The Petrology and Geochemistry of Archean Volcanics, Western

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THE PETROLOGY AND GEOCHEMISTRY OF ARCHEAN

VOLCANICS, WESTERN VERMILION DISTRICT,

NORTHEASTERN MTNNESOTA

A THESIS

SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL

OF THE UNIVERISTY OF MINNESOTA

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

DECEMBER 1977 TABLE OF CONTENTS i Page LIST OF TABLES ••••.•••••••.•••••••••••••••••••••••••• , ••••••• iv

LIST 0}' FIGURES •••••••••.••••••••.. ••••••••••••••••••••••••••• v

LIST OF APPENDICES ••••••••••••••••••••••••. ••••••••••••••••••• x

DEDICATION ••••••••••••••••••••••••••••••••••••••••••••••••••• xi

ACKNOWLEDGEMENTS ••••••••••••••••••••••••••••••••••••••••••••• xii

ABS TRACT ••••••••••••••••••••••••••••• , ••••••••••••••••••••••• xiii

INTRODUCTION ••••••••••••••••••••••••••••••••••• , •••••••••••••

THE VERMILION DISTRICT ••••••••••••••••••••••••••••••••••••••• 3

General Geology ..••••...•...... •...••...... •...•...••• 3

Stratigraphy .....••..••...... •.•...•....•...... ••..•. 5

Structural Geology ...... •...• • ...... •...... •.. 8

Metamorphism...... •....•...... •• • ..••..•...... • 9

Geochronology ...•.....•...•....•...... ; .•..•...... 10

Previous Petrologic and Geochemical Studies .•...... •.•... 12

NATURE OF THE PRESENT STUDY •••••••••••••••••••••••••••••••••• 16

Specific Objectives ...... ••••..•..•...... •...... • 16

Methods of Study ...... •.•.....•....••....•.•.....•.. 18

GEOLOGY OF VERNILION DISTRICT VOLCANIC ROCKS ••••••••••••••••• 21

Ely Greenstone ...... •••...•...... •..•. , .•...•...... 21

Lower Ely. Greens tone Member ...... •.....•...... •.• 23 Pillowed and Massive Flows ...... 27 Petrography ..•...•.•.•.....•...... •.....•.•.... 28 Volcaniclastic Rocks ..•.....•...•..•..•...... •.•... 29 Petrography ...... • ·.· ...... ••...... •.• 31

Porphyries ...... $ ...... 32 Petrography ...... ••....•...... •...•... •• , ...... 32 Plagioclase - Quartz Porphyries ....•...•.... 32 Plagioclase - Hornblende Porphyries ...... 33 Geologic Environment ...•...... •...... ••.. 33 ii Page Upper Ely Greens tone Member .•...... •.•.•...... •. 38 Mafic Pillowed to Nassive and Diabases ..•...... •... 39 Petro.graphy •...... •.•.• . .•... .•...... •.•.....•• 41 Green Basal ts .•••.•....••..••.....•...... •. 41 Gray Porphyritic Basalts •...... ••.....•••. 44 Felsic Flows ...... ••••....••••..•...•...... •.• 48 Petrography ...•.•.....•.•..•...... •.... 49 Vo lean iclas tic Rocks . •.• ....•••...... •...•..•...• 52 Environment of Formation ..•.••••.•.•...... • 53

Lake Vermilion Formation ...••••.•...•••...... •.....• 55 Petrography •.....•.•...... •..••.•...... 55

Newton Lake Formation ...•...... •...... ••... ·, • • . • . . . . . 56

Fels ic Member. . • . . • . . . • . . • • • . . . • • • • . . . • • . . • ...... • . . . • . 59

Mafic Volcanic Member. . . . . • • • . . • • . • • • ...... • • . . . . . • . • • 60 Cedar Lake Area. . . • . • • • . • • • • • . • • • • . . .. • . • . • • • • • . . . . . • 63 Other Localities and Basalt Types...... 82 Newton Lake Sills and Their Chilled Margins...... 92 Significance of Textures in the Newton Lake Flows... 95 Geologic Environment.. . . • . • . • . . . • . • • . . • ...... • . 107

MAJOR AND TR.ACE ELEHENT CHEMISTRY ....••...•..•...... 109

Introduction. . . • . . • ...... • . • . . • ...... • . . • ...... • . . • • . • 109

Chemical Effects of Alteration ••...••..•.•...... • ,.. 112 Vermilion Greenstones...... • . 114 Conclusion ...... •...••••.••....•..••...... ·.•... 12.8

Classification. • ...... • . . . . . • ...... • ...... • ...... • 130· · Classification Based on Percent of Si0 . ..•.•....••. 132 2 The Method of Irvine and Baragar ...... •.•..•. . • 132 The Jensen Cation Plot...... 134 Conclusion. . . . • . . • • . • ...... • • . . • . • . • • . • ...... • . . • . • 140

Chemical Variation of the Vermilion District Volcanic Rocks. . . . . • . . • . . . . . • ...... • • ...... • . • . • . . 142 Introduction...... 142 Ely Greens tone. . . . • . . • . • . . . . • . • . . . . . • . • ...... • • . 143 Lower Ely Greens tone Member...... 143 Upper Ely Greens tone Member...... 150 Ely Felsic Flows and Vermilion Porphyries...... • • 176 Lake Vermilion Formation Basalts...... • . • • • • . . . . . • • 180 Newton Lake Formation. . . . . • • ...... • . . . . . • • . • • . • . . • • . 182 Fels ic Hemb er .. . . . • . • . . . . • ...... • . . • ...... 182 Yafic Member...... 184 Rb-Sr Isotopic Systematics ....•. , ..•.. • .. . .• 195 Conclusion...... 206 Comparison with Recent Mafic Rocks ...... •... 207 Komatiites...... 207 iii f,age Classification and Chemical Characteristics. 211 Compositional Variations and Comparisons ..•• 213 High Iron Tholeiites and the Type A Basalts. 219 Conclusion ...... · ...... 228

PETROLOGY .•••••.••.••.••...•.••.•..•- •.•••••...••.•••...••..•. 229

Vermilion Cale-Alkaline Volcanic Rocks .•....•••.••.....• 229 Con cl us ion .•••.••••.•••..••...••.•• • .•.••.•••.•..•.·• 235

Upper Ely Greens tone Member ..•.••.••• , .•.•..•...•••...•• 238

Newton Lake Formation ..•..•••.••..•- .••.•.- •••••.••.•.•.•. 262 Pyroxene Chemistry ..•••..•••..••.•.•.•.•.•..•....••. 262 Petrogenesis of the Newton Lake- Basalts •..•••.....•• 271 Type C Basal ts .••.•• -••- .•..•••••...•••.••.•.•...• 271 Type A Basal ts ....••••..•.••...... •.•..••••..•.• 281

Conclusion ...... ,.. o. C> 290

CONCLUSION •.••.•..••.•.....••..•..•.•..•.•.••.•.••••..••..••• 292

Geologic History of the Vermilion Greenstone Belt. .•.•.• 300 Discusssion •..••.••.•...••..•..••••...••..•••...•.•. 303

Tectonic Hodel ...... ••...•.••••.•••..•.•.••..•._ •• 307

BIBLIOGRAPHY •...... •...... •...... •....•. 311

APPENDICES •• ••• _••••••••.••.••.••••••••.•••••••••••••••••••••• 329

Appendix 1 ..••.....••...... ••....•...... •.•...... 329 Appendix la ...... 330 Appendix 1b •....•...•.•..•.•..••.•.••...•.•••••...•• 331

Appendix 2...... • ...... • . . • • . . . . . • . . • . . . . . • . . • . . • • . • . • 338

Appendix 3.. . • . •. • . . . • . . . . . • . . ... • ...... • . • . . • ...... • 345 LIST OF TABLES.

Table

1. Stratigraphic Sequence, Vermilion District ..•• 6 2, Important Rb - · Sr ages ....••.• ,•,• .••.• , .•. !,...... 11 3. Arth and Hanson conclusions for plutonic rocks ...•.•. l4 4. cooling rRte study, 15, quartz nonnative. basalt ..•.. , •.... , ••••• , •. , ••.••..•.. ,., •.. 100 5. a) Summary of the classification of Ely Greenstone volcanic rocks •..•...•.•..••.•••.•..•.•.•...... 140a b) Summary of the classification of Newton Lake volcanic rocks ...•...... •.•••.•••.•.....•...... , • 1 40 b 6. Archean and recent calc - alkaline volcanic rocks •••• 149 7. Low Ti02 basalts, upper Ely Greenstone •...... •.....•. j55 8. High TiOz basalts, upper Ely Greenstone •...... •. 155 9. Archean and recent tholeiitic basalts ..•...•....•..•. 166 10. Rb - Sr isotope data, Newton Lake mafic member ....••. 199 11. Representative pyroxene compositions from gray, high Ti0 basalt, upper Ely Greenstone ....•...... •...••... 239 2 12. Mixing calculations for upper Ely Greenstone basalts.248 13. Calculated primary upper Ely Greenstone ..... 2.50 14. Representative pyroxene compositions from Newton:Lake Formation basalts, ., , .•. ••...•.•..• . •...... •. ·263 15. Calculated Newton Lake sill compositions ...•...... •. 275 16. Mixing C:'!lculations for Type C Newton Lake basalts .•. 276 17. High pressure mixing calculations, Newton Lake basal ts ...... •..• • ...... • •...... , •...... •.... 288 LIST OP FIGURES v Figure Page 1. Geologic map, western VE:rmilion district •.•...... 4 2. Nap of exam]ned lithologic units and specific loca:.. · tions, weste:.:n Vermilion district. ....•....•....••.•. 7 3. Sampling corridors, this study ...... •..•....•....••.. 17 4. Lower Ely Grecnstone, mapped sections ..••.•....•...•. 25 5. Photomicrograph uf plagioclase pheo.ocrysts, . porphy- . ritic andesite, lower Ely Greenstone ••.•. , ••...... • 30 6. Tuff lov:er Ely Greenstone. ..•••• • ·•.••.•.••••• 36 7. Pillow of gray basalt, upper Ely Greenstone ..•...•.•. 36 8. Pho tomicro graph; typical upper Ely Greeas tone .••••••• 42 9. Photomicrogre!ph, ske.leta:L plagicclase microlites in upper Ely Green stoP..e be.sa1. t; ...... 43 10. Photomicrograph, skeletal plagioc.lase microlites in upper Ely Greenstone basalt ....•.•..••.•••..• , ..•• , •• 43 11. Photomicrogr..5.pll of glcneroporphyTitic gray basalt, upper Ely G.t.e£ns to:Qe .•••• . •••••••••••••• . 46 12. Photomicrograph of gra;t upper Ely Greensto01e. 47 13. Photomicrograph of ·glomeroporphyritic pyroxene, gray · basalt, upper Ely Greenstcne ..•.•.•.•.••..•.•.••..... 47 14. Garnet amphibollt,:! in uppet.- E:i.y Greenstone .•....•.••. 50 15. showing inclusions in gan1et porphroblasts ...... •....•.... ·•.• ·.. , ...• ....•. 50 16. Photomir.: rog!'.'aph· of horc.blende garnet laye::..-- in a:mp!-1il>oli·te . . , ...... ,, .. ,...... " .... . 50 17. Phot:::;::r::.crograph !:ihc·.wfr.g puruict:!OUS tuff u;>per Ely ...... •..•..••.. .. •....••...... • 57 18. Geologic map of Cedar La.kc a"!. ea. •••••••••••••••••••••• 61 19. F .. ... r. .-..1 ·- · -- ·· in.. "'••' cw- .. .. on T... , ., • :. ·"'I · ... r .·.·...or ...... , ...... 64 20. in Ne;'ltcn ;:,ake FrJn:1at:L:'lJ. ••••••.••••• 6/f ' 21. l'hotomicrograph of from chilled margin of Cedar L;:;.ke "s-:f.11" .•••••••••••••••••••• • ••.••••••.• 69 22. Ph0tonic:-.:og::.ar11 of frorJ. lo:;-J=-r chilled m&:!:gin of Lake "sj 11" ...... •• , •••. 69 1 1 2J. Pho tc.micrograph of microgabbro, (;eda}· Lake : sil:!.. = •. . 70 11 11 24. Phornmicrcgraph of diabase, La.ke sill • 70 25. Pho tomic.rcgraph of euhedrzJ. i:-yro:i.cenes jn type A basalts, !:!ewtcLJ. Lake FiJrmation ••.•••.••••.•••..•••••• 71 26. Photomicrograph of euhedral pyraxe;:ies in type A basal ts, Newtc."T.l Lake Fc1rma t.ion ...... •...... " .•...••. 74 27. Photomicrograph of slightly skeletal py-coxeoes in type A Lake Formation •...... ••.....••. 28. Photomicro·graph 'of slightly skeletal pyroxene.s in type a bc.sal t, Uewtor. I.<·•ke Fo.:ma t :Lon ••••••••.•. • .•.• • • 75 29. Photomicrograph of high skeletal ,pyroxenes in type A bas2l t, Newton Lake Formation ...... •...••....•• 75 30. Photomicrograr·h of high skr:?letal pyroxenes in type A l'asa.lt, ·.Ne:,.rto11 Le.!<.e ?orl1":,tt:t.on ... ,...... c .... ,. •• ...... 76 31. Pho tcrnicr0graph c. f high ::i.l in r:ype A basalt, N•.:wton .••. ; ...... 76 J,IST OF FIGURES vi Figure 32. Photomicrograph of groundmass fan spherulites in type A basalt, Newton Lake Formation...... 77 33. Photomicrograph showing pseudomorphed ilmenite (?), Newton Lake Formation...... • . • • • • . • . • . . . • . • • . . . • . • . • • . 77 34. Photomicrograph showing highly skeletal olivine in Newton Lake Formation basalt...... 78 35. Photomicrograph showing skeletal groundmass texture of 0livine basalt, Newton Lake Formation...... 78 36. Photomicrograph of basaltic tuff, Newton Lake Forma- t ion .....•.....•••.•....•••...... •..•...... : • • . . • . • • 7 9 37. Photomicrograph showii1.g segregation vesicules in Newton Lake Formation basalt...... 79 38. Photomicrograph of fan spherulites, type C basalt, Newton Lake Formation. • • . . . • • . . • . . . • . • . . • • . . • • • • • • • • • . 83 39. Photomicrograph of ophitic gabbro, type C basalt, Newton Lake Femia tion. . • • . . • • . . . . • . . . • • . . • . . . • . • • . • • . • 83 40. Photomicrograph showing conrrnon type C basalt texture, Newton Lake Formation ., • . . • . • . . . . . • . . . . . • • ...... • . . . • • . 84 41. Photomicrograph of graphic jntergrowth of pyroxene - plagioclase, type C. Newton Lake Formation..... 84 42. Photomicrograph showing skeletal pyroxene in type C New.ton Lake Fonnation. .. • . • . . • . . . • • ...... • . . • 86 43. Variolitic basalt showing fragmenta;L interpillow material, Newton Lake Formation... . . • . . . . • ...... • . . 86 Lf4 .: Vc;.riolitic basalt showing merging varioles, Newton Lake Form::i. tion .•..•.•....•... . •... ..••.•...... • . . • 8 7 45. Photomic:.ograph of variolite, Newton Lake Formation... 87 46. Photomicrograph of skeletal olivine phenocrysts in variolitic basalt, Newton Lake Formation...... 88 47. Photomicrograph of skeletal olivine phenocrysts in variolitic basalt, Newton Lake FoTIL1ation...... 88 lf8. c:.) Photomicrograph showing glomeroporphyritic olivine, . Newton Lake Formation •...••..•.••...•••.•.•.•...•.•••.· 89 b) Photomicrograph showing spherulitic groundmass..... 89 c) showing skeletal groundmass pyroxene ....•..•••...... •..•.....•.•..•..•..•.•. . . . . 8 9 49. Photomicrograph of pillow core texture, Newton Lake Formation...... 91 50. Photomicrograph showing skeletal clinopyroxene, Newton Lake Formation ...••...•...•••••••...... •...•.••.••.••. · 91 51. Photomicrograph showing skeletal pyroxene of sill chilled margin, Newton I.ake Formation.... . • • • . . . • . . • • . 94 52. Photomicrograph showing coarse fans of plagioclase in chilled margin, Newton Lake Formation ..•..••.. :...... 94 53. Crystallj_zation model for Apollo 15 pyroxene - phyric basalts...... 99 54. Photomicrograph of peridotite lens. chilled margin sample, Newton Lake Formation...... 102 55. Photomicrograph of per.idotite lens chilled margin LIST OF FIGURES vii

Figure sample., Newton Lake Formation ...... •.....•.. 56. Photomicrograph of peridotite lens chilled margi..< sample, Newton Lake Formation .....•.•...•.....•.....•. 102 57. Nonr,alization plot, all Vermilion volcanic rocks ....•. 116 53. Na o - K.::>O diagram ...... ••..•...... •..••.•....•... 117 2 59. Hughes alkali diagram ...•.•••.•..•..•.•...... ••...... 119 60. Cao vs _CaO/r./ei?O diagram .••...... ••...... •.•..•.. 122 61. v: MgO) ...... 12lf 62. Nurraalization cliilled margin samples ...•...•..•. 126 63. Normalization plot, Little Long Lake flow (SLN) unit.. 126 Si0 classification for Vermilion volcanic rocks .••... 133 2 65. Normative plagioclase composition vs nonnative color

ind ex• e e • • • e 0 • IF e e e e e • • e a e e a " .. • a .. a 'll a a ID a .. ,_ 0 e #" 11 ,. IF a .- • a .. a a Cf J 35 66. Jensen Plot Classification •••••••...... •.•.•... 137 67. a) Jer..sen C

2. Locations of thesis samples ...... •...

3. Stratigraphic geochemical section for upper Ely Greens tone member ...... •..•..•.....••...... •. xi DEDICATION

This thesis is dedicated to Dr. Gene LaBerge, University of Wisconsin - Oshkosh, who first introduced me to the mysteries of the Precambrian and to Dr. Paul K. Sims, U.S. Geologic Survey, who has shown me how to explore these mysteries. xii ACKNOWLEDGEMENTS

I am indebted. to many people for their help and interest in

this research project. In particular, I would like to acknowledge

Dr. Paul Weiblen, who as thesis advisor, provided stimulating guid- ance throughout this study. Also, Dr. Paul K. Sims of the U.S.

Geological Survey, who provided both financial help and expert in.- struction in the art of field mapping and stimulating discussions, both in the field and during late hours on the front porch of Burnt- side Lodge. I would also like to acknowledge the help provided by

G. B. Morey, J. J. C. Green and V. Rama Murthy through numerous discussions concerning various aspects of the project. I am particularly indebted to Dr. Murthy for Rb-Sr analyses. I am also

grateful to Dr. J. M. Rhodes and K. Rogers at the Johnson Space

Center in Houston, for their patient instruction· in the techniques of x-ray fluorescence analysis. Furthermore, I would like to thank my fellow graduate students for their help during those periods when

I was more confused than enlightened by my research. Last, but not least, I would like to express by thanks to my wife, Charlene, without whose support, both moral and financial> this work could not have been completed.

Financial support for this study was provided through grants from

the U.S. Geological Survey· and Minnesota Geological Survey and

graduatefellowships from the Graduate School and Department of

Geology arid Geophysics, University of Minnesota and the Lunar,

Science Institute, Texas. 1-licroprobe st:udies were supported

through the William King grant to the Department of Geology and

Geophysics, University of Minnesota. Y.i.ii ABSTRACT

The Archean Vermilion greenstone belt in northeastern Minnesota was sampled stratigraphically for petrologic and geochemical study.

The oldest unit, the Ely Greenstone, is divided into three members: lower, Soudan Iron-formation and upper members. The lower member consists of calc-alkaline pillowed flows of basalt and andesite composition and mafic to felsic tuffs and breccias. The majority of flows and breccia fragments are highly amygdaloidal suggesting shallow deposition. Geochemically, the lower Ely volcanic rocks are similar to recent island-arc calc-alkaline rocks, but have lower

Al and Y contents. A model involving partial melting of amphibo-· 2o3 lite or garnet amphibolite is proposed for these rocks.

The upper Ely member consists largely (>90%) of pillowed to massive tholeiitic basalt. Two distinct chemical types are recognized in the upper Ely; a low Ti0 , low FeOT/MgO, and a high Ti0 2 2 high FeOT/MgO group. Very few intermediate compositions have been found. Mass balance calculations using both major and trace elements suggests that these two basalt groups can be related by low pressure fractional crystallization of olivine, plagioclase and pyroxene in the ratios 5:50:45. The apparent abundance of high Ti0 basalts with few 2 intermediate compositions suggests that the tholeiitic magmas fractionated in shallow chambers isolated from the main magma reservoir and were only periodically tapped. The upper basalts are compositionally similar to other Archean basalts and have characteristics in common with ocean floor and island arc tholeiites. xiv The Newton lake· Formation cons is ts of a ma fie member and a fel-

sic member. The felsic member consists of calc-alkaline andesites

.and dacites ma.inly of fragmental nature which are chemically similar

to the lower Ely member · calc-alkaline rocks and probably had a

similar origin.

The mafic member consists of pillowed flows and layered to non-

layered mafic-ultramafic sills. The basalts are distinguished by a

wide variety of crystal morphologies and textural types with many

having skeletal pyroxene phenocrysts in a spherulitic matrix. A

model involving supercooling and changing rates of coolings can

account for the range in observed textures.

Two major chemical b2salt types have been identified. One is

characterized by high MgO, varying ratios and marked iron

enrichment with decreasing Al o . These are similar both texturally 2 3 and compositionally to basaltic komatiites from Australia and Canada.

A model involving fractionation in the shallow layered _sills :ls pro-

posed to explain the range of flow compositions.

The other basalt group is distinguished by having high NgO, FeOT,

CaO and incompatible elements (except Y) with low but constant

Al o /Ti0 ratios and marked iron enrichment with increasing Al o • 2 3 2 2 3 These basalts share similarities to South African komatiites and to

the so called high iron tholeiite ;suite in Munro Tmvnship, Canada.

It is suggested, based on textural and chemical characteristics, that

these basalts may represent a chemically distinct komatiite type.

Petrologic modeling has been largely unsuccessfull in relating

the two Newton Lake basalt: types. Varying degrees of partial melting xv of distinct mantle sources seem necessary for these two types.

and petrologi c observations suggest that the Vermilion

greenstone belt developed through the coalescence of petrologically distinct volcanic centers. Calk-alkaline volcanism appears to have been more or less continuous while basaltic volcanism was intermittent in nature. · Mass balance considerations imposed by the logic and petrolog:l.c concll1sions require rapid recylcling and replenishment oof source material to generate the calc-alkaline volcanic rocks which in turn provided sediment to form the source for

the intrusive "granitic" batholiths. In terms of modern analogs, a

setting seems most compatible with the available data. However, for a better understanding of early crustal evolution, further attention should be directed at determining the unique interaction of tectonic-igneous processes during the Archean. THE PETROLOGY AND GEOCHEMISTRY OF ARCHEAN

VOLCANICS, WESTERN VERMILION DISTRICT,

NORTHEASTERN MINNESOTA

INTRODUCTION

The early (>2.5 b.y.) evolution of the earth's crust is currently a topic of considerable interest and speculation in geology.

Present interest in this area of earth science has been stimulated by advances in the study of the plate tectonic processes of the earth and by the recent findings of the Lunar, Mariner and Viking space programs. The wealth of information obtained in both those areas has provided a new framework within which to view planetary evolution and has shown the need for a re-evaluation of existing models of early terrestrial crust-mantle development. The present study was undertaken to provide new data on a well mapped section of mafic volcanic and associated rocks - the 2.7 greenstone terrane of northeast Minnesota. It was hoped that these data and their interpretation would provide new constraints on general models of early terrestrial magmatism.

A partial record of early crustal evolution is preserved in the Precambrian shields of the continents. Forming integral components of these shields are belts of interlayered volcanic and sedimentary rocks generally referred to as "greens tone belts''. These rocks are typically intruded by granitic and/or tonalitic batholiths of the same age and the overall association is commonly referred to as a greenstone-granite terraine. Such terraines, range in age from about 3.8 b.y. to 2.5 b.y. old, and provide a direct -2- though perhaps incomplete record of early terrestrial magmatism and of the processes which produced a significant portion of the present crust. Detailed studies of the structural, stratigraphic, petrologic and geochemical development of these belts can, therefore, provide much of the critical data required for the formula- tion of models of crust-mantle evolution. -3-

THE VERMILION DISTRICT

The Vermilion district and adjacent areas in northeastern

Minnesota (Fig. 1) comprise a typical Archean (>2500 m.y.) greenstone-granite terraine within the Superior Province of the Canadian

Shield. The district is composed of a nearly linear belt of volcanic and sedimentary rocks 10 to 30 km wide and more than 160 km long bounded by contemporaneous granitic batholiths. The many years of field studies carried out by members of the Minnesota

Geological Survey provides the necessary structural and stratigraphic framework within which to view the petrologic-geochemical evolution of this portion of the earth's crust.

General Geology

A brief SUIIllllary of tQe geology of the Vermilion district is presented below. Detailed discussions concerning the geology of the district are contained in Geology of Minnesota: A Centennial

Volume edited by Sims and Morey (1972), Sims and others (1972) and

Sims (1976).

The Vermilion district consists of a nearly linear belt of low-grade, steeply dipping metavolcanic and metasedimentary rocks bounded on the north by the Vermilion batholith, on the south by the Giants Range batholith and on the east by the Saganaga batholith (Fig. 1). The Keweenawan ( 1,100 m.y.) Duluth Complex trun- cates the Giants Range batholith and the volcanic-sedimentary rocks in the eastern part of the district . EXPLANATION

1 -,.. :.:· ,·:.-......

d.J•: ri5 "'Q; Knife Group n. k:, concbmnol, ""J, t-olc.:mi'c 9r11,,,,,,11cA' I Ill, /,1/s1C •·olCUl)C/ostic IOCkS klJ, lJosol.'iC

---- conc1olo d 15 MILE!J -----· Fc:.111·-Colltd wnenr concooled [ :: .. Ely G:ieMtonfl _____J l\oor'>•lmole b:ottl• w\J, upprr m11ml'1r 1u 15 KILOMETRES pcint lo 10 .... or u, Sc11dcn lron-f.)rmo:i.:m ,tf1.nh6r 1 orod• r('lcki _L, ti, IOW6f' j I I .i:-- Figure f(:or the western Vermilion district (after Sims, 1976). 1 -5-

Stratigraphy

Recent mapping in the western Vermilion district has revealed a complex volcanic-sedimentary pile characterized by interfingering and repetition of rock types (Sims and Morey, 1972; Morey and others, 1970; Green, 1970). The stratigraphic sequence in this area, as defi.ned by gross lithologic and structural relationships, is shown in Table 1.

Four formally designated units have been defined by Morey and others (1970), and Sims (1976). The oldest is the Ely Green- stone, composed mainly of mafic pillowed to massive flows and associated diabase sills. It is divided into three members with the persistent Soudan Iron-formation member separating a lower and upper volcanic member. The Ely Greenstone is overlain stratigraphically in the central part of the district by the Knife Lake Group and in the western part by the Lake Vermilion Formation. Both the Knife

Lake Group and the Lake Vermilion Formation are composed of felsic volcaniclastic and graywacke turbiditic rocks .. The youngest unit of the volcanic-sedimentary pile is the Newton Lake Formation which conformably overlies the Knife Lake Group i.n the central part and the Lake Vermilion Formation in the western part of the district.

The Newton Lake Formation changes laterally from dominantly felsic volcaniclastic rocks and flows east of Newton Lake to dominantly / ( mafic flows and sills to the west (Fig, 2 and Green 1970).

Three varieties of hypabyssal intrusive rocks have been recognized in the volcanic-sedimentary pile . These consist of:

1) Diabasic dikes and sills in the Ely Greenstone and Newton Table 1. Supracrustal Rock Units in the Vermilion District (after Sims, 1976).

Lithology in Estimated Rock Unit order of approximate maximum decreasing abundance thickness (m)

Newton Lake Formation Mafic Member •••••••••• Pillowed to massive >1, 350 Mafic lava Diabasic gabbro Differentiated mafic- ultramafic sills Intermediate-mafic pyroclastic rocks Siliceous, impure marble Felsic Member ••••• •• .• Felsic intermediate >1,000 pyroclastic rocks Felsic lavas Knife Lake Group, ••••••••• Felsic-intermediate '500 pyroclastic rocks Graywacke Slate Conglomerate with mixed epiclastic and pyroclastic clasts Iron formation Mafic lavas Lake Vermilion Formation >3,000 Graywacke Member ••••.• Graywacke Felsic-intermediate pyroclastic rocks Felsic Volcani- •..••.• Dacitc tuff and agglomerate, elastic Member iri part reworked Mixed Volcani- ••.•••.. Felsic-internediate tuff elastic Member breccia Volcanic sandstone Basaltic Lavas Ely. Greenstone Upper Member •••.•••••• Mafic lava, mainly 3,600 pillowed Intermediate-maf ic pyroclastic rocks Diabasic gabbro Iron formation Soudan Iron-•• i ••••• 150 ·Formation Member Intermediate-mafic pyroclastic rocks Lower Member ••..•••••. Mafic to intermediate 3,000 amygdaloidal, dominantly pillowed lava Diabasic gab:iro Mafic to felsic pyroclastic rocks Figure 2. Stratigraphic units examined in this study and location names referred to in thesis (see for key to symbols).

90.

et sa. h qo. ../;;:;;'G---"7 Z'r{4"tclBto.r I vb4 r...., -...J I -8-

Lake Fonnations.

2) Differentiated mafic-ultramafic sills in the mafic por-

tion of the Newton Lake Formation.

3) Dikes and small bodies of quartz-plagioclase porphyry

found locally throughout the volcanic-sedimentary pile.

Several types of plutonic rocks occur in the western part of

the district. The oldest recognized are the syntectonic1 rocks of

granitoid composition that constitute the western part of the Giants

Range and Vermilion batholiths. Younger posttectonic rocks of monzonite-quartz monzonite composition compose the eastern end of

the Giants Range batholith and also occur as small, isolated plutorts

along the length of the supracrustal belt. The youngest plutonic

rocks in the region are posttectonic alkalic syenites and associated lamprophyres.

Structural Geology

The volcanic and sedimentary rocks in the district are steeply inclined and complexly folded and faulted. Geologic evidence indicates that major faulting post-dated the folding and superposed a steep shingling effect to the original fold pattern (Sims, 1976).

Folds have been developed to different degrees in the district, largely in response to the physical characteristics of the original rock types. In general, steep isoclinal folds with short wave-

1 Syntectonic is used for those intrusive rocks that were emplacecl virtually synchronously with folding, and posttectonic for those that post-date folding. -9- lengths (on the order of few tens to hundreds of meters) have developed only in the well-layered sedimentary rocks. In contrast, the more massive mafic volcanic rocks have yielded mainly by brittle fracture and are steeply dipping as homoclines over broad areas

(Sims, 1976). While evidence for superposed folding appears widespread in the district, the pattern of multiple folding has only been examined in the Tower-Soudan area (Hooper and Ojakangas, 1971;

Sims, 1976) and in the western part of the district (Bauer, work in progress). Two generations of folds were identified there by Hooper · and Ojakangas (1971). Re-examination by Huddleston (1976) suggests that the first generation of folds may be related to depositional processes.

Three steep fault sets post-date the folding (Sims, 1976):

(1) vertical dip-slip faults, (2) right-lateral strike-slip faults, longitudinal to the Vermilion district, and (3) transverse (NE- trending) left-lateral, strike-slip faults. Based on available geological data, both sets of strike-slip faults are, interpreted as having formed approximately at the same time, but after the main movements on the dip-slip faults (Sims, 1976).

Metamorphism

The predominant metamorphic mineral assemblages in the supracrustal rocks are characteristic of the greenschist facies with amphibolite assemblages locally developed adjacent to and within intrusive plutons. The amphibolite assemblages attain their greatest width adjacent to syntectonic granitoid plutons, such as in the -10-

western part of the Giants Range batholith, where the metamorphic

aureole is as much as 15 km wide (Griffin and Morey, 1969). The

metamorphic aureoles adjacent to the posttectonic monzonite plutons

are narrow (1 km) and generally slightly lower in grade (maximum-

middle amphibolite facies, Sims, 1976). The metamorphic assemblages

in the district appear to be similar to the Abukuma-type facies

series of Miyashiro (1961), which is characteristic of regions with

steep thermal gradients at low to moderate pressures. The meta-

morphism of the supracrustal rocks is interpreted by Sims (1976),

to have occurred contemporaneously with emplacement of the granitic magmas that now constitute the Vermilion batholith and the western

part of the Giants Range batholith.

Geochronology

The geochronology of the Vermilion district and adjacent

areas has been summarized by Goldich (1972) and more recently by

Jahn and Murthy (1975) and Jahn (1972). Table 2 lists the more

important ages; all have been interpreted as primary ages by the

respective workers. These data indicate that the magmatic event

occurred around 2.7 b.y. ago and within perhaps a 50 m.y. time in-

terval. This places a significant constraint on models for the

geologic evolution of the Vermilion district (see further discussion

below).

A further constraint is provided by the sr87/sr86 initial values (Table 2) obtained by the geochronologic studies . The initial values are all below 0.701 and preclude derivation of the fel- -11-

Table 2. Summary of Important Rb-Sr Ages for the Vermilion District.

Rock Unit Age Initial Reference (in b.y.) sr87;sr86

Ely Greenstone 2.69 + 0.08 0.70056 + 0.00026 1

Newton Lake Fm. 2.65 + 0.11 0.70086 + 0.00024 1

Granite Pebbles* 2.69 + 0.28 0.70078 + 0.00058 1

Vermilion Granite 2.70 + 0.05 0.70041 + 0.00029 1

Giants Range 2.67 + 0.02 0.7002 + 0.0005 · 1 Granite

Saganaga 2.72 + 0.15 0.7009 + 0.0002 2 Tonalite

Icarus Pluton 2.69 + 0.02 0.7008 + 0.0001 2

Northern Light 2.70 + 0.12 0.7007 + 0.0004 2 Gneiss

------·-- ---·------* Pebbles in Ely Greenstone (Green, 1970). (1) Jahn and Murthy, 1975.

(2) Hanson and others, 1971, re-evaluated by Jahn and Murthy, 1975.

All uncertainties reported at 2 sigma -12-

sic volcanic and batholithic rocks from pre-existing sialic crust

that was significantly older than 2.7 b.y. (Jahn and Murthy, 1975).

The initial values are also lower than those obtained for Archean

greenstones in Canada (Hart and Brooks, 1977; Jahn and Nyquist,

1976) and may be an indication of mantle inhomogeneities in Rb and

Sr content.

Previous Petrologic and Geochemical Studies

While much is now known about the structure and stratigraphy of the Vermilion greenstone belt, less is known about the petrologic and geochemical nature of the rocks of the area. This is particularly true for the volcanic rocks, which have long been considered to be monotonous greenstones.

The plutonic rocks have received considerable attention in recent years (Arth and Hanson, 1972, 1975; Viswanathan, 1972;

Southwick, 1972; also see other references in Geology of Minnesota:

A Centennial Volume edited by Sims and Morey, 1972). Arth and

Hanson (1975) have provided the most detailed geochemical study and have proposed models based on trace and rare earth element modeling to account for the origin of the major plutonic phases. Their conclusions are summarized in Table 3.

The sedimentary rocks of the belt were originally described by Gruner (1941) and more recently by McLimans (1972) and Ojakangas

(1972a, 1972b). Ojakangas (1972b), from his study of the graywackes in the western end of the belt, concluded that they were derived in large part from felsic-interrnediate volcanic material (mostly -13- dacitic) and transported and deposited by turbidity currents. Geo- chemical studies by Arth and Hanson (1975), Jahn (1972) and most recently by :Morey and Schulz (1977) have confirmed the dacite par- entage of the Vermilion graywackes.

Banded iron-formations are widely distributed in the Vermilion district. They are more abundant in the Ely Greenstone, where they are generally found at major volcanic contacts (Sims, 1972). The iron-formations consist of several intergradational varieties of fine-grained ferruginous chert, of which jaspilite is the most common. The best known and studied deposit is the Soudan Iron-

Formation (Klinger, 1956), which is continuous from Soudan east- wards to the vicinity of Twin Lakes. Because it is continuously mappable for a distance of 26 km, it has been used to subdivide the

Ely Greenstone into two members as noted above.

Early studies of the volcanic rocks of the Vermilion greenstone belt were mostly of a general nature with the most important being those of Goldich and others (1961), Schwartz (1924), VanHise and Clements (1901) and Winchell (1888). The first detailed study of the volcanic rocks was that conducted by Green (1970) in the

Gabbro Lake quadrangle. In this important contribution, Green provided both field and petrographic descriptions of the major volcanic rock types and twenty-one major element analyses.

Sims (1972) compiled all available major element analyses of

Vermilion volcanic rocks up to 1972. Based on these data, he concluded that the Ely Greenstone consists mainly of basalt to basaltic andesite of tholeiitic affinity, while the Newton Lake volcanic -14-

Table 3. Petrologic Models for the Vermilion Plutonic Rocks (after Arth and Hanson, 1975).

Tonalites Including Dacite Porphyry Trondhjemite:

Derived by 20 to 30 percent partial melting of eclogite or amphibolite at mantle depths, leaving a residue consisting predominantly of garnet and clinopyroxene.

Quartz Monzonites:

Derived by 20 to 50 percent partial melting . at crustal depths of dacitically derived graywacke, leaving a residue predominantly COIT.posed of plagioclase, amphibole, garnet and pyroxene or biotite.

Syenites and Syenodiorites:

Derived by partial melting at mantle de?ths of a mixture of quartz eclogite - undersaturated eclogite - peridotite. This represents the last igneous activity to occur in the evolution of tne Vermilion greenstone -belt.

------·--·------15- rocks, ranging from basalt to dacite, have calc-alkaline affinities.

More recently, Arth and Hanson (1975), Jahn (1972) and Jahn and others (1974) provided trace element and rare earth element data for selected Vermilion volcanic rocks. These studies noted a geochemical similarity between the Vermilion volcanic rocks and those present in modern island arcs. Jahn and others (1974) have suggested that a plate tectonic model may, therefore, also explain the development of the Vermilion greenstone belt. The geochemical data presented in previous studies of the Vermilion volcanic rocks, as well as those obtained for this study are compiled in Appendix 1. -16-

NATURE OF THE PRESENT STUDY

The present study was undertaken to investigate the nature of the volcanic rocks in the western Vermilion district. While the recent studies, referred to above, have provided petrological and geochemical data on these rocks, they have generally dealt only with a small number of samples collected from widely scattered localities and independent of the geologic mapping within the greenstone belt.

These data, while providing a broad. overview of the geochemical relationships between various rock types, have proved insufficient to determine stratigraphic variations in any detail. The recent stratigraphic reconstruction of the Vermilion district by Sims

(1976) ,now provides a framework within which to attempt such a detailed investigation.

Furthermore, the previous studies did not examine the mafic volcanic rocks of the Newton Lake Formation. Work done by Green

(1970) and Schulz (1974) suggested that the Newton Lake volcanic rocks are distinct from the Ely Greenstone, both petrologically and geochemically. The present study was, therefore, undertaken to examine in some detail the stratigraphic variations of the Vermilioµ volcanic rocks, particularly in the maf ic portion of the Newton Lake

Formation. The volcanic rocks examined lie roughly in the area east of Lake Vermilion and west of Newton Lake (see Fig. 2).

Specific Objectives

Three main objectives were set for this study: 1) to charac- terize petrographically and chemically, the extrusive rocks in the Figure 3. Location of sampled _corridors.

Sampling Corridors

v ' "' <:? !fr/ ga / 90.

I Yb4 vv- I "'-... - / {/ i.o.1<1 -- -18- western Vermilion district based on systematic sampling through the structural sequence, 2) to examine the nature of the volcanic rocks which compose the mafic member of the Newton Lake Formation, and

3) utilizing those data and that obtained from the literature to critically evaluate existing models of greenstone belt evolution.

Methods of Study

1) Three summers were spent in obtaining a stratigraphic sampling of the volcanic rocks in the western Vermilion district.

The sampling was done along traverses run perpendicular to structural trends in preselected corridors. These corridors., shown in

Figure 3, were selected based on the following criteria: 1) accessibility, 2) rock exposure, 3) availability of structural and stratigraphic controls and 4) relative stratigraphic position within the volcanic pile based on the reconstruction of Sims (1976).

Sampling was also conducted laterally in those .areas where previous work indicated major lateral changes in lithology. Detailed mapping was done, particularly in the areas between Wolf Lake and the east shore of Lake Vermilion and south of Cedar Lake, to further control sampling at these localities. Over 500 hand samples were collected, representing extrusive volcanic rocks from both the upper and lower members of the Ely Greenstone and the maf ic portion of the. Newton Lake Formation. Volcaniclastic rocks, while locally abundant, were given only a cursory examination.

2) A petrographic study was made of some 300 thin sections prepared from the collected samples. Because of the metamorphism, -19-

special attention was given to determining the nature of the

primary mineralogy and texture of the samples . The metamorphism

was not examined in any detail. Locations for all samples dis-

cussed in this thesis are given in Appendix 2. \ 3) Primary mineral chemistry was determined with the aid of the electron microprobe. The major primary mineral investigated

was pyroxene, as it was commonly the only phase not completely re-

placed during metamorphism.

Analyses were made using the recently automated MAC model 400

electron microprobe in the Department of Geology and Geophysics,

University of Minnesota. All analyses were done on carbon coated

polished t:hin sections using the following operating conditions:

20 kv, 0.050 microamperes, and a beam diameter of approximately 5

microns. Background and peak x-ray intensities were measured on

both standards and unknowns with counting times of 10 seconds for

background and 20 seconds for peaks. Natural pyroxene standards

were used and checked repeatedly during runs. Several points were

analyzed in each sample to determine the range in possible pyroxene

compositions. Data was reduced on line by a PDPll computer.

Accuracy is estimated to be within 2 to 5 percent of the amount

present.

4) Based largely·on the thin section study, 76 samples were

selected for major and minor element analyses . All analyses were

done by the x-ray fluorescence methods of Norrish and Hutton (1969)

in the laboratory of Dr. J. M. Rhodes at the Johnson Space Center,

Houston, Texas. In all cases the heels of the prepared thin sec- -20- tions were used for the major element analyses. The heels were broken by hand with weathered edges removed i f present. Final crushing to <100 mesh was done in a tungsten carbide shaker.

Because of the generally fine grain size of the Vermilion volcanic rocks, it is felt the heels are representative samples.

The analyses were made using glass discs, prepared by fusing a 280 mg aliquant of the sample with a lanthanum-bearing lithium borate fusion mixture (Norrish and Hutton, 1969) • Trace elements were measured on 3g pressed powder pellets using a Ag tube for ex- citation. Corrections were made for dead time, instrumental drift, non-linearity of backgrounds and inter-element interferences. Mass absorption coefficients for matrix corrections were calculated from the inverse of the Ag compton peak, essentially following Reynolds

(1967). Calibration was based on rock standards using pr.eviously obtained values based on primary synthetic standards supplemented by previously analyzed U. S. Geological Survey and National Bureau of Standards rock and mineral standards. Sodium was analyzed by neutron activation techniques. Water was determined with a Dupont moisture analyzer. Duplicate sample analyses were made for all major elements except sodium. Analytical statistics, along with an analysis of BCR-1 determined during this study, are presented in

Appendix la. -21-

GEOLOGY OF VERMILION DISTRICT VOLCANIC ROCKS

Ely Greenstone

Van Hise and Clements (1901, p. 402) named the Ely Greenstone from exposures around the town of Ely where a variety of extrusive, intrusive and fragmental rocks of various shades of green occur.

Clements (1903) and many subsequent workers applied the name to all major bodies of greenstone in the Vermilion district. Recent mapping by members of the Minnesota Geological Survey has led, however, to the restriction of the name Ely Greenstone to only those mafic metavolcanic and associated rocks continuous with the rocks exposed in the town of Ely (Morey and others, 1970). As thus defined, the Ely Greenstone extends from the vicinity of Tower north- eastward to approximately Moose Lake; (Fig. 2) a distance of nearly

64 kilometers.

Stratigraphic relationships within the Ely Greenstone have been complicated by the extensive faulting which pervades the

Vermilion district. The longitudinal strike-slip faults, in particular, have offset units laterally on the order of 16 to 19 km

(Sims, 1976) resulting in a 11 shingling" of units. Based on inferred correlative units, Sims (1976) has attempted to reconstruct the Vermilion district stratigraphy by removing the effects imposed by the strike-slip faults. In this reconstruction the Ely Green- stone forms a crudely almond-shaped lens around which the younger sedimentary and volcanic rocks are tightly wrapped (Fig . 1).

A marked structural feature of the Ely Greenstone is a broad, -22- arcuate pattern convex to the north, first noted in the eastern part of the Gabbro Lake quadrangle by Green (1970; see also Green and others, 1966). Mapping to the west, in the Chad Lake and Eagles

Nest quadrangles, (Sims and Schulz, 1977b) has since revealed a similar structural pattern in this area. This broad arch in the western end of the Ely Greenstone is also found in the Soudan Iron- formation member (see Sims, 1973). Green (1970) has interpreted the pattern in the Gabbro Lake quadrangle as part of a broad, vertically- plunging anticline, faulted off across the bottom. The presence of a similar structure to the·west suggests, however, that this pattern may be a primary feature preserved within the Ely Greenstone. In this regard, it is interesting to note that the major occurrences of dacite porphyry and diabase in the Gabbro Lake quadrangle occur in the core of the arcuate structure (Green and others, 1966). In the structural reconstruction proposed by Sims (1976), the arch in the Gabbro Lake quadrangle lies stratigraphically above and slightly to the east of the arch in the Chad Lake and Eagles Nest quadrangles. The possible significance of this structure will be further discussed in a later section (page 302)

The stratigraphic thickness of the Ely Greenstone is subject to interpretation because of structural complications and because the base of the unit is transected by granite. Green (1970) estimated the Ely to be at least 3600 meters thick in the Gabbro

Lake quadrangle. In the area of Twin Lakes, southwest of Ely,

Morey and others (1970) have estimated the Ely to be at least 6000 meters thick. Sims (1976) has estimated the thickness of the lower -23-

and upper members as shown in Table 1.

Lower Ely Greenstone Member

The lower member of the Ely Greenstone is defined as that part

which underlies the Soudan Iron-fonnation (see Fig. 1 & 2) (Sims,

1976). On the south limb of the large inverted anticline near

Tower (Fig. 1), the lower member is stratigraphically overlain by

the Lake Vermilion Formation. In the area of Bear Head Lake the

Giants Range batholith transects the unit, cutting out the strati-

graphically lower part. The base of the Ely Greenstone is thus

not exposed.

Griffin (1967) investigated the lower Ely member on the south

limb of the anticline near its contact with the Lake Vermilion

Formation. In this area, much of the Ely Greenstone is recrystal-

lized to the amphibolite facies, imposed by the Giants Range batholith. In the Tower and western part of the Soudan quadrangles, where the rocks are still relatively low grade, primary structures

and textures are generally preserved. Griffin found the Ely

Greenstone to consist of extensive massive and pillowed mafic flows

and interbedded volcaniclastic sediments. Primary structures found

in the flows include amygdules filled with calcite or epidote, and

ellipsoidal pillows having epidotized rims and veinlets of epidote

(Griffin, 1967) . The associated sedimentary rocks range from thin- bedded tuffs to fine to coarse conglomerates and breccias composed

of basaltic pebbles . . Iron formation is also common in the area.

The lower Ely Greenstone member was examined for this study in -24- two areas:(l) in a zone running south from the south shore of

Robinson Lake to just south of Eagles Nest Lake No, 4 and (2) in the area southwest of Armstrong Lake (see Fig. 2 & 4). The two areas were selected for several reasons:

1). accessibility,

2) reconnaissance mapping conducted by the Minnesota Geologi-

cal Survey suggested a homoclinal north facing sequence,

3) a fairly thick section could be examined (3 to 4 miles),

4) the metamorphic aureole related to the Giants Range batho-

lith is thin in this area, and

5) this area allowed continuation of traverses run in the

upper Ely member directly to the north.

Because mapping of the lower Ely Greenstone member is largely incomplete, it remains somewhat uncertain how representative the study areas are of the whole unit. However, this study shows that the rocks in these areas are lithologically similar to those examined by Griffin (1967).

Although a structural synthesis is not possible at this time, data obtained during this study suggest that structural complications exist within the lower Ely member. For example, in the area south of Robinson Lake cleavage strikes approximately N75E with steep dips to the northwest. Pillows found southeast of Eagles

Nest Lake No. 4 strike N65-75E with tops to the NW. Stretched amygdules in the NW 1/4 of Sec • 19, T.62 N., R. 13 W., define a strong lineation dipping steeply 85° to the north. However, in the area southwest of Armstrong Lake both cleavage and pillows are I N vi I

Figure 4. Outcrop and lithologic maps two traverses in the

lower Ely Greenstone member. Scale 1:24,000. Geochemical sample

locations shown by stars.

Explanation of Map Symbols

B = Basalt AB = Amygdaloidal Basalt APB = Amygdaloidal Pillow Basalt Py An = Porphyritic Andesite AP An = Amygdaloidal Pillow Andesite T = Tuff TB = Tuff Breccia QP = Quartz Porphyry HP = Hornblende Porphyry G = Gabbro Dike = Young (Middle Pree. ?) diabase dikes = Pillow top direction -26-

\... .,... "' v"' d)i; t.).. c OC')O .:;>-+ &-> 0 ., ., " .. " ..::. {{>3 .. ':.

f .. ' ... ' 0...... • -..... "'

.. L .. ..0 :..:: " \... g "' -27-

found striking to the northwest (cleavage N80W, steep NE; pillows

and tuff beds N65-70W 9 steep NE) . Stratigraphic tops could not be

determined in this area.

Pillowed and Massive Flows

As shown in the lithologic outcrop map (Fig. 4), pillowed to massive flows are the dominant lithology in the two areas examined.

The rocks range from dark green to gray in color and have granular

to diabasic textures. Massive flows with a fine to medium grained diabasic texture are the most common. Because of limited exposure,

individual flow thicknesses could not be determined but are probably on the order of several tens of meters.

A distinctive feature of many of the pillowed flows in both areas is their generally amygdaloidal nature. These flows are mostly grAyish green to light green in color and show considerable epidote, both in fine patches throughout the rock and in veinlets and lenses. Amygdules range in size from 1 mm to about lcm and are generally filled with various assemblages of hornblende + chlorite

± quartz ± epidote + carbonate. Pyrite is present in many amygdules, and as disseminated cubes throughout the rock and along fractures.

The massive flows, dark green in color, are geuerally slightly coarser grained than the pillowed flows and have few amygdules.

Their texture is granular to diabasic. Field relations in the area southwest of Armstrong L3ke. suggest that the massive flow units grade into amygdaloidal pillow basalt, which may, therefore, represent upper portions of single flows. -28-

While the majority of the flows appear to be basaltic in na-

ture, two occurrences were found of ai.i.desitic flows: one in the NW

!t;, Sec. 26, T.62 N., R.14 W.s and the other in the central portion

of Sec. 30., T.62 N., R. 13 W. These rocks are gray to dark gray in

color and fine grained. The flows are pillowed and amygdaloidal at

both occurrences. Vesicles are concentrated near the pillow margins

and are filled with quartz, which tends to stand in relief on weath-

ered outcrops. The pillows are variable in size (.3 to 1 . 5 meters),

generally somewhat lobate, and have thick (2.5 to 5 cm) whitish

rinds. The andesitic flow in the Sec . 30, T.62 N., R.13

W., is porphyritic, dark gray in color, and has a very fine grained

quartz-feldspar matrix.

Petrography:

In thin section all the rocks referred to above show extensive

alteration and usually only poorly preserved igneous textures . The

mineralogy of the amygdaloidal pillowed flows and the massive flows

is the same, consisting of granular epidote, pleochroic actinolitic

hornblende, albite, chlorite, muscovite and quartz, along with minor

sphene, pyrite and altered iron oxides. Epidote generally occurs in

granular patches throughout ·the groundmass and also as larger ( 1 mm)

sub-to euhedral crystals in veinlets andamygdules and occassionally

in the groundmass. The amphibole is generally pleochroic from

. yellowish green to bluish green with non-pleochroic light green

amphibole sometimes present; These may represent hornblende-

actinolite pairs. The pleochroism of the amphiboles increases

to yellowish brown-dark green in samples closest to the Giants

Range batholith contact. Samples from within a mile of the contact -29- do not contain epidote.

The pillowed andesitic flows are distincti.ve in their lower amphibole content. These rocks have a fine grained hypidiomorphic granular texture and are composed of albite, epidote, quartz and fibrous fine amphibole (actinolite). Chlorite, quartz, amphibole, calcite and pyrite are found in amygdules with quartz being predominant. The porphyritic andesite is similar to the pillowed andesitic. flows in mineralogy and groundmass texture. The pl_agioclase phenocrysts show strong normal zoning and a mosaic pattern in thin section (Fig. 5).

Volcaniclastic Rocks

Tuffs and tuff breccias (classified according to Fisher, 1966) are common in both areas examined but are more abundant in the area southwest of Armstrong Lake. Both are apparently interbedded wit:h pillowed to massive basalt flows. The tuff breccias are massive in nature, showing no obvious evidence of bedding or other internal structures. The tuffs may be bedded, however, the development of strong cleavage and the generally poor outcrop make determination of these features uncertain. Cleavage is particularly strong in tuffs south of Robinson Lake.

The best exposures of tuf f breccias occur along the railroad track southwest of Armstrong Lake (NW 1/4, SE 1/4, Sec. 22, T.62 N.,

R.14 W.). The rocks are composed of angular mafic to felsic volcanic rock fragments ranging from millimeters to several centimeters in size. The matrix is finer grained and consists of smaller rock

1 I / -30-

Figure 5. Porphyritic andesite (ENEL-86-33), lower Ely Greenstone. Note mossaic pattern of plagioclase phenocrysts. XP, bar= 0.5 mm. (Analyzed sample, Appendix lb). -31- fragments, broken plagioclase and quartz crystals, fine epidote and amphibole. A distinctive feature of the tuff breccias as well as the finer grained tuffs, is the highly amygdaloidal nature of the lithic fragments (Fig. 6). Amygdules are present in fragments ranging from mafic to felsic in appearance. Some fragments, particularly the more felsic, have plagioclase phenocrysts. Felsic fragments usually have quartz-filled vesicles while those in ·the more mafic fragments are filled by assemblages similar to those described for the amygdaloidal flows. Common in the tuff breccias is an abundance of epidote in mafic fragments, veinlets and lenses throughout the rock. The felsic rock types, which are abundant in the tuff breccias, have not been found as individual flows or flow units in the area.

The tuffs are similar to the tuff breccias in gen.eral appearance except for ·a finer grain size and a. less heterolithic nature . .. Some are lithic lapilli tuffs with crystal fragments of plagioclase and quartz. These are less heterolithic than the tuff breccias with fragments generally mafic to intermediate in appearance . Fragments are angular and highly amygdaloidal to pumiceous. Many of the tuffs are fine grained and recrystallized and may be either pyroclastic or epiclastic in origin.

Petrography:

The tuffs and tu££ breccias have the same mineral assemblages as the flows. Distinctive of many of the tuff breccias, however, is an abundance of epidote occurring both in patches in the mafic- intermdiate fragments and the matrix and in veinlets and patches -32-

throughout the rock. Recrysta,llization has been extensive in:_ many of the tu:l;f s so that while fragments are easily visible in hand sample, fragment boundaries in thin section are poorly defined. Many of the tuff fragments are pumiceous. Broken fragments of plagioclase and quartz are usually present in the matrix.

Porphyries

Dikes and irregularly shaped small bodies of plagioclase- quartz porphyry (dacite-rhyodacite) are widespread throughout the lower Ely Greenstone member, as well as other parts of the Vermilion district. They are typically whitish to light gray or buff and weather white. In the SW l/Lf, SW 1/4, Sec. 18, T.62 N., R.13 W., and in the central part of the section: the porphyries have a greenish. gray color and pink plagioclase phenocrysts .

Petrography:

Plagioclase-Quartz Plagioclase, occurring as euhedral to subhedral blocky crystals 1-5 mm across, is the most abundant phenocryst phase (generally about 35-40 percent) of the dacite porphyries. Slight oscillatory zoning is common with compositions ranging from oligoclase to albite. The cores of the plagioclase phenocrysts are often extensively altered to sericite and brownish saussurite. Oxidation of the alteration products apparently accounts for their pinkish color in some samples. Quartz phenocrysts are second in abundance to plagioclase (about 5-10 percent).

They occur as equant to rounded and embayed crystals ranging from

1 to 10 mm across . Hornblende phenocrysts are also ubiquious and compose 5 to 10 percent of the rock. Hornblende phenocyrsts, -33- usually less than 2 mm ac:rqss, a,:re to calcite, chlorite and epidote. Apatites occurring as. short prismatic is generally present in amounts less than one percent, The groundmass of the porphyries consists of a microcrystalline, granoblastic aggregate of quartz and albite. Chlorite, epidote, sphene, magnetite and pyrite, along with widespread sericite, are common accessories of the groundmass.

Plagioclase-Hornblende The plagiclase-hornblende porphyries are distinctive in their general lack of quartz phenocrysts.

Plagioclase occurs as 1-5 mm subhedral to euhedral phenocrysts showing ocillartory zoning and compositions ranging from oligoclase to albite. The plagioclase phenocrysts, which are present in amounts from 10-15 percent, are variably. altered to brownish saussurite ctnd epidote. Hornblende phenocrysts, which in hand sample look to be euhedral, tabular crystals about 1-2 mm in size, are found in thin section to be replaced by light green prismatic actinolite, granular epidote and fine chlorite; they are present in amounts ranging from

5 to 10 percent. Apatite, occurring as short prismatic crystals, and subhedral opaques (magnetite?), altered marginally to hematite, are present in amounts generally less than one percent. The groundmass, as in the dacite porphyries, is a microcrystalline, granoblastic aggregate of quartz and albite, with accessory sericite, actinolite and magnetite.

Geologic Environment

Tt has proved to be very difficult to assess the conditions -34- unde;r whi_ch. greenstone belts haye developed. Th:i,s is particularly a problem when dealing with metabasalts, because except for pillow s.tructu:res few distinctive indicators of mode and conditions of formation exist. This, coupled with structural ·deformation and metamorphic recrystallization, has discouraged efferts to make meaningful interpretations. In the case of the rocks of the lower

Ely Greenstone member, however, several features were found in this study which help place significant limits on the nature of their enviroment of formation.

The most important feature in this regard is the highly amygdaloidal nature of many of the lower Ely Greenstone member rocks. This feature is common in both pillowed mafic to inter mediate flows and in fragments in the volcaniclastics. Several studies (Moore, 1965; Jones, 1969) have now shown a clear inverse correlation between depth and vesicularity in submarine pillowed basalts. Moore (1965), in a study of deep sea basalts near Hawaii, found both the percentage and size of vesicles to slowly decrease in samples down to 800 meters and then rapidly decrease below that depth. Jones (1969), in a study of Icelandic interglacial basaltic pillow lavas, found a similar relationship with vesicles generally present only in lavas extruded in less than 500 meters of water.

McBirney (1963), based on a theoretical treatment, also concluded that vesiculation in submarine basalts would occur only at water depths less than about 500 meters. For intermediate to felsic lavas, vesiculation could occur at depths as great as 2000 meters, depending largely on the water content of the magma (McBirney,

(1963). -35-

The presence of flows in the lower Ely Greenstone member indicates a submarine environment, The.i.r highly amygdaloidal nature would further suggest, by analogy with the studies described above, that extrusion occurred at fairly shallow depths. A detailed study of the type conducted by Jones (1969) was not attempted due to pool;" exposure in the Vermilion :district; however, a water depth less than 800 meters seems likely.

Several features of the volcaniclastic rocks further indicate a shallow water or possibly even subaerial environment. Foremost, of course, is the amygdaloidal nature of the lithic fragments. A further distinctive feature of some f elsic fragments is scalloped fragment boundaries (Fig. 6), which apparently are the result of broken vesicles. The presence of this feature in the tuff breccias is suggestive of a pyroclastic as opposed to epiclastic origin for the fragments. The extensive vesiculation necessary for the shattering of the felsic lavas is indicative of shallow water or subaerial envrironmental conditions (McBirney, 1963). The presence of pumiceous fragments in the lithic lapilli tuffs further supports this.

The general abundance of volcaniclastic material within the lower Ely member may be further evidence for shallow water conditions. Several studies in modern volcanic areas have shown volcaniclastic material to increase in abundance with shallowing of the volcanic environment (Mitchell, 1970; Donnelly, 1972). A similar stratigraphic variation has also been inferred for several

Canadian greenstone belts (Goodwin, 1968). This is attributed both to greater vesiculation occurring at lower pressure and to -36-

Figure 6. Tuff breccia from lower Ely Greenstone (ENEL-108-42) . Note the arnygdaloidal nature of the volcanic clasts. Dime for scale.

Figure 7. Gray pillow basalt, upper Ely Greenstone south of Shagawa Lake. Note the dark chloritic pillow rims. Pencil for scale. -37- lava composition, as intermediate (andesitic) to felsic (dacitic- rhyolitic) rocks generally form most of the fragmental material in these sequences.

Based on the data presently available, it is concluded that the rocks of the lower Ely Greenstone inember formed in a shallow submarine environment probably less than 800 meters deep. The nature of some of the volcaniclastic rocks fµrther suggests that, at least locally, very shallow or even subaerial environments may have existed.

As previously noted, the Soudan· Iron-forination member stratigraphically overlies the lower Ely Greenstone member. Its thickness and remarkable lateral persistence indicate deposition during a major in- terruption of volcanic activity. The presence of interstratified amygdaloidal pillow basalts along with fine grained volcaniclastic rocks (Klinger, 1956), indicate a shallow submarine environment also existed during iron-formation deposition and that minor volcanic activity continued to occur. A shallow water environment for the iron-formation is thus in agreement with the conclusion based on evidence from the volcanic rocks of the lower Ely Greens tone member.

Both the petrology and geochemistry (discussed in a later section) of the lower Ely Greenstone rocks indicate that they are calc- alkaline in nature. It is suggested that the lower Ely member may represent the upper portion of a calc-alkaline volcanic pile, which may l:iave accumulated as a ·stra tovolcano in an Archean sea. The seces- sion of major volcanic activity was followed by an inactive period during which iron-formation was deposited on the established shallow plat-· form. A lat;er period of renewed major activity, represented by the -38-

upper Ely Greens tone member, was. distinctly different in nature

(see below}.

Upper Ely Greenstone Member

The upper member of the Ely Greenstone lies stratigraphically

above the Soudan Iron-formation member west of the town of Ely and

to the east is in intrusive or fault contact with the Giants Range

Batholith. It extends laterally from the east shore of Lake

Vermilion to just west of Snowbank Lake (Fig. 2).

The upper Ely member has now been mapped in detail in the

Gabbro Lake (Green, Phinney and Weiblen, 1966), Ely (Green and

Schulz, 1977), ShagawaLake (Sims, Mudrey and Schulz, 1976), Crab

Lake (Sims and Schulz, 1977a) and Chad Lake (Sims and Schulz,

1977b) quadrangles. Prior to this study, the only detailed petrologic and geochemical study done was that by Green (1970) on rocks

in the Gabbro Lake quadrangle .

The recent mapping has shown that in the area of Lake Vermilion,

the upper Ely Greenstone member interfingers with elastic rocks of the Lake Vermilion Formation. Clastic units and iron-formation lenses are also common in the upper Ely member at its western term-

inus in the Chad Lake quadrangle (Sims and Schulz, 1977b). At its eastward termination, west of Snowbank Lake, the upper Ely member interfingers with or is faulted against the Knife Lake Group

(Morey and others, 1970). Thin beds of elastic rocks and iron formation are also interlayered with the upper part of the member

in the Gabbro Lake quadrangle (Green, 1970). The field relations -39- clearly indicate that the upper Ely Greenstone was, at least in part, contemporaneous with the elastic sedimentation of the Lake Vermilion

Formation and the Knife Lake Group.

The majority C>90%) of the rocks which make up the upper Ely

Greenstone member are of basaltic composition. The remaining fr.action includes felsic volcanic rocks, chert and banded iron-formation, and elastic rocks. The basalts consist of pillowed lavas along with contemporaneous diabasic rocks showing both concordant and discon- cordant relations. The basalts have been divided during recent mapping into units based on such field criteria as color, presence or absence of pillows, amygdules and varioles, grain size, general pillow shape and abundance of intercalated fragmental material.

A specific objective of the present study was to determine whether the units distinguished by the field criteria also have different chemistry. The upper Ely member was sampled extensively for this study in the Crab Lake and Chad Lake quadrangles. Samples

were also taken in the Eagles Nest, Soudan, Shagawa Lake and Ely

quadrangles. The majority of the chemical analyses represent samples

from the Eagles Nest and Crab Lake quadrangles as this portion of the member has not previously been examined. The Ely Greenstone in the

Gabbro Lake quadranglP. (described by Green, 1970) represents a stratigraphically higher portion of the upper member, in the reconstruction of Sims (1976).

Mafic Pillowed to Massive Flows and Diabases

As noted above, greater than 90 percent of the upper Ely Green- stone member is composed of maf ic pillowed to massive flows and dia- -40- bas.e. The majority of; the basalts. are aphanitic to fine-grained, dark to medium gray-green rocks and are similar to those described in detail by Green (l970). and Sims (;1972), Though local occurrences of amygdaloidal basalts exist in the Chad Lake and Gabbro Lake quadrangles, the majority of the flows are non-amygdaloidal. This is in marked contrast to the volcanic rocks of the lower Ely member.

Distinctive light to medium gray porphyritic pillowed basaltic flows and associated fine-grRined ·diabases have been found south of

Shagawa Lake (Sec. 5 and 6, T.62 N., R.12 W., and Sec. 32, T.63 N.,

R.12 W.) and south-southwest of Mud Lake (Sec. 3, 4, 8 and 9, T.62 N.,

R.14 W.). Those south of Shagawa Lake have bulbous pillows (Fig. 7) with dark chloritic rinds. The rock is light gray in color with small plagioclase microphenocrysts set in an aphanitic groundmass.

Those south and southwest of Mud Lake are light to dark gray in color, with the pillows having WP.11 developed toes and rinds which are generally slightly reddish (oxidized) in outcrop. Many samples are visibly glomeroporphyritic with both plagioclase and pyroxene phenocrysts present (plagioclase>pyroxene). Outcrops in Sec. 8, T.62 N.,

R.14 W., show an alternation of pillowed and diabasic rocks; the diabasic rocks probably representing flow centers·below associat;ed pillowed caps. Based on this succession, flows appear to be about

30 meters thick. Characteristic of the gray basalts at both locations is an abundance of calcite. It occurs both as disseminations throughout some samples and in veins and veinlets, often with abundant

(1-2%) pyrite. -41-

_Petrography:

Green Basalts: The majority of the mafic rocks of the upper Ely

Greenstone member are fine to medium grained and dark to medium

green in color. The nonpillowed rocks, representing either massive

flows or intrusive dikes and sills, are generally coarser grained

than pillowed basalts and have diabasic textures. Many rocks, par-

ticularly those in the Eagles Nest and Crab Lake quadrangles, have

a mottled appearance due to localized concentrations of epidote.

This feature is most pronounced in samples cut by numerous thin

(1-5 mm) epidote + carbonate + amphibole veinlets.

Igneous textures are well preserved in most samples even

though they are replaced by greenschist facies mineral assemblages.

Samples from pillowed flows consist of microphenocrysts of plagio-

clase set in a matrix composed of plagioclase microlites,

amphibole (actinolite), chlorite and epidote (Fig. 8). The plagio-·

clase microphenocrysts occur as 2-5 mm tabular crystals largely

replaced by albite and muscovite. Several samples from the Eagles

Nest and Crab Lake· quadrangles have well preserved skeletal plagio-

clase micro lites showing belt buckle sections, swallow-tail termi- nations and hollow rectangular sections (Figs. 9 & 10). These microlites, where not completely replaced by secondary minerals,

show strong normal zoning. In several thin sections, the micro-

lites show a subparallel orientation indicating flow alignment.

The original texture of pillow basalt samples was probably hya-

lopilitic to pilotiaxitic in nature with the interstitial amphibole,

chlorite and epidote patches representing alteration of devitrified -42-

Figure 8. Typical Ely Greenstone basalt (CLEU- 117-3). Note tabular plagioclase micropheno - crysts set in matrix of fine plagioclase laths, amphibole, chlorite and epidote. Bar = 0.5 mm. -44-

glass.

The massive basaltic rocks also contain microphenocrysts of

plagioclase set in a matrix composed of thin plagioclase laths,

amphibole (actinolite), chlorite, epidote and fine grained opaques.

The present texture in these samples, has a fine mottled appearance due to patches of amphibole and granular epidote. Both the microphenocrysts and groundmass laths of plagioclase are replaced by albite and muscovite. The original igneous texture of these rocks was subophitic to hyalopilitic.

The medium to occasionally coarse grained concordant gabbro sills range in texture from subophite to relict poikilitic. Plagio- clase, in amounts ranging from 40-50%, occurs as tabular laths which have been extensively saussurited. Clinopyroxene, generally completely replaced by ac·tinolite, is present in amounts of 35-50% and occurs as either tabular crystals partially enclosing smaller plagioclase laths or as large (3-5 mm) oikocrysts. One sample, from a gabbro sill in the southeast corner of the Chad Lake quadrangle, also contains altered oikocrysts of orthopyroxene. Large

(.5-lmrn) skeletal opaque grains (titaniferous magnetite), showing well developed octahedrally oriented ilmenite (?) are present in amounts ranging from 1-3%. Quartz, and in a few cases granophyre, are present interstitially in amounts ranging from

1-10%.

Gray Porphyritic Basalts: The gray basalts are quite variable in mineralogy and tex ture. Samples from the gray bulbous pillow basalts south of the Shagawa Lake (road cut and adjacent outcrops, ·-45- center Sec. 32, T.63 N., R. 12 W.) have rounded to subhedral plagioclase microphenocrysts (...... 2%) in clusters of three or four, set in a slightly variolitic matrix of plagioclase microlites and very fine grained alteration products probably after glass (Fig. 11).

The microphenocrysts are mostly replaced by calcite. Diabases associated with these pillow basalts have hypidiomorphic-granular to subophitic textures with unaltered augite (30-40%), strongly saussuritized plagioclase (60-65(0 and sphene pseudomorphs after skeletal titaniferous magnetite or possibly ilmenite (1-3%).

Epidote and actinolite are present in minor amounts. The lack of abundant actinolite and, to a lesser epidote and the usually high carbonate content accounts for the gray color of these rocks.

The gray basalts south of Mud Lake are particularly variable.

Several samples are very similar to those described above with tabular plagioclase phenocrysts and microphenocrysts set in a fine matrix of plagioclase microlites and altered material. Other samples have an intergranular to subophitic texture with fresh clinopyroxene between and partly enclosing plagioclase laths (Fig.

12). One sample (CLEU-182-19, see Appendix lb for analysis) has abundant (-'5%) tabular plagioclase phenocrysts (l-2mm long) and a few (...... 5-1%) clinopyroxene phenocrysts ( lnun diameter) which are slightly zoned and contain small plagioclase laths (Fig.

13). The groundmass is very fine grained variolitic with plagioclase and pyroxene microlites set in a mat of unidentified hydrous minerals. -46-

FiGure 11. Gray ?illow b asalt , upp er Ely Gr een- (E'iW-20- 75) showing glomerop orphyritic p lag i oclase phenocry sts (rep laced by calcite) et in a fine variolitic g roundmass. Dar = 0 . 5 mm. -48-

Felsic Flows

Felsic extrusive rocks, while no.t abundant in the upper Ely

Greenstone member, are present locally. Those occurring in the

Gabbro Lake quadrangle have been described by Green (1970). Green and Schulz (1977) found a sequence of intercalated felsic flows and volcaniclastic rocks in the southwestern portion of the Ely quadrangle. During this study, a traverse was run along the N-S power line in Sec. 3, T. 62 N., R.12 W., to sample these flows for chemical analysis. These rocks are within the amphibolite facies metamorphic aureole of the Giants Range Batholith.

The felsic flows examined show a range in color from pale tan to medium gray and greenish gray and are fine grained. Many have small well developed pillows, which are bulbous to slightly flat- tened with 1-2 cm thick rinds (strike of pillows N55W, tops N30E).

Some samples show a pronounced lineation of biotite and occasional amphibole lenses.

Present with the flows are highly foliated felsic rocks, which probably represent interbedded volc·aniclas tic material. Also present locally, are banded garnet-hornblende amphibolites (Fig. 14).

The garnets, some·!µp , to several millimeters in diameter, are concentrated in.thick: lay-ers. The presence of bands of quartz+ plagioclase + biotite with the garnet-amphibolite suggests that these rocks may represent mafic to intermediate volcaniclastic rocks. -49-

Petrography:

Two samples taken from pillowed f elsic flows were examined in thin section. One sample (EEU-7-2) is very fine grained,granoblastic with a .strong foliation marked by short biotite plates.

The rock consists of strongly pleochroic biotite and lesser amounts of hornblende with quartz, plagioclase, calcite, and minor epidote.

The other sample (EEU-9-4) has a similar texture but consists of the assemblage hornblende + quartz +. plagioclase with minor epidote and chlorite (retrograde. after hornblende). This. sample also has a few coarse quartz lenses which may represent amygdules.

The garnet amphibolite is banded on a centimeter to millimeter scale with bands of garnet + hornblende; garnet + hornblende + quartz; quartz + plagioclase + biotite + garnet + hornblende; and quartz +hornblende. The garnets range from subhedral to euhedral with dark inclusion rich cores (Fig. 15). Parallel fractures, often found cutting garnets, are filled with fine amphibole. Hornblende wraps around the garnets (Fig. 16) with a pinch and swell texture.

Groundmass textures are granoblastic with foliation defined by biotite plates and/or hornblende prisms.

The felsic volcaniclastic rocks consist of broken and partly rounded, altered plagioclase crystals (1 mm in size) set in a granoblastic groundmass of quartz + plagioclase + muscovite + carbonate. A few samples also have euhedral blocky hornblende crystals

( .2 - .5 mm) partly rimmed by light green amphibole along with 1 mm

sized pyrite cubes.

Retrograde alteration is seen in most samples from this felsic -50-

Figure 14. Garnet amphibolite, upper Ely Greenstone. Note garnet rich layers and thin band of plagioclase + quartz + garnet with minor hornblende (center of photo). Penny for scale.

Figure 15. Garnet amphibolite, upper Ely Greenstone (EEU-5-l-3A). Note inclusions of plagioclase and hornblende in cores of garnet porphyroblasts (black). XP, bar= 0.5 mm.

Figure 16. Garnet amphibolite, upper Ely Greenstone (EEU-5-l-3A). Note pinch and swell texture of hornblende grains around garnet porphyroblast. Bar = 0.5 mm. -51-

Fig. 14.

Fig. 15.

Fi g . 16. -52- unit. Chlorite and epidote are often found intermixed with hornblende. Pale biotite is found around some garnets and actinolite is found rimming some hornblende crystals. Fine saussuritization of larger plagioclase crystals is also probably a retrograde feature.

Volcaniclas.tic Rocks

As a study of the volcaniclastic rocks was beyond the scope of the present investigation, only a cursory examination was made of those encountered during sampling. A brief description of their general field relations is given below.

The volcaniclastic rocks occur as thin1 laterally discontinous beds scattered at various horizons in the upper Ely Greenstone member, but are concentrated near the top. They often occur in con-

with iron-formation in the Gabbro Lake (Green, 1970), Chad

Lake (Sims and Schulz, 1977b) and Soudan quadrangles. An agglomerate, found in assoc.iation with an iron-formation north of the Arm- strong River (Sec. T.62 N., R.14 W.), was found to contain subrounded clasts of banded jasper along with mafic to felsic volcanic rock clasts.

Several volcaniclastic units were found J.n the southeast corner of the Chad Lake quadrangle (Sims and Schulz, 1977b), some of which are traceable for 2-3 km laterally. Considerable lithologic variation exists, ranging from very fine grained cherty tuffs (?) to fine to medium grained tuffs or graywackes. Abundant plagioclase is present in the coarser rocks along with mafic to felsic rock fragments. Pyrite is particularly abundant (up to several -53-

percent with massive sulfide locally) in a volcaniclastic unit in

the southern portion of Sections 32 and 33, T. 63 N., R. 14 W.

Volcaniclastic rocks are uncommon in the upper Ely Greenstone

north of the Soudan Iron-formation member in the Eagles Nest and

Crab Lake quadrangles. One exception, however, is a thin (10-15 meters thick) tuff found interbedded with pillow basalts in the

NE !t; NW !t;, Sec. 7, T 62 N, R 13 W. The rock is dark green in color

and consists of 2-3 Imu pumiceous intermediate to felsic rock fragments (Fig. 17) in a niafic groundmass.

Several volcaniclastic units are present in the southern por-

tion of the Shagawa Lake quadrangle (Sims, Mudrey and Schulz, 1976;

Sims, 1972). They range from intermediate to mafic tuff and tuff

breccia to reworked carbonaceous quartz tuff. One unit, though

faulted, has been traced laterally to the west end of Foss Lake

in the Crab Lake quadrangle. A few thin banded iron-formations are present with the tuffs in the Shagawa quadrangle. Felsic to

intermediate tuffs are also present in the Mitchell Lake area based

on drill core data from the area. They are interbedded with mafic

flows and cut by quartz-feldspar porphyry bodies.

Environment of Formation

The volcanic rocks of the upper Ely Greenstone member provide little information concerning their environment of formation. Pil- low basalts are abundant throughout the member and clearly indicate submarine extrusion. However, unlike the basal ts of the lower Ely

Greenstone member, amygdules are uncommon, particularly in the -54- lower portion (directly above the Soudan Iron-formation member, see Fig. 2). Though the presence of amygdules can be taken as . an indication of shallow conditions as discussed above, their absence does not provide unequivical evidence for deep water conditions. As discussed by McBirney (1963), composition is also an important factor in determining the degree of vesicularity. Thus lack of amygdules in the upper Ely ·basalts may simply reflect a lower dissolved gas content in these tholeiitic basalts as compared to the calc-alkaline volcanic rocks of the lower Ely member.

In the stratigraphically higher portion of the upper member

(Gabbro Lake quadrangle), there is a significant increase in interbedded tuffs, conglomerates, iron-formation and dacite porphyry

(Green and others, 1966). Green (1970) has described scoriaceous fragments in volcanic conglomerates southwest of Tofte Lake and has also noted conglomerates in the area of Jasper Lake containing clasts derived from underlying dacite porphyry and greenstone.

These features suggest shallow water conditions for at least the upper part of the member. The presence of clasts, derived from the upper Ely Greenstone member, within the stratigraphically overlying Knife Lake Group (Green, 1970) is a further indication of at least shallow water conditions at the end of Ely Greenstone time.

Amygdaloidal tholeiitic basalts have also been found in the upper portion of the member in the Chad Lake quadrangle (Sims and

Schulz, 1977) and the Gabbro Lake quadrangle (Green, 1970). These basalts have compositions similar to the nonamygdaloidal basalts -55- found elsewhere and may indicate either variation in the depth of formation of the lavas or variations in initial volatile content.

It is concluded that the upper Ely Greenstone volcanic sequence represents the build up of a thick (>3000 meters), tholeiitic (see later section), basaltic pile with at least the upper portion having formed 'under shallow water conditions. Ely Greenstone volcanism apparently terminated rather abruptly after build up of the basaltic pile and was followed by deposition of the dominantly felsic volcaniclastic rocks of the Knife Lake Group.

Lake Vermilion Formation

Several occurrences of pillowed to massive basalt are present within the dominantly volcaniclastic Lake Vermilion Formation.

Two localities south of Tower were sampled for this study (NW 1/4,

NE 1/4 Sec. 11, T.61 N., R.16 W. and SE 1/4, NW 1/4, Sec. 16, T.61

N., R. 15 W.). At both the rocks range from pillowed to massive basalt, medium gray in color on fresh surfaces and light gray on weathered surfaces. Amygdules were not observed, but possible small variolites are present in the exposures in Section 11.

Petrography

In thin section, both samples show a very fine grained almost felty texture composed of fine acicular actinolite needles and skeletal microlites of plagioclase. Quartz, minor iron oxides, and rare epidote are also present. The lack of epidote or chlorite in -56-

these samples is striking and probably accounts for their gray color.

The Newton Lake Formation

The Newton Lake Formation was originally mapped as Ely Green-

stone by Clements (1903) and was later mapped as the "unnamed for-

mation'' in the Gabbro Lake quadrangle (Green and others, 1966).

Green (1970) later showed that the rocks of the "unnamed formation"

overlie the Ely Greenstone and are in apparent depositional contact

with the Knife Lake Group in this area. The name, Newton Lake For-

mation, was formally given to these younger volcanic rocks by

Morey and others (1970), with the type locality designated as the

exposures in the vicinity of the Newton Lake. The formation is

bounded on the north by the Burntside Lake fault (formerly part of

the Vermilion fault) and along strike to the northeast by granitic

rocks of the Vermilion Batholith. At its western end, near Wolf

Lake, the formation is truncated by the Wolf Lake fault. In the

Ely quadrangle, the formation is in fault contact with the Knife

Lake sediments, whereas in the Shagawa Lake quadrangle the contact

is conformable and marked by a thick, highly distinctive pyroxene

tuff unit (Sims, 1972).

The formation has been divided into two informal members

(Morey and others, 1970); a ma.fie volcanic member occurring west

of Newton Lake and a felsic-intermediate volcanic member to the

east (Fig. 2). In the vicinity of Newton Lake the two members in-

tertongue (Morey and others, 1970; Green, 1970). Estimated thicknesses for these members are given in Table 1. -57-

, .., )·;.., .. .) ...... -- \;-...... , ...:- · ... '•

'. '•, "':, :... : - .. • j J,· / "· ,...... ,·1. /': . : ...... '. . ...-- .. ·. ·.·' . ·" .. ·...... -"" ...... , . ' .. ·... _,. .· ;: .· . ' '·

·. ··'

;"'' . ,. ;--'.;,.;;,_ ' .. " .. ; t ......

Fi g ure 17 P umi c eo u s tuff, upper Ely Green- stone (E NEU - 9- 1 6 ). Ang dules filled by q ua rt z and car b o na te. Bar = 0 .5 mm -58-

Several important differences have been noted between the

Newton Lake Formation and the Ely Greenstone (Green, 1970). These

include: (1) an abundance of felsic and intermediate rocks in the

Newton Lake Formation east of Newton Lake as opposed to their minor abundance in the Ely Greenstone; (2) iron-formations and

porphyries are connnon in the Ely Greenstone, but are rare in the

Newton Lake Formation; (3) layered ultramafic-mafic sills are

common in the western portion of the Newton Lake Formation but

are unknown in the Ely; (4) thick sedimentary strata and small

lenses of impure siliceous marble are present in the Newton Lake

Formation, but have not been found in the Ely Greenstone. In

addition, in this study significant chemical differences between

the volcanic rocks of the Newton Lake Formation and Ely Greenstone

(see later discussion) have been identified.

As noted above, the Newton Lake Formation has been informally divided into two members based on the lithologic change which occurs

in the area of Newton Lake. Recent mapping completed in the Shagawa

Lake quadrangle (Sims, Mudrey and Schulz, 1976) suggests that a similar change, from dominantly mafic volcanic rocks to dominantly felsic volcanic rocks, may also occur at the western end of the formation. This interpretation is based on the increase in felsic volcaniclastic material which occurs in the Shagawa Lake quadrangle along with the presence of siliceous marbles and banded iron- forma tion. It is interesting to note that the large body of siliceous marble mapped by Gre.en (1970) in the Gab bro Lake quadrangle, occurs in the area of intertonguing between the mafic and felsic -59-

members.

In a pre-faulting reconstruction by Sims (1976), the dominantly

intermediate composition tuffs and tuff-breccias, which occur on

Pine Island in Lake Vermilion (Fig. 2) are correlated with the

Newton Lake Fonnation. Further mapping, particularly in the area

of Pine Island, will be required to substantiate . this interpretation.

The Felsic Member

The felsic volcanic member of the Newton Lake Formation was

not examined during this study, but has been described in detail

by Green (1970). A brief description, based on Green's work, is

given below.

The majority of the rocks in the felsic volcanic member are

fragmental breccias, tuff-breccias and tuffs along with lesser volcanic arkosic wacke and volcanic graywacke. Pillowed and flow- banded dacite lavas, interstratified with the tuff-breccias, are locally abundant. Some of the flows are amygdaloidal, with one or more of the minerals quartz, chlorite, or calcite filling the cavities. Scoriaceous breccias (see Green, 1970, Fig. 20, p. 44) are present locally.

A distinctive feature of many of the felsic lavas and pyroclastic rocks is the presence of plagioclase ± quartz + hornblende phenocrysts. Hd.crophenocrysts of magnetite and apa;tite are also found in a few samples. The tuffs, volcanic and arkosic wacke and graywacke contain clasts of albite, felsite and andesite with minor mudstone, chlorite, sericite and apatite. The mineral -60-

assemblage in these rocks aside from phenocrysts and clasts, is

as follows in order of approximately decreasing abundance: albite,

quartz, chlorite, calcite, epidote, actinolite, sericite, sphene, magnetite, apatite, ankerite, pyrite, and zircon.

A distinctive rock type, siliceous marble, occurs in the fel-

sic member in the area of Upper Pipestone Falls. The layer, about

150 meters thick and at least 1.6 km long, is composed of fine- grained, recrystallized cherty limestone and recrystallized cal-

careous chert along with several conglomeratic zones of cherty pebbles, cobbles, or granules in a limey matrix. The dominant carbonate in the .rock is calcite. As noted above, similar rocks have also been found in the Newton Lake Formation in the Shagawa Lake quadrangle (Sims, Mudrey and Schulz, 1976).

The Mafic Volcanic Member

The maf ic member of the Newton Lake Formation has been mapped in the Ely (Green and Schulz, 1977) and Shagawa Lake (Sims, Mudrey and Schulz, 1976) quadrangles. During this study, further detailed mapping was done, particularly south of Cedar Lake in the

Ely quadrangle. Good exposure in this area facilitated detailed sampling for petrologic and geochemical studies (Fig. 18). Samples were also collected throughout the formation to obtain vertical and lateral representation (F;l.g. 2).

The mafic volcanic member consists of mafic pillowed to massive flows and layered to nonlayered mafic-ultramafic sills. Felsic volcaniclastic units, iron-formation and siliceous marble are -61-

Figure 18. Geologic map and sample locations of the geochemical samples, Cedar Lake area, Newton Lake Formation. Cedar Lak e Area ' I 1 A'l'B ype A Ba•alt s I I I'°\ Type B Bnoalt• 1 J 't' : C Basalts } If. A oc\• / PendotHe \ ,o Gabbro I f ,.,.-0--1/ J,-i " L Lamprophyre ,/l ,0( *0 Ge o h · I / _,--, l!" I 4 > e . em•eal S 4 ,/ , //,u- ' / 'lj, ' -<' Pillow Tope amplee I >. rzy;, /)(q \ 0 - Contacts c•,_ 0 ,, c , ...,,/ /"/ -1_J ,,,.,v '..l I \I / * / / . Faults '\ , , w./,o'.fi-< \"o•')"' C\. ,9o / \ ·. I @), / ,/ 't/p ,,, / / ' 0 ...... di . ,./ / / d • 0 / / •'T -- ______J_ ------D- ) /- J, ,- , .fVn- e /II

Yt I / "'\: /of \ f-!o -;{;;< "i' Li / / e QIV \..;/,:/ f' I c I p iJ",..o .,a"' I 3 ,"" 0 0 I J2;. ,_:j)1"t}-. ,,,:3 'o ;,z_ ! , mo p _,,,n . __/ '-"'"' ...... !)- - _I/"' - J]; L A I< £ 0. I r"\..l. C\)-0

-- l/l I igure 18. /;; '-----" Zt O'\ N I -63-

locally abundant, particularly in the Shagawa Lake quadrangle.

These units are often lenticular, though a few felsic tuff units

have been traced laterally for a kilometer or more. The felsic

tuffs are composed of mafic to felsic angular rock fragments along

with broken crystals of quartz and plagioclase set in a fine

grained plagioclase-quartz - amphibole matrix. The "' felsic ,, tuffs

have andesitic compositions (Arth and Hanson, 1975).

A coarse paraconglomerate unit was found interbedded with

basalt, gabbro and tuff south of Little Long Lake (NW 1/4, NW 1/4,

Sec. 20, T. 63 N., R. 12 W.). A variety of rock types are present, with the most abundant being rounded pebbles, cobbles and boulders

of quartz.;..rich tonalite (Fig. 19). Fine grained basalt and medium

grained diabase clasts, generally much smaller and more angular

than the tonalitic clasts, are also present (Fig. 20). The matrix

is unbedded and consists of actinolite, with lesser amounts of

chlorite, quartz and plagioclase. A similar tonalite bearing

conglomerate was found by Green (1970) one-quarter mile west of

the northwest end of Newton Lake (Gabbro Lake quadrangle). The

source for the plutonic clasts in these units is unknown. Similar clasts from a conglomeratic unit in the Ely Greenstone, however, have given ages similar to the greenstones themselves (2.69 b.y.,

Jahn and Murthy, 1975), suggesting derivation from intrusives con-

temporaneous with volcanism as opposed to an older granitic basement.

The Cedar Lake Area

The area between Cedar and Fall Lakes (Ely quadrangle) was -64-

Figure 1 9 . P araconglo merate with q uartz rich tonalite cobb les and pebbles . Large st cobble a pp roximately 1 foot in diameter.

Fi g ure 20 . Tonalite pebbles and an g ular b asalt and diab ase clasts in p aracon g lom- e rate. Note ?encil for s cale . -65-:- mapped and sampled in detail (fig. 18). Rock units strike northeast-southwest, have near vertical dips and to the southeast as indicated by pillow tops and sill stratigraphy. They lie on the south limb of an isoclinal anticline, the axis of which is marked by a major fault trending NE-SW through Cedar Lake (Fig. 2).

Green and Schulz (1977), based on previous field data, have suggested that the Cedar Lake "sill" may represent a thick flow with surrounding pillowed flows representing a seawater-chilled carapace. The new data from this study further support this interpretation. include the lack of observable contact metamorphic effects, the undulatory nature of the the similarity in texture between marginal zones and surrounding flows and the similarity in chemistry between these same rocks (see later discussion). Similar layered bodies (peridotite through gabbro) in the Abitibi greenstone belt of Canada have also interpreted to be, at least in part, extrusive in nature (Arndt, 1976).

These also have similar compositions to the Cedar Lake Body.

The Cedar Lake "sill" essentially divides the area into a northern and southern portion. The "sill"s about 300 meters thick at .the center, thins to the southwest and is about 3 km. long. At least four major transverse (both northeast and northwest trending) strike-slip faults have segmented the body (Figc . 18). Contacts are generally conformable though undulatory in nature.

The sill is layered, with peridotite at the base followed up- ward by pyroxenite, bronzite gabbro and quartz gabbro. The peridotite, about 65 meters thick near the center of the "sill", is -66- black to greenish-black, medium grained and connnonly shows l to 2 cm. pyroxene oikocrysts on freshly broken surfaces. In the center of the body, peridotite is in sharp contact with medium to coarse grained pyroxenite, while on the ends, it is in sharp contact upwards with bronzite gabbro.

The pyroxenite is a dark to medium green websterite with large, euhedral (2-3 mm.) bronzite crystals and smaller augite euhedra.

This unit is less than 15 meters thick and passes gradationally upwards into two-pyroxene (pronzite) gabbro. The gabbro forms the thickest unit of the body about 150 meters thick except at the southwest termination) and consists of large euhedral (1-4 nnn.) bronzite crystals surrounded by smaller subhedral clinopyroxene and tabular plagioclase. Locally this unit is gradational upwards into medium to coarse grained gabbro and quartz gabbro.

The general features of the Cedar Lake "sill" are identical to those of numerous other layered sills present within the mafic member; These have been described in detail by Schulz (1974).

The Cedar Lake "sill" is surro_unded by pillowed to massive basaltic rocks of several varieties. The lower contact was found exposed in two outcrops in Section 13, T.63 N., R.12 W., (center and SW 1/4, SW 1/4 of section). Here, pillowed to massive dark green basalt, showing no evidence of contact metamorphic effects, is in contact with a highly altered (hydrated) black rock composed of large (1 mm.), blocky, brownish amphibole (probably after clinopyroxene) ·(Fig. 21) . This rock grades inwards (to the south) over a distance of about a meter, into a fine grained black rock ·with -67- fine, radiating needles of amphibole. In thin section, these amphibole needles are seen to be pseudomorphs after acicular skeletal clinopyroxene (Fig. 22). Similar textures are found in some of the surrounding basaltic rocks (see below). Chemical analyses of samples across this contact are presented in Appendix lb. Their composition is similar to the more maf ic of the surrounding flows and have chemical similarities to basaltic komatiites (see further discussion, p. 189).

The upper contact of the "sillrt is harder to define, particularly in the southern portion of the body. Exposures in tbe

SW 1/4, NW 1/4, Sec. 18, T.63 N., R.11 W. show a transition over a distance of about 20 meters, from coarse grained quartz gabbro to microgabbro (Fig. 23), with abundant ( 2%) opaques followed by a fine grained quartz diabase (Fig. 24), followed by a massive black, medium-grained rock witµ abundant (40-50%) euhedral, slightly zoned clinopyroxene crystals set in an aphinitic groundmass of amphibole and minor plagioclase (Fig. 25). The clinopyroxene rich rock is followed to the south by similar rocks, some of which are pillowed and show skeletal clinopyroxene. A bedded volcanic sandstone containing quartz, plagioclase, and amphibole (after clinopyroxene) crystals forms a thin lens within the pillowed rocks.

Based on field, petrographic and chemical criteria, several basaltic flow types have been identified and mapped in the Cedar

Lake area. Distinctive of many of these is a wide variation in texture, even within a single outcrop. These flows are both texturally and chemically distinct from the Ely basalts and are simi- -68- lar to basaltic komatiites (see p.213 for discussion).

The most distinctive of the flows· are the black, pyroxene rich rocks (hence forth referred to as type A basalts), noted above with regard to the Cedar Lake layered body. Green and Schulz (1977) have recently described these rocks and noted their similarity to basaltic komatiites from other Archean greenstone areas. The flows range from pillowed to massive in nature. As described by Green and Schulz (1977), the pillows are distinctive in having irregular shapes which are tightly molded against one another. Pillow rinds are thin (about 4 nnn) and little interpillow material is present.

In several outcrops large (5mm to several centimeters in diameter) varioles were observed. These consist of plagioclase and lesser amphibole and appear as white spots on the outcrop. In one locality

(SW 1/4, NW 1/4, Sec. 13, T.63 N., R.11 W.), small (3 X 2 meters) lenses of sulfide rich, mafic tuff were found interlayered with the pillowed rocks.

The type A basalts are usually very dark green to black on fresh surfaces and dark greenish gray on weathered surfaces. They show a wide variation in textural characteristics ranging from porphyritic to highly spherulitic.* Many have dark green to black needles of amphibole, in random to subradiating clusters, set in an aphanitic, dark green matrix, while others have clusters of equant amphibole in a similar matrix.

* Terminology used for textures, after Lofgren (1974). -69-

Fi g ure 21 . Lower chilled margin sample , Ce d ar Lake " sill " ( EN L-137-58 B). Sample consists of large, blocky, bro wn hornblende, fine actinolite and c h lorite. XP, bar = 0.5 mm. (Analyzed Append ix lb).

22 . Lowe r c hilled margin samp le, Ce d ar Lake '' s i l l '' S am p le 1 meter fro m contact . h i gh l y skeletal clino p yrox e ne i n actinolite - c il lor i t e g roundoa ss . XP , b a r 0 .5 me . (Ana l y zed s amp l e , Ap pe n dix l b ) -72-

The great diversity in crystal morphology in these rocks becomes apparent in thin section. Those samples with equant mafic grains are seen to consist of euhedral, prismatic to equant augite crystals ranging from 0.2 to 1.0 nun. across (Fig. 25); small euhedral chromite grains are often found enclosed in these (Fig. 26).

In some samples the euhedral pyroxenes are seen to have hollow cores (Fig. 27). There is a complete gradation from these samples to ones having more skeletal phenocrysts (Fig. 28 and 29) through to samples with spherulitic and highly skeletal clinopyroxene

(Fig. 30 and 31). Several samples still retain primary pyroxene, some of which are zoned in irregular to sector patterns.

The groundmass in these samples also shows a variety of textures. Those with euhedral pyroxene crystals have fine radiating mats of actinolite and chlorite surrounding either small, tabular or hollow plagioclase (Fig. 27). The plagioclase, where not completely saussuritized, shows multiple twinning and complex zoning.

In samples with fewer pyroxene euhedra, the groundmass may consist of either coarse plagioclase fans with fine actinolite-chlorite

(devitrification after glass?) between the branches or fan spherulites of clinopyroxene (now amphibole) (Fig. 32) showing highly skeletal shapes with plagioclase, actinolite and chlorite interstitially. Also present in the groundmass of most samples are concentrations of minute sphene (?) granules and dustyepidote.

In the samples with euhedral pyroxene, these often occur in thin platelets -showing a grid texture (Fig. 33) and may represent pseudomorphs after ilmenite (?). Fine grained opaques are also -73- present in some samples.

A notable feature of these rocks (as well as in many other

Newton Lake basalts) is the lack of recognizable olivine crystals or pseudomorphs thereof. This is particularly striking as most of these rocks are high in MgO and normative olivine. Only one exception was found in the Cedar Lake area. The rock (NE 1/4, NE 1/4,

7, T.63 N., R.11 W.) is dark gray and very fine grained. In thin section, the sample is seen to consist of chlorite pseudomorphs after highly skeletal olivine (Yig. 34) set in a very fine grained groundmass of spherulitic, and acicular, skeletal amphibole pseudomorphs after clinpyroxene Crig. 35). ·

The rest of the flows in the Cedar Lake area are light to dark green in color and pillowed to massive in nature. Based on field and chemical characteristics, they have been divided into two types

(types Band C). One of these (B) has been found only in the central part .of the map area, overlying the Cedar Lake 11 sill".

The rocks are pillowed, light grayish green on weathered surfaces and light to medium green on fresh surfaces. Small phenocrysts of pyrcxene and plagioclase are usually visible in handsample. In several outcrops, .quartz filled amygdules were noted below pillow rims. In the NW 1/4, SW 1/4, Sec. 18, T.63 N., R. 11 W., several thin tuffs lenses were found, along with a thin lens of cherty iron-formation. In the SW 1/4, SE 1/4, Sec . 7, T.63 N., R.11 W., a basaltic tuff composed of devitrified tachylyte fragments, some containing plagioclase phenocrysts (Fig. was found interbedded with these flows. -74-

Figure 26. (ENL-44-1·2).

lb) .

Figure 27. Type A basalt, Newton Lake Formation (ENL-44-13) Note hollow, chlorite cores of some pyroxene crystals and tabular nature of the plagioclase. Sample is from same outcro p as sample in Figure 26. Bar = 0.5 mm. -75-

Figure 28. Type A basalt, Newton Lake Formation (ENL-114-54A) with hollow cored and slightly skeletal clinopyroxene phenocrysts. Bar= 0.5 mm. sample, Appendix 1b).

Figure 29. Type A basalt, Newton Lake Formation (E-151A) with long (1-2 mm.) skeletal clinopy roxene uhenocrysts. XP, bar= 0.5 mm. (Analy zed Appendix 1b). -80-

In thin section, the rocks are seen to contain either plagioclase and/or augite phenocrysts. The augite is subhedral, shows slight zoning and ranges from 0.2 to 1 mm in diameter. The plagioclase phenocrysts are generally altered either to saussurite or replaced by calcite and occur as clus.ters of tabular crystals O. 2 to

0.5 mm in length. The groundmass consists of granular augite and small laths of plagioclase along with actinolite, chlorite, epidote and scrappy opaques (including minor pyrite cubes). Minor interstitial quartz is present in some samples. The groundmass texture ranges from intersertal to pilotaxitic .

Chemically, these rocks show calc-alkaline characteristics

(see p. 143 for discussion). This they may be related to the calc-alkaline volcanic rocks of the felsic member to the east.

Texturally similar, though chemically distinct, pillowed basalt flows occur south of the Cedar Lake "sill" in the southern half of Section 13, T.63 N., R. 12 W. These rocks are dark green on fresh surfaces and at least locally amygdaloidal. In hand sample, they show black mafic crystals set in a fine grained groundmass showing small plagioclase laths. In thin section, the mafic crystals are seen to consist of fine rosettes of amphibole and chlorite. These clearly represent altered phenocrysts, though whether after olivine or pyroxene is uncertain. Several samples show what appear to be segregation vesicles (Fig. 37), where late interstitial melt apparently filled existing vesicles (Smith, 1967).

Quartz generally fills the centers of these amygdules. The groundmass consists of small, thin plagioclase laths in random orienta- -81- tion, set in a matrix of amphibole, chlorite, epidote, carbonate and minor opaques.

The other type of green basalt (type C) is found throughout the Cedar Lake area, but is best exposed in the northern half of

Sec. 13, T. 63 N., R. 12 W. These basalts vary from pillowed to massive and are often variolitic. Rocks of similar appearance and chemistry are the most abundant type found throughout the rest of the mafic member of the Newton Lake Formation.

These basalts show a wide range in texture sharing many similarities to the type A basalts from this area. They are, however, chemically distinct (see discussion on p. 184). One of the best · exposures of this basalt type occurs west-northwest of BM 1377 in the NE 1/4, NE 1/4, Sec. 13, T.63 N., R.12 W. On the southern end of the outcrop is a pillowed, variolitic basalt which passes towards the northwest (down) into fine grained massive basalt showing fan spherulites, which in turn grades into fine to medium grained quartz gabbro with abundant leucoxene pseudomorphs of skeletal titanomagnetite. This transition occurs over a distance of about 30 meters and may represent a single flow. Pillowed, variolitic basalt and breccia occur again to the northwest of the gabbro.

In the variolitic pillow basalts, the varioles are small near the rim (about 2-5 mm) and become larger (up to 2 cm) and more abundant inwards toward the center of the pillow. In thin section, they show fine radial textures with fan plagioclase and pyroxene.

Small euhedral microphenocrysts are present in the centers of several varioles; these are altered to fine, fiberous actinolite -82-

and have morophologies suggestive of olivine. Between the varioles

is a fine grained mat of amphibole and epidote representing devi-

trified glass. The more massive basalt has fan spherulites of

plagioclase and pyroxene (now replaced by saussurite and actinolite

respectively) (Fig. 38) and small irregular titanomagnetite grains,

now replaced by leucoxene.

The diabase has an ophitic texture with small, tabular plagio-

clase crystals enclosed in actinolite pseudomorphs of clinopyroxene

(Fig. 39). Abundant (1-2%) skeletal titanomagnetite crystals, re-

placed by leucoxene, are present along with local patches of

slightly granophyric quartz.

Further textural variations are shown by samples from other

locations. The most connnon is shown in Figure 40. These have

clinopyroxene occurring as subhedral laths, as smaller anhedral

grains and also intergrown with plagioclase in a graphic like

pattern (Fig. 41). The pyroxenes show strong sector zoning with

clear cores and brownish rims. Small subhedral phenocrusts of py-

roxene are also present in some samples. Plagioclase occurs as

small laths, some of which have hollow cores. The groundmass con-

sists of fine amphibole, chlorite, dusty granular epidote and o- paques.

Other Localities and Basalt Types

The majority of the basalts found throughout the rest of the

Newton Lake mafic member are similar to the type C basalts from the

Cedar Lake area. These are typically pillowed and variolitic,

light to dark green in color and associated with more massive and -83-

Figure 38. Type C basalt, Newton Lake Formation (ENL-57-25-2) showing fan spherulites of amphibole (after pyroxene) and plagioclase. Bar = 0.5 mm. (Analyzed sample, Appendix lb).

Figure 39. Type C basalt, Newton Lake Formation (ENL-57-25-3) showing ophitic texture. Bar = 0.5 mm. (Analyzed sample, Appendix lb). -85- gabbroic portions. Most are texturally similar to those already described; however, some have abundant needles of amphibole set in a fine grained groundmass. In thin section, the amphibole needles show skeletal shapes (Fig. 42) and have replaced original clinopyroxene. Some samples have more euhedral amphibole pseudomorphs of clinopyroxene, some with chlorite cores. The groundmass in these basalts typically shows fine fan spherulites of plagioclase (altered to saussurite) and amphibole (after clinopyroxene). Chlorite, granular epidote, quartz and fine opaques are also present in varying amounts.

A particularly distinctive flow or flow unit was found on the southwest shore of Little Long Lake. The rocks are mostly pillowed, variolitic basalt, dark gray-green on weathered surfaces and dark green on fresh surfaces. Pillows are large (a meter or more in longest dimension) and have fairly thick (2 cm) rinds composed of angular basalt fragments (Fgi. 43). Small 2 to 5 varioles occur about 2 cm below the rim and become larger and merge going toward the pillow center (Fig. 43 and 44). Many varioles have small mafic clots at their center. In thin section, the varioles show radial fans of fine plagioclase (Fig. 45) and have dark rims. The mafic clots are chlorite formed after euhedral to skeletal olivine (Fig.

46 and 47). Considerable textural variation exists in the pillows with the grain size becoming coarser towards the core. Examples of this textural variation are shown in figures 48 a,b,c, and 49.

Also present in this outcrop is a fine grained, dark green rock showing fine, radiating clusters of acicular amphibole. In thin -86-

Figure 42. Type C basalt, Newton Lake Formation (SEN-231) showing skeletal amphibole pseudomorphs after clinopyroxene in spherulitic groundmass. Bar = 0.5 mm. (Analyzed sample, Appendix 1b).

Figure 43. Pillow basalt sample from south of Little Long Lake (SLN-Ba). Note fragmental interpillow material and the merging of the varioles away from the pillow rim. -89-

Figure 48. Variolitic basalt (south shore of Little Long Lake), Newton Lake Formation (SSN-431).

a.) Glomeroporphyritic olivine in coarse spherulitic groundmass of skeletal clinopyroxene (now amphibole). Bar = 0.5 mm.

b.) Enlargement of groundmass texture. Bar = 0 . 5 mm.

c . ) Enlargement of groundmass texture. Note skeletal nature of acicular pyroxene. Bar = 0.1 mm. -90-

Fig. 48a.

Fig .• 48b.

F{ g . 48c. -92-

section, these are seen to be highly skeletal (Fig. 50) pseudomorphs after clinopyroxene. The groundmass consists of fine fan

spherulites of plagioclase and actinolite along with fine opaques

and epidote. A few altered olivine phenocrysts are also present.

Compositionally this sample is very similar to the associated pillowed basalts (see Appendix lb and discussion p. 189). Tex-

turally similar basalts are found to the northeast along the

south shore of Little Long Lake.

Basaltic lavas similar to those found in the Ely Greenstone

Formation have been found at only a few locations, particularly

at the western end of the member. Tholeiitic, high aluminum ba-

salt (SPN-27, App. lb) was found northeast of Picketts Lake (small

"island" in dry lake bed, NW 1/4, NW 1/4, Sec. 7, T.63 N., R. 11 W.).

The rock is pillowed, with small (1-2 mm) varioles and plagioclase

phenocrysts present in the outer parts of the pillows. It is light

gray on weathered surfaces and medium gray on fresh surfaces. In

thin section, the plagioclase phenocrysts are seen to occur in

small clusters. The varioles, often rimmed by fine opaques, con-

sist of very fine grained amphibole, plagioclase and other uniden-

tified minerals. The pillow centers lack phenocrysts and have

small unoriented laths of altered plagioclase set in a groundmass

of amphibole, epidote and minor opaques. Similar basalts were

also found south of Little Long Lake in the Shagawa Lake quadrangle.

The Newton Lake Sills and Their Chilled Margins

Layered sills, similar to the Cedar Lake body, are common -93- throughout the mafic member. They all have peridotite, pyroxenite, two-pyroxene gabbro and quartz gabbro layers, though their proportions vary between sills. Massive gabbro and quartz gabbro sills are also present (pee Green and Schulz, 1977). The layered sills show all the features typical of layered intrusions including phase and cryptic layering, weak rhythmic layering and well developed cumulus textures. The layered sills and gabbroic bodies have been described in detail by Schulz (l974).

Many of the sills have complex chilled margins which show textures similar to the flows. Examples of some of these textures are shovm in Figures 51, and 52. The characteristic feature of these chilled margins, particularly at the upper contacts, is the presence of long (up to several centimeters), sometimes branching, amphibole pseudomorphs after clinopyroxene. These generally lie perpendicular to the contact and become longer and broader into the sill, forming a texture similar to the spinifex of peridotitic komatiites (Pyke and others, 1973). Similar textures in the chilled margins of sime sills in the Abitibi greenstone belt of

Canada have been called pyroxene spinifex (Arndt, 1976; Arndt and others, 1977).

Compositionally, the chilled margins are high in MgO and low in

Al203, sharing characteristics with the flows of the Newton Lake

Formation. This compositional equivalence and the close spatial association of the sills and flows points to the two being co-magma tic (Schulz, 1974). -94-

Figure 51. Chilled margin sample, Newton Lake Formation (SEN-324) showing highly skeletal pyroxene crystals (now amphibole) in fine spherulitic groundmass. Bar = 0.5 mm.

Figure 52. Chilled margin sample, Newton Lak e Formation (SEN-323) showing coarse fans and tabular plagioclase. Light gray is am ph i b ole. = 0.5 mm. -95-

Significance of the Textures in the Newton Lake Flows

The primary textural characteristic of the Newton Lake basalts

is their wide range in crystal morphology. Their abundant skeletal

and spherulitic forms clearly distinguish them from the tholeiitic

basalts of the Ely Greenstone and suggests that they crystallized

under significantly different conditions from the other basalts of

the district.

Skeletal and spherulitic crystal forms have been described

from ocean floor basalts by Bryan (i972a,b). He noted a wide range

in olivine and plagioclase morphology related to their position within pillows. This variation was attributed to differences in

the rate of cooling experienced in different parts of the pillows, resulting in variations in growth rate and element diffusion (Bryans

1972b). Similar textures have recently been described in Archean

tholeiitic basalts by Gelinas and Brooks (1974), Pearce and Donald- son (1971+) and Pearce (1974). Some Ely Greenstone basalts show skeletal plagioclase morphologies similar to those described by the above authors (see Fig. 9 and 10).

It is noteworthy that pyroxene was never found as equant or skeletal crystals, but only as dendritic or spherulitic masses in thegroundmass in the tholeiitic basalts described in the references above. This is in marked contrast to the Newton Lake basalts, · where clinopyroxene ranges from equant and skeletal phenocrysts to acicular and fan spherulitic groundmass forms,

Textures similar to those found in the Newton Lake samples have been described in Archean basaltic komatiites from Western -96-

Australia (Williams, 1972; Hallberg and others, 1976) and Canada

(Arndt and others, 1977). The Newton Lake basalts also show striking textural (and to a lesser degree chemical) similarity to lunar pyroxene-phyric basalts, particularly the Apollo 12 and 15 pigeo- nite basalts . (Drever and others, 1972; James and Wright, 1972;

Lofgren and others, 1974; Weigand and Hollister, 1973; Dowty and others, 1974). The similarity to lunar samples, suggests that the abundant petrologic and experimental data on the lunar rocks may provide meaningful insights into the petrogenesis of the Newton

Lake rocks.

The principal textural features which require explanation in the Newton Lake samples are: (1) the presence of relatively large, often skeletal pyroxene phenocrysts, (2) the disparity in size and habit between phenocrysts and matrix crystals, and (3) the transitional nature of the textural variations between samples. It has generally been inferred that skeletal crystal forms resul.t from rapid cooling or quenching. Support for this has come from comparisons of natural samples to quenched furnace slags (Lewis, 1971) and quenching experiments. This mechanism alone cannot, however, account for the porphyritic texture of many samples nor the large size of the skeletal crystals.

The difference in size and habit of phenocrysts and matrix crystals clearly represents two stages of crystallization; but, as for the Apollo 15 samJ?les, two interpretations are possible of the coqling history which gave rise to these different types of crystallization. In one the porphyritic texture is related to a two- -97- stage cooling history with phenocrysts forming under equilibrium crystallization at depth and the matrix forming at the time of extrusion (this is the classic interpretation of porphyritic texture). The other interpretation, which has been strongly favored for the lunar samples (Powty and others, 1974), involves a single- stage cooling history involving rapid crystallization under super- cooled conditions.

Both of these interpretations appear applicable to specific

Newton Lake samples. Several of the type A basalts from the Cedar

Lake area have clusters of euhedral clinopyroxene crystals suggestive of cumulus processes. A two-stage crystallization history seems likely for these samples, with the phenocrysts having formed at depth in the magma and accumulated into clusters during ascent and extrusion. This is supported by their compositional similarity to the pyroxene layers of the sills (compare analyses and 108 ,

Appendix lb). The variation in groundmass textures in these particular samples (see Fig. 26 and 27) reflect differences in the cooling rate upon extrusion probably in relation to their position within a flow.

For other type A basalts and the type C basalts, a two-stage history is unlikely. The presence of skeletal pyroxene phenocrysts showing strong normal zoning and pronounced sector zoning suggests rapid, metastable crystallization (Dowty and others, 1974). This is more compatible with a single-stage of crystallization under super- cooled conditions.

Dowty and others (1974) and Lofgren and others (1974) have .:....98- presented a supercooling crystallization model for Apollo 15 pyroxene-phyric basalts (Fig. 53). In the model, a melt supercool.s from temperature A to temperature B where nucleation of pyroxene occurs at a supersaturation equivalent to the length of line B-C.

With further cooling pyroxene growth would continue, while the liquid followed a path (B to D, Fig. 53) always supersaturated in pyroxene. Eventually the liquid would reach the pyroxene-plagioclase cotectic (D, Fig. 53) where the sharp decrease in the equilibrium liquidus slope (Wyllie, 1963) would result in a marked increase in pyroxene supersaturation (reflected by the length of the horizontal lines, Fig. 53) and a significant change in crystal growth and nucleation rate (Lofgren and others, 1974). To form the groundmass pyroxenes observed in the Apollo -15 samples and the

Newton Lake basalts, the nucleation rate must significantly increase.

The actual temperature of nucleation would depend on the specific cooling rAte. The actual path of the cooling liquid in Fig. 53 will also depend on the cooling rate. The faster the rate, the greater the supercooling which will be attained with respect to the temperature at which plagioclase comes on the liquidus (point D) and groundmass pyroxene begins to crystallize. Increased supercooling would result in a finer grained and more skeletal or spherulitic groundmass texture (Lofgren and others, 197Lf).

Lofgren and others, (i974) have tested this model with controlled cooling rate experiments on compositions similar to the

Apollo 15 pyroxene-phyric basalts. Their results, sunnnarized in

Table 4, clearly show that the observed porphyritic textures in the -99-

A B

Pyrox.--.... + Liquid Pyroxo- + Ptaoioclase + Liquid Path tof Fractionating Liquid Liquid Fraction Cornposition

Figure 53. Schematic plot of quartz-normative basalt equilibrium liquidus, and the path of the fractionating liquid during the cooling history. Pyroxene supersaturation proportional to length of the horizontal lines. The vertical dist- between the two liquidus paths is a measure of the degree of supercooling. (after Lofgren and otners, 1975). Table 4. Crystal Morphology as a Function of Cooling Rate in Apollo 15 Quartz Normative Basalt (after Lofgren and others, 1975).

Cooling Rate Olivine Pyroxene Plagiclase Opaques (°C/hr) Phenocryst Groundmass ------·------····---- ·---·- -·- -·---·--·------·-·-·· ------·------·- 1260 branching dendrites .!. nTe 430 coarser dendrites with less branching acicular skeletons none chromite with internal dendrites I - structure 220 thinly tabular,"' parallel growths . 115 acicular skeletons l ..v GO elongatel to subcq- larger; elon- fan spherulites acicular inter- subequant- uant skeletons gate skeletons nucleated at grown with fan phenocryst spherulites chromite marr;ins 20 subequantl skeletons .j, l w[ coarser :fan euhedral spherulites, chromite+ subequant crystals ulvosplinel elonBate to sub- acicular, intergrown equant skeletons with fan spherulites; minor skeletal crystals vI 2,5 none subequant eu-to- subequant to skeletal laths, few chromite, subhetlra! crystals crys- acicular crystals ulvospinel, I with hollow cores tals ilmenite I-' 0 0 I -101-

Apollo 15 samples can be accounted for by a single-stage crystallization history involving varying degrees of supercooling and rates of cooling. Direct application of the experimental results from the lunar samples is not possible to the Newton Lake basalts because of differences in bulk composition and other factors relating to crystallization conditions (i.e., f 02 , HzO content, etc.), but it is clear that their formation was also controlled by these same variables of supercooling and cooling rate.

Both experimental cooling rate studies and theoretical considerations (Kirkpatrick, 1975) indicate that with increased supercooling the rate of crystal growth increases while diffusion coefficients in the melt decrease. The combined result is to produce a larger number of more skeletal crystals at faster cooling rates.

Theoretical models for crystal growth under such conditions also predict (Kirkpatrick, 1975) that increasing instability of planar interfaces in crystals is accompanied by decreases in the wave- lenth or spacing of these instabilities. For example, this means that the branches of fan spherulites would be finer and closer together at faster cooling rates than at slower rates.

Several features observed .in the Newton Lake basalts are consistent with these theoretical predictions. Those samples with the fine grain size typically have highly skeletal textures with a high abundance of phenocrysts set in a fine groundmass of small wavelength fan spherulites (see Fig. 42). As the number of phenocrysts decreases both the phenocrysts and the groundrnass become coarse grained

(Fig. 40). In the coarsest grained samples, the ophitic gabbros, a -102-

Figure 54. Peridotite lens chilled margin sample Newton Lake Formation (SEN-213). Sample from upper contact. Note abundant euhedra of amphibole pseudornorphs after clinopyroxene (many with chlorite cores) set in very fine amphibole matrix. Also note sulfide lined amygdule . Bar = 0. 5 mm. (Analyzed sample, Appendix 1b) •

Figure 55. Same as Figure 54 (SEN-212). Sample from middle of chilled margin. Note long, hollow amphibole pseudornorphs after clinopyroxene and coarser matrix than in Figure 54. Bar = 0.5 nun.

Figure 56. Same as Figure 54 (SEN-211). Sample from near peridotite contact. Note coarse plagioclase fans some of which nucleate on the clinopyroxene phenocrysts. Bar= 0.5 mm. -103-

Fig. 54.

Fig. 55.

Fig . 5 6 . -104- distinction between phenocryst and groundmass no longer exists.

Evidence that the observed textural variations are the result of crystallization at varying cooling rates is provided by the chill margin of a thin peridotite lens within the Newton Lake sequence.

The lens consists of a core of fine to medium grained peridotite

( 8 - 10 meters thick) surrounded by a 25 meter thick chilled zone

(Schulz, 1974). The rock at the upper contact of the chilled zone

(i.e., sample having experienced the fastest cooling rate) consists of small, abundant (55%) euhedra of hollow amphibole pseudomorphs of clinopyroxene set in a very fine grained groundmass of acicular amphibole (Fig. 54). A few sulfide lined amygdules are also present.

Inwards toward the peridotite core (i.e., to slower cooling rates) the clinopyroxene phenocrysts (replaced largely by amphibole) become fewer but larger in size. The groundmass also becomes coarser grained with small fan spherulites of clinopyroxene (now amphibole) and plagioclase (Fig. 55). Closest to the peridotite, the clinopyroxenes make up only 30% of the sample, but are much larger (some up to several centimeters) and prismatic to branching in form

The groundmass consists of coarse fan spherulites of plagioclase and pyroxene (Fig. 56). It should be noted that these textures are similar to those found in the type C basalts.

The olivine bearing pillowed flows from the south shore of

Little Long Lake have features which suggest a modified one-stage crystallization history. The presence_of small clusters of euhedral olivine suggests crystallization may have begun at depth, however, the skeletal nature of some and skeletal overgrowths of others indi- -105- cate that crystallizatio'n occurred at a certain degree of supercooling and changing cooling rate. This variation may be explained if olivine began crystallizing during of the magma and continued to crystallize during extrusion and thus, formed under increasing rates of cooling. A similar model has been proposed for texturally similar Apoilo 12 lunar samples (Donaldson and others,

1975).

It is concluded that the wide textural variation shown by the

Newton Lake basalts can be explained by differences in the degree of supercooling and subsequent variations in the rate of cooling.

Their skeletal porphyritic texture is ·compatible with a single- stage crystallization history. Samples with euhedral phenocrysts may have experienced a two-stage history or a one-stage history at very low degrees of supercooling and slow cooling rates. Experi- mental studies using similar bulk compositions are required to allow a quantitative interpretation of the cooling history of the Newton

Lake basalts.

While the model presented above can account for crystallization history of the Newton Lake basalts, it does not explain why only they have crystallized in this manner. The Ely basalts, also extruded in a subaqueous environment, must have crystallized under varying cooling yet do not show a similar development of skeletal textures. The major factor accounting for this difference is the composition of the respective lavas. Variations in composition will .have important effects on viscosity (and therefore diffusion coefficients), phase relations and liquidus slope. These will -106- influence the degree of supercooling and the rate of cooling necessary to produce specific types of textures. Thus Donaldson (1976) found that lattice and chain olivines form at much slower cooling rates in ocean floor basalts than in Apollo 12 and 15 melts. This difference was attributed to the markedly higher viscosity of the ocean floor basalts (ponaldson, 1976).

Another important factor related to composition is the degree of supersaturation attained by a melt. Donaldson (1976) has shown that highly magnesium melts (i.e., high normative forsterite) will grow skeletal and denritic olivines at much slower cooling rates than olivine poor melts. He suggests that. the spinifex of peridotitic komatiites reflects not a rapid cooling rate, but a rapid growth rate caused by high olivine content and very high eruption temperatures.

The data of Donaldson (1976) may provide a further explanation for the pyroxene textures in the Newton Lake type A basalts. These have high normative pyroxene (see Appendix lb) and high liquidus temperatures as reflected by their high MgO contents. Thus like olivine in the peridotic komatiites, a high degree of pyroxene supersaturation would be expected for the Newton Lake basalts resulting in rapid growth rates of nucleating pyroxenes even without rapid cooling.

It is clear, from the presence of similar textures in basaltic komat:i.ites from other areas (Jj'illiams, 1972; Arndt and others, 1977), that the conditions which controlled the crystallization of the

Newton Lake basalts were not unique. It appears that the distinct textural features of the Newton Lake basalts are in some manner a reflection of their distinctive composition. -107-

Geologic Environment

Several features of the volcanic rocks in the felsic member of the Newton Lake Formation indicate that they formed in a shallow subaqueous environment. These features include amygdaloidal pillowed flows, abundant fragmental rocks, scoriaceous breccias and agglomerates, bedded and graded tuffs and sediments and coarse conglomerates. The intertonguing of the mafic member with the member (Green, 1970) and the presence of felsic tuffs, conglomerates, siliceous marble and calc-alkaline amygdaloidal basalts within the mafic member, indicates that the two formed contemporaneously. It is, therefore, likely that the mafic member also formed in a shallow subaqueous environment. The lack of amygdules in mafic member basalts is probably related to their composition (i.e., lower volatile content).

A significant feature of the mafic member is the high abundance

(about 40 to 50% of the member) of layered and nonlayered sills

(Green and Schulz, 1977). While some of these may represent thick flows with pillow basalt caps (Cedar Lake body), the majority are probably intrusive in nature. This suggests that considerable amounts of magma never reached the surface, but crystallized at depth.

Basaltic magmas rise through the crust due largely to excess hydrostatic pressure (Yoder, 1976). Two alternatives are available to the magma; if its density is everywhere lower than that of the volcanic crust, it may rise to the surface and erupt to form lava flows.

If, the magma has a density curve which at some point crosses that of the crust it may cease to rise. and move instead laterally along an equi-density surface to form a sill. Walker (1975) -108- has shown that such a mechanism can account for the formation of intrusive sheet swanns in the Iceland rift. This mechanism may also account for the sills in the Newton Lake Formation. -109- MAJOR Al-t'TI TRACE ELfil1ENT CHEMISTRY

Introduction

A major objective of the present study was to examine the geochemistry of the Vermilion volcanic rocks based on a stratigraphically controlled sampling. The geochemical nature of the mafic portion of the Newton Lake Formation was of particular interest.

From the more than 500 samples collected, 76 were selected for chemical analysis. The samples were selected to represent the majority of the important rock types recognized from the petrographic study and also provide as complete a stratigraphic sampling as possible. Brief descriptions of the selected samples are given in Appendix 2. An attempt was made to select only unsheared and

samples for analysis. Many of the rocks analyzed contain relict calcic plagioclase and/or pyroxene and most have excellent textural preservation.

All 76 samples were analyzed for Si02 , Al203 , Fe as FeO, MgO,

Cao, Na2o, K2o, TiOz, P205, MnO and H2o . In addition, thirty-five of the samples were analyzed for Rb, Sr., Y, Zr, Nb and nine 87 Newton Lake samples for isotopic Sr /sr86 • The results obtained, as well as calculated Niggli Cata-norms, are presented in Appendix · lb. As Fe2o3 was not determined for these samples, it was obtained by the relation Fe o = (FeOT - .85 x FeOT) (i.e., Fe0/Fe o = .85) 2 3 2 3 (FeOT = total iron as feO) after Nicholls and Whitford (1976) for the normative calculations. The analyses were also normalized to

100 wt. percent free of n o . 2 A total of 58 chemical analyses of Vermilion volcanic rocks -110- have been compiled from published and unpublished sources and are presented in Appendix lb. Comparison of these analyses with those obtained in this study ?:"eveals that,while similar bulk compositions are present in both sets of data, the older analyses generally show greater scatter on variation diagrams and are systematically different in some elements. This appears to be a result of both the in-·. clusion of more highly altered samples in the older data set and interlaboratory biases. The later is particularly evident in the

P205 and FeOT values. 'rhe older analyses are usually higher in

P2o5 than those of this study and the total FeO values of some Newton Lake samples are also much higher (compare E-151A in

Appendix lb with E-151A in Appendix lb). This points cut the general problem of using chemical analyses made at different times, with different techniques and in different laboratories for geochemical comparisons. In the discussions to follow, most petrologic conclusions are based largely on the data obtained in this study.

Before any meaningful petrologic interpretation can be attempted of the Vermilion geochemical data, the possible chemical effects imposed by the complex geologic history of the Archean greenstones must be evaluated. This is a different problem: however, as such submarine volcanic rocks may have experienced any or all of the processes of deuteric alteration, diagenesis, ·sea-water weathering, burial metasoma.tism, hydrothermal alteration, and regional metamorphism. These effects would be superimposed on any priiJJary chemical variations which from fractional crystallization and/or partial melting. -111-

To evaluate the of alteration on recent ocean floor

ha.salts, the genera.l approach_ha.s been to compare visibly altered

samples with fresh basalts froni the same locality (Cann, 1969; Frey

and others, 1974). This cannot be done with Archean lavas, however,

as truly unaltered rocks are not available. In recognition of this

problem, several other approaches have been used by workers dealing

with ancient volcanic rocks. One such approach is based on the

assumption that, while a particular sample may have experienced

chemical changes due to secondary effects, the greenstone belts

themselves have remained essentially isochemical except for intro-

duction of water and carbon dioxide. On this assumption several

hundreds or even thousands of samples are anlyzed and then averaged

to obtain representative bulk compositions (Wilson and others, 1965;

Gelinas and others, 1976). Often used in conjunction with this

approach is a screening of the analyses based on a series of criteria

designed to identify and reject from further consideration , those

samples whose chemistry appears anamolous (Wilson and others, 1965;

Viljoen and Viljoen, 1969; Green, 1975; Stauffer and others, 1975;

Gelinas and others, 1976).

Another approach which has been used is the comparisons with

recent unaltered volcanic suites (Descarreaux, 1973; Gunn, 1975).

This method is based on the assumption that the composition of

Archean lavas are similar to those of recent volcanic rocks. It

remains to be shown, however, that this is really a valid approach

as significant differences in mantle composition and tectonic pro-

cesses are likely to have existed in the Archean (see Jahn and

Sun, 1977). 1,··· i

-112- Recently several workers have attempted to bypass the problems

related to the chemical changes i mposed hy alterati on and metamo r-

phism by considering only those elements known or believed to be

chemically stable during such precesses (Pearce and Cann, 1973;

Floyd and Winchester, 1975; Winchester and Floyd? This method was recently useEl by Pearce (1975) to re-·evaluate the chemi·- cal na..ture of the volcanic·,·rocks from the .•

Clearly each of the aboye approaches has certain a.dvantc;.ges and disadvantages. In the discuss.ion. to follow ; all of these di£- ferent methods will be where appropriate, in evaluating . the geocher.1ical data of the Vermili:on district volcanic rocks .

Chemical Effects of Alteration

To evaluate the nature of possible secondary chemical changes in the Vermilion greenstones , some understanding of hotv major and trace elements behave

Seve.ral recent studies have. examined this particul arly with reference to the chemical changes resulting from alteration of ocean floor basalt s (Aumento and others, 1976; Hart and othe:rs,

1974; Miya8hiro and others, 1971; Shido and others, and many others). Pearce (1975) has s ummarized the results of these studies as follows:

Very Mobile +K20 s -·Cao' -MgO Ho bile -Na2o, -srn 2 Slightly Mobile +FeO, + Ti02 Alz0.3 -113-

Changes During Greens.chis.t Facies Metamorphism

Very Nobile -CaO, -Al 2o3 Mobile +NazO , +Si02, +(MgO + FeO)' -K20 Immobile Ti02 These changes are also accompanied by oxidation and hydration (Miya- shiro and others, 1971). While many studies have confirmed these general patterns of element mobility, others have shown that considerable variation can exist in the type of chemical changes which occur in the oceanic environment (Aumento and others, 1976).

The chemical effects of alteration on lavas outside the oceanic environment have also been investigated, particularly in relation to the formation of spilites (see Amstutz, 1975). Recently, Gunn and

Roobal (1976) examined in detail the metasomatic alteration effects on island arc volcanic suites from the eastern Caribbean. They identified two general types of alteration resulting from the burial metamorphism of these rocks: (1) spilitization, with an enrichment in Na2o and depletion in K20, Cao, Rb, Sr, Ba and Cu and less com- manly (2) poeneitisation, with an enriclunent in K2P, Ba and often NazO, Rb.-a'Il'd Sr. Similar alteration effects have also been noted to accompany zeolite facies burial metamorphism of basaltic lavas in western Iceland (Wood and others, 1976). The end result of such alteration if often the production of non-magnetic bulk compositions reflecting mainly the combined bulk chemistry of the particular mineral assemblage stable under the prevailing metamorphic conditions. (Gunn and Roobal, 1976).

It is clear that the chemical changes which may occur during the metamorphism of volcanic rocks are complex in detail and are dependent on a variety of factors including temperature, pressure, -114- fluid composition, Eh_, pH, grain si_ze, original hulk composition and fluid-rock ratios. Seyeral :recent studies have further noted the importance of hydrothennal systems in influencing the nature of chemical alteration of volcanic sequences (Spooner and Fyfe, 1973;

Taylor, 1974). From this review, it is apparent that the alkali elements, along with Cao and SiOz are particularly susceptible to change, being either added or removed depending on the prevailing conditions. These changes are generally accompanied by oxidation and hydration (Miyashiro and others, 1971). Carbonization seems most common in non-oceanic environments. The components Ti02 , Ni, Cr, Zr, Nb and the rare earths (REE) appear to be stable except during intense alteration (Frey and others, 1975) and the FeOT/MgO ratio also appears to remain stable in many cases (Miyashiro and others, 1971).

Vermilion Greenstones

The Vermilion greenstones, as described above, consist of greenschist facies mineral assemblages with or without relict igneous phases. The typical mineral assemblages in the basaltic rocks is sodic plagioclase + actinolite + chlorite + epidote + magnetite + sphene + quartz ± calcite. Previous studies of Vermilion volcanic rocks have assumed that the formation of the metamorphic mineral assemblages was mainly an isochemical processes requiring only the addition of HzO and co2 (Green, 1970; Jahn and others, 1974). Ex- amination of the available data suggests, however, that open system behavior may have effected some elements, particularly the alkalies.

Determining the degree of change in the alkali elements is of -115- particular importance as they are often used in characterizing vol-

canic rocks (Irvine and Baragar, 1971). Shown in Figure 57 is the

total major element variation for all available Vermilion volcanic

rocks, including the quartz-feldspar porphyries. The data are

sented as normalized values relative to average tholeiite in a man-

ner similar to that used in rare e_arth element studies (see page 142

for a discussion of this plotting This figure clearly

shows that there is little apparent systematic variation of Na and 2o KzO in these samples. For a given FeOT/MgO ratio (as indicated by

the slope of the line joining MgO and FeOT in the figure) or given

SiOz value, the samples show a wide range of Na and KzO contents, 2o though the porphyries generally have the highest amounts. This var-

iation is clearly outside that resulting from igneous processes

(compare to Figures 79a and 79c, pages 173 and 174) and could be

attributed to significant movement of the alkali elements in the

post magmatic environment in the Vermilion volcanic rocks.

Figure 58 shows the range in Na 0/K ratios for these samples. 2 3o Several important features are apparent on this diagram. Most striking

is the maximum shown by the K 0 content of the basaltic rocks of both 2 the Ely Greenstone and Newton Lake Formations. At low values of

KzO (0.5%) the samples show a r:ange in Na from about 0.5 to 5.0 2o wt.%, while at higher values (>.5%) sodium is constant at about

2.5 to 3.0 wt.%. Several factors appear responsible for this pattern.

The samples with low Na 0 (2.0 wt.%) and K 0 (0.5 wt%) contents may 2 2 partly reflect primary chemical features as these are also the most

magnes:i.an_ (i.e. primitive) of the analyzed basalts. This may be

ace en tua ted to some degree by alkali removal as shown by the very

\ -116-

LJ o_ . 0

(__) r--1 ,.,7 CI a: (__)

_ _.jI 0 t---i > I- :7: _ _JI l---. __ _..) cc 1--! _J cc (_)

,-y-- =·-'-- l_t_J ·------

LU . ['-. !..L. L() (__') 2

-l---+---f--!··--1----1---l----1---+----l----t---+--+ Q) (_() D C\J CD mo 0J D u " " " a D D -117-

Figure 58. -Na2Cl vs K20 for Vermilion volcanic rocks.

6 ® ® ® ® ® ® @ ® @ ® --,,- a . 17 ...... 5 ...... ® "'-... • • va® 0 ---r • '• • • @ --, • • ' • 4 II> ' • • • •• ... " .--. @ • 0 • • ...... -+- • • 9 • z.1.0 • • • " "• • ---r '----"> • - ...... 0 • • • • 0 3 0 .. • • -- / C\J 0 •• 0 •" / z 0 . • • • • • 0 • " • • ••" • .. • / . • • • 0 • • • / • • / • • • / 2 • • • 0 • • / • • • • • / . • :•• ./ • " / / • Basalts / @ Newton Lake Felsic Member

/ @Porphyries • • / / / / 0.5 1.0 1.5 0 K2 0 (Wt /o) -118-

low alkali contents in two of the basalts.

The reason for the maximum shown by the K 0 content is more 2 equivocal than for the minimum. The high Na low K 0 samples have 2o - 2 likely experienced sodium metasomatism possibly accompanied by

potassium loss; this type of change is characteristic of spilites

(Gunn and Roobal, 1976). The variable potassium at almost constant

sodium may be the result of potassium metasomatism, similar to that

described by Gunn and Roobal (1976) from Caribbean island arc

volcanic rocks. This is supported by the presence of small grains of muscovite in many of these high K 0 Vermilion samples and by high 2 Rb con ten ts •

The felsic volcanic rocks (including the dacite porphyries)

of the Vermilion district show two distint groups in Figure 58: a

low potassium group consisting mainly of the felsic flows and tuff

breccias from the felsic member of the Newton Lake Formation and a

high potassium group representing the felsic flows of the Ely

Greenstone and the Vermilion quartz-feldspar por.phyries. This latter

group shows a braod negative correlation between Na 0 and K 0 2 2 which may be attributed to sodium metasomatism. Such a relationship

is typical of Keratophyres (Battey, 1955).

The compositional pattern of the Vermilion volcanic rocks. is

generally similar to that described from spilite-keratophyre

associations (see Amstutz, 1974). Hughes (1973) has examined the

relationship of sodium and potassium in spilites and keratophyres

relative to the normal igneous variation using a plot of K20 + Na 20 versus x 100 (Fig. 59). The normal igneous variation or

spectrum is based on a largevariety of recent volcanic rock types 10 I I I Igneous Spectrum 8 I 0 (i) IV ®I -+- ® / 3 .._.. (•) ® (•' I (o \ / 0 I / C\I 6 / Gl ./ + / / 0 .. .. I •• @ (\J ·. . . /// 0 4 . z I ... / ...... • . . / . ·...... ·. . ·. . . . . / . . // 2 .···:::.. -. __.--- . -- • Basalts li' Felsics

.I .2 .3 .4 .5 .6 K 2 0 I K2 0 +Na 2 0

Figure 59. Hughes plot for Vermilion volcanic rocks. lO I -120- including tholeiites, andesites, rhyolites, alkali basalts and

nepheline phonolites. The Vermilion samples show a wide range of

total alkalies and alkali ratios with several samples lying outside

of Hughes's (1973) defined igneous spectrum (Fig. 59). The porphyries,

in particular, tend to lie to the left of the igneous spectrum, while

some of the basalts lie in or near the spilite field, most have lower

total alkali contents. It sould be noted that while the high alkali

. ratio samples lie within the spectrum, their ratios are significantly

higher than many samples with similar FeOT/MgO ratios.

From the discussion above, it would appear that the alkali ele-

ments could have been significantly affected by secondary processes

in the Vermilion volcanic rocks. Documenting similar behavior for

the other major elements is more difficult. As no unaltered samples

are available, it is not possible to fully establish the primary

chemical characteristics of these rocks. Furthermore, primary ig-

neous processes (e.g., fractional crystallization and/or partial

melting) can produce significant chemical variations within a vol-

canic suite such that no one composition can be taken as representa-

tive. The problem is further complicated in that alteration commonly

produces chemical variations similar in trend to igneous processes

(e.g., Sio enrichment, CaO depletion). 2 Some understanding of element mobility can be gained by examining

element variations against a known immobile element. This approach

has been used by Wood and others, (1976) and Coish (1977) to examine

element mobility in recent basaltic suites. Several elements have

been shown to be relatively stable during alteration and low grade

metamorphism including Ti0 , Zr, Y and REE (Cann, 1970; Pearce and 2 -121- Cann, 1971; Loeschke, 1976; Coish, 1977). As only Ti0 data are pres- 2 ently available for all the Vermilion it will be used in

the discussion below. Titanium, like the incompatible elements, has

the added feature of showing in general a systematic variation-in

igneous processes and is; therefore, a measure of the degree of frac-

tionation (in rocks in which a titanium bearing phase such as ilmen-

ite crystallizes late) (Miyashiro and Shido, 1975). Thus any cogenetic rock suite whose chemistry has not been affected by post-

igneous processes should exhibit regular trends of changing element

concentration against changing Ti0 content. If an element shows 2 erratoc varoatopms wjem compared to Ti0 , element mobility by later 2 processes is indicated (Wood and others, 1976). Examinat.ion of the

Vermilion chemical data (Appendix lb) reveals that for a

given Ti0 content, the Vermilion samples show highly variable Si0 , 2 2 Cao, MnO, Rb, Sr and Nb contents and moderate variation in FeOT and

MgO. The systematic behavior of A1 o , P , Zr and Y shows that 2 3 2o5 the Newton Lake and Ely volcanic rocks can be divided into distinct rock suites and that the variability of the other elements may, in part, be due to mobility during metamorphism.

While tP-e. c;ibove .comparison suggests that certain elements were mobilized during metamorphism, it gives little information as to the extent or direction of movement. Numerous studies have shown that calcium depletion often accompanies sodium enrichment in basaltic rocks. This is attributed to the release of calcium through albiti- zation of plagioclase according to reactions of the type: 4- + . ++ 3- CaA12Si208 + Si0 + Na + 2H = NaA1Si o + Ca + Al0 + H 0 4 3 8 3 2

(Dimroth, 1971). Shown in Fioure0 60 is the variation of CaO with I ('\I ('\I r-l I

o Newton Lake 0 10 0 •Ely Gree nst one Porphyries and Felsic Flows 8 I- 0 Cl 0 0 • • 0 0 0 • 0 0 0 coo 0 o• 0 ao Na o o• jJ • 2 Oo • I I • oo• • I- • 4 2 •• 0 •oO () .o • o<98 • • o -ao• • o:.,-0 '° 0 0 • . • 0 0 ·

CaO (wt 0/o)

Figure 60. Cao vs Ca0/ Na 2o for Vermilion volcanic rocks. -123- with the Ca0/Na2o ratio for the Vermilion samples. Calcium shows a fairly strong positive correlation with the Ca0/Na ratio indicating 2o that the high sodimn samples also have low calcium contents. Figure

61 shows that the variation in the Ca0/Na ratio does not strongly 2o correlate with FeO/MgO ratio (i.e., with degree of fractionation).

Also plotted in Figure 61a are some Mid-Atlanitic Ridge greenstones (Melson and Van Andel, 1966). Relative to rresh ocean ridge basalts, the greenstones have similar FeO/MgO ratios but lower CaO/

Na ratios due both to higher sodium and lower calcium (see Melson 2o and Van Andel, 1966, Table 6, p. 175). This same type of variation in Ca0/Na ratio is shown by some of · the Vermilion volcanic rocks 2o and may be an indication of Cao loss in some samples .

An alternative to CaO loss, however, wouid be variations resulting from original modal variations of plagioclase and pyroxene. While such modal variation clearly exists and can account for the varying

Ca0/Na ratios of some samples, others show no evidence of varying 2o original modal mineralogy but have quite different CaO and Na con- 2o tents. Samples having very similar MgO, FeO, Ti0 and trace element 2 contents can show greater than 3 wt. % difference in Cao (see Appen- dix lb, samples ENL-114-54A and 114-54B; and 358-6). Such wide variations in otherwise chemically similar samples seems unlikely to result from normal igneous .processes, assuming the rocks actually belong to a congenetic suite. It is, therefore, suggested that CaO loss may have effected some Vermilion samples though further study is requied to evaluate the magnitude of this process.

The high Ca0/Na o ratios shown by some samples indicates that 2 enrichment in calcium has also occurred, largely through the addition -124-

NEWTON LAKE ELY GRE ENSTONE

• • 10

• Basalts • Basalts

@Felsics @ Porphyries and Fel5ic Flows Fresh Ocean Basalt 0 Average Tholeiite to Rhyolite Trend 8 ® Ocean Floor Greenstones (Helson and VanAndel, 19r:; .) • • • • • • • .." • • 0 6 • N • • • • • 0 • .. z • •• • • 0 • • '0 • (.) • • • • • • • T • 4 • • • • •• I • I • • • • • • • .. • @ • • ® • • •• •• • • s • • • @ • • • • • • 2 •• • @ @• • • @ • • • @ @ @ @ ® @ © @ B @ .5 .6 .7 .5 .6 .1 .8 FeOT/FeOT +MgO rigure 61. vs FeOr/FeOT + MgO the Vermilion rocks. Also shown are ocean floor greenstones (A) and the average tholeiite through rhyolite trend from (1976b). -125- of calcite. For some Newton Lake samples, the high ratios simply

reflect high modal clinopyroxene content.

While the Vermilion data can not be compared with unaltered

samples, some comparisons between samples of similar Ti0 , A1 o 2 2 3 and FeO/MgO ratio is possible. Four analyses were obtained of

samples from the chilled margins of the differentiated Newton Lake

sills. Figure 62 shows the range in the analyses normalized to the

average composition of the four samples. Examination of the figure

reveals that while all four samples have similar overall compositions

and similar Ti0 and A1 o (values are within the error of the 2 2 3

analyses), Cao, Na2o, FeOT and MgO show marked variation from the average values. Closer examination of the data shows that those

samples with the highest Na o content also have the lowest Cao and 2 slightly higher Si0 content than the others .. The variation in 2 FeOT and MgO is not as great as that of calcium and sodium and may

in part reflect primary variation. The trace elements (Appendix lb)

show minimal variation with Y aqd Zr showing very similar values while Rb, Nb and Sr show progressively more variation. While the

Rb content correlates well with the potassium content the Sr shows no correlation with calcium content. None of the elements show a

strong correlation to the total water content of the samples. The

overall variation between these chilled margin samples is similar

to that in the Vermilion volcanic rocks as a whole, further supporting

the previous conclusions on element mobility.

Another example of the possible effects of alteration on Ver-

milion greenstones is shown in Figure 63. These data represent five

closely spaced samples of variolitic pillow basalt from the Newton 'J T • Le r- F ig ure 5 2 . I_, r I 1 L L_ J -- r -· ! I' .I. -· . I .- .. I ,, c + r . D _j t I. -·- I J I . .· r " 0 T iI 0. c.·. i- L: I +l . 6 I "';. . 0 . () ,..., f . ,_,/_ +

i-Jr· MG FE MN SI CR RL Tl NA J 'L \ ;_- Figure 63. - t • l · '--

.-.·" . -'- • Ci

Ml3 . FE MN SI CR F1L TI NR p ) ...,12 7- Lake Formation. As no flow contacts where observed between the

samples , it is assumed that they represent a single flow or flow

unit and thus may have had very similar compositions initially.

These samples are described on page 85. Moore

(1965) has shown that fresh pillow basal ts tend to be homogeneous

in composition between core, rim and selvegde. Figure 63 shows

that, if the initial assumption that these samples had similar comp-

ositions is true, then considerable element migration has occurred.

The general pattern of variation is again similar to that discussed

above for the chilled margin samples with the exception that MgO and

FeOT are more variable. Aluminium and Ti0 again show little varia- 2 tion from the aver_age, supporting the assumption· that the samples

may have had very similar compositions before alteration. The

Cao content, while low in comparison to most other Newton Lake basalts,

shows an inverse correlation with Na The high Na samples also 2o. 2o have the highest Si0 content . While the total alkali content is 2 high and remains about constant, the Na 0/K 0 ratio shows both high 2 2 and low values. This suggests that Na o and KzO metasomatism may 2 have selectively effected parts of this pillowed flow unit.

The chemical variation shown by these samples is reflected in

their mineralogy. The samples with the highest iron have abundant

very fine grained opaques, while the high MgO content corresponds

to high chlorite content. The high K values correspond in two 2o cases to samples with plagioclase visibly replaced partially by

white mica. Though these samples have apparently suffered chemical

delicate quench textures are fully preserved (Figures

4 7 and 48, page 88 ) . This feature is characteristic of the al tera- -128- tion of the Vermilion volcanic rocks.

Conclusion

This attempt to examine some of the chemical effects which accompanied metamorphic alteration of the Vermilion volcanic rocks high- lights some of the problems related to geochemical study of Archean volcanic rocks and reveals the complex nature of the alteration processes which have affected these rocks. While further study is clearly required to understand the complex metamorphic alteration of the Vermilion volcanic rocks, several tentative conclusions can be drawn from the data of this study.

One is that metamorphic alteration may not have been as isochemical process for all elements. Though the present evidence is not conclusive, it does suggest some possible redistribution . of at least

Na K 0, Ca.O, Rb and Sr. While the general pattern was apparently 2o, 2 one of Na o metasomatism with accompanying Ca.O and possibly K 0 loss, 2 2 KzO metasomatism with accompanying Rb addition has also effected some samples. The elements Si , Fe , Mg and Mn show varying degrees of mobility with Si showing some correlation to the Na 2o abundance. The elements Al , Ti p , Y and Zr show less scatter and have apparently remained relatively stable during alteration. Hydration, oxidation and in some cases, carbonatization, has accompanied alteration; however, th.e mobile elements (i.e., alkalies, Si and Ca ) show no clear correlation with degree of hydration. The oxidation ratio (Fe o/Fe0, based on the published 2 data in Appendix le) is generally low in the basaltic rocks (values ranging from 0.1 to 0.5) and higher in the quartz-feldspar porphyries -129- (values range from 0.5 to 3. 74); the oxidation ratio also shows little correlation to total water content. From the evidence presented above, it appears that the general pattern of element mobility is similar to that found by other workers in low-grade metamorphic volcanic terrari.es (Coish, 1977; Loeschke, 1976; Gunn and Roobal,

1976).

The record of apparent immobility of Al Ti , Y and Zr resembles the behavior found by other workers in low-grade metavolcanic rocks (Pearce and Cann, 1971; Loeschke, 1976; Coish, 1977).

Loeschke (1976) has suggested that the general immobility of Al o 2 3 and Ti0 in these environments result _from their forming only slightly 2 soluble hydroxides as opposed to the cations like Na+, K+ and ea2+.

As previously noted, the textural preservation is very good in many Vermilion volcanic rocks. Even delicate quench textures, though replaced by secondary minerals, are faithfully retained. This appears to be a common feature in many Archean greenstone belts (Hallberg and

Williams, 1972; Gelinas, and others, 1974). This high degree of textural preservation at greenschist facies metamorphic grade is not typical of continental greenschist facies assemblages in general

(Turner, 1968), but is apparently a common feature of many spilitic assemblages (see papers in Amstutz, 1974) and -in modem ophiolites.

While some element mobility has apparently occurred in the Ver- milion volcanic rocks, it will be shown that primary igneous variations are discernible, especially in regards to the immobile elements as defined above. Furthermore, alteration was generally not so intense (except locally) to totally change the chemical nature of the -130- volcanic rock suites. Based on data discussed below, it appears that chemical alteration of the Vermilion volcanic rocks has resulted largely in scattering the data to greater or lesser degrees about interpreted primary igneous ttends. This scattering and the absence of valid criteria (e.g., unaltered samples) to evaluate the degree of scatter limits, but does not invalidate, petrologic interpretations based on the geochemical data.

Classification

Any attempt to classify the volcanic rocks of the Vermilion district must rely on chemical criteria, as metamorphism has generally destroyed the original mineralogy. Several methods have been proposed for chemically classifying volcanic rocks, the most recent being by Irvine and Baragar (1971), Middlemost (1972), Church

(1975), LeMaiter (1976), Jensen (1976), S:treckeisen (1976) and

Gelinas and others (1976). Of these, the most commonly used method, at least for Precambrian volcanic rocks, has been that developed by

Irvine and Baragar (1971).

No attempt is made here to critically analyze the virtues and limitatiOns of the various classification methods presently available as such discussions already exist in the literature (for example see Church, 1975; Jensen, 197.6; LeMaitre, 1976). A few comments do seem warrented, however, before applying any of th.ese methods to the

Vermilion district volcanic rocks . The classification schemes developed by Irvine and Baragar (1971), Middlemost (1972) and

Streckeisen (1976) rely heavily on alkalies, calcium and silica -131- values for determination. of rock types and classes. For example, in the method of Irvine and Baragar (1971), the major criteria for determining rock types is based on normative color index and plagioclase composition and the method of Streckeisen (1976) relies almost totally on normative pl.agioclase composition. This strong reliance on the content of alkalies, calcium and silica, makes application of these methods to the Vermilion .district volcanic rocks somewhat questionable as it is just these elements that have experienced the greatest change during metamorphism (see discussion in last section).

Clearly indiscriminant application of the the above methods to altered rocks such as those from the Vermilion district can lead to serious errors in classification (see Spitz and Darling, 1975, . an excel:.... lant discussion of the problems in application of such tion methods to chemically altered rocks).

The methods of Church (1975) and Jensen (1976) attempt to mize possible alteration effects by considering only less mobile elements (Jensen, 1976) or using all the major elements for distinction of rock types (Church, 1975). The Jensen (1976) method is particularly useful as it also attempts to distinguish different types of subalkaline basalts. In the discussion to follow the methods proposed by Irvine and Baragar (1971), Jensen (1976) and a classification based on silica content proposed by Gelinas and others (1976) will be applied to the Vermilion district data. The reason for this multiple approach is to assess both the variability shown by the samples and that imposed by the method of classification. -132-

Classification Based on :Percent of Si0 2

The simplest method of classification is that based on the

silica content of the volcanic rocks. The divisions of Gelinas

and others (1976) are shown in Figure 64 along with the Vermilion

district data. The plotted data are for compositions re-calculated

on a volatile free basis. Plotted with the Ely Greenstone flows are

the quartz-plagioclase porphyries which occur throughout the Vermi-

lion greenstone belt. They are :.plotted here idmply for classifica-·

tion purposes and not to imply any type of cogenetic relationship with the Ely basalts. Also plotted with the Ely data are three analyses of flows from the Lake Vermilion Two of these

plot as andesites, with the third as basalt.

The plot shows that the Ely Greenstone consists largely of basalt, thus, supporting the general field and petrographic :i.nter-

pretations. The porphyries range from andesite to rhyolite though most are dacite and rhyodacite. The lower Ely Greenstone member rocks, while showing a similar spread to that of the upper Ely member, may have a higher proportion of andesites.

The Newton Lake Formation data has been divided into the mafic and felsic members. While few analyses are presently available from

the felsic ' member, those available plot as andesite (4) and dacite (1).

Three of these samp1es are from flows arid two are of the tuf f breccias

(Green, 1970). The mafic portion is largely basaltic with only four andesite samples.

The Method of Irvine and Baragar

Irvine and Baragar (1971) proposed a :system for 1laming fresh vol- . -·------r ·-·----·- ···------I

12 f- NEWTON LAKE !lJ D M (Hafic M. 8 r- I I E] F (Felsic M. I I I 41 I

· ' ·r .,.. •....;i

DUE (Upper ,,,\K

(Lower · 8 f- ELY l I fil[lFF (Flows F.

p ( Porphyrie 4

BASALT ANDI;.:SITE DA CITE 54 62 I 0 SiO 2 (wt /o) (;.) (;.) I Figure 64. Si02 frequency diagram, Vermilion volcanic rocks. -134- canic rocks based on the normative color index ( 01 + Cpx + Opx +

Mt + Il) and normative pl_agioclase composition (Fig . 65). They

have noted that caution should. be used when applying this method

to metamorphosed and altered volcanic rocks· as nonisochemical effects

may significantly change the normative components.

The Vermilion data are plotted on the Irvine-Baragar diagram in

Figure 65. Comparison of this figure with the Si0 plot (Fig. 64) 2 reveals that some change in classification has occurred. This

change is generally to more mafic names for the samples; i.e.' change

from dacite (Si0 plot) to andesite (I-B plot) and andesite (Si0 2 2 plot) to basalt (I-B plot). This change in classification from the

silica plot is influenced by several factors. One is the somewhat arbitrary nature of the silica divisions as used by Gelinas and others (1976). More important, however, are the. possible effects

imposed by the chemical alteration of the Vermilion samples. In the

felsic samples the normative plagioclase composition and color index are generally higher than the silica classification would indicate; this results from varying calcium and aluminum contents. In the mafic samples higher color indexes than suggested by the silica classification may be a result both of silica mobility and some chloritization. It is interesting to note that while sodium addition is the most apparent chemical effect noted in the Vermilion samples, it has not resulted in changing them to less mafic portions of the Irvine and Baragar diagram.

The Jensen Cation Plot

Jensen (1976) has discussed at some length theproblems related to the classification of Precambrian volcanic rocks. To overcome I 11) (Y) ...-!

* Newton Lake • Ely Greenstone

@· Porphyries and 80 t Felsic Flows BASALT >< a> * -0 =*=-¥.· c ·X· ·X- ·'k 60 ·X· \.... 0 7<· ·X· -'U:., * .v. 0 ·X·* ·"!: •L ·'k * .-<\C ·X· 'k •'!' o "" • \\' u J.. .:f ·'/d . • ., () '\.· "k • \;<(,, <(., · ·X· '!.:'# ":'j,.'f: if:I:. ·X· *. .<,,. '(.\ 0 <(,,'?; \ Q) 40 : ., ., • t-. > * 4)• ., •/ o r .. - • . @ 0 . ., E . . ti· . ,.__ @ @ 0 @> $J z 20 ® ® ®/\) \)-()

70 50 20 -

discussed above, .he has ·developed a classification scheme based on

Al , FeO + Fe + Ti0 and MgO cation percentages (Fig. 66). The 2o3 2o3 2 oxides Al , FeO + Fe o + Tio and MgO were selected because 2o3 2 3 2 they are relatively stable even within altered volcanic rocks and because these .cations vary in inverse to one another during differentation. The Jensen Cation Plot is particularly useful in that it .not only allows distinction of all the major rock types found among subalkalic volcanic rocks, but also defines three distinct differentiation trends on the same diagram. It further gives an estimate of the relative color index· that can be used directly in the field (Jensen, 1976).

The Vermilion samples are shown on the Jensen Cation Plot in

Figure 67a and b. Several important conclusions can be drawn from this figure. Considering the Ely Greenstone samples first (Fig. 67a), it can be seen that the majority are tholeiitic basalts with both high

MgO and high FeOT types represented. The samples falling in the calc- alkaline basalts field are mostly those from the lower Ely member and the Lake Vermilion Formation. The felsic flows and porphyr.ies range from andesite to rhyolite and are calc-alkaline in nature.

Note that a higher number of the felsic rocks are classified as rhyolites in this plot than in previous plots.

Some Newton Lake volcanic rocks (Fig. 67b) . appear to be distinct from the Ely Greenstone samples in having lower aluminum for a given

FeOT/MgO ratio. The samples from the mafic member show a range from basaltic komatiite to high HgO basalt and high iron basalt; a few calc-alkaline basalts are also present. The felsic member samples -137-

Rock sample colour (ap;:iroximate)

Black

y v y MgO

C:ttioo Plot c.:ition percent:ige.s or Ali0 31 FeO + Fe2 0J . Figure 66. + TiOJ. :ind FeO + Fe 0 +Ti 0 A 2 3 2

Figure 67a. Jensen Cation Plot for

Ely Greenstone and Vermilion

Porphyries.

• • ELY • Greenstones

® Porphyries and 0 Felsic Flows

( •

...... I '-"OJ I

Cation% f'eU + Fe 2 03 + Ti02 B

•:.

Figure 66b. Jensen Cation Plot for

Newton Lake Volcanic Rocks.

• • • • • • •• • • •I'• • •

NEW'rON LAKE • Mafic Member 0 Felsic Member l ....w \C I Cati on% -140- plot as basalts, andesites and rhyolites of the calc-alkaline type.

The following points appear of particular importance from the

Jensen Cation Plot:

1.) The upper Ely Greenstone volcanics show a distinct tholeiitic

iron enrichment trend, while the lower Ely member and Lake

Vermilion Formation samples have a calc-alkaline trend.

2.) The felsic flows and porphyries are calc-alkaline in nature.

3.) The ma fie member samples of the Newton Lake Formation have

distinct compositions from the Ely Greenstone and include

basaltic komatiites.

4.) The felsic member of the Newton Lake Formation as well as a

few samples from the inafic member have a calc-alkaline nature.

The petrologic significance of these conclusions will be discussed in a later section.

Conclusion

Shown in Table Sa and b is a comparison of the resulting classification of the Ely (Sa) and the Newton Lake (Sb) volcanic rocks using the silica plot, Irvine and Baragar plot and Jensen Cation Plot. methods.

It is clear from this comparison that the majority of samples are similarly classified by each of the three methods used. This is particularly true for ·the basalts. The felsic rocks do show some variation with the Jensen Cation Plot classifying many of these rocks as rhyolites as opposed to dacites.

The fact that the Vermilion volcanic rocks are generally similarly classified by three different chemical methods suggests that either -140a- Table Sa. Results of the Classification of Ely Volcanic Rocks and Vermilion Porphyries.

Si Plot I & B Plot .Jensen C.P . fl Basalt 36 41 37 II Andesite 10 9 4 II Dacite 11 8 2 fl Rhyolite 2 1 8 ... fl Cale-alkaline basalt 0 0 8

Table Sb. Results of the Class:lfication of Newton .Lake .Volcanic Rocks

Si Plot I & B Plot Jensen C.P. ff Basalt S6 S9 S6 II Andesite 7 s 2 " 11 Dacite 1 0 0 II Rhyo1ite 0 0 0 II Cale-alkaline basalt 0 0 6 -141- chemical reconstitution by post-magmatic processes has not greatly changed the overall composition of these rocks or that such changes have systematically effected all elements or that significant changes did not occur. From the data available, it is concluded that the Vermiiion volcank rocks still essentially reflect their original compositions. - 142- Chemical Variation of Vermilion

District Volcanic Rocks

Introduction

Examination of the recent pretrologic literature reveals that

a multitude of graphical schemes are used for portraying chemical

data for igneous rocks. Furthermore, there seems to be little

agreement as to the best method for such portrayal. Wright (1974)

has recently discussed the shortcomings inherent in many of these

methods.

In recogni.tion of these problems, Weiblen and Roedder (1976)

have proposed a new method for examining major element data based on

a normalization-:. procedure similar to that used in rare earth element

(REE) studies. There are two immediate advantages in this method:

(1) it allows portrayal of the total major element variation in a

two dimensional plot and (2) compositions which are related by

simple fractionation effects can be readily recognized on the normalization plots. The method has the further advantage of allowing

ready comparisons between large numbers of samples and also with

other chemical suites.

As noted by Weiblen and Roedder (1976), there are two problems

related to the application of the normalization method to major

elements. One is that there is no obvious choice for a reference

composition against which to normalize the data. The other is that

there is no fundamental reason for selecting a specific sequence of -143- major elements for the normalization plot.

For this study the average tholeiite composition of LeMaitre

(1976) was arbitrarily selected as the normalizing composition

('.fable 9) . As this composition is mainly based on ocean-floor basalts it allows ready comparison of the ·Vermilion greenstones to this important basalt group. The sequence of elements used for the plots was chosen to provide easy evaluation of petrologically significant element :ratios (i.e., .Fe/Mg ·. : CaO/Al, Al/Ti). The verticle scale in all the normalization plots represents the log of the normalized value (i.e., log (sample) ). All values below zero are standard negative and those above zero positive.

As an example of the normalization method, Fig. 68 shows the normali zed major-element variation of the picrites from Baffin Bay

(Clarke, 1970) compared to the average tholeiite of LeMaitre (1976).

This plot illustrates the systematic increases in all oxides except

NgO and FeOT, associated with olivine fractionation and suggests that olivine fractionation was mainly responsible for the chemical variation of the picrites (compare with Figs. 2a-d, Weiblen and

Roedder, 1976). This conclusion is in agreement with that of Clarke

(1970) .

Ely Greenstone

Lower Ely Greenstone Member :

Figure 69 shows the normalized major element variation of samples from the 10':1er Ely Greens tone member. Included are six samples from this study and one from Sims, (1972, see analysis -144 ..,.

r1gure 68. plot of the log of the ratios of major-elemeni data for Baffin Bay picrites (Clarke, 1970) . normalized to the average composition of LeHaitre (1976b). Data in weight percent in this figure (and all other normalization plots below) have been divided by (i.e. normalized to) the equivalent data for tne average tholeiite (Table 9). Numbers below zero on the ordinate are negative on all plots. Lables above and below 0.8 on the ordinate should read 1.0 and 1.2 respectively. Changes in the slope of the lines joining adjacent oxides reflect changes in oxide ratios. Q_

cc z en _J l-- cc ()) _J CI a: CD cc >- u er m 1--1 ()) ./'/' 1--1 . z Ll_ > LL cc aJ . 0) w (!)

c.o •.-l µ. ("J 0 CD CO C'\J 0 0J T CO CO 0 ('J a a a a a q a a a a o a Figure 69. LO\·/ER t.I_ Y GREENSTONE ') lines enclose compositional variation of the upper Ely Greenstones. :0 t a 8 T u 6 + /.q l g • I ,,. , ..... '2 ,, # 0 t I '2 l Q -! '6 t , a T

D 0 l "2 ..t. ....I .r- m MG FE MN SI CR RL TI NR K P I -147- Appendix lb). Comparison of this plot with that for the upper Ely

Greenstone member reveals some significant differences. While showing a similar range in MgO content, the lower Ely samples as a group are distinctly lower in FeOT and MnO and higher in Si02 than the upper Ely greenstones. They are also generally higher in Al203 and lower in Ti02 contents and have lower FeOT/HgO ratios. Two of the lower Ely samples do not share these features: one (ENEL-35-29) is comparable to the majority of upper Ely greenstones though higher than most in Al203 while the other (ENEL-25-22) has high FeOT, TiOz similar to the high titanium upper Ely samples. One sample (ENEL-84-

32) is also anomalous in that it has high MgO (8.5%) and Si02(56%) while having low FeOT, TiOz arid CaO contents and a very low FeOT/MgO ratio. Petrographic examination of this sample suggests that abundant modal chlori te may ·account for the chemical anemaly.

The Jensen Cation Plot (Fig. 67a) showed that the lower Ely

Greenstone samples are calc-alkaline in nature. This is also illu- strated in an AFM diagram (Fig. 71). Six of the analyses clearly fall in the calc-alkaline field as defined by Irvine and Baragar

(1971). The one analysis falling in tholeiitic field is the high

Fe-Ti sample (ENEL-25-22) noted above .

It is concluded from the presently available major-element chemistry thi:!.t the lower Ely Greenstone member is calc-alkaline in nature, though some tholeiitic basalts are also present. The calc- alkaline nature of the yolcani.c rocks. may explain the high. proportion of i.ntermediate to fels.ic tuff$ and tuff breccias which occur and also partly account for the a.mygda.loidal nature of these rocks . -148- As only a minimal amount of chemical and geologic data presently exists for the lower Ely Greenstone member, little can be said about possible chemical and lithologic changes with respect to stratigraphic position. The samples collected in this from south of Armstrong Lake, suggest that there may be considerable inter- laying of calc-alkaline basalts and more felsic rock types in this area. However, of the 6 analyzed the basalt and basaltic andesites occur at a higher position in the sampled sequence than the andesites.. Whether this reflects a stratigraphic variation can not be determined without further mapping to better define the structure in this area. The tholeiitic sample from the lower Ely member

(ENEL-25-22), is from what appears to be a dike cutting amygdaloidal pillowed andesite on the east shore of Eagles Nest Lake No. 4 and may be related to the later volcanism of the upper Ely Greenstone member.

Trace element data are available for only three of the lower

Ely samples. Zr, Y and Nb show an increase with decreasing MgO content (Appendix la). The Rb and Sr contents are variable, but in . . the .range of values reported by Jahn and others (1975) for the Ely

Greenstone. The Rb/Sr ratio ranges from 832 to 489. A distinctive feature of the three samples is their low Y content relative to the upper Ely Greenstone samples (Fig. 74). Such low Y values are typical of Archean calc-alkaline volcanic rocks (Lambert and Holland,

1974; Condie, 1976).

The lower Ely data compared with both Archean and recent calc-alkaline volcanic rocks in Table 6. In comparison to the Table 6 . - Archean and Recent V?icanic Rocks ·----·-·-- ·---·-·-----·- -·-- ·---·------Lower I!.!J_ APchean Recent 4 Canadian1 Aust.2 Rhodesia 3 C-A I.A. C-A 35-29 66-33 Bas. And : And , !las. And. Bas. And. And.

Si02 48.49 58 . 76 52.07 59. '.l4 59.30 52 . 19 58.40 50.20 57.30 59.00 AJ.203 16.00 15.50 15.61 16 . 03 14.63 15.35 15 , 58 16.40 17 .110 17.10 FcO'I' 11 . 84 6.18 11. G1 7.90 8 . 62 9.30 7.25 10:11 7 . 30 6.12 HeO 7.63 5.44 6.61 3.90 4.34 0 . 77 6 . 35 6.30 3.50 3 "40 ·CaO 1 1. 37 8.50 9. 04 5.07 7.26 9.19 7.18 10 . 70 8 . .70 7.00 Nt.1 20 2.66 4.37 2.21 3.85 3 . 53 3.37 2.95 3 , 00 2.63 3.68 K20 0. 77 0.33 0.33 0.87 0.81 0.15 0.00 0, 110 0.70 1. 60 1'i02 0.80 0.59 1. 05 1. 04 1.11 0 . 65 0.60 1. 00 0.58 0,70 P205 0 . 02 0 . 11 0.19 0.23 HnO 0.21 0 . 12 0.20 0'15 y 9. 5 16 15 14 --- 22 15 25 25 20 Zr 24 134 105 60 --- BO 75 100 90 110 Rb 14 . 6 5. 6 8 30 --- 2.4 24 10 10 30 Sr 134 152 164 263 202 2111 330 215 385

FI If:' 1. 55 1.14 1. ?G 2.02 1. 99 1. 06 1.14 1. 61 2,08 1. 80 K/Rb 438 489 332 232 --- 518 238 340 580 440 · Rb/Sr 0.11 0.04 o.os 0.11 --- 0.01 0.12 0.03 0.05 0.08 Zr/Y 2.5 0.4 7. 0 4.3 --- 3 . 6 5.0 4.0 3.G 5. 5 ...-F-c07/HgO---·-----·- ·-·-·------.. .. -----· -- ·------·-·------I f-' References: 1. Baragar and Goodwin (1969); 2 . Glickson (1970); 3. Condie and + Harrison (197G); U) 4 . Condie ( 1976). I -150- average Archean calc-alkaline basalts, the lower Ely sample is lower in Si02 and slightly higher in A1 2o3 • In terms of Ti02 and FeOT/MgO ratio, it is intermediate between the Canadian and Rhodesian averages. The average modern calc-alkaline basalt is notably higher in SiOz and Ti Oz than the Ely The Y and Zr contents of the

Ely sample are ·both distinctly than the average Archean and modern calc-alkaline basalts.

The Ely andesite is similar in major elements to the average

Archean examples with a particularly striking to the

Rhodesian sample. The Y content also is similar to the other Archean andesites, while the Zr is slightly higher. In comparison to .the modern andesites, the Ely sample is distinctly lower in Al203, Y and and FeOT/MgO ratio and higher in MgO and Zr (Table 6).

Upper Ely Greenstone Member:

Figure 70, shows the nomalized major-element variation for the basaltic rocks of the upper member. Those analyses which are not complete in terms of major-elements (mainly those done prior to

1960) have been deleted along with those showing extensive chemical alteration (as indicated by high HzO, COz, calcite content and highly anomalous element ratios). The overall major-element variation is fairly systematic, with decreasing MgO content usually being accompanied by increasing FeOT, MnO and TiOz and decreasing CaO and Alz03.

The alkalies show little systematic behavLor relative to increasing

ratio; reflecting the possible effects of metamorphic alteration.

Plotted on an AFM diagram (Fig. 71), the upper Ely Greenstone c · '( 0 Figure 70 . LJ·ppr-R I t L_ L u REENQ'Q['I ,_) I ·-t

c 2 t · a 0 a8 6 -r

a2 cO a2

p Ll

a6 I a 8 t

a 0 I 1 I-' (.)1 a2 I-' I MG FE MN 51 CR RL TI NA K P

· • . p • • • • • • • • • t:, .... Figure 71. AFM diagram for Ely Greenstone,

0 • • • ••• •• •@ •• •

@ (o)_ @ '-'(:!9 @ @ ® @ G> ® ® ® ® ELY GREENSTONE Gl ® Gl • Upper Member

® @Lower Member

@Lake Vermilion Formation

® Porphyries and Falsie Flows I I-' VI wt% N I -153- samples lie within the tholeiitic field and show a trend of iron enrichment. This agrees with conclusions based on the Jensen Cation

Plot (Fig. 67a). Two samples (35) and ( 63-16) show calc-alkaline characteristics.

An attempt was l!l8.de in this study to collect samples that could be related to stratigraphic position within the upper Ely

Greenstone member. A composite geochemical section for the upper

Ely member was constructed from these data and that of Green (1970) based on the stratigraphic reconstruction proposed by Sims (1976)

(Appendix 3). This geochemical: section can not be considered complete, however, due to:

1) inadequate exposures along the individual sections for

which samples were collected,

2) difficulties in correlating between adjacent stratigraphic

sections,

3) uncertainties in the possible repetition of units along

some sections by unknown folding and faulting, and

4) the limited number of analyses presently available of

the upper Ely volcanic rocks.

It is also necessary to assume that the widely-spaced samples, which were used to construct the chemical section, are truly representative of the chemical variation. With these constraints, interpretations based qn the geochemical section must be considered tentative until further detailed stratigraphic-geoche;ro.ical investi- gations are undertaken,

The chemistry of the volcanic rocks varies in an irregular manner with stratigraphic height: (Appendix 3). It is apparent, however, from the Ti02 yariation that at least two distinct basalt types, one high (1.33 to 1.64 weight percent) and the other low

(0. 78 to 1.11 weight percent) in relative Ti02 content, are interlayered throughout the upper Ely member. A striking feature of the geochemical section is the apparent lack of intermediate compositions

(except for samples CL-6 and number 56) between these two basalt types. The samples from the base of the section (ENEU- 3-12, 6-14 and 8-15) are of the low Ti02 type and have slightly lower FeOT/MgO ratios than the stratigraphically higher basalts. The other samples show that some compositional variations exist among the two basalt types but these do not appear to vary systematically with stratigraphic position.

Fur·ther examination of the two basalt types identified in the stratigraphic-geochemical section suggests,however, that all the samples may not be directly related to the same magma type and/or that some samples have been significantly effected by alteration

(Appendix 3). Tables 7 and 8 show the Ti02 and Al203 contents along with the FeOr/MgO, Ca0/Al2o3 , and Al203/Ti0z ratios of the low Ti02 and high Ti02 samples, respectively. The samples show quite a range in Alz03 content and FeOr/MgO ratio. In terms of Alz03 - FeOr (Fig.

72), some marked variations exist. Samples 31 and 3·2, which represent the gray basalts mapped by SimE1 (Sims, Mudrey and Schulz, 1976)

:Ln the southern portion of the Shagawa Lake quadrangle, are notably high in Al203 and FeOT and may a distinctly high aluminum basalt type. Samples 14, 15 and 19 also have higher Al203 than the Ily Greenstone Low Ti02 Basalts -155-

Sample

2 0.78 1l}.76 1. Lf 3 0.57 13.9 3 0.78 15.38 1. 54 0.51 19.7 12 0.87 1 lf .35 1. ,, 3 0.65 16.5 13 0.82 15.17 1. 77 0.71 18.5 36 0. 86 1 5 .90 2.09 O.G1 18.5 Ave.3+5+8 0. 86 15. 7 5 1. 5 5 0. 7v, 18.3 107-1 0.90 14.99 1.70 0 . 3 5 16. 6

15 0. 81 16.03 1.50 0. G2 19.8 31 0. 89 16.00 1. 67 0.50 18.0 14 0. 9 5 17.28 2.15 0.60 18.2 CL-6 1. 01 16.56 2.19 0.74 1 5 . 1+ 56 1. 05 1 Lf . 8 0 1. 54 0.70 57 1. 00 14.42 1. 6 5 0.81 14 . lf

4 3;"; 1.11 15.91 1. 74 0.66 14.3

From basalt unit north of Saganaga Tonalite (Arth and Hanson, 197.5).

Table 8. Ely Greenstone Hi gh Ti02 b asalt s ------·------·------··--·------·------

Sample TiO 2 Al2 0 3 F e 0 T /l-1 g0 CaO/Al203 Al203/Ti02 ------·- -··-· ----- ··------1 1. 5 4 15.67 2.04 0.51 10.2 17 1. 64 13.97 2.82 0.61 8.5 19 1.36 16.39 1. 7 5 0. 5 8 12. 3 -20 1. 47 14.12 2. 3 5 0.77 9.6 1. 5 9 14.18 2.02 0.58 8 . 9 45 1.37 14.04 1. 84 0. 5 8 10.2 47 1. 50 14.87 2.18 0.53 9. 9 137-8 1. 50 14.53 2.37 0.80 9.7 123-4 1. lf 5 14.05 2.15 0.63 . 9. 7 144-10 1 .4 1 1l}. 0 4 2.22 0.68 9 . 7 171-18 1. 5 3 1lf. 17 2 .19 0.76 9 . 3 182-19 1. 5 2 14.60 1. 86 0.53 9.6 14-8 1. Lf 3 14.81 3.38 0. 7 6 10.4 2 5-2 2 ;'; 1 . t; 9 14.21 2. 26 0.62 9.5

* From greenstone dike in Lower Ely Greenstone. 18

17 I- Cale-Alkaline

-0 15 2.- f<) 0 14 C\J

8 10 12 14 16 18 ....I FeOT (wt%) V1 c:1\ I Figure 72. Al203 vs feOT for upper Ely basalts. Line shows trend for Galapagos speading center and Bunch, 1976), -157- majority of upper Ely basalts; samples 14 and 15 are notably higher

in Ti02 (1. 01 and 1.36 respectively) than basalts w;i.th similar Al203 . Green (1970) recognized relict labordorite phenocrysts in these rocks and it is possible that these samples have been enriched in plagioclase and no longer represent liquid compositions. Alteration may also have contributed, a,s both are high in K20 (0.64 and 0.68), possibly reflecting the presence of mica. The higher Al203 in sample 15, described as a fine to medium grained greenstone with an ophitic texture (Green, 1970), may reflect a higher modal plagioclase content. Sample 12, also described by Green (1970) as having an ophitic texture, has low Alz03 and high FeOT similar to the high TiOz group, though having low Ti02 like sample 15. Whether this difference is a primary or secondary feature is not clear from the data currently available.

Four other samples from the low TiOz group appear chemically distinct. Samples 35 and 163-16 have low Alz03 , MgO, FeOT, and Ti02, but have high SiOz and appear to be calc-alkaline in nature. Samples

56 and 57 are both from a pillow breccia unit in the southern portion of the Shagawa Lake quadrangle (Sims, Mudrey and Schulz, 1976).

As breccias and tuffs are generally highly susceptible to alteration (Hallberg, 1971), it is uncertain how representative of the original basalt these samples are (pate the varying FeOr content and alkalies).

Among the high Ti02 basalts three samples appear somewhat anomalous. One pf these (sample 19) was discussed previously. Scnnples

1 and CL-6 both have significantly lower FeOr for their Ti02 content -158- than the other high TiOz samples. Their higher water content re-

lative to ·the other samples suggests alteration may, at least in

part, account for this difference, It is interesting to note that

in terms of FeOT/MgO ratio and TiOz content, sample CL-6 is inter-

mediate between the two groups while sample 1 falls within the

range of the high TiOz samples.

If the "anomalous'' samples discussed above are removed from

the two groups of basalts, the remaining samples show considerably

less scatter and when plotted on the normalization diagram (Fig.

73 a,b) show fairly systematic variations. Comparing the two groups

in Fig. 73c reveals the high TiOz group also has higher FeOr/HgO

ratios and Mn content with CaO and Alz03. A notable feature

is the lack of intermediate composition between the two groups when

the "anomalous" samples are removed. This is not thought to be due

to sampling bi.as as samples from the present study, as well as the

. published data, show this gap in compositions. The high FeOT/NgO

ratios of the high TiOz basalts suggests that they are fractionated

compositions and do not represent a distinct and unrelated magma

type (see further discussion below).

One of the aims .of the present study was to examine whether

units distinguished by color also differ in chemistry. As already

discussed, the gray pillow basalts from the southern portion of the

Shagawa Lake quadrangle do appear chemically distinct from the other

upper Ely basalts. They are characterized by relatiyely low SiOz,

and high Alz03, FeOT and .MnO and, if not the res.ult of alteration,

may represent a distinct basalt type. Different from these, . however, Figure 73 . -159-

.l,_ . 2 I .0 tl .8 I .c ...... u L1 t ") -!-I • L -l

• nu +I -+ 'l I ::::. ·t· . I .• Li l .6 .6 t tI I . 'lL T I . 2 T FE SI CR AL TI p

; !

Lnw- · Tin?.._...c-_ FLY- . .._; ,I 11 Ee'-l ., . t Ii I • v +i . 8 -i- l. • r-•,J· I Li 1 • I I . :::...)· t . (j + ,-! / l . -- I. . -r . 6 l . 8 -

" 0 .-) i . c:. + A.

D MG I 11 ·, CR RL TI ! -160-

\ c.

Figure 73c. Comparison of upper Ely Greenstone high Ti02 (solid line) and low TiD2 (dashed line) basalts. -161-

are the gray pillow basalts from south of Mud Lake (Crab Lake

quadrangle). Two of the samples from this. area (171-18, 182-19)

contain altered plagioclase and fresh clinopyroxene phenocrysts and

are chemically similar to the high. TiOz group discussed above. This was a surprising res.ult as most of the other high TiOz samples are

typically green rocks and have only microphenocrysts of plagioclase.

Thus color in this case does not relate to any significant difference

i .n chemistry. Studies in Canadian greens tone belts (Gelinas and

Brooks, 1974; Pearce, 1974) suggests that gray colored basalts are often less effected by metamorphic alteration. This appears to be also true of the two Vermilion samples, suggesting that variability

in metamorphic replacement may account for the color differences.

One other gray pillowed basalt was analyzed from the area south of Mud Lake (sample 163-16) . This sample has small glomeropor-. phyritic clusters of plagioclase set in a fine grained plagioclase and pyroxene groundmass. This sample is characterized chemically by high SiOz (54%) and low FeOT (9%) and appears to be calc"-alkaline in nature (see Fig. 71).

Five samples from the upper Ely member were analyzed for the trace elements Rb, Sr, Nb, Zr, and Y. Samples from both the high and low TiOz groups were selected, as well as one of the calc-alkaline basalts. Shown in Figure 74 ::ts the variation of Y, Zr and Nb with changing .FeO T/NgO ratios for both the lower and upper basalts of the Ely Greenstone. Also included is. one basalt sample from the

Lake Vermilion Formation. There is a marked difference in Y content between the calc-alkaline lower Ely samples and the tholeiitic upper -162-

.. 14 A 12 @ 10 • Nb 8 .. 6 • 4 • • 2

B 140 • 120 100 • Zr 80 • 60 • • @ • 40 $ 20

40 c

0 30 • • y • 20 •@ • o Upper Ely Greenstone • e Lower Ely Greenstone 10 fl @ Lake Vermilion Form.

I I I .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2:4 2.6 2.8 3.0 FeOT /MgO Figure 74. y' Zr, and Nb vs FeOT/MgO diagram for Ely Greens tone volcanic rocks. -163- Ely basalts, with the tholei.itic samples having higher Y and lower

FeOT/MgO. . The three lower Ely samples lie on a line of increasing

FeOT/MgO with increasing Zr and Y content. The upper Ely samples als.o show Zr and Y, but with decreasing . The linear behavior of these trace elements, in both the lower and upper member samples, suggests that they may be related to distinct liquid lines of descent. This will he examined further in the following section. I.t should be noted that the calc-alkaline basalt sample from the upper Ely member shows a low Y content similar to the lower Ely rocks, The Lake Vermilion sample also has a low

Y content, reflecting calc-alkaline affinities.

The Nb variation is highly irregular, particularly when compared with the behavior of Zr and Y. This is probably related to analytical uncertainties (Nesbitt and Sun, 1976) and possibly redistribution during metamorphism.

The Y/Zr ratios of the upper Ely basalts are close to chondri·- tic (Fig. 75) though the high TiOz samples have slightly lower ratios .

The calc-alkaline basalts have lower Y/Zr ratios than the tholeiitic basalts; this is most pronounced in the lower Ely samples.

The K, Rb and Sr contents of the Ely basalts are quite variable and .nonsystematic, reflecting mainly the effects of metamorphic alteration. The K/Rb ratios and Rb/Sr ratios are similar to those reported by Jahn and qthers (1975} for the Ely Greenstone. As reported by Jahn and others (1975), thes.e ra,tios . a.re similar to those of island arc tholeiites, though considering the possible effects of alteration, it is uncertain whether such comparisons are valid o Upper Ely Greenstone 40 @ Lower Ely Greenstone

Lake Vermilion Formation • 30 •

y 20

(ppm) @ 10 ·-

7 I 20 40 60 80 100 120 140 160 .

I Zr (ppm) I-' (j) + Figure 75. Y vs Zr , Ely Greenstonc. M = Archean mantle ratio; C = Chondite I ratio (both from Nesbitt and Sun, 1976). -165-

(Jahn and Sun, 1977).

In Table 9 and Figure 76, the upper Ely basalts are compared with Archean grP.enstones from Australia, South Africa and Canada.

There ii:; clearly a marked similarity in both major and trace · elements between the Ely samples and other Archean greenstones.

The similarities between the Ely basalts and those from Western

Australia (Table 9, number 3 and Fig. 76c) and Sturgeon Lake,

Ontario (Fig. 76b) are particularly striking. This marked similarity suggests. that the processes involved in the formation of the upper

Ely basalts were probably similar to those generating greenstones throughout the Archean,

Archean tholeiitic basalts have been compared to and referred to as either ocean floor type basalts (Glickson, 1970, 1971, 1976;

Hallberg, 1972; Viljoen and Viljoen, 1969; Anhaeusser, 1973; Hallberg and Williams, 1972) or low-K island arc tholeiites (Folinsbee and others, 1970; Hart and others, 1970; White and others, 1970; Goodwin and Ridler, 1970; Jahn and others, 1975). In some cases, basalts from the same sequence of greenstones have been regarded as ocean floor basalts by one worker and island arc tholeiites by another

(e.g.; Hallberg and Williams, 1972; White and others, 1970). This points to the general problem of distinguishing between tholeiitic basalts from different tectoni.c settings,

This problem wa.s recently highlighted by the controversy which ha,s deyeloped oye::;- the geqche.m;lstr:y qf the Troodos Complex of Cyprus.

The Troodos Complex has long been regarded as the classic ophiolite sequence and has served as. a model for the oceanic crust (Moores and Table 9. Archean and Recent Thqleiitic Basalts.

Upper Ely Archean Recent c Ave. Low Ave, High Ave.a R,ise.c Ave.b Arc. High;d Ave.e 1 Can. 2 Bnrb , 3 Thol,Bas. Mor. Thol, AL Bas, Thol. Ti02 TiOz Aust.

SiO - 49.82 51. 55 52.27 52.07 51.5 51.1 49 ; 3 49,3 51.1 51. 7 49.58 Al2d3 15.75 14.28 15.05 15,61 14,8 16,2 16.0 17,0 16,1 16 , 9 14.79 FeOT 11.22 13.06 . . 10,62 11.61 11.8 10.4 9.3 8. 6 10.0 10.4 11.07 MgO 7.23 6.08 6,81 6.61 7.2 6, 2 7.5 7.2 5.1 6.5 7.30 cao 11.69 9.75 10.88 9.84 10.5 9.9 11.2 11 , 7 10.8 11.0 10:36 Na2o 2.17 2.71 2,74 2.21 2,5 2.5 2.8 2. 7 2.0 3.1 2.37 K20 0.36 0.51 ·0.18 0.33 0.23 0.7 0.14 0.16 0. 30 0.4 0.43 Ti02 0.86 1. 48 0.93 1,05 0.83 1,60 1.5 1,49 0,83 --- 1. 98 P205 0.05 0,ll 0.13 0.19 0. 08 0.22 --·· 0,16 -- -- 0.24 MnO 0.19 0.24 0,21 0,20 0.19 0.17 --- 0.17 ------0.18 y 2o+ 31+ 22 15 32 30 43 20 20 Zr 50 94 60 105 100 100 .. 95 60 100 Rb 9, 3 14.4 9 8 30 1 10 5 9.6 Sr 100 84,5 105 164 135 400 130 225 328

F/H* 1.55 2. 15 1 , 56 1.76 1.68 l .24 1.19 1.96 1.60 1.52 K/Rb 321 332 1200 700 Rb/Sr 0, 09 5,9 0.09 0. 05 0.075 0.007 0.077 0,022 0, 029 Zr/Y 2.9 3.0 2.7 7.0 3.1 3.3 2.2 3,0 5.0

+Trace elements based on one sample a - Majors, Manson, 1967, Trace, P'I"inz, 1967. b - Engel, et.al., 1965. *FeO/MgO, FeO as total iron I 1 .- Kalgoorlie - Norseman Ave., Hallberg, 1972, c .- Condie, 1976. ,_. 2 - Baragar & Goodwin, 1969. d - Taylor, 1969. (fl 0-. 3 - Viljoen & Viljoen, 1969. e - Le...'1aitre, 1976. I r: ;:· 1L' T ;J Figure 76. RBITIBI U- - '41 1....J -167- (Upper Ely Greenstone basalts shown by dashed line)

A. and others, 1976) p MG FE MNSI NR I

STURGEON LAKE VOLCRNICS .2 . 0 l T . 8 T I . 6 + /1 -1 I

II I ("'I I ' .• l . . ' I +I ') C:. -'-I . . tI .6 .o 1t- .C I ) T B. (Beggs, 1975) • c_ + r11 '.{ TT. j,. ir':,01· \ MG FE MN Sl ('R._.. r. L... . \l . -168- Figure 76 . HRLLBERGS THJL. . 2 .0 .8

. 0 . 2 t. /I •I I "6 + : f . 2 + C. (Hallberg, 1372) MG FE MN SI CR RL TI NA K

BARBERTON THOLEIITES •") :0 t . 8 t . 6 T • Lj +I . ?-- +' . c t .• Lj21 . 6 . 8 I . 0 ...,: (Viljoen and Viljoen, 1969b) I D. -a ?L- ·r

0 MG FE MN SI CR RL TI NR K I -159- Vine, 1971). Miyashiro (1973), however, has suggested that this sequence represents not oceanic crust but the lower portions of a basaltic volcano of island arc origin, Ris conclusion was based on the apparent abundance of calc-alkaline volcanic rocks within the lower pillow lavas. Several recent studies have attempted to resolve the question of the nature of the Troodos Complex using basalt geochemistry and Senechal, 1976; Smewing and others, 1975; Smewing and Potts, 1976; Pearce, 1975). While the general consensus remains that the Troodos Complex represents an oceanic crustal segment, the studies have shown that the alteration within the sequence has resulted in changes in whole rock chemistry such that a clear distinction between ocean floor and island arc basalts can not be made

(Kay and Senechal, 1976; Pearce, 1975).

Jahn and others (1975) have proposed that the Ely Greenstones represent island-arc type low-K tholeiites based on trace element

(Rb, Sr, K, Ba, REE) comparisons. Arth and Hanson (1975) reached a similar conclusion from an examination of REE patterns in the Ely basalts. In light of the previous ·discussion, however, further comparisons seem warranted.

Pearce and Cann (1973) and Pearce (1975) have shown that the elements Y, Zr, Sr and Ti can be effectively used to distinguish basalts from a variety of tectonic settings, Figures 77 and 78

the Ely basalts. plotted in terms of the parameters used by

Pearce and Cann (1973). lt ;Ls s.een that in Figure 77a and b the upper Ely samples. fall within the. ocean floor basalt field while in

Figure 78 they fall in between the field ocean floor tholeiites and . -- --. ·------··------

Ti/100 Ti/100

(a ) MAFIC VOLCANICS MAFIC VOLCANICS

@ /_/ c- ZR Yx3 ZR SR/2

O Upper Ely Greenstone@ Lower Ely Greenstone ® Lake Vermilion Formation Figure 77. (a) Ti-Zr-Y diagram, Ely Greenstone; (b) Ti-Zr-Sr diagram. Field A - ocean floor basalts, Field B - low-K tholeiites, Field c- I calc-alkaline basalts, Field D - within plate (after Pearce I-' -..J and Cann, 1973). 0 I 10000 9000 s 8000

...... 7000 6000 .__,0. 5000 f= 4000 i'f) 3000 $ Upper Ely Greenstone

2000 Lower Ely Greenstone 1000 Vermilion Formation

10 20 30 40 50 60 70 80 90 100 l\O 120 Zr (ppm)

Figure 78. Ti - Zr diagram for Ely Greenstonc volcanic I I-' rocks. Recent ocean floor basalts plot in fields £ and -..J I-' C, and island arc tholeiites in Fields A and B (after I Pearce and Cann, 1973) . -172- transitional low-Kbasalts. At least in trace elements, the

upper Ely Greenstone basalts show a similarity to recent ocean

floor tholeiites.

In Figure 79 the normalized major element variation of the

Ely basalts is compared with both ocean floor basalts and island arc tholeiites. Also shown are the metamorphised basalts from the

Troodos and Great Valley (California) ophiolites. It is clear from

the figure that the Ely basalts have features in common with both island arc and ocean floor basalts. In comparison to the island arc basalts, the Ely samples show lower Al2o3 and K20 and greater FeOT and Ti02 enrichment. Relative to the ocean floor basalts the

Ely samples show lower FeOT/MgO ratios, somewhat higher SiOz and greater variation in CaO and Alz03. The Ely basalts generally have higher FeOT and lower Alz03 and TiOz than modern basalts with similar

FeOT/MgO ratios. These features appear characteristic of Archean basalts in general (Glickson, 1971). It has been suggested that the higher FeOT content of Archean basalts reflects a higher FeOr/MgO ratio in the Archean mantle, while the lower Alz03 and TiOz may be related to shallow depth of generation (Glickson, 1971).

It is concluded from the comparisons above, that the upper Ely greenstones are transitional in major element chemistry between ocean floor and island arc tholeiites while their trace element chemistry is siTIJilar to recent ocean floo]'." tholeiites, For the compositional similarities to .reflect si.milar tectonic environments requires that the following a..ss.umptions be true; 1) that the Archean upper mantle was little different in composition from the recent Figure 19 IC THOL.E I I TES -173- --i 0 T(Upper Ely Greeastone basalts s"J.own by dashed line) • 0 I

• uQ tT

. 6 T

. +I . . 2 t :. +t . 6 1 o 8 I

: i A. (Shido and others, 1971) p MB FE MN SI CR AL TI NR '

) + . I I c ·"u

r, TI . 0 T

J 5 li .1 i . J.. ., I . L.

< u ') -i r_ rr . I I I Lj ·r . I c:, ! . T i . 8 I I l] I . +I I B. (S[iido and 1971) '") i others, . (_ T

iVji: tvfi\I C. T ,...._ L.) n !L T T .,j./ i ·"-' I L I li'I 0_ l... 1 • ri1 : l ··: il I\ :\ ; -- \ I I ·;/ Fig. 79. -174- . 2 r . o T . 8 J. . 6 . ...,/I

!v \ p

FIG i·· FIRST r"'vC'L, I L'-.. i:- VOLCRN I cs

. 2. _..I . 0 ("' • (j - .6 . l • c.. T . 0 t . t • I T i::' r • l ..) + I . 8 t • LJ ,..) r ""t°

0) MG FE 1 11 5-1 virR H"'L -I I K r r· D I I ! q \ ! C; C: Fig.79. L Yp ,RI u ' S L, 0 \,,.; L;:_- n;-, 1 T_ :'- ._ 0 \1, L, • , • • 0 -175-

. 021 . 8 i : t . 2 + . 0 t . 2 + /1 •I LI . 5· +I . 8 t "0 .j. i E. :(Boores and Vine, 1971) • 2 T• D MG FE MN SI CR AL TI NR ,\v.

'- a :i .. , ! a LJ t i ·:) 0 :..J ·t • i.J t! a .:..1" T I ') • c_ + • Cl ....._ / I D .:'...- t Li I . I T 0 6 +I • uQ .L

. 0 F. (Bailey and Blake, ? -t. f\,1 r r:- r:- T RI 11 u '!...... I. ... J .l. I._, r , ...... -176- upper mantle; 2) that the major tectonic regimes were similar in the Archean to those active today and 3) that different tectonic

settings can be adequately distinguished based solely on the geochemistry of associated volcanic rocks. As discussed by Nesbitt and Sun (1976) and Jahn and Sun (1977) these assumptions are unlikely to be true for the Archean. Thus, the geochemistry of Archean greenstones may reflect a unique set of tectonic and magmatic conditions. In veiw of this, the general overlap of geochemical characteristics between Archean b.asalts and modern ocean floor and island arc tholeiites may have particular significance.

Ely Felsic Flows and Vermilion Porphyries

The majority of felsic (dacite to rhyolitic) rocks in the Ver- milion district occur as fragmental material within the Lake Vermi- lion Formation and the Knife Lake Group. Small intrusive quartz- feldspar porphyries are widespread throughout the district and pillowed to massive felsic flows occur locally, within the Ely

Greenstone. Of the felsic rock types, only the porphyries have been studied in any detail. Arth and Hanson (1972, 1975) and Jahn and others (1975) have conducted detailed geochemical studies on selected

Vermilion porphyries and have shown that they have calc-alkaline

characteristics. Furthermore 1 these studies have shown tha.t the porphyries can not b.e :related hy any fractional crystallization scheme to the basalti.c lava$.. o:f; the district.

Only two previous analyses a:re available of f elsic flows occurring in the Ely Greenstone (numbers 5, 6, Appendix le and -177- Green, 1970). Both. samples are from flows in the Gabbro Lake quadrangle and are desc:ci.bed hy Green (1970). To further evaluate the nature of the felsic flows, two samples, collected from pillowed flows within the Ely Greenstone, in the southwestern corner of the Ely quadranglP, were analyzed for this study (numbers

EEU- 7-2, EEU - 9-4, Appendix Both samples are from within the metamorphic aureole of the Giants Range Batholith (Fig. 2) and are at the garnet-amphibolite facies.

The two new analyses and the two from Green (1970) are shown in Figure 80 normalized to average tholeiitic basalt (Table 9).

I:t is apparent that these samples show a wide range of MgO, FeoT, and even for similar Si02 contents. Sample EEU-7-2 similar to the two samples of Green (1970) has low FeOT, MgO and high Na20, but has much higher A1 2o3 and CaO and lower Si02 . The other sample (EEU-9-4) is considerably more basic than the other three with high

FeOT, MgO and CaO and low Si02 , Al2o3 and Na2o. For the two new analyses, their chemical features can be directly related to differences in their mineralogy. Sample EEU-7-2 contains calcite and muscovite, which accounts for itsi high Al203 and CaO content.

Sample EEU-9-4 has abundant amphibole and minor biotite accounting for its' high FeOT, MgO and CaO. This sample also has quartz "eyes" which probably represent amygdules and thus the silica value may be

M.gher than in the origina,l rock. From the presence of granitic yein& throughout s.o.rne qutc.:i;-ops ;it is. possible that significant changes in the chemis.tr:y of ·samples may have occurred, making it u.ncertain what thei.r primary chemical characteristics were. Figure 80. FELS IC Fl_OWS -178- .- ,-i !I ._1 r, 11 r:1 -- • [3 . . 2 r-1 c: ,_, ' a 2 i·

a Lj . 6 t .u0 .D .2 I MG FE MN SI CR RL TI NR K P

Figure 81 • PORPHYRIES . 2 l

u '.i. 0 () t

0 6 + • L! +

0 .2 • n .LI . 2 t • .:i I' 1=: • '-1 I . 8 t .n ·I! • c:... -179- The compositional variation of the quartz- feldspar porphyries

from the Vermilion di.strict is shown in Figure SL Comparison of

the f elsic flows with this figure reveals that the two samples

from the Gabbro Lake quadrangle are more felsic than most of the

porphyries.. The two new sa.mples lie within the range of values shown

by the porphyries. This general similarity in major element

chemistry supports the contention that the felsic flows are geneti-

cally related to the porphyries and represent their extrusive equi-

valents. This genetic relationship i .s further supported by the

similarity in the REE pattern of one of the Gabbro Lake felsic flows

(sample 7288a) to those of the porphyries (Jahn and others, 1975).

Arth. and Hanson (i972, 1975) have also shown that the plutonic

tonalites of the Vermilion district, namely, the Saganaga Tonalite

and Northern Light Gneiss, are chemically similar to the dacitic

porphyries and share a common origin.

The Vermilion felsic volcanic rocks compare closely with other

Archean felsic volcanic rocks from Australia, South Africa and

Canada. All show high NazO/KzO and FeO/MgO ratios

as well as low levels of K, Rb, Ba, REE and Rb/Sr ratios. The

marked depletion in heavy REE is a further distinctive feature .

.-, ,,, Jahn and others (1974) have shown that many of the characteristic

chemical features of the Vermilion felsic volcanic rocks are similar

to thc;ise 0 £ the island a;rc calc.,...alkaline series (Jakes and White,

1972), Distinctive, however, from these is the marked depletion in

heavy REE in the Vermilion samples; this i .s a feature not reported

from modern island arc calc-alkaline volcani.c rocks (Condie, 1976). -180- Coleman and Peterman Ci975) have recently defined a suite of oceanic plagiogranites based on data from the Troodos Complex,

Cyprus. These rocks consist principally of quartz and plagioclase, accompanied by minor ferro-magnesion phases and are chemically characterized by high S.i02, moderate alumina, low FeOT and MgO, and extremely low potassium (_ 0,7%). They also have light REE depleted patterns with negar.ive Eu These features contrast with those of the Vermilion felsic volcanic rocks (and Archean felsic volcanic rocks in general) which usually have higher KzO, a wider range of SiOz and show heavy REE depleted patterns.

It is concluded from the comparisons above, that the Vermilion felsic volcanic rocks are chemically similar to Archean felsic rocks from other greenstone belts around the world. Furthermore, while sharing some chemical characteristics with modern island-arc calc- alkaline series, they are distinguished by their unique REE patterns.

Thus, like the Vermilion basalts which also show some similarities to modern volcanic suites but are still chemically distinct, the felsic volcanic rock.compositions point to a unique set of conditions for magma genesis during the Archean.

Lake Vermilion Formation Basalts

Two pillowed basalt units occurring within the Lake Vermilion

Formation, were sru:npled for this study. The analyses and catanorms for the two samples aJ;e given in Appendix lb. (_numbers TLV-1-3 and

TLV-2-4). GJ;out (1926) presented an analysis of a basalt from

Sioux Pine Island in Lake Vermilion. While the stratigraphic re- -181- lationships on this island are still uncertain, Sims (1972) has considered it part of the Lake Vermilion Formation. The analysis from Grout is included in Appendix lb (number 27).

Both of the samples from this study are classified as calc- alkaline basalt on the Jensen Cation Plot 67a). The two samples are similar in composition, with high Si02 and Al203 and low FeOT co:rnpared to the Ely Greenstones. They are similar chemically to the lower Ely member calc-alkaline volcanic rocks.

One of the samples (TLV-1-3) was also analyzed for Rb, Sr, Y,

Zr and Nb (Appendix lb). Yttrium is low when compared to the values found for the upper Ely member basalts, but is similar to the values found for the lower Ely member calc-alkaline rocks (Fig. 75). The

Zr/Y (3.1) and K/Rb (250) ratios are lower than the ratios in samples with similar silica contents from the lower Ely member, while the

Rb/Sr ratio (0.09) is similar.

The analysis from Grout (1926) is markedly different from those obtained for this study. Though having similar silica (54.3%), it has higher MgO, K2o and Ti02 and lower Al2o3 and CaO. These features, along with high co2 (1.0%), suggest that the sample may be extensively altered. Unfortunately, a description of the sample was .not presented by Grout (J926). The sample does show some chemical silllilarity to the basalts from the Newton Lake Formation, particularly with regard to the low Al203 and high MgO, The significance of this. can not be eyaluated without further informat:Lon. -182- Newton Lake Formation

Felsic Member:

Green (1970) presented five analyses of samples from the felsic member. The samples range from andesite to dacite and have calc- alkaline characteristics (Green, 1970; Sims, 1972). The felsic member was not directly sampled in the present study. However, based on chemical analysesJ four basalts having calc-alkaline characteristics were found within the mafic member and are thought to be related to the calc-alkalirte volcanism of the felsic member.

They are therefore, included ·w.fth the felsic member, in the discussion below.

The compositional variation of the felsic member samples is shown in Figure 82. They are very similar to the lower Ely Green- stone samples (see Fig. 69) except for having lower MgO contents .

In comparison with the porphyries of the Vermilion district the

Newton Lake samples have higher CaO and lower K20 (see Fig. 58).

Only one sample was analyzed for trace elements (sample ENL-30-

9 from the mafic member). This sample has very high Sr (315 ppm) but low Y (12 ppm), similar to other calc-alkaline volcanic rocks of the .Vermilion district. Further Rb-Sr data for the felsic member are presented by Ja,hn and others (;I..975) , The average Rb-Sr and K contents for these samples are similar to modern andesites from island-arc environments. Figure 82 ...... " c' ,-! '• l_) r .B T ...... o _,/o + . I + ') . '- t . 0 tI .-/ . c ·- 1· ,q I . ' .6 I . 8 .l I' 5 "'], . f . '- + M G ,_ E" tv' N .-. - , 'R p I I l - i-1 J L CR FIL_ TI -1 K -184- Mafic Member:

A total of 54 major element analyses (Appendix lb) were obtained on samples from the mafic member of the Newton Lake Forma- tion (four of these were discussed in conjunction with the felsic manber above. The trace elements Rb, Sr, Y, Zr, and Nb were determined for 23 samples by x-ray fluorescence spectrometry (XRF). Nine samples were also analyzed by Dr. V. Rama Murthy for Rb, Sr, Ba, K, and sr87/sr86 by isotope dilution techniques (Appendix lb). It should be noted that the Sr values determined by XRF are significantly higher (6 to 21 %) than the isotope dilution values for the same samples. The cause for this disparity in the Sr values is presently unknown; the XRF Sr values determined for the standard sample BCR-1 show good agreement with the value of Flanagan (1973) for BCR-1

(see Appendix la). For the sake of consistancy the XRF values are used in the discussion below.

As previously noted, the basalts can be divided into two major types (A and C) based on field and petrographic criteria ( the B basalts were discussed with the felsic member). The compositional variation of the two groups is shown in Figure 83. Comparison of the plots reveals that the type A basalts are mostly higher in MgO,.

FeOT and lower in Al203 than the type C basalts. They also have significantly lower Al203/TiOz ratios and generally higher CaO/Al203 ratios. On an AFM diagram (Fig. 84), the basalt groups overlap, both plotting mostly in the tholeiite field. They show a wide scatter in alkalies and a broad trend toward iron enrichment. q TYPE I I I! ·-''-·-TS -185- . ? lT . 0 I . 8 t r I . o T • Lj + . ? • 0 I . 2 t

• LII t. I . b i "8 T r· . o,.... A . .LI ·r

I p MG FE MN SI CR .R L TTJ. NR K

TYPE C un'.:"JnSRL TS . 2 r : I • Ll I • I T . 2 T I "0 t . ? LI . I T - I I : t . 0 . ) + B. -,- T I , c: T I (\ p MG MN .J - CR RL ! !. Figure 84. AFM diagram for Newton Lake Format- ion volcanic rocks.

• • • •• • • • • • • • •n • • •\• •••• • ...., •. .. ® • • ® • ®

NEWTON LJ\KE

" Mafic Member

® Felsic Member ....I 00 wt% °'I -187- The basalts can he readi.ly divided on the basis of their

Al2o3-Ti02 variation (Fig. 85). The type C basalts show decreasing Al 03' from about 15.5 wt. % to 14,0 wt.%, with increasing Ti0 2 2 content. Relative to the tholeiitic basaltsof the Ely Greenstone, the Type C samples have slightly lower A1203 and higher Ti02 (Fig. 85). Note that most of the analyses cluster at the end of the trend defined by the gabhros. from the layered sills. The other group

(Type A basalts) has markedly lower Al2o3 ·content which increases from about 6.5 wt,% to 14.5 wt.% with increasing Ti02 content.

I:n both. groups, increasing Ti02 is also accompanied by increasing FeOT, MriO, alkalies and Pz05, though not significantly related to any fractionation index such as FeO/MgO-

The two tholeiitic basalt samples are similar in composition to the Ely basalts except for slightly higher MgO contents. They have lower TiOz and higher Alz03 than either the type A or C basalts

(Fig. 85).

The chilled margin samples, except for those from the Cedar

11 Lake "sill , are very similar in composition and characterized by high MgO and Si02 , low Al2o3 , Ti02 and FeOT/MgO ratios (Fig. 86). The two samples from the Cedar Lake 11 sill11 are higher in MgO 8nd

FeOT and lower in S.i02 , Al2o3 , Ti02 and FeOr/MgO. However, these two samples show· signi:f;icant diJ;ferences with the higher MgO sample

(ENL-137-58B, from the nearest sill contact) having lower Si02 , CaO and TiOz and hi.gher Al2o3 and FeOT• The high degree of hydration and alteration which has. affected th:ts s.ample sugge.sts. that it may no longer reflect an original liquid composition, -188-

16 •

14 0

• • 0 12 :'!'...... c · 0 0 - 10 / r<> 0 :7.0 N <( 8

/ • Type C Basal ts 6 o Type A Basalts D Sill Units I Cedar Lake Sill 4 Tholeiitic Basalts 0 Type B BCl.sal ts

.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 Ti02_ (wt 0/o) Figure 85. AL203 vs Ti02 diagram for the Newton Lake Mafic member . Lines connecting solid and open squares represent cumulate variations in the layered sill. For the type A basalts the line connects assumed parent-differentiate pair. Fields are shown for the Ely Greenstones. -189- .The chilled marg:;ln samples can also be readily distinguished on the basis of the content (fig. 85). Sample f;rom the Cedar Lake 11 sill't lower contact, plots with the type A basalts. A sample analyzed from the upper chilled contact (Schulz,

1974) also plots with the tyJ?e A basalts, but has lower MgO than the basal contact sampies. The other chilled margin samples plot in a small cluster separate from the flow compositions, with an average Al203 and Ti02 content of 12.2 and 0.74 wt. percent respectively.

The distinctive olivine bearing flows from south of Little Long

Lake show variable compositions with a wide range of Si02, FeOT, MgO and alkali contents but very similar Al203 and TiOz (Fig. 87). The sample showing skeletal pyroxene pseudomorphs from this locality

(SSN-430, see Fig. 50) has a composition similar to the average of the olivine bearing samples, except for slightly lower FeOT and Ti02 contents. The unsystematic compositional variation shown by these samples may be the result of alteration of the originally abundant glass component of these pillows. Their average composition is similar to the type C basalts except for the slightly higher Ti02

(Fig. 85).

Ten of the analyzed samples a;re from units in the layered sills

(2 peridotites, 2 pyroxenites., 6 gabbros). These, along with similar

1 analyses from the Cedar Lake '-'sill ' ($chulz 1 are shown in

Figui:-es 88 and 85. Note that the. .m.qst differentiated gabbros have compositions similar to the type C b.asalts,

The K-Rb relationships are shown in Figure 89. The K/Rb ratios .·-- I I T I I '., :Vj .H·; P, 1_J· ·.:.. .-Qj . .. :_ :-. -190- · Ln i L._._1_ ..., Li'l JI 11 11 l -\- ·....I Figu.re 86 . I'· ' ' ..)

C> ,. 1.·_1 I I I

. 6 +I :; t . 0 ·t : f . 6 l "8 • Cl tT . 2 -!. ·, i::-c MG 1L MN SI CR AL. TI NR r\ p

Figura 87 • . .)'.:_ . t n I . u r . 8 t . 6 + . t . . L I o il·-' ·+I : lI . 6 t ' . p + I . 0 t· . 2 +

'-J ::JI 'c. !!- -1·· Tl. j\j R . MG: Ft MN s I C[ I -191- r · (__- j I l Figur e 88. ...) .i.. '._ -: ..._, .. 2 rI

:. 8116 .

• LJ r ) .t • L

• 0 I

0 2 • Lj . 6 .8 • Cl . 2 ,.., T MG FE MN CR . T-1, NH p -192- range from 1046 to 310 and are similar to those found for the Ely

Greens.tone ba,salts (Jahn and others, 197 5, and this study). A. peridotite sample," from a thin lens southwest of Camp Lake, has a much lower ratio of 157. The samples cluster into two groups, one with an average K/Rb ratio of about 500 and the other with a ratio of 350. The type A basalts are distinct in having the highPst ratios (Fig. 89). The samples with the lowest K/Rb ratios also have the highest K and Rb concentrations, but do not show any correlation with FeOT/MgO ratio or any other differentiation index.

The Rb/Sr ratios range from 0.012 to 0.288 in the basalts; the peridotite sample has a very high ratio of 0.828. The Rb...:.sr values cluster into two groups (Fig. 90); one has a mean Rb content of about

4, with a wide range in Sr from 7 to 136, while the other has significantly higher Rb ranging from 16 to 33 and a range in Sr from

77 to 147 ppm. Only one sample (ENL-137-58) is intermediate between the two groups (Rb-10, Sr-166). The type A basalts belong to the first group above (Fig. 90) except for one sample (ENL-120-55) and have the lowest Sr content of any of the Newton Lake samples.

It may be significant that the least altered sample of this group

(ENL-114-54A) has a Sr content (l09 ppm), similar to the other

Newton Lake samples. The Sr content of most of the l-lewton Lake

SaJ!lples is lower than found in the Ely Greenstone bas.alts which have an average Sr content of 150 ppm (Jahn and othe:i:-s,1974 ) .

91 a through d illustrates the relations.hips. between

Ti-Zr, Ti-Y, Zr-Y and Zr-Nb, for the Newton Lake samples, For reference, · the chondritic ratio and estima,ted Archean mantle ratio -193-

I 60000

10000

5000

I; (ppm)" 1000 NEWTON L.Z:..KE

500 O Type A Basalts O Type C Basalts

. I . 5 1.0 5 10 50 Rb (ppm)

Figure 89. K vs Rb for Newton Lake Formation volcanic rocks. fields are for Ely Greenstone basalts (low Rb) and Vermilion porphyries (high Rb)(From Jahn and others, 1974). 18 [ ®

Newton Lake Formation • e 161 • Type C Basalts 141 @> ·Type A Basalts *Type B Basalts 12

E • -0. J _,c.. _a er:: s r· • . 0 6 f- • @> @> II.\ @> 0 © 0 e • ® @> • * 2 I- @> @ • 0

I I - \ 1-J 20 40 60 80 100 120 140 160 180 ' 310 lO -I=" . , tigure 90. Rb vs Sr diagram for Hewton Sr (ppm) Lake basal ts. -195- for each pair of elements (Nesbitt and Sun, 1976) is also plotted.

The type A and C basalts are clearly distinguished on the Ti-Zr,

Ti-Y and Zr-Y diagrams, but not on the Zr-Nb diagram (Fig. 91). The lack of correlation between Nb and the other trace elements as has been found in other basalt suites (Nesbitt and Sun, 1976) could be attributed to redistribution during metamorphism. A similar lack of correlation between Nb and the other trace elements was noted for the Ely _Greenstone basalts.

The Ti/Zr ratios of the type A basalts 91 ) averageB about

110, similar to the suggested Archean mantle ratio of Nesbitt and Sun

(1976). The type C basalts have varying Ti/Zr ratios (from 100 to

68) but the data define a line which intersects the Ti axis at about

2500 ppm Ti. This suggests that titanium is not acting as an incompatible element but is being retained by some phase (most likely clinopyroxene). A similar variation is shown by the Ti/Y ratios of the type C basal ts (Fig. 91 ) .

The type A basalts have much higher Ti/Y ratios than the type C basalts (Fig. 91 ). Comparison with Figure 91 reveals that this is due to lower Y content. This. is also shown in Figure 91 .; where the

Zr/Y ratio of the type A basalts is higher (3.5) than that for the type C basalts (J.O). Note that both basalt types have higher Zr/Y ratios than the sugges.ted Archean mantle or chondriti.c ratios.

Rb-Sr Isotopic Systematics:

Nine samples from the mafic member of the Newton Lake Formation were analyzed for Rb, Sr, Rb87/sr86 and sr87/sr86 by isotope dilution , -196-

y 20 (ppm) 10

20 40 60 100 120 140 160

. 10000 B

0 8000

6000 Ti (ppm) 4000 o Type A Basalts

• Type C Basalts 2000

20 40 60 80 100 120 140 160 Zr (ppm) Figure 91. Y - Zr - Ti - Nb variations for Newton Lake .Basalts. l1 = Archean mantle ratio; C = Chon- drite ratio. (both from Nesbitt and Sun, 1976). -197-

10000 c

8000

0 6000 Ti {ppm) 4000

o Type A Basal-cs 2000 • Type C Basalts

10 20 30 40 Y(ppm) 24 D

16 Nb {ppm} .. • 12 •

20 40 60 80 100 120 140 160 Zr (ppm)

Figure 91. cont. -198- methods. These analyses were kindly supplied by Dr! V. Rama Murthy of the Department of Geology and Geophysic, University of Minne- sota. The data is present in Table 10 with all uncertainties reported at the 26 level.

The nine samples show a well defined positive correlation of

Rb87;s_r86 verses sr87/sr86, which if defined as an isochron, yields on apparent age of 2.03 + 0.06 AR with a sr87/sr86, initial ratio of

0. 70165 + 0.00028 (Fig. 92). Shown for reference in the figure is the 2.65 b.y. isochron determined for the felsic member of the

Newton Lake Formation by Jahn and Murthy (1975).

Several alternative interpretations are possible for the isochron "age".

1) The Newton Lake mafic member basalts were emplaced at about

2.0 AE ago (i.e., the "age" is primary). The minimal

scatter of the data about the isochron (actually giving a

smaller error than the 2.65 b . y. isochron of Jahn and

Murthy, 1975) and the not unreasonable sr87/sr86 initial

ratio of 0 . 70165 are certainly compatible with this inter-

pretation. However, the geologic relationship between the

mafic and felsic members clearly rules out this possibility.

As describ.ed by Green (1970.} the maf ic and f el sic members

intertongue in the a.rea of Newton Lake and the mafic member

also has flows and volcaniclastic rocks

simila:r; to those found :i.n :the felsic member discussion

a.hove). As Rb-Sr dating of the felsic member has shown it

to be 2.65 b,y. in age a similar age is indicated for the Table 10. Whole Rock Rb-Sr Isotopic Data

Sample No. Rb Sr Rb87 /Sr86 Sr87/Sr86 + 2 (ppm) (ppm) ·(a) (b)

ENL-114-54A 4.03 . 114.21 0.10251 0.71470 + 8 ENL-114-54B 4.91 43.43 0.3266 0.71116 + . 8 E-151A 1. 79 19.81 0.2615 0.70852 + 11 ENL-44-12 5.81 19.90 0.8438 0.72573 -+ 7 ENL-25-7 2.37 25.52 0.2689 0.70965 -+ 8 ENL-123-61 3.41 94.33 0.1044 0.70464 + 9 ENL-71-33 5.35 68.29 0.2266 0.70808 + 6 SEN-231 3.10 95.35 o.. 0940 0. 70411 + 8 SEN-213 5.01 114. 02 0.1269 0.70539 + 8

(a) Estimated errors of 3%. (b) Statistical errors correspond to the last figures given and are equal to + ·2 , where 61'\= standard error.

I ...... (,.'.) l.O I -200-

Figure 92. Rb-Sr isochron plot for Newton Lake mafic member .

•736

.732

.728

.724

...co .720 "- (/') ... .716 -en"L . •712

.708

.704 APPARENT T = 2.03± 0.06 APPARENT I= 0.70165 ± .0003 (2

.2 .4 .6 .8 RbB7/ srso - 201-

mafic member.

2) The 2.0 AE isochron age may reflect a major resetting of

the isotopes in these samples. Such resetting could have

been the result of a significant event (_of undetermined

nature) occurring about 2.0 AE ago, Evidence for such an

event is provided by similar ages obtained for diabase

dikes present thoughout the Lake Superior region (Gates and

Hurley, 1973). and also by metamorphic dates on rocks from

Northern Nighigan (Van Schmus and Woolsey, 1975) . However,

if a major event of regional scale did effect the Vermilion

district, it is difficult to explain why other rock units

of the district were not also reset.

3) The age may reflect a local event; Faulting or reactivated

faulting may have locally disturbed the isotopic system,

resulting in lower agei:;·. Goldich -. and others (1961) obtained

low K-Ar ageR on samples close to faults in the Knife Lake

area. Also ages obtained for the rocks of the Rainy Lake

area by Rb-Sr techniques are slightly lower than U-Pb ages

(Peterman and others, 1972) and may have been lowered in

response to faulting (Z. Peterman, written communication,

1977). Also, in thi.s regard, it may be significant that

the highly fractured Giants. Range batholith samples, dated

by Jahn and }'lurthy (1975) giyes ages ranging from 2,08 to

2.48 b.y. There is no however, that the present

sample locations. are any closer to known faults than other

Vermilion samples. Further sampling, particularly in and -202- near known fault zones, is required to properly evaluate

the possible effect of faulting on the Rb-Sr isotopic

systematics.

Several recent studies have discussed the general problem of interpreting well fitting Rb-Sr isochrons which give geologically unreasonable ages (Bickford and Mose, 1975; Page, 1976; Van Schmus and others, 1975; Roddick and Compston, 1977). Of these, the study by Bickford and Mose (1975) on the felsic volcanic rocks of the

St. Francois Montaines, Missouri,-·.is the most complete. They examined both. Rb-Sr and U-Pb systematics in the felsic volcanic rocks and found that the Rb-Sr ages were significantly lower ( 200 m.y.) than the corresponding U-Pb ages. From a comparison between the

Rb-Sr ages and volcanic rock chemistry they found:

1. A strong positive correlation between model Rb-Sr ages

(using an assumed initial) and Sr concentrations.

2. An inverse relation between Rb/Sr and Sr concentration, and

a negative correlation between model age and Rb/Sr.

3. No correlation between Rb-Sr age and Rb concentration.

From these observations, Bickford and Mose (1975) concluded that the Rb-Sr ages of the volcanic units were lowered principally by

Sr loss from the whole rock system, probably during a hydrothermal event. Similar conclusions wel;'e also reached by Page (1976) concerning low Rb-Sr ages for the Mount Isa igneous rocks in Australia.

4. The Rb-Sr relationships shown b.y the Newton Lake samples

a:re similar to those · described by Bickford and Mose (1975)

and may i.ndicate Sr loss as a mecha,nism for the lowering of the Rb-Sr age. -203- Figure 93 shows the correlation between model Rb-Sr _ages

(<::alculated for each sample using the sr87/sr86 initial

value determined for the Ely Greenstones 0.7006), Jahn and

Murthy, (1975)and the Sr concentrati.ons. for the Newton Lake

samples. The data shows a good positive correlation with

the low Sr samples having the lowest calculated ages. There

is also an inverse relationship between Sr concentration and

R,b/Sr ratio (Table 10) etnd a negative correlation between

model ages and the Rb/Sr ratio (rable 10) for these samples,

indicating that the Rb/Sr ratios are mostly controlled by

Sr concentration. Rubidium shows no correlation with either

Rb/Sr ratios or calculated ages. If Sr loss has occurred, the

2. 0 AE age would reflect the time at w'hich Sr removal ceased.

The obtained Sr87 /sr86 initial ratio given by the obtained

isochron would have no significance in this case,

being simply the result of rotation of the isochron.

5. The Rb-Sr isochron may also be interpreted as a mixing line.

This explanation, which also appears compatible with the

observed Rb-Sr relations, would require mixing contami-

nation of the Newton Letke basalts wi.th a very low Sr, high 87 86 sr /sr ratio sou:i;-ce, The age de.(ined by the i .sochron

would ha, ye no geologi.c $igni :eica,nce in this cas.e.

Support for this inte1;'preta,ti.on is proyided by the hyperbolic

;i;-ela,ti.ons.hip between Sr and sr87 /sr86 revealed in Figure 94.

As discussed by Bell and Pow:eli (1969) ", mixing of two end

members produces a hyperbolic pattern when for the resulting 120 I ® @ .. 1001 @@ I E 80 -0.. c. @ lo- I -fJ') 60

@ 40

2200 2400 2600 2800

CALCULATED AGE (M.Y.) I Figure 93. for Newton Lake I\) 86 0 basalts. Ages calculated assuming Sr /Sr initial ratio of +: 0.7006 (value for Ely Greenstone, Jahn and Murthy, 1975). · I ---.. ···------

.730

.724

CJ) co I... .718 (f)

OJ,_ Cf)

.712 I 0 0 0 1 .706 o

20 40 60 80 100 120 140 160

Sr (ppm) I N 87 86 0 Figure 94 . Sr /Sr versus Sr. diagram for Newton Lake basalts. en Line is best visual estimate to the data. I -206-

mixtures, a rati.o of two parameters is plotted against any

single value parameter. They state that they know of no

other process that could produce hyperbolic patterns on

graphs involving sr87 /sr86 ratio.

The composition of the contaminating source indicated by

the mixing model (i.e., high Sr87 /sr86 with low Sr) suggests

that contanlination did not occur during mantle partial mel-

ting and ascent through the crust. This is also suggested

o.y lack of correlation of other trace elements (i.e., K, Rb,

Y and Zr) with. the isotopic strontium ratio. A more likely

source would be sea water or hydrothermal fluids. Spooner

and others (1977} have shown that hydrothermal metamorphism

by convecting sea water could explain high initial Sr87 /sr86

ratios measured in the ophiolitic rocks of the Troodo Massif,

Cyprus. Similar processes may also have affected the vol-

canic rocks of the Newton Lake Formation. Unfortunately,

neither the time of mixing or the composition of the mixing

fluid can be determined from the present data. Considering

the very high sr87 /sr86 ratio required if mixing occurred in

the recent past, contamination sometime s.hortly after for-

Iijatj..on may be more reasonable,

Conclusion: The Rb-Sr isochron for basalts from the mafic member of the. Newton Lake Formation gives a geologic.ally unreasonable young

.a.ge.: An unambigous choi.ce among the other alternative explanations of resetting, due to a major metamorphic event, local faulting, Sr -207- loss or mixing with some radiogen.ic source, can not be made with

the present data. I:t i .s the authors opinion, however? that the mixing model may be the most likely explanation, as it appears most

compatible with all the presently available information.

Comparison with Recent Mafic Rocks;

As classified by ·the Jensen Cation Plot (Fig. 676), the Newton

Lake basalts range from basaltic komatiite to magnesium basalt and high iron basalt. Chemically they are mostly characterized by high

MgO (14.5 to 5.5 wt%), lowA.lz03 and low FeOi/ MgO ratios. In Figure 95 a through d the Newton Lake type A and C basalts

a)'.'e compared with recent ocean floor basalts, picrites and me:ta- mqrphosed ophiolitic basalts. Compared to the ocean floor basalts both Newton Lake basalt types have higher FeOT and lower Al203 · for similar MgO contents. Compared to the picrites they also show higher Si02 • I't is apparent that the Newton Lake basalts, though overlapping in composition with ocean floor basalts, are chemically distinct.

The trace element content of the Newton Lake basalts is compared with that qf mo.dern tholeii.tes in :Figure 96 a and b. As revealed by

the figure, the Ne'Xton Lake. samples plot in the field of ocean floor

thole:ti.tes. defined by Pearce and Cann (1973) except for some being markedly lower in Sr content (these are type A basalts-).

Komatiites:

The compositional similarity of the Newton Lake basalts to -208- !.. 'C01N · 1- (-.. nL -H·. t:11 I :---- -: -I -·-.: --,...·- , Fig. 95 OvL-1 I l_, V I ,Ul-L.-- J. l I L-V

/ • <- (Daa hed lines represent type C Newton Lake !:>asalts) II 0 I (Solid lines represent type A Newton Lake . 8 basalts) tI "-"" t • L1 + ""\ • C:.. ,. ., . u . 2 • Lj ·""' II < 0 t . fj I

TI "0 T..,.. A. (Shido and others, 1971) . D 2 ., ...,... T D FE MI [\; Sl I .i. . . , I K I

BRFFI1\i BRV, , cH2ri I .J

.2 +! • CJ + 8 tI < 6 L: • I . ? fT I q Cl + _J

I i ') I .. c_:_

....i: · •·' " I 1 ( '"' ! ... """!' l c l. • l-1 i . 0 + I ,.) I . ....-.:. + B. (ClaI'ke, 197·J) . •, .\ I rq Ft_:· Si >-;I TT i\!C. MG ' L. ;vi I '1 ...... J. CR I .l i 'II t p f/Ti T '( f -209- ;'\_ L R!'FliJ_r; J. r I l ...J ._I

') _,_ . '-· !

nl_J i

fI .8 ,.. • 0 it I I Lj t . ::::.'""' .0 i I . 2 T .4 I .6 .+lI i" . 8 I i i I ,0 T . ? c. (Gunn, i9 71) . ('(') Tl p MG FE MN ..__, .... pi,_ ! l NR K I

"vp0.115 p I c1.i i LI i 1U LO\{';!_,/CR I _._t_Ti ,,,, L1 ./1 '·-!

:) .J.. u ._ i I • 0 I . a I . 6 li_ /1 T

- '"'c:. +I I .L 0 I ? 1 -i c L I Lj ' • I T . 5· t . 8 + n i • U I I ''""': , , -1- D. (Moores and Vine, 1971). f/ MG FE ·MN S I Hl_ T I r\ D . - ·· -- ···------·------

Ti/100 Ti/100

ZR Yx3 ZR SR/2 Figure 96. (a) Ti-Zr-Y diagram, Newton Lake basalts; (b) Ti-Zr·-Sr diagram. Field A - ocean floor basalts, Field B - low-K tholeiites, Field c - calc- alkaline basalts, Field D - within plate basalts (after Pearce and CBnn , 1973).

0"' I -211- basaltic komatiites has been alluded to several times above.

Recently Green and Schulz (1977) have suggested that the type A basalts ·may represent a che:mically distinct type of basaltic koma-

tiite. / Considering the imJ?ortance placed upon komatiites in recent models of greenstone belt evolution 1976), furhter evaluation of the simila.rities (?rid differences) s.hown by the Newton

Lake basalts to these distinctive rocks is warranted.

Classification and Chemical Characteristics: The name komatiite was originally given to a series of mafic and ultramafic rocks, showing evidence of an ·extrusi:ve origin, from the base of the Archean Swazi- land Sequence in the Barberton Mountain Land, South Africa (Viljoen and Viljoen, 1969a). The rocks were found to be characterized by high MgO (>9%) and low Al203 contents, high CaO/Al203 (>1%) and very low alkali contents ( O. 9%, K20). They were divided into two major groups, peridotitic komatiites and basaltic komatiites.

These were further subdivided into two and three categories respectively, based mainly on chemistry (Viljoen and Viljoen, 1969a).

Besides their chemistry, the presence of "quench textures" (spinifex and other skeletal morphologies) helped establish the komatiite series as dis.tinct from other ma:l;i_c and ultramaf ic rock · types.

$.ince their initial description, numerous other occurrences of texturally and chemically similar rocks have been found including localities from Rhodesia (Bickle and others, 1975), Western Australia

(J-Ia,llberg and Williams, 1972; Williams, 1972; McCall and Leishman,

1971), India (Viswanathan, 1974) and Canada (Pyke and others, 1973; -212- Arndt and others, 1977). Few.: occurrences have been found outside

Archean terranes with the notable exception of lower Ordovician basaltic ko.matiites from Newfoundland (Gale, 1973; Upadhyay, 1976).

An early observation made for the ultramafic and mafic rocks f'.t'om western Australia and Canada was that they did not strictly conform in chemical cha,racteristics to the type locality komatiites from S.outhAfr:lca (Williams, 1972; Pyke and others, 1973). In particular, they do not have the high (>l) Ca0/Al2o3 ratios of the Barbe]'.'ton samples. This led to the use of terms like Archean peridotite and high magnesium basalt rather than komatiite for the rocks from these localities. I.n recognition of this difference, new definitions for komatiite have been proposed which, in essence, broaden the characteristics to include the lower Ca0/Al2o3 ratio samples from Western Australia and Canada (Brooks and Hart, 1974;

Arndt and others, 1977). Schulz and Weiblen (1976) have proposed that two distinct komatiite suites may exist, one having high CaO/

Al2o3 ratios and the other low Ca0/Al2o3 ratios (

A critical proole.m. 7 only, recently addressed .in the literature, -213- . is disti.nguishing lQw:· M,gO komatiites from their tholeii.tic counter- parts. Most classifications: have tended tq limit the use of the

term komatiite to samples with greater than 9 wt% MgO content.

Recent studies by Nesbitt and Sun (1976) and Arndt and others (1977)

this study) clea,rly shqws, however, that lower MgO rocks may also be related to the komatiite suite. Thu$ the arbitrary re-

striction to a set MgO content seems unwarranted. Nesbitt and Sun

(;l.97p) suggest that higher Cr and Ni and lower Y and Zr contents in comparison to tholei.ites are characteristics of low MgO komatiites.

Arndt and others 0.977) suggest that komatiites will also have lower

TiOz than thole;lites with. similar MgO contents and often show skeletal textures.

Compositional Variations and Comparisons: Shown in Figure 97 a,b,c is the normalized (to average tholeiite, Table 9) compositional variations for South Africa, Australian and Canadian kornatiites respectively. Represented for all localities are samples ranging from about 36 wt% MgO (i.e. , peridotitic komatiites) down to about

9 wt% MgO (i.e., basaltic komatiites). Comparison of the South

African komatiites (fig. 97a} with the Australian (Fig. 97b) and

Candian (fig. 97c) clearly reveals their highe.r CaO/Alz03 ratios and gene;rally lo•xer. Al2o3 /Ti02 ;i;-a,tios.. Nqte that the normalized pa,tte:i;-ns. fo:r the A..us.tra,lian a,nd Ca:n;:i,dian sa,mples. a,re essentially ide.nti.ca,l, except for apparently lower P205 in the Canadian peri- dqtitic komatiites.

l:n :Figure 98 a through. d, the Newton La,ke type A. and C basal ts are compared with. the South African and Australian komatiites. Con- -, .,, i-1 L1 -214- Figure 97. jl:u ''

'")

o l.. 1I . II • lJ I . + . 2 i . o I ) -r

:. u tI . 8 + . 0 + A. . 2

I , .J . I'u0· 1 , er:·:..... MN 0:1L- oI (\ ',_ er CR I TI . :N .

I ' q ·.,i .!. T- ' '- RU5TRFIL I Rt\! j'\ 0 I· 11 ' . . •

.. c: i

. 0 .11_ r . . 0 t .. I

. ; ll. -, L I -I ,.... i .) ·t- , "--· I L1 + t l

..r·J l B. . ?.._ l ..,..T I , r! ! Ml\j ,. ,.__ I l ;-\ I I " SI -215-

-- ,. ... -. f"""""'!. -:- 7 I .- · ·-; l 1_ I I '- CRNROIRN f nUi Ii l

I ' TTI J. . V\ MG FE MN sr . ('Fl\.., • 'IPL NR I\

was . . d in ....L' ig.. 97 comp iled by the au t hor. from allData avalibl e literature. Figure 98.S;JLJ TH -216-

-j- (Dashed line shows Variation for Type A N.L. basalts) . '- i I -- " 0 + q i . .._, T . c::: J. /I .,I . ! T I . ? .1. . (Iu

' 2 . c . 0 TI I ! • 8 T

nw . +j •) A. ,. c:.. -!- i=- ;:.· . ,-·(' MG I L. MN I; TI· ING I I I

(0i'"tp•- I T -···- RII . . . '-·· rJ.. CJNI . . I I I J. : r_.:J 2 ·r " .. i ; I I r u '!" (") i , 0 t i ..l. . 6 I i ...... ') T . - I 0 ·-j . I } ·' I 4 r . ' r . '-" n () T.... n I . 0 . ·; t B. I,' MG t\ p SOUTH HFF1 I r\0'-'iqT' ' . I • 1 I Tr:--:l- '--'. -217-

·' ") . (Dashed line shows variation for Type c N.L. basalts) t] -- (J 0 ..:... 6 -4 2 ("_J -1 2 r Li _Lt <· v 8 I T 0 + I c. 2 +

FE· MN . .._)er - . co.,,; I FlL. TI NA }·\ p

10 RUSTRRL l f\OMRT I I TES

'"'•.J · -1- .,, I :-, I . u j· . ,_, .-. . • u.... ·· I

• . I '.:"°) t . -. ... .L 0 -1· ,-) / '-- Tt . I _.(.! t i 6 i n 0 ;-t 0 D • • ')c_ .I.

(''!! T- 0 -ii ., l v I MC Fr-·...... Mi'! ST..... -'- ... _ f\jP. ' \ -218- . sidering first the type A basalts (Fig. 98 a and b) the figure

reveals that these are higher in FeOT, CaO, Ti02 and P2o5 and slightly lm-ler in Alz03 for a given MgO content than the Australian

samples. In comparison to the South African komatiites, the type

A basalts have slightly higher FeOT ,·higher Ti02 and P2o5 contents,

but have similar CaO contents and CaO/Alz03 and A1 2o3 /Ti0z ratios. Based on these simila,riti.es to the South African komatiites, Schulz

and We;i.blen (1976) have previously suggested that these Newton Lake

basalts are members. of the high CaO/A,lz03 komatiite suite.

The type C Newton Lake basalts (including the chilled margin

samples) s.how greater similarity to the Australian komatiites (Fig. 98

d), having similar CaO/Alz03 and Alz03/TiOz ratios but slightly higher

FeOT, TiOz and PzOs and a greater range in MgO and Al2o3 • The greater range in MgO and Al203 reflects the more fractionatated nature of many

of th.e Newton Lake samples. Based on the similarity of type C Newton

Lake basalts to Australian Canadian) komatiites, it is concluded

that they belong to the low CaO/Alz03. komatiite suite of Schulz and

Weiblen (1976). Though many of the type C basalts have lower MgO and

highe.r Alz03 and TiOz than present komatiite definitions allow, their

coarse skeletal textures, s.;Lmilar composition to the gabbroic portions

of the komatiite s.i.lls and chemical continuity from high to low MgO

samples indicates that they are komatiites.

:t:.t should he noted that both t:yl>es of Nei;'(ton La,ke basalts a,re

characterized by higher FeOT, Ti02 and Pz05 than the reported komatiites

of Canada, Austnalia and South Africa. The presence of the same,

though less. pronounced, characteristics in the· Ely basalts suggests -219- these features may reflect a difference in mantle source composition from the other localities, rn this regard, it is interesting to nQte that some of the amphibolite enclaves found in the 3.5 b.y. gneiss terrane of the Minnesota River Valley also have high FeOT and Ti02 and low·Al2o3 compositions (Weib.len and others, 1976; Schulz and Weiblen, 1976). suggesting that similar were also produced at 3.5 b.y.

Very little trace element data is presently available for komatiites. Nes.bitt and Sun (;1976). have,however, presented some trace element and REE data for Australian peridotitic to basaltic komatiites and have shown that they have close to chrondritic Ti-Y,

Ti-Zr, Y-Zr and A1 2o3-Ti02 element ratios. The Newton Lake type C basalts, as shown in Figure 91, have close to chondritic Ti-Y and

Ti-Zr in high MgO samples but have lower ratios in the more fractionated basalts. The Y-Zr ratio is lower than chondrites (see Figure 91) due to slightly higher Zr contents for a given Y content. The type A basalts are also distinct from the Australian komatiites having lower Y/Zr and higher Ti/Y ratios reflecting their lower Y contents.

It should be noted that the type C basalts also have close to chondritic Alz03/Ti02 ratios in high MgO samples, but that the type A basalts have much. lower ratios (Fig. 85).,

High Iron Tholeiites and the Type A Basalts: Arndt (1976) and Arndt qnd others (1977) have identified a of htgh iron tholeiitic basalts associated with the komatiites of Munro Township (Ontario,

Canada). Naldrett and Goodwin 0.977) and Jolly (J.975) have shown -220- that compositionally similar basalts are abundant in the Abitibi greenstone belt. Naldrett and Turner (1977) have also noted similar basalts from Western Australia. Associated with the high iron thole- iittic basalts in the Abitibi belt, are thick differentiated flows with minor peridotite and abundant pyroxenite, norite and normal gabbro capped by high FeO-MgO hyaloclastite (Arndt and others, 1977).

These thick differentiated flows and differentiated sills like the

Dundonald sill (Naldrett and 1968) are suggested by Arndt and others (1977) to be petrologically related to the high iron basalts.

The normalized compositional variations of Munro Township samples

(Arndt and others, 1977) is in Figure 99.

Arndt (1976), Arndt and others (1977) and Naldrett and Goodwin

(1977) have used a plot of .Al o ·versus FeOT/(FeOT + MgO) to distin-- 2 3 guish between the komatiite series and high iron tholeiite series of the Abitibi greenstone belt. Figure 100 shows the data of Arndt and others (1977) plotted on thiS diagram. The high iron tholeiite series is distinguished by higher FeOT/.MgO ratios a:nd also higher Ti02 (Fig. · 101) than the koinatiite series. Note that the low A1 o ( 12 wt%) 2 3 samples of the high iron series· represent the cumulates and flow top units of a thick differentiated· flow (Theo's Flow) .

The Newton Lake samples are plotted in terms of Al versus 2o3 FeOT/ (FeOT + MgO) in Figure 102. The type C basalts (including most of the chilled margin samples) plot on the high FeOT/MgO side (komatiite side) of the boundry; They range from the komatiite field to the transitional basalt field at higher Al contents. The Newton Lake 2o3 type A basalts (including the chilled margin 'samples from the Cedar

Lake "sill") plot to the right of the boundry in Figure 102, falling -221-

H FE :6 99 • . 8 ,..... • D • L1 . ? . () . 2

I'\1G FE -MN SIL,,,,·•-rR C:L tr K -222-

16 Munro Township, Ontario

• Komatiite Flows 0

o Layered Komatiite Flow • • 0 0 14 Hi.gh Iron Tholeiites 00 0 0 • • 0 Layered Flow (High Fe) •• 0 0 • • C-0 12 • • 0 • 0 • 0 -0 ..... 0 ...... 3 0 • 10 0 0 0 0 N • • ••

.2 .4 .6 FeOT/(FeOT+MgO) Figure 100. Al o versus FeO /(FeO + MgO) for Munro Township 2 3 komatiites and liigh iron basalts. T -223-

14 / 0 I -P I 0 I I / . 0 ...-- / 0 0 12 e / 0 I / 2,- / t<) 10 I 0 C\.I I

Newton Lake Formation 6 Type A Basalts D 0 @ Chilled Margins 0 Munro Township, Ontario 4 0 High Iron Tholeiites o Layered Flow (High Fe) 2t

.4 .8 1.2 1.6 2.0 2.4 2.8 TiQ,., {wt%) L Figure 101. Al versus Ti0 for Munro Township in Newton 2o_1 Lake volcanic roe.Ks. Note the2 similarity between the Newton Lake type A basalts and the Munro Township high iron basalts.

r.·e/c) <."'cfores 1-( ...... v-o ft>W\o\$\..-.:.p ,'5 Tre.v-,v .<>f Alol.\-',b( ;,..cv-. ot .Jelly c :s r-c...t-:o OMd s..... "') /lil(t.J. -224-

I- .2 .4 .6 FeOrl(F eOr +MgO)

Figure 102. Al o versus FeOT/(FeOT MgO) for Newton Lake 3 + basalts and sills. -225- in the field defined by the differentiated flow and high rion tholeiites of Arndt and others (1977). As shown in Figure 101, the A1 o - 2 3 Ti0 variation of the type A basalts is also similar to the Munro 2 Township high iron basalts. Therefore, according to the criteria used by Arndt and others (1977) the type A Newton Lake basalts should be considered part of the high iron · tholeiite series and not as part of a kornatiite series.

The compositional similarity bet:Ween the Newton Lake type A basalts and the high iron tholeiite series of Arndt and others (1977) is clear

(compare Figures 83 and 99); however, the designation of these rocks as tholeiitic seems both inappropriate and misleading. As stated by

Arndt and others (1977), . the differentiated flows of this series are dorninanted by pyroxene fractionation as are the associated high iron basal tic flows. This agrees with the clinopyroxene rich nature of the

Newton Lake type A basalts. This is not a characteristic, however, shared by tholeiites, which generally are dominanted by olivine and plagioclase fractionation. The great textural diversity shown by the

Newton Lake samples is also not a typical feature of tholeiites. In particular, coarse skeletal textures involving clinopyroxene have not been reported from tholeiitic rocks.

Recent tholeiites plot in the transitional to high Al fields 2o3 on the A1 versus FeOT/(FeOT: MgO) diagram (Fig. 103). With dif- 2o3 ferentiation, these move downwards and to the right across the boundary and into the field occupied by th.e Archean high rion ·basalts. This trend is largely the result of plagioclase fractionation in the tholeiitic basalts. Picrites and ocean floor peridotes also tend to plot on the low FeOT/MgO ratio side of the diagram (Fig. 103). Also, -226-

0 0

a 12 0 -0 +-· .__.3 rn 10 0 C\J -..... • c:::r 8

6 Recent Tholeiites

0 0Ave. Ocean Floor Basalts DAve. Leg 37 (DSDP) Rocks 9 Differentiated OFB

@:Kilauea Picrites BB Baffin Bay Picrites T21-

.2 .4 .6

FeOT/(FeOT+MgO)

Figure 103. Al versus FeO /(FeOT + MgO) for recent 2o1 tholeiites. Ave, ocean floor tasalts from Melson others 1976; ave. Leg 37 (DSDP) rocks from Gunn and Roobal, 1976; differentiated OFB from Claugue and Bunch, 1976; Kilauea picrites from Gunn, 1971; and Baffin Bay picrites from Clarke, 1970. -227- cumulate rocks (not shoi;.ffi. in Fig. 103) from the classic tholeiitic layered intrusions Skargaard, Stillwater, etc.) plot in the komatiite field of the figure. Thus no recent tholeiites are found to plot along a line of increasing A1 o at the high FeOT/MgO ratios 2 3 shown by the Archean samples.

A further complication to calling the Newton Lake type A and Munro

Township basalt tholeiites is that high iron · basal ts fanned as a result of "normal" tholeiite fraction · (i.e., olivine-plagioclase control) are also present in the Archean greenstone belts. Jolly

(1975) has described a high iron tholeiitic suite from the Abitibi greenstone belt, which shows decreasing Al203 content with increasing

The· intermediate members of this series appear identical in major element composition to the high iron basalts described by Arndt and others (1977) as related to differentiated pyroxene rich flows in the :Munro Township area of the Abitibi belt. As described by Jolly (1975) the tholeiitic basalts of his series are characterized by an abundance of plagioclase and little or no pyroxene.

Thus, there appear to be at least two types of high iron .basalts, one derived by largely plagioclase controlled fractionation and the other by largely pyroxene.

It was already noted above that the Newton Lake type A basalts share many similarities with the South African (Barb.erton) komatiites.

Comparison of Figures 97a and 99 reveals that the high iron series of

Arndt and others (1977) share these same similarities. The South

African komatiites, though not showing the same degree of FeOT and

Ti0 enrichment also plot to theright of the proposed komatiite - 2 tholeiite boundary in Figure 102, further supporting the compositional -228- similarity between these basalt types (Schulz and Weiblen, 1976). It appears, based on composition, texture and geologic association: (high iron basalts as described by Arndt and others (1977) are only found associated with kornatiites) that these high FeOT - low Al o basalts 2 3 have greater similarities with komatiites than to tholeiites. It is, therefore, suggested that these basalts may represent a chemically distinct komatiite type related to the high Ca0/Al o ratio suite of 2 3 Schulz and Weiblen (1976).

Conclusion: The higher· MgO (least fractionated) type C Newton Lake basalts are chemically similar to Canadian and Australian basaltic komatiites, though having slightly higher Ti0 . The lower MgO samples 2 have more normal tholeiitic compositions but with lower A1 o and 2 3 ge.."Lerally lower FeOT/MgO ratios than · the Ely Greens tone basal ts.

Their skeletal textures and chemical similarity to the gabbroic portions of the kornatiite layered sills indicates, however, that they are also related to the k6matiite suite and represent more fractionated compositions.

The type A Newton Lake basalts have many chemical similarities to South African (Barberton) basaltic komatiites, but have higher

FeOT' Ti0 and P • They are also similar to the high iron basalt 2 2o5 suite identified by Arndt and oithers (1977) from the Ahitibi greenstone belt (Ontario, Canada). Because of their skeletal textures and non-tholeiitic chemical variation it is proposed that these basalts represent a chemically distinct kornatiite type. -229- PETROLOGY

Vermilion Cale-Alkaline Volcanic Rocks

The general similarity of the petrographic and chemical characteristics of the calc-alkaline volcanic rocks from the Vermilion district suggests they may have a cominon mode of origin. A significant point in this regard is the low Y content sho"Wn by these rocks.

Numerous models have been proposed for the origin of calc.-alkaline volcanic rocks (Boettcher, 1973). Models involving crustal contamination of basaltic magmas and anatexis of sialic crust have found little support and are not further considered. The two models which have received the most attention and support are (1) fractional crystallization of basaltic parents (Osborn, 1959, 1962; Cawthorn and O'Hara,

1976) and (2) partial melting of eclogite (T. H. Green and others,

1966; March, 1976) or wet mantle peridotite (Mysen and Boettcher,

1976; Yoder, 1969).

The presence of calc-alkaline basalts in both the lower Ely

Greenstone memher and the Newton· Lake Formation requires that a tional crystallization model be considered for the derivation of the more felsic samples. Osborn (1959, 1962) has discussed a fractional crystallization model for calc-alkaline volcanic rocks based on comparisons with experimental studies in the system MgO-Fe0-Fe o - 2 3 Si02 (Muan and Osborn, 1956). He noted that at high partial pressures of oxygen, the stability field for magnetite on the liquidus of that system was greatly expanded. Thus, the early precipitation of mag-

. I netite, rather than olivine or pyroxene at lower oxygen pressures, -230- would result in the accumulation of Si0 and depletion in FeO in the 2 . T residual melt. Osborn (1959) showed that the experimentally derived liquids had similar trends to the Cascade calc-alkaline volcanic rocks on plots of Fe)T/(FeOT + MgO) and FeOT versus Si02 supporting the magnetite fractionation model.

Figure 104a and b shows the Vermilion samples plotted in terms of the variable used by Osborn. It is apparent that they show a trend similar to the Cascade lavas and by analogy to Osborn (1959) to crystallization under high and nearly constant oxygen pressure. Though Os- · ·. born' s model is attractive in many ways, iittle evidence can be shown\ to support it (Boettcher, 1973). One problem, is that a considerable quantity of magnetite must be fractionated from basalt to derive an andesite or dacite. This produce abundant magnetite rich cumulates, however, none have been found in e±ther recent or Archean cal.c-alkaline areas. Furthermore, · magnetite phenocrysts are not commonly abserved in calc-alkaline basalts and none have been found in the Vermilion samples.

Recent experimental studies of natural systems cast further doubt on Osborn's model (Allen and others, 1972). The studies of Allen and others (1972) indicate that magnetite crystallized in more than trace amounts only from· a basalt parent at oxygen pressures greater than the hematiite-magnetite-water (HM) buffer. As oxygen activities greater than those of the nickel + nickel oxide + water (NNO) buffer probably do not occur in natural basaltic magmas (Boettcher, 1973), magnetite fractionation in the amounts required by Osborn's model is unlikely.

An alternative to Osborn's model has been proposed by Cawthorn ""'.231-

". l

VERMILION CALC-ALKALINE VOLCANIC ROCKS

14 A

T 'Tholelitc Trend 12 s--J A 1.hut.hn Volc'<1nlcs @ ®o C Valcan lcs IO

0 0 1 8 " I- 0 6 • Porphyrie s e Lnwer Ely r.u:e nstone 0 4 0 Uppet' tly Crrc:istonir: • • @ take ion • 0 • • 0 • 2 @ WU for::i:1t1on • • a •

1.0 B

.8 • + • c1 b .6 a

'b .4

50 55 60 65 70 l Si02 (wt%) .11 Figure 104. (A) FeOr versus Si0 • (B) FeO /FeO + MgO I 2 versus Si02. Tholei1te and Cascade trends lrolll.'-cJ'sborn, 1959. I Aleutian trend from Marsh, 1976. -232- and O'Hara . (1976). Their based on experimental studies in the system CaO-MgO-Al o -Si0 -Na o-H involves fractionation of 2 3 2 2 2o, amphibole. Experimentally produced amphiboles were found to be low in Si0 (38-42 wt%) with high Fe/Hg ratios. Fractionation of such 2 amphiboles from a hydrous basaltic magma would produce a trend of silica enrichment and iron depletion similar to that observed in calc-alkaline volcanic suites (Ringwood, 1974).

Evaluating the possible role of amphibole fractionation for the

Vermilion samples is difficult with the data presently available. In support of this model, is the presence of amphibole phenocrysts in the more felsic of Vermilion calc-alkaline rocks. The highly amygdaloidal and fragmental nature of many of these rocks further suggests high water pressures, which would also be compatible with the early crystallization and fractionation of amphibole.

The trace element data, however, provides a more effective test of amphibole fractionation. Amphiboles have solid-liquid distribution coefficients for Kand Rb of about 1.0 and 0.3 respectively (Arth, 1976).

Thus, during fractionation, amphibole would retain K in preference to

Rb resulting in residual melts with progressively lower K/Rb ratios.

Examination of the K and Rb contents of the Vermilion calc-alkaline samples reveals that they have low K; however, their K/Rb ratioss while variable, are not much lower than found in the tholeiitic Ely basalts. Though the possible mobility of both K and Rb in these rocks prec·ltides a definitive it would appear that amphibole fractionation alone could probably not . have produced the observed compositional range in the Vermilion calc-alkaline volcanic rocks. -233- This conclusion is further supported by the Y data for the Ver- milion samples. Lambert and Holland (1974) have shown that Y is strongly fractionated by hornblende, resulting in Y depleted residual liquids. Figure 105 shows the CaO-Y variation for three analyzed samples from the lower Ely Greens tone member. As revealed by the figure, the samples show increasing Y with decreasing CaO similar to . the Tongan lavas (Ewart and others, 1973). This trend is not compa t- ible with hornblende fractionation alone, but is compatible with plagioclase + pyroxene fractionation for these samples. Such a relationship between the lower Ely samples is further supported by their increasing Sr and Zr contents with increasing Sio . Thus, while 2 amphibole fractionation alone can not account for the compositional variation of these samples, plagioclase +pyroxene ·(+ amphibole ?) appears compatible with the data.

A model involving fractional crystallization alone can not, however, account for the Y depletion of the calc-alkaline basalts nor the depleted heavy REE's contents and low total REE of the Newton Lake dacite flows and Vermilion porphyries (Jahn and others; 1975; Arth and Hanson, 1975). These features suggest either amphibole and/or garnet involvement as these are the· only minerals known to. retain both

Y and the heavy REE' s. Arth and Hanson (1971) and Jahn and others

(1975) have proposed tha·t Vermilion porphyries and the Newton Lake dacites: :were derived· by partial melting of eclogite. VJhile REE data is not available for the calc-alkaline basal ts, their low Y contents suggest that they may also be depleted in heavy REE's and may have been derived by higher degrees of partial melting of an eclogite source. 12 I 35-29 I -0 10 -.;t -0 0 u 8

10 20 30 y (ppm)

Figure 105. Y versus CaO for lower Ely Greenstone volcanic rocks. I Note the similarity.between the Ely trend and that for Tonga calc- N w alkaline lavas (data from Ewart and others, 1973). I -235- Condie and Harrison (1976) have tested such a model for chemically

· similar Archean calc-aikaline basalts from the Midlands greenstone

belt in Rhodesia. They found that both the major and trace element

contents of these samples were compatible with about 50% partial

melting of ecologite having a composition similar to Archean tholeiitic

basalt.

The data for the· lower Ely Greenstone samples suggests that the

basalts were derived by pigh degrees of partial melting of an amphi-

halite or eclogite source but that the andesitic samples were derived

by fractional crystallization · of plagioclase + pyroxene + a high Fe/Mg

phase (amphibole ? , magnetite ?) • · Further data, particularly the

rare earth elements, are needed to evaluate whether the more felsic

rocks in the lower Ely Greenston.e member are also related to the

basalts by fractionation crystallization.

Conclusion

Arth and Hanson (1972) have presented a detailed model to account

for the origin of the Vermilion porphyries and other tonalitic rocks,

based in part on the model proposed by Hanson and Goldich (1972) for

the Saganaga Tonalite. This model, as noted above, involves the

partial melting of quartz eclogite or garnet amphibolite at a depth

greater than 30 km., with the parent assumed to have the composition

of Archean tholeiitic basalts. Such partial melting would leave a

residue consisting largely of garnet and clinopyroxene, with garnet

causing the derived liquid to be depleted in heavy REE's.

Barker and Arth (1976) have revised this model based on new REE

partition data for hornblende. They found that the REE partition -236- coef fic ien ts for hornblende/dacite glass are similar to those . for garnet/dacite glass. Thus, the REE patterns for equilibrated silicic liquids will be similar for hornblende and garnet fraction. Barker and Arth (1976) suggest that partial melting of amphibolite, having a composition similar to A.rchean metabasalts, would therefore, serve as a suitable parent for deriving tonalitic to trndhjemitic melts.

An amphibolite, as opposed to quartz eclogite, source is also supported by field observations. As noted by Barker and Arth (1976), eclogite and garnet amphibolite are apparently rare in Archean terranes.

Green (1975) has further suggested· that Archean geotherms may have been so steep that eclogite would not be stable. Even for slightly lower geotherms, however, the 15 to . 30% partial melting required to produce tonalitic-trondhjemitic melts would have occurred well before the parental amphibolites reached depths sufficient for garnet to form (Barker and Arth, 1976).

The model of Barker and Arth (1976), for the generation of tonalitic calc-alkaline magmas like those parental to the Vermilion dacitic porphyries and related pluntonic equivalents, may be summarized as follows:

1. Accumulaticin of a thick basalt · pile and metarnorphiSm of the

lower parts to hornblende + plagioclase .± quartz amphibolite. ·- 2. 15 to 30% partial melting of the amphibolite at less than

10 kb total pressure ( 40 km) producing tonalitic-trondhjemitic

melts; the resulting residue would consist largely of horn-

blende, pyroxene and minor magnetite and quartz.

Such melts would , on ascent, crystallize quartz (T. Green and Ring- wood, 1968) which would at lower pressures, not remain on the liquidus -237- and would be replaced by plagioclase. This would account for the common presence of partly resorbed quartz phenocrysts accompanied by plagioclase in the Vermilion tonalites. Higher degrees of partial melting could account for the calc-alkaline basalts and andesites found in the lower Ely member and the felsic portion of the Newton

Lake Formation.

This modal has important -.implications relating to the evolution of the Vermilion greenstone belt. Cale-alkaline igneous rocks presently make up only a small part of the Vermilion volcanic flows.

The dacitic composition of the Lake Vermilion and Knife Lake sediments

(Ojakangas, 1972; Morey and Schulz, 1977a and b) clearly indicates, however, that large of such magmas were generated and that significant calc-alkaline volcanism occurred.

Based on the presently exposed abundance of rock types in the

Vermilion district, -_it would appear that calc-alkaline volcanism and its' resulting products were as or more abundant than tholeiitic basaltic volcanism. It is clear, however, that genenation of large volumes of calc-alkaline magma requires a considerably greater abundance of basaltto form the amphibolite parent. This requires that a thick sequence of basalts existed below the presently exposed volcanic sequence to provide a source for the calc-alkaline magmas •.

It ma¥ be that the amphibolite bodies, present within the Vermilion and Giants Range batholiths, represent renmants of this sequence. The implication of the Barker-Arth model for the evolution of the Ver- milion greenstone belt will .be considered further in a later section. -238- Upper Ely Greenstone Member

The majority of the upper Ely volcanic rocks are tholeiitic in nature and show a pronounced iron enrichment trend (see Fig. 70).

This trend corresponds to a change from generally olivine normative basalts to quartz normative basalts and basaltic andesites. As was shown in Figure 73a and b, there is also a fairly systematic variation in major elements from the low FeOT/MgO, low Ti0 samples to the 2 high FeOT/MgO, high Ti0 This systematic behavior suggests 2 that fractional crystallization may be responsible for the observed range in bulk compositions.

Many of the bas·altic samples contain m:icrophenocrysts and occassionally phenocrysts of plagioclase (gnerally. An _ ). Olivine 80 60 pseudomorphs have been identified in a few basalt samples (Green, 1970), and plagioclase and clinopyroxene phenocrysts have been observed in the high Ti0 gray basalts from soth of Mud Lake. From the observa- 2 tional data, therefore, it appears that plagioclase and possibly olivine were liquidus phases in the low Ti0 basalts and plagioclase 2 and clinopyroxene in the high Ti0 basalts. 2

The phenocrysts and groundmass clinopyroxene of the high Ti02 gray basalts from }rud Lake were analyzed with the electron microprobe.

Representative compositions are given in Table 11. The phenocrysts generally s.how a slight zoning towards more iron rich compositions

(core Wo En Fs ; rim wo En ls ), while the groundmass clino- 38 49 13 37 4 16 pyroxene is markedly higher in iron (Wo En Fs The groundmass 37 33 30o. clinopyroxenes are also higher in Ti02 and MnO than the phenocrysts (Table 11). These features are similar to those generally observed -239-

Table 11. Representative Pyroxene Compositions 3asalt*, Ely

Core Rim Groundmass

s i02 51.88 51. 85 48.52

Al203 2.45 2.22 2.93 FeO* 8.28 10.41 18.02

HgO 17.41 16.97 11. 22

Cao 18.44 18.96 17.43

Na 2o 0.01 ND ND Ti02 . 0.36 0.33 0.89 HnO 0. 17 0.17 0.36

cr203 0.27 0.25 ND Total 99.27 101.16 99.37

Cations Based on 6 Ox y gens

Si 1. 924 1. 912 1 .889

Al 0.107 0.096 0. 135

Ti 0.010 0.009 0.026

Fe 0.257 0.321 0.586

Hg 0.963 0.933 0.651

Ca 0.733 0.749 0.727

Na 0.001

Nn 0.005 0.005 0.011

Cr 0.008 0.007

Tota l 4.008 4.032 4.025

Wo 38 37 37 En 49 47 33 Fs 13 16 30

* FeO as total iron. n All from CLEU-171-18 -240- in tholeiitic pyroxenes (Muir and Tilley, 1964). The significantly higher Fe and Ti content of the groundmass pyroxene indicates that significant titanomagnetite or ilmenite did not occur prior to crystallization of the groundmass

From the petrographic evidence, · it appears that and clinopyroxene should be the major phases controlling any fractionation of the upper Ely basalts, with olivine perhaps important in the early stages. The possible role of these phases in a fractionation model can be evaluated with the aid of Figure I Ob a, b and c. In the figure MgO, Cao and Ti0 have beenploti::ed versus the Al 0/Si0 ratio 2 2 2 following the method of Murata (1960) . . The advantage of this type of graphical approach is that it allows the compositional variations of the rocks to be examined in relation to variations in the proportions of the major minerals (Murata> 1960). This can .be most easily done when only one or two phases are controlling the compositional variations. If three or more phases are involved, such graphical approaches can not be unambiguously evaluated. In the Figure and member olivine (01) and plagioclase (An, Ab) have been plotted along with the phenocrysts (P) and groundmass (G) clinopyroxene composition from Table 11. Only the basalt samples analyzed during this study are plotted.

It is apparent from the figure that the samples define two trends, the one (A) showing a greater decrease in Al o /sio than the other 2 3 2 (B). From the MgO (Fig. 106a) and Ti0 (Fig. 106c) variation, it 2 would appear that either olivine + plagioclase or clinopyroxene + plagioclase fractionated in varying portions, could account for the compositd.onal changes. The CaO variation (Fig. 106b) revels, however, .. Figure 106. MgO versus Al 0 I SiO after Murata, 1960. OL Points labeled P and· G 2 in groundmass 30 I-A A pyroxene compositions from gray pillm;, basalt, upper Ely Greenstone (see Table 11) . Note two trends labeled A and B in figures below.

20 IS IOP 0 01 16 2 14 12 10 8 6 4 2 I N .2 .4 !=" .6 .7 An.a ·"' ...... I Al 20 3/Si02 Figure 106B. CaO versus

B 20 ' R.-----7An 18 G 16 I 14 r- 1 • 0 12 a •A'· u ·;/ 10 1: I. 8 ;: 6 4 2 I I I I I I .I .2 Ab , 3 .4 .5 .6 .7 .8

Al 20 3 /Si02 I N N I Figure 106C. Ti0 versus A1 /Si0 . 2 2o3 2

2.0 c 1.8 1.6 A\.\ I 1.4 C\l 0 1.2 I- 1.0 . 8 •• .6 .4 .2 ¥oL I I I I I . I .2 Ab.3 .4 .5 .6 I N .p. Al203/Si02 w I -244- that fractionation of clinopyroxene+ plagioclase ±olivine is the

most likely as olivine + plagioclase alone would result in increasing

or slowly decreasing Cao in the fractionated liquid. Thus the compo-

sitional variation shown by the upper Ely basalts is compatible with

fractionation of mainly clinopyroxene + plagioclase with a small

amount of olivine The two trends can be accounted

for by varying the proportions of· clinopyroxene and plagioclase frac-

tionated (i.e., trend A, Pl>Cpx; Trend B, Cpx>Pl).

These conclusions can be further evaluated with the aid of simple

subtraction calculations wherein a set composition and proportion of

phases are successively subtracted· from some initial composition. Sev--, ·

eral calculations of . this type 'ivere done using the average low Ti0 2 Ely basalt (Table 9) as the initial composition. The results are displayed graphically in Figure 107 using the normalization procedure

previously described. Tt is apparent from the comparison of Figure

107a and b with Figure 69 that the clinopyroxene + plagioclase frac-

tionation alone can not .account for the. observed compositional varia-

tion of the Ely basal ts. · For · P1 > Cpx, MgO is seen to increase along with iron during fractionation (Fig. 107a),, while Cpx >Pl results in

increasing Al o (Fig. 107b). The subtraction of 01 +Pl+ Cpx, how- 2 3 ever, gives a pattern similar to that of the Ely basalts (Fig. 107c).

In order to evaluate quantitatively, the degree fractionation,

the minerals involved and the relative importance of the fractionating phases, a linear least squares computer mixi_ng program designed by

Wright and Doherty (1970) was used". This program gives the calculated

percentate of a given differentiate that can be generated from a

given parental magma by crystal fractionation of calculated percentages ,.... p + DI RG c nr f-!i-. 0 i" n T r ;-! '..I ._, x' I L_ I I . I -r I ' ,, I .l l_. ' I -245- Figure 107. (60% plagioclase and 40% pyroxene) .

') ,,(. .. ,,, . '- .0 . ") • Lj ... ·.•: ...·.... . 6 ''i . 8 . 0 A • . 2 I•,., MG FE MN SI CR RL TI NR !\ p

CPX + PLRG FRRC .f IONRTIGN

(40% plagioclase and 60% pyroxene).

.o()

.2 . 0 i" ') . '- .-i .Lt .. I \; ..: .6 j ';i .u0 . 0 lt ) B. • L l

MG FE SI "L..R rLr'I TI + C:PX +

•" ) . L TI (55% plagioclase and 42% pyroxene and 3% olivine). 0 i ...... T.L b I I F..) ·Tr -,.I · 1 <- 0 ,...,: lI ·l- <-:: I Lj i ! i .... 0 + j_ . e i ("'> \ I I ,_, T . ') ;:... L c. ..I. . T 1'.iq I.• MC 'Er- MN SI CR •"IHL ! : .. ;\ F' ,-\

.·' -247- of phenocrysts of known composition. Owing to the variable effects

of alteration on the' Ely samples$ average compositions for high and

low Ti0 basalts (Table 9) were used in the calculations. The major 2 elements were weighted such that the presumed least mobile elements

(Al 0 , MgO and Ti0 ) contributed the most to constraining the 2 3 2 resulting solutions.

The selection of suitable mineral compositions presents the

greatest difficulty to applying the mixing program to the present

samples. The program has the option of calculating plagioclase and

olivine compositions from their end-members. Trial runs showed, how-

ever, that better solutions were obtained if the olivine composition was specified, thus only the plagioclase option. was used. The clino-

pyroxene compositions from Table 11 along with reasonable mineral

compositions selected from the literature were also used for the

calculations.

Three parent-differentiate pairs were tested with the mixing

program: (1) average -low Ti0 basalt-average high Tio basalt, (2) 2 2 average low Ti0 basalt-intermediate Ti0 basalt (sample Cl-6) and 2 2 (3) intermediate Ti0 basalt (sample CL-6) - average high Ti0 2 2 basalt. Several different mineral assemblages and compositions where used in testing each parent-differentiate pair. The best solutions

for each of these pairs is given in Table 12. All the solutions

involve olivine, plagioclase and clinopyroxene as major phases with

three solutions also including magnetite. The degree of fit between

the observed and calculated oxides, as indicated by the sum of the

s.quares of differences Table 12). is remarkably good considering ...... ___ -·------·

Table 12. Ely Greenstone Mixing Calculations.

la lb 2a 2b 3

Residual Liquid, % 56.7 48.2 7 8. 8 68.8 7 4. 7 Crystallized, % '• 3. 3 51. 8 21. 2 31. 2 25.3 Minerals Removed: Olvine, % 3. 6 6. 1 1. 5 2.8 1. 9 Plagioclase, % 2 3. 1 26.6 8.5 13. 7 1 7. 0 Clinopyroxene, % 15. 4 19. 1 9.8 13. 6 6. 5 Magnetite, % 1. 3 -- 1. 4 1. 1 01 Comp. Fo 69 69 81 81 69 Pl Comp. An 7.4 . 6 71. 5 100 7 9. 8 61. 5 Cpx Comp. Wo-En-Fs 3.7-47-16 37-34-29 37-47-16 37-36-27 38-46-16

Pl:Cpx:Mt:Ol 6.4 : 4.3 : 0.4 : 1 4.t. : 3.1 : 0 : 1 5.7 : 6.5 : 0.9 : 1 4.9 : 4.8 : OA : 1 8.9 : 3.4 : 0 : 1 0.0063 0. 001.3 0.0027 0. 0013 0.0107

Phosphorous Fractionation: Residual Liquid, % 45 71 64 Crystallized, % 55 29 36

Zirconium Fractionation: Residual Liquid , % 53 79 67 Crystallized, % 47 21 33

la and b - Average low Ti02 basalt to average high Tio2 basalt. 2a and b - Average . low Ti02 basalt to basalt CL-6. 3 - Basalt Cl-6 to average high Ti02 basalt.

I N .I:' 00 I -249- the uncertainties inherent in the calculations.

The involvement of magnetite in three of the mixing solutions

deserves further attention: When calculations were run for the

parent-differentiate pairs 1 and 2 above, including a very iron rich

pyroxene as well as magnetite;" the resulting solutions either used the

iron rich pyroxene alone (case lb, Table 12) or used both the pyroxene

and a smaller amount of ·magnetite (case Table 12). This bias in

the solutions for high iron pyroxene + magnetite suggests that some

high-iron phase must he involved. in the fractionation process. While

magnetite results in good. solutions, it is unlikely that pure magne-

tite would have crystallized· from magmas with Ely basalt compositions.

For example, ocean floor basalts typically have titanomagnetite rich

in ulvospinel molecule (Bence and others, 1975). Calculations made

using titanomagnetite have, however, yielded unsatisfactory results,

particularly for titanium.

Clague and Bunch (1975) have noted a similar bias toward high-

iron pyroxene and magnetite in model mixing calculations done for

differentiated ocean floor · basalts. They s.uggested that fractiona-

tion of small amounts .of. pyrrhotite (a conrrnon sulfide phase in sub-

marin e basalts) could account for the high Fe/Mg ratio of the cal-

culated pyroxenes. This su.ggestion ·is supported by the sulfide saturated

nature of submarine basaltic melts at liquidus temperatures (Mathez,

1976). Recent studies on sulfur content in Canadian Archean basalts

suggests that they were also · sulfur saturated at the time of extrusion

(Naldrett and Goodwin, 1977). Thus pyrrhotite fractionation at

. least in part, account for bfas toward high iron obtained in the

mixing calculation for the Ely basalts. · This can not be evaluated -250- further without more mineral data and a better understanding of the effects of alteration on these samples.

The results of the mixing calculations suggest that the high

Tio basalts may be derived from the low Ti0 basalts by about 50 to 2 2 60% fractional crystallization of olivine, plagioclase and clinopyroxene. It should be noted that the result obtained in going directly from the low Ti0 basalt to the. high Tio basalt (cases la and b, 2 2 Table 12) is essentially the same as the result utilizing an intermediate composition (cases· 2a and b and 3, Table 12). This supports the conclusion that sample CL-6 represents intermediate composition in the fractionation sequence even though it is slightly anonalous in ilL'ori con tent (see discussion· above, · p. 158).

The percentage of crystallization indicated by the mixing calculations may be checked using the available trace element data. .Anderson and Greenland (1969) have shown_ that any element which is largely excluded from minerals aud enriched in the liquid can serve as an approximate indicator of the amount of. crystallization. They proposed that the ratio parental magma/P in residual 2o5 liquidU could be used for this purpose as phosphorus has very small distribution coefficients (P in mineral/P in liquid) for olivine,

·plagioclase and augite (all ( 0. 04). Zicronium, which is also a highly incompatible element, can be used in a similar manner. For the case where th.e distribution coefficient approaches zero, the equation des- cribing trace element behavior during fractional crystallization CL CL,..,_ 1 reduces from! (Arth, 1976), where Ci to Ci - F' F' is the fraction of liquid remaining, C. is the· concentration in ·.the ]. -251- original melt, CL is the .concentration in the differentiated liquid.

Thus, the concentration of an element depends only on the extent of

solidification.

Both phosphorus and zironium fractionation · ratios· were calculated

for the three parent-differentiate pairs (Table 12).. The amount of cr¥stal fractionation calculated. by the two ratios for each parent- daughter pair are similar and compare well with the results calculated using the" linear least squares" computer mixing program.

Arth and Hanson· (197 5) have ·done similar calculations using the

REE and have found that the rare earth patterns predicted for 40-60% crystallization. of equal amounts of" clinopyroxene and plagioclase were in close agreement with the. observed patterns. These results are in excellent agreement with those obtained above, except that olivine fractionation was not indicated. by the REE data. Olivine, however, does nbt ·fractionate the REE and considering the small amounts indicated by the mixing calculations, would not have a sigriificant effect on the calculated REE patterns. The results obtained are, therefore, considereCl• compatible 'tvith those of this study .

. From the observations presented above, it may be concluded wi.th a fair degree of confidence that the high Ti0 basal ts could represent 2 the residual liquids formed by so...:60% fractional crystallization of olivine, clinopyroxene and plagioclase from the low Ti0 basalts. 2 Considering the degree of fractionation indicated for the Ely basalts, it is surprising that more intermediate compositions are not represented

(only sample CL-6 is intermediate in FeOT/NgO and Tio2). If this is not due to a sampling bias (considered unlikely), the lack of intermediate compositions places an important constraint on the magmatic -252- evolution of the upper . Ely volcanic rocks. Furthermore, if the

presently available analyses . of. the. upper Ely member are truely

representative of the unit as a whole, it would appear that the

fractionated (i.e., high Ti0 ) . basalts -are more abundant than the 2 low Ti0 2 In the discussion· above, · it has been shown that the low Ti0 2 basalts may be parental to the high Ti0 basalts . The question re- 2 mains, whether the low Ti0 basalts are truely primary melts 2 from the mantle or are themselves fractionated from some more primi- 2 tive liquid. Studies of .Mg/ + Fe +) ratios (Mg values) in basalt

(Green and Ringwood, i967; 1970, 1971; Nicholls, 1974) and 2 the partitioning of and Fe + between olivine and basaltic liquids

(Roeder and Emslie, 1970; . Nicholls;. · 1974; Mysen, 1975) indicate that

liquids in equilibrium with olivine of supposed upper mantle materials 2 (eg., Mg/(Mg + Fe +) = 0.84 - 0.94; Fujisawa, 1968) must be highly 2 magnesian (Mg. (Mg + Fe +) 0.61). Green and others (1974) have noted

that betwen 20 and 30% partial melting of a peridotite mantle (assuming 2 pyrolite composition; Mg/ (Mg + Fe +) 0. 90) will, therefore, generate

liquids having Mg values equal to 0.70 + 0.02 (assuming

- liquid= 0.30; Mg Mg Roeder and Emslie, 1970). The subalkaline ocean floor basalts, which

form textural and trace element chemistry are considered the most

primitive (Frey arid others, 1974), have Mg values that converge on

O. 70 + 0. 02 and have liquidus phases supporting these conditions "

A frequency . diagram of · Mg values for the upper Ely basal ts is

given in Figure 108. Before calculation of the Mg value, ferrous iron

in all analyses was arbitrarily adjusted to FeO = 0.85 x FeOT, a ------··-- ······--·- ·-

6 >-0 z w :::> 4 0w 0::: LL 2

.6 .55 .5 .45 Mg/( Mg+Fe 2+)

2 Figure 108. Atomic Mg/(Mg + Fe +) frequency diagram for upper Ely Greenstone basalts.

I . N \JI w I -254- relationship similar to that determined for basal tic to basal tic

andesite liquids equilibrated at wUstite-rnagnetite buffer oxygen

fugacities (Fidali, 1965; Nicholls and Whitford, 1976). Similar

oxygen fugacities are believed to prevail in the upper· mantle (Boett-

cher, 1973). It is apparent from Figure 108 that all the upper EJ.y

lavas have Mg values to low «0. 59) for them to have been in equili-

brimn with mantle all.vines (Mg values) 0.84). This suggests that none of the analyzed Ely basalts represent primary mantle liquids and

that fractionation of some ferrornagnesian phase probably occurred

after separeation from the mantle residuum. This holds, however,

only if the Archean mantle had the· same Fe/Mg as that proposed for

the· present mantle. ·

The simplest model that may be tested for the derivation of the

low Ti0 Ely basalts would involve olivine as the major crystalline 2 phase fractionated from some initial mantle liquid. Nicholls and

Ringwood (1972, . 1973) have shown that silica saturated basaltic magmas ·

produced by melting of hydrous peridotitic mantle beneath island arcs are likely to precipitate olivine as they move to the surface.

The occurrence of olivine-poor basalts associated with abundant basal-

tic andesites in a number of island arcs is often given in support of

such a model (Nicholls and Whitford, 1976).

Nicholls and Whitford (1976) have examined the importance of olivine fractionation in basaltic lavas from th.e Sunda arc, in the

islands of Java and Bali. They found that less than 15% addition of olivine ranging from Fo to Fo was required to bring the island 82 87 arc basalts into equilibrium with mantle peri dotite (i.e., Mg values of .0.61 - 0 . 69). Similar calculations for the Ely basalts suggest -255- tha t the addition of 15 to 20% olivine would be required for the liquid

to be in equilibrium with mantle peridotite (caluculated Mg values range from 0.68 to 0.70).

The resulting liquid compositions for 15 and 20% olivine addition to the average low Ti0 Ely basal t are presented in Table 13 (col. A 2 and B). While the simple addition of olivine is probably an over- simplification . of the natural the resulting compositions closely resemble, except for FeOT and A1 o , those calculated by 2 3 Nicholls and Whitford (col. E, Table 13), as well as primary ocean floor basalts (coL F, Table 13), and olivine tholeiites experimentally produced by partial melting of pyrolite (Green, 1973; Green and others,

1967; also table 13, coL C and D).

Experimental studies on modelmantle compositions (Green and

Ringwood, 1967; Green, 1971, 1973; Kushiro, 1972) reveal that olivine normative tholeiitic magmas may be· generated by between 20 and 30% partial melting at depths· ranging from about 80 km (water undersaturated) to 30 km, depending upon their normative olivine content. Depending largely on the subsequent depth of equilibration, these melts may have .low· A1 o or alkalic characteristics (T . Green and others, 1967). 2 3 Green (1971) has summarized these results in a petrogenetic grid for mantle-derivedbasaltic magmas which suggests that for tholeiitic magmas produced by high degrees of partial melting ( 30%) .only olivine and orthopyroxene would remain in the residue. Also; at such high degrees of melting, magmas generated from a hydrous mantle source would not differ substantially from those developed from an anydrous source .1971).

Applying the results of the experimental studies to the Ely Table 13. Primary Basaltic Liquids.

A B c D E F

.Si0 2 48.70 48.29 48.5 49.7 l19. 1 50.0 Al203 13.69 13.04 12. 2 15.9 17. 2 17. 4 Fe OT 12.09 12.17 9. 6 8.5 9. 7 8.22 MgO 12.20 13.82 11. 3 l 0. 5 10.3 9. 81 Cao l 0. 16 9.68 10. 7 10. 7 9. 7 12.9 Na 2o 1. 89 l. 80 2. l 2. 6 2. 6 2.28 K2o 0.31 0.30 0.5 0.2 0.3 0.02 Ti02 0.75 0. 71 2. 7 1. 6 0. 9 0.79 P205 0.04 0.04 NnO 0. 16 0. 16 0.2 0. 2 0. 2 0.13 Mg value 0.68 0.70 0.72 -- 0.69 0. 71 Or l. 82 l. 7 5 3.01 1.17 1. 7 5 0. 11 Ab 16.84 15.95 19.18 23,05 23.02 19.92 An 27.74 26.27 22.78 30.74 33.90 3 6. 18 Di 17. 61 16.70 24.99 17.35 10.85 20.87 Hy 12.20 11. 4 7 12.26 8.75 . 7.48 5.07 01 22.67 26.80 13. 9 6 16.74 21. 7 7 16. 77 Im 1. 04 0.98 3.83 2.20 l. 24 l. 07 Ap 0.08 0.08

A and B - Ely low Ti02 basalt, A, +15% 01Fo 85 ;· B, +20% 01Fo 87 • C - Green, 1973, 27% melting of pyrolite at 20 kb. D - Green and Green and Ringwood, 1967, olivine tholeiite II. E - Nicholls and Whitford, 1976, calculated primary liquid, Galunggung volcano. F - Frey and others, 1974, primary ocean floor basalt, Leg. 3, DSDP.

I N V1 °'I -257- basalts suggests that the primary magmas were most likely produced by 25 to 30% partial melti_ng of mantle peridotite at .depths of about

30 to 40 km under water undersaturated conditions. Studies by Green and Ringwood (1967) show that such liquids, in equilibrium with olivine and enstatite, would contain about 20% normative olivine, 12-14% hypersthene and which are essentially the. characteristics of the calculated primary Ely basalt (Table 13, col. A and B). The

REE data of Arth and Hanson· (1975) for the Ely basalts also suggest about 25% partial melting for the primary magmas, supporting the conclusion above. If Archean geothermal gradients where significantly higher (Green,. 1975) than shallower depths for · partial melting may be possible.

The generally saturated to quartz-normative nature of the Ely basalts requires that the initial magmas fractionated at depths of less than 15 km (T. Green and others, 1967). At such shallow depths, fractional crystallization of initially olivine, followed by olivine + plagioclase + clinopyroxene could produce the observed compositional variation in the Ely basalts .

.T£le interpretation of the observational data on the Ely volcanic rocks provides a basis for elaborating on the above petrologic model.

The important obsenrations.·:are:

1.) Shallow-level crystal-liquid fractionation, involving olivine,

plagioclase and clinopyroxene, ·can relate the observed basalt

compositions.

2.) Intermediate compositions between the apparent parental low

. Ti0 basal ts a"nd the· differentia.ted high · Ti0 basalts are 2 2 very rare. -258- 3.) The high Ti0 b.asal ts are apparently 2 volumetrically more abundant than the low Ti0 basalts. 2 4.) Both high and low Ti0 basalt types are apparently inter- 2 layered throughout the upper Ely sequence.

Intuitively, one would expect crystal-liquid fractionation to pr_oduce ,an extended range of liquid compositions such that sampling of a sequence of flows would define a continuous liquid line of descent. Therefore, . given the validity of obser\ration (1) above, observation (2) implies· that the compositional variation of the Ely

Greens.tone basalts was affected by processes which restricted the range of observed compositions.

The complex interplay of geologic setting and tectonic evolution might provide restrictions on the continuous tapping of progressively differentiated lavas from a fractionating magma chamber. For example, detailed studies of the· magmatic processes at Kilauea Volcano, Hawaii

(Wright and Fiske, 1971) suggest that fractionated tholeiitic basalts develop preferentially in reservoirs that are relatively far from the main eruptive vent and are infrequently Magma stored in the suumit reservoir at approximately 3 to 5 km depth, differentiates by olivine crystallization and summit lavas olivine controlled compositions (Wright and Fiske, 1971). In contrast, the lavas erupted from shallow (approximately 1 km) chambers along rift zones far from the central cauldera, are highly fractionated, showing olivine, plagioclase, and cl:i.nopyroxene controlled compositions (Wright and Fiske,

1971). The magma in these shallow rift reservoirs apparently ates over a period of several years and is forceeLout the chamber

(erupted) by the subsequent injection of a new magma batch from the -259- summit reservoir. During .replenishment, mixing of the new magma with the differentiated melt may occur; resulting in hybridized lava compositions (Wright and 1971). Thus the tectonic setting in Hawa±i is such that only two contrasting tholeiitic lava suites are produced from one magma

Recently, Clague and Bunch (1976) have suggested that a model similar · to that for ·. the Kilauea volcano may also account for the formation of differentiated basalts along mid-ocean ridges. Based on mass ·balance calculations, · they suggest that some ferrobasalts may represent residual liquids formed . by greater than 74% fractional crystallization of "normal" ocean floor tholeiites. This indicates that these magmas must have resided for long periods of time in shallow chambers beneath the ridge . .

Is it possible that the. upper Ely Greenstone basalts provide a record of a similar magmatic evolution related to a unique geologic and tectonic setting ? Incorporating the conclusions drawn in this study with the general model of tholeiitic fractionation described by Wright and Fiske ·(1971) s.uggests the following model for the evoli,1tion of the upper Ely Greenstone basalts (Fig. 109).

1.) Generation of an olivine tholeiite magma by 25 to 30% partial

melting of. water undersaturated mantle peridotite at 30 to

40 km depth. ·

2.) Ascent of the magma to · shallower· depth, accompanied by olivine

and olivine plus plagioclase fractionation and lowering of

the .Hg 1va lue o;f O:the melt.

3.) Extrusion of this magma in limited quantities into a -260-

Figure 109. Magmatic model for the upper Ely Greenstone (adapted from Wright and Fiske, 1971).

LOW Tio2 Summit Basalts HIGH TiOz Lavas

OI.+ PL+CPX // ... ""l Km Rift . / Reservoir / R1It _p ee a.e:r . A . Svstem / / ..

Shallow Central ...... 5 Km Magro.a Reservoir _ :,__:/

Surnmi t Feeder System

Zone of Pa.rtial .-.35 Km Melting -261- aqueous environment (this stage would be represented by. the

low Ti0 basalt summit eruptions (Fig. 109). 2 4.) Continued fractionation of the remaining magma in one or

more shallow chambers, with plagioclase, clinopyroxene and

lesser olivine and sulfide (?) as fractionating phases.

5.) Subsequent eruption of the differentiated magma from the

shallow chambers possibly accompanied by injection of new

magma ba.tches. · (ThiS stage represented by the high Ti0 2 basalt rift lavas (Fig. 109).

6.) Continued .periodic eruptions of-both high and low Ti0 2 basal ts through time. ·

This model does not require or imply that the upper Ely Green- stone basalts evolved in geologic and tectonic settings identical to those suggested for recen:t basalts, but only that similar tectonic factors (namely a tensional environment) may have helped control the range of evolved compositions. if a modern analogue is sought for the upper Ely basalts, the fact that they (1) overlie calc-alkaline vo1canic rocks lower Ely Greens tone member) and (2) have intercalcated ca.le-alkaline flows and volcaniclastic rocks and iron- formation suggests an evolution - in an environment similar to that described for the island of Viti Levu, Fiji (Rodda, 1967) or Talasea

Volcano, New B.ritain (Lowder and Carmichael, 1970) may be applicable. -262- Newton Lake Formation

Pyroxene Chemistry

Although the Newton Lake basalts consist of low grade greenschist facies mineral .relict clinopyroxene was found preserved in several samples. The ' pyroxene compositions were determined for seven samples by using the electron microprobe. Representative analyses · are presented in Table 14 and shoi:m in the pyroxene quadrilateral in Figure 110'. The pyroxene trend determined for the Newton

Lake layered sills (Schulz," 1974) is shown for comparison. As shown by. Figure llO, the' pyroxene from. samples ENL-30-9, ENL-63-27 arid ENL-

46,-16 (Table 14) have iron enrichment trends,: with samples ENL-46-16 and .ENL-63-27 showing increasing CaO and

The Ti-Al variation for · the analyzed pyroxenes is shown in

Figure ll l. The pyroxene phenocrysts from sample ENL-30-9, a calc- alkaline pyroxene-plagioclase phyric basalt from the .Cedar Lake area, have lower Ti/Al ratios . (0.05) than the basaltic .komatiite samples with the exception of sample ENL-46-18. These phenocrysts show a trend of decreasing Al at .almost constant Ti while the groundmass pyroxene-has significantly higher Ti but similar Al to the rim composition of the phenoci:ysts. The· decreasing Al correspo1i.ds to a slight increase in the Fe/Mg ratio of the phenocrysts,with a much higher ratio for the groundmass pyroxene (Fig. 112a and b) . . The marked difference between the phenocryst rim composition and the groundmass pyroxene the phenocrysts did not. equilibrate with the groundmass (residuai liquid) bulk This disequilibrium probably resulted during the' extrusion and rapid crystallization of the Table 14. Representative Pyroxene Analyses, Newton Lake nasalts •

. Sample: ENL-ll4-54A El\1..-63-27 ENL-30-9 ENL-46-18 ENL-46-16 E-151A SSX-358-2 Nw;iber 1 2 3 . 4 5 6 8 9 io ll 12 13 14 15 16 17

5102 52 .01 51.81 51.85 51.49 53.25 51.27 51.09 51.19 51.,00 51.67 51.54 52.80 53.40 53.20 54.10 52.70 53.50 .u203 2.92 2.54 3.18 2.46 2.87 2.84 2.49 2.47 4.47 2.92 2.76 2.34 0.75 0.24 0.35 0:26 FeO* 6.69 6.95 B.15 7.34 8. 70 9.36 . 14.55 16.99 6 •. 9.04 12.21 5.43 7.80 12.70 7.13 11. 60 10.90 l{gO 18.13 17.34 16.92 17.26 16.67 16.57 13.60 12.50 16.78 16.84 15.36 18.10 15.20 10.90 13.90 13.20 13.60 Cao 19.71 19.80 19.71 19.82 19.73 17.87 17 .1,4 17.56 19.83 19.05 18.75 20.00 22.70 23.00 23.50 20. 70 21.40 N::i 0 0.15 0.29 0.03 2 Ti0 0.43 0.47 0.50 0.40 0.43 0.45 0.63 o. 72 0.37 0.36 0.65 0.18 0.14 0.10 0.17 0.11 0.04 2 11\0 0.16 0.31 0.33 0.11 0.29 0.28 0.35 0.22 0.12 0.20 0.05 0.19 0.19 0.30 0.24 0.20 0.48 0.30 0.11 0.14 0.13 0.02 o·.01 0.56 0.15 0.01 0.96 0.211 0.30 0.08 cr,.o3 Total 100. 53 99.52 100.75 99.02 lPl.65 93.78 100.10 100.79 99.80 100.15 101.48 99.86 100.18 100.25 100.25 99.49 100.01

Cations Based on 6 Oxygen Si l. 899 . 1.915 1.900 1.915 ).942 1.918 · 1.928 1.908 1.876 1.909 1.905 l.930 1.980 2.012 l.999 l.994 2.005 Al 0.126 0.111 0.137 0.108 0.124 0.126 0.111 0.111 0.194 0.127 0.121 0.101 0.033 0.011 0.034 0.016 0.011 Ti 0.012 0.013 0.014 0.011 0.012 0.013 0.010 0.021 0.010 0.010 0.018 0.005 0.004 0.003 0.005 0. 003 0.001 Fe 0.205 0.215 0.250 0.228 0.265 0.293 0 .460 0.540 0.202 0.279 0.377 0.166 0.242 0.402 o. 220 0.367 0.342 Hn 0.005 0.010 O.OLO 0.003 0.009 0.009 0.011 0.007 0.004 0,006 0.002 0.006 0.006 0.009 O.OOB 0.006 ?-jg 0.987 0.955 0.925 0.957 0.906 0.925 0.765 0.708 0.920 0.927 0.846 0.986 0.840 0.614 0.765 o. 71,4 o. 759 Ca 0.771 o. 784 0. 775 o. 790 0.771 0.717 0.706 0.715 0.782 0.754 0. 7113 0.783 0.902 0.932 0.930 0.839 0.359 Na ------0.011 0.021 0.002 Cr 0.014 0.009 0.003 0.004 · o. 004 0.001 0.016 0.004 0.028 0.007 0.009 0.002

Total 4.019 4.012 4.014 4.016 . 4.020 4.005 3.998 11,014 4.007 4 .014 4.016 4.001 4.008 3.980 . 3.981 4.001 3.998

\.lo» 39 40 40 40 40 37 36 36 41 39 38 40 46 48 118 43 44 Fn 50 49 47 48 47 48 40 36 48 47 43 Si 42 32 40 38 39 Fa II 11 13 12 14 15 24 28 11 14 19 9 12 20 12 19 17

*Fe 0,23 0,31 0.14 0,36 0.22 0,'.13 Fe+ Mg 0.17 0.18 0.21 . 0.19 0,23 0.24 0.38 (,1,43 0,18 0.22 0.31

I *Atomic units. N 9 E core; 10 • rim; 11 • groundmnss. w "'I Di Hi .. e ENL-114-54A oof 0 ENL-30-9 0 0 ENL-46-.16 . i(J 13 @ Q "'\ )/- ENL-63-27 . D SSN-3,58-2 • E-151A * ENL-46-18

Figure 110. Pyroxene quadrilateral diagram for analyzed Newton Lake samples. I (Solid line shows trend of pyroxene composition in Newton Lake layered sills, N Schuls, 1974), °'I * .024 •

.020 * •

(f) . 016 -0 lO ...... • I- .012 0 ""---<> -- .---6 0 .008 • ENL-114-541\ 0 ENL-30-9 • 0 ENL-46-15 0 ** -le ENL-G3-27 .004 D SSN-358-2 • E-l51A *ENL-46-18

I I I I I .04 .08 .12 .16 .20 .24 Al/60 1s I Figure. 111. Ti ·versus Al for analyzed pyroxenes. (Solid line for sample 0\ ll1"' ENL-30-9 contents core, rim, and groundmass compositi.ons.) I A. . 24

.20 o,o 0

.16 _J 0 \ •R <( - -- 0 •!\ ,,, --*--==o - \ ,,... .12 'oc ,,.. • Q - \ \ .08 \,

.04 o· . 0 Do8 O 0 I

.I .2 .3 .4 .5 Fel Fe+ Mg (pyroxene)

Figure 112. Al versus Fe/Fe+ Mg for analyzed pyroxenes. (Lines connect core, I N Q'\ rim and groundmass compositions for samples ENL-30-9 and compositional zones in Q'\ pyroxene phenocrysts in sample ENL-114-54A. I -267-

.024 0 ENL-114-54A 0 ENL-30-9 .022 0 ENL-46-16 -i< ENL-63-27 0 SSN-358-2 • E-151A .020 * ENL-46-18 .018 */ .016 I

fl .014 J • I J / -*

.006 • • * • 0 .004

.002 0 0 0 B. .10 .20 . .30 .40 FeO/ FeO+MgO (pyroxene) Figure 112. Ti versus Fe/FeO + MgO. (Lines same as for Figure 112A). -268- lava. The pyroxene compositions, in conjunction with the petrography,

suggest the following crystallization model for this sample:

1.) Crystallization of . pfagioclase and clinopyroxene at depth

with increasing plagioclase crystallization depleting the

melt in Al o and enriching it in FeO. This change in melt 2 3 composition is reflected by the slight increase in Fe/Hg

ratio and decreasing Al of the pyroxene phenocrysts from

core to rim.

2.) Rapid ascent and extrusion , of the magma into a subaqueous

environment resulting in rapid crystallization of the ground-

mass and diseqidlibrium between the phenocryst and ground-

mass

The pj.roxene compositions of sample ENL-30-9 are similar to calc- alkaline pyroxenes described by Heming (1977) and Ewart (1976),

for which similar crystallization models have been described.

The basaltic komatiite pyroxenes have variable Ti and Al con-

tents and can be divided into three groups: (1) a high Al group

(samples ENL-114-54A and ENL-63-27); (2) a low Al group (samples

SSN-:358-2, E-151A and ENL-46-16; and (3) an intermediate group

(sample ENL-46-18). The relatively high Al pyroxenes are found in both type A (ENL-114-54A) and type C (ENL-63-27) basalts. Sample

ENL-114-54A has hollow cored to slightly skeletal clinopyroxene phenocrysts (see Fig. 28) showing a variety of zoning patterns. As revealed by Figure 112a and b, these phenoci:ysts have slightly iron

enriched composition, but show no systematic variation in Al or Ti.

The unsystematic nature of these variations may reflect differences

liquid composition and diffusion rates established adjacent to -269- the rapidly growing crystals. In contrast; the other high Al pyroxene

sample (ENL-63-27,; see.Fig. 40) shows. very regular variation in

both Al and Ti with increasing Fe/Mg ratio (Fig. 112a and b). Alum-

inum decreases slightly from core to rim and Ti increases markedly.

This same variation .with increasing Fe/Mg ratio, · is shown in the compo-

sitional variation of the type C basalts which s.uggests that these

pyroxene compositions may reflect changes in bulk composition with

crystallfaation. If this is the case; the systematic increase in Ti

arid the Fe/Mg ratio in the pyroxenes and whole rock compositions

indicates that a high Ti bearing phase such as ilmentite did not

crystallize from the melt.

In an earlier section. (p.\95"). it was shown that the type C

basalts have linear Ti-Zr and Ti-Y ratios· (see Fig. 9la and b) which

intersect. the Ti axis rather than passing through the origin. This

suggests :that titanium did not behave in an incompatible manner, but

was controlled by some phase or . phases. The titanium content of the

core pyroxene in sample ENL-63-2T . (Table 14, analysis 6) is about

2700 ppm and corresponds approximately to the point of intersection

for the whole rock Ti-Zr and Ti-Y trends. The simultaneous increase

of Ti in the basalts and the pyroxenes suggests that pyroxene crystal-

lization may explain the incompatible behavior of . Ti in the type C

basalts.

The intermediate Al pyroxenes (Fig. 112) occur as abundant euhedral

. phenocrysts in a type A basalt (sample El).'L-44-18). Microprobe scans

across several grains revealed no detectable zoning. These pyroxenes

have the highest Fe/Mg ratio of all the analyzed pyroxenes, reflecting

the high magnesium content of the crystallization melt. The lack of -270- zoning · in these pyroxenes suggests that they crystallized under near equilibrium conditions, with their low Ti content reflecting, at least in part, the low titanium content of the original melt.

The low Al pyroxenes are from two type A basalts (samples

E-,151A and ENL-46-16) and a · chilled margin sample · (SSN-358-2). In samplesE-151A and SSN.:..358-2 the remnant pyroxenes are found in the cores of highly skeletal crystal· which · are now . largely replaced by amphibole. · In sample ENL.:...46-16, the pyroxene occurs as small remnent grains within euhedral amphibole pseudomorphs of clinopyroxene (see

Fig. 25). The remnant pyroxene in these three samples is much smaller in size (S0.1 mm) and generally intimately mixed with amphibole as opposed to the larger ( 0.2 mm) and less replaced pyroxenes discussed above. Compositionally, these pyroxenes are distinct from those discussed above, having not only lower Al, bµt also lower Ti and much higher Ca (Tab1e 14). They also tend to have higher Fe/Hg ratios than most of the higher Al pyroxenes> These compositional features are not those typically found in normal basaltic .igneous pyroxenes

(Vejnar, 1975). Some clinopyroxenes from alkali basalts do have similar calcium contents;. but tihey are much higher in aluminum and titanium . than the . Newton . Lake calcic pyroxenes. Similar co·mposition · pyroxenes have been reported, from recrystallized. and spilitized gabbros (Smith, · 1970; Vejnar, 1975).

Several features of · the· calcic-low Al Newtori Lake pyroxenes suggests that their compositions are the result of re-equilibration during metamorphic

l.) These compositions were only found in samples where pyroxene

was mostly replaaed by amphibole. -271- 2.) No zoning was found in the calcic pyroxenes. Samples ENL-

46-16 has two< distinct ·1ow Al pyroxene compositions (Table

14), no intermediate compositions were found and

each appears compositional homogeneous. The high iron

pyroxene grains are closer to the edges of the pyroxene

phenocrysts and may reflect original zoning.

3.) Texturally similar samples · from the Newton Lake (compare

ENL-46-18 and ENL-46-16), which are not as altered, have

more normal .igneous pyroxene compositions.

4.) The low Al-Ti and high Ca compositions are similar to

metamorphic pyroxenes (Vejnar, 1975).

It is concluded that aluminum and titanium may have diffused out of the pyroxenes during metamorphic re-equilibration with amphibole.

The exact nature .and conditions under which this re-equilibration and replacement of the pyroxenes occurred is not determinable without a more complete examination of the phase chemistry (particularly the amphibole) of these samples. Such a study was beyond the scope of this investigation and is .left for others to pursue.

Petrogenesis of the Newtbri Lake Basalts

Type C Basalts:

It will be shown in this section that the type C basalt compositions can be related by near surface fractionation of pyroxene and plagioclase, as they follow a well defined trend of decreasing }IgO, CaO and AI with increasing FeOT, Ti0 and MnO. The 2o3 2 pyroxene compositions discussed· above and the linear variations of -272- the incompatible trace elements support a relationship by crystal- liquid fractionation.

Assuming that crystal-liquid fractionation can account for the ob.served compositional trends' it is necessary to establish the nature of the fractionated phases and of the primary parental magmas from which these rocks were derived. The layered sills, with gabbros similar in composition to the type C basalts, provide direct evidence bearing on these questions. As desribed by Schulz (1974), the units of the layered sills formed by progressive differentiation of magmas having the composition . of . the analyzed chilled margins. The observed crystallization sequence in the sills, based on the· mineralogy of the

Clli

Shown in Fig. 113 is the variation in Al o -Ti0 for the analyzed 2 3 2 sill units. Also shown are sill samples from Schulz (1974) and the

fields occupied by the Newton Lake chilled margin samples and basaltic flows. If the chilled margin samples represent initial parent liquids and the flows are directly related to these compositions, then it should be possible to relate the flow compositions to the chilled margin compositions by a mechanism of progressive fractionation. Revealed by the

trend lines in Fig. 113, crystallization of peridotite and pyroxenite would produce liquid compositions similar (at least in terms of A1 2o3 and Ti0 ) to the loweJ;" Tio ·o.earing· flows. Fractionation of clinopy- 2 2 roxene and plagioclase in changing proportions at this stage (gabbros -273-

I -...... 0 I 3 I -r<) I 0 N _J

T i 0 2 (Wt 0/o) Figure 113. Al o versus Ti0 . Heavy lines represent 2 2 layered sill units.3 Outlying fields represent observed compositions in Newton Lake basalts. Light lines with labels depict possible liquid and cumulate paths for differentiation o.f the proposed type C basalt parent. Note rhat sill units can represent the cumulate phases required to produce the observed variation in basalt composition. -274- of the sill) would produce the decreasing A1 o -increasing Ti0 2 3 2 trend shown by the flow compositions. · It, therefore, appears that the various sill units could represent the cumulus fractions required to explain the observed compositional trends in the basaltic flows.

1his model can be' tested further by calculating liquid compositions from the analyzed layered sill units, assuming that .the sue.,... cessive units, at their present level of are representative of the units as a whole, and that the thickness of each unit is proportional to its volume (Williams and Hallberg, 1973). The resulting liquid compositions, calculated in this manner, are presented in Table 15 and are also plotted in Figure 113. The calculated liquid L , which •represents the liquid remaining after the 3 removal of the peridotite and pyroxenite units, is very similar to the lower Ti0 type C flows; · though having slightly lower Al o and 2 2 3 MgO ; Considering the errors inherent in such a calculation and the possible variations whfoh to occur in the degree of frac.:.. tionation within any one sill, the agreement above may be considered cons±stant with the suggested crystal fractionation model •

.Mass balance calculations provide further support for this model. Using the analyzed layered sill units as the

"phases", calculations were carried out to examine the possible relationships between the flows compositions. The result of these calculations, examples of which are given in Table 16, show that the sill units have the necessary bulk compositions to have trolled the observed liquid line of descent. The residuals obtained in these calculations (Table 16) are slightly higher than -275-

Table 15. Calculated Liquid Compositions for Newton Lake Layered Sills.

Ll L2 L3 L4 5 6 7

Si0 50.3 50.2 2 52.5 52.4 52.69 52.80 50.1 Al o 10.7 13.2 14.6 14.31 12.14 10.2 10.115 2 3 FeOT_ 11.8 11.2 11. 7 12.53 11.53 12.2 12.00

MgO 14. 9 8.1 6.7 5.74 9.65 15.2 15.05

Cao 9.2 11.5 10.8 10.06 9.26 9.0 9.10

Na o 1. 82 2.33 2.55 2.91 1.87 2 3.01 1.92 KzO 0.13 0.16 0.16 0.18 0.42 0.21) 0.20

Ti02 0.65 0.80 0.88 1.06 0.74 0.63 0.64

P205 0.05 0.01 0.01 0.12 0.05 0.04 0.04 MnO 0.17 0.16 0. 16 0.17 0.17 0.18 0.18

Hg* 0.73 0.64 0. 72 0.73

Ll Bulk composition calculated for layered sill using 27% peridotite, 12% pyroxenite, 18% bronzite gabbro, and 43% gabbro. L2 Bulk composition - peridotite. L3 Bulk composition - peridotite + pyroxenite. L4 Quartz gabbro (sample SEN-348). 5 Average chilled margin composition (4 samples). 6 Bulk composition for peridotite lens using 30% peridotite and 70% chilled margin. 7 Suggested primary magraa composition (average Ll and 6). 2+ . 2+ * Hg value = Ng/Hg + Fe a tornic (Fe calculated from FeO = 0. 85 x FeOT). -276-

Table 16. Mixing Calculations for Type C Newton Lake Basalts.

Type C Calculated A SEN-351 .. 13 t--.58D Parent Liquid Difference

Si02 44.99 53.27 50.98 50.35 50.22 0.13 AJ..203 4.28 5.97. 15.37 10.48 10.98 -0.50 FeOT 13.33 9.11 10.75 12 . 10 . 11.15 0.86 MgO 33.35 14.82 7.83 15.11 15.47 -O.J6 CaO 3.06 14.98 10 . 44 9.11 9.51 -0.40 Na 2o 0.49 1.18 2.98 1.88 2.05 -0.17 K 0 2 0.05 0. 10 0. 42 0 .20 0.27 -0. 07 Ti02 0.25 0.42 0 . 92 0.64 0.67 -0.03 P205 0.01 0.01 0.09 0.04 0.06 -0.02 MnO 0.19 0.18 0.22 0.18 0.21 -0.03 Solution % 24 . 73 18.40 57.45

Calculated B ENL-46-21 SEN-349 57-25-3 57-25-2 Liquid Differences

50.96 Si02 51.47 51. 72 51.44 51.40 0.04 Al 17.57 15 . 13 12 . 73 14.92 14.75 0.17 2o3 FeOT 8. 77 9.56 15.59 11.34 11.50 -0.16 MgO 7. 32 8.97 6. 84 8.35 7.89 0.46 Ca.O 12.05 12.41 8.18 9.93 10.85 -1.12 Na 2.08 1.68 2.96 2.03 2.20 -0.17 2o 0.46 0.14 0.08 0.69 0·.18 0.51. Ti02 0.59 0.45 1.54 1.00 0.86 0.14 P205 0.04 0.01 0.13 0.10 0.06 0.04 MnO 0.15 0 . 19 . 0.22 0.19 0. 19 o.oo . Solution % 19.60 45.42 34 . 86

A.) Type C parent = ENL-137-58D + SEN- 351· + SEN-350 • .

B.) ENL-57-25-2 = ENL-57-25-3 + ENL-46-21 + SEN-31f9. -277- those obtained in mass balance calculations for the Ely

stones above, particularly for Cao and Na o, however, as these 2 components show· the greatest variability between samples, part

of their variation may be the result of metamorphism.

Trace element data is not presently available for the sill

units, however, fractionation factors (see p. 250) were calculated

using the aver.age chilled margin composition as the pa,rent liquid.

The Y and Zr variation for the flow · is compared with the predicted

variation in Y andZr with differentiation of the parent magma. in

Figure 114 (assuming distribution · coefficients of zero during dif-

ferentia tion). There is good agreement between the observed and

predicted variations. ·The type C flows, by comparison with the

predicted variation (Figure 114), would represent liquids derived

by about 20 to 65% crystallization of a magma having a composition

of the average chilled margin.

It is concluded that the type C basalts represent a liquid line

. of descent controlled by clinopyroxene and plagioclase fractionation

with the more magnesium flow compositions representing about 20-40%

··crystallization (olivine-pyroxene controlled fractionation). It

has been .shown that the layered sill units have the compositions and

phases required to produce the observed flow compositions. It is

proposed that the sills may represent shallow chambers in which the

basaltic komatiite magmas the layered units representing

the cumulates formed by the differentiation process. This suggestion ------... ---····--- ··

60------.

o.3 50 I- . {( Type C Basalts

Chilied Margins 40 .- {( o.4 E 0.. 0...... 30 >- o.7 20 I- o.s IOI

I I 20 40 60 80 100 120 140 160 180 Zr (ppm)

Figure 114. Calculated Y versus Zr for assumed type C basalt parents . compared to observed Y versus Zr variation in the type C basalts. (Calculations were done assuming distribution coefficients of zero I N during idfferentiation). -..J co I -279- is supported by theobservation that although different layered sills have different proportions of cumulate layeres, they all have similar chilled margin compositions (i.e., similar parent magma compositions)

(Schulz, 1974).

In . the discussion above, . the observed chilled margins were used as the primary magma compositions. As shmvn in Table however, the calculated bulk sill composition Ll is higher in MgO than the obs.erved chilled margins. · These compositions can be related by 12-13% olivine (plus minor· chromite) additions to the chilled margin compositions, · su_ggesting that the initial magma may have had olivine in suspension at the time of intrusion. Alternatively, it may indicate that differentiated liquids .were tapped off during crystallization of the sill.

To evaluate the validity of these two the bulk composition was also calculated for an ultramafic lens from the Newton

Lake Formation . (Table 15). As described by Schul::; (1974), the lenses consist of thin 0'10 meters) peridotite cores enclosed by thick (.--25 meters) chilled margins spinifex like pyroxene,

Ge:ologic features suggest that olivine inay have been carried in suspension at the time of intrusion and was concentrated by settling and flow. differentiation . toward the center of the lens (Schulz, 1974).

The analyzed peridotite and chilled margin samples from one of these lenses· (Appendix lb, samples SEN-:-210 and SEN-213) lie along olivine control lines on chemical variation diagrams (e.g., Fig. 85) with the· chilled margin similar in composition to those from the layered sills. The calculated bulk composition for the lens (Table 15, col. 6) is essentially identical to that obtained for the layered -280- sill above and suggests . that (1) this may be the initial magma compo-

sition and (2) that some olivine had crystallized during ascent to

the surface.

The presence of suspended olivine in the magma may help explain

the occurrence of the many layered sills in the Newton Lake Formation.

In a previous discussion above (p.107), the intrusion of the sills was atributed· to a density contrast between the inagma and surrounding

country rocks. The presence of suspended olivine would significantly

increase the density of the magma (Botdnga and Weill, 1970), possibly

producing the necessary density contrast to prevent its hydrostatic

rise to the surface . .

The average of . the. two calculated bulk compositions above Table

15, col. 7) is .very similar to high MgO Canadian and Australian

basaltic komatiites (Arndt and· others, · 1977; Hallberg and Williams,

1972). The high Mg value of this composition (O. 725) is compatible

with it being a primary mantle melt formed by a high degree .of partial

melting. It is estimated from the chilled margin data that this compo-

sit ion would have approximately the following' trace element con tent:

Zr = 40 ppm, Y 13 ppm, and Ti = 3800 ppm. Assuming that these

elements b.ehave as true incompatible elements during partial melting

(i.e., are essentially completely fractionated into the melt), the

modal melting equation· of (1970) can be used, in conjunction

with the proposed Archean mantle composition of Sun ·and Nesbitt

(1977a), to estimate the degree of partial melting. Using Zr=ll, · and Ti=l259 for the initial mantle composition (Sun and Nes- bitt, 1977a), the calculations suggest that the primary magma for the type C basalts may represent from 30% to 40% partial melting of mantle peridotite. The experimental work of Arndt on -281- bulk compositions similarto that calculated above, suggests that they would have l:Lquidus temperatures of about 135cf C with olivine as the liquidus phase.

As discussed by Nesbitt and Sun (1976) and Cawthorn (1975), the high temperatures and · high degrees of partial melting that are required to generate koina tii te inagmas are unlikely to occur at shallow depth. Even for very high geothermal gradients, · as are often suggested for the Archean 1975), melting at shallow depth would result in liquids being generated and separated long before high degrees of partial melting could .be reached. Nesbitt and Sun (1976) present a model ·involving the riSe and partial melting of mantle diapirs from ·deep in . the· mantle ()400. km for peridod.tic komatiites).

Green (1975) has presented a similar model based on experimental melting studies of· peridotitic k.omatiites. For 30 to 40% partial melting, as is suggested· for the Newton Lake type C basalts, mantle diapirs would probably have started ascending from at least 400 km.'

The fact that higher MgO komatiites peridotitic kornatiites) are not found in the Vermilion district, may imply that diapiric ascent from greater than about 400 km depth did not occur. Though the · depth of segr.egation from the rising mantle diapir can not be uniquely determined from the presently .available data on Newton Lake basaltic komatiites, Sun and Nesbitt (1977b) suggest a . depth of segregation of about 20 to 25 km from· a residue of · olivine and orthopyroxene, for compositionally similar basaltic koinatiites from Australia.

Type A Basal ts:

The type A basalts show increasing Al , Ti0 , FeOT, MnO, P 0 , 2o3 2 2 5 -282- y and Zr with decreasing MgO and CaO. This variation is compatible with clinopyroxene but not clinopyroxene + plagioclase, fractionation.

As previously described above, the most magnesium of the type A basalts . (eg., sample ENL..:44-12, Appendix lb) have abundant ( 50% · euhedral clinopyroxene phenocrysts with minor chromite euhedra and have a bulk composition similar .to the· pyroxenite layers in the layered sills. As shown in Figure 113., the type A basal ts lie along a clinopyroxene control line extendi.ng from ' the pyroxene compositions determined in two of the flow Mixing calculations using the determined pyroxene compositions give reasonable results in tenns of relating the mo·re fractionated samples to those higher in MgO.

Two analyzed samples do not · fit this simple model of clinopyroxene controlled These two samples (ENL-137-58A - chilled margin sample, · Cedar Lake "sill" and E-151A - pillowed type A flow) have very similar textures and chemistry, with high FeOT (13 and 14%) ,

MgO (11.8 and 13:4%) and Ti0 (1.13 and 1.04 wt.%) but also higher Y 2 and Zr than other type A basalts with lower MgO (i.e., more fractionated These chemical differences, particularly the Y and Zr variations, are difficult to reconsile by crystal-liquid fractionation and if not the result of chemical may indicate significant · inhomogeneities. Mixing calculations, attempting to relate these samples to the other type A basalts by crystal give very poor results, especially for FeO and

Ti0 . Further data, particularly the REE, will be needed to more 2 critically ·evaluate their .relationship to other type A basalts.

rn ·an (p. 65), the type A basalts were suggested to form a carapace . to the· Cedar Lake "sill". In -283- support of this, it was pointed out .that a sample fromwhat is inter-

preted as the lower chilled margin of the layered body

is similar both chemically and texturally to some of the surrounding

type A pillowed .However, as revealed in Figure 113, where the

analyses from th.e· peridotite gabbro units of the Cedar Lake

"sill11 (Schulz, . 1974) are the type A basalts and also the

s.uggested chilled margin sample are to high in Ti0 to represent the 2 parent composition from which the cumulate units formed. A sample

from · .the upper contact of · the "sill" cs·chl.ilz, . tho.ugh lower in

.MgO than the sample from the lower contact, also has this high Ti0 2 characteristic. The chemical trend shown by the Cedar:: Lake "sill"

units is similar to the. trend defined. by cumulates related to type C

basalt compositions (Fig. 113) . . The simplest interpretation of these

results is that the· type A basalts (including the interpreted chilled

margin) are not genetically related to the Cedar Lake "sillno The

geologic relations, however, especially along the lower contact where

the suggested chiilled margin samples were collected (see p. bS' ); sug-

gests .that some type of genetic relationship does existbetween these

rocks. · One possible explanation· for this chemically anomaly is that

the Cedar Lake 11 sill11 is complex in nature, possibly having been tapped

at an early stage of differentiation and refilled by type C magma.

There is some support for such a complex origin in that two units of

J. peridotite-pyroxenite have been found in the Cedar Lake 11 sillrr (Schulz,

1974). Further mapping and sampling, particularly along the contacts

of the layered units, will .be required to further evaluate this pos-

sibility . .

For a given FeOT/MgO ratio, · the type A basalts have higher FeOT, -284- TiOZ' P o , Cao, Y and Zr and lower Al o than the type C basaltic 2 5 2 3 komatiites. · Similar enrichments in these same components has been

attributed by other· workers to melting of an undepleted mantle source

(Schilling, 1975; Sun . and· Nesbitt,. 1977a; Naldrett and Turner, 1977).

Naldrettand Turner . (1977), have proposed that high · iron tholeiites

from Western .Australia, formed· by low degrees of partial meltfog (< 20%)

of a rising diapir of. undepleted. mantle mantle material not

previously involved in a melting event). Though this model may account

for the generation . of . low .MgO; •nigh ·FeO basalts (however, see Sun and

Nesbitt, 1977b), · it is uncertain that high MgO , high FeObasalts could

also be formed in the same manner. Furthermore, unless the mantle

source had a high CaO/A1 o ·ratia, · this model would not account for 2 3 the low Al o of the .Newton . Lake samples. · 2 3 As previously discussed above; the Newton Lake type A basalts have many features in cominon with the South African, Barberton basaltic komatiites. In particular, they share the characteristics of low Al , 2o3 high CaO/ Al o ratios . (iri the more MgO rich samples) and low Al o /Ti0 2 3 2 3 2 Green . (1975) has proposed that the. low Al o nature of the 2 3 Barberton. samples could be. the result of early garnet fractionation within the rising mantle diapir fromwhich they were derived . Recent

REE data for several· Barberton samples (Sun and Nesbitt, 1977b) tends

to support this model, with the samples showing s_ignificant heavy REE

This is also · true of some low Al amphiholites from 2o3 Isua, Greenland (Sun and .Nesbitt, 1977b).

Though REE data is not presently available for the Newton Lake

samples, the Y datadoes allow a preliminary evaluation of the garnet

fractionation model for these Lambert and Holland '(1975) and -285- Nesbit t and Sun (1976) - have shown that Y behaves chemically like the heavy REE and, therefore, provides some measure of the heavy REE content of a particular In Figure 9lb (p.IOh) above, the Ti-Y variation for the· type A basalts was compared to the average ratio in choml:rites and an estimated Archean mantle composition. It was noted that the type A basalts have higherTi/Y ratios than the estimated mantle or · chondrite ·composition · (and also higher than the type C basal in terms of Ti-Zr (Fig. 9la) the type A basalts have similar ratios to the mantle composition :(and chondrites) of

.Nesbitt and Sun (1976). the conclusions of Nesbitt and Sun

(197 6) that high MgO magmas primary basalt compositions) have nea"L chondritic .trace. element the similar Ti-Zr, but higher

Ti-Y ratios of the Newton.Lake samples suggests that the high Ti/Y ratios maybe the result of low Y contents. Considering the otherwise high incompatible element content of the type A basalts (i.e.,

Ti0 , : P o and Zr), the .. low Y, maybe attributed to control by some 2 2 5 phase such as garnet. This though not conclusive, is compatible with the garnet fractionation model proposed by Green (1975) to explain the lowA1 o content (arid high Ca0/Al o ratios) of the 2 3 2 3 Barberton komatiites. · Nesbitt and Sun (1977b) suggest that less than 20% garnet removal is required to produce the observed REE patterns in the Barberton komatiites (assuming an initial chondritic

Ti/Al ratio). It seems likely that this would also apply to the

Newton Lake REE data is required to adequately evaluate this possibility.

An al.ternative .explanation: to the garnet fractionation model for the Barberton komatiiteswas suggested by Cawthorn and Strong (1975), -286- who proposed that the low A1 o , high Ca0/A1 o nature of these 2 3 2 3 komatiites reflect . the chemical nature of the source material (i.e., a high Ca0/A1 o is a characteristic of They suggest 2 3 that mineralogical zoning· may exist in the mantle, with clinopyroxene preferentially enriched -at .shallow. depths· and garnet· at greater depths (shallow and deep not being defined). Thus, high degrees of partial melting of . the· "shallow"clinopyroxene enriched zone.would account for the chemical features. of the derived basalts. While this model could also ·account for the .major . element chemistry of the Newton

Lake type-A basalts, ·.it is uncertain if the low Y (and heavy REE's in Barberton komatiites) can be explained in this manner. Considerir1g the lack of evidence for the minerolgic mantle zoning proposed by

Cawthorn and Strong . (1975) and the difficulty of generating high MgO melts .at shallow depths (Nesbitt and Sun, 1976), this model is considered unlikely for the Newton Lake basalts.

The close spatial association of the µype C and A basalts in the

Newton Lake requires that the possibility of a direct genetic relationship between them. also be considered. Both from the petrographic and chemical data; however, it appears that a model of

crystal-liquid fractionation cannot relate the two basalt types. For example; the enrichment factors for Zr, Y, . Ti and

A1 o in the type A basalts relative to the type ·c basalts of similar 2 3 FeOT/MgO are 1.2, 1.1, . 1.4 and 0.8 ·respectively. No reasonable combination of olivine, pyroxene and plagioclase can account for the

enrichment · in Zr, Y and Ti with accompanying depletion in Al2o3 . An alternative inodel ..:rould involve crystal fraction under high

pressure · conditions at depth. · ·Such a model, involving eclogite -287- fractiona tion, has been proposed by Clarke (1970) to relate two suites of picritic basalt from the Baffin Bay area, which also differ in incompatible element contents at similar FeOT/MgO ratios. To examine this possiblity for the Newton . Lake samples, · the procedure of Wright and Helz (1976) has been used. They suggested that the least squares mass balance solution to the relation:

Parent Liquid A = Differentiated Liquid B + En + Fs +

Wo Jd + A1 + Ti0 + Si0 + 2o3 2 2 could be used to evaluate whether two liquids could be related by high pressure fractionation. · For· high pressure fractionation, the. bulk composition of the fractionated componenti:i Z(En + Fs + Ho +

Jd + A1 o + Ti0 + Si0 )_, shoUld approximate a reasonable composi- 2 3 2 2 tion for high pressure pyroxene + olivine + garnet. If the resulting combination is not compatible th the combined composition of known high pressure minerals, no· genetic relationship may exist between the tested liquids or they maybe .related by differing degrees of partial melting (Wright and Helz, 1976).

The calculated primary composition for the type C basalts (Table

15, col. Ll) was used as the parent composition for these calculations and two of the type A basalts (samples ENL-114..-54A and E-151A) were selected as possible differentiates. As an example of these calcula-- tions, Table 17 shows . the . results for the relation:

Primary Liquid (Ll) = Sample ENt.--114-54A + En + Fs + Wo +

It is apparent from the· low residuals that a good fit was obtained, with the calculated composition only significantly different in Na20

(Table 17). The calculated components (i.e., L_(En + Fs + Jd + A1 203) 17. High Pressure Mixing New.ton Lake basalts.

Type C Calculated En JAD A1 114-5/iA Parent Liquid Difference Fs 2o3 (En -i- Fs + Jd + Al) SiO 59.85. 45,50 59.45 51.30 50.35 50.40 0.05 49,5 A1 100.00---- 9.63 10.48 10,48 12.5 233 ------o.oo FeOT --- 54,50 ------12.19 12,01 11,99 0.02 12.0 HgO 40.15 ------9.68 15,11 15,06 0,05 25.7 Cao ------13.68 9.11 . 9.. 02 0.09 Na2o ------15.33 --- 1. 93 1.88 1. 36 0.52 0,31 Kf O --- 0.27 0.20 0.18 0.02 T·o ------0,64 2 ------1. 04 0.69 -0,05 P205 ------0.04 Q,04 0,03 0.01 MnO ------0,23 0.18 0.15 0.03 Solution% 21,62 7.25 0.58 3,98 65.92 ------,. Type C = ENL - 114-54A + En + FS + JAD + A1 20 Parent · 3

I OJ OJ"' I -289- have the following bulk composition

Si0 = 49.5 .MgO 25.7 2 = 12.5 Na o = 0.31 Al.203 2 Fe OT = 12.0

This composition is markedly different from reported eclogites (O'Hara and others, 1975) having no CaO and higher MgO. However, modally this composition could consist of about equal proportions of orthopyroxene and garnet. Other calculations using sample E-151A gave similar though with higher residuals for the calculated parent composition. It, therefore, appears that at least the major element chemistry of the type A basalts could be the result of about 35% fractionation of garnet plus orthopyroxene from the calculated primary magma composition for the type C bas al ts. ·

The Y data, however, tends to rule out extensive garnet fractionation. Garnet has a very large distribution coefficient for Y

( 10, Condie and Harrison, 1976) such that garnet fractionation would rapidly deplete Y relative to the initial melt. Such extensive de-

, pletion in Y is not observed in the type A basalts (though some depletion in Y is see discussion above).

In view of the above, it is the authors opinion that high pressure fractionation, while compatible with the major element data, is not consistent with regards to Y content. It, therefore, is .concluded that crystal fractionation, either shallow-level or high pressure, can not relate the two Newton . Lake basalt types.

Green (1971) suggested that the present day low velocity zone may be compositionally with large ion lithophile (LIL) elements preferentially concentrated near the top of this zone. Sun and . -290- Nesbitt (1977b) have adapted this model to the Archean and have postulated that a much thicker low velocity zone may have existed because

of higher m3.ntle heat production . at that time. Thus basalts enriched

in LIL elements could have been derived from the enriched upper zone, while relatively depleted basalts would have their source at the bottom or below the deep low velocity zone. This model is attractive, in . that it provides a simple mechanism to account for the presence of high MgO basalts . with differiri...-g ·incompatible element contents within a .restricted range Of Space and time. l.Jnfortunetly, this model does not lend itself to rigorous testing, especially for the Archean.

Con Cl us ion:

Based on the presently available data it is concluded that:

1.) The Newton Lake type C basalts can be related by crystal-

liquid fractionation of the phases observed in the layered

sills. ·

2.) The distinct chemistry of the type A basalts seems to re-

quire a distince mantle source from that of the type C

basalts.

3.) Both basalt types formed. by high degrees (>30%) of partial

melting of mantle

The model pu:t forth by Sun and Nesl:iitt (1977b), involvi_ng a thick

Archean low .velocity zone with a LIL element enriched upper portion, may account for the two distinct mantle sources suggested above.

While other models could be equally applicable, this model is considered a viable alternative to explaining the origin of the Newton

However,· unless a low Al 0 content (and a high Lake basalt types. -· 2 3 -291- Ca0/ Al203 ratio and· varying Ti/Y ratio) is attributed to the initial source, $Orne early fractionation of a high A1 o (and Y) phase 2 3 such as garnet· may be· required in the evolution of the type A basalts. -292- CONCLUSION

The new observations made in this study which are relevant to understanding· the evolution of the Vermilion greens tone belt are summarized below. Specific which have been drawn, : are discussed with the information on each major lithologic unit (the relevant referen:aes in the thesis are enclosed in parentheses). A synthesis of these conclusions is incorporated in a tentative geologic history for the evolution of the Verinilion greenstone belt.

1.) Lower Ely Greens tone Meriilier·

Observations:

A. This unit is calc-alkaline in nature (p . 143-149).

B. It consists of amygdaloidal pillowed basalts and ande-

sites with locally aoundant tuffs and tuff breccias

(p. 27 and 29).

C. Based on the analyses of three samples, this unit is

characterized by low Y _contents (p. 150).

Con cl us ions:

A. This unit formed im,a shallow submarine environment (p,. 33)

B. The calc-alkaline basalts in this unit could have been

derived by high. degrees of partial melting of amphibo:-

lite or eclogite (p. 235).

C. The andesites can be related to the basalts by plag-

ioclase + ryroxene + amphibole ·fractionation (p. 233). 2) Upper Ely Greenstone Member -293-

Observations:

A. The upper member consists largely ( 90%) of pillowed

to massive, genera,lly non-amygdaloidal basalt and

contemporaneous diabasic rocks. Pillowed

and felsic (calc-alkaline) volcaniclastic units are

locally abundant and show a concentration near thz

upper portion of the member. The upper member is

interlayered with the volcaniclastic rocks of the

Knife Lake Group and the Lake Vermilion Formation.

and iron-formation is present locally. (p. 39).

B. A broad arch has been identified in the upper member

based on strikes of pillowed units and volcaniclastic-

iron-formation layers. The arch has a structural style

unlike that reported for other fold patterns in the

Vermilion :district . (Sims, 1976) (p. 22).

C. Twelve. samples from this study, combined with the data

of Green (1970), indicate a tholeiitic nature for the

upper Ely member. Irregular compositional variations

exist with respect to relative stratigraphic position

and no ey;:tdence for a significant compositional change

with. t;llI!e (:;i._,. e., height) was found (Appendix 3) (p. 150).

D. Two chemi.c;;i,11¥ dist;i_nct bas.a,lt type$ were identified: a

high Ti0 (> 1. 30) and low Ti0 (< 1.11) (p. 15,4). Based 2 2 on available data, intermediate compositions between the

two groups are rare. The high Ti0 basalts have high in- 2 -294- compat;lble element and total REE contents (p. 163).

Arth. and Hanson, 1975). Compos:Ltionally both groups

of upper basalts are intermediate between recent

ocean floor and island arc tholei.ites (:p. ·H2).

This intermediate chemical nature is also shown by

the trace elements Ti-Y-Zr-Sr, which lie in the

fields of overlap for ocean floor and island arc

tholeiites as defined by Pearce and Cann (1973) (p.170) .

E. The two basalt types are found interlayered through-

out the upper Ely Greenstone member.

Conclusi.ons:

A. The major and minor element differences between the

two basalt types are compatible with derivation of

the high Ti02 group from the low Ti02 group by a process of fractionation of 50 to 60% olivine + plagio-

clase +pyroxene (p. 238-261).

B. Assuming that the Archean mantle had a similar FeOT/

MgO to that inferred for the present mantle, the low

Mg/(Mg + Fe2+) ratios of the low Ti02 basalts suggests

they can not represent primary m&ntle melts (i.e.,

their equilibrium olivfoe compositions (Fo 76) are to

iron rich to have been in equilibrium with mantle

assemblages) (p, 254).

C. If sufficient olivine i .s added to the low TiOz compo-

si.tion to rP,ise :i:.ts Mg/}l:g + .Fe2+ ratio to a mantle

equllibrium value (i.e., ....,..71 - ,69), this primary -295- rnagma (using the model of Green (1971)) could have

by about 25 to 30% partial melting of mantle

peridotite at depths of about 30 to 40 km. under

water undersaturated conditions (p. 255). If Archean

geotherll}al g:i;-adi.ents· were higher than at present, as

suggested by Green (1975), a shallower depth of par-

tial melting may be possible.

D. The presence of two distinct compositional groups

throughout the upper member and the lack of inter-

mediate compositions require a more complicated

mechanism than simple single-stage fractionation (p . 259).

An analogy can be drawn between the upper Ely member

basalts and the proposed evolutionary model for

Kilauea volcano, Hawaii, (Wright and Fiske, 1971) (JJ. 260).

Based on this analogy the following model is proposed:

1) ascent of the primary magma into a shallow central

reservior, followed by occassional extrusion of low

Ti02 basalts, 2) injection of magma into shallower chambers, followed by progressive fractionation of

olivine + plagioclase + pyroxene and 3) eruption from

these shallow chambers as. a result of new injections

of :more p;ciJI!.atiye m.a,gma (high. Ti02 basalts) (p, z5q).

3). The La,ke Vermilion Formation

Observations:

A. Ba,sed on ·the a.nalyses of samples from two separate -296- localities, calc-alkaline basalts with similar major

element chemistry and low Y. contents to the lower

Ely volcanic rocks qccur in this unit (p. 180-182) .

Conclus_ions:

A. An origin similar to the lower Ely basalts is postu-

lated for those in the Lake Vermilion F. (235-238).

4. Newton Lake Maf:Lc Member

Observations:

A. Detailed examination of the basaltic rocks of the

maf ic Newton Lake member has revealed that they have

distinct textures·, characterized by a wide range in

skeletal pyroxene and plagioclase morphologies (Dowty

and others, 1976) (p.63-95).

B. Two chemically distinct basalt types are identified:

1) Type A basalts have abundant clinopyroxene pheno-

crysts (euhedral to skeletal) and have chemical

similarities to South African basaltic komatiites (Fig.

98a with high MgO, Ca0/Al20l>l),low Al203/Ti02 , high incompatible elements ('fi,Zr,P) and FeO contents

21 Type C basalts, having pyroxene or pyroxene and

plagioclase a.s crystallized phases, have chemical

similarities to Canadian and Australian basaltic

komatii.t; es with. high MgO, low CaO/ Al2o3 varying Al /Ti0 , low FeOT/MgO and linearly varying trace 2o3 2 element contents (p. 184-195). -297- c. Chilled margins from associated layered ultramafic

sills are generally chemically similar to the type C

basalts, except for higher MgO and lower Al2o3 contents (p. 189).

D. The type A basalts are enriched in incompatible ele-

ments but they have low Al and Y contents. The type 2o3 C basalts have varying Ti/Zr ratios (100 to 68) which

define a line intersecting the Ti axis. The Ti/Y ratios

show a similar linear variation which also intersects

the Ti axis.(p. 196-197).

Conclusions:

A. The textural variations of these basalts may be a re-

sult of varying cooling rates with differing degrees

of supercooling (p.105). The lack of a similar range

:i.n textures in the upper Ely Greenstone basalts but

the textural similarity with chemically similar

Australian and Canadian basalts is interpreted to be

a reflection of the unique compositions of the Newton

Lake basalts (p. 106).

B. It is proposed that these basalts represent high to

low MgO basaltic komatiites (p. 228). The type A

basalts may represent a previously unrecognized basal-

tic komatiite type (p 228 )_ .and their compositional

variations can be related by pyroxene fractionation

(p. 290) . . -298- C. Based on trace element :restrictions the two Newton

Lake basalt types cannot be related genetically by

near surface or high pressure crystal-liquid fraction-

ation (p .. ·239). The low Al a,nd Y contents of the 2o3 type A basalts may reflect minimal garnet fractiona-

tion as proposed by Green (1975) and Sun and Nesbitt

(1977b) for chemically similar Archean basalts. Both

basalt types are high in MgO and probably represent

relatively high degrees of partial melting ( 30%).

Accepting the arguments of Nesbitt and Sun (1976),

it is postulated that the type C basalts ruay be de-

rived from a diapir rising from approximately 400 km.

depth (p. · 281) .

5) Vermilion Porphyries

Observation:

A. The subvolcanic porphyries are calc-alkaline and have

strongly depleted heavy REE' s ()?. 179); Arth and Hanson,

1972, 1975) .

Conclusions:

A. These rocks can be derived by melting of am-

phib.ol:l.te or eclogite sources (Barker and Arth, 1976). Low Y contents ;ln calc-alkaline basalts suggest higher

degrees of partial melting of a similar source

(p. 235-237) . -299-

6) Additional Geologic. and Geochemical Data

Observations:

A. volcanic rocks (both flows and volcani-

elastic units) are found interlayered and intertonguing

with both the upper Ely Greenstone and maf ic Newton

Lake members (p. 21-23) .. 87 86 B• Th e .. · 1 s·r /sx va1 ues d etermine . d f or ma f"ic an d

felsic volcanic rocks, subvolcanic porphyries and in-

trusive plutonic rocks are all lower than 0. 7 010 (p. 10 ,

Table 3) and the spread in the isochron ageR is on the

order of 50 m.y. (p. 56-59) .

Conclusions:

A. During the evolution of_the Vermilion greenstone belt

calc--alkaline volcanism was more or less continuous,

while tholeiite volcanism was cyclic and inter-

mittant (p. 305).

B. The low sr87 /sr86 initial values preclude derivation

of any of the rocks from the Vermilion grP.enstone belt

from pre-existing sia,lic crust older than about 2.7 b.y.

All the ;i.gneous. ;i;<;:>cks.7 tram has.alt and dacite to post-

kinematic syenite, originated from the mantle or

rapidly recycled (_ 50 m. y ! ) mantle-derived material

(p, 304). Geochronologic studies indi_c1=1.te that the

tectonic-igneous- evolution of the. Vermilion green.,..

stone belt occurred over a 50 m.y, time interval (p.

10). -300- Geologic History of the Vermilion Greenstone Belt

Based on the geol.ogic data and compatible with the conclusions and observations discussed above, the author offers the following geologic history for the Vermilion greenstone belt. An interpreta-

tion of the magmatic relationships prior to deformation in the wes-

tern part· of the belt is presented in schematic form in Figure 115.

Cale-alkaline volcanic rocks, formed in a shallow submarine environment,: represent the basal portion of the presently exposed volcanic sequence. The presence of coarse tuf f breccias suggests at least local explosive centers. Iron formation formed during quiescent periods, probably in local depressions on the surface of the shallow calc-alkaline pile. These may be the direct result of volcanic emanations, as proposed by Goodwin (1962) for similar

Canadian deposits.

With the wanning of calc-alkaline volcanism, iron-formation deposition occurred over a large area to form the Soudan Iron-formation.

The presence of intercalated amygdaloidal flows and volcaniclastic material (Klinger, 1956) suggests continued intermittent volcanism during this time.

Iron-formation deposition was apparently abruptly terminated by the commencement of tholeiitic volcanism east of Armstrong Lake (the upper Ely Greenstone member) and renewed calc-alkaline volcanism to

.the west (felsic volcaniclastic member of the Lake Vermilion Forma- tion). Coarse agglomerates and abundant quartz-plagioclase porphyries in the area of the east shore of Lake Vermilion (Sims and others,

1972) suggests the presence of a calc-alkaline center at approximately -- -FELf- MEMOER

----1c-A1ka1 i ny ,--"-::>- I "'-J L___ / _

KN I FE LAKE I I GROUP I I i-7

< - J--J )/_- sic Mern.ber I ;J f,cl'ltOn La kc f . J Feeder System J/ Feeder System for r.1af1c N.L. for Knife La ke I • 'I<-!>- Group I I/ /;#}I cnrn1STONE LOW Ti0 Of\Sf\L T 2 J I 1 01 + Pl /1 Feeder System Figure 115. Proposed magmatic for Ely \ model for the Vermilion Green- Grcenstt>ne I Feeder System for . l t Ely Grecnstcne s tone belt. (see text for discussion).

I 1 w 0 i1 ...... 11\ I -302- this location. The transitional gradation, both laterally and verti-

cally from this area (Ojakangas, · 1972) into reworked tuff and agglom-

erate, further suggest a major calc-alkaline center in this area.

Tholeiitic volcanism is suggested· to have occurred in two ways,

(1) directly from one or more (?) major reservoirs to form the low

Ti0 basalts and (2) from shallower subsidiary chambers producing 2 high Ti0 basalts by fractionation of olivine plagioclase 2 + + Lava from both sources was intermittently extruded throughout upper

Ely Greenstone time with quiescent periods marked by local iron-

forma tion deposition.

Tholeiitic volcanism continued contemporaneously with calc- alkaline activity both to the west (Lake Vermilion Formation) and

to the east (Knife Lake Group). There was apparently a wanning in

tholeiitic magmatism upwards in the sequence such that by the end of upper Ely Greenstone time, calc-alkaline volcanism was predominant.

Local tectonic readjustment may, at this time, have subaerially

exposed parts of the Ely Greenstone and felsic (Lake Vermilion and

Knife Lake) volcanic sequence, resulting in erosion and formation

of local conglomerates interbedded with volcanigenic graywackes and argillites. The center of tholeiitic volcanism is interpreted to have been related to the broad arch still observed in the upper

Ely Greenstone. The presence of a thicker basalt pile in this loca-

tion could explain the thinning of the Knife Lake Group rocks west- ward from Snowbank Lake.

Active volcanism commenced again abruptly, with deposition of

dominantly andesitic and dacitic lavas, tuffs and breccias east of

Newton Lake and high MgO basalts (komatiites) to the west. Felsic -303- volcanism is also suggested to have continued in the Lake Vermilion

area such that the high MgO basal ts of the Newton Lake Formation

intertongue in both directions with calc-alkaline volcanic material.

The general lithologic features of the felsic material (i.e., amyg-

dules, pumice fragments and abundant fragmental material) suggest

deposition under shallow submarine conditions.

The high MgO basalts were apparently extruded in a generally

crystal free state with supercooling and varying cooling rates

relative to position within a determing the resulting texture

of the crystallized lava. Direct intrusion (and extrustion ?) of

olivine bearing magmas accompanied the ri1afic volcanism. These magmas

differentiated at shallow depths and,at least in some cases, were

tapped at various stages to produce flows.

There is no further record of supracrustal deposit.ion in the western portion of the Vermilion greenstone belt. The intrusion of

the Vermilion and Giants Range batholiths have obliterated evidence

of any further volcanic activity. The intrusion of the batholiths was accompanied by folding and metamorphism of the supracrustal sequence and at later stages of this cycle by major faulting. Intrusion of

the late syenitic and lamprophyric rocks the end of Archean igneous activity in the area and may have been accompanied by thickening and stabilization of the crust in this part of the craton (Sims, 1976).

Discussion

Several features of the proposed geologic history deserve further consideration. The presence of a thick ( 3000 meters) calc-alkaline -304- pile at the base of the supracrustal sequence requires, based on the petrologic model discussed above for these rocks, that a thick basal-

tic sequence have originally underlain this sequence. This agrees with the general observation .made for other greenstone belts around the world, that the supracrustal sequences comffiense with basal tic volcanism and pass upwards into calc-alkaline rocks (Goodwin, 1968.-.

The abundance of calc-alkaline material throughout the Vermilion district, especially if the· thiCk volcanigenic sediments are included, requires even greater thicknesses of parental basalt (to form amphibolite and/or eclogite). While these basalts may have evolved as part of the tectonic-igneous cycle of the Vermilion Greenstone belt; another alternative may be considered, namely, that .the supracrustal sequence evolved on a basaltic (oceanic ?) crust.

The evidence for this is largely but is consis- tant with the known geologic and geochemical constraints. Green

(1970) originally proposed that granitic to tonalitic clasts in conglomerate units in the Ely Greenstone represented detritus derived from a granitic crust present prior to volcanism in Ely Greenstone time. Recent dating of these granitic cobbles (Jahn and Murthy, 1975) has revealed that they have an age similar to the greenstones and thus can not represent an older crust. Other evidence for an older granitic crust is lacking in the Vermilion district. It is suggested that deposition on a basaltic (oceanic ?) crust would alleviate the necessity for tremendous volumes of tholeiitic basalt being generated

1n a restricted geologic area. Partial melting of such a crust would -305- also be compatible with the Sr isotope data, as the ratios in the basaltic crust would be similar to those in the Archean mantle. Re- gardless of the validity of this it is clear that to produce calc-alkaline volcanic rocks thro_ughout the evolution of the Vermil- ion_ greens tone belt, $Ome replenishment of the basal tic source must occur (Condie and Harrison, i976).

With regard to the presently exposed .supracrustal section, there is a clear cyclical nature in the volcanic products doing from: calc- alkaline volcanic rocks Ely Greenstone member), to tholeiitic basalt (upper Ely Greenstone inember), to calc-alkaline perhaps largely, erosionally derived (Knife Lake Group) to high MgO basalts (Newton Lake Formation). This pattern appears, however, to be a result of intermittent tholeiitic and komatiitic volcanism, as calc- alkaline volcanic rocks are found interlayered with both basal tic sequences. This type of variation is not unique to the Vermilion district, but has been described from numerous Archean greenstone belts

Naldrett and Turner, 1977; Arndt, 1976; Gelinas and others, 1976).

This variation . in the nature of the volcanic material may reflect fluxuations in the geothermal gradient and/or other unknown parameters influencing the degree and depth at which partial melting occurs. Thus the calc-alkaline volcanic rocks may represent fairly shallow and relatively low temperature melting, while the tholeiitic and high MgO basalts represent progressively higher degrees of partial melting (also requiring higher temperatures for fornia tion). If the arguements of Nesbitt and Sun (1976) and Cawthorn (1975) are accepted, the komatiitesof the NewtonLake Formation would also be required to -306- have originated from mantle diapirs rising from deep in the mantle.

Arth and Hanson . (1975) have presented a model for the orig·in of the abundant quartz monzonites of the Vermilion and Giants Range batholiths (see Table 3) based on trace and REE data. Their model proposes that the quartz monzonites are the product of 20 to 50% melting of meta-gnaywacke similar in composition to those of the

Lake Vermilion Formation and Knife Lake Group. This model implies that the quartz-monzonites postdate formation of the thick volcanigenic graywacke-argillite sequences, as is observed. The derivation of the quartz monzonites from the. Verinilion graywacke sequences suggests that these rocks and, therefore, their parent calc--alkaline daci·tic material were even more abundant in the Vermilion district than present exposures would indicate.

From the discussion above, · it is apparent that the evolution of the Vermilion greenstone belt was, · in part, controlled by mass balance constraints with respect to the availability of basalt to form amphibolite, which in turn allowed derivation of (1) the calc- alkaline volcanic rocks, (2) the grawwacke-argillite sequence and

(3) the quartz monzonites.

Considering the great volume of quartz monzonite presently exposed in the Vermilion batholiths, tremendous thickness of sediments are required. It is uncertain, however, whether such thick sediment piles could develope within the time span of the tectonic-igneous event observed in theVerinilion district. Thus, while the Arth and

Hanson (1975) model can account for the geochemical characteristics of the plutonic the mass balance considerations suggest fur- -307- ther attention should be given the granitic batholiths to (1) determine their volume relations and (2) further examine the validity of the Arth and Hanson (1975) graywacke partial melting model.

Tee tonic Hodel

The geologic and petrol.ogic observations summarized above suggest that the Vermilion greenstone belt developed through the coalescence of petrol.ogically distinct volcanic centers. The restriction to only a two dimensional view bf the supracrustal sequence, however, sever- ally limits attempts at reconstructing the geologic-tecto::iic environment.

A multitude of models have now been proposed to explain the evolution of greenstone belts (eg., see papers in Windely, B.F., 1976, eed., The Early History of the Earth). These can be divided into two major namely:

1. models, having the supracrustal sequences post-dating and

sometimes overlying granitic crust (eg., Hargraves . 1976;

Hunter, 1974); or

2. models involving oceanic and/or island arc volcanism on

basaltic crust with granites as a late addition (eg., An-

haeusser, 1973; Glikson, 1976).

Recently Glikson (1976) has suggested that two distinct. greenstone sequences may exist; 11 primary 11 greenstones consisting of mafic-ultramafic rocks which formed an early crust for the earth and "secondary" greenstones consisting of bimodal mafic-felsic volcanic assemblages and/or of basalt-andesite-rhyolite cycles· which may have formed in -308- linear troughs developed in partly era tonized r_egions.

The lack of understanding of . the effect if a higher geothermal

gradient and the nature of the. mantle during the Archean, places major limits on evaluating the validity of existing models. McKenzie and Weiss, (197 5) conclude that higher geothermal gradients and faster mantle convection rates· during the Archean would result in numerous, small crustal plates experiencing rapid recycling (i.e., subdaction); whereas Lambert (1976 concludes from similar observations that, a thin crust and high geotherinal gradients would prevent formation of eclogite and inhibit subduction during the Archean.

Jahn and Sun (1977) note that distinct chemical differences between the present and Archean mantle further complicate interpretations of greenstone belts in terms of modern analogs.

An added complication alluded to by Glikson (1976), is that, just as today, different active volcanic environments may have existed during the Archean. Thus, all greenstones may not have formed under the same conditions or. environments. The latter two observations could help explain the litholigic and geochemical variations noted between various greenstone sequences (Glikson, 1976;

Naldrett, and Turner, 1977; Sun ·and Nesbitt, 1977b).

For the Vermilion greenstone belt, any proposed tectonic model must be compatible with the geologic and petrologic constraints established in this study. (see discussion above). Perhaps foremost amorig these is conclusion 6b, which requires relatively rapid recycling and resupplying of urldepleted volcanic material during the evolution of the supra.crustal sequence. Fresh undepleted (by partial -309- mel ting) basalt must be continually provided to allow the generation of the voluminous calc-alkaline rocks. This constraint implies a mobile as opposed to static tectonic environment.

Tarney and others (1976) have noted a similarity between many greenstone belts and island---arc-marginal basin sequences such as the Rocas Verdes complex· in southern Chile. · Like the greens tone belts;. the marginal basin rocks occur as megazenoliths or elongate discontinuous be£ts within engulfing "granitic" rocks; they also have volcanic sequences"ranging from mafic-tholeiitic to felsic- calc-alkaline and volcaniclastic sedimentary (Tarney and others;

1976). Tarney and others (1976) suggest that Archean greenstone belts maybe fossil marginal-basin-island-arc sequences formed by plate-tectonic generally similar to those posulated for modern equivalents.

The model of Tarney and others (1976) can account for many of the features observed in the Vermilion greenstone belt. The subduction of basaltic crust, followed by partial melting at depth would provide an adequate mechanism to continually generate the

Vermilion calc-alkaline volcanic rocks. Futhermore, in this model the Ely Greenstone and Newton Lake Formation basaltic volcanic rocks could be interpreted as having formed .in a marginal basin environment adjacent to an active calc-alkaline arc. The behind arc basin would also provide a site for the deposition of the vol- canigenically derived sediments..

Tarney and others (1976) relate the formation of the marginal basins to continental crustal thinning which occurs behind the -310- active arc as opposed to rifting as posulated for modern marginal basins. As previously noted above, there is no evidence in the

Vermilion district fora pre-existing continental (granitic) crust. either underlying or adjacent to the forming supracrustal belt.

However, as described by Gill (1976) for the Lau basin in the south.-, ern Pacific Oceani . behind -arc and interarc basins can form in a totally oceanic envrironnen t.

The marginal basin-island-arc model of Tarney and others (1977) appears to provide a viable analog for the observed Vermilion district volcanic-sedimentary sequence. It may not be as important,

to define a modern analog for the Archean greenstone-granite terrane as it: is to define the specific and uniqii.e conditions which controlled its developinent. In this regard further study is particularly required of the vast batholiths to determine their petrologic and tectonic relationship to the supracrustal se- quenaes. The mass balance constraints provided by such studies will aid our understanding of the evolution of the early crust. -311- BIB.LIOGRAPHY

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Viswanathan, S., 1971, A petrological and geochemical study of granitic and metamorphic rocks in and adjacent to western part of the Giants Range batholith, northwestern Ph. D. thesis, University of Minnesota, v.l and 2, 656 p.

-----, 1974, Contemporary trends in geochemical studies of early Precambrian greenstone-granite complexes. J. Geol. Soc. India, v. 15, p. 347-379.

Walker, G.P.L., 1975, Intrusive sheet swarms and the identity of crustal layer 3 in Iceland. Jl. Geol. Lond., v. 131, p. 143-161.

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Weiblen, P.W., Schulz, K.J.,and Nielson, B.V., 1976, Compositional variations of Minnesota River Valley amphibolites. Abstract, 22nd Inst. Lake Superior Geology, St. Paul, 68 p. . I

-327- Weigland, P.W. and Hollister, L.S., 1973, Basaltic vitrophyre 15597: An undifferentiated melt sample. Earth Planet. Sci. Lett., v. 19, p. 61-74.

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ADDENDUM

Beggs, D.G., 1975, Petrology and geochemistry of Archean volcanic rocks of Sturgeon Lake, Geotravers e Workshop, Toronto.

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Spooner, E.T.C., Chapman, H.J. and Smewin9, J.D. , 1977, Stron- tium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos Massif, Cyprus. Geochim. Cos mochim Acta, v. 41, p. 873-890.

Yoder, H.S., 1976, Generation of Basaltic Magma , National Academy Sci., Washington, D.C., 264p. -329-

APPENDIX I

Geochemical Data ··-·- ·- ·--·- --·-· -·------

Ap pcn,di x la. Analytical Statistics - BCR-1

Mean Value x: s N Flanagan (1973) Reported Range

SiO 54.51 0.278 6 54.50 54. 13 54.50 Al 13.50 0.057 6 13. 61 13.35 13. 6 7 2b3 FeO * 11. 96 0.017 6 12.06 11. 91 12. 16 NgO 3.42 0.067 6 3.46 3.28 3. !19 Cao 6.90 0.004 6 6. 9 2 6. 9 2 7. 0 1 Na 2 o 3. 3 9 0 . 004 3 3.27 3.23 3.35 K20 1. 68 0.006 6 1. 70 l. 68 l. 76 Ti02 2.24 0.009 6 2.20 2. 22 2.32 P205 0.35 0 , 023 6 0.36 0.35 0.37 MnO 0. 19 0.022 6 0.18 0. 17 0. 19

Rb 45 1. 19 6 3 46.6 Sr " 335 5.022 3 330 y 35 0. 110 3 37 Zr 194 l. 02 7 3 190 Nb 12 0,268 3 13. 5

FeO* = total iron a s FeO X = arithimatic mean S = standard devi a tion N = number of determinations All elements except Na 2o by x-ray fluorescence analysis. Na20 by neutron activation analysis.

I w w 0 I Appendix J.b. Major element compositions of Ver:milion volcanic rocks. · SAMPL.F.: l 2 3 4 5 f.. 7 0 9 l 0 11 12 13 14 15 ROCI', 86-33 35-?.9 22-19 40-22 2'5-::>2 6 c. n1-a· 123-4 144-!0 CL-6 171-18 10?.-19 EEu-111-ou-J-12 s102 58.76 48,49 53.27 55.12 52.49 sn.aq so.e4 s1.4n · 5.t.67 52.0:5 5lo44 411,q13 so.11 AL203 15·03 15.50 16,00 16.15 15.76 14.21 17ol6 14.53 14.05 14•04 15.91 14.17 14•h0 14,IH 15.69 FEC 6.65 6,10 ll,a4 a.n6 a.79 13,46 9.26 l?.65 14,oo 13.54 10,93 12,36 12. 71.1 15.07 11.:12 MGO a.so '5044 7,63 6021 5. 0 6 5,34 G.49 6·l1 6.27 5.66 6.tl3 4.q.5 6063 cno 7,10 o.sa 11,37 7,q9 n,09 9,57 11.67 a.n9 9.62 io.so l0,77 7.!11 1l. 31' 12 ol6 4,04 4,37 2,66 4,n6 3,Qq J.?1 2.61 2.16 2·66 2.71 2" 7Ll 3, :57 ?.!>2 ?..38 K?.C .42 ,33 ,77 .60 ,14 ,73 ,33 ,92 .og .40 .14 1 . 07 ,57 ·36 0 1 1rc2 ,s7 ,59 ,oo • s .02 t.49 ,01 1.so i.us 1.41 loU. . l. 53 1·'32 l 0 13 o.'\9 ol1.• 11 ,02 ol<'.! o)3 009 .10 .ll P205 ,14 '10 .01 .12 • ll oQ5 l;.NO .10 .12 . ,21 ol?. .13 ,09 .!6 •21 •21 ·22 .23 .26 .32 •2l TOTAL . .99·88 99,90 99,79 99.04 99.50 gqo7R 99070 99075 99.80 99070 q9,77 99.72 99,aO NORM oz -o :;,05 -u -o 2 o60 3.12 -o ,73 . -o .. 0 69 -o -o -1) co -() -o -o -o -o -o •·O -o -o -o -o -o -o OH 2.42 1.93 4,55 A.21 ,84 L'JO 5,55 ,54 2.38 ,84 6.::'16 3,46 2.14 AB 42039 3/l.88 19,83 30.1 •1 27.29 23 o 16 2a.r,o 23069 19,Ri p4o36 24·55 2soos 30o47 23027 21 • 'i7 AN 1·1.60 21.51 29o5t 23,87 2s.25 . 27o7G 30·03 27,49 .26.49 ;>6·63 Jo.34 26043 21. 70 2s.20 :;11.:16 OI 15-17 151165 2,45 20.n2 a. ·n 1::'1. :::n 2110 71 J.IT.32 ,1.0& 17,•;1 21·75 13·27 22.1.jq 23.;17 t!Y 11·114 15 091 21,51 2.?1 26.39 3no11 5,72 1'1.66 31.36 280 112 20,4!) 22.10 1 t .f.u o.•1a 9,97 CL 9o97 -o -o 11.27 -o -o l6o70 3.9u .16 -o 3.12 -1) 1•1ol3 11.s3 1 n .2ri IL ,77 '131 20,96 1,31 2o 1 l.·25 ?ol3 2,06 2.00 1. 5(, 2.17 2ol3 ? • 011 1-25 .!\P .22. .• 22 l , ll .?4 .29 .70 '10 o2l ·21 ·?.3 .25 ·23 ,4') .in 1-S • l7 l tl 19 . ·20 2i. 22 23 25 26 27 2 11 30 P.0C'i". IJ-8-15 107-l 163-16 1 17 19 2() 4!; u5 47 2 3 l 2 13 src2 '19·69 '-'9·67 49,66 54. 21'\ 53·51 51.74 St ·"8 52·07 53.J6 S2•92 s2.91 52099 52·06 50 .12 5o.96 15·61 15·% l''· 99 15. 15·67 ! :';.97 .t<>·39 1t1, 12 . 14-16 1 i: •04 14-87 14 · 76 15-3fJ 1u.36 !S 0 1 T FEC ll ·O'l 11.34 ll. 30 ; ., ?4 ll-69 1 1?. 75 lfi 0 '18 12.;1 .· :;.3.03 12 .. 70 12.ao 11.12 l. O ,43 12. c'.ll 1?. .112 MGO 7. '.:4 7 .. 53 5. (,6 6.o:i 5,73 ·20 :;. u l $·% 6f'39 5·60 7.7[l f>·76 ll.9•; 7.n1 CAO 11.:i2 11.53 !2, 72 9 .. 79 7.99 8.51 C? .. in.07 e.21 s.10 7.ll6 9.85 ? .. 34 10.77 ., NA20 2·"7 ). • 67 2,37 2.;s 2.<:;2 :; :"': t;. 2.t() ...... c;a. 2.55 3.,70 3 .. r.. 5 1 · /lL 3.115 2 .. n1 1·% .1() i<20 : .. ;,97 ,98 1. 5'-' • 2fi .,7f3 ·25 ,4,j .u2 .. ::!2 "16 .76 .10 .t2 Tl02 ,35 .. .. ()C • 15 ! .6<; j, ·:'..6 47 1 ... :,q ! .. 3i 1.so .78 .;u .117 .. F\2 P205 ·06 .04 ;::i:; .. 01..:. ·26 .;>5 • :>O ·13 .. :2 .. 12 .22 .. 12 .; 5 "16 •Of\ .. 17 .19 .18 .;rn. ·24 31 • !S ·22 ·26 ·21 ot24 .20 ·20 .23 .2::1 TCliAL 99,79 ?9.79 99 , r,4 1CO·UO 9q'l' 7G 99.75 99.79 99,c9 !.r,0·05 100.01 99.50 99·1l3 99. 711 99,511 NORI/, no ., u.22 -\l .2A GZ ·0 -o -0 3.23 IJ o 41 1 .. q5 1 .. 32 2-72 -o 1 -a -(j .-o ". -o -o -(! -c -o -o -u -c co -o -o -o -o 4,4 7 • :;9 .12 . 5,78 5.81; .59 1 .. c-;7 1.!.f,/ J., so 2 .• SFJ 2.119 ..J."l.Jj. "96 OR 6·:.I'> 2So30 5.1.91 Mi 22.24 l.5· ll' lt): 9·1 24.70 26·62 27. CJ6 1q•i2 2:;.29 23.?.7 :;:;i , 40 3.l,.29 1s . n1 3'+·36 26035 AfJ 2a.110 33.52 ,27,61 30.fl2 :50·11 24.;>0 330117 26·95 26oil l. 20•57 21;.6'3 :52o50 22·33 16.113 f () l 22.02 19.39 7·8 7.31 p,1)3 1() .. q1 2t.a1 11.36 l5o23 13-0ll .05 15 , 26 2<).·'!7 -(j -o 1$.72 OL 19.,26 -c 1. 21 l. :1J 1.27 2. )r; l .')2 2. 0<) 2. l.1 .1u .12 • 10 .11 t 16 olb .03 .o5 • 011 .n1 ·04 fJ.NO .23 .19 ,19 .?.9 .:i2 .?3 .;:>3 .i 7 .21 .21 .24 .1a .23 .23 .16 TOTAL 99,114 99,97 99,73 99.63 99,75 100,<'19 9"l.80 99.. 61 CJ9·95 99.85 99.84' 99,77 99,77 9'l,A2 NORM oz ,59 1.53 1,75 -o -o -o -o -o -o -o 1·116 3.27 -o -o -o co -o -o 2, 17 -o -o -o -0 -o -o -o -o -o -o -o -o OR 3086 1,97 1,B2 6.41 .1 a 4,JF.1 1.oc 3.01 1.79 0.47 2"011 2.13 lo6l 2.02 2.5A A[l ' 19·25 15.42 24,26 23.;>6 23.50 34.ll5 13·:'19 22·96 :>boll 20.33 26.05 17·!'2 25.76 11·62 /IN 34,5u 35,50 34,34 211.cio 36·00 23.90 32·12 1n.49 21.110 1.a.so 32·18 31 •Sil 16·94 13.96 10.7?. DI 2.1:11 . 10. 73 -o .17 12.67 19.14 00 4.20 24.14 14'13 19.51 16·16 41·00 29.01 44,57 HY 18·30 33,41 35,63 14. J 5 14·29 ,59 . 3l•A2 211,9& 7,54 ;>6·86 22.70 19.35 12•23 11 •. so 16·63 OL -o -o -o -o 11.79 15.F.6 5.112 -o 14·40 4·'10 " -o -o 9d2 10.05 12.n7 IL 1·43 1. !.4 25 • .'.'19 1.34 1. ?(l 1. r,n 1ll.31 l •110 1·49 lo24 1.32 1•40 1.52 1·11 AP .19 .21 -o 1.24 .21 .;>S 1.115 ,33 -o .06 .10 .no .14 .n11

SAMPLE 1!6 47 41'1 49 50 51 5? 53 54 55 56 57 5ll 59 no ROCK 7 • 123 61 120-55 57-?.5-2 55-;>4 96-117 E-151A 69-31 70-:52 SPN-:51 SPrl-22 71-33 SEM-130 5!02 49ol8 50·60 50,30 51.33 51. 5.8 52.n1 4A.S7 :;3.117 50•00 49-18 52·00 52· 7l 53,q3 53,43 llL203 12.22 15·24 13. 96 14.119 12.70 14.55 5410. ""' ;>t: r,.au 14.69 14·52 16.52 12 • .?.o 14·26 12.00 13.74 FE.O 14.04 11·51 15,25 !1.32 lS.5!:; 11.21 ll .;>2 14·01 13.30 ,4,35 11·06 12.10 12·2J 11.lB 11.00 'lol6 a.76 6, 72 l:l.33 6·82 l'\.68 ir..no l:'i.39 II 0 :'i3 7.37 7,09 9.0ll 5,q2 9.F.6 7,75 Cl\O 10.10 10.73 6,67 9.<'11 Ad6 7,;;11 A·30 11.99 11•71 a.95 12·86 10.10 10·13 q,06 N,i20 z,57 1·45 3,03 2. (l:'i 2,95 4. :'!5 1.11 ,53 2.54 lo84 l o69 1·73 2.65 3,75 K20 1115 .16 1,64 .69 .ou .14 • ] 0 .21 ·04 ·09 .13 "13 o.l l .26 • lll TI02 1. 1.03 1,47 l.!10 1.54 i.n 1. 20 1. 04 1 .. 37 l . 5u ,88 1.44 1. 4() ,1\1 1.43 P205 ,09 .io ,12 .. 10 .13 .11 ,) 0 .01 ·12 • 11 .as • lll • J.2 .os .14 .20 .21 ,57 ' 1.9 .22 .19 .1.3 .17 .21 .24 .17 .lA .19 ,1a .1 ll TOTAL 99,74 99,79 99,73 99,n 99,73 99,110 99,79 9c;,76 99,77 q9·75 99,1a 99,70 99,77 99,70 99.1\0 ;\;ORM oz. -o .59 -o -o -o -c -o 12.60 -o -o 2.18 4,97 ,23 -o co -o -o -o -o .. o -r, -u -o -o -o -o -o -o -n OH .09 ,95 9,83 ·'.Io l 0 048 , 111 o'l9 1.21.1 . ,;>4 ,54 ,,77 .70 066 '1053 1.06 AD 23.29 l::lo 16 ?..7,62 i8,35 27.04 J7.:'i3 315.na 1'>.37 4,97 :'!3· 18 16.69 15.4! t5·A6 23.03 33.72 20,16 AN 21.57 34,,90 19,95 29.69 21.6?. 39.29 ;:-a.41 36.82 25.72 31.48 20.11 I DI 25, 14 111.54 10,28 . 15.33 150013 13.?.6 211,(,5 311.(,<; 1&·49 ,2.79 21,00 19.51 15·:>4 19.66 l?..o! 6 w MY 7,35 ,34.09 10,30 27.37 29.04 ·7. 31 16·''3 ti. 24·13 ?.5·27 14035 33,95 29·51 33,37 26.08 w 1 lfl,n ·14 o2b ·2J ol'l .30 ·?.5 .10 .;>9 --·-·- - - -··--·- - - ·- ··· - · -· ·------·------···- ---··---- -·- ·--- -· -·-·--·- . - ... . ···---- - ·- --···---- ·71 . . SAMPL.E: 61 62 63 64 65 66 67 68 69 70 72 73 74 75 ROCK SEN-133 SEN•231 SEN-72 80-39 74-36 157-72 63-27 137-58F :'10-9 36-10 17-1 19-3 SNL•4 SLN-2 5102 50.11 51.59 53,93 so.65 57.83 53,74 4Q.116 so.as 51.72 c;3.72 54.51 53o6.7 so.<:>2 · 5:?o01 53,99 AL203 14·21 1•1.64 111,40 16·71 14.37 111,QO l "i, 311 13.ao ,s.5o 13.64 111.s1 t4o27 14.90 111,40 FEO 11.41 11.42 9,69 13.119 6.97 10.75 11.48 1n,73 12.15 8.62 10.60 10,01 J 3, 10 llJ.50 11,39 l·IGO 9, 13 a.41 6,75 6.42 4.27 1.no Q,43 7,81 0.11 a.111 4,3;, 7o27 6·112 7,90 6o"i2 CAO 9o58 a.111 9,112 9.(.6 9.1)7 Ao2"i ll o l.6 1no42 8075 a.114 11052 8062 9-63 10.57 7.1)6 NA20 2.s2 3,73 2,96 l ,q11 306/l 3,:n 1. 411 2.97 4,17 4,33 4 o 14 3o37 l 0913 4.1)4 K20 ,47 .19 ,86 .36 1,03 .14 .42 .110 023 006 .19 .12 ,78 024 TI02 1.39 1 .10 l,10 loP.7 .76 1 on l 1.115 ,q2 .92 062 .54 1.10 1ol9 ,78 lol6 P205 olb .12 ,10 ,;>I) 009 • 10 ,, l 009 • 07 . .26 oOO .12 olO .05 .12 MNO .21 .19 ,22 .?.O .13 , J 7 019 .22 .?.2 ol6 .16 .lA o?.5 .ts .28 TOTAL 99,79 99 .CIO 99,(13 99.76 99o!\7 9

oz -o -o ,41 2o36 1.11 -o -o -o -o -o -o -o -o -o -o co -o -o -o -o -o -o -o -o -o -o -o -o -o -o -o OR 2.78 1.11 5,10 ,24 2.13 6,09 .fl3 :!.48 2.36 1.33 ,35 1.11 .71 4.63 1,41 AO 22·68 33,32 26,60 l 7, P.6 33.n9 29.77 26.66 27·41 :'16ofl9 39.0'l 36.96 30•51 17.139 4lt54 AN 26.13 22.53 23,56 32.40 211ooc- 21o'IS 33.116 27.28 22.00 ?.2·56 17.70 20.33 23o6S 29.117 17.71) 01 16-36 14056 19,76 11.'10 13·14 15.20 16.97 l'lo05 16ol5 13·62 31.39 11.13 19-09 lllo06 13.t 5 HY 19.tlS 10.13 22,71 31.94 15.19 21027 29.'18 Q,03 17.63 l 3oll 4o84 13.95 ! IJ ·61 21.21 13 o.91 OL l0.30 16.S4 -o -n -o 4. r,13 3 .113 1110 OU 12020 11000 5.67 A. 71. 13.Sl r.11 10 ,40 lL 1.94 1.52 1,53 2of>7 1.06 104 (j lo l:7 1.20 1·28 .as .75 1.s2 l o67 1.09 l. f> l AP ,33 025 ,21 .60 0 ltl • :>o o?.3 dc'.l ol4 053 .16 .25 • 21 ol(J ·25

SAMPLE 76 ?7 7!l 79 AO fl 1 fl?. 113 84 85 06 117 (Ill (19 90 ROCK 5LN-3' Sl_M-5 SU1-6 SLM-7 SSM-430 SS'!-3941 SSJJ-3581SSi•-3586S[tl-213 <;EM-347 13 7-!;0A 137-5130 46-! 9 4(,•21 21-4 SI02 so ,C)l 52031 54039 SO.'i9 52067 411050 52. o:'\2 53.90 Slo'l6 <;3o0j 48092 46oS9 51 ·110 so.oa 51 .in AL203 13.13 14.211 14,li7 14.22 14.28 16.no i2-"3J 11097 12.0& 12°19 9.oe 10047 15·00 17.54 16. tO FEO 13·20 12·30 12.26 14. 19 11 .. 05 12.sa 11.tO 11 .39 . 12. 04 11 ·60 12°06 13.39 11 • ?.O (1.76 9.9Q !'1GO 10.19 7,93 6,S4 7,03 7ol12 8 0"l 3 ln.n7 l\.Qj 10.48 a.45 13-411 8040 10.111 12003 11·32 NA20 2.so 2.47 4,19 3.06 3020 ?. .')ll 3o28 3.65 2·56 2°57 2.09 1.51 1.116 2o IJ6 2,qQ K20 .co 1.16 ,35 1.05 .71 ,72 ,r,3 ,35 033 ,1,7 .14 .14 o'l6 • 16 TI02 1.11 1.12 1,16 1.11 1. 08 .!19 ,"!t> .73 .74 ;77 1.13 .79 ,99 .s9 ,77 P2os ,10 .11 ,11 009 ,(19 ,03 ,n4 .ns .os .10 .07 .oa .04 .06 Jt,NO .28 .28 .28 .:'6 ol7 ol9 .t6 .j,{) .iu ol9 .11 .21 -14 .15 ·19 TOTAL 99.77 99·7& 100,77 99o7S 99.79 99,79 9

NORM oz -o -o -o -o -o -o -0 -o -o -o -o -o .46 -o -0 co -o -o -o -o -o -o -o -o -o -o -o -o -o -o -o OR s.oa 6.91 2.05 6.26 4.19 4.25 3. ll 2.05 1.95 2. 77 .a2 .61 1.73 2.71 AB 23·20 22·36 ·.37 .35 27.73 29.45 23.l '5 2'?.;.2 3;:>.57 23006 ;:>3·04 17-33 l3o21l 16.117 18.69 26·112 AN 2.:,.74 24.55 19.50 22.17 22.15 30. l 6 11.34 l 'i, J 5 20.s1 ;>0·31 15·01 2Q.9S 32.ri6 37.19 30.n1 12.:.'13 16·9B .aa 1s.ss 17020 17.112 20-57 01 11.511 11. 12.43 16.11 25·72 1"·30 25·07 I HY 21.35 28oll6 21,42 11."5 17,31 4,?3 7.17 1q.1e 18.73 30.21 40.92 9,66 30.n1 17.111 9.1:'\ w OL 15.31 4.26 6,33 113.ng 9.0!} '15 16.20 10,61 9.s; 5.4<) -o 311050 -G 4.1\4 11.30 · w 1 C:-1'\ 1.n2 23.23 lo 07 . 1.39 .A2 1.07 w IL loS4 1 .. 57 1 0f>O 1.% 1.:>3 l. 01 l • 0., l • O"/ I AP 020 .23 ,?2 ol9 .• '"'l IJ .n6 • no .10 . l 1,1 olO t.57 ol4 .J 6 .08 ol2 ------···· ·------· ------... - - · -···-··· ·-· ·-·--·-·--·---····- ·- . ·-··----- SAMPLE 91 92 93 94 c;i5 .'?f> c;>7 98 99 tOO 101 102 104 105 ROCK 37 :'.10 . 39 40 41 21 20 46 58 SEP-8 E 46A SES 26 SF.S 27 SES ?.9 5102 50.34 46.95 47.92 50.23 50.16 5?..69 52.sa 49.62 52e44 52e69 41e23 39.53 46.24 411.19 51.42 f•L203 10·62 9,44 8,33 11.117 10.21 12·'58 16.08 13.73 ,2.99 6033 4 .24 7,57 1n.05 13.59 FEO l4o97 l::l.'H 15,01 11.75 l?..96 11."i9 12·'12 11.SO 12,90 ,2.59 19.06 17.20 11.'l7 <1.!6 13.33 MGO 7.43 13,37 11,A6 11.13 ll .42 7,<"1;:> 7ofl9 6004 6·37 0.21 27.32 34.44 22.91 16.11 7.7?. CAO l0.05 11. 79 H,40 9,q7 A.42 1t0 :'16 12.13 12.35 9.06 1.02 4.23 3.oo 10.111 14.37 'l.P.ll t2 .n9 .13 .IA .15 .19 • 14 • 09 ,07 .20 MNO .20 .22 026 • 21 .20 ,t9 .22 .23 .18 .23 .22 .20 .17 .l'l TOTAL 98e53 97,57 97,88 97.62 98.12 99.61 9Q,t;S 9q,54 99,95 Q9o59 99,42 99.36 99.1\2 99,04 99.66

oz -o -o -o -o -o -o 4,50 -o -o -o -o -o -o -(I -o co -o -o -0 -o -o -o -o -o -a -o -o -o -o -o -o OR 1.03 .n4 ,36 1.21 4,49 ,111 ,4(l 1.55 1.ss 3,133 .40 .so .22 ,34 .60 AO 31.49 14.09 16,35 15,73 24.24 24.i3 io.ri5 21. 76 29.16 37,28 .10 ,53 1.110 9,70 20,79 /\N 13·'•6 15.71 15, 03. 23,r,1 13·91 20. :rn 29.46 3?..83 22.55 t4•P.2 16·20 9101 19'19 20.66 26.98 DI 28.96 34,43 33,73 20.49 21.78 2a.13 26.10 22.74 17,:'\9 15.16 2.39 .69 211.28 1.110 17.19 HY 4.66 4.67 11,26 32,n9 13·63 23./\ll 21.111 <1,93 21\. 51 8063 14•6°5 2.68 15·60 38.96 3! o62 OL 18031 27.70 210313 19.91 ,nl -o <"l,39 2.22 t0.62 64.25 -o 38,72 -o 1.02 IL· 1.ss 1.35 1, 110 1. 04 1.62 1.110 • ns l o48 2.21 . 1·32 .9o 85.20 .29 26.37 1.34 AP ,47 .36 , 11s ,117 ,40 ·"6 • 19 ·27 .30 ·31 .38 o2A eltl ,39 ·42

SAl>'.PLE 106 ·101 lOo 109 110 111 112 113 114 t15 116 117 118 119 120 ROCK SEM-348 SEM-349 SEr:-350 SEM-352 SF.N-351 srn-210 EfOIJ-7-2 EF.'U-9-&i. 5 I> 7 e Z9 10 28 5102 52069 51 ·30 53,12 52.73 44.88 45.76 511.10 61>.00 70o75 70·86 62·45 69.oo 63·03 6:1.66 71.94 llL203 14131 15 .io 5,90 6,n4 4,27 5.119 lq·"l 14·07 16.62 ,1 . 37 15·51 g,. OB 16.97 17.45 t5•81 FEO 12.53 9. 511 9,0!.\ 9.no 13.30 12 . 49 2·?4 4 , 97 2·42 1.03 s.20 2.56 3.41 7.21 1oA3 M(·O 5,74 . 8095 14,70 14.73 33.27 21J,<'l2 1·10 4.70 l.Oo . ,59 4.49 ,95 3,75 3.26 ,53 CAO 10·06 12.39 14 . 911 15. ?. n 3,05 5,55 !_i.07 f>-12 2·69 1·80 3o40 3.29 6·02 4o60 1.94 N,,20 2.91 1068 1018 1.30 .45 5.r,o 3,27 3,44 5,95 5.61 5,79 5.?.8 3.?.6 •;. 70 1\20 .18 ·14 , 19 .?.?. .. os ' l 1 ,q5 1.13 2.14 1.15 2.17 1.79 .7iJ .26 1.41 l•N> .115 • '15 .u2 ·25 ,35 ·66 046 · 62 ·38 .s2 -29 ·1'6 ,45 ,59 P2v5 .12 .01 , Ol .02 • (ll .113 .to .17 .09 .13 .01 ol 7 .17 ol 4 ol 7 • 19 .10 • 18 .19 .21 .114 .09 .02 .01 .01 . 03 .o5 .05 ·03 TOTAL 99, 77 99.83 99,83 99. PA 99,75 99,77 99,04 9<;>,9l) 99.07 o9·'l3 99,55 9'lo85 99·84 100046 99.92 NORM oz ,55 -o -o -o -o -o 12·45 11\.35 :;2.03 22·26 11.sa 17.41 919 ------·-· ---·-······-- ··- · ···--·-·-- - -··- -- ·- - ·- SAMPLE 121 122 12:'1 124 125 1?.'l' 128 . 129 1:50 131 132 .• 1°33 I 134 ROCK 30 34 35 48 4q so 51 52 S3 22 23 24 25 ?.6 SI02 64.10 68009 55,:59 55,49 61.23 67.07 7P..88 63.30 1.1. 59 60.99 65.00 61.61 5/l,52 AL203 17·28 15.a1 14,75 17· 10 lll.04 15.".19 15.78 17· 15 14 .26 16.40 14.21 16·27 15.llO FEO 3.85 2.12 1(1,40 7.26 6.12 1.63 l. 02 ::>.oo 2,94 7.73 7.69 s.09 5.40 7,54 MGO 2.10 2.23 3,65 6.50 3.09 1. 76 ·?4 ,79 3,75 2.86 4.61 4,09 3.56 4.03 CAO 4.50 3,96 10,70 7.54 5,66 4, IJ3 3.33 ?.87 5.68 7.62 2.97 4.36 7.07 8.49 N/120 5,30 4,93 3,43 4,34 4o41 s,r.8 5.so !i.35 4,55 4o09 s.no 5,34 4·21 3,52 K20 1.so 1.10 • 01+ 2.26 1. 52 1.26 J .1)7 1.139 2 .10 .21 .44 .n., ,73 TI02 .3d .20 ,no • 111 .77 .19 .2a .:'12 .114 .69 .62 .54 .n1 P:?.05 .19 .13 .14 ,34 .t2 • 10 .ri1 .12 .05 .25 .1'1 .14. .11 • I 8 MliO .01 .03 ,25 .14 .14 • fl3 ·03 .03 .03 .21 .10 .09 .oa .12 TOTllL. 99,07 99.93 99,71 99,95 100-16 99.99 100·01 99.99 99.95 a9•69 99·60 Q9.38 99·72 99.74 NORM QZ 10.86 18.15 4,61 -o 8.87 14.44 23·90 22.16 9.35 13061 6.98 13.75 11.05 8.55 co -o -o -o -o -ll -o -o -o -o -o lo69 -o -o -o OR 8.75 9.97 .24 13.14 8.94 7.31 901\l 11ol0 12.r,9 1.43 1.22 2.'59 s.13 4,34 r.s 47.04 44,40 31,32 38.:'16 39,45 51.87 49.) 0 47.79 40·27 ;!7•21 51.56 47.70 3lo/l4 AN 18·71 15.65 25, 15 15.46 2?·30 18,77 13.03 l:'lo 38 19065 ;>0.11 13.68 13·'15 22.<11 25.34 DI 1·70 2.11Q 22,65 15.34 4,54 .ta 1 • (,3 -o 6.so 13·23 .-o 5,75 9ol4 12.12 HY llo99 a.1s 14,lfb 3,,,7 14 .79 (J,'15 o'l7 .11.91 11·011 12·66 23.60 15.51 12.90 15.66 OL -o -o -o l2oll9 -o -o -o -o -o -o -o -o -o -o IL .s2 038 1,24 1.Ql J, 06 .40 .30 • 113 1 olll ,

L Rock refers to field, laboratory, or literature number.

2. Norm refers to catanorm ·as used by Irving and Barragar (1971). qz=quartz, co=corundum, or= orthoclase, ab=albite, an=anorthite, hy=hyperstene, ol=olivine, il=ilmenite, ap=apatite.

3. Stratigraphic distribution of samples: Lower Ely 1-7, Upper Ely 8-39, Lake Vermilion Formation 40-42, Newton Lake Fonnation 43-100, Newton Lake layered sill units 101-111, Vennilion Porphy- ries and felsic flows 112-·134 ( 130-134 are from the Newton Lake member).

4. Data Sources; Sims (1972) 7, 20-23, 40, 96, 114-120, i30.:..134lJnpublished data from the I w Minn. Geol. Survey 34-36, 97, 121-123. Green and Schulz (1974) . 2-4, 91, 95. Schulz (1974) w 1..11 101-105. Arth and Hanson (1975) 24-26, 37, 99, 124, 129. All other data from this study. I

-----····-4·----·---·- --

Appendix lb. Trace element data, Vermilion volcanic rocks.

Rb S"t' y Zr Nb Ba ID KID Rb ID srID

ENL-ll.4-5LfA 3.6 109 16 56. 6.9 48.5 2332. 3.92 104.4 El:\TL..-114-54B 4.6 '.)0 :;1-6 59 10.2 53.8 2822 4.91 43.4 E-151A j_ ! 7 21 19 61 3.0 36.8 1743 1. 79 19.8 ENL-44-12 24 11 39 6.9 107.1 3652 5, 71 19.8 El:\TL-27-7 2.2 30 20 81 6.8 28.6 1245 2.37 25.5 SEN-213 4,6 137 16 45 6.0 63.2 2739 5.01 114. ENL-123-61 3,8 114 26 78 4.4 32.9 1328 3.40 94.3 SEN-231 2.9 116 29 85 7.2 76.2 1577 3.10 95.3 ENL-71-33 5.2 83 16 58 7.6 56.4 2158 5.56 68.3 SEN-347 7.4 128 15 48 2.7 SEN-210 5.8 7 6.5 22 1. 9 ENL-57-2.5-2 16 117 27 75 7.0 ENL-57-25-3 1. 3 80 . 37 105 5.1 SSN-430 19 147 29 90 11.4 ENL-137-58A 1. 4 32 20 71 10.0 ENL-137-58B 2.7 21 19 60 3.0 ENL-137-58D 10.1 166 22 58 7.2 ENL-19-3 1.3 110 32 100 13.3 ENL-17-1 3.6 136 30 92 13.3 SNL-4 17 129 11 43 4.8 SLN-3 21 77 30 98 4.6 SLN-6 6.0 lOt: 30 97 5.3 SSN-358-6 6.6 81 15 50 5.3 I w Et-11..-120-55 33 134 22 84 5.0 w SPN-31 2.6 111 32 111 6.6 O'I ENL-30-9 3.6 315 12 78 5.1 Appendix lb. Trace element data continued.

Rb Sr y Zr Nb

CLEU-182-19 24 6.7 29 94 5.8 CLEU-163-16 2.9 133 18 48 3.9 CLEU-137-8 4.9 10.2 33 94 28 ENEL-86-33 5.6 152 16 134 18 ENEL-35-29 14.6 1% 9.5 24 5.0 ENEL-22-19 15 176 14 84 7.6 ENEU-3-12 9.3 100 20 50 17 CL-6 18 89 25 63 8.9 TLV-1-3 12 13Lf 17 52 10.6

wI w --.J I

r -338-

APPENDIX II

Sample Locations Appendix 2. Geochemical Sample Locations

Sample No. Location Nanc Photo Page

(LOWER ELY MEMBER) ENEL-108-42 NW shore Eagles Nest Lake No. 1, SW, SW, Tuff breccia 36 22, 62 N., 14 W. ENEL-84-32 NW, NE, 30, 62 N., 13 W. AmvP.:daldidal pillowed andesite (?) ENEL-86-33 NE, NW, 30, 62 N., 13 W. Porphyritic andesite with abundant plagioclase phenocrysts 30 ENEL-35-29 SW,SW, 18, 62 N., 13 w.. Amygdaloidal pillow basalt ENEL-22-19 N. shore Eagles Lake No. 1, NW, SE, 22, Massive fine grained 62 N., 14 w. basaltic andesite ENEL-40-22 SW, SW, 18, 62 N., 13 w. Massive diabasic andesite ENEL-25-22 NW, NW, 26, 62 N., 14 W. Dike of diabasic tholeiitic basalt (high Ti0 ) 2 (UPPER ELY MEMBER) ENEU-3-12 SE,NE, 7, 62 N., 13 W. Pillowed fine grained basalt 42 ENEU-6-14 SW, NE, 7, 62 N. , 13 W. " " " ENEU-8-15 NE, NW, 7 , 6 2 N., 13 W. " " " I CLEU-137-8 SE,SE, 2, 62 N., 14 Massive fine grained w w. w basalt \.!) I CLEU-123-4 SE,SE, 2, 62N., 14W. Pillowed basalt Sample No. Location Name Photo Page CLEU-107-1 NW, SE, 1:, 62 N., 14 w. Pillowed basalt CLEU-144-10 NW, NE, 1, 62 , N., 14 w. Diabasic, massive basalt CL-6 NW, NE, 4, 62 N., 14 w. Fine grained, gray pillowed basalt CLEU-163-16 NE, SE, 3, 62 N., 14 w. Light gray basaltic andesite (calc-alkaline) CLEU-171-18 SW, SW, 3, 62 N., 14 w. Gray, porphyritic pil- 47 lowed basalt CLEU-182-19 .SE, SE, 3, 62 N,m 14 W. Gray, porphyritic pil- 47 lowed basalt EEU-14-8 Roadcut, NE, SW, 63 N., 12 w. Diabasic, fine grained basalt from below pillows EEU-7-2 Powerline, SE, NE, 62 N., 12 w. Light buff, dacite, pillowed flow. EEU-9-4 " ll II II " II II II II (LAKE VERMILION FORMATION) TLV-1-3 NW, NE, 11, 61 N., 16 W. Gray, pillowed basalt TLV-2-4 NE, NW, 16, 61 N., 15 W. Gray, fine grained basalt

(NEWTON LAKE FORMATION) (Type A Basalts) I ENL-114-54A Along trail to Cedar Lake, SW, NE, Black, medium grained py- w .i::.. 7, 63 N. , 11 W. roxene rich basalt 0 75 I E-151A SW, NE, 7, 63 N., 11 W. Black with skeletal pyro- 75 xene phenocrysts Sample No. Location Name Photo Page

ENL-114-54B SW, NE, 7, 63 N., 11 w. Black, fine grained "quench" basalt ENL-·44-12 SW, NW, 18, 63 N., 11 w. Massive, medium graines, 74 pyroxene rich basalt ENL-25-7 SE, NE, 13, 63 N., 12 w. Black, pyroxene-oxide rich basalt . ENL-120-55 NE, NE, 7, 63 N., 11 w. Dark green, amygdaloidal pillowed basalt SPN-31 SW, sw, 6, 63 N., 11 w. Fine grained dark gray pillowed basalt ENL-137-58A Center, 13, 63, N., 12 N. Chilled margin, Cedar Lake sill ENL-137-58B " II " " " II . " " ENL.:..96-li 7 NW, SW, 7, 63 N., 11 w. "Quench" textured, black 76 basalt (Type C Basalts) ENL-17-1 NE, NW, 22, 63 N., 12 w. Fine grained variolitic ENL-19-3 II II II II II Variolitic pillowed basalt ENL-123-61 NE, NE, 7, 63 N., 11 w. Gray, very fine grained quench olivines ENL-57-25-2 Along trail to Cedar Lake (BM 1377) Variolitic basalt. 83 I NE, NE, 13, 63 N., 12 w. w

II II II ft I-' ENL-57-25-3 " " Ophitic gabbro 83 I ENL-55-24 11 11 II 11 II 11 Variolitic pillowed basalt Sample No. Location Name Photo Page

ENL-69-31 At N. end of Cedar Lake, NW, NW, Gray fine grained 5, 63 N., 11 w. basalt ENL-70-31 " " II II " " " " Pillowed fine grained basal.t ENL-71-33 SW, NW, 6, 63, N., 14 w. Pillowed, porphyritic basalt SPN-22 NE, SW, 6, 63 N., 11 w. Pillowed fine grained basalt with felty texture SEN-130 SW, . NW, 22, 64 N. , 11 W. Dark gray, fine grained pillow basalt SEN-133 NW, SW, 32, 64 N., 11 W. Massive, diabasic basalt SEN-231 NE, NE, 1, 63 N., 12 W. Pillowed basalt with 86 "quench" texture SEN-72 NE, SW, 29, 64 N. , 11 w. Green, medium grained pyroxene-phyric basalt ENL-80-39 NE, NE, 18, 63 N., 11 w. Pillowed, dark green basalt ENL-157-72 SE, sw. 13, 63 N., 12 w. Pillowed, dark green amygdaloidal basalt ENL-63-27 S. shore of Cedar Lake, NW, NE, Dark gray, fine grained 84 13, 63 N., 12 w. basalt

ENL-137-58D Center sec. 13, 63 N., 12 W. Fine grained diabasic I w basalt .I:>. N ENL-137-58D " II II 1! " . " " " I Sample No. Location Name Photo Page

SLN-2-7 s. shore Little Long Lake, SW, SW, Dark green, pillowed vario- 86-91 17, 63 N., 12 w. litic, olivine-phyric basalts SSN-430 II It It It II It " Green pillowed basalt with 91 fine grained plagioclase micro lites (Type B Basalts) ENL-30-9 SW, SW, 18, 63 N., 11 W. Pillowed, light green, porphyritic basalt ENL36-10 NE, SW, 18, 63 N., 11 W. Pillowed, amygdaloidal, light green basalt ENL-74-36 SW, SE, 7, 63 N., 11 W. Pillowed amygdoloidal, plagioclase-phyric basalt SNL-4 SW, SW, 34, 63 N., 13 N. Pillowed gray basalt (Tholeiites) SPN-27 Small island in dry lake bed, SW, Pillowed, plagioclase:phyric SW, 6, 63 N, 12 N. gray basalt SSN-394-1 SW, SW, 16, 63 N., 12 W. Green, pillowed basalt with fine grained plagioclase microlites (Chilled Margins) SSN-358-1 and 6 North shore Little Long Lake Fine grained, "quench" tex- NW, NW, 16, 63 N., 12 W. tured, with skeletal pyroxene SEN-213 SW of Bright Lake, NW, NE, 6, Abundant, small euhedral pyro- 103 63 N., 11 W. xene phenocrysts with a few I w amygdules it::> w I Sample No. Location Name

SEN-347 SW, SW, 9, 63 12 W. Texturally like SSN-358-146 (Sill Units) SEN-351 From sill south of Dry Lake on the Peridotite Echo Trail, SW, SW, 9, 63 N., 12 W. SEN-350 " " 11 " " " " " Pyroxenite SEN-'352 " 11 11 " " " 11 11 Py•roxeni te SEN-349 " 11 11 " " " 11 11 Bronzite gabbro SEN-348 11 11 II 11 11 11 " 11 Gab bro SEN-210 Peridot:i.te from lens south of Bright Peridotite Lake, NW, NE, 6, 63 N., 11 W. ENL-21-4 Gabbro ·from large gabbro sill, Gab bro SE, SW, 15, .63 N., 12 W. ENL-46-19 Upper quartz gabbro Cedar Lake 11 sill 7 Gab bro SW, NW, ·13, 63 N., ·.H W. ENL-46-21 " " 11 " 11 " Gab bro Other Locations

Paraconglomerate, Mafic member Newton Lake Formation: North of Shagawa Lake, Sec. 20, 63 N,, 12 W. , NW, NW.

I Pumiceous Tuff, Upper Ely Greenstone, NE, NW, Sec. 7, 63 N., 11 W. w ""'I SW, SW, 22, 62 N., 14 W. represents SW!t;, SW!t;, Sect. 22, T.62 N., R.14 :-W. \

...,345-

APPENDIX III

Chemical Stratigraphic

Sections for the Upper

Ely Greenstone -346-

Because of the lack of adequate sampling and the problems related to correlating widely seperated samp-

le locations, only relative height is shown in the

following figures. The TiOz plot suggests that two

groups of basalts are present in the upper Ely member and are interlayered throughout. Note the lack of

intermediate compositions between the two groups.

From the present data it remains uncertain whether destinct cycles in the nature of the tholeiitic volcanism are present within the sequence. RELATIVE STRATIGRAPHIC HEIGHT

LOWER ELY GREENSTONE UPPER ELY GREENSTONE I UPPER ELY GREENSTONE 0 .P> 0 GABBRO LAKE EAGLES NEST & CRAB LA Kr

"' 0

+ . G) 0 0 ,...... rt en . elf> 0 '.J r --...... __ I /\

I '\ I '\ I .co --Iv 0

...... 0 0 I w -..J "'"I RELATIVE STRATIGRAPHIC HEIGHT

LOWER ELY GREENSTONE UPPER ELY GREENSTONE UPPER ELY GREENSTONE b:l ()I . EAGLES NEST & CRAB LAKE GABBRO LAKE 0 I .-..J 0

.<.O Q '"rJ trJ 0 >-3 f-"' .f-"' ,,...... 0 ::: r-t

...._; f-"' w

f-"' ()1

I w co """I RELATIVE STRATIGRAPHIC HEIGHT

LOWER ELY GREENSTONE UPPER ELY GREENSTONE 1 UPPER ELY GREENSTONE n 'O .

,_ EAGLES NEST & CRAD LAKr GABBRO LAKE

0 1., ro I 1 i-' 1--J H 0 10 I ,...... :0-: t-' rt en \ I '-'

I w .i:. \.0 I