but weathering to a pale reddish-brown color.

Structure

Large tabular euhedral potassium feldspar crystals up to 3 -^ cm. long lie in a coarse-grained ground mass of quartz, plagioclase, and potash feldspar. The potash feldspar phenocrysts commonly show a dual foliation pattern with respect to their internal position in the pluton; this

evidently is a weak flow foliation which reflects the pattern of flow of the original magma of the intrusive during its

emplacement. The foliations commonly show a north-northeast 62 and a west-northwest orientation. Commonly one of the foliations is much stronger than the other, but the strong foliation may have either of the two prevailing trends, depending upon where you are in the pluton. The foliation pattern on Figure 1 within the outcrop area of the Pole cat pluton represents the writer's best estimate of foliation strike where observed; in places this was mea sured on poor outcrop, and may deviate to some degree from the true foliation.1 The pluton would well repay a detailed granite-tectonic study.

Age

Age data on this pluton have been obtained by

K-Ar and Rb-Sr techniques. The resultant ages present a complex picture which is fully discussed in Chapter 10.

For the purposes of this chapter, it may best be said that the time of its initial intrusion into its present level in the crust and its present outcrop outlines, as well as the time of its initial crystallization from the melt, was 1425 million years ago. A strong disturbance of this crystalline terrain, probably not involving creation of any molten phase in the Polecat, took place at 1000 million years ago.

Plutons Similar to the Polecat Quartz Honzonite

The Rattlesnake granite of Sabins (195?b) is 63 defined by him as that granite which outcrops in Rattlesnake point in the northeastern Chirichahua mountains. The writer has made a map of the Precambrian basement in this local area, and finds that the field characteristics of the intrusive at this locality are identical with those in the Polecat quartz monzonite. The Rattlesnake Point locality is ten miles east of the easternmost Polecat exposure, however, so the correlation is quite tenuous.

It is worth noting, however, because later Rb-Sr- analysis and chemical work may be able to establish the identity of these two different Precambrian plutons.

It should be noted here that the term "Rattlesnake granite" has been used in a casual way for the rapakivi quartz monzonite stock exposed and east of the village of Dos Cabezas in the southern part of the range. This is an erroneous use of the term as the two bodies are not similar, except in the respect of being coarsely porphyritic granitoid plutons,

Precambrian Apllte Dikes

In the easternmost part of the Dos Cabezas moun tains, north of the Paloosoic-Hesozoic sedimentary se quence, several long thick splite dikes cut the Pinal

Schist. The southern ends of some of these dikes pass underneath the basal Paleozoic Cambrian quartzite where it lies atop the Pinal Schist. This feature serves to 64 identify these dikes as PreCambrian in age, probably related to one or another of the large Precambrian plutons. The dikes are unfoliated, which makes it most likely that they are related to the Polecat quartz monzonite, the only large unfoliated pluton of Precambrianage in the northern Dos Cabezas mountains* This would, if correct, fix their age at about 1425 million years.

These dikes have not been examined petrographically in any detail, but are generally very simple leucocratic biotite-free quartz-feldspar rocks of quartz monzonitic composition.

Several quartz and dacite porphyry dikes of pro bable mid-Tertiary age occur in the same area as do the aplite dikes, and discrimination between the dacite por phyry and aplite dikes is occasionally difficult because the aplites are aphanitic in a few places, and these aphanitic parts are megascopically quite similar to the dacite porphyry in many cases.

The last major intrusive in the Precambrian group of leucocratic plutons in the Dos Cabezas mountains is the rapakivi quartz monzonite pluton exposed in two areas in the southern part of the range; one lies around the village of Dos Cabezas and the other in Apache Pass.

This unit is here called the Dos Cabezas rapakivi quartz monzonite. The genesis of rapakivis is a matter of some controversy, and in the interests of a thorough discussion of the problems concerned with the unit it is discussed in the next chapter. /

CHAPTER 4

DOS -CABEZAS MPAKJVI QUARTZ MONZOKITE

Most of the southern Precambrian terrain in the

Dos Cabezas mountains is made up of a pluton of very coarse-grained alkalic quartz monzonite porphyry, whose two exposure areas underlie some 16 square miles of out crop,1 The large potash feldspar phenocrysts in the rock

■are often mantled by plagioclase rims, and are frequently rounded or ovoid in section; this texture served to identify the pluton as a rare pluton acidic rock type called rapakivi. Rapakivi plutons are found in only a few localities on earth.

Most of the original work on these bodies was done in the southern part of Finland; some more recent study has been done on two plutons in this country

(Terzaghi, 1940; Volborth, 1 9 6 2 ) and one in France (Thomas and Smith, 1932)# The body in the Dos Cabezas is here called the Dos Cabezas rapakivi quartz monzonite. Due to its unusual nature, the writer decided to do a detailed study of the petrogenesis of the pluton, and of rapakivis in general. 66 6?

Characteristics of the Classic Ranakivis

The tern rapakivi was first coined by the Finnish geologist Tilas before 1?35 (Sederholn, 1928). The word is Finnish, and means crumbling stone.’ The name refers to the rapid disintegration that characterizes much of the rock on weathering. Additionally, Wahl (I9 2 5 ) has defined two characteristic facies of rapakivi.: The first • of these is the wiborgite type, in which the potassium feldspars have ovoid forms and bear plagiocla.se rims; the second type, pyterlite, has oval but unrimmed potassium feldspar phenocrysts.

The field characteristics of the classic Finnish rapakivis include the above feldspar phenocryst texture, sharp contacts with the walls, absence of chill zones at the borders, lack of marked internal foliation, rarity of pegmatites, and rarity of rapakivi dikes not connected

to the parent pluton at the present erosion level.

Iliarolitic cavities are often present. Rapakivis are higher in potassium content than most granites, and higher in iron and fluorine content too. They often

contain two different general ground mass size ranges

of quartz and feldspar crystals. The type rapddvis

in Finland have common characteristics that are often

shared in whole or in part by the rapakivi plutons de

scribed from other parts of the world. The Dos Cabezas 68 rapakivi pluton shares all these classic characteristics

save for the absence of modal fluorite and miarolitic

cavities; the fluorite is the expression of the high fluorine content found in most rapakivis."

The Dos Cabezas Rapakivl Pluton

The rapakivi pluton in the Dos Cabezas mountains

is well exposed, where present, but is cut off by a

fault along its northern boundary and covered by Paleozoic

sediments along the southern margin of the mountains.

Its full original size is hence unknown. It is presumed

that the two areas of exposure now visible are part of

the same pluton; it may be noted in this regard that whole-rock Hb-Sr samples from the two parts lie along

the same whole-rock isochron (see Chapter 10),

Field Characteristics

The map (Figure 1) shows the general character

of the pluton (see Figure 10). It is quite heterogeneous

from outcrop to outcrop within a single local area, al

though there is a general tendency for the eastern part

of the pluton to be less coarsely porphyritic and coarse

grained than the western. Internally, the pluton shows

a weak but persistent generally dual foliation which

appears to be best interpreted as magmatic flow folia

tion orientation of the feldspar phenocrysts and, 69 occasionally of the ground mass. Locally, as in the area south of the Mascot mine, the pluton has a gneissoid facies in which the orientation of the ground mass minerals is quite obvious (see Figure 11b). The gneissoid facies merges imperceptibly with the more weakly foliated main facies of the pluton. The pluton contains large numbers of gneiss, quartzite, and micaceous quartzite inclusions, up to a hundred feet on a side or so (see Figure 10b); the number of these inclusions varies greatly from place to place. Each inclusion has sharp contacts with the sur rounding igneous rock. The contacts of the whole pluton with its wall rocks are uniformly sharp and frequently cross-cut primary structures such as foliation in these units. Ho dikes of rapakivi wholly separated from the main pluton cut into the surrounding rocks, but a few small dikes of rapakivi cut the inner parts of the pluton itself, especially in SU 1/4 Sec. 2, T1

~~centage than in the main pluton. Ho rapakivi pegmatites are present. Except for the absence of miarolitic~~

~~cavities within it, the Dos Cabezas rapakivi quartz monzonite has all the field characteristics of the classic~~

~~Finnish rapakivis.~~

Other investigators (Volborth, 1958; Backlund, 70 1938) have noted that many rapakivis show no apparent orientation of their constituent minerals while others have noted the reverse (Terzaghi, 19^0). The gneissoid zones in the Dos Cabezas.rapakivi show marked orientation of minerals; these gneissoid zones may represent tongues of the liquid-crystal mush which rose along narrow and possibly tectonically active fissures in the magma's roof. The gneissic zones appear to have been essentially resorbed along their margins as the main mass of the magma rose around them; the gneissic rock masses are essentially large cognate inclusions of an early-appearing part of the rapakivi magma.

Detailed examination of.the texture and structure of the more apparently isotropic parts of the rapakivi mass show that a preferred orientation of at least the more tabular of the large potash feldspars exists.

Figure 13 is a map of 10 square feet of rapakivi surface, showing all the potassium feldspar phenocrysts and their relative rim thicknesses. On simply viewing the surface, local alignments of a few crystals tend to draw the eye toward certain possible foliation directions. To determine

if this qualitative impression corresponded with reality, a rose diagram was made of the long axis orientations of the mapped potassium feldspar phenocrysts in the area of Figure 13. The resulting diagram (Figure 14)

~~Figure 10: Primary Structures in the Dos Cabezas Rapalcivl Quartz Honzonite~~

(a): Surface of the rapaMvi showing rounded ovoids and euhedral crystals side by side. Rims;of oligoclase on the crystals are generally thin; only a few of the thicker ones can be seen in the photo. Mote the wide variety of phenocryst types and sizes, the coarse-grained ground mass of small white plagioclase crystals and dark mineral clusters, and the absence of any marked foliation in the rock. This rapalcivi is a wiborgite phase* of medium coarseness. The picture was taken in the central western part of the pluton.

(b): Rapalcivi with a large gneiss block xenolith in it.' Rote the fairly sharp to very sharp contacts between the two types. Note the erratic distribution of potassium feldspar phenocrysts in the rapalcivi. The out crop pictured is about four feet across." z7

~~Figure 10: Primary Structures In the Dos Cabezas Rapaklvl~~

~~Quartz Honzonite IN Figure 11: Contrast Between the Rare Gneissic and Common Little-Foliated Rapakivi Quartz Honzonite.~~

(a): A slab of rapakivi showing non-foliated tex ture. Rims of oligoclase are visible on some of the large potassium feldspar phenocrysts. Smaller white plagioclase phenocrysts and clots of dark minerals lie between and within them, Hote the marked unevenness in the distribu tion of the large phenocrysts.

(b): Slab of rapakivi showing the well-foliated character of its ground mass. The foliation trends horizontally across the flat face. This specimen well Illustrates the marked ground mass foliation found in a few parts of the rapakivi pluton. Figure 11: Contrast Between the Rare Gneisslc and Common

~~Little-Foliated Rapaklvi Quartz Honzonite pyrtr,’, •*r s>a,o j e. •*vnon+‘*<3 /D.^Gn ocsj J < x33>~~

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~~MAP OF D/STF/8UT/ON OF MANTLED M/CROCL/NE PHENOCRYSTS /V DOS CABEZAS RAPAK/V/ PLUTON F/gure /3 N~~

~~SCO/S .* 2?/nm. o/ong Q /*&}? a / phan&csysf- onanted~~

~~figure, td A?ose /D/ogrom of *Sfym//rc Or/mnrof/ons of /d3 /^otos^s/u/n fm/dspers *s/>oMng CXd/oog Out///)*s, hf/TWo o /0 *sg(/are foot /Qrco /n trim jDos Co&e^os &gpo/r/W Cdc/ortjr Ado/tJto/iJfm* 76~~

~~shotTs tiro clear orientations which trend at approximately right angles to each other, and which presumably define~~

~~the horizontal intercepts of two steeply dipping foliation planes in the rapakivi* In a few areas within the pint on,~~

~~these planes can be observed in"section, and always dip~~

~~steeply to vertically. A third subhorizontal plane is weakly expressed in some outcrops.5 Presumably the folia~~

~~tion orientation of the feldspars indicates the flow pat~~

~~tern of the partially crystalline rapakivi magma during~~

~~its intrusion into this level of the crust. The internal~~

~~tectonics of the pluton leave little doubt that the pluton~~

~~passed through a magmatic stage.~~

~~The large number of Pinal Schist and gneiss blocks~~

~~in the pluton, especially in the.H, center of Sec. 2,~~

~~T15S R.2 7 2 , indicates that in this region the pluton was~~

~~just completing destruction of the roof at this level.~~

~~A number of aligned quartzite-topped hills in the eastern~~

~~part of the pluton may point to disruption of a quarzite~~

~~horizon in the roof in this area.~~

~~Another interesting feature of the rapakivi pluton~~

~~is that the proportion and size of the mantled phenocrysts~~

~~and the thickness of their mantles, varies from area~~

~~to area and often varies within a few square feet of area.~~

~~Figure 11 shows this quite well. All the phenocrysts are~~

not the same shape, either, and crystal sections ranging 77 from tabular .and euhedral to circular and wholly anhedral are present in various proportions in most outcrops. The internal patterns in these crystals vary also. Some show cores of dark minerals, some seem internally homogeneous; almost all show small apparently randomly located inclu sions of dark minerals and plagioclase crystals.

The textures in the phenocryst type and distri bution imply to the author that not all the phenocrysts presently assembled in any one spot originated in that spot; that is, mixing of various magma parts with various types of phenocrysts has occurred. The writer would like to emphasize that the variation in phenocrysts is not

~~just a matter of scale.~~

~~It seems likely, from some evidence in the pluton~~

~~(the large number of included blocks and the fine-grained ground mass) that we are observing a part of the pluton~~

~~that was very near its top. It seems likely to the author~~

~~that continuous advance of the magma into its roof, with~~

~~tongues and dikes of magma continuously rising into~~

~~shifting local zones of weakness, must have resulted in~~

~~considerable mixing of various magma fractions. Variations~~

~~in the amount of magma reaction with walls, temperature,~~

~~pressure, exact original chemical composition, and local~~

~~prehistory in the magma could all lead to observed types~~

~~of variation in size, shape, rim pattern, core pattern. 78 and inclusion pattern in .the potassium feldspar crystals, in terms of the rapakivi genetic model proposed below.~~

Hiring of partially crystalline magma masses with such accidental variations could produce phenocryst textures in the pluton just like those in Figure 13.: The mixing would have to have taken place before the last major in

~~trusive pulse which produced the foliation of the pheno-~~

~~crysts.~~

~~In the Apache Pass area at the eastern end of~~

~~the Dos Cabezas mountains occur dikes and masses of pyterlite rapakivi with unmantled ovoids and euhedral~~

~~crystals of potassium feldspar. These rocks share all~~

~~the other outcrop characteristics of the main wiborgite rapakivi. With respect to the genetic theory proposed below, the reason for the appearance of pyterlite rather~~

~~than wiborgite seems to be essentially compositional;~~

~~they have much more potassium-feldspar rich modes and higher Bb/Sr ratio than the wiborgite, and seem to have~~

~~lower iron contents as well (see Chapter 10).~~

~~Rapakivi Petrography~~

~~The next few paragraphs discuss in detail the writer's theory for the origin of rapakivi texture in the~~

~~described pluton. This theory is considered by the author~~

~~to be a successful one, in that it accounts reasonably for 79 the observed features of the rock; however, it is certainly possible that some parts of it will require modification at a future date.~~

~~Microscopic petrographic evidence is decisive in determining the exact genesis of the rapakivi pluton.-~~

~~Over 5° thin sections and over a dozen slabs of the rock have been studied. All slabs and sections have had their potassium feldspar stained yellow to facilitate modal~~

~~studies. Figure 12a, b, c, and d show many of the most~~

~~important relationships.~~

~~The rapakivi has a grossly porphyritic texture, . -vV ; in which mantled and unmantled potassium feldspars in phenocrysts up to 10 cm. in diameter lie in a ground mass~~

~~of quartz and plagioclase crystals and dark mineral clus~~

~~ters all averaging about 1 cm. in greatest dimension.~~

~~The ground mass, which is partly composed of the just- mentioned crystals and clusters, has a dual size mode.~~

~~The large quartz and plagioclase crystals and dark mineral~~

~~clusters, which make up the coarse-grained part of the~~

~~ground mass, lie in a finer-grained ground mass component~~

~~of quartz, microcline, and occasional plagioclase crystals,~~

~~all of which are about 1-2 mm. in greatest dimension.~~

~~This "doubly porphyritic" texture is characteristic of~~

~~the Finnish rapakivis as well. 80~~

Past efforts to decipher the genetic significance of the rapakivi texture have centered upon interpretation of the large mantled or unmantled potassium feldspar phenocrysts." The large potash feldspars have all the classic characteristics over which discussion has "been carried on, and discussion of these crystals seems applica ble to those of most rapakivis." In the Dos Cabezas mountains, almost the whole pluton is wiborgite rapakivi,

~~and the genesis of this type will be discussed first.~~

~~The most obvious characteristic of the mantled potassium feldspars is their variability. There are, perhaps, a minimum of three groups of these feldspars.~~

~~These are: first, ovoids of all sizes with their mantles~~

~~of plagioclase which vary in width by a factor of no more~~

~~than two or three around the circumference of the crystal;~~

~~second, ovoids, generally small, with deeply embayed potash feldspar cores and thick mantles of plagioclase whose gross thickness is about equal to the diameter of~~

~~the potassium feldspar core; and, third, rather tabular~~

~~and euhedral potassium feldspars with thin mantles of~~

~~plagioclase whose width also varies by a factor of no more than two or three around the circumference of the~~

~~crystal. Any of these three classes may have one or more~~

~~internal rings of closely spaced but generally uncon nected plagioclase crystals. The more euhedral and tabu~~

lar feldspars make up about 75^ of the feldspar phenocryst 81 population. The embayed core ovoids of the second type are very rare, but are occasionally found in the pyterlite rapakivi, where they are the only rimmed variety.

Internally these large potassium feldspar crystals are very complex.' The main crystal itself is usually perthitic to some degree, and usually has orthoclase optics." Local zones and patches of microcllne twinning are present, and imply a beginning of structural inversion of orthoclase to microcline. Presumably this potash feldspar is a "minimum microcline" (Goldsmith and Laves,

195^) still in the process of unmixing and moving toward a more markedly triclinic structure. The perthite ob served is always string perthite, and appears to be a simple result of unmixing of an originally homogeneous higher-temperature feldspar. Carlsbad twins are the only

~~twin types observed; these are odd in that the twin plane occasionally becomes irrational and deviates in an ir regular manner from parallelism with (0 1 0 ),~~

~~All the large potassium feldspar crystals have numerous inclusions. The most common of these are small blocky euhedral plagioclase crystals, generally 1 -2 mm.~~

~~long. Figure 12a shows their relationship." Large numbers of small 1 -5 mm. long quartz crystals, with markedly anhedral outlines composed of linked short curved~~

segments, are quite common, and often lie along the 82 irrational twin planes. Small 1-2 ram. crystals of horn blende are common; the optical properties imply that it is a high-soda, high-iron content variety similar to ferrohastingsite. This hornblende is frequently partial ly replaced by chlorite and some biotite. Small euhedral quartz crystals are occasionally included in the potassium feldspar, as are small euhedral magnetite crystals.

Frequently a potassium feldspar crystal will have a com posite dark mineral core composed of hornblende, chlorite, biotite, magnetite and plagioclase, which in all comprise a quite fine-grained aggregate of crudely circular section,• commonly 3 -5 mm/ across overall.

All the examined potassium feldspars share these general characteristics. Most, but not all, show also some development of more or less continuous sheetlike marginal rims or mantles of plagioclase which are, in detail, made up of large numbers of individual blocky plagioclase crystals in contact with one another around their margins (see Figure 12a);

~~Composition of the potassium feldspar is unknown."~~

That from analyzed rapakivis is almost always very close to Ory^Abg^, and there is no reason why the phenocrysts from the Dos Cabezas rapakivi quartz monzonite should not have the same composition. Certainly the large amount of plagioclase in the perthite in these crystals indicates 83 a considerable amount of sodium having been present in the high-temperature predecessor feldspar which unmixed to form the present perthite." Host of the included plagioclase, in both perthite stringers and included blocky crystals, has reacted to form a mass of minute crystals of epidote, sericite, and untwinned plagioclase (albite?).

The few unreacted crystals which could be determined are oligb.clase of about An^Q composition. No sign of weather ing decomposition is visible in the sample (8 2 9 ) on which the detailed study was done, and it is presumed that the reaction breakdown of the plagioclase is a deuteric re action.'

~~A few large, generally ovoid quartz crystals about~~

5 mm*' across which show a faint blue color are present in all sections of the rock.' They form perhaps of the mineral mode. Some of these, where included within potas sium feldspar crystals, still retain a presumably original euhedral character. These were probably originally high- temperature beta-quartz crystals, and the outlines of those not included in feldspars have been lost due to partial resorbtion or overgrowths later in the magma*s history.

~~Internally the crystals show low-temperature quartz optics, resulting from structural inversion following crystal lization."~~

The ground mass surrounding the potash feldspars has many distinctive characteristics also. The hornblende outside the potash feldspar crystals shows very consider able reaction to biotite and epidote (see Figure 12c).

~~In addition, books of biotite several millimeters across are also found outside the potash feldspars and show no~~

~~signs of having been formed by reaction from hornblende.~~

~~These hornblende-biotite-epidote •crystal aggregates and biotite crystals are associated with large anhedral to~~

~~euhedral crystals of magnetite; these latter are often~~

~~rimmed by a thin rim of a high-birefringence mineral~~

~~tentatively identified as sphene. Magnetite often makes~~

~~up 30-50;£ of these dark mineral clusters. Small euhedral~~

~~apatite crystals are very numerous and are included in~~

~~all the other crystal phases.~~

~~Plagioclase crystals make up the greater part~~

~~of the phenocrysts which comprise the coarser part of~~

~~the ground mass.' They are euhedral with thin overgrowths~~

~~around their outer margins, and are generally 1 -2 cm. in~~

~~greatest dimension. Microscopically they are unzoned,~~

~~except at their outermost margins, where rapid change to~~

~~ward more sodic composition takes place.1 They are twinned~~

~~on the Carlsbad and albite lawsThey include small blocky~~

~~crystals of potassium feldspar and small anhedral quartz~~

~~crystals. They have generally reacted nearly completely~~

~~to form masses of epidote, sericite, and albite (?). 85'~~

~~Those whose composition could be determined are about An^Q in composition. There seems to be no essential difference between the plagioclase within or without the phenocpysts of potash feldspar.-~~

~~As mentioned earlier, all the above large and small phenocrysts (true potash feldspar phenocrysts and smaller phenocrysts of the coarse groundmass phase) are contained~~

~~in a fine-grained ground mass of microcline, quartz, and~~

~~occasional plagioclase, all averaging 1 -2 mm. in greatest dimension. The proportions of the mode of this material~~

~~are about quartz, mi crocline, and 10% plagioclase/~~

~~The pyterlite variety of rapakivi found in Apache~~

~~Pass has some petrographic characteristics which are dis~~

~~tinctly different than those of the wiborgite phase/ The potassium feldspar phenocrysts in the pyterlite are not mantled with plagioclase except, as earlier mentioned,~~

~~for a few deeply embayed and thickly mantled potash feld~~

~~spar ovoids described earlier as one of the main types in~~

~~the wiborgite rapakivi phenocryst groups. These unrimmed~~

~~potash feldspars may have either euhedral or ovoid outlines,~~

~~and a certain amount of magma mixing has evidently taken~~

~~place here too. The potash feldspars, no matter what their~~

~~outline, include plagioclase, quartz, biotite, and mag~~

~~netite in a manner similar to the wiborgite phenocrysts/~~

~~Rather than having a plagioclase crystal mantle or rim. 86~~

~~they have a ragged anhedral optically continuous over growth of potassium feldspar along the circumference of~~

~~the crystal (see Figure 12b).~~

~~The melanocratic mineral assemblage forms clusters~~

~~in the pyterlite assemblage as well as in the wiborgite,~~

~~but there are some marked differences between the wibor~~

~~gite dark minerals and the pyterlite clusters. The main~~

~~one is that the pyterlite shows no trace of the present~~

~~or prior existence of hornblende. Rather, the dark~~

~~clusters in the pyterlite are made up almost wholly of~~

~~large crystals of chlorite with biotite Interlayers, in~~

~~cluding magnetite crystals and euhedral apatite crystals.~~

~~The dark minerals occasionally include potassium feldspar~~

~~and muscovite (see Figure 12).~~

~~The pyterlite rapakivi has been rather thoroughly~~

~~fractured, and some movement of quartz and feldspar com~~

~~ponents has occurred along these fractures, as evidenced~~

~~by thin sheets of the above minerals lying in and cementing~~

~~these cracks.~~

~~• The petrography provides a key to understanding~~

~~the formation of the rapakivi texture. First, the general~~

~~paragenetic sequence for the wiborgite type of rapakivi,~~

~~about which most controversy has developed, may be out~~

lined and used as a basis for a genetic discussion; then 87 the pyterlite type of rapakivi may be discussed as a variant of the above viiborgite type*

~~In the vriborgite rapakivi, apatite, hornblende, and magnetite were the first phases to begin crystallization.~~

~~They are included in the large bluish beta-quartz crystals, and these are in turn included by potash feldspar, but do not include potash feldspar* Occasional euhedral~~

~~crystals of epidote may be primary, and occur in all phases. Potassium feldspar and plagioclase include phenocrysts (small euhedrally-outlined crystals) of each~~

~~other, and also include quartz, hornblende, apatite, mag netite, and epidote. The included crystals are quite uniformly although randomly, scattered throughout the~~

~~host potash feldspar crystals. No single or multiple~~

~~crystals of biotlte are found within the potash feldspar~~

~~crystals; this seems to imply that the original calcium-~~

~~iron-magnesium silicate was hornblende. Sphene, present~~

~~on the margins of many magnetite crystals, has an uncer~~

~~tain position in the paragenetic sequence.~~

~~Dark mineral clusters outside the feldspar cry~~

~~stals include small blebs of quartz and potassium feldspar~~

~~and are hence at least partially contemporaneous with~~

~~potash feldspar crystallization; this is, of course,~~

~~also pointed out by the inclusion of these dark minerals~~

and dark mineral aggregates in the feldspars, where they 88 often make up the core of the crystal. Hornblende crystals within and without the feldspars have usually, though not always, broken down to form mineral assemblages with lower

~~temperature stability ranges, among which the hornblende~~

~~components are portioned out. Hornblende included in~~

~~the potash feldspars has generally broken down to form~~

~~chlorite, chlorite plus biotite, or chlorite plus biotite~~

~~plus epidote. Hornblende outside the potash feldspar~~

~~crystals has broken down to form biotite and epidote,~~

~~chlorite generally being totally lacking.~~

~~Magnetite "is strongly associated with the dark~~

~~minerals, being included in the biotite and chlorite and~~

~~also growing between them; it is far more common outside~~

~~the light mineral phases than in them, both as inclusions~~

~~and as crystals in the dark mineral aggregates, and seems~~

~~to occupy a position of late prominence in the paragenesis~~

~~of the dark minerals, even though in the beginning it~~

~~was one of the first minerals to crystallize. Some of~~

~~the magnetite may be a reaction product of the breakdown~~

~~of hornblende, but much of it is in large biotite crystals~~

~~showing no trace of a hornblende parent,~~

~~Breakdown of plagioclase throughout the rock to~~

~~form epidote, sericite, and albite (?) is almost complete.~~

~~Only rare unreacted crystals were found. This is thought to be a deuterlc reaction.~~

~~The presence of included euhedral crystals of the 89 two feldspars in each other indicates that both were crystallizing out simultaneously. Presence of clots~~

Of dark minerals in the potash feldspars and of small anhedral inclusions of potash feldspar and quartz in the dark mineral clots show that their crystallization periods overlapped. Thus, it appears that quartz, potash feldspar, plagioclase, hornblende, and magnetite were crystallizing simultaneously over at least part of their crystallization range. A-very complex crystallization involving four or more components was involved.

~~It is here that the critical argument for the writer's idea as to how rapakivi forms begins.~~

It is, first of all, of prime importance to realize that the potassium feldspars were growing at the same time as the plagioclase, and vice versa. Large numbers of small plagioclase inclusions are scattered randomly throughout the large potash feldspar crystals; these inclusions are occasionally arranged in oval rings deep inside the host crystal. The plagioclase crystals which make up part of the coarse ground mass assemblage contain small euhedral potassium feldspar inclusions.

With reference to the potash feldspar crystals, the following should be noted about the formation of rims of plagioclase on them. In the outer few millimeters of some, but not all potash feldspar crystals, the quantity 90 of included plagioclase increases rapidly toward the outer most margin of the crystal from a point a few millimeters below the surface. Often, a single included plagioclase crystal located at some point on the periphery of the host potash feldspar, and having a simple euhedral blocky inner contact with the host, becomes wider rapidly as one moves across the crystal from inner contact with the host to the outer surface of the plagioclase. Neighboring crystals of plagioclase on both sides of the first crystal observed show the same tendency; the net result is that a number of plagioclase inclusions, originally separated by some finite amount of the matter of the host potash feldspar, become wider in their outer parts and gradually take the place of the host that lay between them until they butt against one another. VIhen this occurs the outer surface of the host feldspar becomes covered with a multi crystal marginal rind or mantle of plagioclase crystals.

This rim largely obscures the essentially buried host and presents the surface of a complex plagioclase crystal to the magma. The mantle or margin may then grow to various degrees, becoming wider and having a complex internal crystalline structure.

This picture is one of changing abundance of the two feldspar types toward the conclusion of growth of the large potash feldspar crystals. • The most likely reason 91 for this would be the changing abundance of Na, Ca, and

~~K in the liquid phase in contact with the growing crystals.~~

~~The mechanism responsible must be able to rapidly change~~

~~the composition of the liquid phase of the magma, evidently~~

~~in the direction of relative increase of sodium and calcium with respect to potassium.~~

~~The mechanism for such a magma change is the re action breakdown of hornblende in the magma outside the potassium feldspar. It breaks down to form biotite and~~

~~epidote, thereby extracting potassium from the liquid phase in considerable amounts. An essential “basification",~~

~~if one may use the word, has taken place in the magma over a relatively short period of time. The following~~

~~calculations indicate the probable course of the crystal~~

~~lization history of the magma during such a reaction breakdown 'and consequent basif ication.'~~

~~Model Rapakivi Magma~~

~~Let us make a model of a quartz monzonite magma «* and its subsequent crystallization. Assume a simplified~~

~~quartz monzonite of only four components: potassium feld~~

~~spar, plagioclase, quartz, and biotite. Give this as~~

semblage the mode of the analyzed sample of the Dos Cabezas rapakivi quartz monzonite: 40;£ potassium feldspar, 25% plagioclase, 30% quartz, and 5% biotite. Mow assume this material to be wholly molten, and add a few percent by 92 weight of dissolved volatiles, predominately water. The minerals which will appear in the course of the crystal lization of this magma are arbitrarily assigned the follow ing compositions, which appear reasonable in the light of analyses of minerals from Finnish rapakivis.

~~Potassium feldspar is OrygAb^Q~~

~~Plagioclase is An^Q. It is assumed that no potas sium is present in it.~~

~~Quartz is pure SiOg.~~

Biotite is a highly ferrous variety, with a K/Na ratio of 7/1 in the alkali cation site and an Fe/Kg ratio of 2,^3 (70/ Fe) in the other cation site. It is assumed that no calcium is present in the biotite. The following percentages (by weight) of significant elements are present:

~~K 6 .63^ Na 0.9%% Fe 24.90/ Si 17.90/~~

Hornblende is also a highly ferrous variety, similar to ferrohastingsite. The mineral analysis for the model comes from an analysis of a hornblende from a Finnish rapakivi reported in Geochemistry, by Rankama and Sahama, p. 150. It has the following percentages of those elements

~~judged to be significant in the reaction process in the model. 93 K l.-30;1 . Na 1.11/5 Pe 2k.20% Si 17.86# Ca 7.62#~~

Vie begin with a wholly liquid magma, containing some dissolved water and the liquid equivalent of the modal minerals above, In the model, crystallization will follow the sequence of paragenesis noted in the Dos Cabezas rapakivi. To begin with, quartz (beta-quartz), plagioclase, potash feldspar (probably low sanidine) and horn blende crystallize out essentially simultaneously as tem perature drops in the magma. At a point in the thermal history and crystallization history of the magma where

75# of the potash feldspar, 80# of the plagioclase, all the hornblende, and 66# of the quartz are crystallized, and the residual magma is 30# of the original molten ma terial, by weight, the temperature drops below the lower stability temperature of hornblende, which then reacts with the magma to form biotite.

At this point, the following chemical relationships apply. The overall potassium content of the original magma is 4 .62#. 29# of this is still in the liquid at the re action temperature of hornblende (1.35# bulk). 1.5# of the overall potassium in the original magma is in the hornblende. When reaction occurs, the hornblende 94

~~spontaneously "but slowly disintegrates and recomposes into~~

~~biotite, with a new internal structure and a new chemical~~

~~composition. This requires addition of some components~~

~~from the magma and dispersal of unwanted components into~~

~~the magma/ Notice that, with reference to the previously~~

~~given chemical compositions, the amount of addition or~~

~~release of silicon, iron, and sodium is very small or~~

~~negligible. Calcium from the parent hornblende, on the~~

~~other hand, must be wholly expelled from the ferromag-~~

~~nesian crystal area, while a great deal of potassium~~

~~' must be brought in from the magma to produce the biotite.~~

~~The amount of potassium in the reacting mineral must~~

~~increase about sixfold; in the process the liquid phase~~

~~of the magma loses 24,v of its potassium.~~

~~The calcium which is released during the reaction~~

~~goes at least partially into the formation of epidote.~~

~~That which does not is released into the magma1s liquid~~

~~fraction and is added to the plagioclase which later~~

~~crystallizes from it. Addition of enough calcium at this~~

~~point might be expected to cause reverse zoning in the~~

~~plagioclases; those in the rapakivi in the Dos Cabezas~~

~~mountains are generally so badly broken down to lower-~~

~~temperature minerals that the character of their original~~

~~zoning is lost, and it is hot known if reversed zoning 95 appeared in them. In any case, the fate of the calcium not used by epidote, if any, is not of prime importance~~

~~in this -model .~~

~~To return to the main points of interest in the model's course of crystallization, let us look at the~~

~~liquid part of the magma during the reaction breakdown~~

~~of the hornblende. As the slow breakdown of hornblende~~

~~to biotite takes place (necessarily slow because the~~

~~first-formed biotite armors the outer surface of the hornblende, after which the components moving in or out~~

~~of the.magma must diffuse out through the biotite or~~

~~along cracks between biotite crystals) feldspar crystal~~

~~lization has been continuously going on. As the amount~~

~~of potassium in the liquid diminishes relative to sodium~~

~~and calcium, there is a gradual slowing of crystalliza~~

~~tion of potassium feldspar and an increase in crystalliza~~

~~tion of plagioclase. The increase in plagioclase crystal~~

~~lization is largely seen in the rapid growth of partly-~~

~~buried inclusions in the potash feldspar crystals, al~~

~~though the free-floating plagioclase crystals in the magma will also grow.~~

~~In phase diagram terms, the feldspar crystalliza~~

~~tion has changed due to a shift from the potassium feld-~~

~~spar-plagioclase feldspar cotectic line into the plagio~~

clase field, caused by a change in the composition of 96 the system. Continuing with this approach to discussion, the course of crystallization in the plagioclase field is toward the cotectic plagioclase-potash feldspar line, and as temperature drops in the system the composition of the crystallizing feldspar moves in this direction.

~~When the cotectic line is reached again, the two main feldspar types will crystallize out together as before.~~

~~This implies that the reaction breakdown of hornblende has essentially ceased, so that the composition of the system is not continually being moved back into the plagioclase region of the diagram.~~

When the cotectic line is reached the second time, crystallization of potash feldspar and plagioclase should begin again. Those potash-feldspars which have not been completely mantled by plagioclase should continue growth; because many of the potash feldspar crystals are only partially mantled, a considerable proportion should show such evidence of regrowth if the two feldspars have both begun to crystallize. If no such potash feldspar over growths are found beyond the plagioclase rims on partially rimmed crystals, it can be.assumed that the trend of crystallization was interrupted before the cotectic line could be reached. Let us, in the model, assume this latter to be the case. Mo potash feldspar growth takes place after onset of formation of plagioclase rims on the 97 large potash feldspar phenocrysts until something dras tically alters the crystallization pattern of the magma.

At a point in the model*s thermal and crystal lization history where hornblende is about 80^ reacted and plagioclase mantles have become quite continuous on many crystals of potash feldspar, let us assume that the roof of the pluton fractures along breaks that reach the surface.' This will cause no great change in the litho- static pressure on the solid-liquid magma, but will cause a considerable decrease in confining gas pressure. Vola tiles, concentrating since the onset of crystallization in the residual liquid phase of the magma, begin to escape rapidly from the liquid phase; Escape of volatiles raises the liquidus temperature of the crystallizing system quite rapidly, causing final crystallization of the 30$ of the magma which is still liquid. Approximately equal propor tions of quartz and microcline (microcline forms due to the lower temperature) and a trace of plagioclase crystal lize out rapidly in small anhedral crystals. The rock is now wholly solid.

The model, as evolved, has the same gross textural characteristics as the Dos Cabeza^ rapakivi pluton. It has large potash feldspar phenocrysts frequently wholly or partially mantled by plagioclase, ccrtaincd in a ground 98

~~mass with two markedly different size ranges.1 The coarser~~

~~ground mass has plagioclase crystals, quartz, and clots~~

~~and single crystals of ferromagnesian minerals in it; the~~

~~finer ground mass contains the large phenocrysts of potash~~

~~feldspar and the coarser ground pass phase within Itself,~~

~~and is composed of a fine-grained, essentially aplitic~~

~~quartz-microcline crystalline mass.~~

~~The model differs from the Dos Cabezas rapakivi~~

~~primarily in the probable homogeneity of its large mantled~~

~~potash feldspar phenocryst assemblage. The very hetero-~~

~~• geneous phenocryst population in the actual Dos Cabezas~~

~~rapakivi pluton is probably due to mixing of a number of~~

~~different parts of its magma with slightly different pre-~~

~~histories; in.terms of the model, it involves mixing of~~

~~model magmas whose crystallization histories have varied~~

~~considerably in their final development, with respect to~~

~~amount of hornblende reaction talcing place, amount of~~

~~hornblende originally present, and so on.~~

~~The model also differs from the Dos Cabezas rapa~~

~~kivi in being mineralogically simpler. Magnetite, sphene,~~

~~apatite, zircon, etc., are ignored. The omission only~~

~~seems serious to the writer in the case of magnetite, which~~

~~is present in some abundance in some dark mineral clusters,~~

~~and which may partly be derived from iron released by '~~

~~hornblende breakdown. On the other hand, the associa~~

tion of magnetite with the dark minerals may have some 99 more subtle reason for being; perhaps magnetite nucleates more easily in such surroundings. Magnetite is not ob served to form in small blebs and tongues along the edge of disintegrating hornblende crystals, as are biotite and epidote; there is no direct textural evidence as to whether or not it forms from hornblende iron during reaction breakdown.

The model, then, represents in a fairly detailed way the course of crystallization thought to have taken place in the Dos Cabezas rapakivi quartz monzonite, minus the late mixing of various magma fractions and consid eration of some minor rock minerals. The writer finds no discrepancy or inconsistency in the pattern outlined, and believes the model illustrates all the essential features

~~of rapakivi crystallization from a magma in a single~~

~~cooling process.~~

~~Conclusions~~

~~Field and petrographic evidence indicate to the writer that this rapakivi pluton in the Dos Cabezas~~

~~mountains appeared at its present level in the crust as~~

~~a simple partially crystalline intrusive magma. The~~

~~unusual texture of the rapakivi is a consequence of its~~

~~chemical composition and cooling history, as especially~~

~~the result of reaction breakdown of the hornblende float~~

~~ing in the magma, which released calcium and used up 100 potassium. Ho non-magmatic processes are needed to ex plain the origin of rapakivi texture in quartz monzonite plutons.~~

The question notr arises as to why rapakivi plutons are so rare, since the above sequence of crystallization involving breakdown of hornblende to biotite might be expected to take place in many granitoid magmas around the globe. The author harbors a suspicion that rapakivis are probably a lot commoner than often supposed, as none of the three geologists who did work on the rapakivi ter rain before this distinguished it as such. Shields (19^0) refers to it as a Laramide body in part and a Precambrian body in part, and does not mention the rapakivi texture.

~~Jones and Batcheller (1953) who made a road log through the Dos Cabezas for a New Mexico Geological Society field trip, measured at least one stratigraphic section in the~~

Paleozoic whose base is on the rapakivi, without noting its character. Sabins (1957a., 1957b) has studied the stratigraphy of the Dos Cabezas and the geology of its eastern portion, and also did not note the unusual character of the Precambrian pluton in the southern part of the range.

~~These authors treated the Precambrian terrains in the Dos~~

Cabezas mountains in a reconnaissance manner, which was in keeping with the main purpose of their studies, which were more concerned with the stratified rocks of Paleozoic 101 and younger ages* Many geologic studies, especially of areas of coarse-grained granitoid rocks, are reconnaissance in nature in all or part of the area covered, and it may well be that many rapakivis are overlooked by workers who are uninterested in plutonic rocks or who are not ac quainted with rapakivi texture. In this light, concerning recognition, the author notes that a small Laramide pluton in the Dos Cabezas shows local antirapakivi structure around crystals of plagioclase feldspar; thus out of thirteen plutonic bodies of all ages in the Dos Cabezas mountains, two show rapakivi or antirapakivi structures.

~~Bapakivis may not be rare.~~

The writer does not pretend to wholly understand the crystallization conditions which lead to rapakivi formation. Some generalities about rapakivis do seen to emerge from consideration of the few published analyses.

They are generally high in alkalis and high in iron; they may be either whole plutons or zones in plutons; they frequently, if not always, show some indication of having solidified rapidly toward the end of their crystallization history. Perhaps the combination of high iron content and high alkali content causes them to follow some special course of crystallization. Possibly they represent rela tively normal granitoid magmas whose normal course of crystallization was interrupted by rapid cooling toward the end of crystallization. Certainly, more work on 102 rapakivis would be productive.

~~Rapakivi granites, because of their unusual tex tural patterns, have been the subject of much discussion.~~

~~Classic arguments concerning the magmatic or\solid-state~~

(granitization) origin of these rocks have taken place in previous years. The author will next examine the classic and recent work on rapakivis, to see how his present theory accommodates itself to those offered in the past.

~~Studies on Raoakivi Previous to Present Work~~

The classic studies on rapakivis were conducted in the early part of this century in Finland, largely under the direction of A. A. Sederholm, The initial theories for the origin of rapakivi were essentially con cerned with the formation of the rocks from some non- rapakivi parent by a solid-state transformation caused by influx and exit of appropriate quantities of appropri ate ions; that is, by granitization. In more recent times, such ideas have fallen on hard times, but still have some proponents (Raguin, 1 9 6 5 ). Magmatogenic .ideas are now the common ones in the literature of recently studied rapakivis. In the interests of brevity the writer will quote from recent works of proponents of granitization and magmatic origins, and examine the ideas of each. 103

~~Solid-State Granitization Hypotheses~~

~~In recent years H. H. Read (1944) and H. Backlund~~

~~(1938) have been the most active proponents of this idea; many European geologists, at least, still follow their~~

~~thoughts (Raguin, 1 9 6 5 ). Raguin quotes Read (1944):~~

“The rapakivi texture is intimately connected with replacement intimately associated with the general process of feldspath ization. It is evident that the rim of plagioclase is clearly a replacement rim, and the corrosion of the potash feldspar is just as evident. -- --these rapakivi rocks represent arrested stages in the replacement of potash by soda."

~~The work of Backlund (1938) is of considerable~~

~~interest because he describes the characteristic textures~~

~~of the rapakivi in some detail, and has, outside of the writer * s work, the most detailed description in English~~

~~of the potassium feldspars and their mantles. Backlund~~

~~is impressed with the large number of inclusions of both~~

~~dark and light minerals in the ovoids and with their~~

~~general rounded outlines. He notices inner inclusion~~

~~rings parallel to the rounded outlines of the surface of~~

~~the crystal. He correctly points out that the existence~~

~~of small euhedral plagioclase crystals in the ovoids means~~

~~that the plagioclase was in existence before the ovoids were; he feels this mitigates against the ovoids having~~

~~foraed early, or at high temperatures. So far as other 104~~

Interpretations go, he feels that the rounded, outlines of the ovoids and repeated inclusion rings in their interiors point to repeated resorbtion rounding of the crystals, due to temperature fluctuations very late in the rocks genesis.

He feels that this opposes a magmatic origin for the rock, and proposes instead solid-state origin for these bodies resulting from addition, at the proper time, of plagioclase constituents followed by orthoclase constituents to a parent rock he identifies with the uppermost Precambrian

~~Jotnian sandstones of the area.~~

Backlund, in this paper, gives several tables showing the lime, soda, potash, and alumina contents of a number of rapakivi plutons and lavas, and of Jotnian sand stone; the figures show that the rapakivis are higher in all four of these constituents than are the sandstones, which leads him to say:

•'The figures cited show that there seems to be no formal difficulty in transftirming, without essen tial volume change, a Jotnian sandstone suite into a rapakivi granite by addition of material approxi mating in composition to sodium, potassium,(and calcium?) aluminates." (Backlund, 1938, P. 38?)

~~Backlund explains the details of the rapakivi tex~~

ture, including the inclusions in the ovoids, by means of pulses of 'emanations' of various ions at various times and temperatures during recrystallization and metasomatism of the pre-existent sandstone. 105 Regarding the famous oligoclase mantles, he says:

“A sudden rise of temperature would favor the possi bility of the crystallization of plagioclase and of the diffusion of GaO necessary for this purpose. The force of crystallization of the larger potas sium feldspars would have already swept aside such lime-bearing material, and now, at this stage the plagioclase mantles would be formed, in part at the expense of the new influx, the homogeneous ovoids or clusters of mesostasis serving as cen ters or foci of crystallization." (Backlund, 1938, p. 391) Backlund makes the point that the easiest-crumbling zones in the rapakivi in quarries tend to be concentrated along near-horizontal planes often marked by unusual mineral concentrations, and states that this is probably due to in complete cementation of granitized coarse sandstone layers in the replaced Jotnian units. He further states:

"With the rapakivi originating as an equivolume replacement, as pointed out above, its struc tural properties may have been inherited, with some modifications, from its sandstone stage of evolution. A second case of heritage may be the gray and red colors which, respectively, character ize well-defined age variants of the rapakivis (e.g., Nystad) and which are by no means explain able by simple differentiation." (Backlund, 1938, p. 388)

~~Read (195?) notes the general characteristics of rapakivi texture in some detail, including the inclusions of the groundmass phases in the potassium feldspar ovoids and says:~~

~~"It is clear, even from this skeleton statement, that if the mantled ovoids have arisen through processes of magmatic crystallization, then these 106~~

must have been ------extensive and peculiar." (Read, 1957, p. 137) As can be seen in the above quotations, the pro ponents of a granitization origin of rapaidvi simply believe that normal magmatic processes are incapable of

~~producing it. The writer has pointed out what he regards~~

~~as a perfectly acceptable simple magmatic course of crystal~~

~~lization which can produce it, and will of course disagree~~

~~with Backlund and Read. He regards their explanations~~

~~essentially as weak hypotheses; neither experimental proof~~

~~that diffusion can supply and remove enough material to~~

~~produce the presumed transformation of the sandstone, nor~~

~~any detailed consideration of the sequence of events in~~

~~1 emanation’ source areas nor any detailed consideration~~

~~of proposed mineral transformations has been offered. Why,~~

~~for example, should euhedral potassium feldspars spon~~

~~taneously begin to dissolve around their margins until~~

~~rounded outlines are developed on them, solely because~~

~~the sodium ion content of cracks and grain boundaries in~~

~~the area of the crystal had increased? Why should potas~~

~~sium ions migrate away from these disintegrating crystals~~

~~while the sodium ions which presumably cause the disinte~~

~~gration remain in place? How are the numerous inclusions~~

~~in the potassium feldspars maintained under conditions~~

~~which allow diffusive transfer of materials for great 106~~

must have been ------extensive and peculiar." (Read, 1957, P. 137) As can be seen in the above quotations, the pro ponents of a granitization origin of rap aid. vi simply believe that normal magmatic processes are incapable of

~~producing it.1 The writer has pointed out what he regards~~

~~as a perfectly acceptable simple magmatic course of crystal~~

~~lization which can produce it, and will of course disagree~~

~~with Backlund and Read. He regards their explanations~~

~~essentially as weak hypotheses; neither experimental proof~~

~~that diffusion can supply and remove enough material to~~

~~produce the presumed transformation of the sandstone, nor~~

~~any detailed consideration of the sequence of events in~~

~~1 emanation* source areas nor any detailed consideration~~

~~of proposed mineral transformations has been offered. Why,~~

~~for example, should euhedrai potassium feldspars spon~~

~~taneously begin to dissolve around their margins until~~

~~rounded outlines are developed on them, solely because~~

~~the sodium ion content of cracks and grain boundaries in~~

~~the area of the crystal had increased? Why should potas~~

~~sium ions migrate away from these disintegrating crystals~~

~~while the sodium ions which presumably cause the disinte~~

~~gration remain in place? How are the numerous inclusions~~

~~in the potassium feldspars maintained under conditions~~

~~which allow diffusive transfer of materials for great 107~~

~~distances? Until some detailed discussion of such problems~~

~~is forthcoming from those favoring solid-state transforma~~

~~tion, it seems to the writer that workable magmatic hypo~~

~~theses are to be preferred.~~

~~• Magmatic Hapakivl Genesis Hypotheses~~

~~Several studies of recent decades have emphasized~~

~~the role of magmatic processes in producing rapakivi tex~~

~~tures.1 Thomas and Smith (1932) describe a rapakivi con~~

~~tact zone, several feet wide, of a normal granitoid pinton~~

~~in Franco. In this zone the granite has oligoclase rimmed~~

~~microcline microp'erthite phenocrysts and increases in its~~

~~content of modal oligoclase, biotite, and hornblende over~~

~~the mode of these constituents in the normal granite.~~

~~Oligoclase of the mantle has grown from a ring of oligo~~

~~clase inclusions which spread and merge toward the outer~~

~~parts of the crystal. The oligoclase is often myrme-~~

~~kitically intergrown with quartz. The phonocryst phases~~

~~are bordered by a fairly coarse quartz-orthoclase micro~~

~~pegmatite. Green, sieve-structure, hornblende crystals~~

~~are common, as are other dark minerals; some dark zones~~

~~in the rock are almost wholly composed of hornblende,~~

~~dark-brown biotite, quartz, sphene, and magnetite.~~

~~Thomas and Smith note that the rapakivi zone oc~~

~~curs where the granite cuts a norite country rock, the 108 granite is charge with inclusions of it. They feel that~~

"basification" of the liquid phase of the partially crystalline granite took place by means of reaction be tween the basic inclusions and acidic magma. The xenoliths tend to react or dissolve in the magma, losing magnesium, calcium, sodium, and gaining potassium, among other thingsThe authors feel that the rapakivi mantles have appeared and grown because rapid increase in plagioclase components above orthoclase components occurred in the liquid phase of the granite in the near-contact zone.

They specifically point to the potassium-subtractive ef fect of the formation of numerous biotite crystals as a second basifying effect which couples with the inclusion reaction in producing a marked enrichment of sodium re lative to potassium in the local magma volume. Their ex planation of rapakivi formation is almost identical with that of the writer; perhaps the sole real difference is that they consider the inclusion reaction mechanism to be the main factor in magma composition change. Perhaps, in the case that they discuss, it is; not all rapakivis need have identical genesis.

~~Volborth. (I9 6 2 ) describes a large, complex pluton of rapakivi from the Gold Butte area in southern Nevada.~~

This body is granitic to granodioritic in composition, displays no internal foliation, and has only local flow structure. It has sharp contacts with its walls and 109 produces no contact metamorphic effects; in all field as pects it is a classic rapakivi body. The internal tex- . tures are classic, too, with large ollgoclase-mantled potassium feldspars, large beta-quartz crystals and large crystals of hornblende, biotite, epidote, sphene, mag netite, and fluorite all lying in a finer-grained leucocratic ground mass. The characteristic 1 doubly porphy- ritic1 texture is present.

~~In explaining the genesis of the mantled feldspars, he confines his attention to the feldspar phases, and follows Tuttle and Bowens’ (1958) view of rapakivi growth.~~

Tuttle and Bowen point out that in every rapakivi pluton some unmantled feldspars are present along with the mantled ones, and that this texture must be explained. They ex plain the relationships on the basis of the liquidus dia gram for the orthoclase-albite-silica system at j)00kg,/ sqVcm.' pressure. Using an average rapakivi mode recalculated to 100/j quartz and feldspars, they develop a cooling paragenesis as follows: First, potassium feldspar (Or^Abgg) crystallizes from 800 degrees until 15$ of the magma has been used up, at which time quartz begins to crystallize.

~~At about 660 degrees 22$ of the liquid is left, and plagio-~~

clase begins to crystallize. The two feldspars will be nearly identical in composition at first, but will diverge • . 110 in composition as.cooling continues. The crux of Tuttle and Bowens1 argument is that, essentially randomly, some of the feldspars will zone toward plagloclase and some toward orthoclase; those which zone toward plagioclase will develop rapakivi textures.

Apart from the fact that they arc using a phase diagram not determined for the full system of rapakivi composition, Tuttle and Bowen are using a mechanism de ficient on at least three counts. First, they cannot account for the small plagioclase inclusions in the center or near-centers of potassium feldspars. Second, they would produce too little plagioclase; a little less than half the modal feldspar in the Dos Ca'oezas rapakivi is plagioclase. Third, there is really no evidence that when a plagioclase begins to crystallize only a portion of the feldspar crystals will zone toward one or the other feldspars as cooling continues. This last idea is only a hope. Tuttle and Bowen ignore the dark mineral phases of the rock completely.1

Terzaghi (19^0) has studied a rapakivi pluton in Maine which has all the classic characteristics ex cept modal fluorite. She attempts to explain the observed feldspar pattern and texture solely on the basis of rela tive growth rates and abundance of the feldspars. She postulates that the potassium feldspar crystals grew very Ill rapidly, and thereby depleted the magma in their immediate

~~area in potassium feldspar constituents, while super~~

~~saturating the same surrounding volume in plagioclase~~

~~constituentsV Hence, plagioclase grows on the margins~~

~~of the potassium feldspar crystals," She ascribes this~~

~~hypothesis to Sederholm.~~

~~Terzaghi* s ideas presuppose that, in a buried~~

~~pluton, the rate of crystal growth in the magma exceeds~~

~~the rate at which diffusion of the constituent elements~~

~~of the magma can keep the composition of the remaining~~

~~magma homogenized; to the writer, this seems very unlikely.~~

~~She also fails by this hypothesis to explain the presence~~

~~of plagioclase Inclusions in the cores of the potassium~~

~~feldspars.~~

~~Terzaghi proposes an alternate hypothesis for~~

~~magmatic generation of the rapakivi texture as follows.~~

~~She proposes that an increase in water pressure in the~~

~~magma may result in increased orthoclase solubility,~~

~~leading to plagioclase crystallization/ In the first~~

~~place, the writer does not see why this should follow,~~

~~and in the second, more recent work by Tuttle and Bowen~~

~~(1958, p. 5^-56) has shown that addition of water to an~~

~~albite-orthoclase-silica melt causes an increase in the~~

orthoclase stability field. Terzaghi makes only a 112 brief mention of the presence of biotite, and makes no men tion of the ground mass character or the textural relation ships of the phases at all.

The writer, to conclude, would regard the inter pretations of Thomas and Smith (1932) and himself as the ones which most nearly completely explain the rapalcivi texturev He regards all the solid-state formation hypo theses he has read as lacking in provable mechanisms and reactions, and regards most other magmatic theories as being Incomplete due to lack of work on the dark mineral phases of the rock.' The writer believes that rapalcivis can be explained as the.results of the single cooling of a magmatic body while it moves upward into its roof. CHAPTER 5

~~YOUNGER PRECA1-1BRIAM AI© POST-PRECAKBRIAN~~

~~■SEDIHENTATIONAL HISTORY~~

~~The post-Older Precambrian stratigraphic history~~

~~of the Dos Cabezas mountains covers several hundred mil~~

~~lion years, and contains much of the history of the range~~

~~during that time. The bedded sequence in the range con-~~

~~• tains a good representative sequence of all the Paleozoic~~

~~formations up to the lowest formation in the Naco Group,~~

~~as well as a thick sequence of Cretaceous clastic rocks.~~

~~Younger Precambrian Sediments~~

~~The stratigraphic units of the larger part of the~~

~~Apache Group of younger Precambrian age cropout in the~~

~~Little Dragoon mountains, only thirty miles to the west~~

~~of the Dos Cabezas mountains (Cooper and Silver, 196*1').~~

~~Some 450 to 600 feet of these strate are present there.~~

~~The stratigraphic character of the described units gives~~

~~no indication of the shoreline having been nearby in~~

~~Apache time.~~

~~The outcrops of the Apache Group in the Little~~

~~Dragoon mountains are the southeasternmost exposures of~~

~~the group. Given the absence of nearshore facies in~~

~~113 the group where exposed, the writer was curious as to why the Apache Group doesn’t show up in the Dos Cabezas range.~~

~~The reason apparently lies in post-Apache and pre-Paleozoic intrusive and structural activity in the Dos Cabezas moun tain area. •~~

The answer could lie, of course, in the non- deposition of the Apache Group units within the area of the present Dos Cabezas range, but this is thought to be unlikely, as mentioned above, The writer believes that the answer can be found in the character of the Polecat quartz monzonite, which underlies much of the northern

Dos Cabezas mountains♦' A reheating and recrystallization event of 1000 million years age, and of considerable severity, is recorded for the Polecat and its Precambrian wall rocks to the west. This reheating is probably re lated to deep-seated plutonism and consequent high heat flow in a 1000-million year old disturbance recorded in other areas in the southwest. There is evidence from the northern Dos Cabezas mountains that considerable structural development was locally associated with this event (see Chapter 6). The Apache group is at least

~~1150 million years old (Damon, Livingston, and Erickson,~~

~~1962) and it seems quite likely that during this 1000 million year old event, the area of the Dos Cabezas was uplifted enough so that the Apache group sediments were 115 removed "by erosion. .~~

~~Nature of the Contact Between the Rapaklvi Quartz~~

~~Monzonite and the Overlying Paleozoic Sequence~~

~~The base of the Paleozoic stratigraphic sequence~~

~~in the Dos Cabezas mountains rests everywhere upon a rather~~

smooth planar surface cut upon the underlying ?recambrian rocks. Sabins (1957a) says “at all exposures, the pene plain is a smooth planar surface, except for a large monadnock formed by a thick, resistant quartzite member of the

~~Pinal Schist at the west side of Bowie mountain.“~~

~~The monadnock referred to above by Sabins has a~~

~~smaller equivalent in the Dos Cabezas mountains. In the~~

~~N E l A SIT 1/4 of Sec. 3 Tl^S R27E, there are a number of '~~

~~inclusions of Pinal quartzite of considerable size in the~~

~~rapakivi quartz monzonite. Where the largest of these is~~

~~cut by the nonconformity surface carved upon the rapakivi~~

~~(see Figure 15), there is a marked change in the nature of~~

~~the basal sediments of the Cambrian sequence. The base~~

~~of the Bolsa quartzite to the east of this hill of quartzite~~

~~has no basal conglomerate, while to the west of it there~~

~~is a .basal conglomerate some 10 feet thick, full of very~~

~~angular quartzite boulders and cobbles.~~

~~There has been considerable discussion as to whether~~

the contact between the base of the Paleozoic strata and 116 the underlying rapaltivi quartz monzonite is intrusive or not. The first thoughts that the contact might not be a sedimentary nonconformity were generated by the first .

K-Ar age on the rapakivi, which was Paleocene; it was also noted that there is an obvious local alignment of mantled potash feldspars within the rapakivi parallel to the Bolsa-rapakivi contact (see Figure 16b).

~~.1 believe that there is compelling evidence, how ever, of both a classic and a geochronologic nature that the contact is indeed a classic sedimentary nonconformity.~~

~~The classic evidence lies first in the presence of several large slabs and boulders of the rapakivi within the basal conglomerate of the Bolsa quartzite. See Figure 18a and~~

~~18b for pictures of two of these. These rapakivi boulders are found in a restricted area of the conglomerate which also contains some enormous boulders of Pinal quarzite~~

~~(see Figure 17). Secondly, at several places at the base of the conglomerate the surface of the Bolsa-rapakivi contact can be seen (see Figure 18a and 16a) and it is a~~

~~smooth planar surface quite separable from the Bolsa above; it dips to the southeast conformable with the bedding of the conglomerate and the overlying Bolsa strata.~~

~~It should also be noted that there is no deviation in the position of the contact for some miles along the strike of the hogback of Paleozoic sediments Which forms~~

~~Figure 15: Primary Structures in the Procambrian and Cam brian Rocks Hear the Unconformity Surface Cut on the Rapakivi Quartz Honzonite~~

(a): Pinal Schist quartzite lying under the uncon formity surface below the Cambrian Bolsa quartzite# This Pinal is a large xenolithic block in the rapakivi, on the high eastern slope of hill 5 6 6 0 , in Sec. 3« T15S R27E. b is the Bolsa quartzite, and pi is the Pinal on the figure. Note the foliation symbol in the Pinal and the strike-dip symbol in the Bolsa; the two are at near right angles.' Here the Pinal xenollth has been a local source of quart zite debris along the old unconformity surface.

(b): Pinal quartzite showing relict bedding. The outcrop pictures is about three feet wide, and lies at the south margin of a small hill on the low northeastern slope of hill 5 6 6 0 , Sec, 3, T15S R27S. The bedding strikes SSV7 and dips steeply U.’ //f

~~Figure 15J Primary Structures In the Pre-Cambrian and~~

~~Cambrian Rocks Near the Unconformity Surface Cut on the~~

~~Rapaklvi Quartz Monzonite Figure 16: Rock Structures’ and Textures Along and Near the Unconformity Between the Bolsa and the Rapakivi~~

(a): Picture of the exposed unconformity surface below the Cambrian Bolsa Quartzite.' This surface was cut originally on the Precambrian rapakivi quartz monzonite which occupies the right-hand part of the picture; the basal quartzite boulder and cobble conglomerate of the Bolsa lie atop the surface to the left.

(b): A picture of the surface of the rapakivi near the Bolsa quartzite, showing a weak west-northwest strike marked out by subparallel orientation of weathered feldspars on the weathered surface. This foliation is parallel to the unconformity surface trace in the local area. //a

Figure 16: Rock Structures and Textures Along and Near the Unconformity Between the Bolsa and the Rapaklvl I ISfct Figure 1?: Primary Depositions! Structures in the Basal Conglomerate of the Bolsa Quartzite

~~(a); Large Pinal Schist quartzite boulders in the basal conglomerate of the Bolsa Quartzite. The exact locality is unknown but is in Sec. 3 or 4 of T15S R27E.~~

(b): Large rounded boulder of Pinal quartzite with load cast compaction structure formed when the boulder dropped into its present position and squeezed up arkosic debris under it. Picture from the west slope of hill 5660, Sec. 3, T15S B272. Figure 1?s Primary Depositional Structures in the Basal

~~Conglomerate of the Bolsa Quartzite-1~~

~~Figure 18: Primary Depositional Features in the Basal Conglomerate of the Bolsa Quartzite~~

(a): Large slab of rapalcivi quartz monzonite in the basal conglomerate of the Bolsa quartzite/ The slab pictures is quite angular and about three feet long and about ten feet above the base of the conglomerate. The white fragments are Pinal Schist quartzite/ This picture taken just east of the summit of a small hill in the MS/1/4 Sec. 4, T15S R27E.

(b): Three large subrounded boulders of Pinal Schist quartzite with a boulder of rapakivi quartz monzonite to their right sitting in the basal conglomerate of the Bolsa quartzite. Mote the exposure of the old unconformity sur face in the right-hand part of the picture; this surface is carved atop the rapakivi and presently dips about sixty degrees to the southwest, concordant with the dip of the overlying Bolsa sediments/ /20

~~Figure 18: Primary Depositional Features in the Basal~~

~~Conglomerate of the Bolsa Quartzite -2. 121 the southwestern boundary of the range (see Figure 1).~~

~~The contact is always located at the base of the Cambrian~~

Bolsa Quartzite, and always below the basal conglomerate of the unit, if the conglomerate is present. No dikes or apophyses of the rapakivi invade the Bolsa or higher units,' This behavior may be contrasted with that of the

~~Juniper Flat Granite in the Mule mountains, which is of~~

Jurassic age (Gilluly, 1956).' This pluton sends numerous tongues up into the Paleozoic sediments whenever it is in contact with them. Additionally, it should be noted that the basal Bolsa continues on beyond the rapakivi onto the Cienaga gneisses which the rapakivi obviously intrudes (see Figure 1) without any change in the posi tion or character of the Bolsa, It seems obvious that the Bolsa was laid down atop the rapakivi and the Cienaga gneisses after they had been eroded down to a rather smooth peneplain, as originally described by Sabins.

' Geochronologic evidence for the true age of the rapakivi quartz monzonite lies in a whole-rock PVb-Sr isochron age of 1380 - 30 million years, which almost certainly precludes the possibility of this plutons being of post-Cambrian age.' Three K-Ar ages from widely separated areas in the pluton are all Tertiary, but these ages are common in all but the westerhmost part of the range in all the Precambrian units, and are thought to be 122 due to heating by largely buried Tertiary plutons. See

Chapter 10 for a full discussion. The alignment of large mantled potassium feldspars in the rapakivi with the Bolsa-rapakivi contact is felt to be due to a rather interesting combination of regional tectonic features of long duration. In the field it was noted that there is a VJlft/ trending foliation in the rapa kivi parallel to the trend of the Bolsa-rapakivi contact.

It has been found by the writer that internal flow folia tion, although often weak, is characteristic of the rapakivi pluton; this usually takes the form of two foliation planes marked out by feldspar orientation and trending at near right angles to one another. One of these foliation planes commonly has a WNVI strike in most parts of the pluton. This foliation is an internal flow folia tion produced by intrusion of a partially crystalline magma; the west-northwest orientation of one of the planes is due to the structural control exercised on the rising pluton by major west-northwest trending fractures in the deep basement. These fractures also are responsible for the orientation of the west-northwest trending faults which cut the range, such as the Apache Pass fault.

The fault blocks cut out by these present west- northwest trending faults dip to the south-southwest at right angles to the fault's strike. This means that any 123 strike line on the bed surfaces of strata involved in the faulting will have a west-northwest trend, parallel to the major fault trend. The outcrop line of the Bolsa- rapakivi contact is essentially a strike line at the base of the Bolsa, drawn on one of these tilted fault blocks.

Since this strike line has a west-northwest trend, and since the flow foliation in the rapakivi also has a west- northwest trend, the feldspar orientation must be parallel • to the contact between the rapakivi and the Bolsa Quartzite.

The correlative directions are not a coincidence, but neither are they chronologically equivalent; rather they are due to the control of Dong enduring structural features over two separate events separated by several hundred million years of time.

~~The Bolsa-rapakivi contact has been tilted about~~

~~60 degrees to the southwest by post-Paleozoic tectonic ac tivity. A considerable amount of shearing from this event is evident along the Bolsa-rapakivi contact in both the~~

Bolsa and the rapakivi, and a considerable amount of iron staining has developed in these shears, probably by simple . weathering. Also, as mentioned in Chapter 10, there is mineralogic and geochronologic evidence for an intense early Cenozoic thermal event in this area; this probably accounts for the present rigid character of the tilted 12k

~~and once highly sheared material along the contact.~~

~~Numerous healed fractures are visible in both the rapakivi~~

~~and the Bolsa along the contact.~~

~~Paleozoic Sedimentational History~~

~~As mentioned in the introduction to the disserta~~

~~tion, I have not focused my attention on the details of~~

~~the post-Precambrian stratigraphy of the Dos Cabezas, pre~~

~~ferring to leave that to someone with a better understanding~~

~~of the record of those times. Nonetheless, certain signifi-~~

~~’ cant features of the strata have impressed themselves upon~~

~~my mind; in addition, a brief review of post-Precambrian~~

~~sedimentational history is in order in this overall chrono~~

~~logic and petrographic study of the Dos Cabezas mountains.~~

~~The exposures of the post-Precambrian sediments in~~

~~the Dos Cabezas mountains are large and complete enough so~~

~~that some fair idea.of the depositional history of the~~

~~times can be gained. The structural attitudes of the units~~

~~presently exposed in the range are discussed in Chapter 6~~

~~and the characters, of the sediment blocks in the volcanic~~

~~breccia are discussed in Chapter 7 . Outcrop areas of the~~

~~various formations are shown on Figure 1. The names ap~~

~~plied to the various formations are those of Sabins (1957a)~~

~~based on his study of the stratigraphy of the northern~~

Chirichahua and Dos Cabezas mountains, unless otherwise 125 noted. In places where the writer was unable to clearly separate the known formations, the terms "upper Paleozoic undifferentiated" or "lower Paleozoic undifferentiated" may be used.

~~Bolsa Quartzite The basal Paleozoic formation in the Dos Cabezas mountains is the Bolsa Quartzite. Its stratigraphic top~~

~~is defined as the zone where the sandy shales of the upper~~

~~Bolsa give way to the carbonate-rich elastics of the overlying El Paso Formation. Its base is defined as the bottom~~

~~of the Paleozoic sequence. Jones and Batcheller (1953) have measured its thickness in two places; the results~~

~~are 440 and 320 feet.~~

~~The Bolsa is, in general, a medium to coarse-grained pink to reddish quartzite, generally thick-bedded. Cross bedding is generally present. A basal conglomerate of~~

~~quite variable thickness is present at the base of the~~

~~unit in most exposures seen by the writer.~~

~~I have examined the basal conglomerate in many~~

~~places in the Dos Cabezas mountains. It.varies from zero~~

~~to about bO feet in thickness, and often thickens and~~

~~thins quite rapidly along strike. It is composed almost~~

~~wholly of quartzite fragments of white color contained in~~

~~a dark purple to red medium-grained quartzitic matrix. 126~~

The quartzite debris is generally an assortment of well- rounded cobbles and boulders of white quartzite, although locally quite large rounded boulders or angular slabs of quartzite are also present. In the conglomerate at the base of the Bolsa where it overlies the rapakivi east of

~~Rancho Sacatal road most of the quartzite debris is quite angular, probably reflecting its local derivation from blocks of Pinal quartzite in the rapakivi or from the~~

~~outcrops of Pinal quartzite west of the rapakivi. In~~

~~this area several large angular boulders of the rapakivi~~

~~itself are present in the conglomerate, together with~~

~~several spectacular boulders of Pinal quartzite up to~~

~~twenty feet in diameterv~~

~~The contacts of the overlying sandy Bolsa with~~

~~its basal conglomerate are usually quite sharp, and may~~

~~be a disconformity. In many places, the Bolsa has numerous pebbly stringers in it a considerable distance above the~~

~~conglomerate. The Bolsa in general appears to get a little more fine-grained and thinner bedded as one goes from~~

~~bottom to top of the formation.~~

~~Sabins (1957a) reports definite upper Cambrian~~

~~fossils in the upper Bolsa at Blue Mountain in the Chiri-~~

~~chahuas, and lower Dresbachian forms lower in the upper~~

~~Bolsa southeast of Dos Cabezas village. Thus, a definite~~

~~upper Cambrian age is established for at least the upper 12?~~

~~Bolsa; Sabins believes that the lower Bolsa in the Dos~~

~~Cabezas-Chirichahua area may be as old as middle Cambrian.~~

~~He also noted that the Bolsa varies a good deal in thiclc- ness and locally vanishes altogether, as in Sec. I?, T15S~~

~~R29E in the Chirichahuas. •~~

~~El Paso Formation~~

The El Paso Formation is next above the Bolsa in the Paleozoic sequence. In the Dos Cabezas the unit is a sequence of carbonate beds which contain silt and sand in the lower part of the formation, and some chert in the upper part." The bedding is very thin. On weathered surfaces the rocks take on a laminated and crumpled appearance marked by thin wrinkled siliceous bands which protrude beyond the carbonate on the weathered surface. The formation ends at the base of an overlying sequence of carbonate-bearing siltstones. These strata are about 360 feet thick at Dos

~~Cabezas village and up to 600 feet thick two miles to the west of the first locality (Jones and Batcheller, 1953)•~~

~~Sabins (1957a) reports 715 feet of thickness for these strata southeast of Blue Mountain in the Chirichahuas.~~

~~The upper part of the limestone strata bear middle~~

~~Canadian (lower Ordovician) fossils in considerable abund ance near Dos Cabezas village (Sabins, 1957a). An upper~~

~~Cambrian fossil (Sabins, 1957a) from the lower part of 128 the El Paso sequence near Blue Mountain implies that the formation transgresses the Cambrian-Ordovician boundary.~~

Sabins (1957a) believes that the upper part of the El Paso formation has been removed by erosion, and that the time interval between the end of El Paso depo sition and the deposition of the overlying Devonian elastics contains the time of a broad epirogenic upwarp of the Chirichahua-Dos Cabezas area.

~~Portal Formation~~

~~The Portal Formation lies next above the El Paso~~

Formation in the Dos Cabezas area. The rocks in the Portal are a sequence of limestones and carbonate-bearing and carbonate-free shales and sandy shales, which Sabins (1957a) has divided into four members. The various members of

Sabins’ type Portal will not be distinguished here because there is little upon which to base such a discrimination in the Dos Cabezas mountains. The formation’s base is marked by'transition from the thin-bedded limestones of

~~the 21 Paso to the generally olive colored carbonate bearing shale; the top is marked by transition from Portal rocks to grey, cliff-forming Escabrosa limestone. Exposures~~

~~of the Portal are almost uniformly poor in the Dos Cabezas mountains; it underlies topographic depressions in general.~~

~~It varies considerably in thickness, from 400 feet two miles west of Dos Cabezas village to 165 feet at the village 129~~

~~(Jones and Batcheller, 1953) to 300-330 feet in the Blue~~

~~Mountain section of Sabins (1957s.).~~

~~Sabins discusses the sparse fossil evidence and refers the Portal sequence to the middle and upper Devonian.~~

He introduces the name Portal Formation for the sequence since it is lithologically unlike the Martin Limestone and is too far from Devonian clastic units in the New Mexico area to allow certain correlation,

~~Sabins’ evidence indicates that the upper Ordovician, and all Silurian, and lower Devonian time are represented by no extant depositions! units in the Dos Cabezas-~~

~~Chirichahua mountains area. A time period of some ?0 million years (Kulp, I96I) is thus unrecorded in the strata of the~~

~~Dos Cabezas mountains; Sabins, again, thinks this is due to~~

~~a broad epeirogenic upwarp of this area during this time.~~

~~Bscabrosa Limestone.~~

~~Above the Portal carbonate-bearing elastics and~~

~~limestones lies a thick sequence of generally thick-bedded~~

~~crinoidal and cherty gray limestone. The upper contact~~

~~is marked by the transition to overlying thin-bedded lime~~

~~stones of the Horquilla Limestone, No clastic beds are~~

~~present in this thick-bedded limestone series. The sequence~~

~~is about 800 feet thick two miles west of Dos Cabezas~~

~~village (Jones and Batcheller, 1953), some 630 feet at 130~~

~~Blue Mountain.and 730 feet near Portal in the Chirichahuas~~

~~(Sabins, 1957a). Fossil types imply a lover Mississippian age ranging up to Meramecian or mld-Hississippian time.~~

~~Sabins refers this unit to the Mississippian Escabrosa~~

~~Limestone. In the northern Chirichahua mountains the Escabrosa~~

Limestone is overlain by a. thin sequence of dark shales and thin-bedded limestones containing an upper Mississippian fauna, referred to as the Paradise Formation." This unit does not outcrop in the Dos Cabezas mountains. Its posi tion in time corresponds to the disconformlty between the

~~Escabrosa and the overlying Horquilla Limestone. Whether the Paradise Formation is missing because of non-deposition or post-depositional erosion is not known.~~

~~Horquilla Limestone~~

~~In the Dos Cabezas mountains the Escabrosa Lime stone is overlain by a sequence of thin to thick bedded limestones and shales. This sequence is the Horquilla~~

~~Limestone (Sabins, 1957a.) • The base and top of the sequence are both difficult to locate,, but for different reasons.~~

The base can only be located within 10 feet, due to litho logic similarity between the basal Horquilla and the under lying Escabrosa; for convenience, the base is placed at the level in the sequence where the first fusilinid 131 fossils appear in the limestone. The top of the Horquilla

~~Limestone has been removed by erosion wherever that formation is found in the Dos Cabezas mountain area, and hence its exact original'thickness is unknown. The~~

Horquilla Limestone, where present, is usually overlain by the Cretaceous Bisbee Formation, which rests atop it on a gentle but persistent angular unconformity. Although the upper part of the Horquilla has always been removed by erosion where found, the amount of this erosion has been slight enough so that in at least some places much of the Horquilla is still preserved. Hr. Tom Dirks identified a fossil assemblage from the Horquilla just a few feet below tho overlying Bisbee Formation in the

~~SW1/4 Sec. 27, T14S R273 as characteristic of the upper~~

~~Horquilla. The Horquilla is often quite fossiliferous and it is of Pennsylvanian age.~~

~~Concha Limestone~~

~~In an isolated hill west of the village of Dos~~

Cabezas in the SW1/4 Sec. 2 7 , Tl4s R2o3 a moderately thick- bedded limestone unit is exposed. The unit is a dark- grey-brown limestone which weathers grey on exposed surfaces. It has a fossil assemblage containing abundant

Dictyoclostus sp. brachiopods, gastropods, several types of bryozoa, and crinoid stem fragments. This unit was 132 identified by Cooper (i9 6 0 ) as the Permian Concha Limestone of the Naco Group; on examination of the hill, Hr. Dirks held the same opinion, and it seems quite certain that this is the Concha.1 The distance on the ground between this isolated hill and the nearest Paleozoic outcrops is about a half a mile, and the interval is covered by al luvium.1 The nearest Paleozoic unit near this hill is the

~~Pennsylvanian Horquilla Limestone, also of the Naco group.~~

~~There are several Permian and Pennsylvanian-Permian units in the normal stratigraphic sequence of the Naco Group between the Horquilla and the Concha (Earp Formation,~~

~~Colina Limestone, and Sherrer Formation); none of these have been found anywhere in the Dos Cabezas mountains.~~

~~Given the normal absence of Naco Group units above the~~

~~Horquilla Limestone, the writer suspects that this local~~

Concha outcrop has been preserved by some special event; presumably the Earp, Colina, and Scherrer are also present below or beside it to some degree, but are hidden from view by alluvium. The special event was, in the writer's view, probably downfaulting of the upper Naco units in

~~this local area, so that somewhat younger, units show up~~

~~on the old pre-Bisbee Formation erosion surface than is normally the case. It is possible that the Concha is here preserved as a hill or nonadnock on the pre-Bisbee~~

~~surface, but this is thought unlikely, as no higher Naco 133~~

~~Group formations are preserved in this manner anywhere with in the range. No final resolution of the problem is possible at present.~~

~~Mesozoic Sedimentational History~~

The largest hiatus in the post-Precambrian strati graphic record in the Dos Cabezas mountains is that be tween the time of deposition of the Permian Concha Limestone and the time of deposition of the lower Cretaceous Bisbee

~~Formation; No known Triassic or Jurassic strata are present in the Dos Cabezas mountains; the nearest ones are in the~~

Little Dragoon mountains thirty miles to the west (Cooper and Silver, 1964). The area of the Dos Cabezas mountains may have been uplifted and dry throughout this time, or it may have lost early Mesozoic sedimentary cover by pre lower Cretaceous erosion.

~~Terminology~~

~~The writer will not use Sabins* (1957a) terminology~~

~~for the Cretaceous rocks in the Dos Cabezas mountains, which was largely derived from Sabins* study of these rocks in~~

~~the northern Chirichahua mountains; some of Sabins * units~~

~~cannot be found or discriminated in the Dos Cabezas range.~~

~~Ransome (1904) used the term Bisbee Group for the lower~~

~~Cretaceous rocks in the Mule mountains; outside of this 134 range, hovrever, his various component formations of the~~

Bisbee Group become difficult or impossible to separate, and various workers in several mountain ranges in south eastern Arizona have all used terms for lower Cretaceous sediments which retain some of the features of Ransome* s names but have a terminological character more suited to the local situation/ Gilluly (1956) uses the term Bisbee

~~Formation for all the Cretaceous sedimentary rocks in the~~

Cochise county area outside the Mule mountains, and the writer will adopt this terminology for the Dos Cabezas mountains because it seems best adapted for the local situa tion. The basal conglomerate of this undifferentiated

~~Bisbee Formation occupies the position of the basal~~

~~Glance Conglomerate formation in the Bisbee group type area. Consequently, the name Glance will be used for this conglomerate where it appears; in this application in the~~

~~Dos Cabezas mountains "Glance" should not be considered a formational name.~~

~~Glance Conglomerate~~

~~These basal coarsely clastic units in the Dos~~

~~Cabezas mountains which are presumed to be equivalent in position, at least, to the type Glance, are of two types.~~

At the base of the Bisbee Formation in the central and eastern parts of the Dos Cabezas range a moderately thick 135 (0 -5 0 feet) quartzite cobble conglomerate rests on an erosion surface cut on the underlying Horquilla and

~~Escabrosa Limestones. In one area what appears to be a local stream channel cut into the Horquilla holds about~~

~~200 feet of this unit. The cobbles are of a white quart zite, very like those in the basal conglomerate of the~~

Cambrian Bolsa Quartzite; they are held in a matrix of quartz sand. The unit is only moderately lithified, and individual pebbles and cobbles can be pried out of the matrix." This unit pinches out along the base of the

~~Bisbee Formation just east of the old Mascot mine and north of the village of Dos Cabezas.~~

~~West of the Silver Camp stock and east of Camel- back Mountain, in the N. center Sec.' 3» Tl4s R26E and~~

~~in SWl/ty Sec. 3^, T13S R26S, there outcrops some two~~

~~square miles of a predominately carbonate sedimentary unit of an anomalous type for the Dos Cabezas mountains. It~~

~~is quite unlike any unit in the Bisbee anywhere in the~~

~~Dos Cabezas range, and also quite unlike any Paleozoic~~

~~unit, but does bear some resemblance to carbonate clastic~~

~~units reported from the Glance Conglomerate in other~~

~~areas (Cooper and Silver, 1964). This unit, in the Dos~~

~~Cabezas mountains, is composed predominately of thin beds~~

~~of lime-pebble conglomerate interlayered with siliceous~~

~~limestone and sparse shale beds. The total assemblage 136~~

Is some few hundreds of feet thick and rests with profound unconformity atop the Pinal Schist, The beds of the unit presently lie in a homocline striking north-northwest and dipping some twenty degrees south-southwest. There is no

~~structural congruence between this exposure and the Bisbee~~

~~strata exposed about four miles to the east, just north of~~

~~Dos Cabezas village (see Figure 1); these latter strata dip~~

~~steeply north and are overturned, and are very sheared and~~

~~crumpled,'~~

~~In spite of the lack of lithologic and structural~~

~~similarity between the carbonate clastic sequence atop the~~

~~Pinal Schist and the rest of the Bisbee Formation in the~~

~~Dos Cabezas range, it seems most reasonable to put this~~

~~unit in the Bisbee as a local facies of the basal Glance.~~

~~Cooper (i9 6 0 ) chose to do this, and the writer feels that~~

~~this is the best choice at present.'~~

~~Bisbee Formation~~

~~Atop the conglomeratic strata in the Dos Cabezas~~

~~range rests a wholly clastic sequence of considerable~~

~~thickness. At least 5000 feet of strata are present above~~

~~the Glance in the easternmost part of the range. The top~~

~~of the formation is missing due to faulting or intrusion~~

~~throughout the range.' The beds are predominantly fine~~

~~grained shales, very thin bedded, dark maroon to black in 137~~

~~color, and unfossiliferous.' A few beds of medium to fine grained sandstone up to several feeb thick are locally pre~~

~~sent,' The Bisbee Formation along the Apache Pass fault~~

~~trace is much reddened and more resistant to erosion than~~

~~elsewhere, especially within 0 -5 0 feet of the fault line.~~

~~No age relations can be established directly for~~

~~the Bisbee Formation in the Dos Cabezas mountains, be~~

~~cause no fossils were found in it. On purely regional~~

~~lithologic grounds, the best guess that can be made is~~

~~that the Bisbee strata in the Dos Cabezas mountains are~~

~~the same general age as those in the ranges to the south~~

~~and west. Gilluly ( 1 9 5 6 ) Cooper and Silver (196*1-) and~~

~~Sabins (1957a) are all of the opinion that the Bisbee in~~

~~the areas they have studied is of lower Cretaceous~~

~~(Comanchean) age and this age is postulated for the Bisbee~~

~~rocks in the Dos Cabezas. The age of at least the upper~~

~~part of the Bisbee in the Dos Cabezas area may be Albian,~~

~~as little as 100 million years (Kulp, 1961).~~

~~After deposition of these lower Cretaceous strata,~~

~~deposition was essentially halted by destruction of the~~

~~Cretaceous seaway in the strong deformation of the late-~~

~~Cretaceous Laramide orogeny. A long period of tectonism,~~

~~volcanism, and plutonism comprises the next sequential part~~

~~of the geologic record in the Dos Cabezas mountains, and~~

~~lasted well into the Cenozoic. 138~~

~~Cenozolc Sediraentational History~~

There are no known older Cenozolc sediments in the Dos Ca'oezas mountains.' The range is, however, sur rounded by basins of varying depth which are filled to a large degree with clastic deposits of probable Cenozolc age. Most of this clastic alluvium presumably post-dates final uplift of the Dos Cabezas and formation of the neighboring valleys, but I am quite uncertain as to how much of this uplift took place during Laramide times and how much in the mid-Tertiary. About all that can be said with some certainty is that almost all this .basin-filling unconsolidated alluvium is of Cenozolc age.

~~The Cenozolc alluvium in the basins bordering the~~

Dos Cabezas mountains is a minimum of several thousand feeb thick in places; Jones and Batcheller (1953) report several well sites where depths of 3~^000 feet were ob tained in this unconsolidated alluvium near Bowie, San

~~Simon, and Hillcox. The writer has spent no time studying the young alluvium beyond the margins of the Dos Cabezas mountains.~~

Very young alluvial deposits-are present along modern stream channels in the range, and some of the flat-lying low relief areas in the northwest and south center of the range have patchy thin blankets of recent 139 alluvial cover.' These alluvial deposits are the youngest sedimentary units in the range, and are provisionally classed as Quaternary alluvium.

~~Total Post-Precambrian Sedimentation~~

~~The total minimum thickness of the Paleozoic units in the Dos Gabezas mountains is some 2800 feet according to Jones and Batcheller (1953)•' If the Earp, Colina, and~~

~~Scherrer units of the Haco Group are present below the~~

~~Concha Limestone in the area where it outcrops, the origi nal thickness of Paleozoic units could have been much greater.~~

~~Cretaceous units are upwards of a mile thick (Figure~~

~~1 ), and considerably more may have been present before erosion and faulting removed their upper part.~~

~~Sabins (1957a) estimates that 7.600 feet for the~~

~~Paleozoic strata and 350° feet for the Mesozoic strata may be taken as average stratigraphic thicknesses for these eras in the eastern Dos. Cabezas mountains and the northern~~

Chirichahua mountains. The author regards this as quite a minimum figure for the Dos Cabezas as a whole. About two miles, perhaps a little more, of post-Precambrian sediments were present before the structural developments of the

~~Laramide and mid-Tertiary occurred.~~

~~It should be noted again that about four times the JUt-0 above thickness, at a bare minimum, is present in the meta morphosed Precambrian sediments which largely make up the~~

Precambrian Pinal Schist; the major part of the overall depositional history of the Dos Cabezas mountains area is contained in them, rather than in the somewhat more con spicuous and more thoroughly studied Paleozoic and Mesozoic strata,' CHAPTER 6

~~FAULT STRUCTURES OF THE DOS CABEZAS MOUNTAINS~~

This structural survey is based so lely upon the present work. Only the major structures, such as large faults and folds, were investigated,1 because of the semi reconnaissance nature of the study. Folding of any size is largely confined to the Pinal Schist, and has already

~~"been described; the discussion in this chapter is solely about the faulting of various ages found within the range.~~

The fault structure of the Dos Cabezas mountains is essentially simple (see Figure 1 and Figure 19); it is composed of older west-northwest trending steeply-dipping faults, and a younger set of north-northeast trending faults that cut the former types. The age of initial formation of the west-northwest trending breaks is open to some question, but a definite age for formation of the north-northeast trending faults can be demonstrated.

~~West-Northwest Trending Faults~~

~~The writer has observed five essentially separate large west-northwest trending faults in the Dos Cabezas mountains. The Apache Pass Fault~~

A very lstrongly expressed fault which is cut across the southern part of the mountains is one of these. It is the direct continuous western extension of the Apache Pass fault, named by Sabins (l95?b) from its outcrops in Apache

~~Pass, which divides the Chirichahua and Dos Cabezas moun tains The fault is given the sane name in this study.~~

This large Apache Pass fault structure cuts west- northwest across the south central and southeastern parts of the Dos Cabezas mountains.' It divides two markedly dif ferent Precambrian rock terrains from one another, and is also the locus of considerable post-lower Cretaceous deformation.' In the Dos Cabezas mountains, the Apache Pass fault has a visible strike length of some thirteen miles, and undoubtedly extends much further west under the al luvium southwest of the range; it also extends for many miles into the Chirichahua mountains. It seems to be a simple structure, although much of the zone it has af fected has been obliterated by later geologic activity, and the deduction of its simple nature has been based on fragmentary evidence to some extent, Figure 20 shows two views along it.

~~The main trace of the fault lies along the south western boundary of a mass of west-northwest trending 4~~

■

~~I I £P/ 144~~

~~Paleozoic and Mesozoic sediments, which rest on the Pinal~~

~~Schist where their base is exposed, and dip steeply to the southeast over most of their exposure (see Figure 1)V~~

The steepness of dip remains constant at about 6 0 -7 0 degrees S in the eastern part of the range, but in the central and western part of the range the formations in crease in dip as one proceeds to the west along the strike of the beds. The formations are generally over turned in the last three to four miles of their exposure, and dip steeply to the north at some 60 to 80 degrees.

The dip of the fault piano itself is about 60 degrees southwest in the eastern part of its exposure, but appears to increase in the western parts of the fault’s exposure, and the fault plane may actually dip to the north-northeast at the western end of the fault’s exposure, if the fault planes’ attitude conforms to that of the sediments just north of it.

~~The history of movement along the Apache Pass fault * will be considered next, beginning with the nature of the pre-Bisbee, pre-lower Cretaceous movement.' In addition to~~

~~the Laramide movement recorded along the Apache Pass fault,~~

~~there is evidence in the Precambrian terrain on both sides~~

~~of the Apache Pass fault to indicate the existence of con~~

~~siderable fault movement along this break in Precambrian~~

~~time/ Host noticeably, there is no correlation at all~~

~~Figure 2 0 : General Features of the Apache Pass Fault Zone in the Central Dos Cahezas Mountains~~

(a): The Apache Pass fault zone as seen from the Mascot mine, looking west.' The area south of the fault is Pinal Schist (or Mazatzal?) quartzite; that to the north of it is volcanic breccia and some Bisbee formation, marked by Kb on the picture/ q is a symbol designating a large quartz dike cutting the Bisbee formation north of the fault (see Figure 1)/

(b): The Apache pass fault zone as seen from the Mascot mine, looking east.’ The dark hill on the topo graphic ridge in the middle distance is Bisbee Formation, heavily iron stained and exceptionally strongly lithified, lying just north of and parallel to the fault zone. The low hills to the south of the fault are of rapakivi. The peak on the center skyline is Howard Peak; volcanic breccia lies to the left of it, with sedimentary units invaded by the breccia lying to the south on south-sloping surfaces.

~~DCrqm = Dos Cabezas rapakivi quartz monzonite~~

~~vb = Volcanic breccia~~

~~rp = dacite porphyry plug~~

~~Kb = Bisbee Formation~~

~~Ph = Horquilla Formation /45~~

~~Figure 20: General Features of the Apache Pass Fault~~

Zone in the Central Dos Cabezas Mountains 146 between the structures and rock types on both sides of the fault, aside from the one small outcrop of probable Baton gneiss south of the Apache Pass zone which correlates with the great mass of this unit north of the fault. It is really remarkable that there is so vast a difference between the

~~two Precambrian terrains, and it seems that such a major discord must imply a great deal of relative displacement~~

~~of the two terrains along the fault line. It should be~~

~~emphasized that the differences are not just due to depth~~

~~or type of erosion; if one follows along the base of the~~

~~Paleozoic sedimentary sequence to the north and then to the~~

~~south of the fault, one can observe that the Precambrian~~

~~units on which the nonconformity was cut are quite different~~

~~in the two areas.~~

~~Since the rapakivi and the Cienaga gneisses are~~

~~cut by the fault, it must postdate the 1380 million year~~

~~age of the rapakivi, at least where it cuts, that body.~~

~~Possibly there was some movement along an ancestral fault~~

~~in the same area as far back as the time of deformation of~~

~~the Pinal Schist. The fault is certainly a major feature~~

~~of the Precambrian terrain. The simplest hypothesis for~~

~~the age of the fault is that it corresponds to the age of~~

~~the post-1 0 0 0 million year old faulting in the northern~~

~~part of the range, discussed below, on the basis of a~~

~~hypothesis of structural compatibility of all the large 14? vrest-northwest trending faults in the PrecanVorian terrains of the Dos Cabezas.~~

~~The orientation of the net slip along the fault trace is unknown. It is thought that the major part of~~

~~the movement is strike-slip, due to the marked disparity~~

~~in the character of the two terrains; some dip slip was probably also present, however.~~

~~There is some suggestion of a post-Paleozoic pre lower Cretaceous fault movement in and around the Apache~~

~~Pass Fault zone/ The suggestion comes from local preserva~~

~~tion of a small block of Permian Concha Limestone southwest~~

~~of the present fault zone about two miles, in an area of~~

~~Paleozoic sedimentary rocks of pre-Concha age. This small~~

~~outcrop of Concha has locally escaped the erosion which~~

~~removed it in the rest of the range, and it may be a~~

~~fault-bounded block downdropped relative to the other~~

~~Paleozoic rocks around it, before pre-lower Cretaceous~~

~~denudation of the area. No actual faults have been observed~~

~~around this block, which is wholly surrounded by alluvium,~~

~~and the foregoing discussion is quite hypothetical.~~

~~The map (Figure 1) shows obvious pos t-lower~~

~~Cretaceous (post-Bisbee) deformation by faulting in the~~

~~Dos Cabezas range, all along the Apache Pass fault. The~~

~~character of the sediments to the north of the fault zone have been described above; the south wall of the fault 148 is either Cienasa gneiss or rapakivi quartz raonzonite of~~

Precambrian age. Along most of the western part of the fault zone, the sediments just north of the fault have been invaded by intrusive volcanic breccia of Cretaceous age. The sediments north of the fault zone are broken by several other short west-northwest trending breaks parallel to the main fault, as well as by numerous north- northeast trending faults to be described later.

~~There are no other west-northwest trending fault .~~

segments of length comparable to the main Apache Pass fault in the vicinity of that latter fault, but there might once have been; east of the volcanic breccia a fault probably exists which cuts east-west through the Pinal

~~Schist and the Paleozoic sediments for several miles at~~

~~least, and probably ran on to the west much further before~~

~~the intrusion of the volcanic, breccia in the later part of~~

~~the Cretaceous. The trace of this fault is largely ob~~

~~scured by talus and much of its course is inferred from~~

~~joints parallel to its length.~~

~~Several west-northwest fault segments are present~~

~~in the zone of faulted Paleozoic and Mesozoic rocks north~~

~~of the Apache Pass fault (see Figure 1). In the NS part~~

~~of Sec. 19» and the NW part of Sec. 20 T14S R27E, there~~

~~are two west-northwest striking blocks of Pinal quartzite~~

~~faulted up into the Bisbee Formation along west-northwest 149 trending faults whose exact extent is difficult to determine.~~

In the east part of Sec. 22, T14S B27E, Pinal Quartzite is separated from Horquilla Limestone along a west-northwest trending fault which has been cut by a younger north-south trending fault; this last west-northwest trending fault has a strike length of only a third of a mile.

In Sec. 12, T14S R26E, and Sec. 7, 8 , 15, 16, 17 and 18,. T14S R27E, among others, large masses of Paleozoic and Mesozoic sedimentary rocks arc found within the volcanic breccia which makes up the core of the range. Identifica tion of these units except as undifferentiated limestone or quartzite has not been attempted. Quartzite is sometimes present near the base of the exposed part of blocks composed mostly of limestone; this assemblage looks like the Bolsa-

E1 Paso Cambrian sequence; no fossils were found to confirm such ideas. Many of these sedimentary blocks are a mile or more north of the Apache Pass zone, and may indicate a much larger former extent for the area of Paleozoic and

~~Mesozoic rocks disturbed by the faulting along the Apache~~

Pass fault, and for the area of former outcrop of these units on this level. The southward dip of the overturned sediments along the Apache Pass fault would, however, pro ject their surface position out over the present breccia outcrops so that the sediments would lie several thousand feet above the present outcrops of the exotic blocks in the 150 breccia.- It is, therefore, quite possible that these large blocks have sunk in the breccia during its intrusion, and

~~that the fault zone as extended upwards in former times had~~

~~a nature like that shown at the present level.~~

~~The post-lower Cretaceous movement along the Apache~~

~~Pass fault has evidently had both strike-slip and dip-slip~~

~~characters. First, the presence of PreCambrian quartz~~

~~monzonite and quartzite on the south hanging wall of the~~

~~fault and Cretaceous Bisbee Formation in the north footwall~~

~~shows a reverse dip-slip component in the overall slip.~~

~~Second, if one projects the beds in the hogback along the~~

~~southern margin of the range over to the Apache Pass fault~~

~~zone, their base lies six miles above their correlative~~

~~units on the north side of the zone. This would imply~~

~~that six miles or so of dip-slip displacement took place~~

~~along the Apache Pass fault in the Laramide (post-lower~~

~~Cretaceous) erogenic movements. So large a displacement~~

~~seems unlikely; the rapakivi between the hogback on the~~

~~southern margin of the range and the Apache Pass fault is~~

~~quite broken up by numerous faults, and it is more likely~~

~~that a number of blocks across the rapakivi have tilted to~~

~~the southwest during the Lar amide deformation of the area,~~

~~and that most of the supposed six-mile net slip displacement~~

~~along the Apache Pass fault is illusory. Dip slip might 151 well be a mile or two, however.~~

~~Although the strike of the sedimentary units in the~~

Apache Pass fault area is rather uniform, dips of the af fected sediments vary a good deal. Tills seems to he due to shear-induced wrinkling and folding of these units as the Precambrian block on the southwest rode up over the northern footwall.'

The strike trace of the fault in its western and central parts in the Dos Cabezas mountains is fairly smooth and uniform, and would allow strike-slip movement of the walls. That such movement may have occurred is in dicated by a feature of the sedimentary sequence north of

~~the fault. The strike of the sedimentary units in the more~~

~~intensely deformed central and western parts of the fault~~

~~zone systematically trends more toward the northwest than~~

~~the strike of the fault itself. This implies to the writer~~

~~deformation due to a couple caused by right-lateral strike~~

~~slip motion along the Apache Pass fault plane.~~

~~There is no way to estimate the probable magnitude~~

~~of this strike-slip movement, at least from evidence in the~~

~~Dos Cabezas mountains. In the easternmost Dos Cabezas range,~~

~~and more especially in the northern Chirichahua mountains,~~

~~the trace of the fault becomes very jagged, and this would~~

~~seem, to prohibit much strike-slip motion in these areas.~~

~~The writer thinks it likely that the recorded strike-slip 152 movement is local in character, and that the major movement . along the Apache Pass fault is reverse in character.~~

In summary, the west-northwest trending Apache Pass fault shows traces of a major Precambrian strike-slip displacement of unknown magnitude or direction, and strong reverse displacement coupled with local strike-slip displace ment of right-lateral character in the Laramide orogeny.1 .

~~Other West-Northwest Trending Faults~~

~~The four smaller west-northwest trending faults in~~

~~the northwestern part of the Dos Cabezas, some distance from the Apache Pass zone, show similar displacements and appear to be related. All four have steep to vertical dips.~~

~~No names have been given to these faults as they are rather~~

~~small in terms of present exposure. All four are in Pre~~

~~cambrian rock units (see Figure 1 and Figure 19), but whether~~

~~or not the faults are Precambrian in age cannot be directly~~

~~stated with complete assurance.~~

~~Fault A . This fault shows a little less than one-~~

~~half mile of apparent left-lateral displacement. No~~

~~noticeable drag features appear along the fault and its~~

~~displacement is inferred from offset of a large amphibolite~~

~~sill and the Sommer gneiss-Pinal Schist contact along its~~

~~trace (see Figure 1). Whether this apparent offset is a 153 product of pure strike slip is not known; dip-slip could have produced the same offset with the north block of the fault being downthrownv~~

~~Fault B, Fault B (see Figure. 19) shows perhaps~~

2.5 to 3 miles of left-lateral offset of the contact between the Polecat quartz monzonite and the Pinal Schist.' The plane of this fault is nowhere directly exposed. Since this contact seems to be essentially vertical, it seems unlikely that this offset has been produced by a predomin antly dip-slip motion, and the fault is presumed to have a predominantly strike-slip net slip. The marked strike-slip

~~character of this fault, and its close relationship with~~

~~fault A, which shows a similar offset, implies that the~~

~~latter is really a strike-slip fault too.'~~

~~The western part of the trace of fault B has not been observed directly. This is due to its being covered by a thin cover of young alluvium. Numerous west-northwest •~~

~~trending joints which cut the exposed rocks lie parallel to~~

~~and near to the strike projection.of the visible part of the~~

~~fault, and its trace is thought to be reasonably well lo--~~

~~cated.~~

~~The intersection of faults A and B is complex. The~~

~~area of intersection is covered in many places and exposures~~

are often poor. Fault A appears to carry into the block northeast of fault B for perhaps a mile, but appears to fade into parallel east-northeast trending joints beyond that poiriV Ho evidence of any great displacement along this latter segment of fault A was observed. Two explana tions for the observed relations are possible: first, fault B nay have preceded fault A and Fault A was formed by pure normal faulting with the north block downthrown and the faulting displacement dying out to the east along its strike; second, fault A.nay have preceded fault 3 in time and was cut off at its eastern end by fault B; later disturbances caused fault A to move slightly after fault

B was inactive, and caused fault A to extend itself slightly into the block northeast of fault B.' No real choice can be made between these two hypotheses at the present tine, although the author prefers the latter, as will be seen.

~~Faults C and D . Two small west-northwest trending faults, labeled C and D on Figure 19, cut the westernmost~~

Pinal Schist exposures in the Dos Cabezas mountains. Both faults are largely inferred from topographic and joint evidence. Fault C cuts the Sommer gneiss, and the evident displacement of the Sommer-Pinal contact is such as to in dicate a left-lateral strike slip character for the fault, with a net slip of about a half a mile. This is exactly the displacement along fault A to the east of fault C, and 155 as can. be seen on Figure 1 the two faults have parallel traces; that of fault C is offset about 1000 feet to the south of Fault A. It seems quite likely that the two lines are part of the same fault, and that this fault has been broken into two parts by a later crosscutting fault.

~~It is thought likely that there is such a fault running north-south through the topographic low in the north center of Sec.' 23, T13S R25B.~~

~~No markers of displacement were found along fault~~

~~D, and its character is unknown in any definite sense; since it seems to be a part of a group of left-lateral strike-slip faults, its displacement is probably of that sort.~~

~~Age of the West-Northwest Trending Faults~~

All these faults (A, B, C, and D) cut only Precam- brian units, although there are some small basaltic dikes lying in the sheared area along Fault C; these are probably late Laramide in age and are not particularly fractured.

~~The youngest PreCambrian unit cut by .the fault system is~~

~~the Polecat quartz monzonite, and the fault that cuts it~~

~~(B) must be younger than 1000 million years (it is con~~

~~sidered unlikely that the fault zone could have remained~~

~~as sheared and broken as it presently is through a strong~~

recrystallizing event such as that which affected the Polecat pluton 1000 million years ago). Although there 156 is no definite evidence that the other faults in the group need to be this age, there is no reason why they could not be, and for reasons of simplicity all the faults of the group are assigned to this post-1000 million year age.' If the basaltic dikes in the shear zone along fault C are Lara- mide, then they define the last major movement along it as being pre-basalt in time. Again, the other faults of the group may be presumed to have stopped moving at the same time. It should be noted that, although the faults can reasonably be assigned a post-1000 million year age of formation, there is no need for them to be PreCambrian in age. They could be, or they could date from the tine of the post-Permian pre-lower Cretaceous deformation recorded in the Mule mountains (Gilluly, 1956) and the Little Dragoon mountains (Cooper and Silver, 1964-). They could also be early Laramide (post-lower Cretaceous) faults. Their time range cannot be uniquely determined from the evidence in the northwestern Dos Cabezas mountains alone.

~~A more detailed picture can be assembled by as suming that the four faults discussed above are syncronous in time of origin and periods of major movement with the~~

Apache Pass fault, which lies to the south of the other four. The strikes of the two fault groups are parallel, and both show evidence of strike-slip character during their first formation. Because the Apache Pass fault 157 shows considerable evidence of ProCambrian movement, all five of the west-northwest trending faults discussed are probably Precambrian in age; the most reasonable time of origin is in a deformation at about 1000 million years time; this dates the youngest Precambrian events yet found in Arizona, and an older age for the formation of these faults is unlikely.

Since the Apache Pass fault shows a major dip-slip reverse movement in the Laramide, it may be presumed that the other four in the northwest part of the mountains did too. This may be when the extension of fault A into the block north of fault B took place. Since no post-PreCambrian marker units are present along the four faults in the north western part of the range, there is no way of determining what displacement, if any, they had during the post-Paleozoic deformations of the area. The basalt dikes along fault C probably indicate a cessation of movement in the north western Dos Cabezas at about the same time as in the central part of the moutains, at about the onset of the Cenozoic.

~~The North-Northeast Trending Faults~~

The entire Dos Cabezas mountains have been cut by a large number of generally vertically-dipping north- northeast striking faults which always offset and cut the west-northwest trending faults and structures. These 158 north-northeast trending faults show apparent left-lateral displacement of faulted beds and contact markers in most

~~cases, which nay be due either to their having that charac~~

ter or to normal faulting with the upthrown side on the west, or a combination of the two. The intensity of the faulting is the greatest in the east central part of the range, and dies away to the. east and west. Most of the above faults are quite small, but at least two (see Figure

~~1 and Figure 19) are large, with displacements of over~~

~~100 feet. There is no trace of this fault direction in the~~

~~Precambrian structures of the area; it seems to be a Lara- raide structural innovation. There are a number of north-~~

~~south or northeast-southwest trending canyons on the~~

~~north side of the range which may indicate the presence of~~

~~other major faults of this trend not yet discovered.~~

~~These north-northeast trending faults cut all the~~

~~units of the Paleozoic and Mesozoic sedimentary sequence,~~

~~and are quite definitely Laramide in age.~~

~~Many of the faults of both northeast and northwest~~

~~trend in the zone of faulted sedimentary rocks north of~~

~~the Apache Pass fault zone have been entered by tongues~~

~~of the pre-Paleocene volcanic breccia which makes up the~~

~~core of the Dos Cabezas mountains. The Apache Pass fault~~

~~and the disturbed zone around it have been locked solid 159~~

~~since the onset of Cenozoic time. The fault zone does not~~

~~appear to have been active in the mid-Tertiary Basin and~~

~~Range orogeny in this area.~~

~~There is only one definitely recorded faulting movement in the Dos Cabezas following intrusion of the~~

~~volcanic breccia, and it involves a north-northeast trend~~

~~ing fault which cuts the Silver Camp stock of earliest~~

~~Paleocene age.' The fault as now seen must be post-Cretaceous~~

~~in age, but whether it is a late-forming member of the~~

~~Laramide north-northeast trending faults or is of con~~

~~siderably younger age, is unknown.' It appears to be cut~~

~~by a mid-Tertiary dike at its northern end, but because~~

~~the position of the fault in this area must be inferred~~

~~from joints which presumably parallel the fault trace,~~

~~this is by no means certain.~~

~~This fault has a puzzling character. Its dis~~

~~placement where it cuts the stock is considerable, and yet~~

~~it is quite difficult, if not impossible, to locate only a~~

~~couple of miles north, A southern outcrop of the same~~

~~fault probably is responsible for the offset at the western~~

~~end of the hogback of Paleozoic sediments at the south~~

~~western boundary of the range. It is thought that this~~

~~fault is a major break, and that it assumes a hinge-fault~~

~~character as it passes through the Silver Camp stock. 160~~

~~The fault may die out about two miles northeast of the~~

~~Silver Camp stock.~~

The trace of this north-northeast fault marks a very subtle feature of the Dos Cabezas mountains. If one extends the fault where it passes through the Silver Camp stock down to the southwest, a marked difference in the character of the observed sedimentary units on both sides of the fault line is observed. On the east, one has two separate strips of southwestward dipping and west-northwest striking series of Paleozoic and Mesozoic sediments.

Where the Bisbee Formation is present, it overlies a thick sequence of Paleozoic units. The westernmost ex tension of these tilted Paleozoic and Mesozoic sediments occurs at the westernmost end of the hogback along the southwestern boundary of the range, just west of the Dos

~~Cabezas-Willcox road; as one approaches this end of the hogback, structural deformation by folding and faulting increase in intensity up to the point where the units dis appear.~~

West of this major fault trace there are no Paleozoic rocks exposed. The line-pebble conglomerates and limestones of the probable Bisbee Formation lie in a honoclinal, southward-dipping block, directly atop the Pinal Schist.

~~It seems reasonable to the author that the trace of this 161 post-Cretaceous fault marks a much older major fault zone which was active in post-Permian and pre-Cretaceous time.~~

~~Presumably the Paleozoic sediments west of the line were stripped away and contributed to the lime-pebble con glomerates in the Bisbec. Any finer-grained clastic ma~~

~~terial of the Bisbee Formation which once might have overlain the conglomerates has been eroded. It should be noted~~

~~that the obvious existence of post-Permian pre-Cretaceous~~

~~fault activity in this zone makes it very likely that the~~

~~small block of Concha Limestone lying slightly to the~~

~~southwest of the range was downfaulted in this period of~~

~~time.-~~

~~Kid-Tertiary Structural Development~~

~~The writer has observed no definite trace of a mid-~~

~~Tertiary uplift or faulting due to the classic Basin and~~

~~Range orogeny block-faulting anywhere in the exposed hard~~

~~rock area of the Dos Cabezas mountains. There are a large~~

~~number of dikes and one large stock dating from the mid-~~

~~Tertiary in the Dos Cabezas, and it is apparent that the~~

~~area of the Dos Cabezas range was tectonically active in~~

~~this period. Mo major faults, normal or otherwise, de~~

~~finitely dating from this period of time can be shown to~~

~~occur in the range. Indeed, there is strong evidence that~~

~~all faults of large size exposed in the Dos Cabezas range~~

~~have been inactive in the middle Tertiary, with the sole • 162 exception of the fault which cuts the Silver Camp stock.~~

This is not to imply that normal faulting related to ten sion in the mid-Tertiary did not occur in the area of the crust in which the Dos Cabezas lie. Sabins (1957b) has shown good evidence for a long normal fault, upthrown on the south, paralleling the northwestern boundary of the

Dos Cabezas and northern Chirichahua mountains; the fault lies a mile or two north of the mountain front and is buried under the alluvium.' Throw on this fault is perhaps as much as a mile in some places.

~~The presence of numerous small narrow dikes of mid-~~

~~Tertiary age and of erratic local strike, occurring in all parts.of the range, implies the existence of tensional conditions throughout the Dos Cabezas during the mid-Tertiary.~~

~~Probably the crust in the area was pulled apart a small amount, with various randomly located fractures opening up as magmatic material rose from below.~~

There is evidence for a boundary fault only along the northeast boundary of the Dos Cabezas; there is none for faults along the southwestern, eastern, or western boundaries. The steep southwestward dip of the Paleozoic sediments along the southwestward boundary of the range carries these sediments down to lower elevations rather rapidly, and there is no need for a fault along this boundary; the basin of the Sulfur Spring Valley to the 163 southwest of the Dos Cabezas mountains could be essentially synclinal, so far as surface evidence shows/ To the northwest of the Dos Cabezas, there is no basin of the same type as the Sulfur Spring valley or the San Simon valley to the southwestward or northeastward of the range. To the west of the range there are low hills like the Circle Hills and the Spike E hills, which appear to rise above a shallowly buried rock surface extending westward from the Dos Cabezas mountains. The southeastern boundary of the range is quite artifical, as the Dos Cabezas merge with the Chirichahua mountains in that direction. The boundary in this study has been drawn at the road through the Apache Pass, which is just a topographically low area which is the lowest part of the mountains in the area.

The range stands quite a bit higher than the floors of the Sulfur Spring and San Simon valleys to the northeast and southwest; as mentioned in the previous chapter, drill holes in these valleys have penetrated several thousand feet of unconsolidated alluvium before hitting solid rock bodies at the floor of these basins/ Again, only part of

~~this relative uplift can be.assigned to faulting.~~

~~Indicators of Perth of Crystallization of~~

~~Various Parts of the Dos Cabezas Mountains~~

~~If coarseness of crystallinity is any guide to~~

~~depth of crystallization of igneous rocks, the plutons in 164~~

~~the Dos Cabezas mountains record considerable variation in~~

~~depth of cover of the present surface at various times in~~

~~the past. The sequence is roughly as follows.~~

~~The northern Precambrian granitoid rocks (those~~

~~north of the Apache Pass fault) in the range have generally medium to coarse grained textures, implying considerable~~

~~depth of burial at time of crystallization according to~~

~~the simple hypothesis above. The southern Precambrian~~

~~rapakivi quartz monzonite and Cienaga gneisses, on the other~~

~~hand, have quite porphyritic textures and fine-grained~~

~~ground masses reflecting fairly shallow cover when cooling.~~

~~Since these two terrains lie on opposite sides of a major~~

~~fault, the two texture types may reflect uplift of the~~

~~block north of the Apache Pass fault with respect to that~~

~~south of it; probably, that would have happened as a~~

~~secondary component of the net slip along the fault during~~

~~the primarily strike-slip Precambrian movement.~~

~~The accumulation of Paleozoic and Mesozoic sedi~~

~~ments in the range implies considerable downsinking of the~~

~~crust in the area over that period of tine. The onset of~~

~~the Laramide revolution presumably raised the area con~~

~~siderably, since all the Laramide bodies in the Dos~~

~~Cabezas have a fine-grained character. Many of these are~~

~~of a volcanic, near-surface type. This high-standing low-~~

cover state seems to have persisted until the igneous 165 intrusives of the nld-Tertiary had largely finished in truding the range, The early mid-Tertiary dikes are porphyritic with a very fine grained ground mass.

At this point cover must have thickened locally in the southern Dos Cabezas range, at least, because the later mid-Tertiary Ninemile granodiorite stock is very coarsely crystalline, and has a number of quite coarse grained pegmatites associated with it. In fact, in overall character the Ninemile is the coarsest grained igneous pluton in the whole range. Local growth or development of cover may be due to accumulation of lavas related to older mid-

Tertiary dikes lying west of the Ninemile stock. Any cover of this type has long since been stripped off by erosion, of course, and all the various plutons now exposed in the range show no trace of their original cover rocks.

It should be noted that the writer considers the preceding discussion somewhat hypothetical, since other factors beside depth of burial control the grain size of plutonic rocks. Chemical composition of magmas and their content of volatiles are probably at least as important as depth of cover in determining grain size. CHAPTER 7

~~CRETACEOUS INTRUSIVE VOLCANIC BRECCIA TERRAIN~~

~~The core of the Dos Cabezas range, an area of some~~

~~16 to 17 square miles size, is composed of a complex terrain~~

~~of angular xenolith-bearing and xenollth-free aphanlte and~~

~~aphanite porphyry rocks of a predominantly dark color. It becomes obvious after a short amount of investigation that~~

~~these are not ordinary tuffs or flows, and the picture that~~

~~the writer has finally assembled about their origins implies~~

~~that the area is one of the more remarkable geologic terrains~~

~~in Arizona.'~~

~~General Characteristics of Breccia Units~~

~~On detailed macroscope and macroscopic investigation~~

~~the xenolith bearing units are nearly always found to con~~

~~sist of fragments of various rock types, ranging in size~~

~~from large blocks many feet to tens of feet across down to~~

~~nearly microscopic mineral and rock fragments. The over~~

~~all texture may generally be described as seriate, with a~~

~~continuum of fragment sizes being present in approximately~~

~~equal amounts. The rocks are obviously classifiable as~~

~~breccias. The fragments they contain are fritted or fused~~

together, producing, generally, a tough hard rock of extreme 166 16? competence. The term "welded" is used to describe this character of the breccias, as it has come into general use for volcanic rocks whose once-separate constituents are solidly fused together as a result of the high temperatures under which they came together .'

Rock fragments observed in the breccias are of many kinds. Dark colored aphanite and aphanite porphyry fragm.ents are the great majority observed, but light- colored aphanite and aphanite porphyry fragments make up the greater part of some units. Fine-grained phaneritic rocks do not appear as fragments. Frequent fragments of coarse-grained quartz monzonite, along with schist, ar gillite, phylllte, quartzite, and limestone are found in various local areas, as will later be described. The predominance of aphanitlc rock fragments in a fragmental ground mass obviously contained in vent-type structures leads to the term "volcanic" in discussion and naming of the breccia units.

~~Contacts of the breccias with surrounding rocks and with each other are sharp, steep to vertical, and often disruptive of the contacted unit. With reference to~~

Figure 1, note the contact of the purple volcanic breccias with Bisbee Formation (?) limestone in Sec. 3» T14S R26E, or contact of volcanic breccias with Polecat quartz mon zonite in the SW1/4 of Sec. 11, T14S R27E, where canyons EXPLAAMT/Q/V

~~qd anctss/re porphyry tvh/re iro/coo/c Ar&cc/o pish parp/e tso/cao/c~~

~~gsh green iso/eorsc~~

~~P'/gure *?/ P/on one/ Z. o car/on of Oos Cohexos i/Ve/dod Pnrrus/s* IrWcon/c j9recc/o G>^/ Figure 2 2 : Some General Characteristics of the Volcanic Breccia Terrain and Breccia Rock/~~

(a): Camelhack mountain from the north/ This peak is a purple volcanic breccia vent; the breccia lies within the area marked vb in the picture. The purple breccia here cuts up through Pinal Schist and Cretaceous (?) Bisbee Formation (?), and is the westernmost part of the main breccia outcrop area in the Dos Cabezas mountains The peak is in the SvIl/4 of Sec. 28, T13S.R26E/

(b): A slab of the green volcanic breccia taken from a sample collected about 1500 feet north of the crest of the range in the western part of Sec. 16/ T14S R27H. This specimen shows a typical texture and average fragment size. The eraser is about two inches long. Figure 22: Some General Characteristics of the Volcanic

~~Breccia Terrain and Breccia Bock l o~~

~~I Figure 2 3 : Internal Foliation in the Green Volcanic Breccia. Along the Northern Boundary of the Breccia Terrain~~

~~(a): Outcrop of green volcanic breccia showing marked foliation parallel to the pen used for scale. The foliation has a vertical dip.' The outcrop is about a mile ' southwest of Cooper Peak.1~~

(b): Northern contact zone of the green volcanic breccia with Polecat quartz monzonite in the bottom of Howell canyon, in the SW1/4 of Sec.’ 11, T14S R27E, Note the pen in the center of the picture, used for scale. Massive Polecat lies about twenty feet to the right of the picture downstream. In the picture, large blocks of Pole cat quartz monzonite and various aphanitic volcanic rocks are arranged in a crudely parallel foliated pattern; the foliation trends west-northwest and dips vertically.- The outcrop is in SW1/4 of Sec/ 11, Tl^S R27E. Figure 2 3 ; Internal Foliation in the Green Volcanic

~~Breccia Along the Northern Boundary of the Breccia Terrain I "11(? Figure 24: Breccia Bodies in the Northern Part of the Volcanic Breccia Terrain~~

(a): White volcanic breccia in a typical exposure just east of hill 6845 in Sec/ 11, T14S R27E. The breccia here is composed of small rhyolite fragments in a minutely fragmental rhyolite ground mass. The mass possesses a p la n a r foliation parallel to the large right-hand dipping joints; these joints are spaced about a foot and a half apart and the outcrop is about five feet wide.1-

(b) : Gobble breccia dike which cuts green volcanic breccia east of hill 6845 in Sec. 11, T14S R27S. The large block in the dike which is labelled da is dark volcanic aphanite; the blocks labelled pom.are Polecat quartz monzonite.1 Ground mass of the breccia is composed of quartz monzonite fragments of sand size or smaller. Figure 24: Breccia Bodies in the Northern Part of the

~~Volcanic Breccia Terrain~~

~~Figure 25: Textures in the Purple Breccia Dike Cutting the Bisbee Formation Near the Southeastern Margin.of the Breccia Terrain~~

(a): Slabs of pale Bisbee formation shale in a matrix of purple volcanic breccia.' Note the scattered . small dark fragments and the cavities in the dark ground mass material between the stale fragmentsThe dark frag ments are volcanic aphanites/ The cavities may be from gas bubbles. Notice the near-vertical foliation in the face photographed; the face is about three feet high.' This picture was taken on a purple breccia dike cutting the Bisbee formation in the center of Sec,: 3 0 , Tl^S, R28E;’

(b): A photograph of an area in the same dike as above, showing purple aphanite fragments aligned in flow foliation in the darker purple fragmental ground mass. The same general location as in (a). Figure 25: Textures In the Purple Breccia Dike Cutting the Blsbee Formation Near the Southeastern Margin cf the Breccia Terrain 174 of the overall breccia mass. Figures 22, 23, 24 and 25 show common features of this terrain including characteris tic fragment types and general nature of the foliation.

~~The units illustrated in the figures are somewhat coarser- grained, with average of fragment sizes of 1-5 mm. being the usual case.~~

In many places the breccia shows no internal folia tion, but contains a randomly-oriented collection of various shapes of fragments in an apparently isotropic fine-grained ground mass. Perhaps a majority of the outcrop area of the breccias shows internal foliation to some degree, how ever.

The vertical, non-faulted contacts, characteristic steep to vertical foliation, and the presence of tongues and dikes of breccia cutting into surrounding units show these breccia units to be truly intrusive into their surroundings.

The term intrusive weldedbreccia is used hereafter to de scribe these units. It is tentatively presumed by the writer that the breccia terrain represents a complex of old volcano vents, and is the site of a vanished ancient vol cano of great size.

The petrology of an individual breccia unit can be described in general terms. The outcrop will usually dis play from ten to eighty percent of fragments of a roughly uniform size, ranging in local areas from sizes a few 175 piillimeters across to fragments a foot or more across on. the average. On close examination, large numbers of smaller fragments will be found Interspersed among them, and among these are interspersed smaller still, and so forth (see

Appendix A, Figures 39 and ^0). Microscopic investigation reveals that this textural pattern carries on to the finest visible size range. This fine-grained fragmental material is interspersed with a few percent of fine-grained plagioclase crystals and crystal fragments. The visibly separate fragments and crystals are all contained in a few percent of a microcrystalline heterogranular aggregate of unknown character. The very high proportion of angular rock frag ments of very small size coupled with the small percentage

Of interstitial material, and the absence of significant melting or reaction between most of the fragments and the matrix in which they lie implies a system which was not a simple xenolith-bearing magma during its emplacement.

The fragments are of many types as previously men tioned. Usually several types of aphanlte and aphanite porphyry rocks can be distinguished, and any one rock type will usually make up a large number of the local fragments.

The fragments of any type seem to be as likely to be small as to be large. The texture would indicate disruption of larger blocks and masses of these rock units to produce the large number of similar fragments of various sizes. 176

~~Microscopic examination reveals most of these fragments to~~

~~be andesites and andesite porphyries. True basalts arc~~

~~very rare. Rhyolite to dacite porphyries are common in~~

~~some units. Non-volcanic rock fragments of shale, lime~~

~~stone, and granitoid phaneritic plutonic rocks are generally~~

~~less than one percent of the rock, but may be quite con~~

~~centrated locally; see petrographic description of the dif~~

~~ferent rock units for discussion of this.~~

~~Separate Breccia Units~~

~~In terms of petrologic character, there are a number~~

~~of separate breccia units, each with its own characteristic~~

~~type or style of fragment assembly and color. The writer~~

~~has grouped all the breccia units exposed in the Dos Cabezas~~

~~mountains in a crude classification system based on ground~~

~~mass color of the breccia units; the three units are green~~

~~ground mass breccias, purple ground mass breccias, and~~

~~white ground mass breccias. These units are called the~~

~~green, purple, and white breccias for purposes of discussion.~~

~~Only a very crude chronology of appearance of these breccia~~

~~units can be set up, correlated with the crude type clasi-~~

~~sification used. The green breccias are generally older~~

~~than the purple ones, which in turn are older than the~~

~~white ones. The green breccias are in general larger than~~

~~the purple ones in area, and the white are the smallest. 177~~

~~Green Volcanic Breccias~~

The largest, and probably the oldest units in the breccia terrain are the green volcanic breccias which make up most of the eastern half of the breccia units, but are not of the same exact age. Most of the breccia in the

~~eastern part of the main breccia outcrop area is green;~~

~~that in Sections 6, 7, 8, 9. 10, 11 13, 14, 15, 16, 22,~~

~~23, and 2^ in T14S R27E has a grossly congruent charac~~

~~ter and a.pale to medium-gray-green colored ground mass.~~

~~Figure 1 shows the distribution of this unit. The local~~

~~color varies a good bit but is usually some shade of light~~

~~green on fresh surfaces•~~

~~Petrography. In field exposures the green breccia~~

~~shows 20--5-0/O fragments, commonly a few centimeters in~~

~~maximum dimension, in a fine grained color-mottled ground~~

~~mass of visible fragments and mineral crystals of varied~~

~~sizes, together with aphanitic material lying between the~~

~~discernible constituents. Again, no sharp distinction~~

~~between ground mass fragments and larger ones is possible,~~

~~because all sizes of fragments are present in a seriate-~~

~~assemblage. Small white plagioclase crystals and crystal~~

~~fragments are ubiquitous. In most of the western part of~~

~~the unit small clusters of epldote crystals are present in fragments and ground mass. Quite a variety of fragments 178~~

~~of red, white, green, grey, and black colors are present in most outcrops, lending a multi-colored spotted or mottled~~

~~appearance to the rock. Fragments of limestone, quartz monzonite, shale, phyllite, and quartzite are present in most outcrops, but form only a percent or so of the total~~

~~fragment quantity; the rest of the fragments are fine~~

~~grained aphanites and aphanite porphyries of various colors,~~

~~types, and sizes. See Appendix A for a detailed petrographic~~

~~discussion of the green breccia.~~

~~Contact Relations. The contact relations of the~~

~~green volcanic breccia are quite striking. It has quite~~

~~markedly invaded the Apache Pass fault zone along its~~

~~northern boundary, sending breccia dikes out from the main~~

~~unit along old joints and faults in the sedimentary rock~~

~~units disturbed by the original Laramide faulting. Much~~

~~of the old faulted sedimentary terrain has been destroyed~~

~~by the breccia intrusion. The outcrop pattern in Sec. 22~~

~~and 23, T14S R27E, or in the northern parts of Sec. 19 and~~

~~20, T14S R27E, as seen on Figure 1 are quite explicit.~~

~~Large blocks of Pennsylvanian Horquilla Limestone and~~

~~Cretaceous Bisbee Formation (?) have been completely~~

~~separated from their former neighboring blocks in the fault~~

~~zone. A number of blocks of various sedimentary and~~

~~igneous units are presently found quite far in the interior~~

of the breccia; whether they, have migrated laterally to 179 their present positions o have sunk from above cannot be stated with certainty, although the writer prefers the latter idea. Note, for example, the large quartzite block on the east face of Cooper Peak, in the SW1/4 SVI 1/4

~~SE1/4 of Sec. 15, T14S B27E and the limestone block a few hundred feet to the west of it. A very large block of~~

Polecat quartz monzonite has been wholly separated from the main body of that unit in the Nl/2 Sec. 9. T14S R27E, largely due to Intrusion of volcanic breccia. Other masses of quartzite and limestone can be seen in the area west of the Dos Cabezas peaks. A large mass of presumed

~~Bisbee Formation north of the Mascot stock was presumably originally surrounded by the breccia before intrusion of . the Mascot stock.~~

In some places the contact of the breccia with the various units is quite smooth, and conformable or nearly so with some marker horizon in the unit it cuts, over some distance; note relations in the IF.il/4 Sec. 21, T14S R27E.

On a larger scale, however, the breccia gradually transects these marker horizons in the units it cuts, and eventually cuts re-entrants in the units, although there is consider able jointing in the breccia in places. As mentioned before, all the observed evidence points to the breccia being truly intrusive into its wall rocks. 180

~~Purple Volcanic Breccia~~

The next breccia type to be considered is the purple breccia group; these are smaller than the green breccias in outcrop size, and appear to be generally later in the developmental sequence of the overall breccia terrain*

~~Several of these bodies will be described below.~~

~~,Main Purple Breccia. The western part of the breccia~~

~~terrain in Sec. 2, 3 , and 11, R14S H26S is a purple vol~~

~~canic breccia area of rather uniform character. In Sec. 33#~~

~~T13S B26E small outlying masses of this unit are found which~~

~~are separated from it at this level of erosion but are~~

~~identical with it petrographlcally, and are probably con- .~~

~~nected with it at depth.~~

~~The main mass of the above breccia has a dark-~~

~~purple-colored ground mass speckled with numerous small~~

~~white plagioclase crystals and small varicolored rock~~

~~fragments a centimeter or so across. Most of the rock~~

~~fragments are red, green, or purple in hue, and together~~

~~with the purple ground mass give the overall breccia a~~

~~marked purple color. In some parts of this unit the~~

~~plagioclase is sparse in abundance or lacking. (See Ap~~

~~pendix A for a detailed petrographic discussion of the~~

~~purple breccia.) 181~~

Other Purple Breccias. Other masses of the purple volcanic "breccia lie in other parts of the breccia ter rain.' Some are associated with purple andesite porphyry dikes carrying no fragmental material, and in some areas both are present and frequently merge with each other.

The writer suspects autobrecelation of the nearly solidi fied magma due to gas pressure buildup by the second boiling phenomenon, in cases where this merging of breccia and nonbreccia units occurs.’

~~The outlying neck of Camelback mountain in the~~

Sl/2 of Sec. 28, Tl4s R26E makes up a purple volcanic breccia mass of about 1/4 square mile outcrop area. This mass is quite similar to the main purple breccia mass de scribed above, but has a higher proportion of angular but subrounded fragments of about 1 -2 cm. average size, which may compose up to 80^ of the rock. No pumice or hornblende is present.'

In the center of Sec. 4, T14S R26E, at the end of a prong of Cretaceous Bisbee Formation limestones and lime- pebble conglomerates running out to the southwest from the mountains, there is a small amount of purple intrusive breccia of a striking character. It contains numerous cobbles, boulders and pebbles of limestone, derived from•

~~Paleozoic units, together with the typical small 1 -2 cm. aphanite and aphanite porphyry fragments found in the 182 normal purple breccia.~~

~~Over the central part of the breccia terrain, in~~

~~Sees/ 8 , 9, 16, 17, 20 and 21, T14S R27E, appear a large number of purple breccia dikes and plugs of small size.~~

Most of these show two types of intermixed and associated purple units; one is a coarsely brecciated unit of purple aphanite porphyry fragments in a fine-grained purple ground mass, while the other is a predominantly porphyritic fine-grained unit containing a few percent small plagioclase crystals in the aphanitic purple ground mass, and containing no fragments. The two types have a dike like penetrating character, a,nd penetrate one another about as often as they penetrate the wall rocks of their combined intrusion. The two types often occur side by side in the same intrusive body, and when they do neither type is usually demonstrably older than the other.

~~A large purple breccia dike cuts the Bisbee for mation in Sec. 29 and 3 0 , T14S R28E, and Sec. 2 5 , T1%S~~

~~R27E. This body, is wholly contained within the Apache~~

Pass fault zone and has perhaps a square mile of surface outcrop in all. The fragments in the breccia are almost wholly of purple aphanites and aphanite porphyries, but large fragments of Bisbee Formation shales are found in some parts (see Figure 2 5 (a) and (b)). In general, this breccia body is quite like the other purple breccia masses 183 of the fragment-bearing type.' A small plug of the same unit cuts the Bisbee Formation just west of the west end of. the above large dike.'

~~Two small intrusions of purple breccia of unknown extent occur at the southwestern margin of the range in~~

~~Sec. 31, T15S R27E. Contact relations are obscured by alluvial cover, but the breccia appears to be cutting up through the Escabrosa limestone. An outcrop of the~~

Escabrosa just east of the eastern purple breccia body has an unusual texture of small fragments of limestone lying in a coarsely crystalline calcite cement; the rock looks as though it may have been affected by gas-streaming fluidization related to the breccia intrusion.

~~Two small purple breccia volcanic breccia dikes cut the Polecat quartz monzonite and the Pinal Schist in~~

~~Sec. 3k and 35» T13S R26E. The dikes contain large and small fragments of purple andesite and quartz monzonite fragments. They are generally quite similar to other dikes of this class.~~

~~A small red breccia outcrop in the SMI/4 of Sec.~~

10, T13S R26E, at the northernmost extremity of the range, has a most unusual character. It is composed of granitic fragmental debris in the form of quartz, plagioclase, and potassium feldspar crystal fragments, 0 .2 -0 .3 mm. and less, in a seriate size pattern. Very rare anhedral magnetite 184 crystals and rare epidote and carbonate crystals appear, and a few small quartzite fragments are present. The breccia in the field occurs fairly near a fault trace pro

jected by Cooper (i9 6 0 ) which the writer agrees is likely to exist, though he has not noted it on his own map. In the field and in section, however, the rock shows no indications of being a shear or crush breccia such as is often found in conjunction with large faults, but rather

~~seems to be a mass of fragments cemented together after~~

~~initial breakup and mixing. The writer suspects this mass of fragments was both formed and mixed by rising gases re lated to the other intrusive volcanic phenomena of the~~

~~Cretaceous in the Dos Cabezas mountains/ The hand speci men shows an interesting pseudo-porphyrltic texture; spots~~

~~of white breccia "fragments" lie in a red breccia ground mass.1 The two color areas of breccia, however, are~~

~~identical in section, both being aggregates of very small~~

~~fragments; the only variation is that the red areas have~~

~~small masses of hematite crystals in them, while the white~~

~~areas do not/ The writer does not understand this hema~~

~~tite distribution.~~

~~• A small purple breccia plug in the NE1/4 of Sec.~~

~~25 T13S R26E closely related to this just-discussed area~~

~~has up to k0% 1.0-0.2 mm. plagioclase crystals and crystal~~

fragments with circa euhedral hornblende pseudomorphs, 185 which now consist of magnetite rims around sericite-chlorite cores, all contained in a ground mass of minute plagioclase crystals and crystal fragments - 0 .0 1 mm. in size with slightly smaller crystals of magnetite, biotite, and epidote.

~~White Volcanic Breccia~~

The least common and smallest units in the volcanic breccia terrain are the white volcanic breccia units. Like the purple type, they occur in several small intrusive bodies. They are thought to be the youngest group of breccia bodies in the overall terrain.

~~A number of small white breccia units (see Appendix~~

~~A for a detailed petrographic discussion of the white breccia) are present in various parts of the breccia ter rain.1 There is a good deal of variation among them.~~

They are characterized by assemblages of predominantly white to grey rock fragments and/or by a white to yellow ground mass. Numerous dark fragments may be present, but the light outcrop color is their most outstanding field characteristic. The white breccias characteristically oc cur as small dikes or plug-like bodies cutting earlier breccia bodies. One dike or stock-like mass to this breccia group in the eastern part of the breccia terrain is about a quarter of a square mile in area, but most are much smaller; a few hundred square feet in size is about 186 average for these units. The white breccias seem, in general, to be the youngest breccia types in the range.

~~The purple breccias appear to occupy an intermediate position in size and time, although because there are no overlapping boundaries for many of these units, some of~~

~~the purple units could be younger than the white ones.~~

~~(See Appendix A for a detailed petrographic discussion~~

~~of the white breccia.) All the above units of green, purple, and white volcanic breccia form a large breccia complex which ap~~

~~parently marks out the former site of an old eruptive~~

~~breccia of great size. In the next section the question~~

~~of the origin and development of this vent is considered.~~

~~Genesis of the Volcanic Breccias~~

~~The general character of the volcanic breccia units~~

~~in the Dos Cabezas mountains implies an intrusive frag~~

~~mental natureThe mechanism which, to the writer, seems~~

~~responsible for this activity will next be discussed.~~

~~In mos t of the samples examined no clear record of~~

~~a true liquid silicate melt of any kind of pervasive~~

~~character could be seen in the interfragmental regions of~~

~~the rocks. The various breccia units are, however,~~

~~solidly welded together in some way, even though they~~

~~largely consist of fragmental debris. Internal foliation. 18?~~

~~generally steep to vertical in dip, is characteristic air~~

~~though not ubiquitous.' It is defined, again, by a rough~~

~~planar array of fragments in the outcrops of the breccia.~~

~~Megascopic Fragments~~

~~The fragments, especially the larger sizes, are~~

~~mostly rock fragments evidently once part of much larger~~

~~and more continuous units. The disruption of these pre~~

~~existent units, as well as the arrangement of their present~~

~~parts, must be accounted for. The sharply angular nature~~

~~.of most of the fragments does not appear to indicate a~~

~~detrital source for them, nor do they bear any resemblance~~

~~to small joint-outlined blocks.~~

~~The rocks from which the fragments were derived were~~

~~probably fractured initially by shrinkage and/or tectonic~~

~~factors during their early history. Breakup of these~~

~~presumably good-sized pieces of rock into the small particles~~

~~present in most of the breccia must involve some subse~~

~~quent, powerful mechanism capable of disintegrating large~~

~~masses of rock in a most intimate and thorough way. Leva~~

~~(1 9 5 9 ) described the continuous impact, abrasion, and size~~

~~reductions of particles in a fluidized bed; spalling of~~

~~fragments from the surface of impacting particles produces~~

~~larger and larger numbers of smaller and smaller bits and~~

~~leads to the gradual destruction of all large blocks. 188~~

Another important mechanism which causes size reduction is provided by the splitting or wedging action of gas in mi nute crevices when the pressure in the gas phase is fluctuating rapidly and strongly, as it must in volcanic gas-blast type eruptions.

The rock fragments are very predominantly "volcanic"; that is, they are aphanitic porphyries or aphanites with very fine-grained ground masses produced by rapid cooling and crystallization of the liquid part of their respective magmasThe rock types are predominantly andesitic in composition/ They seldom display any conspicuous develop ment of carbonate minerals or of hydrous minerals such as epidote, except as secondary reaction-formed minerals de veloped following disintegration of the parent masses.

~~Ground Mass Fragments and Crystals~~

The ground mass phases of the breccia units characteristically contain large numbers of minute rock fragments of several kinds, and also free crystals and crystal fragments of plagioclase feldspar, together with similar but less common fragments and crystals of quartz, potassium feldspar, and hornblende (?), The latter mineral is not seen except as masses of secondary reaction products within the original subhedral or euhedral outline of the crystal form which these masses formed. A gross dacitic 189 or trachyandesitic composition for their source is general ly implied by the visible free mineral assemblages of the fragments and crystals. These crystals and crystal frag ments are free of pre-breccia rock matrix themselves, though, of course, they are contained in the rock ground mass of the present breccia. These identifiable or re solvable parts of the ground mass are, again, contained in a very fine-grained but patchy ground mass of minute weakly birefringent areas which are only partially re solvable into separate parts.’

~~Breccia Formation Mechanism~~

In considering mechanism for the formation of the breccia, the production of rock fragments and free crystals, among other things, must be accounted for. The writer be lieves that a single genetic process can be described that is rational, possible, and capable of producing all of the observed features of the breccias in the Dos Cabezas moun-’ tains.’ The basic mechanism appealed to is gas fluidization.

The large number of aphanitic rock fragments point to presence of a large amount of volcanic rock which crystallized quite rapidly/ Presumably, this occurred on the surface or very close to it, within a few tens or hundreds of feet at the most. It is noteworthy in this respect that no fragments of fine-grained phaneritic 190

~~Plutonic rocks, such as those not infrequently found in~~

~~local Tertiary plutons of all types, have been found in the~~

~~breccias in the Dos Cabezas range. This seems to the~~

~~writer to mean that the volcanic fragments observed are~~

~~not just the upper parts of some fairly deep-seated~~

~~plutonic body, but are surface flow or near-surface shal~~

~~low plutonic rocks; further, since the crystallized~~

~~throat of a volcano should become slowly more coarsely~~

~~crystalline as one goes down from the surface, the absence~~

~~of phaneritic rocks of a fine-grained type from the breccia~~

~~suite implies that the present level of erosion is not far~~

~~below the land surface onto which or just below which the~~

~~pre-fragmental volcanic rocks appeared. The presence of~~

~~trachytic flow structure in a large proportion of the~~

~~fragments in the breccias implies strongly to the writer~~

~~that the parent rocks for the breccia fragments were, in~~

~~deed, ordinary surface flows. This surface cap or pile~~

~~of lavas was disrupted rather thoroughly by gas-blast~~

~~activity during the formation of the breccia.~~

~~Flow of escaping gas upward along a negative pres~~

~~sure gradient from a deep source through a mass of fragments~~

~~will cause fluidization of the fragments if the gas~~

~~velocity passes a certain lower limit. Fluidization, in~~

~~turn, will cause an entrainment of fragments below a~~

certain size level, and rounding, size reduction, and 191 size sorting of fragments will occur. Also, if fluidiza tion conditions remain at all constant, foliation should develop among the fragments as discussed below.

The gas source for the fluidization must lie below the present erosion level, if it was a magmatic body or bodies. As will be mentioned later, samples of this magma may be present in numerous small dikes which cut the breccia and which show, internally, some evidence of having had a high volatile content at some point in their history.

~~The only nonmagmatic source the writer could imagine for~~

~~the vast amount of gas needed to cause fluidization of the breccias would be some very deep-seated natural gas stream~~

~~caused by heating of ground water at great depth; appear~~

~~ance of gas from a deep geothermal source seems unneces~~

~~sarily fortuitous to be a reasonable hypothesis for the~~

~~formation of this breccia mass/ Normal evolution of gas~~

~~from a rising, gas-charged magma is thought to be a far more reasonable type of - genesis for the gas. This latter~~

~~hypothesis is examined below.~~

~~A magma which is saturated with one or more gaseous~~

~~elements or molecules, such as water, and.which lies at~~

~~some deep position below the crustal upper surface, will~~

~~lose some of this gas if it rises to a shallower position~~

~~in the crust where it lies under a new, superincumbent~~

load pressure. If the magma rises to its new position 192 very slowly, gas should be released from It slowly; con versely, if the external pressure on the magma changes rapidly, release of the gas may occur rapidly. Gas re lease will take place by appearance of small bubbles in the magma; these bubbles will then rise through the magma toward lower pressure areas, and will escape at the upper magma surface."’ The bubbles will probably coalesce into larger masses of gas as they contact one another while rising through the magma, and all the escaping gas will merge into one rising stream once it has escaped from the magma. If the magma is saturated with gas in all its parts, and if the pressure drop is felt throughout its

~~interior, gas release will occur everywhere inside it as it~~

~~rises into new, lower-pressure surroundings.~~

~~A second contribution to the cause of gas evo~~

~~lution from the rising magma comes from the ”second boiling"~~

~~phenomenon.’ Once crystallization of minerals has begun in~~

~~the magma, the residual magma increases in gas content in~~

~~direct proportion to the amount of magma which becomes~~

~~crystalline. The only situation in which1 this might not~~

~~be true is when magmas of low gas content whose primary~~

~~gas is water are crystallizing out hydrous phases, in~~

~~which case the gas content of the residual magma might~~

go down/ Crystallization of non-hydrous phases such as 193 quartz and feldspar, however, which essentially exclude water and other gases, will cause a rise in the partial pressure of all the gases contained in the magma. In a gas-rich magma which is crystallizing as it rises, both pressure release and crystallization of non-hydrous mineral phases will cause gas evolution from the magma.

It should be pointed out that the second-boiling gas production in a crystallizing magma has a damper on it, however, since in crystallizing the minerals forming will give up their latent heat of crystallization, thereby raising the temperature of their enclosing magma and op posing more crystallization. Normally the temperature of a magma from which a single pure component is crystallizing will remain constant until crystallization is complete, and the rate of crystallization will be the same as the rate of heat escape from the magma. The thermal conduc tivity of the crystallizing system is the main control.

~~Because of this crystallization damping mechanism, the amount of gas produced by the "second-boiling" mechanism~~

~~is liable to be quite small over short periods of time.~~

Contribution of gas from this source to the overall gas released from this magma will be a function of the cooling history of the body, while the evolution of gas due to pressure drop on the magma is solely a function of the rate of rise of the magma, which of these two mechanisms is more effective will determine which gas source is most 194 potent; In any case, a gas-saturated magma which rises a few miles in the crust and crystallizes a fair amount of its substance is bound to release a lot of gas as it rises.

A link between the two mechanisms which might produce considerable gas release driven by both mechanisms is as follows. If the magma has cooled to the point where some crystallization has begun, a sudden upward pulse of the magma toward the surface might cause a major release of gas due to the pressure-drop mechanism. Such a release of gas would cause immediate raising of the llquldus temperatures in the system, the amount of raise being proportional to the amount of gas released. A loss of half the gas in the magma might cause llquldus temperatures to go up several hundred degrees Centigrade. In such a case, crystallization of a great deal of the still-liquid part of the magma would take place; this would not stop until evolution of latent heat by crystallization had raised the magma* s temperature to that at which it would again be in equilibrium with its contained crystals. The rapid, crystallization of a good part of a magma*s sub

stance in this manner would cause a further major gas release by the second-boiling mechanism.' Once equilibrium was re-established, further evolution of the magma could be either slow or fast, depending on rate of rise and . 195 rate of cooling.

~~' Let us now consider what happens if the superincum bent load pressure on a rising, partially crystalline magma~~

~~suddenly drops. Such an event might occur, if, for example,~~

~~the roof of the magma suddenly fractured through to the~~

~~surface; another case would arise if the magma were suddenly~~

~~squeezed like toothpaste in a tube by tectonically-produced~~

~~earth movements which compressed the magma chamber and~~

~~caused the magma to rise by punching or prying its way~~

~~through the overlying rocks. In such cases, the magma~~

~~would become unstable throughout its mass with respect to~~

~~gas escape, and gas bubbles would appear throughout the~~

~~liquid part of the magma. As described, the liquidus tem~~

~~perature for crystallizing components would thereupon be~~

~~raised, causing some crystallization and evolution of more~~

~~gas by the "second-boiling" mechanism. If the initial pres~~

~~sure drop were severe, the resulting gas escape might~~

~~essentially disintegrate the magma, and escaping gas would~~

~~coalesce into large masses and begin to move to the surface~~

~~quite vigorously. Any liquid particles of magma carried~~

~~along by the gas would continue to evolve gas as they rose,~~

~~adding to the gas content of the system. In this way a~~

~~catastrophic event is initiated, in which a disaggregated~~

~~mass of liquid particles entrained in a gas rush toward~~

the surface along conduits or fractures above and around 196 the magma chamber. This mechanism is essentially the same one postulated for the development and extrusion of large ash-flovr sheets in nufee ardGntes type eruptions (Ross and

~~Smith, I9 6 5 ). and it would seem to be a frequently-occurring mechanism in volcanic activity,r~~

With operation of the above mechanism below the surface, there seems to be no fundamental difficulty in obtaining a large gas supply below the surface in an area where surficial flow eruptions are occurring. The explosive and catastrophic rush of gas for such a gas-liquid system as described above toward the surface would probably explosive ly disintegrate large parts of the overlying flow stack, as expanding and rising gas rushed into every tiny fracture in the solid rock. Thus, a vast mass of fragments would be produced from a series of massive flows in a comparatively short period of time.

~~At the same time, the disintegrating liquid portion of the gas-source magma below would contain the early crystals produced in the magma before it disintegrated.~~

The crystals, and the numerous drops and particles of magmatic liquid, would both be entrained in considerable numbers by the rising gases during separation and rising of the gases from the disintegrating magma. One would expect to find small crystals and minute aphanitic blobs in among the large rock fragments formed by gas-blast 197 eruptions .*

Tile action described above accounts for the finely brecciated character of the fragments in the intrusive volcanic breccia in the Dos Cabezas mountains, and also ac counts for the existence of small crystals and a small proportion of aphanitic irregularly crystallized ground mass in among the breccia fragments. It also accounts for the fact that the composition of the crystal plus aphanitic ground mass composition seems to be different than the composition of the fragments contained in this ground mass, as mentioned earlier. See Figure 26(A) for a schematic illustration of this process.

If the above theory corresponds to reality, it seems notable that no solid original masses of the capping flows remain. There are no post-Precambrian true igneous surficial flow units anyuhere in the Dos Cabezas mountains.

It is not likely that this is due to complete destruction of former capping units by gas blasting, but rather it seems that erosive removal of the old terrain on which the ancient volcano sat has taken place. Thus the level of erosion we now see in the breccia complex exposes part of the mass which was some distance below the land surface at the time it formed. This requires that all the cap rock fragments we see have arrived at their present position by a net sinking from their original level. /9Q

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~~Fluidization~~

In order to account in some more detail for the formation of the main breccia fragments, the writer would like to enter into some discussion of gas fluidization of particulate systems, and application of the chemical engineering studies of the phenomena encountered to the natural gaseous systems encountered in nature.

Leva (1959) presents quite a detailed account of fluidization phenomena. The geologic system should be much more complex than any model industrial one, due to likely fluctuation in throat sizes, wide particle size distribution, and variation in gas flow, but the basic laws and ideas of Leva’s study should still apply.

To Leva, there are two main types of fluidization; in one the particles move with.respect to each other, but the bed (accumulation of particulate fragments) does not move, and in the other the particles move with respect to each other and are also carried along by the gas, so that the bed moves tooy The former is called the dense phase of fluidization and the latter the dilute phase. In fluidization of the dense-phase type, the phenomena of channeling, slugging, and spouting occur; these all seem likely to have important geologic applications.' In channeling a channel or open space is blown through the 200 bed of fragments by the gas (or other medium) which causes a great increase of gas velocity in the channel and cessa tion of fluidized conditions in the rest of the bed.

~~Spouting is a variety of channeling in which bed material creeps into the gas channel, is entrained (carried along with.the moving gas) and is blown or spouted up above the~~

~~surface of the bed.1 Slugging occurs when large masses of gas begin to blow clumps of fragments up through the bed;~~

~~it usually occurs near the onset of dilute-phase conditions.~~

~~Channeling, slugging, and spouting are all enhanced by ad dition of gas at the base of the particle bed from small~~

~~channels or cracks rather than from a uniform membrane of~~

~~some sort; the former set of conditions seems more likely~~

~~to prevail in the geologic situation. The type of gas~~

~~feeder, namely crevices, which one would expect to find in~~

~~the earth, would seem to insure that a great deal of~~

~~spouting, slugging, and channeling will occur.~~

~~During fluidization of either major type, the bed~~

~~will behave as if it were a fluid, and has a characteristic~~

~~viscosity and specific gravity. If the bed is essentially~~

~~free. from obstructions and has no boundary at its base,~~

~~objects of higher specific gravity can sink right on down~~

~~through it. Less dense ones will float on the bed top or~~

~~rise through it. In the case of objects having the same~~

~~specific gravity as the particles in the bed (not the 201~~

~~same as the specific gravity of "the bed itself) the flow~~

~~ing gas will have a certain maximum size of particle which~~

~~it can support. Larger ones will sink through the bed, while very small ones will be entrained by the gas and~~

~~swept from the bed.1~~

~~The particles in the fluidized bed strike one~~

~~another continuously, causing continuous comminution and~~

~~cracking of the fragments. Pressure fluctuations in the~~

~~gas phase will cause a wedging or splitting action in the~~

~~fractures in the various fragments and cause a continuous~~

~~spallation. In this respect it is interesting to examine~~

~~sample 762 (see Figure 41 in Appendix A) which is a piece~~

~~of Polecat quartz monzonite from the margin of a very large~~

~~block in the northern part of the main green breccia out~~

~~crops/ In this piece the individual mineral crystals have~~

~~been broken up along old fractures and the individual grain~~

~~fragments separated slightly; small bands of sericite~~

~~crystals now wholly surround many of these original crystal~~

~~fragments. The quartz monzonite has been essentially~~

~~disaggregated and the fragments recenmted by sericite;~~

~~the sericite may be a vapor-phase crystallization product.1~~

~~A continuous size reduction of the fragments~~

~~should take place, accompanied by a rounding of their~~

~~shapes. In this regard, the fragments in the breccias~~

in the Dos Cabezas mountains are seldom perfectly rounded, 202 but are often well-rounded and are nearly always equant in shape — seldom are they very elongated.’ In detail, their edges in section often take on a scalloped or dished out line and the corners on even the angular fragments are rounded.

The fluidized bed, acting as a rising fluid, has a tendency to invade its walls and to enlarge the openings it creates by the same weeding process that allows fragments in the bed to be broken apart, coupled with straight abrasion by the rapidly moving bed particles. The bed, moving in its dilute phase, will have a tendency to persist as it moves out into these crevices, and will form dikes of breccia in the wall rocks surrounding the breccia mass.

~~Several such dikes have been described earlier.~~

~~The original cap rock units broken apart by the initial vent clearing explosions would break into pieces of widely varying sizes, whose fate in the bed will vary.~~

~~The larger fragments will sink into the bed, and continue sinking until they pass from the bed or are broken into pieces which are small enough to be supported by the bed.~~

~~Initial fragments of the right size will be just supported by the bed, while those that are finer-grained will be entrained by the rising gas and swept from the bed.~~

Now, since abrasion and splitting apart of the fragments is presumed to be talcing place on all levels in the bed, the particles which happen to be sinking at 203 any level In the "bed will be undergoing the process of being broken into smaller ones; at every level, some of these will be of a size which can be just supported by the gas rising through the bed, and others will be of a size so small that they are entrained. Thus, at any point in space or time within the bed, there will be fragments sinking, standing, and rising, relative to some marker outside of the bed,' Continuous size reduction of all fragments will take place.

If the present level of exposure of the breccia is far below the old land surface, then the observed rock fragments must represent pieces torn from foundering blocks sinking down through the level now observable or carried up from levels lower down. The breccia in any one outcrop usually has a marked upper size limit of about one to five centimeters for the fragments it contains, and there are usually a lot of these fragments in the mass of breccia.

~~These probably represent the material just being supported by the bed,1 The finer material is that which was being swept slowly upward past the more stationary fragments.~~

The large blocks up to several tens of feet across, which are found in parts of the breccia, are presumed to be foundering blocks arrested in their present position by the cessation of fluidization conditions in the bed. 204

The complex picture of mixed particle motions is further complicated by the great likelihood of consider able variation in the gas flow through the bed, mostly due to channeling, spouting, and slugging, but also due to waxing and waning of the quantity of gas passing upward from the deep sources from which it comes, at any moment in the bed’s history. Channeling, spouting, and slugging all have the effect of concentrating the flowing gas into narrow channels; when this happens, the fluidized state of the rest of the bed is diminished or ceases, and simul taneously the local concentration of rising gas will enable larger-than-average fragments to be supported in the breccia, and local zones of exceptional coarseness may form in breccia of generally smaller fragment size ranges. Under conditions of non-homogeneous flow, one would expect to find considerable textural variety in the former fluidized material.' Just such a textural pattern characterizes large parts of the breccia terrain, and seems adequately

explained by the mechanism presented. See Figure 24-(b) for an example of an exceptionally. coarse-grained breccia zone. *

In areas as large as, say, the main green or purple breccia outcrops, it seems unlikely that the whole system was continuously fluidized. Probably various channels or conduits served for gas entry into the base 205 of the fragmented bed at various times, as tectonic move ments occurred, and as various magmatic gas sources waxed and waned. Spouting and channeling were probably the . common forms of fluidization near the feeder fractures.

Leva (1959) points out that spouts and channels are prone to dissipate and diffuse away at higher levels in the bed, due to the action of individual particles in the spouting or channeling bed in breaking up the gas stream and dif fusing its energy, in a manner analogous to the scattering of light by the dust particles in the air. In the higher levels of the fluidized bed more uniform dilute and/or dense phase conditions should be the general ones; in this regard, the breccias of the Dos Cabezas mountains have quite a variety of contained fragment sizes, but the maxi mum size commonly found does not vary over wide areas by a factor of more than two or three; intra-breccia dikes of exceptionally coarse material are not common. The writer feels that the present exposure level in the breccia is far enough above the main inlet level for the gas so that rather more uniform fluidization conditions prevailed in the material exposed than were present in the material . near the vents.' Occasional coarse breccia dikes in the other breccia bodies implies persistence of channeling to a higher-than normal level.1 206

~~Foliation~~

~~A characteristic feature of the breccias in many outcrops is a marked to weak internal foliation. The foliation is generally northwest in strike, and dips~~

~~steeply to vertically. It is outlined by the parallel or~~

~~subparallel orientation of the non-equant rock fragments and by flow banding visible in the ground mass fragments or crystals. The writer believes that this foliation had~~

~~its origin in the flow of gases through a bed of particles~~

~~of various sizes, in the following way.~~

~~The foliation is produced by a combination of three~~

~~effects, each of which is responsible for one aspect of~~

~~the foliated pattern of the fragments. In Figure 26(B) we see an initial assemblage of fragments produced during~~

~~initial breakup of the cap-rock units.' Let us first con~~

~~sider the coarser fragments and the possibility of their~~

~~orientation, given a powerful stream of gases rising up~~

~~through them.' The gas flow will, first of all have a marked upward path, as it moves from an area of high to~~

~~one of low pressure. It will also tend to move faster in~~

~~the center of any conduit than along the sides, due to~~

~~friction with the sides. When the gas moving through the bed creates a fluidized bed, the bed will behave as a~~

fluid in its own right, and will have essentially the same 20? flow pattern as the initial gas flow. That is, if the bed is moving, it will move upwards along its conduit or chan nel, and it will have a velocity gradient from center to walls also/

Consider the vertical and probably upward motion of the gas in the bed and the bed itself. Most of the fragments in the bed have a crudely triaxial character; that is, three axes of unequal length at right angles to each other can be used to describe their overall shape/

If these fragments do not have their center of gravity at their geometric center, the pressure of the rising gases on the fragments will cause them to turn until this pres sure is minimized, and the fragments will then assume an oriented position in the flowing gas stream.' The part of the fragment which is furthest from the center of gravity of the particle will be swung "upstream”. If the fragment has its center of gravity very close to its geometric center, it will probably just spin in the gas stream.

An assymetrlc fragment will align itself permanently in the flowing gas, its more distant part acting as a rudder to keep the nose of the fragment pointed down into the rising gas. This will occur whether the fragment is rising, sinking, or holding at a constant level in the gas stream. This accounts for one of the factors of orientation of the fragments. 208

If the velocity distribution of the rising gas was constant at any position at any level in the bed, the only orientation tendency for the fragments would be that al ready described, namely that for them to be aligned with their long axes parallel to the direction of gas flow.

Sec Figure 26(b). If, however, there is a velocity gradi ent across the bed at any level in it, there will be a second orientation produced by it. If there is a fragment such as one of those in Figure 26(b) which has a crudely elliptical section at the level concerned, the outer part of the fragment will be exposed to a higher pressure due to its being in. an area of higher gas flow. The inner face, nearer the boundary of the conduit, is exposed to lower gas velocities and hence lower pressures. Tills means that higher pressure will be exerted on the outer face of the fragment, and the fragment will tend to turn away from this high pressure. In doing so, it will move around parallel to the lines of equal flow rate in the gas, as the

"prow" of the fragment leaves the highest-velocity part of the gas stream.1 After the fragment is aligned parallel to the lines of equal flow rate in the gas, it will still have more pressure on its outer surface than on its inner, and as a consequence will tend to move sideways through the bed toward the wall of the channel in which the bed is located. This in turn means that the foliated fragments 209 will be packed closely together. Thus the second type of foliation is produced. This type too is produced independ ent of whether the fragment in question is sinking, rising, or stationary in the bed.

~~If gas flow conditions in the Dos Cabezas mountains breccias were very constant, one would expect to find a very marked well-sorted and clearly fol i ated assemblage of~~

~~coarse tabular or elliptical fragments in every outcrop.~~

~~The fact that this ideal is only partially achieved is not~~

~~surprising, in view of the likelihood of markedly non-~~

~~uniform gas flow conditions in the beds, as pointed out~~

~~earlier.' Sudden spouting or slugging of the bed should~~

~~obliterate earlier foliation and leave behind a markedly~~

~~ill-sorted and unfoliated aggregate.~~

~~The third type of foliation is the foliation marked~~

~~by flow lines in the ground mass, around the outlines of~~

~~the larger fragments of the breccia. It is due to en~~

~~trainment and gross flow of the smaller particles in the~~

~~breccia, travelling as a dilute-phase fluidized bed, which~~

~~flows and swirls around the larger fragments.~~

~~Foliation of all three types will occur in the bed~~

~~unless it is composed of nearly spherical fragments, so~~

~~long as the onset of fluidization has taken place. Gas~~

~~velocity will only control the presence or absence of this~~

onset and the maximum size of particle which can be supported 210 by the bed, and not the foliation character itself, which is rather independent of particle size and gas velocity once fluidization conditions have been established. In the event that a vast majority of the particles in the bed are nearly spherical in shape, only the third type of foliation caused by ground-mass entrainment will develop.

~~When the gas velocity drops below that necessary to con tinue fluidized conditions, the bed will freeze and the foliation and other internal features will be preserved.~~

~~Solidification of the Breccias~~

The toughness of the breccia indicates a strong fusing or cementing together of the constituent particles of the mass. This could be due to vapor-phase crystal lization of silica or other cements, but the writer notes no trace of this in thin section study. Some of this action no doubt took place, but the writer thinks it more likely that the main solidification of the breccia is due to fritting or fusing together of the fragments in the breccia, coupled with some slight recrystallization of the fritted mass. This would require that the gas phase be at high temperature, and there is plenty of evidence in’ the breccia to suggest that it was. The pronounced re action of the green breccia with the production of epidote " in large quantities is the most obvious example. Many of 211

~~the breccia fragments show colored margins, and in thin~~

~~section these rims are often seen to contain new mineral phases formed by reaction breakdown of the original minerals~~

~~of the breccia fragments.~~

~~The question of the exact temperature of the gas~~

~~phase in the breccia and, hence, essentially of the whole~~

~~system, is quite open. Presumably, gas escaping from a~~

~~crystallizing magma will be at a temperature of more than~~

~~500 to 600 degrees Centigrade. As it rises and expands,~~

~~it must cool, unless the gases are chemically reactive, in which case the gas phase could heat up. In any case, the~~

~~gas will give up some of its heat to the sinking cooler~~

~~fragments in the bed, and will cool to one degree or another~~

~~depending on the temperature difference between the frag~~

~~ments and gas. Thus there are a number of factors which~~

~~are quite difficult to quantify which must influence the~~

~~temperature of the fluidizing gas phase in the breccias;~~

~~their overall effect is difficult to evaluate. It suffices~~

~~to say that mineralogic evidence indicates that sometime~~

~~after the bed ended its fluidized state in many of the~~

~~breccia units, considerable reaction and fusing together~~

~~of the various fragments in the bed occurred, and that~~

~~gas-phase heat of a high degree was probably responsible~~

~~for it.~~

~~The evidence of reaction is strongest in the green 212~~

~~breccia, which Is, indeed, generally green because of the~~

~~presence of ubiquitous epidote.~~

~~The later purple and white breccias seem to show~~

~~much less evidence of reaction, although they are quite~~

~~tough physically/ The crude time sequence of green-purple-~~

~~white breccia color seems also to be the order of de~~

~~creasing reaction. Some of the white breccias are not~~

~~really too well lithified. This all may point to a~~

~~diminution of the heat content of the gas phase with time,~~

~~The length of time involved in the emplacements~~

* of these breccias can only be crudely estimated. No geo-

~~chronologic evidence was obtained on the breccia system~~

~~by direct dating. It would seem that development of so~~

~~large a vent complex would take many thousands of years at~~

~~least, but could have taken much longer. There could have~~

~~been long quiet spells during its formation. It is note~~

~~worthy in this regard that the breccia contains no fragments~~

~~composed of other fragments cemented or fused together,~~

~~which implies that the length of time over which the system~~

~~in general developed was short enough so that no part of~~

~~the fragmented bed became cemented or fused and then~~

~~broken apart again. About all that can be said with~~

~~certainty is that the whole breccia mass was emplaced and~~

~~solidified within the post-Alblan part of Cretaceous~~

~~time. See Chapter 10 for a discussion of this. 213~~

One may ask how certain the interpretation is, that the breccia is tough and hard because of end-of-fluidization fusing together of the breccia fragments. There are numerous post-breccia intrusives, of stocklike and dikelike character, within the breccia terrain, and there is strong evidence of a general Paleocene heating of the part of the

~~Dos Cabezas mountains which includes the breccia terrain.~~

May not this fusing together of the breccia fragments be just a contact-metamorphic development? The writer thinks not, and as his best evidence would point out that the breccia is tough and hard right up to its contacts, and the units which it cuts are generally much less hard and competent, whether they are sediments or granitoid intru sive rocks."’ These surrounding units, especially the

Polecat quartz monzonite, often show deep weathering pro files of considerable age, which are unaffected by any recrystallization, while nearby breccias show very little chemical weathering. The fusing together of the particles in the breccia by contact metamorphism would seem to have been a very high-temperature process, and should have left its mark on the surrounding units but did not. The differ ence in degree of lithification of the breccia and its surrounding units is most clearly seen by the fact that there is about a 1000 foot topographic break which occurs along the breccia-wall rock contact all around the breccia 214 mass*’ The "breccia weathers much less rapidly than the wall units around, it. It is also obvious that the lithified character of the breccia does not change along its northwest contacts, where K-Ar dating results Indicate no contact metamorphic event has occurred for 1000 million years.'

~~Intrusive Ifacgnatio Bodies Related~~

~~To The Intrusive Breccias~~

~~The main intrusive breccia bodies have been cut by.a larger number of small-to-medium sized intrusive bodies of an original magmatic liquid nature, and of many~~

~~shapes and sizes. These intrusives are of a diverse~~

~~character, but most are porphyritic aphanites of a purple~~

~~or green color; the rest are nonporphyritic dark aphanites.~~

~~There is, as has been mentioned, some tendency for the purple breccia units to have purple nonbrecciated associ~~

~~ated dikes and even be partly unbrecelated themselves~~

in the same outcrop area. There is no real correlation between green breccia and green aphanite magmatic units; most of the green aphanitic magmatic units lie in the west central part of the breccia terrain near the Silver

~~Camp stock; see Figure 1.~~

~~Six samples of these units have been investigated~~

~~in thin section. All appear to be true andesites and all~~

~~show strong reaction breakdown of primary phases to 215 deuterlc assemblages of epidote, carbonate, and sericite.~~

~~See Appendix A for a detailed petrographic discussion of some of these units.~~

The non-breccia units show a less siliceous character than do the materials of the breccia ground masses, and a marked similarity to types of rock repre sented abundantly in the rock fragments of the breccia.

This seems odd, in view of their post-breccia character; one might expect them to be magmatic upwellings of the parent magma of the gas, which also contributed the ground-mass crystals of the breccia, i.e.:, if the author’s ideas are correct. The most obvious characteristic these nonbreccia units have in common is severe deuterlc break down of their primary mineral phases; the marked carbonate content of their ground masses is especially notable. It seems likely to the writer that the marked deuteric breakdown in these rocks implies a high volatile content, especially of water,' in the magmas from which they came.'

Given this, the writer presumes as most likely the idea that these dike rocks represent a differentiate of the gas-charged magma from which the gas for the formation of the breccia came." They are regarded as being separate from the pre-breccia flows in terms of time and evolu tionary relation to the dacitic magma from which the gas came to form the breccia bodies. 216 Of the original vent system and the volcano or volcanoes which once lay over the area of the Dos Cahezas mountains, no trace now remains.! Whatever surface vent structures existed have long since been destroyed by erosion. It should incidentally be noted that the writer has found no evidence for caldera-like subsidence of any part of the Dos Cabezas mountains; such collapse struc tures might be expected from such a vast outpouring of material as has taken place in the range, but such struc tures have not been found. It may be that there are some buried under the alluvial fill of the Sulphur Springs valley, but none can be seen at the surface.

~~Other Examples of Intrusive Volcanic Breccias~~

The purpose of this short section is to point out that the only really unusual characteristic of the breccia terrain in the Dos Cabezas mountains is its size. In the. following paragraphs, intrusive breccia bodies from the state of Arizona and from other places around the world will be described.

~~Mayo (I9 6 3 ) has produced a very detailed study of the Piedmontite Hills area in the Tucson mountains, in '~~

Arizona, which lie about 150 miles west of the Dos Cabezas mountains.' In the area he has studied, several pyroclastic bodies of intrusive character occupy an area of about one 217 and a half, square nilesThe area of exposed pyroelastics is cut off on the north by a fault and on the south by a younger arkose, and is larger than its visible outcrop by an undetermined amount (see Figure 2 7 ).

~~The northwestern part of the area described by~~

Mayo contains a block of a ferruginous shale formation perhaps 1000 by 200 feet in size. To the south, east, and west, this block of redbeds is in contact with volcanic breccia and conglomerate of andesitic composition, which penetrate the redbed block along innumerable reentrants.

The breccia and conglomerate display a frequently wrinkled and distorted but broadly homocllnal structure dipping shallowly to the east.- The strikes of the redbeds and the breccia-conglomerate units are perpendicular to each other.

~~Within the volcanic breccia and conglomerate lie many large and small blocks of redbeds, many of which are in structural continuity with the large main block previously mentioned. Mayo says:~~

“Near the contact between the coarse, fragmental volcanics and the long, narrow belt of redbeds at the northeastern edge of the hills, many small and large steep-sided lenses, pipes, and irregular bodies of the fragmental volcanics are present in the fine-grained red rocks. At numerous places the same phenomenon can be seen in the isolated red- bi o cks. Apparently, the fragmental volcanics have actually invaded the redbeds, as is suggested also by the previously mentioned very irregular contacts." (Mayo, 1 9 6 3 , p. 72) s/e iSSl

~~Co^c<~~

~~-300 ’ OOO ■~~

~~D/VI Por/oAyry Tnrrua/ves~~

[ 1 >4/770/W /3r/rose Jgn*07&-//* 1e (CZ)?/y-(/a/Ke^ ^ \ > I 9 / 9 I jOass/G/y sosne exTru&vB \ ^ L,, tt?A! C/ost/c pjpes /n xo/c &~ecc/o ^ t t ? ,'■ r .’ jl 'Sondstene /o tso/con/c drecc/o X V # v 1 j^>fco7zc drecc/a 6 coog/o/nerare B R /?e

~~fogure 27 Geo/ogy o-f f/ie f^/edmont/Te /////s Tucson A/ftns , /Qr/zono S//np//f/&d -from A/foyo (/&63) 219 Regarding the mechanism for this activity, Mayo says:~~

“The most active ingredient seems to have "been the sandy matrix of the volcanics; this material appears in the smallest, initial (?) projections into the redbeds. The matrix appears to have been followed, as the projections increased in size, by entrained pebbles. By continuation of this intrusive process, the red bed blocks must have become isolated, the red chips distributed throughout the volcanics, and large volumes of the red beds completely disinte grated." (Mayo, I9 6 3 , p. ?2)

~~The volcanic breccia and conglomerate are cross-cut~~

~~by a large number of small intrusive clastic pipes, con~~

~~taining exceptionally coarse andesitic clastic debris;~~

~~Mayo ascribes these to channeling during fluidization, pre~~

~~sumably while the main mass of breccias and conglomerates~~

~~was invading the redbeds.~~

~~In several places the breccias and conglomerates~~

~~•were invaded by intrusive ignimbrite; fine-grained, presumably~~

~~Yielded, tuffaoeous material. These are possibly related / to the large upper Cretaceous ash flovis which make up~~

~~much of the mountains nearby.~~

~~Mayo1s viork is the most detailed study of an in~~

~~trusive breccia that the writer has yet found, with the~~

~~exception of the lack of thin-section viork. The field •~~

~~relations are quite explicit, however, and petrogrzphlc~~

~~study would add little to the conclusions of. the study.~~

~~With respect to the breccias in the Dos Cabezas mountains,~~

~~the most important point is the widespread development~~

of planar orientation (foliation) in the volcanic breccias 220 and conglomerates, evidently produced during their invasion of the redbeds/ Marked foliation characterizes the in trusive ignimbrite as well (see Figure 2 7 ).

The only other study of intrusive breccias in any detail which the writer has yet found is that of the Sud bury breccias at Sudbury, Canada. In many places around the great gabbro Intrusive there are large dikes or complexly outlined areas of breccia; this breccia is generally acknowledged to be intrusive by one means or another into the rock in which it is found. Host of the rock fragments are rounded, and derived from the wall rock types;' a few fragments in all exposures, however, are from some distance awayThe matrix of the breccia is fragmental, frequently showing a high abundance of very

~~small sized fragments of the wall rocks and in general~~

~~shows marked flow structures around the large fragments.~~

~~The largest of these breccia bodies is evidently~~

~~that at the great Frood ore body, Zubrigg et al (1957) presents a good map of its form around the mine area. The breccia mass evidently underlies several square miles of~~

~~the surrounding country, in a somewhat dikelike pattern~~

~~trending oast-northeast. Host authors (Falrbair and Robson,~~

~~1942; Zubrigg et al, 1957; Speers 1957; Hawley, I9 6 2 )~~

~~ascribe the origin of the breccia to some type of fluidized~~

or gas-blast intrusive mechanism.' There is no real 221 distinction which can be made between the internal textures in this body and the intrusive breccias found in other parts of the world. The main points of interest with respect to the breccias in the Dos Cabezas mountains are the great size of the terrain and the ground mass folia tion^ ' *

~~In the Tertiary volcanic province of northern~~

Ireland, Pitcher and Head (1952) describe a small Intrusive breccia body, composed of calc-silicate rock and quartzite fragments in a generally dark hornblendic cement evidently due to high-temperature reaction breakdown of the calc- silicate fragments. The .quartzite fragments have risen at least 1000 feet above their source horizon; they show a remarkable planar orientation. These quartzite fragments are 3-5 inches across, and quite platy in shape. They lie in a markedly planar array, and in addition the long axes of the fragments are aligned within that plane. The main point of Interest here is the well-developed foliation.

In the same part of Ireland, Patterson (1963) dis cusses 20 separate volcanic "agglomerate" vents cutting the Upper Chalk and Tertiary basalts, and gives a number of criteria for recognition of their intrusive character.

He cites vertical mixing, presence of chalk .among the basalt fragments, intrusion of vent walls, upward orientation of contacts, and rounding of larger fragments. Host of 222 these criteria apply to the "breccias in the Dos Cabezas mountains

In the volcanic fields of the Absaroka volcanic field in Wyoming (Bouse, 1937* Parsons, i9 6 0 ) enormous quantities of breccia have been disgorged from numerous vents over a large area.' A number of intrusive breccia vents are exposed in local mountains or canyon walls. The key relationship of the Absaroka breccias to those in the

~~Dos Cabezas mountains, however, lies in the presence~~

~~within the Absaroka breccias of enormous limestone blocks~~

~~up to several hundred feet long and tens of feet thick~~

~~which have been moved and often lifted some distance from~~

~~their original horizonsThis is not to imply that the~~

~~large sedimentary blocks in the Dos Cabezas breccias~~

~~have moved up from below, although they might have; rather,~~

~~it is to point out that very large blocks can persist for~~

~~some time in fragmental breccia of generally much finer~~

~~size fractions.~~

~~Thus the vertical contacts, wall-penetrating dikes,~~

~~internal foliation, and large included blocks of Paleozoic~~

~~arid Mesozoic sediments, which are all features of the Dos~~

~~Cabezas mountains' breccias, have their counterparts in~~

~~other breccia terrains in other places. The great size~~

~~of the breccia terrain in the Dos Cabezas mountains is its~~

~~only really unique feature, and that is perhaps accidental 223 and due to the local level of erosion, A little more or a little less elevation and much less might be exposed. CHAPTER 8.~~

~~EARLY CENOZOIC (LARAI1IDE) STOCKS AID DIKES~~

The Dos Cabezas mountains contain several medium- sized to small stocks and dikes of predominantly mesocratic to melanocratic aspect, ranging in age over the span of Cenozoic Laramide events (Paleocene-early Eocene) in the Basin and Range province in southern Arizona.'

~~Evidence for early Laramide (late Cretaceous) stocks and/or dikes is present (i.e., diabases) but is somewhat tenuous/~~

~~The intrusives are post-breccia in age, and two of the larger stocks in the group cut the breccia.~~

~~Dlabaslc and Ouhltic Intrusives~~

The earliest of these stocks and dikes are a group of diabase intrusives scattered across the northwestern part of the range (see Figure 28 ) and the complex of pseudo-ophites which are intrusive into the easternmost part of the mountains.

~~Diabases~~

~~The dlabaslc units have been classified together on the basis of an apparent similarity in petrogenesis and time of intrusion. They tend to be grouped along the~~

224 225 northern border of the Dos Cabezas mountains in its central part. They all appear to have undergone strong, isotropic crushing after development of primary diabasic texture and to have developed a later essentially metamorphlc texture which borders on one produced by simple deuteric reaction in some samples.1 Recrystallization in the large diabases in and around the Government Peak area (see Figure 1) has been so strong that the rock now has an ophitic texture in hand specimen; this accounts for the name pseudo-ophite, as the rock is not just a simple ophite produced by initial crystallization of a magma.'

The evident crushing in these rocks seems isotropic, and perhaps has involved little overall displacement of the borders of these bodies. All are intruded into PreCambrian granitoid rocks or Pinal Schist units which are more or less

sheared and a small amount of displacement of these wall rocks might have produced considerable pressure on the dia base units while they were in the crystalline state without any appreciable effect being noted in the field.™ In the field, however, these diabases show no outward signs of

~~their internal brecciation and recrystallization, and their outer borders show no shear offsets. The rock surrounding~~

~~them is not especially sheared, either. In plan they are rather Irregular and polyobate in shape, but this appears~~

~~to be an original intrusive outline. The writer suspects #?5£: fl26E~~

~~Legend s/a Mode -ao/7io/e 643 Z>c7ZV/y -samp/e.~~

~~E/gure 23 L orom/de Afogmot/c Jofr vs/Ve s j &b/) cyxd Loco/-/os? o/~ ~3o/np/es 6 ^~~

~~i \~~

~~227 that the crushing took place before intrusion of the units to their present positions. Whatever its source, the~~

~~crushing has been quite pervasive and has been one of the~~

~~.factors in the development of a new mineral suite in the rocks.' See Appendix A for a detailed petrographic dis~~

~~cussion of these units.~~

~~The following mechanism for the development of breeolation is proposed.' It seems most reasonable that~~

~~at some level below the pre sent one the partially crystal~~

~~line diabase magma was compressed and a filter-pressing process took place which drove most of the liquid from between the crystals and allowed the crystals to come~~

~~into contact with one another and to transmit stress among~~

~~themselves; brecclation of the crystals then followed.~~

~~After this, before the magma completely crystallized, the~~

~~crystal mush with interstitial liquid in some proportion~~

~~was intruded into some higher level of the crust, and~~

~~there solidified.’ The hornblende crystals with the aci-~~

~~cular needlelike form were produced by rapid crystal~~

~~growth from very small nuclei! which were probably minute~~

~~fragments of the original hornblende crystals. Plagioclase~~

~~either crystallized after this development of acicular~~

~~hornblende or had a much stronger tendency to reheal and~~

~~re-establish a more blocky crystal form than the hornblende exhibits/ • .. 228~~

~~Government Peak Ophite~~

~~The area around Government Peak (see Figure 28 and~~

~~Figure 1) has a small complex stock and several dikes of a melanocratic rock referred to here as the Government~~

Peak ophite.i This unit is really a strongly recrystallized diabase and is truly a pseudo-ophite. It appears, in out crop, to be almost wholly composed of dark minerals, A general ophitic texture is present,1 The texture is not really an original crystallization texture, as previously mentioned,’ The great similarity of the internal character of this mass and that of the diabases in the western part of the range causes the writer to classify them together.

Both of these samples must have been profoundly recrystallized after their original complete or, more likely, partial crystallization from a magmatic state. The general pattern of the recrystallization is the same as that

in the diabases in the western Dos Cabezas mountains, and presumably is due to the same type of genetic history. It may well be that all these presently separate recrystallized diabase masses are part of the same mass of crushed crystals which originally lay at some distance below the present surface.’

~~Age of Diabase and Ophite Intrusives~~

The age of the diabase-pseudo-ophite group of 229 plutons and dikes is unknown in a strict sense.' No trace of the crushing event which profoundly shattered these units' constituent hornblende crystals, or of the deuteric

~~event producing the characteristic acicular hornblende~~

~~crystals, is found in any other stock or intrusive body~~

~~in the Laramide or mid-Tertiary assemblage in the mountains.~~

~~Based on this observation, it would seem that the diabase~~

~~intrusion predates the intrusion of the other stocks and dikes in the range. The oldest of .these uncrushed stocks~~

~~is the earliest Paleocene Silver Camp quartz diorite, which has an age which places it at the Mesozoic-Cenozoic boun~~

~~dary.1 It would.seem that the diabases and the events which modified them must be latest Mesozoic in age. There~~

~~is no direct evidence as to their age, but the Government~~

~~Peak ophite seems to be cut off by the intrusive volcanic~~

~~breccia on its southern boundary, and it is likely that the~~

~~diabases date from the close of the Laramide deformation and~~

~~predate the breccia. The evidence is somewhat tenuous,~~

~~however, as the contact between the ophite and the breccia~~

~~is seldom clearly exposed.- No crushing like that seen in~~

~~the ophites shows up in neighboring breccia, however, so~~

~~that the author is quite convinced that the diabase-ophite~~

~~assemblage is pre-breccia, perhaps 90 million years old or~~

~~so... • ' ' 230~~

~~Quartz Diorite Stocks~~

Three small quartz diorite stocks invade the central and western Dos Cabezas ♦' Figure 1 shows their distribution." They have been given provisional names for purposes of discussion. The northwesternmost is named the

~~Cowboy stock, after Cowboy canyon which cuts it. The cen tral one is the Silver Camp stock, after Silver Camp canyon, which cuts it. The southeasternmost is here called the~~

Mascot stock after the old Mascot mine, which is partially in.the stock.' Several satellite dikes of quartz diorite are associated with each of the stocks, and small dikelike ore bodies of magnetite, siderite, pyrite and chalcopyrite are associated with the eastern two. Each stock will be discussed in its turn.'

~~Cowboy Stock~~

The Cowboy stock is a. rather complex small quartz diorite stock of some one and one-half square miles surface outcrop; much of the center of the stock is buried by al luvium. The stock cuts Final Schist units and Polecat quartz monzonite, and cuts into the boundary fault which separates the quartz monzonite terrain from the Pinal.

~~The stock has been investigated in some detail.~~

Examined samples indicate a general quartz diorite 231 composition, with local complexities*’ There are two main phases, called here A and B., together with apparent cog nate xenoliths of earlier phases, as well as dikes which follow A and B in time* Not all of the latter dikes are diorites. The largest part of the stock is composed of main phase A; perhaps' it makes up to 80$ of the surface exposures. Phase A is a medium-grained mesocratic quartz 'i diorite with phenocrysts of plagioclase, biotlte, and/or hornblende in a fine-grained ground mass of quartz and potash feldspar* See Appendix A for a more detailed petrographic discussion.1

Phase A can locally vary a good deal in composition, as in the west central part of sec. 2 3 , T13S R26E, where hornblende-rich and biotite-rich zones alternate in rapid succession. Overall, the phase is equigranular and nonporphyritic, of average grain size about 1 mm, and sometimes shows a very weak flow structure.

~~Main Cowboy phase B is about three times as coarse grained as phase A, and appears mostly in the easternmost~~

~~J>art of the stock, where it has obvious crosscutting and dikelike apophyses which penetrate phase A.' It shows no obvious compositional variation from phase A.~~

The main phases of the rock show small aggregates of ferromagnesian minerals a few millimeters in diameter, 232 which lend a spotty appearance to the rock. 1 These are horriblende-biotite-iaagnetite aggregates. On a larger scale, the rock frequently contains inclusions of melanocratic or mesocratic fine-grained intrusive rocks; the xenoliths are well-rounded and appear to contain horn blende, plagloclase, and biotite, and are interpreted as cognate xenoliths of the stock itself.

~~The stock gives a K-Ar age of 59*0 - 1.8 m.y. on biotite from a stream-bed outcrop (on sample 8 0 6 ) which marks it as early Paleocene in age.~~

~~Silver Camp Stock '~~

In the central western part of the volcanic breccia terrain, the faulted Silver Camp quartz diorite stock crops out in the central western "part of the volcanic breccia terrain (see Figure 28). This unit has a general quartz dioritic composition, and shows perhaps less in ternal variation than does the Cowboy stock. The rock has the same texture as the Cowboy stock. See Appendix

~~A for a thorough petrographic discussion of the unit.~~

~~Three small masses of magnetite and siderite are present in and around the stock, as can be seen in Figure~~

~~1. These small ore bodies have been developed to some~~

~~extent in the past, but no present operations are in progress. 233 The large fault which bisects the stock is thought to be due to reactivation of a major Precambrian break.~~

~~The trace of this fault is always a low area, but its position may generally be plotted by following joint patterns beyond the local alluvial cover.~~

. The K-Ar date on this stock gives 62A - 1.9 m.y." on biotite from a blasted outcrop in the northern part of the stock.' It Is latest Cretaceous to earliest Paleocene in age; if, following the work of Follinsbee et al (1961),

~~63 m.y.- is used as the border between the Mesozoic and~~

~~Cenozoic, this stock straddles the time boundary between two eras."~~

~~Mascot Stock~~

~~The third of the quartz diorite intrusives is the~~

Mascot stock. It trends west-northwest through the green phase of the volcanic breccia in the central Dos Cabezas, and has a rather dikelike aspect (see Figure 28). It locally cuts into the Apache Pass fault zone near its southern margin (see Figure 1) and helps confirm the absence of post-Mesozoic movement in the fault zone. No potassium-argon data were obtained on the stock because of the intense deuteric alteration of its primary mineral phases, but its age is presumed to be the same as that of the other quartz diorite bodies in the range. In Its field exposures the rock is quite similar to the other two bodies just described.- It is a mesocratic fine-grained rock, whose average crystal size is about

2 mm,: The rock has in it a large number of inclusions of the volcanic breccia and purple basalt porphyry units, and has borders which break down into numerous short parallel dikes at its western and eastern ends. It is quite deeply weathered and most outcrops are quite weak and crumbly/ Inclusions are of small size and are rare.

~~See Appendix A for a detailed petrographic discussion.~~

It is interesting to note that aerial photographs of the Mascot mine area show a marked halo of stained ground around the intrusive, extending out to perhaps half the intrusive width from its boundaries. This is thought to have been due to reactions caused by hot fluids escaping from the magma in the closing stages of crystallization.

~~Ore samples from the Mascot locality are the same as those from the Silver Camp stock, and serve to con firm the genetic tie between the two bodies. Several~~

~~small dikelike ore masses the same in position and mineral ogy as those at the Silver Camp site were found around the Mascot.'~~

~~All these stocks seem grossly similar, and the~~

concordant K-Ar ages on the Silver Camp and Cowboy stocks 235 would confirm this general impression of congruence. It seems almost certain that the three quartz diorite stocks are parts of a single large magmatic body which was pre sumably differentiating over time, so that the composition of the surface of the rising pluton kept changing. Potas sium-argon data do not allow discrimination between the separate pulses or Intrusion into the present stock system, since the analytical data are not that precise.

~~4503 Quartz Diorite~~

~~In the easternmost Dos Cabezas lies another small quartz dioritic body similar in mineralogy and texture to those just described (see Figure 28). It invades the~~

Pinal Schist units in the area, and has a quarter-square mile outcrop at most.- It will provisionally be given the unromantic designation of the 4503 quartz diorite, since there are no named geographic features near it; it is named after the elevation of the hill it underlies.

Modally the sample is similar to the Mascot stock, though it shows little deuteric alteration. There is no raineralogic criterion which could be used to separate it from the other Laramide quartz dlorites in the range, and it is thought to be part of the proposed quartz diorite group of essentially congruent or time-equivalent stocks of this character." See Appendix A for a detailed petro- ,x graphic description. 236

Not only are all the above described quartz diorites similar to each other, but the Cowboy, and especially dikes in it, are quite similar to the diabases which preceded them in the range although they lack the crushed crystals characteriziiig those units.' The petrographic and field relations indicate that there is a strong likelihood that all these units are part of one overall magmatic intrusive group, whose activity probably spans the later part of the

Cretaceous and the early part of Pal eocene time. The wide distribution of the diabase and quartz diorite units in the range indicates that their presumed parent magma is at • least as widespread, and may underlie a long northwest- southeast trending zone at some depth below the present erosion surface. Implicit in this model is the idea that the magma smaples represented by the present exposed plutons and dikes have risen essentially vertically from their source at depth, and that the aforesaid source underlies the present plutons with respect to geographic position.

I have at present no clear idea how the diabase and quartz diorite plutons and their hypothetical parent magma relate to the source magmas for the volcanic breccia that lies in time between at lease some of the diabase units (Government Peak ophite) and the quartz diorite plutons. It may be that the breccia parent plutonic 237 masses, "being of dacitio or andesitic character, are part

~~of a differentiated prong of the quartz diorite-diabase~~

~~parent* It would seem that they are separate magmas entirely,~~

~~but in the absence, especially, of chemical data and more~~

~~complete age data, I would rather not draw tenuous con~~

~~clusions about their relations,i~~

~~Buckeye Dike Group~~

~~•’ A system of five large, thick biotite-rich quartz~~

~~monzonite dikes outcrop on the flanks of Buckeye canyon~~

~~and Cement canyon on the eastern margin of Maverick moun~~

~~tain in the east northern Dos Cabezas (see Figure 28 and~~

~~Figure 1).' All these dikes are quite similar in mineral~~

~~ogy and texture; the analyzed sample (see Appendix A) is~~

~~quite representative of them. They will be referred to as~~

~~the Buckeye dike group for purposes of discussion.~~

~~These units are quite dark in the field, and~~

~~display a weak porphyritic texture defined by 4-5 mm,"~~

~~blocky euhedral plagioclase crystals in a 2 -3 mm. biotite-~~

~~rich quartz-feldspar ground mass.~~

~~Maverick Stock~~

~~Northeast of the earlier described quartz diorite~~

~~intrusives, in the center of Maverick mountain in the~~

~~north center of the Dos Cabezas mountains, crops out a 238 large complex stock called the Maverick stock." The name is quite provisional and refers to the mountain mass in~~

~~•which the stock lies (see Figure 1 and Figure 28).~~

The Maverick stock lies, at this level of erosion, in four separate parts which are so closely associated spatially arid so similar petrographically that there can be little doubt of their identity. The unit is a raeso- cratic porphyritic quartz diorite, containing large tabu lar crystals of plagioclase antiperthite up to 3 cm. long in a medium-grained biotite-rich quartz-feldspar ground mass/ The unit shows flowage foliation orienta tion of the phenocrysts in one or two planes; where the two are present they cross at near right angles. As can be seen on Figure 1, the unit contains many large and small blocks of the granitoid rocks it invades, and in general gives the impression of being more like a complex of thick dikes than a coherent stock; it is, of course, quite likely that the separate dikelike masses meet at depth. See Appendix A for a detailed petrographic de scription.

Texturally the rock bears certain distant re semblances to the quartz diorite family earlier described, with comparatively large phenocrysts of plagioclase in a finer-grained ground mass rich in dark minerals and 239

~~containing a fine-grained potassium feldspar-quartz ground mass.' The plagioclase is more alkali-molecule rich than~~

~~that of the quartz diorltes, and more total quartz and~~

~~potassium feldspar is present." These variations of~~

~~mineralogy follow those which might be expected of dif~~

~~ferentiation products of a more basic magma, and are also~~

~~easily related to the chemistry of the rock which obviously~~

~~is more alkaline than the quartz diorite sequence members~~

~~are. •~~

~~The K-Ar age of the stock, determined on biotite,~~

~~is 55*9 - 1*7 m.y., or about lower Eocene. The unit is~~

~~about 3 million years younger than the Cowboy stock,~~

~~which crops out within a few miles of the Maverick moun~~

~~tain area.1 The age data certainly emphasize that the~~

~~stock, is part of the general Laramide intrusive group,~~

~~and allow the interpretation that the stock is later dif~~

~~ferentiation of the same parent magma which has given rise~~

~~to the diabase and quartz diorite units of previous times.~~

~~Basalt Dikes~~

~~The Maverick stock is the last large intrusive~~

~~unit to appear in the Dos Cabezas in Laramide time. There~~

~~is, however, one system of small intrusive units which~~

~~appears in the westernmost part of the Dos Cabezas moun~~

~~tains and dates from pre-Oligocene time. This is a system 240 of basalt dikes which outcrop widely over the northwesternmost Pinal Schist and Sommer gneiss PreCambrian terrain.~~

~~Most of the dikes labelled "b" on Figure 1 are of this~~

~~type. These dikes are characterized in the field by a rather massive character, are only weakly jointed, and weather into large minutely pitted cobbles and boulders~~

~~of a distinctive light gray-green color.~~

~~One of these dikes, the largest, was dated by the~~

~~K-Ar technique applied to a fresh sample of whole rock from a waterfall outcrop. An age of 4?.6 £ 1.4 m.y., about~~

~~middle Eocene, was obtained. Whole rock ages should be~~

~~considered to be minimal, except in the case of very young~~

~~units, when the possibility of excess argon biasing the~~

~~age toward a value that is too old must be kept in mind.~~

~~In this rock, the main modal minerals are hornblende and~~

~~and plagioclase, both of which are frequent carriers of~~

~~considerable quantities of excess argon. Recent data~~

~~(Damon et al, 196?) show, however, that in young volcanic~~

~~rocks the fine-grained ground masses of the rocks do not~~

~~contain the excess argon found in the phenocryst phases,~~

~~or at least contain excess argon in a much smaller quantity.~~

~~Experimental data do not allow any proof of the existence~~

~~of any excess argon within the ground mass minerals~~

~~in this rock The small size of the ground mass minerals evi dently allows rapid diffusional removal of original~~

~~trapped argon from these crystals during the final cooling of the rock.’ At lower temperatures, after solidification~~

~~of the rock, diffusional loss of argon from the ground mass minerals is small, and post-cooling radiogenic argon accumulates without significant loss.- Thus, excess argon~~

~~is not a significant "biasing agent for the age of these~~

~~fine-grained ground mass mineral assemblages.~~

~~Because of the fine-grained size, then, of the mineral assemblage of hornblende and plagioclase which~~

~~primarily composes these basalt dikes, it seems that the~~

~~K-Ar age for the one dated dike is quite reasonable, and~~

~~that the dike group as a whole probably is well dated by~~

~~this ago/~~

~~The age of these units places them at the tail~~

~~end of the series of Laramide events noted in southern~~

~~Arizona, as defined by Damon and Mauger (1 9 6 6 ). According~~

~~to Turner and Verhoogen (I9 6 5 ), the appearance of basaltic~~

~~volcanism at the close of orogenic periods is common;~~

~~certainly this pattern is seen in the closing events of~~

~~the mid-Tertiary events of southern Arizona (Damon and~~

~~Bikerman, 1964). This common location for a major phase~~

~~of basaltic volcanism tends to reinforce the writer's 242 opinion that the K~Ar data represent a true age for the dike and the dike group.~~

~~Whether these dikes are part of the diabase-quartz- diorite-quartz monzonite sequence is difficult to determine,~~

~~The analyzed sample is much richer in hornblende than a typical diabase sample, which is the closest analog to these dikes to be found among the older stocks and dikes.~~

These dikes may represent a crystal concentrate from a parent magma of all these units, but the dikes are suf ficiently younger than and modally different from the older units so that the writer prefers to think that the basalt dikes represent a separate and distinct pulse of magmatic activity. A time gap of some 8 m.y. separates these dikes from the youngest Laramide plutons of other ages, which certainly allows the latter interpretation.

~~The writer prefers the second hypothesis, and it should be noted that this implies the presence of two post-~~

~~Bisbee post-deformation nonbreccia magmatic pulses in the~~

~~Dos Cabezas mountains area in Laramide time. The former was more restricted in time, space and petrography than was the latter, at least at the present erosion level. CHAPTER 9~~

~~HID-CENOZOIC STOCKS AND DIKES~~

After the close of Laramide intrusive activity in the area of the Dos Cabezas mountains, there were no events of record for some 12 million years. Then, in a quite narrow span of time, there appeared a number of dikes and one large stock, which make up the last major group of units in the Dos Cabezas mountains.

~~Mid-Tertiary Dikes~~

~~There are three major dike groups of mid-Tertiary~~

~~age in the Dos Cabezas mountains.~~

~~Turkey Track Dike Group~~

~~The oldest of these is a collection of 33 small~~

~~dikes and plugs of andesitic composition, arranged along~~

~~a uniform single west-northwest trending line in the~~

~~northwestern part of the Dos Cabezas mountains. The~~

~~series of sub-unit dikes runs for more than eight miles~~

~~along the strike of.this trend. The individual dike~~

~~sub-units are up to a little over a half a mile long, and~~

~~some are as short as a few tens of feet (see Figures 1~~

243.. and 2 9 ). Host of the segments, of whatever length, are about twenty feet wide.' Internal flow foliation of feld spar crystals is ubiquitous, and shows steep dips to north or south. Pinal Schist units and the Polecat quartz monzonite arc the host rocks for the dikes.

~~The rock involved is a plagioclase phenocryst bearing andesite porphyry of the type known as "Turkey~~

Track" (Cooper, 1961). Perhaps 60,u of the rock is composed of markedly euhedral and platy tabular plagioclase pheno cryst s up to 3-4 cm. across set in a fine-grained plagioclase-pyroxene-biotite ground mass. See Appendix A for a detailed petrographic description of this unit.

~~This is the earliest mid-Tertiary dike phase.~~

~~K-Ar dating on a plagioclase separate from the dike gives -f- an age of 35*2 -3,1 m.y. for the unit. This is lower Oligocene.~~

~~Hornblende Andesite Dike Group~~

The second major mid-Tertiary unit is a group of hornblende andesite dikes which lie along a north 65 west trending line about one-quarter mile southwest of the line followed by the "Turkey Track" dike swarm and parallel to it, in the northwest part of the Dos Cabezas. Some 20 to 25 short sub-parallel dikes up to two miles in individual length and thirty feet in width make up the group, which is #2 re

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A'/gure £ 9 A//d-7&/~t-/arj Mqg/r>o+/c Jsjrrxts/rms J £>A»n o/Td So/np/e /.ocot/on* about 13 miles long in all (see Figures 1 and 29), Two other dikes which are evidently part of the same group outcrop in the northwest part of sec. 2k, T13S, R25E

(see Figure 1). All the dikes in this group are very deeply weathered in general, and fresh samples are hard to find. The rock is a peculiar yellowish tan color when weathered and is quite distinctive in the field.

In section 2 5 , T13S R26B, a dike unit of the hornblende andesite porphyry cuts the “Turkey Track" porphyry, and is thereby established as younger than that unit. K-Ar dating was not carried out on the.unit, as a pure separate of the hornblende or the plagioclase could not be obtained. This turns out to be unimportant in determining the overall age of the dikes, however, as they can be bracketed in time between the "Turkey Track" + andesite and rhyolite dikes of 33*9 - 1.0 m.y. age. All these dikes occupy a narrow time interval.

As mentioned, the rock is a pale lemott yellow to yellow tan color on outcrops, with local white and red blotches on the surface. A faint near-vertical foliation is occasionally seen, outlined by lines in the ground mass or alignment of white blotches, but it is rare and weak.' See Appendix A for a detailed petrographic description of the unit. 247

~~Dacite Porphyry Dike Group~~

~~The third, major group of Mid-Tertiary dikes in the~~

Dos Cabezas consists of a large number of leucocratic aphanite porphyry dikes, sills, and plugs which are pre sent in all parts of the range; indeed, this unit is found in more places than any other unit. Perhaps half of all the surveyed sections in the range have one or more of these bodies within them/ Figure 14 and Figure

29 show their distribution. It is by no means certain that all these units are of the same age, but their general geologic relations indicate that most, at least, are part of the same dike swarm or group. Some local phases of the Precambrian aplite dikes in the eastern

~~Pinal terrain are apparently aphanitic, and these imply that some of the purely rhyolite dikes in that area are~~

~~Precambrian, as the aplites are. Alternately, local phases of the obviously Tertiary dikes have aplitic~~

~~crystalline facies.~~

~~A K-Ar age was determined on a whole-rock sample from one of these dikes where it cuts the Mascot stock north of Dos Cabezas village. The age obtained was~~

~~33•9 - 1.0 m.y.’ This date indicates that all three of~~

~~the described mid-Tertiary dike groups occupy a narrow~~

~~time span of two million years, pointing to rather marked 248 fracturing and igneous invasion of the crust in the region of the Dos Cabezas at this time.~~

The dikes are generally of a pale lemon yellow er sometimes, white color and in almost all cases show a marked flow banding parallel to their walls; this is de fined by variations in grain size of the very fine-grained ground mass of the rock along linear trends on outcrop surfaces. All are commonly erratic in strike, short, and narrow; some larger dike groups are all linked together in a complex pattern of criss-crossing and T-joined dikes.

Many times, as is especially common in the eastern part of the southern Dos Cabezas, there will be a large number of small dikes of this type which lie along a common trend, very much in the manner of the earlier, older dikes pre viously described.

~~A number of large sill and dike-like bodies of this group invade the volcanic breccia, Pinal Schist, and~~

~~Cretaceous (?) lime units in the Camelback mountain area~~

~~(sec. 33» T14S R26E; - see Figure l). In addition, a number of small plugs or plutons of this unit crop out in the~~

~~Apache Pass fault zone and in the Paleozoic sequence~~

~~southwest of Dos Cabezas village. These small plugs,~~

.dikes, and sills all show the same flow banding, color, and mineralogy that the dikes have, and they are all 2^9 part of the same group of units so far as the writer can tell. Altogether, these bodies record a rather intricate and massive invasion of the local crust in mid-Tertiary time. See Appendix A for a detailed description of this unit. '

On the basis of phenocrysts and the high quartz content of the ground mass, these dikes are dacite por phyries or possibly latite porphyries. The name dacite porphyry will be used in this discussion.

~~Nlnemile Granodiorlte Stock~~

~~. Following intrusion of the aforementioned dike series, a large stock invaded the northeast part of the~~

~~Dos Gabezas range, where it cuts the Sheep Canyon quartz monzonite gneiss and rocks of the Pinal series (see Figure~~

~~29). The rock is given its name provisionally, after the ranch at the base of the outcrops of the stock, and is hence called the Nlnemile granodiorlte. (A picture is given in Figure 30b).~~

A K-Ar date was obtained on biotite from a sample collected near the ranch house which gave an age of rl* 29.0 - 1.7 m.y., equivalent to about mid-to-late Oligocene time. The stock is demonstrably younger than the previously discussed dikes. 250 In field exposures the Ninenile stock is a weakly

~~porphyrltic very coarse-grained leucocratic rock, with~~

~~large tabular euhedral plagioclase crystals up to 4-5 cm.~~

~~length lying in a coarse-grained ground mass of crystals~~

~~5-10 mm.- across. The phenocrysts in some but not all~~

~~outcrops are aligned weakly in what appears to be a~~

~~primary flow foliation. The writer's confidence in his~~

~~qualitative estimate of the strike of this foliation is~~

~~not too high in most of the observed outcroppings, and~~

~~the map of Figure 1 so indicates. In the core of the~~

~~intrusion a poorly delineated, finer-grained phase ap~~~

~~• pears; its boundaries have only been crudely approximated~~

~~in the present survey. Average grain size in the core is~~

~~perhaps 2 -3 mm.~~

~~The Hinemile stock is quite a uniform unit so far~~

~~as can be seen in the field. Modal data (Table 4) were~~

~~obtained from thin-section study (see Appendix A), and~~

~~because of the coarseness of the samples these are not~~

~~regarded as being tightly controlled; the data are only~~

~~sufficient to define a granodioritic rock with a high~~

~~quartz content.- See Appendix A for a detailed petrographic~~

~~description.~~

~~The stock is cut by numerous aplitic and some~~

~~pegraatitic dikes and veins. The aplites are sugary-~~

~~textured quartz-feldspar rocks whose exact mode was not Figure 30: Pictures of Typical Mid-Tertiary Units~~

~~(a): A photograph of a quartz dike which outcrops in Apache Pass, near the road and just north of the crest of the pass. This dike outcrops more boldly than most but is otherwise typical of the type/~~

~~(b): The Ninemile granodiorite stock as seen from the west. The body is quite distinctive in its weathering characteristics, and is very massive. 2 5 /~~

Figure 30: Pictures of Typical Mid-Tertiary Units 252 determined.' The pegmatites are simple quartz-feldspar- muscovite pegmatites, in which no rare minerals were ob served. They are much less common than the aplites, and are, indeed, the only pegmatites observed in the entire

~~Dos Cabezas range.~~

Schlieren are present in some localities, and are defined by long parallel bands of dark minerals in a granitoid ground mass; the ends and sides of these are rather vaguely defined and they appear to have been marginally dissolved by the magma. . Numerous large quartz pods and veins are present in the stock, and numbers of them are present in the surrounding terrain. More will be said about these quartz bodies later.

~~Miscellaneous Granitoid Intruslves~~

~~of Possible Mid-Tertiary Age~~

~~There are a number of small intrusive granitoid bodies of diverse type which crop out along the northern boundary of the range, and which seem likely to be related~~

~~to the Mid-Tertiary pluton group rather than the Precambrian units. Each will be described individually below.~~

~~317 Quartz Honzonite~~

~~In the south central part of sec. 1 5 , T13S R26E, 253 there crops out a small coarse-grained quartz monzonite plug or stock which cuts the Cowboy1 quartz diorite, the~~

~~Polecat quartz monzonite, and the fault zone previously~~

Invaded by the Cowboy, The mode of this unit, the 31? quartz monzonite, is given in Table 4. It is a leucocratic blotlte and hornblende bearing quartz monzonite, and looks in the field not unlike the Ninemile granodiorlte.

In the field it has an outcrop of several acres size, most of which is composed of large rounded boulders partially protruding from the alluvium; solid outcrop is only visible in a few places. The intrusive shows a marked but weak internal flow foliation.' The Cowboy stock seems to be altered along the contact zone, with a marked lightening of the mica being especially obvious.

~~The petrography of the unit is discussed in Appendix A.~~

It is felt that these above-mentioned units are likely to be more acidic differentiates of the Ninemile parent magma; the outer margins of plagioclase crystals in the Ninemile is the same in composition as the body of those in the above aplites and quartz monzonites.

~~All share the presence of modal muscovite, which in the~~

~~Ninemile stock is seen in its associated pegmatites. It is, of course, possible that the units just described are Precambrian or of some other age; no final conclu- • sions can yet be reached. 25k~~

~~Quartz Honzonite Near the Alma Mine~~

About half a mile east of the old Alma mine in the north central part of the volcanic breccia terrain there outcrops a small plug of quartz monzonite of un certain, but post-breccia age. The field sample of this unit has unfortunately been lost, so that no more accurate modal data can be obtained on the unit.

~~Quartz Dikes~~

The last units to appear in the mid-Tertiary history of the Dos Cabezas mountains were a large number of quartz dikes and plugs which outcrop over the whole range.' A picture of a typical dike is in Figure 30(a).

Distribution of these units is^iown on Figure 1. The writer prefers, in the |8dsence of evidence forcing another approach, to assume that all these dikes are of the same age.' The most likely age for the dikes is mid-

~~Tertiary, for the reasons which follow. Quartz pods and dikes are present in the Minemile granodiorlte stock and its environs; quartz dikes cut the Cowboy and Mascot~~

~~stocks of Laramide age; unfaulted quartz dikes and veins are present in the Laramide disturbed zone along the Apache~~

Pass fault zone, and quartz dikes are present parallel to and often interfingering with the mid-Tertiary dacite 255 porphyry dikes which crop out over the whole range. No quartz dikes can be shown to have any older a^e than mid-

Tertiary on the basis of field evidence. Independent evidence for this age for the dikes appearance is provided by the K-Ar age on a sample of rapakivi quartz monzonite at the Old Cottonwood mine, which was taken from among a pile of quartz lumps on a waste dump from a pit dug in a large quartz dike. The age was 32.7“ 3«3 million years, which serves to confirm the field age assignment for the dikes/

The great majority of these units are steeply dipping dikes two or three feet in width and several tens of feet long, but many much smaller veins and many small quartz pods with no particular linear trends are also present in various parts of the range,' A few of the larger dikes are quite flat-dipping. Hineralogically these quartz bodies are generally extremely simple, being composed of 100^ large ahhedral interlocking quartz crystals. Some dikes have small vugs in them, which are lined with quartz crystals. All the quartz is milky.’

~~Joint and fracture surfaces in the dikes and plugs are generally stained a faint to heavy yellow-brown or red- brown color due to iron staining (?).~~

Several quartz dikes have economically significant minerals in them. A large flat-dipping quartz dike in 256 the SHI/4 of the SW1/4- of sec. 3, T14S R27E has small nodules of galena and pyrlte present in it. Tliere has been a considerable amount of past economic development

~~of this body, but no activity is going on at the present day.~~

~~Galena, together with copper carbonates and some gold (based on a sample shown the writer by an old pros pector met coming down the mountain one day) is present~~

~~in small quartz masses associated with the dacite porphyry dikes in Camelback mountain (NW1/4 Sec. 33, Tl^S R26E)~~

~~and galena is present in a small quartz dike in the NE1/4~~

~~of the NE1/4 of the NE1/4 of sec. 2 5 , T13S R26B.1 Galena~~

~~is present in a quartz dike cutting the Cowboy stock where~~

~~it crosses the road to the Klump ranch. CHAPTER 10~~

~~•GEOCHRONOLOGY OF THE DOS CABEZAS MOUNTAINS~~

~~The Dos Cabezas mountains are an unusually complex~~

~~mountain range in the Basin and Range province of southern~~

~~Arizona. They contain an unusually complex Precambrian~~

~~terrain, and a diversified assemblage of post-Bisbee~~

~~(Comanchean) intrusives of various kinds, Most of these~~

~~.bodies of a given general age cannot be interrelated by~~

~~means of classic crosscutting relationships, and in the~~

~~absence of geochronologic data only a very general under~~

~~standing of the chronology of the range can be attained,~~

~~A moderately detailed geochronologlc study, using Rb-Sr.~~

~~and K-Ar techniques, has accordingly been made. Outside~~

~~data on the ages of the sedimentary units of the Paleozoic~~

~~and Mesozoic have been combined with local evidence to~~

~~provide an overall chronology of events in the Dos Cabezas.~~

~~Sampling techniques are described in Appendix B;~~

~~results are discussed in this text,~~

~~Precambrian Geochronology~~

~~Precambrian Pinal Schist~~

The Precambrian Pinal Schist (see Chapter 2) makes 258 up a large part of the Dos Cabezas mountains, and an estimate of its age is of considerable importance in any overall discussion of the geo chronology of the range.

~~There is no. solid geochronologic picture which comes from internal evidence in the Pinal Schist itself.~~

All the age relationships of the Pinal must be determined at present on the basis of known ages of intrusives which cut it or inference from relationships of Pinal outcrops in the Dos Cabezas mountains to dated Pinal exposures in nearby areas.

~~On the basis of data obtained in the Dos Cabezas mountains, there is no evidence that requires an age of more than about 1^70 million years for the Pinal Schist in the range."~~

Assuming that only one major Precambrian raetamorphism of regional character took place in southeastern Arizona, dating that metamorphism anywhere in southeastern Arizona should provide a reasonable value for the age of the

Pinal metamorphism in the Dos Cabezas mountains, which in turn will provide a younger limit for the ages of the sediments and flows deformed and metamorphosed at that time.' Tile Pinal Schist terrain in the Johnny Lyon Hills some thirty miles to the west of the Dos Cabezas mountains has been dated by Silver and Deutsch (I96I). The technique 259 used was the U-Pb concordia Intercept method using zircon samples from the rocks involved* The data give an age of

1720 - 30 million years for the rhyolite flows interbedded in the sediments deformed during the dynamo thermal metamorphism there, and an age of 1660 - 30 million years for the Johnny Lyon granodiorite which intruded the Pinal after it was metamorphosed.' The time of deposition of the Pinal sediments exposed in the Johnny Lyon Hills may then be set at about 1700 - 1750 million years ago, and the metamorphism there dates from somewhere between 1720 and 1660 million years ago.

The Pinal Schist exposed north of the Apache Pass fault in the Dos Cabezas mountains, especially in the western Pinal terrain (see Chapter 2), is quite similar to the Pinal Schist exposed in the Johnny Lyon hills and the Little Dragoon mountains as described by Cooper and

Silver (1964J, and hence I find little difficulty in as suming as a usable hypothesis that the Pinal Schist in the Dos Cabezas mountains and Little Dragoon-Johnny Lyon hills area are part of the same volcanic-sedimentary ac cumulation and were metamorphosed in older Precambrian times simultaneously, about 1700 million years ago.

Since the Pinal Schist by all accounts is an enormous mass of material, and is a minimum of several miles thick in 260 the Dos Cabezas alone, it seems best to expand the time limits found for the Pinal Schist in the Johnny Lyon hills to some extent before using these limits for the Pinal

~~Schist in the Dos Cabezas mountains. There is no present evidence that there are metamorphosed sediments more than~~

1800 million years old anywhere in southern Arizona, and it is assumed that the Johnny Lyon hills metamorphism a little before 1 6 6 0 .million years was continuous with that in the Dos Cabezas range, which provides limits for the possible age of the Pinal Schist in the Dos Cabezas range.

~~Ho more than a general interval of 1660-1800 million years for the time of deposition and deformation of the Pinal~~

~~Schist will be used.~~

As has been remarked in Chapter 2 there is some evidence to support the idea that the large outcrops of quartzite south of the Apache Pass fault in the Dos Cabezas and northern Chirichahua mountains are not true Pinal

~~Schist, but are-rather related to the younger Hazatzal~~

Quartzite described by Wilson (1939)• If so, the simple hypothesis above must of. course be modified, and correla- l tion will be made between the Johnny Lyon Hills and the part of the Pinal in the Dos Cabezas range which lies north of the Apache Pass fault alone.

~~There has not been any detailed dating survey of the type section of the Hazatzal Quartzite, but Livingston 26l~~

(1967) has dated rock units above and below the large masses of quartzite in the Bronco Ledges area along the Salt River in southern Arizona. An ignimbrite sequence under the quartzite gives a whole-rock Rb-Sr age of 1600 - 100 million years, while the Ruin quartz monzonite pluton, which invades the quartz!te-ignimbrite

~~sequence, gives an age of 1420 million years on a whole-rock, biotite, plagioclase, and potash feldspar~~

~~Rb-Sr isochron. Thus the quartzite in this locale~~

~~gives an age of 1400-1600 million years or so; this~~

~~is about 100-300 million years younger than the age~~

~~of the metamorphism of the Pinal Schist in the Johnny~~

~~Lyon Hills. If this mass of quartzite in the Bronco~~

~~Ledges area is even crudely correlative with the quartz~~

~~ites south of the Apache Pass fault in the Dos Cabezas~~

~~mountains some 200 miles to the south (a very tenuous~~

~~assumption) and if the age of the Pinal Schist in the~~

~~Johnny Lyon hills is the same, broadly, as that in the~~

~~Dos Cabezas mountains north of the Apache Pass fault, then~~

~~there is a possible age difference of 1 0 0 -3 0 0 million~~

~~between the Precambrian metasediments on the north and~~

~~south sides of the Apache Pass fault. The correlation~~

~~of the quartzites in the Dos Cabezas mountains with those~~

in the Bronco Ledges may be regarded as an open question; 262 the simplest hypothesis is to assume that all the metamorphic rocks in the range are from the same general mass of metamorphosed strata, and were all metamorphosed in one dynamothermal event. Since no age data refute this hypothesis, it is the one that the writer adopts, although it is far from proven.

~~PreCambrian Plutons~~

~~Seven distinct major PreCambrian intrusives invade the Pinal Schist terrains in the Dos Cabezas mountains.~~

Some of these, thought to be the most critical ones, have been dated, and provide a framework of reference for the undated ones and for the whole Precambrian development of the Dos Cabezas mountain area.

~~Dos Cabezas Rapakivi Quartz Monzonite~~

~~The sample group which gave the best results by the reduction technique used was the suite from the Dos~~

Cabezas rapakivi quartz monzonite. This body is of an unusual petrographic type and is also the largest pluton in the southern part of the Dos Cabezas range, which factors account for its selection for dating. The regression lines for the four samples analysed are almost identical

~~(see Figure 31) and have the following formulas: 263~~

~~Srey S r e6 = 0.7055 +0.0/94 M G?~~

~~SreySr86 = 0 . 7 0 5 5 + 0.0/95 P 6 e7ys r 86~~

The 1-sigma envelope is shown on Figure 32 * 66/2 of all points should fall inside this line; the number of samples is too small to evaluate this criterion. The standard deviation of the slope constant b is 2 .06^, which is the standard deviation applied to the age of the pluton. Age and standard deviation of the age are both rounded off to the nearest 10 million years. The .j. resulting age is 1380 - 30 million years, with an initial

~~Sr8?/Sr86 ratio of 0.7055.~~

~~Eaton Gneissic Granodiorite~~

~~The second PreCambrian pluton analysed by the~~

~~isochron technique is the Eaton gneiss, which on geologic~~

~~grounds seems to be one of the oldest plutons in the northern Dos Cabezas mountains. Four samples were~~

~~analyzed, and produced a good isochron which turned out~~

~~to be somewhat difficult to analyze quantitatively, how~~

~~ever. One regression line was calculated using all four~~

~~samples, but gave an initial Sr8 ^/Sr8^ ratio of 0 .6 9 6 , which appears to be too low for a granitoid pluton. If~~

~~one composes a regression line using samples 854, 841,~~

~~and 845 alone, one gets an initial ratio of 0 ,7 0 3 , which 2 6 4 omX; 'Sr*r/'S/~9* ax y. _y= 0.7034 - O. 0*056 X /vpure 32 lEorosi Gn+/3s/c Quartz A/fonzooita l*//>o/m-&ocA J~socS»ron £ 9 ? J OO .~~

~~Z?ey^e*jyo^7 C~~

~~• ^miaispo.~~

~~iVAtoM /ft>c/r~~

~~/-b/~~

~~Since the formula for a, the initial intercept, contains b, the initial Sr^?/Sr^ ratio cannot be determined accurately for this line, either.~~

The(x, y) values for sample 8^0 lie enough off the line determined for the other three samples so that com putation of a line using all four points increases the slope to an unacceptable value, as mentioned. A visual examination of the isochron reveals no obvious reason why this should be so, but on examination of the data the reason seems clear. The three samples, exclusive of sample 840, which can be used for regression calculation, fit their line so well that addition of another sample which lies just a little below the line pulls it down enough so that the slope of the regression line increases an exceptional amount. Visual examination shows that the regression line computed leaving sample 840 out conforms 268 to the reasonable line through all four points, and this line is used for age and initial strontium-isotope ratio determination for the Eaton gneiss system. 1-sigma width of error for the line and 1-sigma slope error values were determined for the line, although the small number of data points means that these figures are not as accurate as they appear. The age and error are rounded off to the + nearest 10 million years. The age value is 1470 - 30 million years; the two 1-sigma values are essentially Zp each. The isochron for this suite is Figure 3%*

~~Polecat Quartz Monzonite~~

The third PreCambrian pluton dated by the isochron technique is the Polecat quartz monzonite, which is at once the largest pluton in the range and the largest Pre- cambrian pluton. The pluton makes up a large part of the northern Dos Cabezas mountains (see Figure 1).

The Precambrian terrain in which the Polecat crops out is largely difficult of access, and in the areas of access where vehicles can be brought near outcrops the pluton is very deeply weathered. Because of this situa tion it was difficult to obtain large completely unweathered samples for Rb-Sr isochron study. One sample was finally obtained by a day’s blasting of a waterfall, and it was decided to use it for a combined mineral-whole rock 269 Isochron study. The results ere shown in Figure 33*

As can be seen, the Polecat quartz monzonite isochron is different from the others. It should be pointed out that only one whole rock sample has been analyzed so that no final secure judgment can be made about the absolute age or initial S r ^ / S r ^ ratio of the pluton. The present isochron plot is quite suggestive of several age parameters, however, especially when com bined with K-Ar data on the rock.

An isochron line through the biotite, whole-rock, and plagioclase points on the isochron plot conforms rather well to the three points, and the line gives a slope age of 1000 million years with an initial Sr®?/Sr86 ratio of O.8 7 . Error for this line is difficult to evaluate, but is probably about 2%, based on relationship of this age to K-Ar data for the rock. More precise statistical analysis of the isochron plot was not made because it seems that the probable deviation of data points from the line is partly due to incomplete equili bration during essentially solid-state recrystallization of the rock sampled; the variations of the points from the line are not just due to analytical error, and the plot is not susceptible to simple statistical analysis.

The K-Ar date on the biotite from this sample was run-on the same biotite split used for the Rb and Sr 270 + analyses. The K-Ar date is 1010 - 30 million years, and agrees with the isochron age for the rock.

The initial Sr®7/sr^^ ratio of the isochron plot provides the main clue to the rather complex genesis of the pluton. The value, 0.86, is very high, and reflects a considerable history of radiogenic Sr accumulation before development of the present isochron system. The easiest way of explaining the present system would be to assume that an original Precambrian pluton older than the

~~Polecat quartz monzonite was wholly or partially remelted and intruded into a higher position in the crust about~~

~~1000 million years ago. This may or may not have been the case; in the absence of more whole-rock sample analyses of the Polecat; it is impossible to say with certainty.~~

~~If one takes the present analysis of the single whole-rock sample, and presumes the sample had an initial~~

Sr^^/Sr8^ ratio of 0 .70^, the length of time it would take to generate the sample's present Sr®?/Sr®6 ratio of I.I96 would be 14-20 million years. This provides a figure for the maximum age of the pluton.

In the absence of any other whole-rock analyses of the Polecat body, I hesitate to say that a body of near- batholithic dimensions such as the Polecat stock was formed by re-melting of an earlier Precambrian mass. In 271 addition, it should be noted that the plagioclase point on the isochron plot is considerably above the uhole-rock- biotite line; this implies to the writer that all the mineral phases in the rock are not in mutual equilibrium.

It should be noted for purposes of visual examination of this plot that the axes on the diagram are very small in scale compared with those of the other two isochron dia grams (Figures 31 and 3 2 ) and the deviation of the plagio clase point from the whole-rock biotite isochron line is several times the deviation of any of the whole-rock samples from either of the other sample isochron lines in those figures. It is also striking that the potash feldspar point is so far from the Isochron line defined by the rest of the samples in a terrain which has been essentially "cold" since the ending of the 1000 million year reheating. The last point is not a strong, one, since potash feldspar values often fall far off the isochron lines which agree well among other samples of the rocks on which the analyses are run (see Pidgeon, I9 6 7 , p. 295 and 2 9 7 ).

The writer suspects strongly that the present isochron pattern of the samples from the Polecat quartz monzonite results from a marked recrystallization and reheating of the pluton at this 1000 million year point in time, and that the pluton rose into its present 2?2 position in the crust 1^25 million years ago. Nearly com plete redistribution of Sr isotopes took place at this

~~time, viith results as follows.~~

~~The potash feldspar crystals are by far the largest~~

~~in the rock, and this may control the present position of~~

~~the potash feldspar on the isochron plot. This is probably~~

~~at least partly due to flushing of much of the feldspar's~~

~~content of radiogenic strontium out of the sites in the~~

~~mineral in which the radioactive R b ^ originally lay.~~

~~The radiogenic Sr was added to the Sr in the ground mass~~

~~minerals, but this new strontium assemblage didn't equili~~

~~brate with the nonradiogenic strontium inside the feldspar.~~

~~Essentially the Sr^^/Sr®^ ratio outside the potash feldspar 87 crystals is different from that within because Sr migrated oz out of the feldspars during reheating faster than Sr .~~

~~It is alternately possible that the potash feldspar is~~

~~below the isochron line due to slow diffusional leakage of 87 the Sr out of the crystals over the long span of time~~

~~since the 1000 million year event. Both effects may be~~

~~involved in the present disequilibrium picture.seen on~~

~~the isochron plot for the Polecat quartz monzonite.~~

~~Plagioclase taken from the rock is a bulk sample,~~

~~and some of the grains came from inside the potash feld~~

spars, which are richly endowed with plagioclase feldspar 273 Inclusions; It is felt that those plagioclase inclusions inside the potash feldspar picked up a slightly higher proportion of Sr^^ than those in the ground mass; both probably incorporated most of the Sr in their Ga sites.

~~This accounts reasonably for the position of plagioclase on the isochron plot.~~

Biotite lies.wholly in the ground mass of this rock, and is present as rather small crystals. It is felt that this phase is most likely to have been wholly equilibrated with the whole rock in the 1000 million year event; the K~Ar date on the biotite (1010 - 30 million years) serves to reinforce this impression. For this reason, the biotlte-whole-rock line is chosen as the standard for discussion of perturbation of the Polecat pluton.

The exact nature of this 1000 million year old event is difficult to state with any certainty. It is not thought likely that any part of the Polecat was actually remelted, although this is certainly possible.

Probably local high heat flow, perhaps from burled plutons of this age, was responsible. There are plutons of this age in the Franklin mountains and Llano uplift area in Texas (NAS-MEG pub. 1276, I9 6 5 ). There is evi dence of plutonism or thermal metamorphism in the Gold # Butte area in southern Nevada (Volborth, 1 9 6 2 ) and in Arizona in the Yarnell area and in the Hualapai mountains

(Wasserburg and Lanphere, l$66)Thus it is clear that there was considerable geologic activity at this 1000 million year time in many places in the southwest. The age defines what appears to be the youngest group of younger Precambrian events in Arizona.

To recapitulate, the Polecat quartz monzonite is a pluton which first solidified in its present position about 1425 million years ago, and then underwent a strong remobilization, reheating, and recrystallization at 1000-

1010 million years ago. Since that time the terrain has been quite "cool", since the K-Ar age on the rock has not been biased toward the present from that time. Whether all parts of the pluton have been affected in the same way cannot be stated on the basis of present evidence.

~~The absolute dating evidence indicates that the~~

Eaton gneiss is about 1470 million years old; it bears a marked internal foliation which is partly tectonic in origin, and points to a tectonic deformation of at least mild local character in the Dos Cabezas mountains area at this time. The Eaton pluton is essentially synkinematic.

~~The Polecat quartz monzonite appeared in its pre sent position 1425 million years ago or so, and the Dos~~

Cabezas rapakivi quartz monzonite appeared about 1380 million years ago. Both show weak internal feldspar 275 orientation due to flow, but are essentially post-kinematic plutons. Thus it seems clear that whatever tectonic event was involved in the formation of the gneissic structure in

~~five of the seven PreCambrian plutons exposed in the Dos~~

~~Cabezas mountains, the forces involved had ceased by~~

~~about 1425 million years ago.~~

~~Rough Mountain, Sheep Canyon, and Cienaga Quartz Honzonite~~

~~Gneisses •~~

~~Age relations of the Rough Mountain, Sheep Canyon,~~

~~and Cienaga gneisses can only be estimated by inference~~

~~based on intrusive or crosscutting relationships. No~~

~~Rb-Sr data have been obtained on them.~~

~~The Rough Mountain gneiss cuts the Eaton gneiss~~

~~and is cut by the Polecat quartz monzonite, and is, there~~

~~fore, about 1440 - 50 million years old/ It is a syn-~~

~~kinematic pluton which is younger than the Eaton gneiss.~~

~~The Sheep Canyon gneiss can only be dated by~~

~~inference, by assuming that it correlates with the Rough~~

~~Mountain gneiss,5~~

~~• The Cienaga gneiss is cut by the Dos Cabezas rapa-~~

~~kivi quartz monzonite, and is pre-1380 - 3 0 .million years~~

~~in age. There is no reason for it not being part of the~~

~~circa 1450 million year old group of gneissic plutons~~

~~characterized by the Eaton gneiss. 276~~

~~Sommer Quartz Honzonite Gneiss~~

The Sommer gneiss, which lies in the westernmost part of the Dos Ca'oezas mountains, is different from the other gneissic units in the Dos Cabezas in that it contains a large number of amphibolite dikes and sills similar to those in the Pinal Schist around the Sommer gneiss, while all the other gneissic plutons are essentially free of amphibolites. As discussed in Chapter 3, the author

~~thinks that the Sommer dates from before the time of metamorphism of the Pinal Schist which encloses it, and the pluton is thought to be about 1700 million years old.~~

~~Conclusions~~

~~Thus, it can be seen that all the Precambrian~~

~~intrusives in the Dos Cabezas mountains, with the excep~~

~~tion of the Sommer gneiss, can be definitely shown to be~~

~~or reasonably inferred to be part of a 1470-1380 million year old group of plutons, several, of which are gneissic~~

~~in character and indicate a mild tectonic event at about~~

~~1450 million years ago, during the time of their intrusion.~~

~~K-Ar Age Data~~

~~The only area in the Dos Cabezas mountains where~~

Precambrian K-Ar ages were obtained on the Precambrian rock units is the northwestern part of the range; some of 277 these ages are thought to have been biased by the same event which affected the Polecat quartz monzonite 1000 million years ago. Data locations are given on Figure 3 6 .

~~The only Precambrian K-Ar age regarded as being~~

"firm" is that on the biotite from the Polecat quartz monzonite, which date is wholly congruent with the iso chron age for the same unit. The K-Ar date is 1010 - 30 million years, while the most reasonable Rb-Sr isochron age is 1000 million years.

~~Two other Precambrian K-Ar ages were obtained in this area. • The first is on a biotite separate from the "i* Sommer gneiss. It gives an age (see Table 7) of 1100 -~~

20 million years. As previously mentioned, the Sommer has petrographic field characteristics which make the author quite sure that it is about the same age as the metamorphosed Pinal Schist terrain in the northwestern

Dos Cabezas range, or about 1700 million years. The K-Ar age is then about 600 million years too young to represent the true time of appearance of the unit. The reason for this seems to be a marked reheating of the terrain in which the Sommer gneiss outcrops at the time of re mobilization of the Polecat quartz monzonite. The nearest outcrop of the Polecat is about two miles away from the point where the Sommer sample was collected, but the sample collection point is hear the margin of the 2?8 range outcrops, and the# is likely to be quite a bit of

~~Polecat quartz monzonite out under the basin fill of the~~

~~San Simon valleyProngs of the Polecat could extend~~

~~beneath this cover to within a half a mile of the collec~~

~~tion point. Also, the Polecat is several miles across in~~

~~any dimension, and so large a pluton must have had a~~

~~great thermal effect on its walls. There seems no reason~~

~~why the postulated biasing of the Sommer age by the~~

~~remobilization of the Polecat pluton could not have oc~~

~~curred.~~

~~In the hope of determining an approximation to~~

~~the original age of the Pinal Schist and Sommer gneiss~~

~~terrain, a hornblende separate from the coarsest amphi~~

~~bolite in the Pinal was dated. The age obtained was + 1180 - 35 million years. The age, although older than~~

~~that obtained on the Sommer gneiss, is still much younger~~

~~than the probable true age of the Pinal in the area. The~~

~~Pinal in this terrain must be older than 1470 million years,~~

~~since it is invaded along its eastern boundary by the~~

~~Eaton gneiss. This amphibolite, on all petrographic and~~

~~field evidence, is part of the Pinal terrain, and the~~

~~age must be biased toward the present. The amphibolite~~

~~sample was taken from a point about a mile from the lo~~

~~cation of the Sommer gneiss sample, and it is presumed~~

~~that biasing of the amphibolite sample was also caused by 279 the remobilization of the Polecat quartz monzonite 1000 million years ago. The age of the amphibolite, as given~~

~~by the hornblende, is less biased toward this 1000 million~~

~~year, time than the Sommer gneiss biotite age because~~

~~hornblende is more resistant to thermal degassing than~~

~~biotite is (see Hart, 1964).~~

~~The 1000 million year old reheating recorded by~~

~~K-Ar and Rb-Sr data on the Polecat quartz monzonite~~

~~can thus be seen to have affected the Precambrian terrain~~

~~to the west of the pluton as well.~~

~~Tertiary Reheating of Precambrian Terrains~~

~~There has been extensive biasing of K-Ar ages~~

~~toward the present day in most of the Dos Cabezas mountains.~~

~~Figure 36 shows the distribution of Tertiary K-Ar ages~~

~~in the Precambrian and other rock areas of the range.~~

~~Four samples from the Precambrian terrains give Paleocene~~

~~ages, and one gives a mid-Tertiary age (Table 1). Two~~

~~other Paleocene ages in non-Precambrian rock units are~~

~~suspected of also being biased or of representing the~~

~~biasing agent. The only part of the Dos Cabezas mount~~

~~ains Precambrian rock exposure to escape this markedly~~

~~uniform and very strong biasing of its ages was the~~

~~westernmost part of the terrain, and even there some~~

~~Precambrian age biasing went on, as just discussed. The 280 TABLE 1~~

~~Table of Laramide and I-IiA-Tertiary Biased K-Ar Ages~~

~~Sample Rock Age, 106 years~~

~~RCE-653-64 rapakivi 33 (32.7 ± 3*3)~~

~~RCE-829-65 rapakivi 49 (49.1 ± 1.5)~~

~~PED-25-61 rapakivi 52~~

~~RCE-840-66 Eaton gneiss 58 (58.1 - 1.7)~~

~~RCE-853-66 Rough Mountain 51 (51*1 - 1.5) gneiss~~

~~RCE-851-66 Purple volcanic 53 i (52.6 t 1.6) breccia~~

~~RCE-655-65 Maverick stock 56 (55*9 - 1.7) mean age and standard deviation of Precambrian samples~~

~~RCE Nos. 829, 840, and 853 and PED-25-61 is:~~

~~53 - 3*4 x 106 years~~

~~Mean age and standard deviation of all six samples thought likely to represent terrain biased by Paleocene heating or to represent the agent of biasing:~~

53 - 3*0 % 106 years 281 reason for this marked biasing of Precambrian ages into the Tertiary appears to be strong heating of the Precambrian rocks in the affected areas during the- Paleocene, due to largely still-buried plutons associated with the Laramide intrusive events recorded in visible plutons in the range.

~~Within the thermally perturbed area essentially none of the argon present in the rocks at the onset of~~

Paleocene time has been retained. It is, at first, sur prising, that more petrographic evidence of this marked thermal metamorphism is not present, but investigation of diffusional losses of argon from biotite and feldspar (all the affected dated samples were of one or the other of these minerals) reveals that the temperature of the rock mass during argon loss need not have been very high, or if high need not have been of long duration. In addition, most of the rocks in the affected area are previously- metamorphosed sediments or granitic intrusives whose main mineral phases have a wide stability range.

~~There is some petrographic evidence of metamorphism, however, which seems to date from this event. The~~

Escabrosa Limestone in Apache Pass has been converted to marble (Sabins, 1957a), presumably by shallowly buried plutons. The plagioclase in pre-Paleocene rocks in most of the Dos Gabezas range is heavily sericitized and 282 epidotized, and this may be due to this Paleocene heating, although it is perhaps more likely to be a deuteric break down phenomenon.

~~The K-Ar data in the biased Precambrian terrain~~

(see Table 1) are remarkably homogenous, giving an average age of 53 - 3*^ million years for four samples from widely separated areas in the range. One other sample gives a mid-Tertiary age, and it will be discussed later. In addition to these four Paleocene ages from Precambrian rock outcrop areas, there are two other Paleocene dates from the range which may reflect aspects of the same ther mal metamorphism recorded by those in the Precambrian rocks.

~~The first of these latter is that of a plagioclase sample from the purple volcanic breccia in the NE 1/4 sec.~~

4, Tl4s R26E.' The breccia in this local area cannot, on the basis of field evidence along, be shown to be older than its present K-Ar age (52.6 - 1.6 million years), but two miles east of the sample outcrop the Silver Camp stock of earliest Paleocene age cuts up through the breccias, which implies that the breccias cannot be truly Tertiary rocks. This causes the writer to think that the age from the plagioclase is biased toward the present by the same Paleocene thermal metamorphism which shows up in the Precambrian terrain. There are a large number of small dacite porphyry dikes and plugs which cut the area 283 (see Figure 1) but it is thought that if these had been the biasing agency that the age found would have been younger than it is, as these dacite porphyrys are of mid-Tertiary age. The congruence of the presumably biased breccia age with the biased age average in the Precambrian terrain is strong evidence for a common biasing event and time.

~~The Paleocene Maverick stock gives an age which puts it in the group of potentially biased ages (55«9 -~~

~~1.7 million years). This unit might be older than the~~

~~K-Ar data indicate, but this is thought to be unlikely.~~

~~The Maverick is weathered very little compared to the~~

Precambrian rocks which surround it, and bears no petro graphic similarity to them. It has, rather, close af filiations with the Paleocene quartz diorite assemblage, as already described.' Tectonically, the rock is not nearly as jointed or sheared as the Precambrian rocks it invades.

~~All this leads the writer to feel that the Paleocene age of the stock is very likely its true age. It seems very likely that this pluton is one exposed part of the group~~

of biasing plutons which caused the Paleocene thermal metamorphism in the range. Other examples of these plutons may be visible in some of the small intrusive bodies which cut the rapakivi quartz monzonite in the southern part of the range. 284

~~If these two rock ages are added in with the four from known Precambrian rock exposures, the average becomes~~

53 - 3.0 million years. This grouping is slightly tighter than before and has the same mean age; this provides very strong evidence that all six areas bear the age of the same event, namely the Paleocene thermal metamorphism.

~~It is of interest to examine the limiting conditions for this remarkable Paleocene degassing of the Precambrian rocks, especially insofar as the heating temperature and/or times needed are concerned/~~

~~Hart (i960, p, 522) quotes an infinite series solution to the differential equation for diffusion from a slab of half-thickness L:~~

~~/ - e / n z T , [(//A'2y) e - ( ^ ^ 0 ]~~

~~Here B = and F = fraction of gas lost; D L. is the diffusion coefficient. For cases of severe gas loss, the series converges so rapidly that only the first term is significant, and~~

~~F- /- &/?is (e ~ Assuming F = 0.99, and thickness of the mica plate~~

~~= i o o u ,~~

~~Dt = 4.7 x 10“^cm^~~

This model is deemed appropriate since Evernden and Curtis (i960) point out that their experiments on 285 phlogophite indicate that low-temperature diffusion seems to take place normal to the cleavage. Diffusion along fractures and grain boundaries is assumed to be instantan eous, which would mean that argon would escape a rock mass very- rapidly compared with the time it would take it to escape a mineral crystal.

~~Using the above Dt value, if t = 3.2 x 10^3 sec.~~

(106 y.), D = 1.46 x cm.2/sec. This value agrees well with Everenden and Curtis' value of D for phlogophite losses of argon at 200-300°C (Everenden and Curtis, i960, p. 593)• In a time less than the standard deviation ofthe biased ages, it is quite likely that essentially all the pre-Laramide argon could have been removed from the base ment rocks of the central and eastern Dos Cabezas mountains by a heating event of quite moderate intensity.

Hart (1964) has studied maximum temperature versus distance relationships in the contact zones of an intru sive stock. His data apply rigorously only to heat flow from the side walls (radial to) a pluton; heat flow to ward a nearby overhead surface in contact with the atmos phere would be greater, although by how much is difficult to evaluate. Hart's data (1964, p. 510) indicate that a temperature of 200°C could be reached within a zone from 1500 to 12,000 feet from the pluton, and temperatures 286 of 300°C could be attained within a zone from $00 to 5000 feet away from the magma, depending on the model of the

Intrusive chosen. Thus, using Hart*s data, the biasing plutons under the Dos Cabezas mountains could be buried as little as a few hundred feet or as much as a couple of miles, again depending on the heat-flow model of the pluton or plutons causing the heating. If heat flow toward the surface was higher than that to the sides of the pluton, the plutons could be buried deeper still. Also, if the plutons which caused the Paleocene heating are larger than the rather small stock studied by Hart, they could be buried quite deeply. The size of the biased terrain causes the writer to feel that the biasing magma • or magmas are quite large, perhaps of batholithic dimen sions; in such a case, the bodies could be buried perhaps three or four miles below the present surface. Thus, it would not be surprising if there were fairly little surface expression of the intrusions. Again, there are some plutonic bodies, such as the Maverick stock, which seem to be samples of this magma mass. There seems, then, to be no fundamental difficulty in accounting for the biasing of the K-Ar ages in much of the Dos Cabezas mountains to ward Paleocene ages, on the basis of present data.

One of the dated samples gives a mid-Tertiary age (32.7 - 3 .3 million years). This is a piece of rapakivi 287 quartz raonzonite taken from the easternmost edge of the western part of the pluton, and was picked up from a dump of mixed quartz fragments and rapakivi at the old Cotton wood mine. This mine was developed along a large quartz dike; these dikes are thought on the.basis of other dates and general geologic evidence to be mid-Tertiary in age.

~~Their age is probably equal to or younger than that of the~~

~~Ninemile granodiorite stock (2 9 .0 - 1 ,7 million years).~~

~~This implies that after the Paleocene thermal metamorphism,~~

~~there was a mid-Tertiary thermal metamorphism of a very local character, which removed some 85% of the argon which had accumulated in the dated biotite since that Paleocene reheating.~~

~~The geochronologic data from the range then show~~

~~three separate thermal metamorphisms of varied ages. A~~

~~circa 1000 million year heating event is recorded around~~

~~the Polecat quartz monzonite in the northwestern part of~~

the range. A widespread Paleocene (53 million year old) heating and a very local and weak mid-Tertiary heating are both recorded in the range, also. Probably other thermal metamorphic events would have been picked up if wall rocks

~~around the volcanic breccia and the circa 1450 million year old plutons had not all been biased so strongly in~~

~~the Paleocene. 288~~

~~Late Cretaceous-Eocene (Laramide)~~

~~Geochronoloftic Results~~

~~In addition to the Paleocene ages in the biased~~

~~Precambrian terrain and the biased volcanic breccia area,~~

~~four dated intrusive bodies fall into this general early~~

~~Tertiary time period,. and indirectly point to late Cre~~

~~taceous ages for some units not directly dated.~~

~~The oldest of these visible early Tertiary plutons~~

~~is the already mentioned Silver Camp stock, which gives an~~

~~age of 62 A - 1.7 million years on biotite. The biotite~~

~~was extracted from a sample taken from a dynamited road~~

~~cut in the northeastern part of the stock. If 63 million~~

~~years is accepted as the age boundary between the Mesozoic~~

~~and Cenozoic eras, this stock has a position in time which~~

~~lies right on that time boundary and is latest Cretaceous~~

~~. or earliest Paleocene in age.~~

~~As has been previously mentioned, this stock cuts~~

~~the purple volcanic breccia complex in the west central~~

~~part of the breccia terrain, and would seem to place that~~

~~mass in the later Cretaceous as an upper limit on the~~

~~time of its intrusion.~~

~~The Cowboy stock, petr©graphically nearly identical~~

~~with the Silver Camp stock and only a few miles from it~~

geographically, gives nearly the same K-Ar age. A date 289 of 59.0 - 1 .8 million years was obtained on biotite col lected from a strearabed outcrop sample. This stock invades a large fault along its northern boundary, and shows that the fault has not moved since Paleocene time.

These two dated bodies have ages whose standard deviations just overlap, and it seems likely that they are truly slightly separate in time of intrusion; quartz diorite plutonism in the Dos Cabezas mountains may have covered a span of several million years.

~~The third of the major quartz diorite stocks in the range, the Mascot stock, was not dated for two reasons.~~

First, all its primary mineral phases except the potash feldspar have been strongly reacted to secondary mineral phases like sericite and chlorite, which are unsuitable for K-Ar dating; second, the stock outcrops in the area of biased Paleocene ages and might have given a biased age.

These quartz diorite stocks are all slightly older than the biased age terrain near them, and do not seem to represent the main biasing event directly. They may,. however, be older offshoots of the main magma which caused the more widespread later biasing, having left that main mass when it was at deeper levels in the crust.

~~Perhaps the main plutonic mass later rose to a higher 290~~

~~level, skewing the ages in the invaded terrain around the~~

~~magma'to a value somewhat younger than the age of the~~

~~earlier-arriving quartz diorites which are presently ex~~

~~posed at the surface. It seems obvious that the later~~

~~Paleocene biasing event did not affect the areas where the~~

~~earlier Paleocene stocks were; it is to be assumed that~~

~~the later rising magma used channelways which had not been~~

~~blocked by the earlier plutons.~~

~~The already mentioned Maverick stock gives an age~~

~~of 55•9 - 1 .7 million years, which is distinctly younger~~

~~than those of the quartz diorite assemblage. It is the~~

~~same age as the average.age of the biased terrain, and~~

~~again is presumed to be a sample of the biasing magma.~~

~~One of a large number of olivene basalt dikes in~~

~~the western Dos Cabezas mountains was dated by a whole-~~

~~rock technique. The method used was simple: a fresh~~

~~'unweathered piece of the rock was taken from a solid out~~

~~crop in a waterfall and was cut into slabs on a water-~~

~~lubricated diamond saw, and each slab was cut up into a~~

~~number of oblong blocks, each just large enough (about 15~~

~~by 10 by 10 mm.) to fit into the molybdenum crucible of~~

~~the fusion apparatus. All the blocks which, in their~~

~~original positions, had surrounded the block chosen for~~

~~argon analysis, were ground together in a Pica mill and~~

~~the aggregate powder was used for potassium analysis 291 sample to correlate with the argon analysis obtained, from~~

~~the single block.~~

~~By the above mentioned technique, the dike gave~~

~~a date of 47.6 ~ 1.4 million years, which makes it several million years younger than the older intrusives of the~~

~~Cenozoic. As mentioned earlier, these dikes are not~~

~~thought.to be a part of the diabase-quartz diorite-quartz~~

~~monzonite system of intrusives, but to represent an es~~

~~sentially different and later stage of magmatism with a~~

~~different source area.~~

~~All the dated units in the Dos Cabezas mountains~~

~~which come from the Baramide have Tertiary ages. They lie~~

~~in the younger half of the great pulse of Baramide mag~~

~~matic events which have been chronicled by Damon et al~~

~~(1965). These newly dated bodies fall into the same~~

~~general time zone as the other Baramide intrusives of~~

~~' Arizona, and are not chronologically distinguishable from~~

~~them. See Figure 34.~~

~~All age data on the large volcanic breccia ter~~

~~rain must be obtained essentially by inference or outside~~

~~sources. This large mass in some areas cuts Cretaceous~~

~~Bisbee Formation of Comanchean and, possibly, Albian age.~~

~~This would imply a maximum age of about 100 million years~~

for the breccia. In addition, the Cretaceous sediments 292 were folded and faulted in the Laramide revolution before intrusion of the breccia, which presses the time of breccia intrusion toward the present day. The oldest stock which cuts the deformed terrain produced in the Laramide revo lution is the Schieffelin granodiorite in the Tombstone

~~Hills, which is 72 million years old (Creasey and Kistler,~~

~~1962). .~~

If one assumes that the breccias predate all in trusive s in southeastern Arizona, the breccia*s age is between 75 and 90 million years. Presumably, the flows from which the breccia fragments came were extruded between circa 90 million years and the time of breccia formation; this pushes the age of the breccia toward the 75 million year boundary. All the foregoing is rather speculative, however, and the breccia can only be given a *' solid" age of post-baramide faulting and pre-Tertiary on the basis of present internal evidence from the Dos Cabezas mountains alone/

~~Mid-Tertiary Oligocene-Kiocene Plutonism~~

As has been discussed in Chapter 9, three large dike complexes rose in the Dos Cabezas mountains during mid-Tertiary time. The oldest of these is a dike swarm of “Turkey Track” andesite porphyry. This unit dates at

35.2 - 3.1 million years on plagioclase taken from an A/u/Tiber o f /D & fes/Soreryo/ fC-/) /4g ~~£?on$e one/ / ra e n e G f o Asfotc/t/ng /sor/ Rfrsc Oo/r>of? •ro/J Rof+restce A/^srogro/n fry 4 are S A?66. 4 3 p 0 9 ny y /n ff-Ar RTCOS~~

~~293 unvreathered sample. According to the work of Mielke (196^1-)~~

~~and Halva (1961) this rock is a true member of the "Turkey~~

~~Track" dike group in southern Arizona, both chemically and petrographically. The discovery of this dike extends the~~

~~area in which known "Turkey Track" andesite outcrops con~~

~~siderably,1~~

~~The plagioclase on which the date for this unit was made is not thought to contain any significant amount~~

~~of excess argon, based on the relationship of its age to~~

~~that of a dacite porphyry dike which cuts it (see below).~~

~~Excess argon is often a problem in dating plagioclase from~~

~~young intrusions (Livingston et al, 1967), but the amount v of excess argon encountered varies quite a bit, and in~~

~~this case seems to be so low as to not bias the date" on the~~

~~sample significantly,~~

~~A large number of dacite porphyry stocks and dikes~~

~~cut the Dos Cabezas range in all its parts. One of these~~

~~dikes, from a blasted outcrop on an old loading platform at~~

~~the Mascot mine north of the village of Dos Cabezas, has~~

~~been dated by the whole-rock technique previously described, + and gives a date of 33*9 - 1,0 million years. Presumably~~

~~the whole-rock age is a minimal age, as the ground mass~~

~~mineral phases are microcrystalline and some diffusional~~

~~loss of argon is bound to have occurred from then. It~~

is notable, however, that only about 1 million years in 295 mean age separates the age of the “Turkey Track" from that of the daoite porphyry, and the dacite porphyry clearly cuts the “Turkey Track" in field outcrops; thus the field and geochronologic relationships are correlative.

Any tendency for the plagioclase to hold significant ex cess argon or for the dacite porphyry whole rock to lose significant argon would have widened the age gap "between the two units, and it seems quite certain that the ages given these two units are well within one standard devia tion of being absolute.

. Between these two dike systems in time, cutting the “Turkey Track" and cut by the dacite porphyry, is a very long dike swarm of hornblende andesite porphyry which generally parallels the "Turkey Track" swarm, a little to the south of it. Based on known ages and field relations, the hornblende andesite porphyry is 3^*5 ~ 1*0 million years old.

The last intrusive event of mid-Tertiary time was the intrusion of the large Ninemile granodiorite stock into the northeastern Dos Cabezas mountains area. It + gives a K-Ar age of 2 9 .0 - 3*5 million years on biotite from a sample collected from a stream-bed outcrop. This is the last dated event in the range. The only discernible younger event is the appearance of the young quartz dikes, at least a few of which cut the Ninemile stock. As 296 discussed in Chapter 9» it is thought likely that all these quartz bodies in the Dos Cabezas sire of the sane general mid-Tertiary age. Based on crosscutting relations with the

~~Ninemile stock, it can be said that these dikes are about~~

~~30 million years old, or less. Mo meaningful younger limit can be put on their age.~~

~~The mid-Tertiary events in the Dos Cabezas moun tains fall into the early part of the mid-Cenozoic pulse of magmatic events (Damon et al, 1966; also see Figure~~

~~34). The*spread of dated plutonic events, at least, in~~

~~the Dos Cabezas mountains is less than the overall spread~~

~~in the Basin and Range province of the southwest in general, with a total of some 3^ million years being~~

~~covered by definite dates. Part of this clustering may be illusory, as the breccia terrain is older than any dated non-Prccambrian intrusive.~~

~~Overall Chronology of the Dos Cabezas Mountains~~

~~The Dos Cabezas mountains contain an extensive~~

~~Precambrian and Tertiary plutonic assemblage. They also~~

~~have a representative selection of Paleozoic and Mesozoic~~

~~sedimentary units from the area. Figure 35 shows all~~

the dated and bracketed events described in the foregoing work placed in one overall pictorial chronology. 297 The overall pattern seen on the figure illustrates the four gross units of the time sequence. The first covers the sequence of Precambrian sedimentation, meta morphism, and plutonism, and lasts perhaps 750 million years/ The second covers the youngest Precambrian time period, in which no events of any kind are recorded, for some 4-00 million years. The third period is the long period of Paleozoic and Mesozoic sedimentation and minor epeirogenic deformations. The last unit is that in which the very strong Laramide deformation and the later locally weaker Basin and Range orogeny (disturbance?) took place in the late Cretaceous and early half of the Cenozoic together with intensive Lar amide and mid-Tertiary magmatism. Each succeedingly younger part of the record is somewhat better detailed, in part because of better preservation of the records of the times, but also in part because very de tailed geo chronologic work has not been done in much of the Precambrian of Arizona as yet. This especially applies to the enormous mass of Pinal Schist metasediments in the

~~Basin and Range of southern Arizona, which in the Dos~~

~~Cabezas mountains alone holds four times as much strati graphic information as does the entire Paleozoic and Meso zoic sedimentary record.~~

~~Recorded sedimentation in the Dos Cabezas range / falls into two time periods of very unequal length, 29S~~

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300 separated by almost a billion years in which no preserved units, are present. It is considered unlikely that no stratigraphic units were laid down in this vast expanse of time, but rather it seems likely that the units which were laid down were removed by erosion during that vast

1000 million year long interval; obscure epeirogenic and even orogenic uplifts and disturbances, like the 1000- million year old event recorded in the northwest part of the mountains, probably took place in this interval.

The sole dynamothermal metamorphism recorded is that of the Pinal Schist, the oldest unit present in the range. Other thermal metamorphism is found, however, at several times in the past. These is a general corres pondence between intensity of metamorphism of either kind and intensity of structural deformation, as might be expected.

Thus, the broad pattern of the Dos Cabezas mountains is drawn. A range which owes its present high elevation and geographic position to recent and rather weak events in its history has preserved one of the most complex patterns of sedimentation, metamorphism, tectenism, and plutonism presently exposed in southern Arizona. It is the writer's hope that the integration of numerous comparatively detailed studies such as this one will result in construction of a 301 chronology of events for southern Arizona which reveals

~~Its past In something like its original form and complexity. CHAPTER 11~~

~~THE DOS CABEZAS MOUNTAINS IN THE FRAMEWORK~~

~~OF THEIR IMMEDIATE SURROUNDINGS~~

The discussion of the Dos Cabezas as an entity unto itself, unrelated to the surrounding mountain ranges, would be partial and incomplete. The range shares a number of common features with those ranges surrounding it, and also contains some unusual features of its own.

~~The writer will briefly sketch the history of the Dos~~

~~Cabezas mountains in terras of its major parts, and then consider each part in its turn and relate significant features in surrounding mountain masses to it.~~

~~A brief outline of Dos Cabezas features in a chronological order would be as follows:~~

~~1. Precambrian deposition of sediments and flows~~

~~2. Precambrian metamorphism and tectonism~~

~~3. Precambrian acidic intrusion~~

~~4. Precambrian thermal perturbation~~

~~5. Paleozoic deposition and deformation~~

~~6. Post-Permian Pre-Comanchean deformation and volcanism~~

~~7. Comanchcan Bisbee deposition~~

~~8. Post-Comanchean Pre-Paleocene deformation 302 303 9. Post-Comanchean volcanism~~

~~10.. Post-volcanlsm Intrusion~~

~~11. Basin and Range faulting and uplift~~

~~The surrounding ranges to be mentioned In the dis cussions are the Pelloncillo, Chlricahua, Dragoon, Little~~

~~Dragoon, Mule, Galiuro, and Winchester mountains, and the~~

~~Pat Hills and Johnny Lyon Hills.~~

~~Precambrian Deposition~~

Deposition of Pinal (and, possibly, Hazatzal) stratigraphic units in the Dos Cabezas has been described earlier. There is considerable variability in the Pinal, with thick conglomeratic sequences, thick quartzite se quences, numerous volcanic flows, and one small intrusive unit present along with the more usual metamorphosed sedimentary arkoses and shales.

Extensive Precambrian metasedimentary outcrop areas are present in the nearby Mule, Chlricahua, Dragoon, and Little Dragoon mountains. Extensive Precambrian ter rains in the Graham range north of the Dos Cabezas moun tains are essentially ununvestigated.

~~Tne Precambrian sediments in the Mule mountains~~

~~(Ransome, 1904; Gilluly,1956) are typical Pinal Schist composed of metamorphites formed from fine-grained pelitic sediments. Minor intercalated basic flows are present. 304~~

~~Precambriam sediments in the Chirichaliuas (Sabins, 1957a,~~

~~1957b) contain, south of the Apache Pass fault, large quartzite units similar to those south of the fault in~~

~~the Dos Cabezas. The two terrains are thought to have been essentially continuous before intrusion. Other~~

~~Precambrian metamorphosed units north of the Apache Pass~~

~~fault are, where visited by the writer, "normal" meta morphosed politic sediments. Precambrian sediments in~~

~~the Dragoon range (Gilluly, 1956) were pelitic quartz~~

~~and feldspar bearing sediments with minor intercalated~~

~~basalt flows. In the Little Dragoons there are several~~

~~distinctive units in the Pinal, including a thick grey-~~

~~wacke sequence containing small lithic fragments, a large~~

~~basalt flow or group of flows, and a thick rhyolite flow,~~

~~together with the usual metamorphosed quartz rich pelitic~~

~~sediments. A few thin and discontinuous pebbly lenses~~

~~are present in these latter.~~

~~The available data indicate that the Precambrian~~

~~sediments of the Dos Cabezas mountains were exceptionally~~

~~diverse in character, relative to the Precambrian terrains~~

~~exposed in the surrounding mountains. Perhaps the most~~

~~extensive accumulation of volcanic units known in the •~~

~~Precambrian of southern Arizona occurs in the range, along~~

~~with an enormous thickness of pebbly cross-bedded sediments~~

~~which are evidently unique to the Dos Cabezas, and a thick 305~~

~~sequence of quartzite beds not directly relatable to any units in the surrounding ranges. Large areas of politic~~

~~sediments, similar to those making up most or all of the~~

~~Pinal in surrounding ranges, are also present. The only~~

~~type of Precambrian stratigraphic unitnot yet observed in~~

~~the Dos Cabezas but present in the surrounding ranges is~~

~~the greywacke found in the Little Dragoons.~~

~~Precambrian Iletamorphism and Structural Development~~

~~The Precambrian sediments of the Dos Cabezas have~~

~~been dynamothermally metamorphosed into the greenschist~~

~~facies of regional metamorphism, together, with a local~~

~~superimposed second thermal metamorphism which caused~~

~~nonaligned books and small crystals of biotite to be~~

~~developed in local areas. The Pinal is characterized by~~

~~a few broad, open folds; Precambrian faulting, if present~~

~~in the Pinal, is obscure. Foliation strikes northeast~~

~~to east, with steep to moderate dips.~~

~~In the Mule mountains to the southwest, and in~~

~~.the Dragoon mountains north of them, the Pinal has general~~

~~ly a greenschist facies of metamorphism, with basic flows~~

~~represented by oligoclase-epidote-chlorite-hornblende~~

~~assemblages and the sediments characterized by chlorite-~~

~~muscovite -quartz assemblages. Occasional detrital (?)~~

oligoclase and microcline are present. The outcrops are 306 only small and scattered, and Gilluly gives only his general impression that the foliation has a predominant northeast trend; dips shown on his map are generally steep; no data on folds are available but none are obvious . on the map.'

~~In the Little Dragoon mountains and Johnny Lyon~~

Hills to the west of the Dos Cabezas, the Pinal again has a general greenschist facies rank, in which meta morphosed basalts have an albite-quartz-chlorite-epidote- muscovite-(calcite) facies assemblage. Metamorphosed sediments have a quartz-chlorite-biotite-sericite facies assemblage. In the Johnny Lyon Hills and Little Dragoons, the foliation strikes are northeast for the most part; in the Johnny Lyon Hills the dips arc steep and generally to the south, and the beds are overturned to the north. In the Little Dragoons, the foliation is north-northeast to northeast generally, with steep dips to both north and

~~south, and beds which generally are upright and face northwest; there are local isoclinal folds which cause~~

~~the beds to face southeast. No large folds are visible~~

~~in the Pinal in these mountains.'~~

~~Generally, the Pinal Schist netamorphics in the~~

~~Dos Cabezas range have a common greenschist facies metamorphic mineralogy with the Pinal in other nearby ranges.~~

~~Structurally the Dos Cabezas Pinal has an average strike 30? and dip of foliation, but an anomalous fold pattern.~~

~~Precambrian Acidic Intrusions~~

~~A number of Precambrian intrusives of quartz monzonitic character (seven in all) and of various sizes and ages invade the Pinal Schist in the Dos Cabezas.~~

~~Intrusive bodies of generally unknown but presumed Pre cambrian age cut the Pinal in many surrounding ranges, or appear as separate phaneritic crystalline basement blocks.~~

~~Only one of the observed bodies bears much resemblance to any Precambrian intrusive in the Dos Cabezas. All the larger surrounding Precambrian intrusives will be very briefly described.~~

~~In the Tombstone Hills, Gilluly (1956) mentions an albite granite on the west slope of Ajax Hill. This rock has suffered marked deuteric action, and now shows~~

~~circa 2 cm. orthoclase phenocrysts with circa 4 mm.~~

~~albitized oligoclase phenocrysts, small chlorite crystals,~~

~~and quartz. A weak gneissic texture is present.~~

~~In the Dragoons, Gilluly notes two outcrops of~~

~~extensively chloritized and brecciated quartz monzonite.~~

~~They contain sausseritized plagioclase, microcline-~~

~~perthite, muscovite, quartz; and chlorite. He assigned~~

~~these to the Precambrian on the basis of their contact~~

~~with the Bolsa quartzite, which appears to rest on them 308 atop a sedimentary nonconformity.'~~

~~In the Little Dragoons, Cooper and Silver (1964) describe a complex of "rhyolite porphyry" intrusions of~~

Precambrian age which cut the Pinal. These stocks are really dacite porphyries, containing partially resorbed quartz crystals and sericitized sodic plagioclase cry stals (circa with minor magnetite-ilmenite- bio.tite aggregates in a very fine-grained ground mass of quartz, albite, and potassium feldspar." This unit is grossly quite similar to the small dacite porphyry stock in the Precambrian terrain in the eastern Dos Cabezas.

~~The Johnny Lyon granodiorite makes up much of~~

~~the Johnny Lyon Hills, and has been studied by Cooper and Silver (1964). The rock is a medium-grained to~~

~~coarse-grained somewhat porphyritic hornblende-biotite granodiorite. 1 0 -1 5 mm.' plagioclase (zoned normally from~~

~~An32~25 An20-22^ crystals, 1-5 mm." microcline crystals, quartz aggregates up to 15 mm., biotite and hornblende~~

~~up to 6 mm,, and sphene crystals to 2 mm, are all present.~~

~~Epidote family minerals are ubiquitousNo foliation or~~

~~flow textures are mentioned. The rock is more basic~~

~~than any Precambrian unit in the Dos Cabezas. The unit~~

~~is also older than any definitely dated unit in the Dos~~

~~Cabezas; Silver and Deutch (1961) report a U-Pb concordia~~

~~age on zircons of 1660 - 30 m. y. This is the only 309 Precaiabrian intrusive in the mountains surrounding the Dos~~

~~Cabezas which has been dated.~~

~~The Tungsten King granite crops out on the west~~

~~side of the Little Dragoon mountains, and has been de~~

~~scribed by Cooper and Silver (196^). AIMtic plagioclase~~

~~in 1-10 mm. sizes full of sericite and epidote; micro-~~

~~cline perthite crystals up to k- cm.' long, 1-2 mm. biotite~~

~~and muscovite blocks, and interstitial (?) quartz grains, make up the rock. It is more acidic than any Precambrian~~

~~Dos Cabezas pluton.'~~

~~The Granite Gap granite makes up a large fault 1 block in the central Pelloncillo mountains (Gillerman,~~

~~1958). It is a medium-grained granite with microcline~~

~~perthite, quartz, small biotite crystals and sparse plagio~~

~~clase.~~

~~In the Chirichahua mountains, described in part~~

~~by Sabins (I95?n, 1957b) the Precambrian intrusives within~~

~~a few miles of the southern boundary of the Dos Cabezas~~

~~are simple extensions of the intrusives found there, for~~

~~the most part. The Dos Cabezas rapakivi quartz monzonite~~

~~and the Polecat quartz monzonite are both represented in~~

~~areas in the Chirichahua mountains which the writer has~~

~~visited. Sabins did not break down his Precambrian in~~

~~trusives into many separate units, so the number of other 310 units present is not known. The writer knows of at least one undescrlbed granitoid unit between Apache Pass and~~

~~Bowie mountain on the south side of the Apache Pass fault.~~

~~The writer has no description of the other Precambrian intrusives exposed further to the south in the Chirichahuas.~~

~~It seems interesting to the writer that the Dos~~

~~Cabezas mountains alone contain as many Precanbrian in trusive acidic units as do all the surrounding mountains.~~

~~Probably detailed study of the Graham range, for example, would reveal many more, but at the present time the Dos~~

Cabezas contain the largest single collection of separate described Precambrian Igneous rocks of any range in south eastern Arizona.' It would seem that the range is located along some zone which was exceptionally active in the

~~Precambrian as far as intrusive activity is concerned.~~

~~Another interesting point is that the bodies of~~

~~intrusive rock in the Precambrian outside of the Dos~~

~~Cabezas mountains are generally either more acidic or more basic in character than the large plutons in the~~

~~Dos Cabezas. It seems that the Precambrian quartz raon-~~

~~zonitic plutons in the Dos Cabezas range form a very~~

~~coherent magmatic group, even over the some 200 m.y~~

~~that is presumed to be covered by their history. 311 Precambrian Thermal Perturbation~~

The Precambrian granitoid and metasedimentary rocks in the Dos Cabezas in its northern and western parts show evidence of a strong thermal metamorphism at 1000 million years ago. Both Rb-Sr and K-Ar data (see Chapter

~~10) reveal marked perturbation of their true ages and resetting of the geochronometric systems at that time.~~

As discussed in Chapter 10, the exact nature of this thermal event is unknown. Since 1000-million year old plutons are known from Texas, the simplest hypothesis is to ascribe this heating to 1000-million year old plutons which lie below the present erosion surface in the Dos

~~Cabezas mountains.~~

~~Paleozoic Sedimentation~~

~~The eroded surface on which the Paleozoic sedi ments are deposited locally shows considerable relief.~~

~~In the Dos Cabezas mountains the amount of relief is~~

~~seldom more than a few feet, and the surface is quite~~

~~planar. Cooper and Silver (1964) discuss the Bolsa-~~

~~Apache Group unconformity at some length, showing that~~

~~considerable relief exists along it in the Little Dragoon~~

~~range, with small valleys up to 200 feet deep present in~~

~~part of the Apache terrain. Sabins (1957a) mentions 312 that, in the Chiricahua mountains, the large quartzite mass in Bowie mountain existed as a monadnock on the old~~

~~PreCambrian surface which was otherwise quite flat~~

~~(peneplained). Gilluly states that in central Cochise county the Bolsa was deposited on a flat, smooth erosion surface largely cut on the Pinal Schist.~~

The sequence of Paleozoic sediments is much the same over the whole area. The most significant feature of the sequence is probably the absence of Silurian rocks from the section, which is generally the case over all of

~~Arizona (Wilson, 1 9 6 2 ). This fact seems to imply a broad epeirogenic uplift over the whole of Arizona at this time.~~

Possibly, similar upwarps are represented by the disconformity separating the Devonian and I-lississippian sequences, which apparently (Gilluly, 1956) are separated by a considerable time period, although no hiatus can be observed in the field.

~~Post-Permian Pre-Comanchean Deformation and Volcanism~~

~~Between the time of the deposition of the Concha~~

~~Limestone and the Lower Cretaceous Bisbee Formation~~

~~(Comanchean) a period of tectonism recorded mainly by small displacement vertical faults, and an accompanying period of volcanism appeared in an area around the Dos~~

Cabezas mountains. 313 In the Mule mountains, Gilluly (1956) records a large anticlinal structure or homodine (?) which dates from this time and is the most well-developed and exposed structure in the area being considered. The tilted Paleo zoic beds are overlain by the nearly horizontal Glance

~~Conglomerate, with a profound angular unconformity. These tilted Paleozoic sediments were invaded by the Jurassic~~

~~Juniper Plat Granite (177 ia.y. by Rb-Sr)(Creasey and~~

~~Mistier, 196^), which has also been eroded and then covered by the Glance Conglomerate. No volcanics are present below the Glance in the Mules.~~

~~In the Dragoon mountains, Gilluly mentions~~

~~Cretaceous (?) andesites in the South Pass which are thought to be pre-Bisbee, These are bounded by faults of unknown age.~~

~~Cooper and Silver (196*1-) mention the pre-Bisbee~~

Walnut Gap Volcanics, which cover some faults in the under lying Paleozoic units (Cooper and Silver, 196^, p.; 103) and are also cut by pre-Bisbee faults, These volcanics were evidently deposited during the midst of the pre-Bisbee structural deformation,

Gilluly points out that the basal Glance Con glomerate of the Bisbee formation lies, in the Dragoon mountains, on units ranging in age from older Precambrian (Pinal Schist) to Permian (various formations of the Naco

~~Group). The same situation exists in other ranges near the Dragoons, including the Dos Cabezas mountains, Sabins,~~

~~(1957b) shows, for example, that the basal Glance Con glomerate of the Bisbee rests upon various units of the upper Paleozoic section at various places in the northern~~

Chirichahua mountains and the southeastern Dos Cabezas mountains. The Glance lies atop one or another of the formations of the Naco Group in most of the ranges sur rounding the Dos Cabezas range, and it is not thought that the pre-Bisbee surface was particularly rough in most places.' This is not so in the Mule mountains, of course, and is not so in one area of the Dos Cabezas mountains.

~~In terms of classic erogenic terminology, this generally rather mild deformation and erosion interval before the deposition of the Bisbee Formation corresponds to the~~

~~Nevadan orogeny of Jurassic age; the degree of correlation is difficult to determine, as little of the pre-Bisbee deformation in the Cochise County area can be or has been dated accurately.'~~

Within the Dos Cabezas mountains there is evidence of major structural disturbance at this time along a line trending north-south through the present Silver Camp stock area a few miles west of Dos Cabezas village. East of 315 this line the Paleozoic sequence is present and overlain by

~~fine-grained elastics of the Bisbee Formation. No basal~~

~~Glance Conglomerate is present in the Bisbee strata.~~

~~West of this line the Paleozoic and Mesozoic strata ex~~

~~posed east of the line have vanished, and limestones and~~

~~lime-pebble conglomerates of probable, but not certain,~~

~~Bisbee age sit on the older PreCambrian Pinal Schist.~~

~~A locally preserved block of the Permian Concha Limestone,~~

~~a unit not normally found in the Dos Cabezas range, is pre~~

~~sent along the trend of this line, and probably owes its~~

~~existence to downfaulting during the development of this~~

~~feature, which must essentially be a north-south trending~~

~~normal fault upthrown on the west.~~

~~Structures in the Bisbee (?) units on both sides~~

~~of the line area also distinct. The lime-pebble con~~

~~glomerate on the west side of the line lies in a simple~~

~~homoclinal block dipping some 20° to the southwest, while~~

~~the Bisbee elastics on the east side of the line are part~~

~~of an overturned, steeply-dipping sequence of Paleozoic~~

~~and Mesozoic sediments.~~

~~Deposition of the Bisbee Formation~~

~~The Bisbee sequence is best developed in the Mule~~

~~mountains (Gilluly, 1956)» where it can be divided into 316 four formations: these are the Glance Conglomerate, the~~

~~Morita Foramtion, the Mural Limestone, and the Cintura~~

~~Formation. In the Mules these are easily separable, but to the north and east the Mural pinches out and the~~

~~Morita and Cintura become inseparable. The name Bisbee~~

~~Formation (Gilluly, 1956) seems most appropriate for the~~

~~Bisbee units outside of the Mule range.~~

The Bisbee sequence is always thick, and outside of the Mules consists almost wholly of fine-grained pelitic sands and shales, with a coarse conglomerate unit at its base. The basal Glance Conglomerate of the Bisbee is usually quite thick, and "runs upwards of a thousand feet in places. It thins toward the Dos Cabezas mountains, and in most of the range is only a few feet to tens of feet thick; one considerable channel fill about two hundered feet thick is present in a local accumulation at the base of the Glance in the east central Dos Cabezas mountains

~~(see Figure 1).~~

~~Post-Comanchean Pre-Paleocene Deformation~~

~~The major post-Precambrian deformation in south eastern Arizona took place in the interval between deposi tion of the Bisbee Formation and the onset of Cenozoic time.~~

The deformation is characterized by a certain set of features in the Dos Cabezas mountains and all the surrounding 317 ranges.1 In all the ranges in Arizona under discussion, there are numerous steep reverse faults and shallow

~~thrusts whose planes dip to the southwest or west and which strike northwest to north, in general. The move ment is generally with the southwestern block upthrown~~

~~over the northeastern one, and more than one such fault~~

~~is usually present in a given mass of rock more than a~~

~~square mile or so in area/ In all cases the affected~~

~~part of the range is along their southern or southwestern~~

~~portion, and the central and northern part of the ranges~~

~~form the generally stable block onto which the upward~~

~~and northeastward moving blocks advanced. In all the~~

~~ranges these thrusts and reverse faults are broken by~~

~~normal faults transverse to the strike of the former,~~

~~and the displacement along these offsetting faults is~~

~~usually small.~~

~~It seems quite clear that the deformation in the~~

~~Dos Cabezas which dates from this period is part of an~~

~~amazingly uniform structural pattern developed during~~

~~this time in southeastern Arizona. This deformation, at~~

~~least crudely if not in an absolute chronometric sense,~~

~~corresponds to the classic Laramide orogeny of the western United States.'~~

~~- It is interesting to note that the deformation in~~

~~the Bisbee varies locally quite a bit. The Bisbee in the 318~~

~~Mule range lies essentially in a homocline with a very~~

~~shallow easterly dip, while some 15 miles to the north~~

~~the Bisbee in the Dragoons is overturned to the east and~~

~~severely faulted along north-northwest trends. In the~~

~~Steele Hills, east of the Little Dragoons, there is a marked anticline-syncline fold set with horizontal axes~~

~~trending northwest. In the Dos Cabezas, the Bisbee~~

~~gradually overturns to the north from an originally south~~

~~dipping position as one goes from the south to the center~~

~~of the range. The style of deformation of the Bisbee~~

~~varies from place to place, and serves to emphasize a~~

~~certain irregularity in the Laramide deformation of the~~

~~Dos Cabezas and the surrounding area.~~

~~Cooper and Silver (1$)6^) point out that a large~~

~~west-northwest trending fault zone in the southern Win~~

~~chester mountains is almost exactly on strike with the~~

~~Apache Pass fault in the Dos Cabezas mountains to the east~~

~~and the Mogul fault in the Santa Catalina mountains to the~~

~~west. South of this line there is another, largely~~

~~buried west-northwest trending fault (Antelope Tank fault)~~

~~some 30 miles in length. They point out that these fault~~

~~strands are part of the structural belt known as the Texas~~

~~Lineament, which cuts across the general grain of the~~

~~post-Precambrian structures in Arizona, Nevada, and 319 California with a general' west-northwest trend. It runs from west Texas to California.~~

~~The change in trend of the mountain mass of the~~

~~Chirichahua~Dos Cabezas range from northerly to north westerly to west-northwesterly as one goes from the central~~

~~Chirichahuas north toward the north end of the Dos Ca’oezas is one of the more striking regional features of southern~~

Arizona. Jones (I9 6 3 ) and others have pointed out that this may be due to swinging of the strike of deep base ment structural breaks of the area into line with the trend of the Texas Lineament. The pattern is that which would be produced by left-lateral strike-slip in the Lineament zone.' This origin for the main trend of the Dos Cabezas seems quite plausible to the writer, and is regarded as the most reasonable present hypothesis for the trend and gross outline of the range.

~~Several intrusions which invade these major reverse and normal faults in different ranges have been dated by~~

K-Ar techniques. The Silver Camp stock in the Dos Cabezas gives 63 m.y. (this paper), the Texas Canyon quartz monzonite gives 53 m.y. (Damon et al, 1 9 6 6 ), the Schieffelin granodiorite in the Tombstone Hills gives 72 m.y.', and the Uncle Sam porphyry in the same area gives 63 m.y.,

~~(Creasy and Kistler, 1962). 320~~

~~The major Laramide faulting activity seems to have ceased by the onset of Cenozoic time, and perhaps much earlier,~~

~~Post-Bisbee Volcanism~~

~~There are extensive volcanic terrains in all the mountain areas investigated around the Dos Cabezas, None~~

~~of them are free from some structural deformation, but most appear to postdate the majorLaramide faulting and~~

~~folding. Many of the units are true flows, and many are~~

~~ash-flow tuffs of predominately welded character, or are~~

~~air-fall tuffs. Breccias and agglomerates are markedly~~

~~subordinate and are generally thin. No geochronologic~~

~~data are available for these units, and as the separate~~

~~outcrop areas are separated by alluvium-covered valleys,~~

~~and the lithologic patterns in the various groups are not~~

~~correlatable to any marked degree, no clear picture of the~~

~~volcanic stratigraphy can be attained. The writer feels~~

~~that it is curious that so little breccia is present in~~

~~the surrounding units, considering the vast amount of it~~

~~present in the center of the Dos Cabezas range.~~

~~Post-Volcanism Intrusion~~

~~A very large number of stocks and dikes, mostly~~

~~small, and probably largely of Cenozoic age, cut the~~

volcanic sequences in the surrounding ranges. The intrusives 321 are of a number of petrographic types and are seldom de scribed completely; no correlation except very local ones can be made on general lithologic or structural grounds.

The only age data available outside the Dos Cabezas are on the Stronghold granite in the Dragoons, which gives a K-Ar age of 22 m.y. (Damon and Bikerman, 1964). In the absence of reliable petrographic criteria and geochronologic data,

~~the writer feels that it would be pointless to attempt any~~

~~tight structuring of origin times for these bodies.~~

~~Basin and Range Uplift and Faulting~~

~~Uplift of mountain masses as horsts between mar~~

~~ginal normal faults has been given as the mechanism of~~

~~formation of many Arizona ranges of the present day.~~

~~Such bounding faults are generally obscure due to~~

~~burial by alluvium from uplifted blocks, and must~~

~~generally be inferred from secondary evidence. In the~~

~~area around the Dos Cabezas mountains, several inferred~~

~~or visible faults are present. As mentioned earlier,~~

~~Sabins (1957b) infers a boundary fault on the northeast~~

~~side of the Chiricahua mountains on the basis of drill~~

~~hole data indicating a mile or so thickness of alluvium~~

~~a mile or so from the margin of the hard-rock exposures«~~

~~Small normal faults presumably parallel to it are exposed~~

~~locally in the northeast part of Sabins' area. A few 322 faults of normal displacement, quite local and small, are present on the other side of the Chirichahuas southwest of~~

~~Bowie mountain, and may reflect a major normal fault beyond the range margin.~~

~~A large normal fault is present on the west side of the Gunnison Hills in the area investigated by Cooper and Silver (196*0.' The South Camp fault in the western~~

~~Little Dragoons may be such a bordering fault. A normal fault along the central axis of the Steele Hills, though showing small displacement, may be another such fault.~~

~~Gllluly (1956) mentions a fault on the east side of~~

~~Tombstone.inferred from a thickness of over 1000 feet of alluvium only 1000 feet from the nearest Bisbee outcrops, and also a small group of normal•faults near South Pass on~~

~~the west side of the Dragoons; he feels that the best~~

~~evidence for border faults comes from the marked linear~~

~~trends of most of the ranges of this area.~~

~~No direct evidence for boundary faults was seen~~

~~in the Dos Cabezas. An extension of the probable northern~~

~~boundary fault of the Chirichahuas along the northeastern~~

~~margin of the Dos Cabezas is thought likely, but no direct~~

~~evidence compels this interpretation." The dip of the~~

~~Paleozoic sediments on the southwestern boundary of the~~

~~range accounts for the location of the boundary of the~~

~~range in that position without recourse to faulting. although, of course, a fault might be present beyond the exposed rock. APPENDIX A~~

~~DESCRIPTIONS OF PETROGRAPHICALLY ANALYSED SAMPLES~~

~~This section of the dissertation contains detailed descriptions of individual rock samples and compilations of modal data for groups of rocks of some igneous suites.~~

Sample location is keyed in two ways: first, samples are located on the figures in the main text which show the location and outcrop area of the units described in the different chapters (as on Figure 3 in Chapter 2, for example, for all Pinal Schist samples); second, location is recorded by section, township and range coordinates placed just after sample numbers in the following text.

The described samples are grouped into units which match the coverage of the different chapters. Drawings of microscopic views of thin sections are provided where they aid materially in understanding the petrographic character of the units.

~~Petrography of Pinal Schist Samples~~

~~Samples of the Pinal Schist selected for clear~~

~~Illustration of the textures and mineralogy of the major lithologic types arc described below. Some sample descriptions are accompanied by drawings of microscope~~

~~324 325 thin-section views in Figures 37 or 3 8 .~~

~~Samples from the Western Pinal Schist Terrain~~

Phyllites. Samples 4l (HVJ1/4 Sec. 15, T13S R25S), and 258 (SE1/4 Sec. 11, T13S R25S) are simple weakly meta morphosed phyllites. They are composed of very fine grained quartz and sericite containing randomly scattered minute magnetite crystals, all three of which are generally less than 0.1 mm. in crystal size, Biotite is present in some sections of this kind, and usually occurs in contact with magnetite crystals. The quartz and sericite are usually arranged in a well-foliated pattern revealed in section by elongation of quartz crystals and preferred orientation of sericite crystals; this parallels composi tional banding shown by parallel zones containing varying proportions of quartz, sericite, and magnetite. Biotite, where present, tends to form foliation-paralleling ag gregates of rather randomly oriented crystals. These phyllitic units might represent either original clastic shales or pyroclastic air-fall tuff units. The writer tends to favor the first parent rock type, because of the finely laminated character of the phyllites and the high quartz content of some of the laminae. It would seem that tuffs would not, upon metamorphism, show thin laminae varying widely in composition, nor have any 326

~~laminae containing over seventy percent quartz. However,~~

~~some tuffaceous component of these fine-grained meta~~

~~sediments may exist, because there was a great deal of~~

~~volcanisri in the period of time during which the original~~

~~sediments of the western Pinal Schist terrain accumulated~~

~~(see below)♦~~

~~Peldspathic Argillites and Feldsnathlc Phyllites.~~

~~Samples 116 {SEl/k Sec, 23, Y13S R26E), 332 (SE1/4 Sec. ?,~~

T13S R25E), and 35^ (NE1/4 Sec, 24, T13S R25S) are weakly metamorphosed arkoses. In hand specimen they contain conspicuous sand-sized quartz and feldspar crystals, and were called feldspathic argillites or phyllites during

~~the field work, In this section, one sees rounded and weathered grains of plagioclase and rounded grains of~~

~~quartz, all lying in a well-foliated presently phyllitic ground mass. The premetamorphism sediment was evidently~~

~~an arkosic shale or arkose. The ground mass in these units generally have the same character as the phyllites~~

~~just described above and are interpreted the same way.~~

~~Figure 3?f shows a thin-section view of an arkose from~~

~~another Pinal Schist terrain which has the same petro~~

~~graphic character as the meta-arkoses in the western~~

~~Pinal Schist terrain.~~

~~Phyllites Containing Tuff Crystal Debris. Samples 32? 168 (HE1A Sec. 15. T13S R25S), 3^0 (SU1/4 Sec. 1?, T13S~~

~~R26E), 362 (mi/4 Sec. 24, T13S R25S), 936 (SV/l/^l- Sec. 1?,~~

~~T13S R26S), and 939 (SW1/4 Sec. 1?, T13S R26S) are feld-~~

~~spathic argillites similar to those described above in hand specimen. In thin sections of these specimens, how~~

~~ever, the feldspar detritus is seen to consist wholly of plagioclase in sharply angular fragments or markedly~~

~~euhedral crystals (see Figure 37)* These plagioclase~~

~~crystals generally display complex twinning patterns with~~

~~an interplay of twinning on the pericline, albite, and~~

~~Carlsbad laws; these twin patterns have not, perhaps~~

~~surprisingly, been erased by metamorphism. These plagio~~

~~clase crystals are almost universally unzoned. The com positions of the plagioclase, as determined by albite~~

~~twinning extinction angles and optic sign, are commonly~~

~~An27“An^^, but more albitic compositions in the range An^-An^ also appear.~~

~~Quartz in these plagioclase-rich units is usually~~

~~present as sharply angular crystal fragments or as cry~~

~~stals of rather euhedral outline showing resorbtion ea-~~

~~bayments (see Figure 37e), although some well-rounded •~~

~~grains also appear.~~

~~All of these sand-sized crystals, most of which~~

~~have obviously not been transported any great distance 'b'Vt Figure 37$ Hetasediments and a Dacite Porphyry Stock From the Pinal Schist~~

e. Phyllite containing pre f. Arkose. Shows rounded sumed air-fall crystal and recrystallized tuff minerals showing quartz, plagioclase, resorption embayments. and potash feldspar Some sand-sized crystals in phyllite ground- may have "been transported a mass with superimposed significant distance. secondary biotite. p - plagioclase p - plagioclase q - quartz b - biotite q+s - quartz + sericite s+q - sericite + quartz m - magnetite q - quartz k - potash feldspar m - magnetite

Sample 168; NEl/4 Sec. 1 5 , Sample 4 9 6 ; NWlA NWlA T13S R25E N E l A Sec. 35, T14S R28E g. Ferruginous oolite. • h. Dacite porphyry stock, Oolites now broken down with quartz, plagio to spherical assemblages clase and rare potash of magnetite and sericite. feldspar in a quartz- Large leaf-green chlorite sericite-feldspar crystals in groundmass, ground mass. Stringers which is otherwise seri of biotite and musco cite with sparse magnetite. vite lie parallel to the foliation. cl - chlorite k - potash feldspar b - oolite body m - magnetite m - magnetite q - quartz p - plagioclase b - biotite mu - muscovite

~~Sample 937; N W l A S E l A Sample 756? N E l A N E l A SU1/4 Sec. 17, T13S R26E S E l A Sec. 12, T14S R27E 323~~

~~e 2/)y///?e. US/tf) Tuff d /r- yOrAase Z b// Cry3to/ Detritus ^Somp/c. 2 9 3 •3o/np/e /6<9~~

~~fierruy/naus Oof/te /Docsre Porphyry 3toc/r - 5 o m p fe 9 3 7 TSompJe. 353~~

~~f/gore 3 3 A / e r o j « o'/rner?rs one/ o Ooc/te. f^orpriyr-y SrocA From the P/no/ Schist~~

~~Figure 38: Monsedimcntary. Units From the Pinal Schist~~

a. Weakly metamorphosed b. Weakly metamorphosed basalt flow. Original basaltic sill. flow texture preserved; Epidote-chlorite- later chlorite and albite assemblage epidote developed. developed. p - plagioclase phenocrysts e - epidote pm - plagioclase m icrelites cl - chlorite car - carbonate p - plagioclase cl - chlorite sp •• sphene e ** epidote m - magnetite gm - ground mass

Sample 40; M l A Sec. 15, Sample 6^2; SElA NElA T13S R25E NElA Sec. 13, T13S R25E c. Hornfelsed amphibolite d. Andesite flow. Euhedral dike. Radiating hornblende plagioclase with resorb crystals in plagio tion texture. Former clase ground mass; ferr omagne sian min original foliation erals broken down preserved as a relict. to magnetite and biotite; weakly p - plagioclase sheared. hb - hornblende m+b - magnetite + biotite b - biotite p - plagioclase q - quartz gm - ground mass of plag ioclase , potash feldspar, biotite, and sericite

~~Sample 927; WEI A MWlA Sample 3^5; top Hill 5 IS7 S E lA S e c . 27, T A S R28E Sec. 21, T13S R26E 32<7~~

~~A/ornfe/se.d A/np/t/bo/it* dodes/re F/o**. Z)//re , So/np/a PFF Somp/e. ^545~~

F'/gur-e Von^ect/mg nrory Un/te /n The P/no/ >5c/?/sT 330 by running water, lie in a phyllitic ground mass of fine grained and markedly laminated quartz, sericite, and magnetite, quite like that of the phyllites previously described, except that occasional small crystals of chlorite and epidote are seen in the phyllitic material presently described. No potash feldspar or plagioclase is present in the ground mass. Biotite is present in some units in foliation-paralleling aggregates or clusters of rather randomly oriented crystals. The high quartz content of many of the ground mass laminae and the ab sence of feldspar from all the laminae indicates to the writer that this ground mass is essentially a normal politic sediment (now metamorphosed, of course); presence of the epidote and chlorite in the ground mass probably reflects a small amount of original admixed tuffaceous dust and ash.. The sand sized grains are interpreted as a component most likely added to the sediment as air-fall crystal tuff detritus. Of the five specimens of this type studied, one shows quartz in the sand-sized fraction while four do not; the tuffaceous component in the former is dacitic while that in the latter is andesitic.

~~Metamorphosed Lava Plows (M etavolcanics). Samples~~

~~133 ( m / 4 Sec. 27, T13S E26E), 130 (NW1/4 Sec. 34, T13S~~

~~R263), 141 (NE1/4 Sec. 3 2 , T13S R26e ), 143 (SW1/4 Sec. 2 9 , 331 R13S R26S), 149 (MW1/4 Sec. 30, T13S R26E), 311 (Bil/4~~

~~Sec. 19, T13S R26B), 309 (SEl/4 Sec. 13, T13E R2?E), 345 ( m / 4 Sec. 21, T13S R26E), and 392 (SSl/4 Sec. 28, T13S~~

~~R26E) appear to have "been true extrusive surface flows~~

~~(possibly in some part near-surface intrusive sills) ranging in composition from rhyolite porphyry (sample 1 3 3 ) to quartz latite porphyry (sample 1 3 0 ) to dacite porphyry~~

~~(samples 3 9 2 , 149, 3 4 5 , 141) to andesite porphyry (samples~~

311, 143, 309)• In these rocks, as seen in thin section, all the phenocrysts of feldspar have sharply angular or euhedral outlines, and quartz, if present, generally shows marked resorbtion texture in many of the crystals (see

Figure 38d). In some of these rocks, masses of muscovite or sericite, together with biotite in some samples, lie in angularly outlined clumps which are thought to repre sent potash feldspars which have suffered reaction break down to these aggregates during metamorphism. Plagioclase in these rocks is not generally severely broken down, although partial reaction of plagioclase to form sericite and epidote is not uncommon in some samples. The com position of the plagioclase in these flows is the same as that in the tuff-contaminated sediments previously described; that is, An^Q-An^ in general, as measured by albite twin extinction angles and optic signs. This. 332 plagioclase, like that In the tuff-contaminated sediments,

~~is unzoned. Identification of these units as flows rather~~

~~than as tuff-contaminated sediments is based on their ground-mass character, as discussed below.~~

~~The ground mass of the units identified as meta morphosed flows is marked by the presence of one of three~~

~~textures which imply to the writer original crystallization~~

~~of the rock ground mass from a melt. These primary tex~~

~~tures, as previously noted, are only partially obscured by metamorphism in most cases.~~

~~In the first of these presumed originally igneous~~

~~textures, the ground mass is composed of minute inter~~

~~locking crystals of plagioclase, potash feldspar, and~~

~~quartz, with minor epidote, sericite and chlorite. The~~

~~ground mass shows no tendency toward lamination or layered~~

~~compositional variations, and is sheared along separate~~

~~discrete shear planes a few millimeters apart.~~

~~In the second of these textures, the ground mass~~

~~is composed of extremely fine-grained and nonfoli a ted~~

~~aggregates which appear to be an original aphanitic or~~

~~glassy ground mass recrystallized during a subsequent~~

~~metamorphism. Again, the ground mass shows no tendency~~

~~toward compositional banding, but is massive and sheared~~

~~along discrete separate shears a few millimeters apart. 333 The third type of" ground mass is composed of small~~

Interlocking regions of graphic intergrowths of quartz and potash feldspar together with myrmelcitic intergrowths of plagioclase and quartz. Again, the ground mass is mas sive and homogenous and is cut by separate and somewhat isolated shears. A kind of micro-boudinage structure is common in some o f t h e s e sh ea red ground m a sse s.

These volcanic flow rocks cannot in general be separated from the essentially sedimentary feldspathic argillites (metamorphosed arkoses and tuff-contaminated sediments) by any cursory examination of field outcrops or hand specimens. Both show a porphyritic texture and weak foliation. Thin section examination is needed to dis tinguish them.

~~Metamorphosed Limestone.' Sample 886 (HW1/4 Sec. 15,~~

~~T13S R25E) represents a very small weakly metamorphosed limestone body from a predominantly phyllitic sequence at the very western end of the western Pinal Schist terrain.~~

This body is perhaps one hundred feet long and up to five feet wide, and shows marked foliation and banding parallel to the phyllite foliation above and below it.' In thin • section the unit is seen to consist of circa 0.1 mm, crystals of carbonate with deformed twim lamellae and cleavages, containing some five percent of separate 334 scattered round quartz grains. Local bands and clumps of tremolite-actinolite, which contain radiating star-like aggregates of elongated tremolite-actinolite crystals, are present cutting the carbonate

~~Metamorphosed Oolitic Ferruginous Shale. Sample~~

937 (StJl/4 Sec. 1?, T13S R26E), is, apparently, a very weakly metamorphosed oolitic unit once composed of ferrugi nous oolites in a clayey matrix; the present texture con sists of a large number of 2-3 mm. dark gray-black masses showing internal concentric rings marked by bands of various shades of gray, these round masses being in peripheral contact for the most part and lying in a grayish white ground mass (see Figure 376). Mo signifi cant shearing was observed in this unit. In the field this unit covers a small area, perhaps 100 by 5 feet, and lies parallel to the foliation of the surrounding units.'

Microscopically, the rounded masses consist of sericite aggregates containing interspersed crystals and rods of magnetite and spliene (?). The individual sericite crystals are nearly submicroscopic and the magnetite rods are never more than 0.1 mm. long; these latter dark crystals are arranged in crudely concentric zones of varying concentra tion, though each magnetite crystal is always surrounded by sericite. The ground mass between the recrystallized oolites is mostly sericite and magnetite of equal fineness 335 to that in the oolites, "but also eontains numerous leaf- green, low-birefringence, nonpleochroic, poikilitic crystals up to 1 ram. in size, which may be a ferruginous

~~chlorite," These green crystals usually include one or more nonpleochroic, nonbirefringent, bright red-brown~~

~~crystals of high releief about 0.2 ram. in size, which~~

~~are probably ferruginous garnet.~~

~~Metamorphosed Basalt Flow. Sample 40 (MU1/4~~

~~Sec. 15, T13S R25E) is taken from a thick amphibolite body once thought to be a sill like all the others in~~

~~this terrain. Examination of a thin section and re~~

~~examination in the field show clearly that the unit is~~

~~a basalt flow which has been weakly metamorphosed. In~~

~~thin section (see Figure 38a) it shows numerous small~~

~~blocky plagioclase crystals less than 1 mm. in length,~~

~~markedly euhedral and -*-n composition, sitting in a~~

~~ground mass of minus 0.1 ram. microlitic crystals of~~

~~plagioclase of An^g composition (compositions determined~~

~~by albite twin extinction angles for the larger crystals~~

~~and by microlite extinction angles for the small ones),~~

~~together with elongate hornblende and equant magnetite~~

~~crystals. The ground mass shows a marked primary flow~~

~~structure. Chlorite and epidote have partially replaced~~

~~many crystals of the primary minerals. 336~~

~~In the field this flow has a quartzite boulder,~~

~~enclosed in basalt at its base, which not only helps to prove its character as a flow but also makes it possible~~

~~to show that the Pinal Schist in the area is upright.~~

~~. Amphibolites. Aside from the one basalt flow noted above, all the amphibolites in the western Pinal~~

~~Schist terrain in the Dos Cabezas mountains represent weakly to strongly metamorphosed intrusive sills and~~

~~dikes of basaltic composition. These intrusive bodies~~

~~are really all sills of various sizes with minor dikelike~~

~~projections (see Figure 1). These amphibolite units can~~

~~be easily distinguished from the enclosing Pinal Schist~~

~~units of other types, due to their tendency to weather~~

~~out with a marked green color, whereas most of the meta~~

~~sediments and metavolcanics weather out to a red-brown~~

~~color. These sills vary greatly in thickness and are~~

~~often split into two or more subparallel units along part~~

~~of their length; parts of various sills connect with one~~

~~another locally. There are (in this Pinal Schist terrain)~~

~~a very large number of very small amphibolite sills only~~

~~a few feet or tens of feet long and a foot or two wide ‘~~

~~which could not be mapped on the scale at which the~~

~~mapping was done for this project. • 337 Samples 16$ (milA' Sec. 23, T13S R25B), 300 (mi/4 Sec. 18, T13S R26B) f 642 (IJEl/4 Sec. 13, T13S R26E), 828~~

~~(NE1/4 Sec. 3, T13S R26E), and 938 (SE1/4 Sec. 1?, T13S~~

~~R26E) are such amphibolites from the western Pinal Schist~~

~~terrain; they are only a small proportion of the total number of samples taken but present the major features~~

~~of the whole assemblage. Host of those sampled show an~~

~~original hornblende-plagioclase-magnetite assemblage now~~

~~replaced in part by epidote and chlorite; some units~~

~~show epidote-chlorite-plagioclase-magnetite assemblages~~

~~(see Figure 38b). The units are generally fine-grained~~

~~(crystals size below 0.1 ram.) and show well-developed~~

~~internal foliation. A few basic dikes and sills in this~~

~~western Pinal Schist terrain show no netamorphic textures,~~

~~a different mineralogy, and different weathering character~~

~~istics than do the amphibolites, and are generally thought~~

~~to be part of a dated late Laramide group of dikes and~~

~~sills which occupy the same general area as the main~~

~~concentration of amphibolites do in the schist. On~~

~~Figure 1 amphibolites are labelled 3 while the later~~

~~Laramide dikes and sills are labelled b.~~

~~Those metamorphic amphibolite units which do~~

~~show foliation show it in varying degrees; in some it is~~

~~marked, in some less so, and in some it is seen in thin 333~~

~~section but not in hand specimen. Some amphibolites show~~

~~lineation in the foliation surface and some do not. The~~

~~textural range in these units implies to the writer that~~

~~these amphibolites were intruded over a long period of time~~

~~during which the metamorphic intensity in the area gradual~~

~~ly waned.~~

~~Sample 6^3 is of an amphibolite which is quite~~

~~unusual. It is composed of about 80;2 small (minus 0.1 mm.)~~

~~euhedral to subhedral plagioclase crystals, all strongly~~

~~,sericitized, in a very fine-grained ground mass composed~~

~~of sericite and chlorite. This unit may represent an~~

~~anorthosite sill, or it may be some type of surficial~~

~~deposit largely composed of crystals from an airfall~~

~~tuff from which most of the fine material has been removed.~~

~~The ground mass appears to be rather rich in iron and~~

~~magnesium, and this causes the writer to prefer the sill~~

~~interpretation; were the ground mass full of quartz, he~~

~~.would prefer the other.~~

~~The number and size of these amphibolite bodies,~~

~~is the greatest in the westernmost part of the western~~

~~Pinal. Schist terrain, and both size and number drop off~~

~~rapidly as one goes eastward toward the center of the~~

~~range. Neither of the other two Pinal Schist terrains~~

~~have anything like the number of amphibolites that this one does. 339~~

~~Samples From the Eastern Pinal Schist Terrain~~

~~Phyllites. Samples 9 (SW1/4 Sec. 36, T14S R28E),~~

~~919 (ME1/4 Sec/ 6, T14SR28E), 921 (SW1/4 Sec. 6, Tl4s~~

~~R28E), and 929 (SW1/4 Sec. 1, T14S R2?E) are phyllites from the eastern Pinal sequence. They are identical in character to those already described from the western~~

~~Pinal Schist terrain.~~

~~Feldspathic Argillites and Feldspathic Phyllites.-~~

~~Samples 496 (NE1/4 Sec. 35t Tl4s R28E), 7^9 (NE1/4 Sec. 24,~~

~~T14S R27E), 896 (Hilltop 758?, Sec. 20, T14S R28E), 898~~

(SE1/4 Sec. 20, Tl4s R28E) are weakly metamorphosed arkoses similar in many ways to those already described from the western Pinal Schist terrain. Sample 896 is a typical rock of this group. It shows circa 50/» sand-sized grains of quartz and potash feldspar contained in a fine-grained ground mass of sericite and quartz (quartz generally pre dominating ), together with many small euhedral magnetite

crystals. Quartz sand detritus is about four times as abundant as feldspar sand-sized detritus. The quartz grains are anhedral to blocky crystal fragments showing abundant granulation and recrystallization dating from before deposition in their prernetamorphic sedimentary

~~environment. The potash feldspars are brown, filled with 340 sutomicroscopic crystals of alteration mineralsf and occur in rounded crystal fragments up to 0.2 mm. in size. See~~

Figure 37f for a thin-section drawing of a similar arkosic metamorphite from the same area/ Tile rock was an arkosic quartz sandstone before metamorphism, and has only under gone mild recrystallization and weak shearing during its

~~subsequent metamorphism.~~

. Ketaconglomerate. A thick sequence of beds in the westernmost part of the eastern Pinal Schist terrain is largely composed of weakly to strongly metamorphosed con glomerates.' The pebbles and cobbles in these conglomerates are uniformly well-rounded and have ellipsoidal shapes in

~~section and round shapes in plan. In some parts of the~~

~~conglomerate horizons the pebbles are markedly stretched~~

~~and elongated, while in other parts the original texture has hardly been disturbed, and the rock could pass for~~

~~some much younger unit.~~

~~Most of the pebbles are composed of quartz or some~~

~~leucocratic aphanite rock, but dark aphanitic rocks are~~

~~common and make up the larger part of some zones. No~~

~~detailed study of pebble types and characteristics was • made, but undoubtedly much could be learned about the~~

~~pre-Pinal Schist rocks in the Dos Cabezas area by such~~

~~a study. The writer knows of no comparable sequence of conglomerates anywhere in the Precambrian metasediments of~~

~~southeastern Arizona.~~

~~The matrix of the pebbles in these conglomerates~~

~~is arkosic material like that previously described.~~

~~Metamorphosed Lava Flows. Only two lava flow~~

~~units were found in the eastern Pinal Schist terrain,~~

~~samples 49? (SE1/4 Sec. 25, T14S R28E) and 7^9 (HE1/4 Sec. 2k% T14S R2?E).~~

~~Specimen 7ky is from a rhyolite porphyry unit~~

~~containing angular fragments of quartz, showing rcsorbtion~~

~~embayments, together with crystals of potash feldspar,~~

~~both up to 1.5 mm. in size, contained in a uniform non-~~

~~laminated ground mass of quartz, potash feldspar, and~~

~~sericite crystals all less than 0,05 mm. in size.~~

~~Quartz is about four times as abundant as is feldspar in~~

~~the phenocryst phase.~~

~~Specimen 497 is from a dacite porphyry flow~~

~~• showing rounded and rcsorbtion-textured quartz crystals~~

~~together with crystals and crystal fragments of plagio-~~

~~clase of composition An^g, all contained in a ground mass~~

~~of quartz, potash feldspar, plagioclase, and sericite.~~

~~Phenocryst crystals are about 1.5 mm. in size, while ground~~

~~mass crystals are about 0.1 run. in size. The ground mass~~

is homogenous and nonlaninated, but shows traces of poorly 342 developed shears cutting through out*

~~Amphibolites * There are several large amphibolite bodies in the eastern Pinal Schist terrain-, represented by samples 491 (SE1/4 Sec. 35. T14S R28S), 899 (SVJ1/4 Sec. 21,~~

T14S H28E), and 92? (821/4 Sec. 27, T14S R28E). These amphibolites show two stages of internal fabric develop ment, and are hence more complex than the corresponding amphibolites of the western Pinal Schist terrain. These

~~amphibolites from the eastern terrain show, in hand~~

~~specimen, a clear foliation marked out by color banding and~~

~~apparent mincralogic banding on specimen faces. This banding is presumed to represent a primary metamorphic~~

~~foliation produced during initial metamorphism of the~~

~~rock, and is also presumed to correspond to the common~~

~~texture of the amphibolites in the western Pinal Schist~~

~~terrain; both are primary dynamothermal metamorphic~~

~~foliation produced during the metamorphism of the Pinal~~

~~sediments into which the basic intrusives from which~~

~~the amphibolites were produced had been intruded.~~

~~Under the microscope this primary mineralogic~~

~~banding can still be seen, but is somewhat obscured; what~~

~~dominates the view in the thin section are crudely linear~~

~~aggregates of radiating stars of hornblende crystals up~~

~~to 1 ram." in length, sitting in an interstitial matrix of quartz or plagioclase. Magnetite is also present (see~~

~~Figure 38c). The original mineral suite of these amphi bolites, so far as can be told, has been replaced by a second nonfoliated mineral suite developed in a second wholly thermal metamorphism.~~

~~Hornblende in these amphibolites is common horn blende. The leucocratic ground mass varies in nature~~

~~from sample to sample; in samples 491 and 92? plagioclase~~

~~is the gEoundmass phase, while in sample 899 quartz and~~

~~only a little plagiocla.se are the ground mass phases.~~

~~The plagioclase in these ground mass areas is untwinned~~

~~and biaxially positive; its composition seems to lie in~~

~~the range An^-An^ .'~~

~~This second metamorphic texture is not associated~~

~~with any marked shearing, and is thought to not be dynamo-~~

~~thermal in character. Rather, it is thought that this~~

~~texture points to a thermal metamorphism of considerable~~

~~strength which took place after the principal Pinal dynamo-~~

~~thermal metamorphism. Since these rocks are Precambrian~~

~~amphibolites, it might at first seem reasonable to as~~

~~sign this thermal metamorphism to the same event which~~

~~produced the biotite-bearing Pinal metamorphites, which~~

~~are also the products of contact metamorphism postdating~~

~~the primary dynamothermal metamorphism of the schist. This biotite producing event seems to have clearly been~~

~~Precambrian in age, as no trace of it can be found in post-~~

~~Precambrian rocks. As will be later described, however,~~

~~the metamorphic effects just described from the originally~~

~~Precambrian amphibolites in the eastern Pinal Schist ter~~

~~rain also show up in late Cretaceous diabase plutons~~

~~found all through the western Dos Cabezas mountains, while~~

~~Precambrian amphibolites in the same western terrain show~~

~~no trace of this thermal metamorphism.' Because of this,~~

~~the thermal metamorphism of the amphibolites in the eastern~~

~~Pinal Schist terrain is regarded as a Cretaceous thermal~~

~~metamorphism separated from Precambrian biotite-forming~~

~~thermal metamorphism which shows up in the schists by a~~

~~time period of several hundred million years.~~

~~Metamorphosed Dacite Porphyry Stock. Sample 756~~

~~(SE1/4 Sec. 12, Tl^S R27E) is representative of a weakly~~

~~metamorphosed dacite porphyry stock in the eastern Pinal~~

~~' Schist terrain. The stock has some three square miles of~~

~~exposure (see Figure 1). The rock is composed of some~~

~~15/3 2-3 mm. across quartz and feldspar phenocrysts in a~~

~~very fine-grained ground mass of 0.01 mm. crystals of~~

~~quartz, feldspar, and sericite, and slightly larger crystals~~

~~of biotite and magnetite (see Figure 37b).: Plagioclase is~~

~~about ten times as abundant as potash feldspar in the pheno-~~

~~cryst phases. The plagioclase is An ; the potash feldspar~~

is orthoclase with broad and 3^5 poorly defined twins. The post-crystallization development of stringers of muscovite and bictite along shears in the rock implies that this rock took part in the Precambrian thermal metamorphism recorded by biotite development in the schists of the western and eastern Pinal Schist ter rains, and is the best present evidence for assigning a

~~Precambrian age to the stock;~~

~~Samples from the Southern Pinal Schist Terrain~~

~~Quartzite. Samples 20 (SU1/4 Sec. 30, T14S B27E),~~

~~21 (SElA Sec. 30, T14S R2?E), 4l? (SW1/4 Sec. 20, T14S~~

~~R2?E), 451 (MElA Sec. 30, T14S R2?2), and ^57 (KElA~~

~~Sec. 29, T'AS R28E) are quartzites of various kinds from the southern Pinal Schist terrain." A typical specimen is massive, hard white quartzite such as specimen 419.~~

This specimen, in thin section, shows circa 90/S fritted and tightly packed quartz grains 0.1 to 1.0 mm. in dia meter all fritted together and tightly packed, with local small clots and clumps of interstitial sericite, quartz, and magnetite lying between the larger quarts grains.

~~In some places there are large single crystals of mus covite between the large quartz grains; these may be recrystallization products of earlier sericite.~~

Some quartzites in the field (sample 457) are red-brown in color, and in thin section are seen to 346 contain comparatively few large quartz grains, together with a large number of small - 0.1 nun. quartz grains and a large proportion of sericite and muscovite.

~~Phyllites. A typical phyllite specimen is sample~~

19 (1W1/4 Sec.' 25, T14S R26B) collected at the southwestern margin of the southern Pinal Schist terrain. It shows a very fine-grained (0.05 mm.) aggregate of sericite, quartz, minute magnetite crystals, and occasional rare local anhedral biotite crystals; the entire assemblage, with the exception of biotite, is well-foliated and finely laminated, and seems identical in character to the phyllites in the other Pinal Schist terrains, and like them is assumed to be a weakly metamorphosed shale.

~~No metamorphosed volcanic rocks or amphibolites were found in the southern Pinal Schist terrain.~~

~~Petrography of Non-Ranaklvi Precambrian~~

~~Granitoid Intrusive Rocks~~

~~Eaton Quartz Monzonite Gneiss~~

~~Samples 840 (SE1/4 Sec. 27, T13S R27E), 84l~~

~~(SE1/4 Sec. 27, T13S R27E), 845 (KW1/4 Sec. 35, T13S~~

~~R27E), 854 ( m / 4 Sec. 35, T13S R27E), 566 (NE1/4 Sec. 27,~~

~~T13S R27E), and 817 ( m / 4 Sec. 34, Y13S R27E) are from~~

the Eaton gneiss. Modal data were obtained on the first four samples by pointcounting on thin sections and on the last two by counting on a 3 mm. grid on slabs stained to color the potash feldspar yellow. The data, though show ing considerable variation internally, agree on a calcic quartz monzonite composition for the unit (see Table 3).

~~This rock easily has the most calcic overall composition of any Precambrian intrusive rock in the range, with the possible exception of the dacite porphyry stock in the~~

Pinal Schist described in the last section. Another un usual feature of the Eaton rock is that modal sphene is very high in it, running up to about 2^. Sphene is quite obvious in hand specimen as honey-brown crystals up to

~~2 mm. long.~~

Microscopic study of four specimens (see above) gives the following characteristics. Potassium feldspar occurs in large weakly perthitic microcline crystals con taining anhedral blebs of quartz and euhedral plagioclase crystals as inclusions; these latter are up to 5-10 mm/ long. Plagioclase occurs in two compositional types;

AngQ-Ang^ and .An^Q-An^, as determined by albite twin extinction angles on single crystals. Both of these plagioclase types occur in a graded series of sizes, with the maximum being about 5 mm. long. Strong reaction breakdown to sericite, epidote, and low-calcium plagio clase has occurred in some plagioclase crystals. Zoning 348 is common, weak, and normal in character, and the outer zones in the crystals are little different from the inner ones in composition. Biotite forms large groups of concordantly oriented small sheaf-like crystals; the biotite masses contain large euhedral to anhedral sphene crystals, cubed- . ral to anliedral epidote crystals, anhedral magnetite crystals and small euhedral apatite crystals. The biotite is poikilitic with respect to quartz as well. The gneissic texture is weak in thin section, but is marked in hand specimen by the subparallel planar orientation of small clots of dark biotite (see Figure 8a).' Shearing seems to be weak to. nonexistent in the specimens examined, and the foliated texture is thought to be a primary tectonic one related to flow of the pluton during its intrusion.

~~The paragenesis of the Eaton gneiss is as follows.~~

The feldspars and quartz began to crystallize out together at about the same time, and, toward the end of their crystallization, biotite, sphene, epidote, and apatite crystallized out together with the last of the quartz, feldspar crystallization finishing before that of the latter group. Evidently the crystallization of biotite took place after orientation of the feldspars by flow

~~took place. Sample of Second Body of Presumed Baton Gneiss~~

~~Sample 612 (SE1/4 Sec. 32, Tl'JS R28B) is from a small body of gneisslc quarts monzonite trhich looks, in the field, very much like the main mass of Eaton gneiss.~~

Its identity is important since it lies on the other side of the Apache Pass fault from the main body. In thin section the rock shows very large subhedral crystals of microcline perthite which include plagioclase and quartz. Plagioclase is An^g 1% composition as determined by albite twin extinction angles, and shows no marked compositional zoning. The section examined is small, and if a second plagioclase is present it could easily have been missed. Quartz is present in very large 5-10 mm. crystals showing some strain. Biotite is arranged in bands of crudely aligned platy crystals, with included anhedral sphene and magnetite crystals, the latter with narrow rims of sphene (?). All the above petrographic characteristics are shared by the main body of Baton gneiss, and on the basis of the remarkable petrographic similarity of these two masses, the small Eaton-like quartz monzonite body south of the Apache Pass fault is presumed to have once been part of the main mass of

~~Eaton quartz monzonite gneiss now located north of that fault. 350~~

~~Sommer Quartz Konzonite Gneiss~~

~~Samples 2?0 (HE1/4 Sec. 23, T13S R25E), 277 (SVJ1/4~~

~~Sec. 13, T13S B25E), 279 (SElA Sec. 13, T13S R25E), 285~~

~~(IIE1/4 Sec. 13 T13S R25E), 286 (NE1/4 Sec. 13 T13S R25E),~~

~~287 (Bfl/4 Sec. 13, T13S R25E), 288 (EVIlA Sec. 13, T13S~~

~~R25E), and 331 (Wl/^ Sec. 13, T13S R25E) are from the~~

~~Sommer quartz monzonite gneiss.1 The modal data for this~~

~~body point to a quartz monzonitic composition, although~~

~~the data show considerable scatter. Counting of each of~~

~~these specimens is based on a single thin section cut nor~~

~~mal to the foliation of the gneiss, and some of the modal~~

~~variation from one sample to another is no doubt due to an~~

~~insufficient sample size. However, most of the marked nodal~~

~~variations, such as those for the low-plagioclase samples~~

~~270 and 286, are easily matched by visible variations in~~

~~the hand specimens. The percentage data for the major~~

~~minerals, in an individual specimen are regarded as being~~

~~within about 20^ of the true mode of the specimen.~~

~~Because of the difficulty of obtaining unweathered~~

~~sample material, only a few of the modally analysed samples~~

~~were markedly free of the decomposition products of the~~

~~primary minerals. The detailed petrographic description~~

~~below is for the least weathered of all the specimens,~~

sample 33^• 351 In this rock a marked gneissic texture is present, which microscopic investigation showed to he partly due to secondary shearing of the rock and partly due to primary orientation of the primary minerals, especially the potash feldspars. Medium-sized 3-10 mm. long tabular to ovoid crystals of potash feldspar, plagioclase, and quartz lie close together, with thin dark seams of biotite-and-chlorite rich dark material separating them. Numerous small frac tures trend across the larger individual crystals at right angles to the foliation, and these appear to be extension fractures related to shearing in the development of the original foliation.-

In thin section the potassium feldspar is seen to be orthoclase perthite. Euhedral crystals of plagioclase and anhedral quartz crystal blebs are included within it; one potassium feldspar crystal was observed to have a partial rapakivi rim of plagioclase buried just below its surface.: The plagioclase seems to be of two compositions, as determined by albite twin extinction angles on crystals;

~~An23 is the common type and An^g is .rarer. The exact proportion of the two types cannot be estimated, since~~

~~so many of the plagioclase crystals have suffered strong reaction breakdown to sericite and a second plagioclase.~~

~~The plagioclase of both primary types forms euhedral to 352 v anhedral-angular crystals." Only the A ^ o types have been definitely identified as inclusions in the orthoclasc.~~

~~The dark-colored mineral-b earing bands lying be~~

~~tween the light colored crystals are composed predominantly~~

~~of biotite and green chlorite, with minor epidote, sphene,~~

~~and magnetite in small subhedral to singular crystals,~~

~~together with a large number of small dark cloudy masses.~~

~~The writer suspects that these dim cloudy clots are iron~~

~~oxide crystal aggregates formed due to incipient weather~~

~~ing of the rock/~~

~~Shearing of the rock is quite pronounced and af~~

~~fects all phases. Quartz, plagioclase, and orthoclase~~

~~have all been marginally granulated or disrupted and the~~

~~dark mineral phases are commonly strung out along folia~~

~~tion shears. Chlorite and sericite crystals seem to be~~

~~oriented more toward parallelism with the foliation plane~~

~~than do those of biotite. In this respect the biotite-~~

~~chlorite relationships resemble those of the biotite-~~

~~bearing Pinal Schist rocks near the Sommer gneiss, and~~

~~the writer believes that formation of the biotite in the~~

~~Sommer gneiss reflects the same biotite-forming thermal-~~

~~metamorphism reflected in the biotite development in the~~

~~Pinal Schist in all three of its major terrains. As~~

~~mentioned earlier, this thermal metamorphism in the~~

~~western Dos Cabezas mountains may be due to a buried 353 prong of the Polecat quartz nonzonite or some predecessor of it presently buried too deeply to be seen.~~

~~Rough Mountain Quartz Monzonite Gneiss~~

~~Samples 581 (N21/4 Sec. 20, T13S B27E), 853~~

~~(NE1A Sec. 31, T13S R2?E), and 9^0 (HE1/4 Sec. 35, T13S~~

~~R25E) are from the Rough Mountain gneiss. These samples were modally analysed by counting points beneath a cross-~~

~~hatched mylar overlay (see Table 2) on slabbed surfaces~~

~~stained to show potash feldspar as yellow colored areas.~~

~~These data are very concordant and agree on an alkalic~~

~~quartz monzonite composition for the unit.~~

~~In section (especially based on an examination of~~

~~specimen 853), the mineralogy and textures arc as follows.~~

~~The potash feldspar is microcline perthite, in 2-10 mm.~~

~~crystals, containing biotite in small subhedral crystals,~~

~~anhedral quartz blebs, euhedral zoned plagioclase crystals,~~

~~and subliedral magnetite crystals. The included phases are~~

~~all small (0.1 mm.), and most are markedly smaller than~~

~~equivalent crystal types which grew free in the magma~~

~~until the end of crystallization. Large plagioclase~~

~~crystals are present in 1-2 mm. euhedral to rounded cry~~

~~stals; these are poorly twinned, and patchy albite~~

~~twinning is about all that is seen. Zoning, on the other~~

~~hand, is marked, even, and normal. One albite twin 354 suitable for an extinction angle composition determination vras found, and gave an indicated composition of Ang,.. Re~~

~~action brealcdovm of the plagioclase is to sericito; this~~

~~is only poorly developed in most crystals, and the absence~~

~~of twinning in the original crystals is the main reason~~

~~for the difficulty in getting more compositional data on~~

~~the plagioclase in this rock. The plagioclase crystals~~

~~poikilitically enclose biotite, magnetite, and quartz, but~~

~~not potash feldspar. Quarts occurs in large aggregates of~~

~~separate anhedral equant crystals 0.5 to 1.0 ram. across.~~

~~These aggregates have generally elongate cross-sections~~

~~and lie in the plane of foliation. Biotite is in separate~~

~~platy crystals or aggregates of a few subparallel crystals.~~

~~The biotite contains sparse anhedral magnetite, epidote,~~

~~and apatite. The larger crystals in the rock lie in a~~

~~ground mass of quartz, microcline and plagioclase quite~~

~~a bit smaller in grain size than the larger ones. Ground~~

~~mass crystals are perhaps 0.2 - 0.3 mm.* as opposed to~~

~~1 - 10 mm. for the phenocryst phases.~~

~~The paragenesis of the unit based on texture and~~

~~enclosing relations implies simultaneous crystallization~~

~~of biotite, magnetite, plagioclase, potassium feldspar,~~

~~and quartz, quartz and feldspar forming larger crystals~~

~~than the other phases. This larger growth of the light-~~

colored minerals may be a function of their growth rate 355 \ or of their ease of nucleation in the melt. Foliation was produced in the unit "before final crystallization and was followed by final crystallization of a quartz-feldspar ground mass,'

~~Sheep Canyon Quartz Honzonite Gneiss~~

~~The Sheep Canyon Quartz Honzonite Gneiss is repre~~

~~sented by samples 200 (HW1/4 Sec. 23, T14S R28E), 213~~

~~(SE1/4 Sec. 14, T14S R28E), 500 (SE1/4 Sec. 15, T14S R28S), and 502 (NE1/4 Sec. 21, Tl4s R28E). Modal analysis of~~

~~these samples was made by the mylar overlay point counting~~

~~technique earlier described with results shown in Table 2 .~~

~~The modal data are not tightly grouped, and only show that~~

~~potassium feldspar predominates over plagioclase to some~~

~~extent or another in each specimen. The rock is an alka~~

~~line quartz monzonite, which in respect of its internal~~

~~variability is much more like the Sommer gneiss than the~~

~~Rough Mountain gneiss.~~

~~Thin section analysis shows the gneiss to be a~~

~~medium-grained rock with varying degree of secondary shear~~

~~foliation superimposed on a primary flow foliation orienta~~

~~tion of the potash feldspars and plagioclase. Of the~~

~~samples examined, sample 200 has the least reacted~~

~~plagioclase and was hence chosen for the detailed analysis~~

~~below. Shear foliation is anomalously weal; in this 356 t specimen.'~~

~~In the examined sample, the potassium feldspar con sists of large 2-10 mm. crystals of microcline perthite, nhich Include small 0,1 - 0,5 mm. euhedral plagiocla.se~~

~~crystals, anhedral quartz blebs, and biotite crystals.~~

~~Plagioclase comes in tvio compositional variants, as determined by albite twin extinction angles; these are~~

~~An2o“An22 and A n ^ v Many plagioclase crystals are strongly~~

~~reacted to sericite, epidote, and low-calcium plagioclase.~~

~~It is difficult to tell, even in this comparatively un~~

~~reacted sample, what the true proportions of the two~~

~~plagioclase types are. The less reacted crystals generally~~

~~show good albite twins and marked zoning of both normal~~

~~and reversed types. Biotite is quite sparse, and is~~

~~present in very small and scattered crystals interstitial~~

~~to the feldspars and quartz. Muscovite is commonly pre~~

~~sent in single crystals up to 1 mm. in size in the same~~

~~type of interstitial position. Sample 12, for which~~

~~there is no mode due to its weathered nature, shows some~~

~~secondary biotite growth in the shear zones which markedly~~

~~pervade the specimen; this Implies that some biotite~~

~~growth was still occurring while shearing was taking place.~~

~~The potash feldspar in specimen 200 is often optical~~

~~orthoclase in part of a crystal and optical microdine~~

~~in the rest of it. The Sheep Canyon gneiss shows the same paragenetic sequence as the other medium-grained gneissic rocks of the~~

~~Dos Cabezas mountains do. Potassium feldspar, quartz, plagiocla.se, and biotite began to crystallize essentially~~

~~simultaneously at some point early in the history of~~

~~crystallization. At some later time, toward the end of~~

~~crystallization, rate of crystallization increased and a~~

~~finer but still medium-grained aggregate of quartz, feld~~

~~spar, and biotite was formed. Shearing in this gneiss~~

~~parallels the foliation of the primary minerals where~~

~~seen, but apparently is secondary to final crystallization~~

~~of the rock.~~

~~Cienaga Quartz Honzonite Gneiss~~

~~This unit is represented by samples 480 (NE1/4~~

~~' Sec. 7, T15S R28E) and 950 (SW1/4 Sec. 1, T1$S R2?E).~~

~~The modes of these samples were determined by the mylar~~

~~overlay technique described previously.* The two samples~~

~~are modally identical within the precision of the counting~~

~~techniqueThe rock is an alkaline quartz monzonite close~~

~~to a true granite in composition (see Table 2).~~

~~As mentioned in the main text, there are two~~

~~textural phases of this unit; the coarser is called Cgn^~~

~~and the finer-grained porphyritic variety is labelled Cgr^.~~

In thin sections of the main phase Cgn^ the following 358 characteristics can be seen. Potassium feldspar is perthitic orthoclase in large tabular euhedral to subhedral crystals up to 3 by 1 cm. in size. Small quartz blebs, 1 - 2 mm. euhedral plagioclase crystals, biotite crystals, and magnetite crystals are all included in the potassium feldspar. One large plagioclase crystal was observed protruding from the side of a large orthoclase crystal; its outer part was enclosed in a second plagioclase crystal.

In some parts of some sections, orthoclase and plagioclase growing side by side have developed an interpenetrating boundary in which projections of the crystals of the two feldspar types project across the boundary into each other.

Plagioclase is present outside the potash feldspars as 1 - 4 mm/ generally euhedral crystals; they may include quartz in small anhedral blebs and crystals. Twinning and zoning are present in the plagioclase, albeit weakly expressed. Albite twinning extinction observed on one crystal gave a composition of An^g-Angg' Sericitization of the plagioclase is weak. Quartz in the main crystal assemblage occurs as separate 1 mm/ crystals associated in groups; the individual members of the groups are fitted together along their outer margins or are separated from each other by thin anhedral feldspar crystals.

Biotite occurs as small clusters of randomly oriented crystals; the clusters are up to 1 - 2 mm. across, 359 while the individuals, in then are seldom more than a tenth of a millimeter across.' Magnetite is present as small euhedral to subhedral crystals 0.1 - 0.2 mm.' across, generally within or next to the biotite crystals.

The minor gneiss phase Cgn2 shows a marked porphyritic texture with a few percent of potassium feldspar crystals lying in the much more fine-grained ground mass.*

~~The proportion of phenocrysts varies from perhaps five to twenty percent; in size they are about 1 - 3 cm. in maximum~~

~~size. In thin section this minor phase shows the following relationships:~~

~~Potassium feldspar may either be orthoclase perthite or microcline perthite. Shearing has often de~~

~~formed the crystals and fracturing within crystals is~~

~~common. These potash feldspars include euhedral plagio-~~

~~clase crystals, euhedral biotite, and anhedral quartz.~~

~~Muscovite-biotite aggregates are also seen. Plegioclase~~

~~is of two types; one is An-^-A^^, the other is An^Q-An^.~~

~~The low-anorthite plagioclase is the only type which has~~

~~been positively identified in the inclusions in the potash~~

~~feldspar, and it is also present in the ground mass. The~~

~~other, high anorthite plagioclase is apparently only a~~

~~ground mass phase. Most of the plagioclase crystals~~

~~show cores which have strongly reacted to form epidote,~~

~~sericite, and low-calcium plagioclase; the outer parts of 360 these plagioclases show less reaction than the cores.~~

Biotite is present in 0.2 mm. or smaller crystals within microcline crystals or in the ground mass, where it is often found in shear bands which cut the primary crystal lization products. Magnetite is present in scores of small markedly anhedral crystals which locally appear as equant grains 0.1 - 0.2 mm. across, which are often present as discrete individuals in other ground mass components, but are more generally present in fritted aggregates of many crystals. These quartz crystal aggregates are not foliated.

Shearing, which crosscuts the primary crystal phases but is essentially parallel to the main flow folia tion produced during intrusion of the gneiss, is present in sample 581 but not in 481 or 950* The shearing is local in nature, as seems to be the case in some other

~~Precambrian gneisses in the Dos Cabezas mountains.~~

~~The two types of the Cienaga gneiss are modally indistinguishable. As can be seen, however, there are minor mineralogic variations between them. Chief among~~

these is the presence of two plagioclase types in the minor phase and only one in the major phase. The writer feels that this may be a recognition failure rather than a true disparity; the main unit plagioclase crystals are poorly twinned and hard to identify with much precision. 361

No variation observed compels the interpretation that the two types of the gneiss are different bodies in any fun damental sense, and it is thought very likely that the minor phase (Cgn^) is a syngenetic predecessor of the main phase (Cgn^).'

~~Polecat Quartz I-Ionzonite~~

~~This unit is represented by samples 373 (NEl/^~~

~~Sec. 17, T13S R26E), 380 (SE1/4 Sec. 8, T13S R26S),~~

~~586 (SE1/4 Sec. 19, T13S R27E), 603 (SSI/4 Sec. 35, T13~~

~~R26E), 697 (SvJl/4 Sec. 30, T13S R27E), 714 (SvJl/4 Sec. 3,~~

~~T14S R27E), 722 (SSI/4 Sec. 5. T14S R27E) , 803 (SVJ1/4~~

~~Sec. 26, T13S R26E), and 837 (HVJ1/4 Sec. 19, T13S R27E).~~

These samples were analysed modally by the mylar overlay technique described previously; most of the slabs examined were cut from field samples and were only about three inches in length and width. This size is a bit small for good analytical precision on so coarse-grained a rock; therefore, modal data on this pluton are presumed to be no closer than plus or minus ten percent to the true mode of the rock in the area where any sample was collected. In any case, the general nature of the pluton stands out well. It is an.alkalic quartz monzonite in which the relative proportions of the two feldspars varies considerably, although potash feldspar is never 362

~~less than one and a half times and generally two to three~~

~~times as abundant as plagioclase. Sample 373 is a true~~

~~granite, with over four times as much potash feldspar as~~

~~plagioclase in the specimen. Field observation of the~~

~~pluton confirms that the proportions of the feldspars~~

~~varies from place to place, A few zones of masses of~~

~~potash feldspar are present. Most dikes of the Polecat~~

~~in the wall rocks of the pluton are much richer in potash~~

~~feldspar crystals than is the main body of the pluton;~~

~~these dikes commonly carry 60~80$ potash feldspar crystals~~

~~crammed bricklike into the dike with their long axes~~

~~parallel to the strike of the dike. Every indication is~~

~~that the potassium feldspar phenocrysts were pre-existent~~

~~before the pluton reached this level in the crust.~~

~~There is no particular trend to the observed~~

~~compositional variations in the Polecat quartz monzonite.~~

~~In thin section no particular trend in fundamental~~

~~paragenetic relationships or mineralogy was seen. Sample~~

~~802, used for geoclironologic study, was chosen for detailed~~

~~petrographic study as it was the least weathered of all the samples.~~

~~In sample 802, potassium feldspar is present as •~~

~~microclinc perthite in euhedral to subhedral tabular~~

~~crystals up to 3 cm. in length by 1 cm. wide. The perthite~~

~~includes numerous euhedral plagioclase crystals together~~

~~I. 363 with anhedral quartz crystals, both 0.1 - 0»5 mm. across.~~

Plagioolase inclusions near the margins of the potash feldspar host are generally aligned with their long axes parallel to the borders of the host crystals. Plagioclase occurs as two compositional varieties, as is common in these older PreCambrian rocks; compositions of Ang^ and

~~An^Q were determined by albite twin extinction angles.~~

Both varieties were observed in several crystals in the slides in both ground mass crystals and as crystals in cluded in the potash feldspars. Plagioclase crystals in the ground mass have strongly reacted cores where potas sium-bearing plagioclase has reacted to form sericite plus potassium-free plagioclase. Ground mass plagioclase sizes run 1-2 mm. Outer zones in these feldspars show much less reaction than the. inner ones.

~~Biotite is present in clusters and aggregates of randomly oriented crystals a millimeter or less in size;~~

~~chlorite is present as interlayers in the biotite or as~~

~~separate crystals which may or may not be pseudomorphs~~

~~after biotite. Although not observed in section, a few muscovite crystals were obtained from the rock during • mineral separation. Anhedral. magnetite crystals and small~~

~~euhedral apatite crystals are included in the biotite~~

~~and chlorite locally. Quartz is present as large 3-5 mm. 364 fractured and slightly strained crystals, with no inclusions.~~

~~Petrography of Raoalcivi Quartz Honzonite Samples~~

The rapakivi quartz monzonite is a most unusual granitoid rock, and in the interests of a clear and detailed discussion of its genesis, petrographic data on the rock and its internal varieties has been kept in the text of

~~Chapter 4. Modal data are contained in Table 2, however, along with similar data for the other Precambrian grani toid bodies.~~

~~Petrography of Volcanic Breccia Units~~

The volcanic breccia in the Dos Cabezas mountains is a very complex assemblage of large numbers of separate . breccia and nonbreccia intrusive volcanic rocks, and the following descriptions can only give some idea of the great petrographic variety which exists there.

~~Green Volcanic Breccia Units~~

~~Samples ?02 (SU1/4 Sec. 6, T14S R27E), ?21 (SV/1/4~~

~~Sec.. 5. T14S R27E), 729 (NS1/4 Sec. 21, Tl4S R27E), 730~~

~~(SVtl/4 Sec. 16, T14S R27E), 745b (SVI1/4 Sec. 8, T l4s R27B),~~

~~747 (SW1/4 Sec. 13, T14S R27E), 748 (S'Jl/4 Sec. 13, T14S~~

~~R27E), 767 (SW1/4 Sec. 9, T14S R27E), 772 (NE1/4 Sec. 22,~~

~~T14S R28E), 774 {SE1/4 Sec. 23, T14S R28E), 775 (1^1/4~~

~~Sec. 23, T14S R28E), 776 (SW1/4 Sec. 14, T14S R28S), 777 365 TABLE 2~~

~~Modal Data on PreCambrian Rocks~~

~~Rook Unit K-spar Plag. Quartz Biotlto Ka%. Other Total Cts.~~

~~Eaton First four samples by thin section analysis on gneiss 2 mm. grid. Last three by counting overlay on slab; interval~~

~~841 25.1 35.2 31.5 7.4 1.2 0.6 sph. 1288~~

~~845 23.6 36.2 29.2 6.3 1.5 1.4 sph. 1299~~

~~854 27.7 35.1 25.7 6.9 0.8 2.0 sph. 2015~~

~~840 19.2 40.4 21.6 10.9 0.1 1.3 sph. 1341~~

~~817 34.4 28.2 21.7 15.8 354~~

~~566 29.633.8 18.6 18.3 290~~

~~612 (?) 37.8 28.3 22.2 11.6 275~~

~~Sommer All counts on 2 mm. grid on thin sections normal gneiss to foliation~~

~~331 17.5 21.5 41.0 6.9 7.1 ser. 680~~

~~287 18.2 27.9 41.7 3.8 8.8 ser. 741~~

~~285 54.2 9.1 31.2 1.2 2.4 ser. 738~~

~~277 27.8 20.7 42.8 4.6 3.8 ser. 629~~

~~279 20.6 22.8 42.1 4.5 9.9 ser. 494~~

~~270 . ' 41.4 16.6 39.0 - 0.7 ser. 428~~

~~288 28.0 20.2 39.5 0.5 ser. 582~~

~~286 , 37.9 8.6 43.2 6.5 ser. 679. All figures in per cent See Figure 7 for sample locations 366~~

~~TABLE 2-Continued Rock Unit K-spar Flag. Quartz Biotite Hag. Other Total~~

~~Polecat Quartz Monzonite~~

~~380 39.5 25.4 29.O 6.0 334~~

~~383 31.4 23.2 28.2 17.5 359 602 51.0 17.6 27.7 3.7 296~~

~~697 48.2 20.0 26.6 5.7 425~~

~~722 49.5 15.7 21.4 7.7 348~~

~~714 39.8 18.9 33.3 7.9 456~~

~~837 41.8 19.9 25.5 12.7 463 586 42.8 26.7 23.3 7.0 415 ro 0 373 58.7 15.4 0 7.6 419 802 40.1 27.I 29.2 3.6 414~~

~~All samples counted on mylar sheet overlay on~~

~~stained slabs of samples.~~

~~Dos Ca'oezas rapakivi quartz monzonite~~

~~829 37.9 26.4 25.6 6.9 0.7 hb, 1.2 chi.-- (vriborgite) 0.2 sphene 1.1 epidote~~

~~Counts difficult to evaluate for this sample.~~

~~Phenocrysts measured by area on six slabs and ground mass~~

~~counted about 2000 counts on six thin sections. Sole~~

~~rapakivi sample due to great length of time needed for analysis. 36? TABLE 2-Continued Rook Unit K-Soar Pla%. Quartz Biotite IfeS* Other Total Cts.~~

~~Rough Htn.: Gneiss~~

~~581 39.2 21.3 33.1 6.4 281~~

~~940 33.1 26.5 33.2 7.4 580~~

~~853 33.9 2?.6 3I.I 6.5 508 All samples counted with a mylar sheet grid overlay on stained slabs.~~

~~Sheep Canyon Gneiss~~

~~200 .36.7 29.5 30.7 3.3 646~~

~~500 ' 39.5 27.9 22.3 10.1 690~~

~~213 44.2 18.5 31.9 5.7 508~~

~~502 46 ."5 18.6 26.9 7.7 404~~

~~928 50.7 21.7 25.8 2.0 395 All samples counted with mylar sheet grid overlay on stained slabs.~~

~~Cienaga Gneiss~~

~~950 42.2 22.7 28.3 6.8 553~~

~~480 . 45.4 19.0 30.3 5.3 341 All samples counted on mylar sheet grid overlays~~

~~on stained slabs 368~~

~~(NVJ1/4 Sec. 14, T14S R28B), and 803 (B J1/4 Sec. 16, T l4s~~

~~R28E) represent the green volcanic breccia in the Dos~~

Cabezas range. Mo nodal data were collected, due to the extreme variability of composition and the fine-grained ground masses of the rocks; rather, the general charac ter of the units is described below.

On slabs.and in thin sections the following re lationships appear. The dark aphanitic rock fragments in the breccia are invariably andesitic in composition, and no true basalts have been found among the fragments in the unit. A large majority of the fragments showed well-developed internal trachytic flow structures.

Considerable variation in phenocryst size and proportion is noted in the different fragments. In the observed sections, light aphanite fragments were also seen, which were spherulitic rhyolite, rhyolite, dacite porphyries, and rhyolite porphyries. Several granitoid phanerite fragments were seen, as were quartzite fragments and well- foliated microcrystalline carbonate fragments. Quite a varied population of prebrecciation rocks was present in the area now occupied by the breccia (or, more precisely, above the area where the breccia now lies; see discussion in the text)Some of the fragments show considerable

~~evidence of reaction breakdown to new minerals; one former andesite (?) fragment observed had completely broken down 369 to form an assemblage of epidote and quartz. Marked reaction~~

~~rims occur around some, but not all, fragments in many~~

~~localities in the green breccia.~~

~~As mentioned in the general description in the~~

~~text, microscopic study reveals that the ground mass of~~

~~the breccia is mostly made up of small angular rock frag~~

~~ments down to a fraction of a millimeter in size, together~~

~~with plagioclase and quartz crystals and crystal fragments~~

~~a few millimeters to a few tenths of a millimeter in maxi-~~

~~, mum dimension, both contained in a sparse interstitial~~

~~microcrystalline ground mass which shows considerable and~~

~~local variation in grain size (see Figure 39). The very small proportion of this latter phase, which is generally less than 5% by volume of the unit, as well as its con~~

~~siderably varied texture, would indicate that it was not~~

~~a pervasive throughgoing magma phase at the time of ap~~

~~pearance of the breccia. Bather, the ground mass appears~~

~~to be a mass of minute fragments fritted or fused together.~~

~~Opposing this just-mentioned point of view is~~

~~evidence that some of the small crystals in the ground .~~

~~mass.have passed through a growth or resorbtion stage in~~

~~some sort of coherent medium, presumably of magmatic~~

~~character. Occasional plagioclase crystals appear to be~~

whole euhedral crystals, and some quartz crystals show strong resorbtion embayments. Sample 744-, among others, 370 shows some small psendomorphs after euhodral hornblende (?) composed of aggregates of magnetite, plagioclase, and epidote within the bloeky outlines of the former crystal.

However, these crystals need not have remained in a magma during their entire history before the final solidification of the breccia. Crystals can be separated from their parent magmas to various degrees by several known physical processes, such as crystal settling or disintegration of the magma during gas escape. Escape of gas during the latter process might reasonably be

~~expected to cause entrainment of released crystals along with minute particles of glassy or aphanitic solidified magma. This in turn would account for the presence of~~

~~small crystals and crystal fragments in a very fine~~

~~grained but irregularly-sized ground mass, with both~~

~~small crystals and ground mass forming an overall ground~~

~~mass for the larger rock fragments. Such a hypothesis~~

~~agrees with the hypothesis of formation of the larger~~

~~fragments in the breccia by gas-blast disruption of sur-~~

~~ficial lavas and their underlying rocks, with consequent~~

~~entrainment of those fragments in a gas stream of a~~

~~forceful nature emanating from some deep source.~~

~~The breccia ground mass and the intrafragmental~~

ground masses have reacted strongly in most specimens to 371 form minute clear highly birefringent crystals which have been tentatively identified as sericite; these are con tained in a generally obscure weakly birefringent mass of material which is probably low-potassium and low-calcium plagioclase. Small anhedral crystalline masses of epidote are common in these reacted areas. In much of the western part of the green breccia mass, large crystals and crystal masses of epidote also appear, replacing rock

~~fragments, crystal fragments, and ground mass areas of~~

~~the breccias quite impartially. The epidote crystals~~

~~disrupt and crosscut the fragment boundaries and ground~~

~~mass flow lines, and are obviously a post-fluidity mineral~~

~~phase in the breccia. The epidote is essentially a~~

~~deuteric phase formed after movement of the breccia had~~

~~ceased, but before it had cooled off. Epidote is ubi~~

~~quitous to some extent or another in most of the green~~

~~breccia, and is usually responsible for its green color.~~

~~Green chlorite is also common in plagioclase crystals,~~

~~rock fragments, and ground mass, and is another, more~~

~~weakly developed, deuteric phase. Another common ap-~~

~~• parently deuteric phase is a carbonate mineral, probably~~

~~calcite, which cuts across the original structures of~~

~~the rock and forms several percent of the mode in many~~

~~specimens. 372~~

The gross composition of the green breccia appears to be dacitic or quartz andesitic, judging from the visible parts of it. In detail, due to its nature as a mixture of fragments, its composition varies from place to place within the broad limits of the general composition. That is, for example, the proportion of plagioclase crystals of a certain composition in the ground mass of the breccia may vary by five or ten percent in a few centimeters distance. The rock fragment assemblage as a whole is more

~~andesitic than the mineral crystal and crystal fragment~~

~~assemblage; the latter contains numerous quartz crystals~~

~~and crystal fragments, while in the former quartz is quite~~

~~rare.' The ground mass of the breccia seems to have had a~~

~~different source than the fragments which that ground mass~~

~~contains.~~

~~Purple Volcanic Breccia~~

~~Samples 38 (HE1/4 Sec. 11, Tl4S R26E), 402 (HEl/4~~

~~Sec. 4, T14S R26E), 428 (SE1/4 Sec. 28, T13S R26E), 432~~

~~( m / 4 Sec. 33, T13S R26E), 629 (SE1/4 Sec. 1?, T14S R27E),~~

~~734 ( m / 4 Sec. 1?, T14S R2?E), 752 (NE1/4 Sec. 25, T14S~~

~~R27S), 847 (NE1/4 Sec. 4, T l4s R27E), 848 (same as 847),~~

~~849 (same as 847), 873 (SVJ1/4 Sec. 29, T14S R28E), 882~~

~~(center Sec. 30, T14S R28E), and 941 (SVJ1/4 Sec. 28, T13S~~

~~R26) are of purple volcanic breccias from various parts 373~~

~~of the central Dos Cabezas mountains. These ‘breccias are~~

~~composed of dark-colored aphanitic rock fragments in a purple or red-brown ground mass; in the ground mass many~~

~~white plagioclase crystals and crystal fragments can be~~

~~seen.~~

~~Sample 38 is indicative of the character of the~~

~~main body of purple volcanic breccia. Microscopically,~~

~~the rock shows some "^0% rock fragments down to 0.2 mm. in~~

~~size, of a sharply angular character, together with circa~~

~~15/£ plagioclase of composition An^* 2-3'£ hornblende crystals, and circa 5% collapsed pumice fragments down to~~

~~0.'5 mm. in size, all contained in a very fine-grained~~

~~groundmass of plagioclase and considerable epidote, along.~~

~~with hematite and occasional chlorite. See Figures 40a~~

~~and 4la and 4Tb. The texture of the ground mass is usu~~

~~ally holecrystalline,, even, and very fine-grained. It~~

~~locally bears numerous anhedral plagioclase fragments,~~

~~which are generally sericitized; the ground mass material,~~

~~however, is not sericitized. This implies that sericitiza-~~

~~tion is an early-deuteric process which took place before~~

~~final solidification of the breccia occurred.~~

~~The pumice in this unit is the only pumice seen~~

~~anywhere in the breccia. It is collapsed ( Ross and Smith,~~

~~1965) and shows masses of plagioclase and chlorite with~~

~~some epidote, all arranged in a strong linear or web-like pattern which the writer can only interpret as collapsed pumice fragments.'~~

~~The presence of the pumice in this section implies that in places, at least, in this unit, very marked dis ruption of some magmatic phase by gas escape must have occurred.~~

Samples 428 and 941 represent the isolated vol canic breccia neck of Camelback mountain (see Figure 1) which lies west of the main breccia terrain. This mass has a very high proportion of angular rock fragments'(up to 80;5 of the whole) about 1-2 cm. in size. In thin sec tion the ground mass interstitial to the fragments is

~~composed of a mass of minute angular plagioclase fragments,~~

~~strongly serlcitized, together with frequent quartz fragments and 5-157» magnetite/ A few pseudomorphs of magnetite and sericite after biotite were seen (see~~

~~Figure 40a).~~

~~The grain size of the ground mass averages a~~

~~fraction of a millimeter. Sericitization in the ground mass is less than in the coarser-grained plagioclase~~

~~fragments, as reported before. What appears to be very~~

~~minute and rather uncertainly identified crystals of~~

~~second-generation biotite lies in wispy and stringy ag~~

~~gregates along fragment boundaries and flow lines in the~~

~~breccia. This may be a vapor-phase crystallization product.~~

fragment fragment large (note plagioclase crystal) fragment ccia ccia. of of the ground mass q q p quartz - plagioclase - C C andesite- porphyry • k m potash feldspar - magnetite - Note the faint flow banding B quartzite - fragment d. d. White volcanic bre e e - epidote A andesite - porphyry p - plagioclase C C - andesite porphyry q - quartz k - potash feldspar B - andesite fragment b. b. Purple volcanic bre A - collapsed pumice

fragment - quartz - - - plagioclase of of ground mass of the p q plagioclase - quartz - c. c. White volcanic breccia rock m magnetite - Note marked flow banding B C basalt fragment - andesite porphyry - Breccia a. a. Greenvolcanic breccia A felsite fragment - Figure 39: Figure 39: Examples Mainof Phases of Volcanicthe a* A 3Z5

~~2 ' 'Vvx ■I' ■ w~~

~~Gr-eef) iSD/coss'C £rmcc>o Purp/e V&'Coni C p r e t e s t ? 3ofnpSe ■Sampi 'e -5S~~

~~k/'/ure. i/O/co/v/c ^-ecc/2 -Su/T3/3/« S*-i5£>~~

~~frgure 39 Exompie.s of A/Jo/n Pho^ses of ff?e \/o/con/c Brecc/o Figure 40: Special Features of the Volcanic Breccia~~

a* Interfragment zone in b. Fine-grained ground the purple volcanic mass from the purple breccia* Note the volcanic breccia. numbers of small plag- Note the resorption ipclase fragments in embayments in some the zone between the of the small.quartz two main fragments. grains. Dark material is ferruginous (hem- A - fragment of ferruginous atitic); it contains aggregate of minute some anisotropic quartz grains and material. hematite; possibly normal shale A - quartzite grain

~~B - andesite porphyry q - quartz fragment p - plagioclase~~

c. Xenolith of Polecat d. Section of nonbrecc- quartz monzonite. Shows iated andesite porphyry. a separation of the Note marked flow fol original minerals from iation of phenocrysts. • each other, coupled Note pseudomrphs of with marked internal magnetite and sericite disintegration of the after hornblende (?) feldspars along internal Note large, mat of car cracks. Thin seams of bonate in the ground sericite surround each mass. grain• h - hornblende pseudo- q - quartz morph composed of p - plagioclase magnetite and sericite k - potash feldspar p - plagioclase m -.muscovite c - carbonate s - sericite s - sericite 376

~~/■’(ssyo/e Isb/can/c ^recc/o Purp/e IA>/coo/c &r+cc& .Znres- /ragmen t ^Tone p/ne- Sro/nec/ P/?ose Samp/* PV/ 6}Qmp/* 4 0 P~~

~~/^o/sc^Z Xeno/sTt? Andes/f^ Porphyry 'So/Tjp/e 7^62~~

~~P /g v r e 4 0 Spec/o/ Peorc/res of rr>G i/oJc.on/c Ps-ecc/a 377~~

Observed rock fragments in this breccia are of two predominant types; the first are small 1 cm. fragments of quartzite, and the second are plagioclase-sericite-hematite rocks showing strong reaction and breakdown of their primary minerals, which were presumably plagioclase, horn blende, and magnetite. One fragment was observed to be a mass of minute 0.05 mm. quartz and sericitized feldspar crystals locked in a hematite cement. An arkose fragment was also seen.

~~Samples 402, 84?, 848, and 849 are from a most~~

Interesting small outcrop of purple breccia which contains large numbers of limestone fragments along with the custom ary volcanic rock fragments. The limestone fragments are evidently derived from buried Paleozoic limestones. In section, the ground mass of this body is seen to be composed of about 60^ small euhedral plagioclase crystals, all of which are strongly reacted to sericite, carbonate, and epidote-. Numerous small euhedrally-outlined masses of magnetite surround cores of epidote, sericite, plagio

~~clase, or carbonates; these are presumably pseudonorphs after hornblende. Up to 10;» of the ground mass is~~

~~euhedral to anhedral magnetite crystals. Several percent~~

~~of the ground mass is composed of ragged anhedral crystals~~

of carbonate which do not seem to include any other 378 mineral phases. A small proportion of highly birefringent material which is minutely crystalline and cloudy may be a second generation of epidote. In this rock there is no

~~textural evidence requiring the ground mass to have been~~

~~fragmental in nature, and it may be that in this local~~

~~area the ground mass was liquid during emplacement of the~~

~~breccia; certainly the presence of so many euhedral feld~~

~~spars in the ground mass indicates the latter/~~

~~There are areas in the breccia that are free from~~

~~fragments larger than a millimeter or so in size. Micro~~

~~scopic examination of sample 848, for example, reveals~~

~~minute 1 - 0.1 mm. angular to sub-rounded rock fragments~~

~~of many types together with plagioclase and quartz~~

~~fragments. Plagioclase is perhaps ten times as abundant~~

~~as quartz, and makes up 30-40;, of the rock. Plagioclase~~

~~is strongly sericitized; most crystals contain only a~~

~~small amount of original material, which forms a residual~~

~~core which is deeply embayed and looks like a mass of~~

~~worms. This is surrounded by a mat of minute intergrown~~

~~sericite crystals. The minute rock fragments, aside~~

~~from three quartzite fragments observed, are all aphanite~~

~~porphyries of one sort or another. Usually one or two~~

~~crystals and a small amount of ground mass material are~~

~~all that can be seen in any one fragment. All these rock~~

fragments show strong evidence of reaction breakdown; 379 most of the ground mass areas in these fragments is now sericite and the plagioclase phenocrysts are strongly broken down to epidote and sericite. A few former ferromagnesian mineral crystals in the fragments are now epidote rimmed with magnetite, with the outline of the original crystal pseudomorphously preserved. The ground mass between these small rock fragments is composed of plagioclase, sericite, chlorite, and up to 30^ carbonate

~~is present. Rare small hematite flakes are present in~~

~~the ground mass.~~

~~A finer-grained ground mass bearing phase was~~

~~also examined and was seen to be composed of some 80%~~

~~0.1 - 0.2 mm. plagioclase and quartz fragments; the~~

~~plagioclase-quartz ratio: is about 10:1.~~

~~These small crystals in the above sample lie in * a microcrystalline ground mass of undetermined nature~~

~~which is full of sericite. Small areas of this ground~~

~~mass are ringed by magnetite crystals, giving a spotted~~

~~appearance of unknown genesis to the rock. The texture~~

~~might be due to disruption of the ground mass followed~~

~~by vapor phase crystallization of magnetite and subsequent~~

~~rehealing of the ground mass.~~

~~Sample 402, also from this locale, reveals a mass~~

~~of some 80% circa 0.2 mm. and less quartz fragments, many 380 showing resort)tion embayments, together with circa 15;^ 0.2 ima. and less wholly sericitized feldspar crystals; see~~

~~Figure &0t>. The rock is interesting on a hand-specimen scale, as it is an aphanitic purple rock which is quite hard and tough, and would be called a basalt in the field by many geologists.~~

Sample ?3^ is illustrative of a large number of small purple breccia dikes which cut the main green breccia in the central part of the breccia terrain. A section shows a fine-grained ground mass containing numerous small

1 ramV rock fragments, quartz, potash feldspar, plagioclase, carbonate magnetite, hematite flakes, chlorite, and epidote in it. These fragments are small, and the crystals fragmental-anliedral, except for the carbonate, which forms large single-crystal anhcdral masses up to 2-3 mm. across.

In the same large hand specimen from which the fragment-bearing breccia just described was taken, there is a band of aphanitic non-fragment-bearing rock, which was also examined in thin section; it consists of quartz and sericitized plagioclase crystals or crystal fragments in a finer-grained patchy fragmental looking material which is strongly sericitized and carries many small magnetite- chlorite-epidote pscudomorphs after hornblende (?) and numerous minute magnetite crystals. Numerous tiny hema tite crystals are scattered throughout the ground mass. 381

~~A grossly dacltic composition characterizes these units, which appear to bo two textural phases of the sane rock.~~

In the field, such fragmental zones of purple breccia are often associated in the same dikelike body with masses of obviously magmatic andesites which inter- tongue with the breccias; in general the field relations suggest that the breccia material is forming from the magmatic material; this is probably due to escape of gas from the crystallizing magma in the so-called second boiling phenomenon.1 Additional material from below or from the walls of the dike was probably added to the an desite fragments produced by this action.

~~Other purple volcanic breccia bodies arc briefly discussed in the main text.~~

~~White Volcanic Breccia~~

~~Samples 735 (SW1/4 Sec. 8 , T14S R272), 755 (WW1/4 Sec.' 2k, T14S R27E), 761 (SW1/4 Sec. 9, Tl4s R27E), 823~~

~~(SW1/4 Sec. 3, T14S R27E), and 862 (hWl/4 See. 20, T14S~~

~~R27B) are of white volcanic breccias. In the field, they~~

~~characteristically are composed of white or yellow~~

~~aphanitic fragments in a white to gray ground mass.~~

~~Two bodies of the white breccias which were~~

~~examined (samples 823 and 862) appear to display a type~~

of eutaxitic structure or flow banding which is quite 382 pronounced. Both show plagioclase, potassium feldspar, and quartz crystals together with minute rock fragments, all less than 2 mm.: in size, in a fine-grained microcrystal line ground mass.' This ground mass has an interesting bi-textural pattern.' Elon g ated and often wavy strips of the ground mass are completely crystalline and often appear to be bent around the outlines of nearby rock and crystal fragments." These bands are contained in a finer-grained ground mass, also crystalline (see Figure 39)• Whether this is an essentially rccrystallized shard pattern, or a texture resulting from recemntation of a sheared and shattered ground mass, the writer cannot state, although he prefers the former hypothesis. The rock fragments are. andesitic, and rare. As in other breccias, the whole mass is hard, solid, and tough, indicating that its components have been strongly fused together. Sample 862 is trachyandesitic and sample 823 is a quartz latite.

Sample 755 Is of a large white breccia dike. This body shows no eutaxitic texture. It is a quartz latite, composed of sericitized plagioclase, potassium feldspar, quartz showing angular fragmented boundaries with local rcsorbtion eabayments, all together with andesitic rock fragments in a sericite-plagioclase" ground mass. A few magnetite-epidote pseudonorphs after hornblende are present. 383

A large proportion of carbonate is present in the ground mass, and a considerable amount is present in the folia tion shadows of the larger grains and.fragments. A marked flow texture is seen in the ground mass (see

~~Figure 39).~~

~~Intrusive Magmatic Rocks Associated With the Breccias~~

~~Samples 413 (SW1/4 Sec. 3, Tl4s R26E), 4l4 (same as 413), 632 (NW1/4 Sec. 21, T14S R2?E), 701 (NE1/4 Sec.~~

~~6, T14S R27E), ?64 (WW1/4 Sec. 9, Tl4s R27E), and 850~~

~~(NVJ1/4 SecV 5. Tl4s R27E) are from various small dikes and plugs of dark aphanitic magmatic rocks which cut the breccia bodies inside the breccia terrain. All the~~

~~examined rocks appear to be true andesites, and all show~~

~~strong reaction breakdown to deuteric assemblages of~~

~~epidote, carbonate, and sericite.~~

~~Samples 413,and 850 are andesite dikes from the~~

~~western part of the main purple breccia unit (see Figure~~

~~21). Plagioclase forms the phenocrysts which make up~~

~~30-40^ of the rock. They are 0.1 - 1.0 mm. in size, show~~

~~a seriate size distribution, are very strongly sericitized~~

~~and apparently have a present composition near AnQ.~~

~~Numerous small pseudonorphs after biotite and hornblende~~

~~are present. The former hornblende is marked by euhedrallyr~~

outlined masses of magnetite with epidote, plagioclase. 384 or Motite cores. Former biotite is now narked by masses of magnetite with rare bands of sericite parallel to the former cleavage in the biotite. The ground mass of the units is chlorite, sericite, plagioclase, magnetite, and carbonateV plus local epidote, in an intimately intergrown assemblage.

Sample 4l4 lies quite near the two dikes discussed above, and is in general quite similar to them. It shows no ground mass carbonate, however, and the ground mass plagioclase is much less sericitized than the phenocryst plagioclase; most of the deuteric breakdown in this dike took place before final crystallization.

~~Sample 632 is from a purple andesite porphyry carrying about k0% plagioclase crystals approximately~~

4 mm.: long in a fine-grained ground mass of carbonate, sericite, plagioclase, magnetite, and hematite. Typical magnetite-sericite and magnetlte-sericite-biotite pscudomorphs are present after biotite and hornblende respec tively; these latter make up only 3~4£ of the rock.

~~The plagioclase pheriocrysts are very strongly sericitized and only small remnants of the original crystals are left; the feldspar composition cannot be determined.~~

~~Secondary biotite is present at the crystal-ground mass boundary locally.1 385~~

~~Sample 764- is an andesite porphyry. It is composed of about 45,3 plagioclase of composition Ang^, which is~~

~~strongly sericitized, in sizes from 0,5 to 4 mm., together with 15/5 reacted hornblende, marked out by various pseudo- morphous aggregates as noted earlier in other rocks. The~~

~~ground mass of the rock is composed of carbonate, plagio-~~

~~clase, and sericite (see Figure 40).~~

~~Sample 701 is an unusual rock for the Dos Cabezas~~

~~mountains. It is a hornblende andesite porphyry whose~~

~~original dark minerals are still largely unaffected by~~

~~deuteric breakdown or metamorphism.' It is the only sample~~

~~of any rock from the breccia terrain which shows this~~

~~characteristic. It has plenty of other evidence of reac~~

~~tion breakdown, however. Hornblende comprises about 5%~~

~~of the rock and is in acicular needles up to 2 - 3 mm.~~

~~long, and is generally quite unchanged, although a little~~

~~epidote has developed from it locally. Plagloclase is in~~

~~large, blocky crystals which are strongly sericitized and~~

~~epidotized, and compose some 5/5 of the rock. The contain~~

~~ing ground mass is of hornblende in needle-like crystals,~~

~~plagloclase, potassium feldspar, magnetite, and deuteric~~

~~(?) chlorite and epidote.~~

~~Petrography of Early Cenozolc (Paramide) Intruslves~~

Laramide intruslves in the Dos Cabezas range form 386 a complex and diversified'group, whose members can be grouped into a number of separate collections as below. All the groups discussed below show certain overall affinities, which will be pointed out in the discussions.

~~Diabasic Intrusives~~

~~Samples 389 (SE1/4 Sec. 14, T13S R26E), 644~~

~~(NE1/4 Sec. 17, T13S R26E), 645 (SE1/4 Sec. 15, T13S .~~

~~R26E), 879 (NS1/4 Sec. 29 T14S R28E), 897 (NE1/4 Sec. 20~~

~~T14S R28E) are of mafic intrusives in the Dos Cabezas moun~~

~~tains which have an essentially diabasic or ophitic charac~~

~~ter.~~

~~These diabases and ophites show marked granulation~~

~~and recrystallization of their mineral components,~~

~~especially the hornblende.~~

~~The diabase masses near the Cowboy stock (samples~~

~~389, 644, 645) show a number of effects which may, perhaps~~

~~be arranged serially and which seen to be unrelated to the~~

~~nearby stock of quartz diorite. Sample 389 is from a~~

~~body about half a mile north of the east end of the Cowboy~~

~~stock, and shows the original diabasic texture of the~~

~~rock quite well. It is composed of about equal parts of~~

~~former pyroxene and plagiocla'se. The pyroxene remains~~

~~as relict crystals in hornblende in a few places; horn~~

~~blende has replaced the pyroxenes almost completely, 38? retaining their original outline.' The hornblende is common~~

~~hornblende with a maximum extinction angle of 21 degrees,~~

~~A little chlorite and epidote have developed in some~~

~~crystals, but are rare,' The original plagioclase has~~

~~strongly reacted to sericite and epidote, and its original~~

~~composition and twin or zone pattern cannot be determined,~~

~~A fevr anhedral magnetite crystals have partially reacted~~

~~to sphene or epidote.'~~

~~Sample 645 is from a body very near the north~~

~~boundary of the Cowboy Stock, and shows approximately~~

~~equal proportions of hornblende and plagioclase. The~~

~~plagioclase is only slightly reacted but has some epidote~~

~~in it; the composition is A ii^ q .' Hornblende is present~~

~~in two phases; part is in solid crystals and part in~~

~~myriads of isotropically radiating needles gathered in~~

~~sheaflike aggregates. Pigeonite is present in a few~~

~~good crystals and reacted cores in some of the hornblende.~~

~~Some chlorite development has occurred in the hornblende.~~

~~Tile latter sample shows the development of a new~~

~~isotropic hornblende crystal fabric not dissimilar to~~

~~that seen in the foliated amphibolites in the Pinal~~

~~Schist in the eastern Dos Cabezas mountains; the cause~~

~~may be essentially the same. Thermal mctamorpiiism, in~~

~~this case resulting from original internal heat in the~~

~~diabases, caused growth and recrystallization of the 303 fragmented early crystals. The metamorphisn of both types nay be of the sane age.~~

Sample 64^ is from a body a mile and a half Host of the Cowboy stock exposure, but shows' the sane effects as 645 in a very much more pronounced way. Plagioclase is An^Q and often has an odd tan color in plane-polarized light; it shows no trace of reaction except for development of a little epidote. Hornblende is common hornblende; most is present in the fora of sheaflike aggregates of

~~acicular needles, and the needles are bigger than in~~

~~sample 645. No pyroxene was observed. Magnetite is pre~~

~~sent as numerous small circa 0.5 mm. anhcdral crystals~~

~~which make up some Ijv of the rock.~~

~~The overall view one gets of the evolution of this~~

~~group of diabases is as follows: the breakdown of the~~

~~original plagioclase-pyroxenc-magnetite assemblage to the~~

~~present horriblende-plagioclase-magnetite assemblage,~~

~~featuring good crystals of hornblende, occurred before~~

~~the event which developed the acicular needles. It also~~

~~appears that the development of the second hornblende~~

~~fabric took place after the extensive crushing and micro-~~

~~brecciation which has affected these units.~~

~~Sample 879 is from a body of the Government Peak~~

~~ophite on the southeast flank of Government Peak, and~~

~~shows a largely obscured original diabasic texture. A 389 few severely fragmented plagioclase crystals remain, which show an odd tan color like that in diabase sample 644.~~

Their composition is An^ • Hornblende, as in the more reacted diabases, is present in two forms, and the acicular form is greatly predominant. Hornblende as a whole i s a b o u t 80% of the rock, and it might have qualified as an ultramafic hornblendite before its marked recrystal lization.’ Biotite is locally present as minute anhedral crystals, and makes up some 2% of the specimen. About

~~1$ of sparse magnetite crystals are present.~~

~~Sample 897 is of a dike of the ophite from about half a mile south of the.mid-Tertiary Ninemile stock. It~~

~~displays no relicts of diabasic texture at all. About 80%~~

~~of the rock is composed of what at first appear to be~~

~~blocky 3 ** 4 mm.1 long crystals of hornblende. In thin~~

~~section these are seen to have an odd sheaflike character;~~

~~each Mcrystal” is actually composed of dozens of small~~

~~acicular crystals of hornblende with their long axes~~

~~parallel to one another and lying against one another.~~

~~The ends of these aggregates are frayed and ragged looking~~

~~in the extreme. The amphibole is common hornblende.~~

~~Plagioclase is present as untwinned very fine-grained~~

~~aggregates between the hornblende crystal aggregates,~~

~~and seems to have crystallized later than the hornblende.~~

~~It is biaxial negative, and probably a low-anorthite type 390~~

~~"between and An^g. Biotlte makes up about 1.% o f th e rock and is present as numerous small anhedral crystals, generally located near the margins of the hornblende~~

~~c r y s t a l s .4 A few small anhedral magnetite crystals are p r e se n t.'~~

~~The crystal paragenesis in these ophites is essen~~

~~tially the same as that in the diabases described above.~~

~~It is thought by the writer that the two groups of units~~

~~are essentially identical in time of origin and genetic~~

~~h is t o r y .'~~

~~Quartz Diorite Stocks~~

~~There are three separate quartz diorite stocks in~~

~~the central Dos Cabezas mountains, which share many simi~~

~~larities, and are in turn not unlike the earlier diabases~~

~~in some respects.~~

~~Cowboy Stock. Samples 318 (SW1/4 Sec. 15. T13S~~

~~R26E), 324 (551/4 Sec. 22, T13S R26E), 34? (SE1/4 Sec. 16,~~

~~T13S R26E), and 805 (NW1/4 Sec. 23, T13S R26B) are of the~~

~~Cowboy stock; this body is a small mesocratic Intrusive~~

~~which lies in the north central part of the Dos Cabezas~~

~~range (see Figure 28). Modes of the above samples are~~

~~given in Table 3•~~

~~The rock is quite mesocratic in general, being a~~

dark green color in the field. All the samples above 391 except 34? and 318 are part of the main body of the stock; the others are dikes. The main body of the stock is rather uniform/ In thin section, a typical sample shows very strongly zoned plagioclase, with a core composition of AityQ dropping to about An2^ in rims. The plagioclase

~~shows good euhedral forms, up to 0,‘5 mm.' long; these often~~

~~show thin late overgrowths of untwinned and unzoned plagioclase (alblte?).' Hornblende is present in good~~

~~euhedral crystals, pleochroic in green and with a 30-d e g r e e maximum extinction angle.' It has reacted in many spots,~~

~~forming large poikilitic crystals of biotite up to 1 mm.~~

~~long. Hi crocline and quartz are present and wholly~~

~~interstitial to the plagioclase, hornblende, and b iotite..~~

~~Minor sericite and epidote have developed in the plagio~~

~~c l a s e /~~

~~Sample 318 is of a small mesocratic body some 100~~

~~by 75 feet in size, which intrudes the Cowboy stock in~~

~~its northwestern part. The body carries pyroxene with a~~

~~2V of about 70 degrees, which is biaxial positive, has a~~

~~maximum extinction angle of 42 degrees, is nonpleochroic,~~

~~and appears to be diopsidic augite. There is also an •~~

~~epidote group mineral which has not been firmly identified;~~

~~this is biaxial negative, has a 2V of 80 degrees, a maxi~~

~~mum extinction angle of 25 degrees, high relief, a maxi~~

~~mum interference color of first-order red, and occurs in 392 equant crystals•' Both of these two types are generally In cluded poik llitically In anhedral hornblende crystals some~~

1 - 2 ram. across. Biotite occasionally appears on the margins of the hornblende crystals. Rare chlorite occurs on hornblende margins, too. Plagloclase Is very complex and shows wide variations in composition. Zoning is very strong in some crystals and absent In others. Cores as high as An^ have been seen; small crystals late in the crystallization sequence run about Ang^-An^, while the larger crystals run An^Q-An^0 on the average/ This rock body, though small in size, is quite interesting be cause of the large number of reaction steps it has gone through in its history which are still preserved in the mineralogy of the rock. It is a markedly unequilibrated a sse m b la g e .1

~~Sample 3^7 is of another dikelike mass, and is a high hornblende content variety, with large 3-^ mm.'~~

~~crystals of common hornblende. Plagloclase occurs in~~

~~large interstitial crystals 1 - 2 mm/ a c r o s s , w ith compo~~

~~sition ranging from Ang^ to Aj^ q in zoned crystals; most~~

~~unzoned crystals are An^-An^/ Some are as low as Angz~~

~~in composition .4 Local chlorite and epidote have developed~~

~~in the hornblende, and local sericite and epidote have~~

~~developed in the plagloclase. 393~~

~~. Silver Camp Stock.- Samples 440 (W l/^ See. 1, Tl^S~~

~~R26E), # 7 (NElA Sec. 12, Tl4S H26E), 618 (IfcJlA Sec. 12,~~

T14S R26B), 740 (SE1/4 Sec. 7» T14S R27E), and 741 (same as 74-0) are of the Silver Camp stock, which lies in the west central part of the Dos Cabezas range.' Samples ?4-0 and 741 belong to a small satellite pluton of the main s t o c k .4 Modes of these samples are given in Table 3«

~~A l l th e m ain sam p les show a common t e x t u r e .~~

Euhedral to subhedral plagioclase crystals up to 2 mm. long, showing considerable zoning, lie in a finer-grained ground mass of small euhedral to anhedral hornblende crystals, anhedral poikilitic biotite, potassium feld spar, and quartz .1 Plagioclase cores are up to An^ with rims down to An^^-Ang^.- The hornblende is common horn blende.' B iotite, where present, is commonly reacted to

Pennine chlorite in large part, It is rarely associated with hornblende but occurs as separate crystals, perhaps indicating separate nucleation in the melt and not re action formation from hornblende.

The satellite pluton represented by sample 740 has no potassium feldspar in its mode, and is generally quite like the Cowboy stock." Its hornblende is again common hornblende. Plagioclase in the examined specimen is largely reacted to epidote and sericite, and epidote is common in the hornblende. No plagioclase compositions could be determined. Biotite is now reacted largely to chlorite, as in the main stock.

The dike represented by sample 740 is a quartz- free syenodlorite, in which the plagioclase shows very strong reaction to epidote and serlcite. Common horn blende is abundant. Chlorite mats and epidote crystals are well-developed in the hornblendes. Potassium feldspar in the ground mass has been unaffected by this reaction.

~~Mascot Stock. Samples 615 (NE1/4 Sec. 21, T15S~~

~~B27B), 617 (SW1/4 Sec. 16, T14S R27E), and 633c (NE1/4~~

~~S e c . 2 1 , T l4s R27E) are of the Mascot stock, which outcrops in the south central part of the Dos Cabezas mountains,~~

Modal data on these samples are given in Table 3*

~~In thin section the rock is characterized by nearly complete breakdown of the primary phases. Plagio~~

~~clase of An^Q-An^tj composition is broken down to serlcite-~~

~~filled low-anorthlte plagioclase pseudomorphs after the~~

~~original feldspar. Hornblende has reacted to epidote-~~

~~chlorite masses, and the biotite originally present (?)~~

~~is represented by laminated aggregates of chlorite,~~

~~serlcite, and magnetite. In contrast to the strong re~~

~~action breakdown of the other primary phases, the potassium~~

~~feldspar in the ground mass of the rock shows no trace of 395 reaction breakdown; this implies that the other minerals broke down before final solidification of the rock.~~

Reaction breakdown of the primary components, al though severe, has not destroyed the primary texture of the stock, which is still, essentially, the same as that in the other two quartz diorite bodies described. The plagioclase is present in euhedrally-outlined masses of plagioclase and sericlte, which lie together with reacted hornblende and biotlte in a ground mass of interstitial potash feldspar and quartz. The three analysed samples

~~show the same mineralogy and only vary slightly in the proportions of the various phases.~~

~~Modal data reveal both sim ilarities to and dif~~

~~ferences from the other quartz diorite stocks in the~~

~~Laramide group. The Mascot stock is by far the most quartz- rich of these, and has a plagioclase content equivalent to~~

~~that of the Cowboy and a potassium feldspar content~~

~~equivalent to that of the average of the Cowboy and Silver~~

~~Camp s t o c k s .~~

~~. 4503 Quartz D iorite. Sample 925 (NU1/4 Sec. 25,~~

~~T14S R28F) is of a very small quartz diorite intrusion in~~

~~the easternmost Dos Cabezas mountains. Its mode is given~~

~~in Table 3. The body is very similar in texture and~~

~~mineralogy to the large stocks just described. It contains 396 plagioclase in a seriate assemblage of euhedral to rare anhedral crystals from 0.5 to 2 mm. long, showing marked~~

~~compositional zoning. The feldspar shows little reaction breakdown,' Observed twin extinctions indicate a composi~~

~~tion of Angy-An^Q for cores to An^-An^ for rims of~~

~~these crystals. The plagioclase is a little less anorthite~~

~~rich than that in the other quartz diorites. It shows a~~

~~weak oscillatory and strong normal zoning. Hornblende~~

~~is present as small 0.5 mm. equant to aclcular crystals.~~

~~Biotite is present as numbers of small crystals in and~~

~~around the hornblende, and as aggregates of small crystals~~

~~poikllitically enclosing quartz. The latter clusters are~~

~~evidently later in the paragenetic sequence than are the~~

~~former. Biotite encloses tabular apatite crystals up to~~

~~0.5 mm. across, anhedral sphene, and spares magnetite.~~

~~All the preceding minerals are in a ground mass of anhedral~~

~~quartz and orthoclase of about 0.$ mm. size or less. The~~

~~textural pattern is identical with that of other quartz~~

~~diorites in the range."~~

~~Buckeye Dike Group~~

~~Sample 816 (SW1/4 Sec. 27, T13S B27E) is representa~~

~~tive of a group of large dikelike intrusives which lie~~

~~in the northeast central part of the Dos Cabezas range.~~

~~These are dark weakly porphyritic units in the field;~~

~~they show 4 - 5 mm. blocky euhedral plagioclase crystals 397 in a 2 - 3 mm.1 biotite-rich quartz-feldspar ground mass/~~

The samples1 mode is given in Table 3*

In thin section the rock is seen to be a quartz nonzonite,1 containing what appears to be two different plagioclase suites. One suite is a seriate assemblage of clear tabular plagioclase crystals 0.5 to 3 mm. in size, of composition An^gAn^ with narrow, more sodic rims, good albite twinning, and numerous small equant epidote crystals in the plagioclase crystal cores. The

~~second suite is a seriate assemblage of 0.1 - 3 mm. some what more blocky euhedral to anhedral pale gray-blue un~~

~~zoned crystals showing no twinning of any kind except for~~

~~rather poorly defined broad anhedral patches with varying~~

~~extinction positions. These crystals show little or no~~

~~development of epidote. Orientation of these crystals~~

~~cannot generally be determined, and their composition is~~

~~unknown." Biotite is present as 0,5 mm. sieve-structure~~

~~poikilitic crystals enclosing quartz and epidote, with~~

~~interlayers of chlorite present in the biotite. Plagioclase~~

~~and biotite together are contained in a fine-grained~~

~~ground mass of microcline and quartz in anhedral crystals~~

~~a few tenths of a millimeter across.' Texturally as well~~

~~as mineralogically, these dikes are like the next-to-~~

~~be-described Maverick stock, and yet 398~~

~~the general texture of the dikes Is not unlike the quartz dlorite stock group already discussed. The albite-twinned plagloclase in the Buckeye dikes is more anorthite rich~~

~~than that in the Maverick stock, and is not antiperthitic~~

~~as is that in the Maverick, It may be that the Buckeye~~

~~dikes occupy some intermediate position in the general~~

~~differentiation and development of the early Tertiary~~

~~magmas in the Dos Cabezas mountains area.~~

~~Maverick Stock~~

~~Sam ples 569 (SMI/4 Sec. 28, T13S R27E), 584 (NEl/4~~

~~Sec. 19, T13S R27E), and 655 (NE1/4 Sec. 19, T13S R27E)~~

~~are from the large and complex Maverick stock in the north~~

~~central Dos Cabezas mountains. The unit is a complex~~

~~mesocratic quartz monzonite, containing large tabular~~

~~crystals of plagloclase antlperthite in a medium-grained~~

~~quartz-feldspar ground mass. Modes of the above samples~~

~~are given in Table 3«~~

~~In thin section, this unit can be seen to be a~~

~~calcic biotite-rlch quartz monzonite. It contains large~~

~~2 - 3 cm. plagloclase crystals and occasional 2 - 3 mm.~~

~~quartz crystals in a 0.5 - 1 mm. medium-grained ground mass~~

~~of biotite, plagloclase, microcline, and quartz. The~~

~~large plagloclase crystals are antiperthitic, with a few~~

~~percent of any section through one being composed of~~

~~wholly anhedral blebs of microcline. They contain numerous 399~~

"blebs of quartz, which may take up 20- 30^ of the crystal area, and also numerous blotite crystals and epidote crystals.' The plagioclase crystals are generally uniform until very near their margin, where strong compositional zoning takes place, as revealed by albite twin extinction angles." The cores, but not the rims, of most plagioclase crystals, are strongly serlcitized, and compositional determinations could only be made on a few grains. The core plagioclase is biaxial negative, with an angle be tween the two albite twin extinctions of 6.5 degrees; this indicates that it lie s between An^r, and An^^ in composition.

~~Around some of the plagioclase phenocrysts a tex~~

~~ture which might be called antlrapakivi is developed; a broad anhedral rim of untwinned potassium feldspar wholly~~

~~or completely surrounds the plagioclase constituents to ward the end of crystallization, which same is Indicated~~

~~also by the sudden development of marked zoning in the~~

~~plagioclase very near the completion of crystallization.~~

~~The composition of the outermost layers of the plagioclase~~

~~crystals is that of nearly pure albite; this should form~~

~~a good locus for crystallization of microcline when sodium~~

~~supplies are exhausted and potassium, aluminum, and~~

~~silicon are still available." The availability of micro-~~

~~cline constituents is proved by the fact that about one-~~

~~half the fine-grained ground mass in the rock is mlcrocline. 400~~

Biotite poikilitically encloses magnetite, sphene, epidote, and quartz, and is wholly enclosed by plagioclasc in places, but is never enclosed by mlcrocline. Biotite occurs in 0.5 - 1 mm.' crystals; quite often several small crystals will lie in contact with each other.

Biotite, quartz, and plagioclase lie in a 0.5 mm. quartz-microcline-plagloclase ground mass. Plagioclase is a small percentage of the ground mass, and occurs in small euhedral to subhedral crystals. The microcline and quartz are somewhat finer-grained than it is, and are wholly anhedral; the plagioclase may be essentially a phenocryst phase which fortuitously did not grow very large.: The microcline in the ground mass is very polkllitlc and encloses numerous blebs of quartz; small zones of mioro graphic intergrowths are present as w ell/

~~Basalt Dikes~~

~~Sample 657 (SE1/4 Sec. 17, T13S B26E) represents a large group of small basalt dikes in the westernmost part of the Dos Cabezas mountains. They are the youngest~~

~~Laramide units in the range. The mode of the sample is given in Table 3» it is a basalt or andesite of uncertain mineraloglc composition. The mode of the rock is about~~

~~60% plagioclase, 17^ amphibole, 10$ epidote, and various small proportions of other minerals (see Table 3). 4oi~~

~~TABLE 3~~

~~Modal Analyses of Typical Members of Laramide~~

~~Stock and Dike Group~~

~~P is a .' Amoh. B io .' S t z . K s p . Man: / Other Total C ts.~~

~~1/ D ia b a se s~~

~~8 7 9 -6 5 3 0 .3 6 2 .0 4 .6 2 .7 3 .0 e p i .: 474~~

~~6^ 4-65 4 9 .2 4 6 .8 3 .0 598 2/ Cowboy s to c k~~

~~806-66 60.0 2 1 .7 13.0 2.6 0.6 1.8 655~~

~~3 4 7 -6 4 6 4 .2 19.2 5 .7 7 .8 1.2 1427~~

~~3 2 4 -6 4 52.2 3 5 .6 0 .5 2 .4 2.6 p y r . 1127 2 .4 c h i.~~

~~3 1 8 -6 4 2 7 .5 62.8 2 .5 0.2 2.0 e p i . 310 3 . S i l v e r Camp • S to c k~~

~~4 4 7 -6 5 4 7 .5 11.8 4 .6 15.8 1 6 .3 2.7 1909~~

~~4 4 0 -6 5 4 6 .2 9 .2 1.0 9 .4 2 5 .4 3 .2 2 .3 m yra. 1400~~

~~6I 8-65 55 .'5 8.-3 3 .3 12.5 12.5 2.5 0.1 sp h . 2084~~

~~7 4 0 -6 5 66.5 1 7 .3 3 .8 3 .6 0.2 3.8 4 .8 h o rn . 524~~

~~7 4 1 -6 5 3 3 .2 6 1 .9 0 .3 6.9 0.1 5 .5 h o rn . 291 M ascot s to c k~~

~~6 1 5 -6 5 5 9 .6 7 .7 3 .7 1 7 . 5 8 .9 1.8 560~~

~~617—65 60.6 10:3 2 0 .4 7.7 0.8 1110~~

~~6 3 3 -6 5 4 8 .7 1 .7 15.8 2 5 .4 8 .3 0 .3 602 402 TABLE 3-Continued tfia ft.1 AmphV B io . Q tz. K sp. Hag_._ Other Total C ts,~~

~~5 .! M averick s to c k~~

~~569-65 3 1 .3 1 4 .5 26.0 20.5 2.6 4 .5 e p i . 619~~

~~5 8 4 -6 5 3 6 .2 1 4 .0 29.0 15.3 1.2 3 .0 e p i . 654~~

~~6 5 5 -6 5 3 5 .0 15.9 32.2 I 3. I 2.2 0 .8 e p i . 1 534~~

~~6 , 4503 6 0 .8 1 1 .2 8 .2 13.4 5.7 0.2 439 Q uartz D io r it e 2 2 5 7v Buckeye 13.9 2 2 .3 17.1 23.4 1.6 8 .6 e p i . 560 816 D ik e 1 2 .7 group~~

~~8.- B a s a lt 6 0 .2 1 6 .6 2 .2 9 .7 e p i.' 549 d ik e 652~~

~~All figures in percent~~

~~Abbreviations not obvious.:~~

~~(1) myna.' is mymekitic Intergrowths~~

~~(2) chi.' is chlorite~~

~~For mineral composition data see individual rock~~

~~descriptions in text.' 403~~

~~There is a fair amount of sericitization of the plagloclase in this rock, together with development of a~~

~~considerable amount of small carbonate g r a in sN o com posi~~

~~tion determinations could be made on the plagloclase. The hornblende has a maximum extinction angle of twenty~~

~~degrees* and has both a brown and a clear color phase;~~

~~numerous clear equant anhedral epidote crystals, mats of~~

~~green chlorite, and sparse subhedral magnetite are present~~

~~also.' Grain size runs about 0.05 mm. or less for all~~

~~these minerals." Modal carbonate in the ground mass areas~~

~~is characteristic of these dikes.~~

~~Although they are much younger, the basalt dikes~~

~~just described resemble some of the magmatic intrusives~~

~~which cut the Intrusive volcanic breccia terrain to their~~

~~southeast, especially in the carbonate-rich character of~~

~~their ground masses."~~

~~Petrography of Mid-Cenozoic Intrusives~~

~~The youngest group of intrusives in the Dos Cabezas~~

~~mountains ore a series of stocks and dikes of mid-Tertiary~~

~~age. They are not as homogenous a group as the Laramide~~

~~intrusives, although like them they share some common~~

~~affinities; in this case, toward a general plagioclase-~~

~~rich character.' The Intrusives are much more closely~~

~~allied with the.Laramide intrusive units than the Precanbrlan on es 404~~

~~"Turkey Track" Plagioclase Andesite Porphyry~~

A large dike swarm in the northwestern Dos Cabezas mountains is composed of coarsely porphyritic andesite porphyry. Perhaps 60% of the rock is composed of markedly euhedral and platy tabular plagioclase phenocrysts up to

~~3 -4 cm. across set in a fine-grained plagioclase-pyroxene- biotite ground mass. The rock is so coarsely porphy ritic that no mode was taken.~~

~~In thin section the plagioclase is quite homogenous to within a few hunderedths of a millimeter of the border of each crystal, where weak normal zoning is present.~~

~~Twinning on the albite and pericline twinning laws is everywhere present. . Extinction angle composition determin ations on albite twins indicate core compositions of about~~

~~An^r? zoning to Angy at the outer margins. Smaller euhedral crystals have less calcic cores in general; An^o was determined for one.~~

~~Pyroxene is present as small circa 2 mm. crystals of euhedral to subhedral character, found in both the ground mass and included in the plagioclase. It has a~~

3? degree maximum extinction angle, is biaxial positive with a 2V close to 80 degrees, is length-slow, nonpleochroic, and clear in plane-polar!zed light. It is tentatively classified as diopsidic augite. Some of the crystals show marked resorbtion indicated by wavy or scalloped edges, while others show simple terminated prisms that are quite

~~euhedral." Some complexities in the cooling history are~~

~~indicated by this,~~

~~Biotite is present as small subhedral crystals up~~

~~to 0.1 - 0.2 mm. in size, which give no indication of having~~

~~formed by reaction from the pyroxene, and is also present~~

~~as small crystals bordering or lying in pyroxene in such~~

~~a manner that a reaction formation mechanism for these~~

~~latter seems quite obvious. It is also present in minute,~~

~~wispy strands in the ground mass. This seems to be a~~

~~late magmatic reaction or crystallization phase produced~~

~~during final crystallization of the rock. Small 0.1 -~~

~~0,n2 mm. crystals of magnetite are scattered through the~~

~~r o c k /~~

~~The ground mass of the rock is a fine-felty ag~~

~~gregate of plagioclase, biotite, and rare pyroxene; the~~

~~average crystal size is about 0.01 mm.~~

~~Hornblende Andesite Dikes~~

~~Lying just south of the above "Turkey Track" an~~

~~desite porphyry dikes is a series of hornblende-bearing~~

~~andesite porphyry dikes. The rocks are generally very~~

~~weathered on the surface, showing a blotchy yellow appear a n c e .1 406~~

In this section, the rock shows small 1-2 mm. hornblende and plagloclase crystals in a very fine-grained ground massv The hornblende, perhaps 5/^ of the rock, occurs as small euhedral to skeletal subhedral crystals.

The hornblende is common hornblende. Plagioclase, per haps 10^ of the rock, is present as euhedral crystals and crystal fragments; twinning is weak in then and so i s z o n in g A tentative albite twin composition determina tion of Angy has been made, but the data are sketchy on

It.’ The ground mass is fu ll of small 0.1 mm. or less m icrolites of feldspar; the feldspar material between the phenocrysts, which is made up of these m icrolites, shows a distinct color pattern. Light colored equant areas 0.1 to 0.2 mm.1 across are present in a web of dark- ground-mass m aterial. No mode was made for this sample since the rock contains a high percentage of microscopic ally unidentifiable ground-mass material."

~~Dacite Porphyry Dike Group~~

The entire Dos Cabezas range is cut by a very large number of small plugs and dikes of an aphanitic yellow to white rock. Petrographically, these dikes usually show a few percent of small phenocrysts of feld

~~spar and quartz in a fine-grained aphanitic ground mass;~~

~~some units are wholly aphanitic." In section, the examined 40? units show small euhedral phonoorysts of plasioclase, of~~

~~composition An^-Ang*^ and up to 1-2 mm.' across. They are markedly twinned crystals, showing little or no zoning.:~~

~~The crystals by and large are reacted to assemblages of~~

~~s e r i cite . and a new plagioclase, in a rather patchy and~~

~~spotty manner.' The ground mass is a very fine-grained~~

~~assemblage of feldspar and quartz crystals with rare local~~

~~biotite. The feldspar crystals are too small to determine~~

~~their composition optically, but the presence of oligo-~~

~~clase in the phenocrysts implies that much of it is~~

~~plagioclaser Feldspar and quartz crystals in the ground~~

~~mass are all very heavily sericitized, and this tends to~~

~~obscure grain boundaries. The biotite is present as~~

~~minute crystals and wispy patches around the other grains,~~

~~and crystallized very late in the cooling history of the~~

~~rock.' In one sample, small bands of carbonate trend~~

~~through the rock, and appear to be a primary feature of~~

~~the dike; Local patches of carbonate are present in~~

~~these dikes in other areas, also.~~

~~Ninemile Granodiorite Stock~~

~~This stock is represented by samples $08a (S171/4~~

~~Sec. 9, T14S B28E), 508b (the same as 508a), 511 (NiIl/4~~

~~Sec. 11, T14S R28B), 516 (no loc.), and 622 (Stfl/4 Sec. 10,~~

~~T l4s R28E). Modes of these samples are given in Table 4.' 408~~

~~The rock is a leucocratlc very coarse-grained and weakly porphyrivic granodiotie.' It contains mmerous quartz pods and dikes.~~

In thin section the stock shows large 1-2 cm. euhedrally to subhedrally outlined crystals of plagioclase with very marked and strong zoning and good albite and perl- cline twin development.' The compositions of the plagioclase vary considerably from crystal to crystal. Cores as high in anorthite as An,.,, were determined in two grains by con- 55 bined Carlsbad-alblte twin extinction methods. The general run of cores was An^Q-An^ zoning to An^Q-Angg and as low as An^ on one crystal. -

These plagioclase crystals are set in a slightly . finer-grained mass of potassium feldspar, quartz, biotite, and magnetite;* The potassium feldspar is present as scattered poikilitic crystals containing quartz, plagio clase, and biotite." The feldspar shows weak fine g rill twinning, and is probably micro dine; it is possible that

~~it is anorthoclase, however. Biotite is present in anhedral masses and platy crystals, and is generally inter~~

~~stitia l to other phases.1 Quartz is present as scattered~~

~~euhedral crystals 0.5 mm. or less in size, and as 2 - 3 mm.~~

~~assemblages of numerous small crystals fritted together.~~

~~Magnetite is present as scattered small euhedral crystals of. complex form.' 409~~

~~31.7 Quartz Honzonite~~

A small body of quartz monzonite exposed several miles west of the Nlnemile granodiorite stock looks quite like It; in the field this unit outcrops over an area of only a couple of acres. Sample 317 (SW1/4 Sec. 15, T13S

~~R26E) is of this unit; its mode is given in Table 4.~~

Petrographically the unit shows 1 - 2 mm.- euhedral to subhedral plagioclase crystals with a few cores as calcic as An^0; most are about An^g-An^, grading to Angg or so at the crystal margins V The crystals are very strongly zoned and twinned. The plagioclase in the rock shows more variation than does that in any other rock in the range.1 The presence of these highly calcic cores in

~~some, but not all, plagioclases, is also a feature of the~~

~~Nlnemile granodiorite stock, and strengthens the field impression of equivalence between the two units.~~

~~The potassium feldspars seem to be anorthoclase,~~

~~and are present as 1-2 mm. crystals showing very fine~~

~~g rill twinning." Some of the crystals show perthitic~~

~~texture. The crystals commonly polkilitically enclose~~

~~small blebs of quartz and small plagioclase crystals.~~

~~Biotite is present as brown crystals with fine needle~~

~~like inclusions of rutile (?). H ornblende i s common~~

~~hornblende.' The plagioclase crystals are set in a ground mass~~

~~of these poikllitic anorthoclase crystals, which also en~~

~~close the biotite and hornblende. The presence of the~~

~~anorthoclase and the general textural sim ilarity to the~~

~~Ninemile granodiorite in terms of the paragenesis of the~~

~~main mineral phases again reinforce the w riter's impres~~

~~sion that the two units are offshoots of the same magma/~~

~~Minor Granitoid Intrusives Along the Northern Range Border~~

~~Along the north central border of the range, sev-~~

~~• eral small and complex granitoid bodies of alaskltic,~~

~~pegmatitic, or aplitic affinities crop out.’ Samples 385~~

~~(SE1/4 Sec. 15, T13S R26E), 390 (B ilA Sec. 24, T13S~~

~~R26E), and 834b and 834c (1^1/4 Sec. 15, T13S R26E) are~~

~~from such units. Samples 385 and 390 have modes reported~~

~~in Table 4, and both are biotite-bearing quartz monzonites.~~

~~Microscopic study was done on samples 834b and c, as fol~~

~~lo w s /~~

~~Sample 834c is from a body of muscovite-bearing~~

~~alaskite and is a local coarse phase of a more aplitic~~

~~unit.4 Microscopically, it contains plagioclase as 1-2 mm.~~

~~euhedral to subhedral crystals of composition An^^-An^^,~~

~~together with approximately equal proportions of micro~~

~~cline and quartz. A few small muscovite crystals were~~

~~observed in thin section, and a certain amount of serlcite~~

~~and muscovite have developed in the plagioclase. 411~~

~~TABLE 4~~

~~Modal Data on Mid-Terblary Rock Units~~

~~Rock Unit K[-soar Flag. Q uartz B i o t i t e Has.' Other Total Cts~~

~~N in em ile Granodiorite~~

508a 1 4 .1 4 9 .2 2 8 .7 6 .5 0 .9 0 .4 c h i. 1850 0 .4 a p . 508b 8 .0 5 6 :5 29.8 4 .6 0 .6 0 .1 c h i . 2106 0 .5 ap . 622 1 7 :2 3 8 .5 32.0 3 .3 0 .6 0 .5 c h i . 1101 0 .4 a p . 511 2 6 .0 51. I 1 7 .7 4 .3 0 .8 2074

~~516 1 4 .2 4 4 .1 3 4 .7 6 .6 ' 1 .2 2000 '~~

~~31? Q uartz M onzonite~~

~~317 3 0 .9 3 4 .4 26.2 4 .3 0 .6 2 .1 h b . 611 1 .4 c h i:~~

~~All above samples counted with 1 mm. grid on thin~~

~~s e c t i o n s .~~

~~Medium- g r a in e d g r a n it o id c u t t in g P o le c a t~~

~~390 3 2 .3 3 1 .9 2 4 .2 1 1 .5 553~~

~~385 3 9 .8 2 8 .6 2 5 .9 5 .4 554 Both above samples counted on mylar overlay grid~~

~~on stained slabs. 4-12~~

~~Sample 834-15 consists of 0.1 mm. crystals of microcline and quartz, wholly anhcdral, mailing together some~~

80/3 of the rock, together with small sheets and wisps of muscovite up to 0.5 mm.' locally, a few 1 mm.' quartz crys tals, and numerous small euhedral to subhedral magnetite crystals/ The rock here is a muscovite granite of aplitic texture, and is unusual in that it is the only body of true granite seen in the range. The writer tends to feel that these bodies are products of Tertiary pluton ism, along with the Ninemile granodiorite and the 31? quartz monzonite, but as the bodies lie in a larger pluton of Precambrian age and granitoid character, this assignment must be tentative/ APPENDIX B

~~GEOCHRONOLOGIC EXPERIMENTAL TECHNIQUES~~

~~This appendix contains a detailed discussion of~~

~~sampling and analytical techniques used by the author,~~

~~together with assembled geochronometric analytical d a ta .~~

~~Sam pling~~

~~The PreCambrian part of the Dos Cabezas mountains~~

~~shows extremely deep weathering in almost a ll parts, and~~

~~it is very difficult to find even reasonably fresh out~~

~~crops of most units. All PreCambrian rock samples~~

~~gathered for age determinations, with the exception of~~

~~that for the Sommer gneiss, were, therefore, obtained~~

~~by blasting the freshest available outcrops. Blasting~~

~~was continued until rock was obtained which showed no~~

~~sign of weathering. Weathering was considered to be absent~~

~~i f :~~

~~1. No iron staining was present along fractures or~~

~~around biotite and magnetite crystals in the rock.~~

~~2. No indication of oxidation, hydration, or carbon-~~

~~ation was present in thin section analysis of the sp e c im e n s.~~

~~413 414~~

~~3.' The rock was very solid and competent, with a pow~~

~~der of fine crystalline fragments on broken surfaces.~~

~~Blasting was carried out by use of an Atlas Copco~~

Company Cobra portable gasoline-powered jackhammer, with which the writer could d rill two to three foot long holes, one inch in diameter, in an hour. Between one and five samples could be collected per day, depending on depth of weathering and luck in choosing a good spot at which to b e g in .

~~Non-Precambrian samples were not generally collected by blasting, as fortuitous exposures in stream beds or road cuts provided fresh outcrop material for these units.~~

Samples collected from such spots were obtained by breaking up the rock with a five-pound maul and a chisel, and selecting fragments which met the requirements for being unweathered mentioned above. Most Laramide and younger rocks have a weathering crust which is quite thin, general ly a centimeter or less in width, and fresh samples can be obtained rather easily/

~~Whole-rock samples were obtained by taking samples at least 1000 times larger than the largest crystal in~~

the sampled rock; more than this was collected for the rapakivi samples, since the feldspars in it are rather unevenly distributed/ Samples were crushed in a large jaw crusher, and the individual samples analysed for con tamination by the jaws, After each successive sample operation, the sample was weighed and compared with its original weight, determined after this initial crushing.

~~Overall loss of dust and larger particles from this point on was never more than 2.% and usually less than 1%,~~

Because many people were using the crushing equip ment used by the writer, and because some dust from other samples must have been shaken out of the machine during use, two procedures were followed after initial cleaning of the machine to eliminate the cross-contamination hazard as much as possible. First, before the sample

Itself was run, several large pieces of the same rock were run through the crusher to clean off the jaws of the machine; after this an air blast was used to clean off the machine again and the analytical sample run through.

The sample fragments produced by the large crusher were mostly hand-sized, although some smaller ones and some dust were produced as well. The second contamina tion control step was to individually blow the dust off. each fragment and examine it closely; any which showed signs of jaw contamination were rejected, as were any which showed signs of weathering. Only a very few frag ments from each sample were rejected in this step. All 416 pieces a centimeter or less across were discarded, as was all dust. Between 95^ and of the sample was retained, and as the writer never noted that any one mineral type tended to be concentrated in the fine fragments, it is felt that no significant sampling bias was introduced using this technique.*

Smaller separation equipment could be cleaned more thoroughly than could the large crusherAfter t h e i r cleaning, samples were run directly, being weighed after each step to check mass losses. After passage through a small jaw crusher and a small roller crusher, the sample was reduced to a collection of fragments sand size and smaller.' This ground material was sp lit down to about ten pounds of material; special efforts were made to re tain as much dust as possible in this step. This split fraction was run through a small Braun pulverizer, until the rock fragments were a ll fine sand size or smaller.

~~This pulverized material was weighed to check for proces sing loss of material, and was then sp lit down.,to about~~

~~50 grams of sample.' This sp lit was then ground to less than 100 mesh in size in a Pica grinding m ill. This ma terial was placed in a polyethylene vial, and used for~~

~~Rb-Sr analysis. The remaining sand-sized ground material was treated for extraction of mineral separates if they were desired. 41?~~

~~Mineral samples were concentrated by various stan dard techniques and were not accepted unless at least 99% pure on a count of 1000 grains. Feldspar samples were~~

~~tested by staining grain mounts with sodium cobaltinitrate~~

~~solution to test for potassium, or by staining with barium~~

~~chloride and an amaranth fruit dye solution to test for~~

~~sodium/ All analysed samples of feldspar were at least~~

~~99% pure/ Between 2 and 30 grams of most mineral separates~~

~~was obtained.'~~

~~K-Ar Analysis~~

~~The K-Ar analyses were a ll done on single samples~~

~~from one vial, from which all the material for all the~~

~~argon and potassium determinations for that sample was~~

~~ta k e n /~~

~~Potassium Analysis~~

~~Potassium determinations were done on an exten~~

~~sively modified Perkin-Elmer 146 flame photometer. A~~

~~combined hydrofluoric acid and sulfuric acid digestion~~

~~technique was used/ 500 ppm sodium buffer and 800 ppm '~~

~~lithium internal standard were added to the sample solu~~

~~tion, which was diluted to 200 ml. volume after addition~~

~~of these standard aliquots and then analysed in the photo~~

~~meter, using a natural gas and air mixture for the flame.~~

Samples were run in duplicate, and the mean value of the 418 two taken as the analysis. If the difference between the determinations of the two samples was greater than l£ of the smaller determination value, the sample was re-analysed.

~~The potassium analysis technique has been fully described elsewhere (Damon et al, 1966) .i~~

~~Argon Analysis~~

~~Argon was run by a standard technique. Between~~

0.15 and 10 grams of the sample (depending on potassium content and age predicted) was placed in a molybdenum crucible within an alumina crucible; the alumina crucible was hung by a platinum wire inside a Pyrex glass vacuum bottle. The bottle was connected to a vacuum system of approximately 0.8 liters volume which contained the gas clean-up apparatus.' The system was baked under vacuum at least one day at 400 degrees E. After this and before sample fusion the vacuum system and the mass

~~spectrometer were tested dynamically for air leaks. This was done by spraying argon from a tank of compressed~~

~~atmospheric argon rich in the mass 40 isotope over the parts of the system while the mass spectrometer, to which the vacuum system was opened, was set to read~~

~~the m/e equals 40 peak. Any leak immediately showed up~~

~~as a rise in the peak. Pressure in the vacuum system,~~

~~in which fusion of the sample takes place, was always less than 10“? torr before fusion.~~

~~Fusion procedure was to raise the temperature of the sample slowly over about one-half hour, monitoring the~~

~~temperature with an optical pyrometer. Normal fusion~~

~~temperatures for biotite and whole-rock samples were about~~

~~li00-1200 degrees C., while feldspar generally fused at~~

~~1300-1^50 degrees C, After fusion, a single individually~~

~~calibrated spike or tracer of Ar-^ was broken open into~~

~~the fusion system and the Ar^® allowed to mix for a half~~

~~an hour at least with the sample gas.~~

~~These argon spikes are nearly pure Ar^®.; Analysis~~

~~of the gas gave the following composition (from Damon et~~

~~al, 1966).'~~

~~TABLE 5~~

~~Argon-38 Spike Composition~~

~~(Zurich III)~~

~~Isotope Holes/cc.- a-36 -12 0.0446 x 10 ,-12 8 8 9 .2 x 10 -12 2.-318 x 10~~

~~Argon spikes were calibrated by comparison with~~

~~aliquots of air of known argon content, and checked by~~

~~analysis of various standards. The current standard is 420~~

~~U.S.G.S. P-207 muscovite.' The analysis of this sample gave~~

~~12.62X10*"*0 moles/gn. of radiogenic Ar^°, which compares well with the present interlaboratory average for this~~

~~standard of 12.67 x 10“*® moles/gm. of radiogenic Ar^.~~

~~A detailed discussion of the Ar sir spike and~~

~~Ar^® tracer spikes preparation systems is given in Damon~~

~~et al (1966). . The analytical data for dated samples from~~

~~the Dos Cabezas mountains are given in Table 6.~~

~~Clean-up of the gases in the fusion system was~~

~~done with artifical zeolite, titanium metal sponge, and~~

~~copper oxide. After initial water absorbtion by the zeolite~~

~~during fusion of the sample, the titanium was slowly heated~~

~~to about 850°C and then slowly cooled to about 350°C. As~~

~~the temperature of the titanium sponge fell, the gas was~~

~~cycled back and forth over hot copper oxide held at 550°c»~~

~~with any water formed by reaction of hydrogen with the cop~~

~~per oxide being trapped in small liquid-nitrogen chilled~~

~~cold traps at either end of the tube through the copper~~

~~oxide furnace.' After this process, the gas was drawn~~

~~onto a small charcoal filled tube (cold finger) by im~~

~~mersing the tube in liquid nitrogen; the gas was held~~

~~there while the fusion system was pumped out for about~~

~~twenty minutes. All effective traces of hydrogen seem~~

~~to be removed from most samples in this manner. Other~~

~~chemically active gases such as oxygen and nitrogen were TABLE 6~~

~~Potassium-Arp;on Data~~

~~Sample and K,# Ar*°rad. A r ^ a t Apparent Mineral x 10""1®m./gm. Age,10oyrs~~

~~Precambrian~~

~~BCE-828-65 0.20 5 ; 84 21.6 1180-35 hornblende~~

~~RCE-331-64 3.7^ 99.79 14.9 1100-20 biotite~~

~~BCE-802-65 5.01 1 1 8 .56 3.7 1010^30 biotite~~

~~Laramide~~

~~BCE-689"65 5.92 6.73 6.5 62.4^1.9 biotite~~

~~BCE-806-66 7.38 7.84 13.3 59.oil.8 biotite~~

~~BCE-655-65 7.46 7.52 12.9 55.9-1.7 biotite~~

~~BCE-657-65 1.68 1.44 15.9 47.6-1.4 basalt, whole rock~~

~~Mid-Tertiary~~

~~BCE-329-65 1.00 O .63 78.7 35.2-3.1 plagioclase~~

~~BCE-636-65 4.93 3.00 14.9 33.9-1.0 dacite porph. whole rock~~

~~BCE-622-65 7.28 3.77 71.6 2 9 .oil.7 biotite 422 TABLE 6-Continued Ar^°rad. Ar^°at ,;t Apparent x 10"10m./gm; Age, 10°yrs~~

~~Ages Pre sumed to be Biased~~

~~BCE-829-65 5.74 5.08 16.5 49.1-1.5 biotite~~

~~BCE-840-66 7.46 7.80 33.4 58.1^1.7 biotite~~

~~BCE-853-66 7.73 7.11 24.4 51.1-1.5 biotite~~

~~BCE-851-66 I .65 1.56 12.1 5 2 .6—1.6 plagioclase~~

~~PED-25-6I 6.34 5.94 14.7 52± ? biotite~~

~~BCE-653-65 6.19 3.63 80.1 32.7-3.3 biotite~~

~~For sample locations and.rock types, see Figure 9*7.~~

~~All ages calculated using the following constants:~~

~~\ e = 0.589 x l O ^ V 1~~

~~X b = 4.76 x 10“10y"1~~

~~K42 1.21 x 10"4g/gK~~

~~All samples discussed in text under appropriate numbers. 423~~

~~removed by combining with the metallic titanium. If,~~

~~during later mass spectrometrlc analysis of the sample,~~

~~the gas was found to contain a large amount of gases such,~~

~~as nitrogen which should have been removed In the clean~~

~~up process, the sample was shut off from the mass spectro~~

~~meter and cleaned again.~~

~~After-clean-up the gas was transferred to a small~~

~~glass volume connected with the mass spectrometer by~~

~~being adsorbed onto the charcoal in a second cold finger.~~

~~After a twenty-minute freezeout, the Inlet into this~~

~~volume was closed, thereby isolating the cleaned gas and~~

~~the mass spectrometer from the fusion vacuum system.~~

~~With the gas still frozen down onto the charcoal finger~~

~~the inlet valve from this finger into the mass spectro~~

~~meter was opened while the machine was monitoring the m/e~~

~~equals 36 peak. If the peak was observed to rise, it~~

~~was assumed that some hydrogen was still present in the~~

~~. sample gas, and that it was escaping from the sample and~~

~~forming H^Cl-^ by uniting with an unidentified chlorine~~

~~source in the mass spectrometer. If the presence of~~

~~hydrogen in the sample was shown by this test, the sample~~

~~was recleaned; if only a small amount of hydrogen seemed~~

~~to be present the sample was often pumped on through the~~

~~fusion system for another ten to twenty minutes and~~

~~tested again; such a pumping often served to remove the last traces of hydrogen from the sample.~~

~~Gas samples were analysed dynamically, once, with the sample being consumed in the process; half-life of the sample ran about 10 to 15 minutes for most runs.~~

Generally, a minimum of five peaks of the 40, 38, and 36 m/e positions were recorded. These peak heights were recorded as a function of time on a strip-chart recorder and the data were then transferred to a semilog graph.

Decay lines were drawn by connecting the peak tops for each m/e position; these lines were run back to t equals zero, and the isotopic ratios at that time used for the argon analysis of the sample gas.

~~The mass spectrometer used for these analyses is a Mier-type, six-inch, metal tube, sixty-degree field, magnetic sweep machine, which is attached to the fusion~~

~~system by a small glass inlet and mounted on the same bench as the fusion system. Sensitivity for argon of~~

~~the instrument under the above operating conditions is~~

~~about 6 x 10~*3 moles/millivolt.~~

~~Overall accuracy in the potassium-argon analysis~~

~~is evaluated as follows.• Potassium data for a sample must~~

~~agree to within 1% for repeated determinations, as de~~

~~scribed above. The accepted potassium value for the~~

~~sample is then ascribed a,value of 1.75/* relative standard~~

~~deviation as a very conservative estimate of precision. 425~~

In the argon analyses, the argon-40 peak can be read with a precision of about 1% and the amount of spike plus atmos pheric argon-40 can be measured within about 2 % t The critical factor is the amount of the m/e equals 36 peal: and the precision with which it can be read.

The overall error in the Ar^0/ ^ 0 ratio is taken as the square root of the sum of the squares of the errors in the potassium and argon determinations. The proportion of atmospheric argon to radiogenic argon in the sample is the main source of the overall error, and a graph showing the evaluation of the overall error is given in Figure 41.

~~Rb-Sr Analysis~~

~~Rubidium-strontium analyses of suites of whole rock~~

~~and/or mineral samples were carried out for three Precam- brian plutons in.the Dos Cabezas mountains. Analytical~~

~~data are given in Table 7.~~

~~Rb-Sr Ratio Determination~~

~~Whole-rock powders and powdered mineral samples~~

~~from rock suites of interest are scanned by X-ray fluor-~~

~~esence to determine sample Rb/Sr ratios and to see if the~~

~~samples have enough spread in Rb/Sr values so that a good~~

~~isochron can be set up. If the preliminary analysis in~~

~~dicates that the sample suite is usable, very precise Rb~~

and Sr analyses are made by the X-ray fluoresence technique. yT JE/~ro/~ C'S & J r o /?c^e^s/7cc /Dc?/r?o/7 ■ So /oo f/gure 4/ ^ / Corr r r o ect/on C A/r ° ^ X JEr/~Qr / A * o t £)ue JE/-/-or /)f°and/he AfJfK40 Es-t/mated&ot/o 7y/>/co/ £br Precjs/on tx>r ^0/7o//sms £>ete/~n7i///t>g Rod/agen/c

~~_re Z > z 7 v 7 7 / z 7 / / 7 y A rrr>oA/o^)«/~/cX Conner/on, c /- q /, /d66, 426 42?~~

It has proven possible to get Rb and Sr compositional data by this technique which are comparable in accuracy and precision to isotope dilution results; all the Rb and Sr analyses in this study were determined by the X-ray fluoresence technique.5

Details of the Rb and Sr analytical techniques are given in Damon et al (1966). The key advance insofar as X-ray analyses are concerned lies in the application of a discovery by other investigators that the intensity of the molybdenum K-alpha Compton-scattering peak is, to a first approximation, inversely proportional to the mass absorbtlon coefficient of the sample; and, for any two samples, the ratio of their mass absorbtlon coefficients is constant for the region above iron. Calibration curves based on samples analysed for Rb and Sr by isotope dilu

~~tion analyses consists of counts/sec. Rb or Sr divided by~~

~~counts/sec. Mo K-alpha Compton plotted against ppm Rb or~~

~~Sr, These graphs are used to determine unknown concentra~~

~~tions of Rb and Sr from the X-ray fluoresence counting~~

~~data.~~

~~Strontium Isotopic Analysis~~

~~Rb and Sr total element concentrations are obtained~~

~~from X-ray data. Sr Isotopic analysis was carried out~~

~~for each sample by Isotope ratio analyses on unspiked 428 samples, according to the following procedure:~~

~~1;’ Samples are digested in Teflon beakers/ First,~~

~~the beakers are cleaned with soap and water, and~~

~~are then- given at least a four-hour soak in 1:1~~

~~HC1 solution. This is followed by at least four~~

~~hours soaking in 1:1 HNCy solution. The beakers~~

~~are then rinsed with a stream of doubly distilled~~

~~water taken from a Barnstead still and then dried.~~

~~2.' Circa 200 mg.' of sample powder from the same vial~~

~~from which the X-ray sample was taken is placed~~

~~in a crucible. Samples are run singly.~~

~~3^ After weighing, samples are moistened with a drop~~

~~of water, and acidified with 2 ml.' of 1:1 I^SO^~~

~~and 10 ml. of HF. 25 ml. of HP is added when the~~

~~first 10 ml.' is gone, and then 10 ml. after that.~~

~~Initial digestion temperature is 250°C. The sample~~

~~is reduced in volume to circa 1 ml." of concentrated~~

**2S04 solution. The sample is then cooled.

~~4. Centrifuge tubes and small column elutrate pickup~~

~~beakers are soaked overnight in 8N HNO^. Centri~~

~~fuge tubes are rinsed with 2N HC1 before loading.~~

~~5. “ 2-3 ml.' of HC1 is added to the sample. The solu~~

~~tion is decanted into the centrifuge tubes and spun~~

~~briefly. Half the liquid is decanted into a~~

~~storage beaker, dried, and stored.' 429~~

~~6 .' Several ion exchange columns loaded with Dovrex~~

~~50WX8 cation exchange resin in 2N EC1 are pre~~

~~pared." These columns are each rinsed with about~~

~~20 ml," of 2N HC1 to make sure the acid in each of~~

~~the columns is at this strength." Plastic thistle~~

~~tubes are then inserted in the top of the glass~~

~~column body; the base of the thistle tube is not~~

~~allowed to touch the resin. The remaining one-~~

~~half of the liquid in the centrifuge tube for~~

~~each sample is decanted into the top of a thistle~~

~~tube.' The acid solution in the columns is at the~~

~~top of the resin before the sample is added. 1~~

~~7»' A total of 135 ml. of 2N HC1, including the volume~~

~~of the sample, is run through the column. The~~

~~last 45 ml.’ is collected, and contains the strontium.~~

~~This is picked up in $00 cc. Vycor beakers. The sample is then heated at about 300°C until the~~

~~sample volume is reduced to about 5 cc; the solu~~

~~tion is then decanted into a 15 ml. Pyrex beaker~~

~~cleaned as described above.’ The samples in the~~

~~beakers are cooked to dryness; the beakers are~~

~~then cooled, labelled and put in the cupboard.~~

~~These small collecting beakers are discarded after~~

~~a single use.~~

~~8 .; For mass spectrometric analysis, 150 microliters~~

~~of 2N HC1 is added to the dried sample in its 430~~

~~beaker, and 25 microliters of the resulting solu~~

~~tion is placed on each of two rhenium filaments,~~

~~where the liquid is evaporated. These filaments~~

~~are then placed in a three-filament mount, which is~~

~~in turn attached to the current leads in the mass~~

~~spectrometer. The center filament is used for~~

~~ionization of emitted atoms, which are produced by~~

~~the two side filaments. The center filament is run~~

~~at a lower temperature than the other two.~~

~~9.‘ Hass spectrometric analysis is carried out on a~~

~~solid-source, six-inch, sixty-degree, first-order~~

~~focusing single collector mass spectrometer at an~~

~~approximate ion current of 10"**®a . for Sr.~~

~~Peaks at m/e equals 85, 86, 8?, and 88 are measured~~

~~rapidly in succession, so that about 20 measure~~

~~ments of each peak are made in twenty minutes~~

~~time.5 This group of peaks plus pre and post run~~

~~backgrounds constitute one run. Three such runs~~

~~are generally made on each sample. Precision of~~

~~the runs now stands at about 0.01% for (Sr®?/Sr^)n~~

Peak heights are calculated, and the data re corded as 87/86 and 88/86 ratios; all 88/86 ratios are adjusted to 0.1194, and the corresponding 8 7 /8 6 ratio normalized accordingly, by the following relationship (Damon et al, 1 9 6 7 ). 431

~~, (SreySr°°)m ( S r ^ / S r ^ s n~~

~~Isochron Calculation for Bb-Sr Sample Suites~~

The appropriate isochrons constructed by the fol lowing technique for the different sample suites analysed are shown in Figures31*32, and33• The isotope ratio values used in construction of these isochrons are given in Table

7. The isochrons shown in the above figures have been determined by a statistical double-regression technique using least squares analysis." The characteristics of the analytical method and the analysed systems prevented full use of the method in two of the three systems studied.

The least-squares technique involves fitting a straight line through an assemblage of scattered points on a Cartesian 2-variable graph, in such a way that the deviations of the points from the line are a minimum. In

~~the simplest kind of least-squares reduction, one of the~~

~~two variables is considered to be perfectly determined~~

~~for each point and all deviation from the line is con~~

~~sidered to be due to variation of the other variable from~~

~~its true value. Minimization of deviations for positioning~~

of the line is along the variable axis. Since the variables in an isochron plot are Hb^/Sr®^ and Sr^/Sr^, and since both are experimentally determined, variation in position of points on the diagram involves variation in both para meters, and this simple system is not strictly applicable.

A crude but simple way to partially over come this limita tion is to use double regression; in this variation of the basic technique, two simple regression lines are computed, holding each variable constant alternately, and the two lines are compared. If the two are very similar or iden tical, it can be presumed that they represent the "true" regression line very closely.

~~The line computed can be represented by y=bx+a, and a scattered collection of N points of Coordinates~~

~~(x, y) is related to a simple regression line by the~~

~~constants a and b, determined by: _ W XI xy - SxZjy w x : x2-(z:x/~~

~~a = Zy-AZ* AZ The 1-sigma standard deviation or “error width" of~~

~~the line can be computed by a value for the variable of:~~

~~ZCy-?)2 5 A/-/~~

~~Here y is the value of the variable at a sample TABLE 7~~

~~RubIdlum-Strontium Data~~

~~Sample Rb§7 (Sr87/Sr86) (Rb87/Sr86) xlO” m/gin xlO . m/gm~~

~~RCE-802-65 1.45 0.0589 1 .1 9 6 24.6 Whole rook~~

~~RCE-802-65 3.40 0.0446 1.576 7 6 .2 K-feldspar~~

~~RCE-802-65 0 .2 2 9 0.0815 0 .9 2 0 2 .8 1 plagioclase~~

~~RCE-802-65 5.18 0.0737 4.93 2 9 0 .9 biotite~~

~~RCE-829-65 0.737 0.189 0.784 3.90 Whole rock~~

~~RCE-830-65 1 .0 2 0 .0 9 9 4 0.910 1 0 .2 3 Whole rock~~

~~RCE-831-65 1 .1 0 0 .0 7 1 2 1 .0 1 15.45 Whole rock~~

~~RCE-8 3 2 -6 5 0 .8 2 6 O .1 2 9 0 .8 2 6 6.41 Whole rock~~

~~RCE-8^0 -6 6 0.681 0.189 0.773 3.61 Whole rock~~

~~RCE-841-66 0.798 0.154 0.810 5 .2 0 Whole rock~~

~~RCE-845-66 . 0.810 0 .1 3 9 0.823 5.82 Whole rock~~

~~RCE-854-66 0.764 0.193 0.785 3.97 Whole rock~~

~~Constants: R’o®^/Rb^^ =2.59~~

~~^B= 1*39 % 10"11years“1 point and y is the value of the variable where the regres~~

~~sion line crosses the variable axis through the sample on~~

~~the graph/~~

~~The standard deviation of the slope constant b can~~

~~be represented by:~~

~~Sfc =~~

~~This error is the error of the age of the suite,~~

~~since the age is a function of the slope of the isochron~~

~~line/ If/ in double regression, both lines are nearly~~

~~identical, the error for one line is essentially the~~

~~error for the "true" regression line. Similar techniques~~

~~have been used by other investigators (see Pidgeon, 1 9 6 7 )•~~

~~The complete system as described was used in the~~

~~analysis of the Dos Cabezas rapakivi quartz monzonite~~

~~(Figure31). In the analysis of the Eaton gneiss (Figure~~

~~3 2 ) the Sr^/Sr®^ values were so close together that when~~

~~used as x they led to a small and very inaccurately deter~~

~~mined difference between two very large numbers, which,~~

~~in turn, led to a very inaccurate determination of the~~

~~denominator in the slope constant formula for b; conse~~

~~quently, only one regression line was computed for this~~

~~sample using Sr8^/Sr8^ as y/* Also, the point on the dia~~

~~gram for sample 840 of the Eaton falls far enough below~~

~~the regression line computed for the other three samples that if it is. used in computation of a four-sample isochron line, the initial Sr®?/Sr®^ ratio for the Eaton system~~

~~comes out at O.6 9 6 , which is too low to "be reasonable.~~

~~Using just the other three sample points an initial Sr®?/~~

~~Sr®^ value of 0.'703 is obtained, which is more realistic.~~

~~Error values were computed for the three-sample isochron~~

~~line.5~~

~~In the analysis of the mineral-whole rock sample~~

~~isochron for the Polecat quartz monzonite (Figure 33 )~~

~~there is sufficient evidence of disequilibrium among the~~

~~samples so that only one regression line was calculated~~

~~as a basic for discussion of the suite. No error values~~

~~were computed for this isochron, as it was felt that they~~

~~would be unrealistic.- REFERENCES~~

~~Backlund, H. , 1938, The problems of the rapakivi granites: Jour. Geol. v. 4o pp. 339-396~~

Cooper, J., i9 6 0 . Reconnaissance geologic map of the Willcox, Fisher Hills, Cochise, and Dos Cabezas quadrangles, Cochise and Graham counties, Arizona: U.S.G.S. Mineral Investigation Field Study MF-231

~~______, 1961, Turkey Track porphyry; a possible guide for the correlation of Miocene rocks in southeastern Arizona: Arlz. Geol. Soc. Digest v. 4, pp. 17-35~~

~~Cooper, J ., and Silver, L., 1964, Geology and ore deposits of the Dragoon quadrangle, Cochise county, Arizona: U.S.G.S. Professional Paper 4l6~~

~~Creasey, S., and Kistler, R., 1962, Ages of some copper bearing porphyries and other igneous rocks in southeastern Arizona: U.S.G.S. Professional Paper 4500* pp. D1-D5~~

~~Damon, P., and associates, 1 9 6 6 , Annual Progress Report No. COO-689-50 to U.S. Atomic Energy Commission~~

~~______, 1 9 6 7 , Annual Progress Report " No. COO-689-50 to. U.S. Atomic Energy Commission~~

Damon, P., and Bikerman, M., 1964, K-Ar dating of Laramlde plutonic and volcanic rocks within the Basin and Range province of southeastern Arizona and adjacent areas: Arlz. Geol. Soc. Digest v. 7 , pp. 6 3 -7 8

Damon, P., Livingston, D., and Erickson, R., 1 9 6 2 , New K-Ar dates for the Precambrian of Pinal, Gila, Yavapai, and Coconino counties, Arizona: New Mexico Geol. Soc. Guidebook, 13th Ann. Field Conf., pp. 5 6 -5 8

~~436 437 Damon, P., and Hanger, R., 1 9 6 6 , Epelrogeny - orogeny viewed from the Basin and Range Province: Trans, A.I.H.E. v. 235, pp. 99-112~~

~~Everenden, J., and Curtis, G., i9 6 0 . Argon diffusion in glauconite, sanidine, leucite, and phlogoplte: A. Jour. Sci. v. 2 5 8 , pp. 5 8 3 -6 0 4~~

~~Fairbairn, H., and Robson, G., 1942, Breccia at Sudbury, Ontario: Jour. Geol. v. 50* PP« 1-33~~

~~Paul, H., 1 9 6 6 , Ages of Rocks, Planets, and Stars: McGraw- Hill, Inc.~~

Fechtig, H., Centner, W ., and Kalbitzer, S ., 1959* Argon- best immungen an Kaliummineralien - VII: Diffusion- sverluste von Argon in Mineral!en und ihre Auswirkung auf die Kalium-Argon Altersbestimmung: Geochem. et Cos. Acta v. 19, pp. 70-79

Follinsbee, R., Baadsgaard, H., and Lipson, J., 1961, Potassium-argon dates of upper Cretaceous ash falls, Alberta, Canada, in Geochronology of Rock Systems, J. L. Kulp, ed., New York Acad, of Sci. Annals, v. 91, Art. 2, pp. 159-594

~~Gillerman, E., 1958, Geology of the central Pelloncillo mountains, Hidalgo county, New Mexico, and Cochise county, Arizona: New Mexico Bur. Mines Min. Res. Bull. No. 57~~

~~Gilluly, J., 1956, Geology of central Cochise county, Arizona: U.S.G.S. Professional Paper 28l~~

~~Goldsmith, J., and Laves, F., 1954, The mi crocline- sanidine stability relations: Geochem. et Cosmo. Acta v. 6 , pp. 100-118~~

~~Halva, C ., 1961, A geochemical investigation of basalts in southeastern Arizona: unpub. University of Arizona H.S. thesis~~

~~Hart, S., 1964, The petrology and isotopic mineral age relations of a contact zone in the Front Range, Colorado: Jour, of Geol. v. 72, pp. 4 9 3 -5 2 5 438~~

~~Hawley, J., 1 9 6 2 , The Sudbury, ores, their mineralogy and origin - Pt. 3» Interpretations - the history and origins of the Sudbury ores: Can. Mineralogist v. 7» PP» 146-20?~~

~~Jones, P., and Batcheller, L., 1953» Measured sections near Dps Cabezas, Arizona: New Mex. Geol. Soc. 4-- Ann. Field Conf.~~

~~Jones, R., 1 9 6 3 , Structural evolution of a part of southeastern Arizona: A.A.P.G. symposium on the Backbone of the Americas, A.A.P.G. Memoir No. 2~~

~~Kulp, J., 1 9 6 1 , The geological time scale: Science v. 133, PP. 1105-1114~~

~~Leva, M., 1959, Fluidization: McGraw-Hill, New York~~

~~Livingston, D., and Damon, P., 1 9 6 7 , The ages of stratified Precambrian rock sequences in central Arizona and northern Sonora: Can. Jour. Earth Sciences v. 5, PP. 763-772~~

~~Livingston, D., Damon, P., Mauger, R., Bennett, R ., and Laughlin, A.W., 1 9 6 7 , Argon-40 in oogenetic feldspar-mica assemblages: Jour. Geoph. Res. v. 72, pp. 1361-1375~~

~~Mayo, E., 1 9 6 3 , Volcanic orogeny of the Tucson mountains (a preliminary report): Ariz. Geol. Soc. Digest v. 6 , pp. 61-82~~

~~Mielke, J., 1964, Trace element investigation of the "Turkey Track" porphyry, southeastern Arizona: Ariz. Geol. Soc. Digest v. 7, pp. 87-96~~

~~Nat. Acad, of Sciences - Nat. Res. Council, 1 9 6 5 , Geochronology of North America: N.A.S.-N.R.C. Pub. 1276~~

Parsons, W., I960, Origin of Tertiary volcanic breccias, Wyoming: XXI Int. Geol. Congress, Norden i960 Part 1 3 , pp. 139-146 439 Patterson, E. , 1 9 6 3 , Tertiary vents in the northern part of the Antrim Plateau, Ireland: Quart. Jour. Geol. Soc. of London v. 119, pp. 419-443

~~Pidgeon, R., 1 9 6 7 , A rubidium-strontium geochronological study of the Willyama Complex, Broken Hill, Ausralia: Jour. Petrology v. 8 , pp. 283-324~~

~~Pitcher, W., and Read, H., 1952, An appinitic intrusion- breccia at Kilkenny, Haas, County Donegal: Geol. Magazine v. 8 9 , pp. 32 8 -3 3 6~~

~~Raguin, E., 1 9 6 5 , Geology of Granite: Interscience~~

~~Ransome, F., 1904, Geology and Ore Deposits of the Bisbee quadrangle, Arizona: U.S.G.S. Professional Paper 21~~

~~Read, H., 1944, Meditations on granite (Part.2): Proc. ' Geol. Assoc. LV, pp. 45-94~~

~~______, 1957, The Granite Controversy: Interscience~~

~~Ross, C., and Smith, R., 1 9 6 1 , Ash-flow tuffs: their origins, geologic relations, and identification: U.S.G.S. Professional Paper 366~~

~~Rouse, J ., 1937* Genesis and structural relationships of the Absaroka volcanic rocks, Wyoming: Geol. Soc. Am. Bull. v. 48, pp. 1257-1296~~

~~Sabins, F., 1957a, Stratigraphic relations in the Chiricahua and Dos Cabezas mountains, Arizona: A.A.P.G. Bull. v. 4l, pp. 466-510~~

~~______, 1957"b, Geology of the Cochise Head and western part of the Vanar quadrangles, Arizona: Geol. Soc. Am. Bull. v. 0 8 , pp. 1315-1341~~

Sederholm, A., 1928, On orbicular granites, spotted and nodualr granites, etc., and on the rapakivi texture: Bull, de la Comm.. Geol. de Finlande v. 83 Shields, J., 19^0, Geology of the Dives and Gold Ridge groups, Dos Cabezas, Arizona: unpub. University of Arizona M.S. thesis

~~Silver, L., and Deutsch, S., 1 9 6 1 , Uranium-lead method on zircons: Div. of Geol. Sciences Pub. 1012, Calif. Inst, of Tech.~~

~~Speers, E., 1957* The age relations and origin of common Sudbury breccia (Ontario): Jour. Geol. v. 6 5 , pp. 497-514~~

~~Terzaghi, R . , 1940, The rapakivi of Head Harbor Island, Maine: Amer. Mineralogist v. 25, pp. 111-122~~

~~Thomas, J., and Cambell-Smith, 0., 1932, Xenoliths of igneous origin in the Trtgastel - Ploumanac’h granite, Cotes du Nord, France: Quart. Jour. Geol. Soc. London v. 88, pp. 274-296~~

~~Turner, F., and Verhoogen, J., 1 9 6 5 , Igneous and Meta- morphic Petrology: McGraw-Hill, Inc., New York~~

~~Tuttle, 0,^ and Bowen, N., 1958, Origin of Granite in the Light of Experimental Studies: Geol. Soc. of America Memoir 74~~

~~Volborth, A., 1962, Rapakivi-type granites in the Precambrian complex of Gold Butte, Clark county, Nevada: Geol. Soc. of Amer. Bull. v. 73, pp. 813—832~~

~~Wahl, N., 1925, Die Gesteine des Wiborgen Rapakivi Gebietes: Fennia v. 45~~

~~Wasserburg, G., 1962, A study of the ages of the Precambrian of Texas: Jour. Geophys. Res. v. 6 7 , pp. 4021-404?~~

~~Wasserburg, G., and Lanphere, M., 1 9 6 5 , Age determinations in the Precambrian of Arizona and Nevada: Geol. „ Soc. of Amer. Bull. v. 7 6 , pp. 735-750 441~~

~~Wilson, E«, 1939» ProCambrian Hazatzal revolution in central Arizona: Geol. Soc. of Amer. Bull, v. 5 0 , pp. 1113-1164~~

~~______, 1962, a resume of the geology of Arizona: Ariz. Bur. of Mines Bull. 171~~

~~Zurbigg, H., and Geological Staff, 1957, The Frood-Stobie mine (Ontario); in v. 2 of C.I.M.M. Geol. Div., Structural Geology of Canadian Ore Deposits, pp. 341-350~~