University of Montana ScholarWorks at University of Montana

Graduate Student Theses, Dissertations, & Professional Papers Graduate School

1977

Small laccoliths and feeder dikes of the northern Adel Mountain volcanics

Celia Kathleen Whiting The University of Montana

Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y

Recommended Citation Whiting, Celia Kathleen, "Small laccoliths and feeder dikes of the northern Adel Mountain volcanics" (1977). Graduate Student Theses, Dissertations, & Professional Papers. 7117. https://scholarworks.umt.edu/etd/7117

This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. SMALL LACCOLITHS AND FEEDER DIKES OF THE

NORTHERN ADEL MOUNTAIN VOLCAN ICS

by

C, Kathleen Whiting

B.S., University of Redlands, 1974

Presented in partial fulfillm ent of the requirements fo r the degree of

Master o f Science

UNIVERSITY OF MONTANA

1977

Approved by:

Chairman, Board o f Examiners

nrad5atebchooi~

Date

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP37918

All rights reserved

INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted.

In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion.

UMT Diassrtation PkAWâmg

UMI EP37918 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code

uest*

ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIRDTAIL BUTTE

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Whiting, C. Kathleen, M.S., 1977 Geology

Small Laccoliths and Feeder Dikes o f the Northern Adel Mountain Volcanics ( 74 pp.) .

D irector: David D. A11

Potassium-rich basalts o f the upper Cretaceous Adel Mountain Volcanics form a thick sequence of flows and volcanic breccias intruded by radial dike swarms, laccoliths, and stocks. Complex dikes radiating from multiple centers approximately 4 miles to the west o f the center proposed by Lyons (1944) and Beall (1973), feed two sets of laccoliths arrayed along circular arcs north of the main volcanic pile. Lyons (1944) and Beall (1973) showed that the northernmost la cco lith s are emplaced in the lower part o f the upper Cretaceous Virgelle sandstone along an arc with a radius of about 20 miles. These bodies are more than h mile in diameter and relatively s ill­ like. Four smaller laccoliths arrayed along a concentric arc with a radius of about 13 miles are steep-sided, their thickness being greater than their diameter. These are emplaced along the unconformable Interface between the Two Medicine Formation and the overlying Adel Mountain volcanic p ile . The laccoliths and their feeder dikes contain prominent euhedral phenocrysts of diopsidic augite and less prominent phenocrysts of plagioclase and either biotite or olivine. Surrounding the pheno­ crysts is a fine-grained groundmass of anorthoclase, plagioclase, pyroxene, and magnetite, altered in some cases to a complex assem­ blage of zeolites. Blebs of magnetite and apatite, apparently re­ flecting phase immiscibility, are pervasive. Pétrographie analy­ sis of porphyritic textures indicates that pyroxene phenocrysts crystallized prior to intrusion of the dikes. These phenocrysts, however, show no flow features. The lack of flow differentiation features may in part be explained by turbulence during magma in­ je c tio n .

11

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

My deepest thanks to David A lt whose Interest and enthusiasm

generated so many interesting discussions during the course of this

work, I also wish to thank Dr. Don Hyndman»who provided fresh in­

sights on several occasions. Dr. Tom Margrave, who read the final

manuscript, and Larry Williams for his assistance with the petrology.

The ranchers of the St. Peter area of Montana are gratefully

acknowledged fo r th e ir h o s p ita lity . I also thank J. R. McBride

for his special interest in this project.

m

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

Page ABSTRACT ...... ü

ACKNOWLEDGMENTS i i i

LIST OF ILLUSTRATIONS...... vi

LIST OF TABLES...... v i i i

CHAPTER

I INTRODUCTION...... 1

Previous Work and Purpose of th is Stu d y ...... 6

I I GENERAL GEOLOGY ...... 9

Sedimentary Rocks ...... 9

Eruptive Rocks ...... 13

Structure of Volcanic P ile ...... 15

Intrusive Rocks ...... 16

D ik e s ...... 17

L a c c o lit h s ...... 19

I I I PETROGRAPHY OF THE INTRUSIVE ROCKS...... 33

General Description and D istrib u tio n ...... 33

Descriptive Mineralogy ...... 38

A Possible Carbonatite in the AdelMountains? . . . 48

T e x tu re ...... 51

Problems Related to Texture ...... 52

iv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS (Continued)

Page

IV PETROGRAPHICPROVINCE OF CENTRAL MONTANA...... 59

Relation of Adel Mountain Volcanics to the Central MontanaPétrographie Province ...... 62

V SUMMARY...... 67

REFERENCES...... 69

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS

Figure Page

1. Location map of Adel Mountain Volcanics and Study Area . 1

2. Geologic Map of Adel MountainVo lc a n ic s ...... 3

3. Geologic Map of Northern Adel M ountains ...... 4

4. Igneous and Tectonic Features Associated with Lewis and Clark Lineaments ...... 5

5. Geologic Map of the Intrusive Rocks of the Adel Mountain Volcanics ...... 10

6. Major Structural Elements of North Central Montana . . . 11

7. Geologic Map of Area Remapped by A u t h o r ...... 20

8. Square B u t t e ...... 22

9. B e a ll's (1973) Suggested Mechanism fo r Radial Extension of Country Rocks ...... 24

10. Block Diagram Showing the Relationship of Intrusive Dikes and Laccoliths to the Sedimentary Formation . . . 25

11. Formation of Small Laccoliths ...... 27

12. G.K. Gilbert's (1877, p. 28) Hypothetical Ideal L a c c o l it h ...... 28

13. Block Diagram Illustrating that the Formation of a Laccolith Causes a Cylindrical Block of Overlying Layers to Move Upward ...... 31

14. Double Triangular Diagram Showing the Representative Compositions of the Alkali-rich Trachydolerite of the Northern Adel Mountain Volcanics ...... 34

15. Olivine-bearing Trachydolerite ...... 36

16. Sector and Fine Oscillatory Zoning in Salite Phenocrysts 39

17. Complex Intergrowths of Clinopyroxene Phenocrysts . . . 40

vi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS (Continued)

Figure Page

18. Clinopyroxene Phenocrysts Mantled by Olivine Crystals . . 42

19. Magnetite Blebs with Embedded Apatite Crystals ...... 46

20. Groundmass Anorthoclase and Plagioclase Microlites . . . 53

21. Sketch Map o f Cretaceous-Eocene Igneous Rocks o f Central Montana ...... •...... 60

22. Variation Diagrams ...... 65

v n

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Page

1. Average modal compositions o f B io tite - and O livine - Bearing Trachydolerite ...... 34

V I I 1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

INTRODUCTION

The Adel Mountain Volcanics» located approximately 56 kilometers

(35 miles) southwest of Great Falls, Montana, form the northern end of

the Big Belt Mountains of west-central Montana (Fig. 1). These volcanics

GREAT FALLS FT. SHAW IMMS

STUDY V AREA

ST. PETER

CASCADE

GREAT FALLS

10 K M V a DEL MOUNTAIN VOLCAN 1 CS Figure 1. Location map of Adel Mountain Volcanics and the Study Area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are a thick (> 900 meters) sequence of Late Cretaceous to early Tertiary

potassium-rich flows, breccias, and agglomerates, and unconformably

overlie the Two Medicine Formation. Complex dike swarms, s ills ,

laccoliths, and stocks, fed from centers within the volcanic pile,

intrude the Upper Cretaceous sedimentary formations surrounding the

northern edge of the volcanic rocks (Fig. 2). The western edge of

the Adel Mountains is bounded by an imbricate thrust zone of

M ississippian to upper Cretaceous rocks (Lyons, 1944).

Two sets of laccoliths form arcurate arrays north of the main

volcanic pile (Fig. 3). The northernmost large, si 11-like laccoliths

studied by Beall (1973) align on a circular arc with a radius of about

32 kilometers. Smaller steep-sided laccoliths, included within the

study area, are about 21 kilometers from the source and only a few

miles north of the volcanic pile. These intrusives form a smaller

arcuate array with visible dikes tangentially intersecting each laccolith.

Erosion of the surrounding sedimentary rocks has le ft the laccoliths

and the feeder dikes exposed as prominent buttes and ridges.

The Adel Mountains lie at the southern end of the Disturbed Belt,

a zone of intensely folded and faulted sediments paralleling the

Front Ranges of Montana and Alberta (Fig. 4). The bend in the Disturbed

Belt near the northern Big Belt Mountains could be in response to trans­

current movement along deep-seated faults (the Big Snowy lineament).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. — y ’ ' fZ' ■ / '. '. ^ ‘ ■ j ' ' • ' ', '■' GEOtOOiC MAP OF THC LEGEND NORTHERN BIG BELT RANGE |JL“£] Mr ## 4>p ihrnmr ê v li0 Fwmttion 3'f#t r*ll» l#k# ...... - Appfwimw# «OMl«t V fhrvi% (»wl| vivcc^*{^*tfrr Znmn Atfti Mipi »ol(ar>jtl Fautl \^>tCOS/O0A'ifr («■• ma formaii*A t ÜVilI Caiaratfri **>•<«

H4*Um«i fa rm a l.M

I luv fermât«#m

M i i i ; 9'*«*

Ma^.van Otvcman Um##p*i*d ^ Cambr»tA Japfr 8*11 ««Mai S«iOl P#l#f . «» •iSaokan* kh«l« { 8rH k«ri««)

NTKvk'.E i&NtOuS NOCmt 1 Ater>|p^'1« (~~ÿp~| fraçbw b*t«n

« m m 1

m lftA 0’

Figure 2. Geologic map of the Adel Mountain Volcanics. Circled area was remapped by author. Area enclosed by dashed lines was remapped by Beall. (Lyons, 1944; modified by Beall, 1973)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SHAW BU TTE

CROWN y/'i'lTSQUARE / ' H BUTTE BUTTE

B(ROTAIL

LIONHEA&s, V h a y s t a c k

SHBACK y /, I fCASCADE K " \r< , •

CASCAD

ÛAL QAI

ADEL INTRUSIVES ^ AND VOLCAN CS I TWO MEDICINE ' FORMATION n i l EAGLE SANDSTONE COLORADO SHALE

Figure 3. Geologic map of the northern Adel Mountain Volcanics showing the sedimentary formations and laccoliths.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■D O Q. C 3 Q.

■D t CD II C/) W IW I^ l 3o' LI O TCRnAm iNTRvsnrc aoocs 3 *TnM««rM tgM M M i* CD 8

(O' o

~ SiS ' ' / / \ «II V ^ V " g I0r«enw«d . ^ •• v • ^ 3 . ' \ w v c « 3" CD * + ^+^f^.4Î / ) \ j w /^9P*KMR.|

CD ■D O Q. C a o 3 ■D O

Lewis AMO a A M UMeAMSMTS eoMsmucMT sm ucrwes CD iSorfero trocoo of *op-soofoO mopooAoorw Q. s i# i 0*o Aomo 1 WVE-eOWlER IWfEAMENT T Cot Crook ontMfew end f M t m o • Big WtaU orOieUno C v d U m n NW# i th n itU Big Snowr # U ff V 1 l a k e b a s in LINEAMENT « ■D # «OMMoh Pochot onfidiM CD t l Showrmit onlielino 3 058URN l in e a m e n t n Big CeuloooHoHMono m m Slnicturt cMlMni «n bM« #f CKw # C/) C/) o Lake SooMt fouti mow #W # m â NuwMW# (CratMMM} u Pfiror lorroc* 1 a c SNODfV UHCAHEHT ' « Clark fork on khoWn MW# » Ookwrn foult lono S CAT CHEEK UHEAMENT If Hop# foiAl CampilM from The Tectenk Mop of Itio Uitlod Slotoo,1Mt « LHIto Bolt UptlM n JwOilh Meunloinc Intruclvoo 20 Adel Mtn volcanic*

Figure 4. Igneous and tectonic features associated with the Lewis and Clark lineaments. tn (From Smith, 1965; modified by Beall, 1973). but it is not clear whether these faults played a significant part

in the development of the Disturbed Belt (Smith, 1965). A series of

west-northwest trending structural lineaments, collectively known as

the Lewis and Clark Lineaments, are thought to reflect deep-seated

lateral movements in the crust or upper mantle (Smith, 1965).

On a regional scale, the Adel Mountains igneous center is a

subprovince of the Central Montana Pétrographie Province defined by

Larsen (1940). Other igneous centers scattered through Central Montana

from the Canadian border to Yellowstone National Park are also character­

ized by early Tertiary alkaline igneous activity and an affinity for

the edge of the stable craton where it is intersected by deep-seated

lineaments.

Previous Work and Purpose o f th is Study

Larsen (1940) recognized the Adel Mountain Volcanics (Larsen's "Area

west o f Cascade") as part o f the Central Montana Pétrographie Province,

but based much of his conclusion on work by Lyons. While conducting

a reconnaissance study of the northern Big Belt Mountains, Lyons (1944)

mapped the Adel Mountains, delineating the extent of the volcanics,

establishing stratigraphy, and analyzing the igneous rocks. Beall

(1973) continued research on the Adel Mountain volcanics by modeling

the stresses favorable for the formation of the dike swarm and the

Three Sisters "stock". Beall (1973) also studied the petrochemical

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. evolution of the large laccoliths to the north of the study area.

Both of these studies raise questions which merit further investigation.

This study pursues questions or ideas suggested in the works by

Lyons and Beall. The in itia l phase of research sought to verify the

suggestion made by Beall (1973, p. 35) that there is probably not a

single magmatic source for all of the dikes. The southwest quarter

of the Simms Quadrangle was mapped to determine i f the dikes point to

a source area other than the Three Sisters "stock". Corollary parts

of this problem include the determination of the significance of the

smaller concentric array of laccoliths in the study area (see Fig. 3),

nested within the larger arc of the four laccoliths to the north; and

why these smaller laccoliths are so different in shape from the s ill­

lik e forms o f Square, Shaw, Crown and Cascade Buttes to the north.

The research further attempted to add petrologic detail to the

knowledge o f the igneous rocks o f the area w ith the hope of elucidating

the origin of potassium-rich magmas. To determine if any compositional

differences exist among the intrusive rocks, I sampled and petrograph-

ically analyzed all of the major intrusive bodies and dikes in the

study area. Birdtail Butte is the only laccolith with a partially

exposed cross-section and so was the only butte to be sampled v e rtic a lly

and petrographically analyzed to determine if there was evidence Of

crystal settling. Determination of compositional and textural variations

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8

in the dike rocks was accomplished by sampling and petrographically

analyzing rock along the length of the dikes and across the dikes,

perpendicular to the length. A relative order of phenocryst crystal­

lization was also determined by pétrographie study.

Another phase of this research attempted to relate textural

features of the northern Adel Mountain dikes to experimental studies

by Bhattarchji and Smith (1964) that suggest phenocrysts w ithin a

flowing magma should 1) become oriented with long axis parallel to

flow direction, if they are rod-shaped, and 2) should move away from

the walls toward the center of the dike (flow differentiation). Rod­

shaped clinopyroxene phenocrysts in the Adel Mountain Volcanics seem

to f it the description of crystals which should show these flow features,

Considerable effort was spent in the field seeking evidence of flow

orientation of the clinopyroxene phenocrysts and evidence of higher

phenocryst concentrations in the dike centers. Possible explanations

for the absence of flow features during magma flow and pertinent ex­

perimental work on flow differentiation are discussed.

Finally, the general relation of the Adel Mountain Volcanics to

the Central Montana Pétrographie Province, based on literary research,

is reviewed.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER II

GENERAL GEOLOGY

The geologic map of Figure 2 and the intrusive rock map of

Lyons (1944) (Fig. 5) show the general relationship of the sedimentary,

extrusive, and intrusive rocks of the northern Adel Mountains. A

summary o f the regional stratigraphy can be found in Lyons' study

(1944) o f the northern Big Belt Mountains. Names o f the formations

used here are based on Cobban (1955, p. 107). The thicknesses of the

Cretaceous formations are from well-core data obtained from the

Montana Department o f Natural Resources and Conservation (John Hug,

personal communication, 1975). Well holes are represented on Figure 7

by crosses.

Sedimentary Rocks

Sedimentary rocks o f the northern Adel Mountains are upper

Cretaceous sandstones and shales. The rocks, o f both marine and non­

marine origin, contain fossils and have been correlated with sedimentary

formations on the northwest flank o f the Sweetgrass Arch (Fig. 6). The

regional structure is a shallow, southwest-dipping homocline. Some

fo ld in g , as in the Crown Butte a n tic lin e (Erdmann, 1959, p. 189),

and some shallow thrust fa u ltin g (Cobban, 1955, p. 139) north o f the

9

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10

SIMMS

1

o CASCADE

\ % " l ( ] ,/

' O 0 WOLF CREEK A;:>:

5 KM Figure 5. Geologic map of the intrusive rocks of the Adel Mountain Volcanics. (Modified from Lyons, 1944)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. n r —

KEVK

HIGHWOOD ' MTNS

EL mUNTAINS

LITTLE LT MTNS.

Figure 6. Major structural elements of north central Montana (from Alpha, 1955).

volcanics of the Adel Mountains probably reflect the proximity of both

the Disturbed Belt of the Rocky Mountain Front to the west and the

volcanic pile of the Adel Mountains to the south. The lack of sedi­

mentary rock outcrops just north of the volcanic rock outcrops makes

evaluation of the extent of tectonic deformation difficu lt.

The Colorado Shale, Lower Upper Cretaceous, is the oldest sedi­

mentary formation outcropping north of the volcanic pile. This forma­

tion is a dark gray, fossiliferous marine shale with interspersed sandy

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12

layers. This shale is 465 meters thick, and has a distinctive

rhyolitic tu ff layer 60 meters above the base (Lyons, 1944). The

Telegraph Creek Formation is transitional from the Colorado Shale to

the Eagle Sandstone and consists o f a medium gray sandy-silty shale

with some fine-grained shaly calcareous sandstone. This formation is

about 100 meters th ic k . The V irg e lle Sandstone member o f the Eagle

Sandstone is a 45 meter thick light gray, concretionary, cliff-forming

sandstone. Cross-bedding and thin dark brownish layers of fine-grained

magnetite sandstone (Cobban, 1955, p. 107; Erdmann, 1959, p. 161)

are common in the upper part o f the V irg e lle Sandstone. The uppermost

Cretaceous formation in the area is the Two Medicine Formation.

Originally more than 600 meters, the formation has been reduced by

erosion to less than 270 meters. The Two Medicine Formation is com­

posed of non-marine variegated shales interbedded with cross-bedded

sandstones and quartz-pebble conglomerate.

The distribution of the intrusive bodies in the sedimentary rocks

shows an interesting pattern (see Fig. 3). The larger sill-like

laccoliths are confined to the Virgelle Sandstone and contrast sharply

with the smaller steep-sided intrusive bodies in the Two Medicine

Formation. This apparent difference in style of intrusion may be due

to the greater rigidity of the Virgelle Sandstone compared to the

relatively plastic shales of the Two Medicine Formation (Lyons, 1944).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13

Dikes Intruded in to the more rig id V irg e lle Sandstone appear to have

been diverted into s ills , whereas the shales of the Two Medicine

Formation could be pryed apart by intruding magma to form wide dikes

(Birdtail and Fishback Buttes) or mound-like structures (Mt. Sullivan,

Haystack, and probably Lionhead Buttes). The smaller intrusive bodies

might also have formed at the unconformable contact between the Two

Medicine Formation and the overlying volcanics.

Eruptive Rocks

Volcanism in the Adel Mountains is thought to have commenced during

the Late Cretaceous as evidenced by a thin rhyolitic tu ff layer near

the base o f the upper Cretaceous Colorado Shale (Lyons, 1944). The

volcanic flow breccias which comprise the major proportion of the Adel

Mountain Volcanics unconformably ove rlie the upper Cretaceous Two Medicine

Formation. The time of the Adel Mountain volcanism, based on the

relative stratigraphie position of the volcanic rocks, is placed at

Late Cretaceous.

The volcanic pile is more than 900 meters thick and consists pre­

dominantly of zeolitized alkali basalt breccia flows. Maroonish-brown

in color, the breccia flows weather differentially, leaving the breccia

fragments as a rubbly surface cover. Breccia fragments range in size

from several millimeters to nearly 4 meters in diameter and comprise about

80 percent of the flows. Most of these fragments are coarse-grained

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

trachydolerite (the same intrusive rock that forms the dikes and

laccoliths), but finer-grained rocks are also common. The mineralogy

of all components is similar, consisting of euhedral phenocrysts or

fragments of clinopyroxene, plagioclase, and some anorthoclase, which

also occurs locally as anhedral crystals. Magnetite is ubiquitous.

Traces of quartz, biotite, pseudoleucite and altered olivine are present

in lith ic fragments. Variations in the texture, color, degree of

weathering, the type and percentage of phenocrysts (Parsons, 1969), and

the degree of zeolitization in the breccia fragments of the Adel

Mountains characterize these deposits as breccia flows.

The matrix surrounding the breccia fragments is exceedingly fine- ,

grained. Very fine phenocrysts of feldspar are usually the only mineral

recognizable in the hematite stained m atrix. Locally the m atrix appears

to be composed of fine-grained broken crystal fragments.

Sills and flows are quantitatively minor (Beall, 1973, p. 11),

but distinctive and conspicuous components of the volcanic pile. The

columnarly-jointed s ill of Skull Butte is prominently visible as a

concordant, lig h t-co lo re d layer dipping 7 degrees to the southeast.

The rocks of the Skull Butte s ill are porphyritic with phenocrysts of

pyroxene (10 percent), olivine (8 percent), plagioclase (20 percent)

and 1 percent magnetite. The remaining 61 percent of the rock is made

up of a fine-grained m icrolitic groundmass of feldspar, pyroxene, olivine,

and magnetite. Other s ill-lik e layers in the volcanics rocks are present

to the west of the study area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15

One thin, brick-red-colored layer which may be a paleo-soil

horizon or possibly an altered ash deposit is exposed near the top of

Telegraph Mountain. This layer is composed of fragmental and inter­

s t it ia l sanidine, a low percentage o f ragged pyroxene, plagioclase,

anorthoclase, magnetite and a trace of hornblende in a reddish-brown

hematitic matrix. A few rock fragments are also present. Discoloration

and increased solidification and fragmentation near the top of the layer

suggest contract metamorphism and disruption, respectively, by the

overlying breccia flow.

Structure of the Volcanic Pile

The breccia flows that form most of the volcanic pile are thought

to be produced underground by gas expansion. The breccia is le f t as

vent fillin g until it is extruded in thick unstratified deposits closely

associated with the main volcanic center. The thickness of the breccia

flow beds varies from a meter to at least 30 meters (Parsons, 1969),

and is consistent with the bedding characteristics of the Adel Mountain

Volcanics. Steep in itia l dips of lower beds may indicate part of the

cone structure (Parsons, 1969, p. 175).

Attempted structural reconstruction of the volcanic centers in

the Adel Mountains through use of outcrop attitudes of layers within the

breccia flows shows only random patterns. A more subjective impression

of the structure of the volcanic pile can be gained by viewing the canyon

w alls from a distance. They convey an impression o f a volcanic p ile

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16

growing from alternate sources and various directions. A sequence of

lower layers dipping at a moderate angle, truncated and overlain by

much shallower-dipping layers is often repeated several times in an

exposure. The overall structure of the volcanic pile consists of five

major northwest-trending anticlines approximately paralleling the

structural trend of the disturbed belt (Lyons, 1944).

No faults in the volcanic rocks in the study area have been

recognized. The d ifficu lty in mapping faults was also noted by Lyons

(1944), and Beall (1973, p. 13) who suggested that recognition of faults

is made more d ifficu lt by the lack of marker beds and distinctive

lithologies. Beall (1973, p. 13) does mention some lineaments trending

through the volcanic pile that could be faults.

Intrusive Rocks

Dikes terminating in laccoliths, stocks and some s ills are the

predominant forms o f the in tru sive phase of the Adel Mountain Volcanics.

These shallow intrusions, as their texture suggests, are trachydolerite,

containing euhedral phenocrysts of clinopyroxene. Zeolitization to

some extent is common in almost all of the intrusive rocks.

Intrusion of dikes through the basal sections of the volcanic pile

indicates a relatively younger age for the intrusive rocks. Faulting,

while not recognized in intrusions surrounded by sedimentary formations,

is noted in some trach ydo lerite and hornblende dikes (Lyons, 1944, p. 455)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

This faulting serves to bracket the time of igneous intrusion -- after

the volcanism, and prior to the deformation accompanying the Laramide

Orogeny,

Dikes. The dike swarm is one of the most striking features of the

Adel Mountain Volcanics. Dikes radiating from centers within the volcanic

pile cut through the volcanic sequence and the underlying Upper Cretaceous

sedimentary formations. Intrusion o f dikes appears to be confined to

the Two Medicine Formation and Colorado Shale. The absence of dikes in

the V irg e lle Sandstone suggests tha t the r ig id it y o f the sandstone forced

intruding dikes to turn and become sills.

The dikes are exposed as prominent ridges that can be followed

laterally up to 8 kilometers (5 miles) in the Two Medicine Formation and

up to 21 kilometers (13 miles) in the Colorado Shale before they ter­

minate in laccoliths or dive beneath the sediments. More commonly

the dikes are shorter segments which in several locations become high-

angle sills.

The source area fo r dikes in the northern Adel Mountains was thought

to be the Three Sisters "stock" (Lyons, 1944). This "stock" is not a

stock in the usual sense, but an area o f exceedingly dense dike concen­

tra tio n (B eall, 1973, p. 16). Dike trends o f the northern Adel Mountains

indicate that there is another source area within the volcanic pile.

Projection of the due North to N 25°W dike trends of the study area

toward an inferred source suggests that this dike swarm originates from

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18

an area about 6.5 km west o f the Three Sisters "stock". Projection

of dikes feeding the larger laccoliths to the north indicates that

these laccoliths may also originate from the same source.

The dimensions of the tabular dikes are most accurately determined

where dikes can be followed from the sedimentary rocks into the volcanic

rocks. The exposure of one light-colored dike within the brownish

breccia flows reveals sharp intrusional contacts, a width of 21 meters

and a dip within 20° of vertical. Generally dikes in the sedimentary

rocks are eroded so th a t only a width o f 9 to 24 meters can be estimated.

Intrusion of dikes has deformed both volcanic and sedimentary rocks.

Deformation of volcanic rocks where the thinner dikes "snake through"

the extrusives appears to be a consequence o f mechanical anisotropy

of the extrusives at shallow depth (Beall, 1973, p. 17). Dikes intruding

sedimentary formations appear to arch the sediments as evidenced by

the slight dip of sediments away from both sides of the dike. Stoping

during dike injection has le ft lenses of sandstone and shale enclosed

in the dike. Discoloration of both the dike rocks and surrounding

sediments and mineralogical alteration of the intrusive rock suggests

that a metasomatic(?) exchange took place, with the sediments supplying

the water required for the nearly pervasive zeolitization.

The dikes are alkali basaltic and are petrographically similar to

the rocks of the laccoliths. Elongate phenocrysts of diopsidic augite

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

(or sali te) are the most striking grains found in the intrusive rocks.

The presence of olivine or biotite serves to divide olivine-bearing

rocks from those that are biotite-bearing. The rare occurrences of

these minerals together can probably be explained by the biotite

being a secondary, discontinuous reaction rim around the primary

o liv in e .

The map of Figure 7 shows that no consistent relationship exists

between the biotite- and olivine-bearing dikes. Both rocks are,

however, present in the same dike, indicatin g the composite o rig in of

some dikes. Injection of the later phase is characteristically along

one side of the dike, rather than through the dike center.

The relative order of injection of biotite- and olivine-bearing

dikes is based on a single field occurrence. To the south of Birdtail

Butte, a sill-like intrusion of olivine-bearing intrusive is intruded

by the biotite-bearing feeder dike of Birdtail Butte. This cross-cutting

relationship is the only evidence found to suggest that olivine-bearing

dikes predate the biotite-bearing dikes.

Laccoliths. Small laccoliths emplaced along an arc with a radius

of 21 kilometers intrude the Two Medicine Formation beyond the north­

western edge of the volcanic pile. Feeder dikes tangentially inter­

sect each la c c o lith . These steep-sided la cco lith s have an exposed

thickness of at least 120 meters and are all less than 800 meters in

diameter. The lens-shaped intrusions of Birdtail and Fishback Buttes

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20

Biotite-Sea ring □ Intrusive Olivine-Bearing Intrusive BIRDTAIL V VI Volcanic Rocks LIONHEAD HAYSTACK 0 X WELL HOLES

FISHBACK &

V SKULL BUTTE

©

0 2 3 KM JU ■■I

Figure 7. Geologic map of area remapped by author.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21

are about 800 meters long and 300 meters wide. Mound-like intrusions

of Haystack and Mt, Sullivan Buttes are about 400 meters in diameter.

Lionhead Butte, eroded in to a horse-shoe-shaped intrusion was probably

originally similar to the mound-like intrusions. No sediments are

visible at the base of these small laccoliths and it is impossible,

because of lack of outcrop, to prove that sediments once arched over

these intrusions. Jointing is well-defined and has developed normal

to the steep sides and normal to the now-missing roof rocks.

The shape of the small laccoliths contrasts sharply with the s ill­

like laccoliths to the north of the study area (Fig. 8). Crown, Shaw,

Square, and Cascade Buttes are topographically prominent intrusions

sitting atop light-colored sandstone. These northernmost laccoliths

are emplaced in the lower part of the Virgelle sandstone along a 27

kilometer arc with a radius of 32 kilometers. Feeder dikes can be

followed through the Colorado Shale into the lower part of the Virgelle

Sandstone where the r ig id ity o f the sandstone appears to have forced

the dikes into sill-like intrusions.

The diameters of these laccoliths range from 1.2 kilometers in

Crown Butte to 4 kilometers in Shaw Butte. Most intrusions are over

180 meters thick (Beall, 1973, p. 56), The sedimentary base of these

laccoliths is visible and enough remnants of an overlying chill zone

remain to indicate an o rig in a l domal-shape roof (B eall, 1973, p. 56).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22

Figure 8. Square Butte. Compare the s 111-like form of this laccolith with the steep-sided form of Birdtail Butte shown in the Frontisepiece.

The intrusive rocks of both small and large laccoliths are

similar. Smaller laccoliths are trachydolerite with some variations

in composition. Lionhead and Birdtail Buttes, the westernmost in­

trusions, are light-colored trachydolerite containing phenocrysts of

diopsidic augite and biotite surrounded by a very fine-grained ground-

mass of predominantely anorthoclase laths. Cavities are fille d with

zeolites. Haystack, Fishback and Mt. Sullivan are dark-colored

trachydolerite with phenocrysts of diopsidic augite, altered olivine,

and plagioclase. The groundmass is also fine-grained, composed o f

coarser anorthoclase microlites and fine-grained olivine and plagioclase.

Z eo lites, p a rtic u la rly thompsonite, are in t e r s t it ia l. Sampling o f a

vertical section through Birdtail Butte revealed no evidence of crystal

settling or flow textures.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23

The rocks of the larger laccoliths are compositional ly similar

to the rocks of Haystack and Fishback Buttes. The texture is generally

coarse with larger anorthoclase phenocrysts and differs from the

porphyritic-aphanitic texture of the smaller intrusions.

Morphological and dimensional differences between the large and

small laccoliths can begin to be explained by Beall's (1973) model

of the formation of the Adel Mountain Volcanics dike swarm. Beall

(1973, p. 36) proposed tha t "the extension needed in the country rocks

to accommodate such a large volume o f dike magma was derived not by

doming o f rocks over a magma chamber, but by subsidence and thinning

in the thick sequence of relatively plastic shale as a consequence of

the load pressure exerted on them by a large volcanic cone." This

suggestion o f subsidence appears to be consistent w ith Lyons' (1944)

observation that the Adel Mountain Volcanics occupy a structural trough

at right angles to the northwest trend of the regional structure.

Figure 9 illustrates Beall's basic hypothesis. The shape of the

base of the cone was changed during separate experiments from circular

to e llip t ic a l. Results showed that an e llip t ic a l base produced a dike

swarm along the major axis of the cone, very similar to that of the Adel

Mountains. Layers with varying strengths of gelatin corresponding to

the strengths o f rocks in the area produced s i l l - l i k e la cco lith s sim ila r

to Crown, Shaw, Square, and Cascade Buttes when injected with fluid gelatin,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■o o cQ. g Q.

■o CD

C/) C/)

8 VOLCANIC c5' CONE

"PLASTIC" 3 3" CD MESOZOIC SHALES VISCOUS FLOW ■oCD O Q. C ZONE OF EXTENSION a 3o "O o RIGID" PRECAMBRIAN AND PALEOZOIC ROCKS CD Q.

TD Figure 9. Beall's (1973) suggested mechanism for radial extension CD of country rocks. C/)(/)

ro 25

In his modeling, Beall was concerned with the formation of the

dike swarm and the larger laccoliths. To evaluate the formation of

small and large la c c o lith s a block diagram (Fig. 10) showing the re­

lationships of the intrusive rocks to the sedimentary formations was

devised.

Dike intrusion in the Adel Mountains is confined to the Upper

Cretaceous formations (Lyons, 1955). Therefore, in the block diagram,

only the Upper Cretaceous Colorado Shale, Virgelle Sandstone, and Two

Medicine Formation are represented. The existence of a central magma

chamber (see Fig. 10) is not clearly established, but a central magma

chamber a t some depth does seem a more a ttra c tiv e source than a batho-

lith underlying an area of 2100 square kilometers (Lyons, 1944).

VIRGELLE SANDSTONE

TWO MEDICINE FORMATION

COLORADO SHALE laccol 1 th dike

5 K M — '

Figure 10. Block diagram showing the relatio nsh ip o f in tru sive dikes and laccoliths to the sedimentary formations.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26

From study of the block diagram, it becomes clear that the sheet­

like feeder dikes of the large laccoliths must have been intruded

into the Colorado Shale, probably with a very low upward trajectory

(refer to Fig. 10 for this discussion). Intersection of the dikes

with the more rigid Virgelle Sandstone forced the dikes to form s ill-

lik e intrusions (i.e . Crown, Shaw, Square and Cascade Buttes). Dikes

in the Two Medicine Formation must have intruded the stratigraphie

horizons above the V irg e lle Sandstone.

The diagram (Fig. 10) also explains why no dikes can be traced con­

tinuously from the Two Medicine Formation into the Colorado Shale

(B eall, 1973, p. 18). Dikes injected from the Two Medicine Formation

in to the V irg e lle sandstone would probably form s ills . I f injected

from below the V irg e lle sandstone, the dikes would be forced into s i l l -

like intrusions by the rigid sandstone.

A greater depth o f emplacement o f the larger la cco lith s appears to

explain the coarser groundmass of the rocks and to be consistent with

G. K. Gilbert's (1877, p. 84) conception of laccolith formation. In

the smaller laccoliths evidence of a fine-grained groundmass and

stratigraphie constraints as indicated in the block diagram suggest

emplacement at a shallower depth than the larger la c c o lith s .

The cause of dike termination and the formation of the smaller

laccoliths is not clear. One observation that may be pertinent is that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27

II

1 /2 K M Volcanic flow breccia

Trachydolerite Dike i.* t, J.i t Two Medicine Shale

Figure 11. Formation of small laccoliths.

I. Dike intersects Two Medicine Formation-Adel Mountain Volcanics unconformity.

II. Laccolith forms by faulting the overlying volcanic breccia flows.

I I I . B ird ta il Butte today. Erosion has renoved the volcanic rocks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28

all of the patches of volcanic rock remaining in the area have a basal

elevation which would be below the tops o f the la c c o lith s . Possibly

these "laccoliths" are not true laccoliths intruded between layers of

sediment, but bodies of magma that were intruded between the sediments

and the base of the overlying volcanic pile (see Fig. 11).

Are the small steep-sided intrusive bodies actually laccoliths?

The form known as a laccolith (Fig. 12) was firs t recognized in the

Henry Mountains of Utah by G. K. Gilbert (1877). In his interpretation

of the laccolith, Gilbert (1877, p. 55) imagined a toadstool shaped

intrusive to form between sedimentary layers with the stem representing

the conduit feeding the body. The generally fla t bottom is oval in

Figure 12. G. K. Gilbert's (1877, p. 28) hypothetical ideal la c c o lith .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29

overall view and the form is similar to half of a partially flattened

sphere. The height was never more than 1/3 of the width and usually

closer to 1/7. No feeder conduits for the laccoliths were identified

in the Henry Mountains. G ilb e rt (1877, p. 20) stated th a t the dikes

represented below the laccolith "are purely hypothetical, since they

cannot be seen. In a general way the molten rock must have come from

below, but the channel by which i t came has in no circumstance been

determined by observation."

Laccoliths in the Adel Mountains differ in several ways from

Gilbert's (1877, p. 84) conception of the ideal laccolith. The four

large laccoliths show the more characteristic form of a laccolith with a

fla t bottom and a small height to diameter ratio. The smaller laccoliths

however, are either steep-sided intrusions with a height to diameter

ratio of about 3 to 1 in the case of Haystack and Mt. Sullivan or

elongate lenses resembling a widened dike in the case o f Fishback and

B ird ta il Buttes. The Adel Mountain intrusives are, however, c la s s ifie d

by Lyons (1944, p. 459) "as la c c o lith s c h ie fly on the form of the

outcrop, the pétrographie character of the rocks and their jointing."

The most significant deviation from Gilbert's model is that the

feeder dikes prominantly intersect most of the intrusive bodies. More

importantly the lateral dimensions of the dikes as well as the modeling

by Beall (1973) are convincing evidence that magma was injected laterally

through dikes rather than v e rtic a lly as imagined by G ilb e rt.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30

Though Gilbert (1877, p. 75) perceived that magma coming up from

below would arch the overlying strata, Johnson and Pollard (1973),

who have studied several laccoliths of the Henry Mountains, suggest that

the injected magma spread out laterally as a thick sheet or s ill until

it gained sufficient leverage to fracture the overburden near its

periphery. The overburden would then be lifted and the intrusion

thickened to become laccoliths. The only problem in relating the in­

trusional process proposed by Johnson and Pollard (1973) to the Adel

Mountain laccoliths might be the application of lateral force of the in­

truding magma to a theoretical model requiring a vertical driving force

to lift the overlying strata.

G ilb e rt (1877, p. 80) observed in the Henry Mountains that there

are no laccoliths with diameters of less than 0.8 kilometers. His ex­

planation for this peculiarity requires analysis of the theoretical

formation of a laccolith. The formation of the laccolith could be

imagined to be a cylinder of uplifted rock bounded by a cylindrical

fault, with the cylinder of rock (Fig. 13) pushed upward by the magma

(Gilbert, 1877, p. 81-85). The driving force caused by the magma pressure

is proportional to the area of the end of the cylinder, so that it is

proportional to the square of the radius. The resisting force is pro­

portional to the surface area of the sides of the cylinder, so it is

proportional to the radius. The driving force decreases more rapidly

than the resisting force with a decrease in the radius. For a given

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

magmatic pressure there is , therefore, a lim itin g radius above which

the magma can l i f t its overburden and below which the magma cannot.

Thus Gilbert reasoned {1877, p. 83) that small laccoliths can not form.

This explanation assumes the overburden deforms elastically,

a condition which is not completely applicable to the actual situation

since rocks can withstand very little flexing without breaking (Johnson,

1970). Johnson and Pollard (1973) point out th a t the overburden fa ils

by shearing along the cylindrical faults or by diking near the periphery

and conclude tha t any in tru sive body which would be called a la c c o lith

rather than a s ill has nearly always deformed its overburden inelastically.

Figure 13. Block diagram illustrating that the formation of a laccolith causes a cylindrical block of overlying layers to move upward.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

What are the small steep-sided intrusives of the Adel Mountains

to be called? Possibly the small intrusions of the Adel Mountains are

intrusions which caused extreme deformation of a relatively thin over­

burden o f incompetent shales or were intruded between the sediments

and overlying volcanics.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I I I

PETROGRAPHY OF THE INTRUSIVE ROCKS

Lyons (1944) described most of the rock types o f the Adel

Mountains. His descriptions, however, do not indicate the composi­

tional difference of the dikes and laccoliths of this study area.

Descriptions of the composition, mineralogy, and texture reported

here are based on investigation by pétrographie microscopy and hand-

specimen identification. Mineral percentages are visual approximations

with some point counts for verification of accuracy.

The c la s s ific a tio n o f a lk a lin e rock is often ambiguous. For

sim plicity, the alkaline rock names used in this paper will follow

the c la s s ific a tio n found in Sorensen (The A lkaline Rocks, 1974, p. 70).

General Description and D istrib u tio n

The igneous rocks of the Adel Mountains are generally classified as

alkali basalts. Composed of olivine, Ca-rich clinopyroxene, plagioclase

(more calcic than An^^) and opaque oxides, all the rocks are silica

undersaturated. Feldspathoids, in particular analcime, if present,

have only accessory status.

The major in tru sive phase o f the northern Adel Mountains is a

slightly alkali-rich trachydolerite (Fig. 14 and Table 1), an alkali

basalt defined by an alkali feldspar content of 10 to 40 percent of the

33

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

Table 1.

BIOTITE-BEARING* OLIVINE-BEARING** TRACHYDOLERITE TRACHYDOLERITE

Phenocrysts + Phenocrysts + Phenocrysts Groundmass Phenocrysts Groundmass

Pyroxene 12.3% 16.1% 11.7% 16.9% Olivine 6.7 11.1 B io tite 2.6 6.9 Plagioclase 17.2 19.4 18.8 28.7 Anorthoclase 35.2 29.2 Magnetite/Apatite 2.8 5.5 2.0 4.9 Zeolites 17.4 9.6 M atrix*** 67.6 62.0

Total 102.5% 100.5% 101.2% 100.4%

* Average percentages based on 17 samples ** Average percentages based on 14 samples ***The matrix is very fine grained anorthoclase and plagioclase. Because of the difficulty in distinguishing these two minerals in thin section, the percentages in the "Phenocrysts + Groundmass" column are at best approximations.

QUARTZ

ALKALI FELDSPAR PLAGIOCLASE

— biotite-bearing trachydoleri te

FELDSPATHOIDS X — olivine-bearing trachydolerite Figure 14. Double triangular diagram showing the representative compositions of the alkali-rich trachydolerite of the northern Adel Mountain Volcanics.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35

total feldspar present and by feldspathoids comprising less than

10 percent of the rock (Wilkinson, 1974). The rocks are porphyritic

with phenocrysts of diopsidic augite, plagioclase along with either

biotite or olivine. Accessory magnetite associated with crystals of

apatite is common. Phenocrysts are surrounded by a fine-grained ground­

mass composed of anorthoclase laths and flakes of magnetite with some

plagioclase, pyroxene, biotite or olivine, and very rarely analcime.

Felsic veins similar to those described by Beall (1973, 1972) in the

larger laccoliths are composed predominantly of anorthoclase laths

and regularly cut the dike rocks of the eastern part of the study area.

In his original reconnaissance study Lyons (1944) described the

intrusive rocks within the study area as homogeneous syenogabbros.

Systematic sampling of the dikes and laccoliths and detailed pétro­

graphie study reveal that the trachydolerites (syenogabbros) are of

two varieties: one olivine-bearing and the other biotite-bearing.

Olivine-bearing rocks are dark gray and typically contain 3-10

percent olivine which varies from nearly fresh to completely altered.

Sub- to euhedral phenocrysts are common, with finer-grained altered

olivine present as part of the groundmass. The overall texture is

usually less altered and more distinct than in the biotite-bearing

rocks (Fig. 15).

Rocks with biotite phenocrysts (3-5 percent) are a medium greenish-

gray. The phenocrysts are euhedral, slightly elongate laths with rounded

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

sa il te

anorthoclas U magnet!te

plagioclase îq

s '

liv in e

s a li te V.

Figure 15. Olivine-bearing trachydolerite. Groundmass anorthoclase and plagioclase laths are slightly coarser than in biotite-bearing variety (see Figure 20).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37

corners. B io tite in the matrix appears as very fin e la th s, which

with the anorthoclase laths, sometimes define a flow foliation.

Only a few intrusive bodies contain both olivine and biotite.

In these occurrences the highly-altered olivine and the corroded-looking

biotite are intimately associated, leading to the conclusion that the

b io tite may be a discontinuous reaction rim around the o liv in e (see,

for example, Wilkinson, 1974). This reaction could also explain why

fine relict olivine grains in the interstitial matrix are slightly

pleochroic. The biotite and olivine volumetrically form only about '

5 percent of the rock.

The map of the biotite- and olivine-bearing intrusives in Figure 7,

discloses no pattern to the distribution. The two small laccoliths to

the west are biotite-bearing whereas those to the east and Mt. Sullivan

are o liv in e bearing. I t is in te re stin g to note that though Mt. Sullivan

and Lionhead Butte appear to originate from the same dike (see Lyons,

p. 459), the buttes are of two different compositions. Both rock types

are found together within several dikes, confirming the composite origin

of some dikes.

The significance and implications of olivine- and biotite-bearing

rock to the paragenesis of the intrusive rocks have not been entirely

established. The single field occurrence by a biotite-bearing dike

cutting through the sill-like intrusion to the south of Birdtail Butte

indicates the intrusion of biotite-bearing magma must have followed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38

the intrusion of olivine magma. The inclusion of both olivine and

biotite grains within the pyroxeme phenocrysts suggests that both

minerals formed or were incorporated in to the magma p rio r to pyroxene

crystallization.

Descriptive Mineralogy

Pyroxene phenocrysts are the most conspicuous mineral in the

tra ch yd o le rite . Dark green to black in handspecimen, the phenocrysts

are euhedral, rod-like crystals that are up to 1.2 cm long with diameters

up to 0.4 cm. In thin section clinopyroxene is light green and rarely

lig h t brown. A green co lo r, probably in dicatin g a s lig h tly enriched

iron concentration (Heindrich, 1965, p. 216), is unusual in a lk a li

basaltic rocks which more conmonly show the lila c to purplish colors of

a higher titanium content (Heinrich, 1965, p. 216; Wilkinson, 1974).

Sector and fine oscillatory zoning (Downes, 1974) are often well-

developed in the clinopyroxene phenocrysts (Fig. 16). Hour-glass

structures, which are reportedly common in undersaturated rocks (Strong,

1969) are not present, or at least were not recognized. Zonation of

inclusions, especially apatite, biotite, and olivine grains gives some

phenocrysts a sie ve -like appearance and indicates that clinopyroxene

must be one of the later phenocrysts to crystallize.

Clusters of euhedral phenocrysts are often joined by complex twins

(Fig. 17). The deep penetration of hour-glass-shaped intergrowths

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

f t b io t it i

%

apatite

sal I t magnetite 1

Figure 16. Sector and fine oscillatory zoning in sali te phenocrysts.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40

c l inopyroxene zeol 1 te

magnetite

e . groundmass

altered b io tite

Figure 17. Complex intergrowths of clinopyroxene phenocrysts.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41

indicates that phenocrysts must have come into contact early in their

growth. Magma turbulence may have promoted the contact (Vance, 1969)

of phenocrysts and the creation of the clusters. Locally phenocrysts

o f b io tite , plagioclase, or o liv in e are Included in the pyroxene

clu ste rs. More ra re ly , fin e o liv in e crystals mantle the pyroxene

phenocrysts (Fig. 18).

The interiors of the phenocrysts are in some cases completely

shattered (and removed during the making of the thin section). Some

fractures within the phenocrysts are fille d with calcite; near the

sedimentary country-rock contacts, the phenocrysts are commonly com­

pletely replaced by calcite.

The pyroxene is o f s a li te composition (Ca^g Mg^^ Fe^g g*

B eall, 1973, p. 58), having a 2V o f 56-60® and an average 2AC o f

42 . Chemically and optically the sali te is characteristic of the

chi nopyroxenes seen in other subprovinces in central Montana (Pirsson,

1905; Witkind, 1969). This suggests that the clinopyroxenes throughout

the intrusive rocks of central Montana have a common origin.

Plagioclase occurs as euhedral phenocrysts ranging from the size

of the groundmass to about 6 mm. Glomeroporphyritic texture is common

with phenocrysts joined by penetration and synneusis twins. Some

zonation is visible in thin section, but An contents could not be

determined on both interior and exterior zones. Rims of anorthoclase.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 017 vine 42 salit

magnetite

o liv in e

Figure 18. Clinopyroxene phenocrysts mantled by olivine crystals.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

recognized by uneven extinction (moire^ effect) (Heinrich, 1965,

p. 347) surround some plagioclase grains. A few plagioclase crystals

fresh enough to yield An contents revealed labradorite composition

( Z , XaOIO and XaOOI). Most plagioclase remains only as the

relict outlines of euhedral phenocrysts. The interiors are considerably

altered to zeolites, particularly thompsonite, as well as calcite,

very fine-grained greenish epidote, and an unidentifiable isotropic

mineral.

Laths of biotite are euhedral with rounded corners. They exist

as phenocrysts and are often adjoined to the exterior of pyroxene

phenocrysts. Pleochroism with x = buff and z = reddish-brown is common,

though few pleochroic green laths are seen lo c a lly . Deformation in ­

dicated by kinking is present in a few laths.

Rare biotite grains with quartz(?) interleaved between the mineral

layers and xenolithic fragments suggest that the biotite may also be

xenocrystic in origin. Philpotts (1974, p. 307) described a similar

occurrence of biotite phenocrysts in an alnbite from the Monteregian

Province, Quebec, and suggested that the biotite, which may be

xenocrystic, could be derived from the partial melting of a biotite

peridotite. In the Adel Mountains, partial melting of biotite

pyroxenite, which is included in some intrusives within the Three Sisters

"stock" (Lyons, p. 461), possibly gave rise to biotite "phenocrysts"

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44

(actually xenocrysts) now seen in the intrusives of the northern Adel

Mountains. Another alternative is that biotite crystallized relatively

early in the potassium-rich magma, as suggested by the inclusion of

biotite phenocrysts in many pyroxene phenocrysts.

01ivine is present as subhedral and euhedral phenocrysts which

are commonly altered to a drab pleochroic olive green bowlingite(?)

in the grain interiors or a brownish red iddingsite along fractures.

Discontinuous reaction rims of biotite surround some olivine pheno­

crysts. The olivine phenocrysts, as large as 5 mm in diameter, often

form glomeroporphyritic clusters or are riimed by granular coronas

of pyroxene phenocrysts. Rarely fine olivine grains surround the ex­

te rio r o f pyroxene phenocrysts.

The olivine composition as determined by Lyons (1944, p. 461)

is high in iron, containing 25 to 49 percent faylite. Wilkinson (1974,

p. 72) notes that in a lk a li basalts the o liv in e composition ranges

from Fa__ to Fa__. The higher iron content o f the Adel Mountains cU Jo olivine could be the result of a more evolved fractionation process.

Since the crystallization trend of olivine is controlled by low pressure

fractionation of alkali basaltic magmas, a decrease in temperature

as fractionation progresses would result in the enrichment of fayalite

(see W ilkinson, 1974).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45

Magnetite is a ubiquitous accessory mineral in the igneous rocks

o f the Adel Mountains. Though there is no chemical analysis o f the

magnetite, pétrographie evidence of magnetite and the low titanium

content o f the rocks (B eall, 1973, p. 65) leads me to the conclusion

that the titanium content is not high enough to qualify the mineral

as ilmenite or titanomagnetite. This conclusion is also consistent

with the lack of high titanium characteristics (i.e., purple colors)

seen in the clinopyroxene.

The magnetite grains are cubic, irregularly blocky, and are as

large as many of the other phenocrysts (about 5 mm). Apatite crystals

which are also a common accessory, are often engulfed or partially

enclosed by the magnetite grains (Fig, 19). These euhedral crystals,

are characteristically less than 0.5 mm long.

Such a constant association of these two minerals throughout the

intrusive rocks suggests their crystallization may have resulted from

the im m is c ib ility o f the two phases, a condition which may be enhanced

by a high Na content in the magma (Philpotts, 1967). A similar invnis-

cib ility of apatite and magnetite is attributed to the formation of

apatite and magnetite rocks associated with some carbonatites (i.e .,

Palabora, Transvall). The likelihood of a carbonatite, as yet un­

discovered, in the Adel Mountains is discussed in the following section

The m icrolitic groundmass surrounding the phenocrysts and com­

prising 70 to 80 percent of most intrusive rocks is composed of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46

b io tite

9

apatite s a lite

agnetite

Figure 19. Magnetite blebs with embedded apatite crystals Salite phenocryst shows zonal arrangement of apatite and biotite inclusions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47

anorthoclase and magnetite with some plagioclase, either biotite or

altered olivine, clinopyroxene, fuzzy brownish glass and minor

anaclime. Anorthoclase which commonly forms spindly euhedral and

blocky subhedral laths makes up most of the groundmass. Anorthoclase

is d i f f ic u lt to id e n tify because i t is so fine-grained; however,

uneven extinction in the groundmass anorthoclase as well as in the rims

surrounding the plagioclase phenocrysts do help to distinguish this

mineral.

Magnetite in the groundmass is always present as very fine cubes

or very fine flakes. Fine splinters of biotite and pyroxene with the

anorthoclase laths delineate flow structures. Interstitial to the fine­

grained groundmass and zeolites is a brownish, slightly isotropic

glass which gives the matrix an overall turbid appearance.

Zeolites are present throughout the igneous rocks of the Adel

Mountains. They occur as cavity fillin g , mineral replacements, and as

in te rstitia l fillin g between groundmass minerals. Common zeolites are

thompsonite, scolecite, analcite{?), chabazite(?) and possibly

phillipsite(?). Many other zeolites are present, but identification

of these minerals was not possible using a pétrographie microscope.

Calcite is also nearly always associated with the zeolites.

A pattern for the intense zeolitization seen in the rocks of the

Adel Mountains is s t i l l not established. Lyons (1944, p. 464) noted

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

that zeolitization seemed to be the greatest where the rocks were highly

fractured or highly porous. I also concluded that in the study area no

zeolitization pattern existed, but observed that thinner dikes contain

more zeolites than the thicker laccoliths. Beall (1973, p. 61),

however, relates the abundance of zeolites to the dark and light layering

phenomenon o f Square,Shaw, Crown, and Cascade Buttes. He (B eall, 1973,

p. 51) concluded from his study that intense zeolitization was caused

by the movement of volatiles (mainly water) into the upper part of each

layer following each magma injection.

Comparative chemical analysis of zeolitized and unzeolitized rocks

by Lyons (1944, p. 464) and Beall (1973, p. 63) show l i t t l e chemical

changes resulted from the zeolitization. The addition of some water

(Lyons, 1944; Beall, 1973) and the oxidation of some iron (Lyons, 1974)

were the extent o f chemical changes. The source o f water added could

be magmatic, as Beall (1973) concluded to explain the layering phenom­

enon, or ground water, or as Lyons (1944) suggests, a combination o f

both.

A Possible Carbonatite in the Adel Mountains?

The occurrence of apatite-magnetite blobs throughout the trachy-

doleritic dike rocks of the northern Adel Mountains suggests a possible

relationship to apatite-magnetite carbonatite rocks. Association of a

carbonatite complex w ith the Adel Mountains would not be extraordinary

since 13 (37 percent) of the 35 major alkaline localities are known to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

have carbonatites (Heinrich, 1966. p. 27). The origin of carbonatites

is also inextricably interwoven with that of alkaline igneous rocks

and that while many alkaline complexes lack a carbonatite member, only

a few carbonatites are not obviously related to alkaline rocks.

Magnetite and apatite are described by Heinrich (1966, p. 157)

as the "most ubiquitous of the non-silicates and probably the most

wide-spread of all accessory species" associated with carbonatites.

In the intrusives of the Adel Mountain Volcanics, the blebs of apatite

and magnetite are pervasive, volumetrically comprising 3 to 5 percent

of the trachydolerite. The euhedral crystals of apatite are most

commonly embedded in the blocky, irregular-shaped magnetite crystals,

but are also present in the groundmass and as inclusions in the

clinopyroxene phenocrysts (Fig. 19). The intimate association of the

apatite magnetite bleds may be caused by the immiscibility of the apatite

and magnetite phases (P h ilp o tts, 1967) in an a lka lin e magma.

Calcite in the intrusive rocks of the northern Adel Mountains is

constantly associated w ith zeolites and appears to be secondary. In

other areas in Central Montana, c a lc ite in the igneous rocks has led

to the discovery of a carbonatite complex, as in the Bearpaw Mountains,

or suggestions that a carbonatite may be closely related, but as yet un­

discovered, as in the Highwood Mountains. Woods (1974, p. 33) in a

study of textural and geochemical features o f the Highwood Mountains

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

described calcite "in primary silicate globules which have textural

features of immiscible droplets." He concluded that silicate immis­

c ib ility should be considered in dealing with the origin of alkaline rocks.

Generally, the rocks closely associated with the carbonatites, a

subdivision of nepheline syenite (miascitic branch), are similar

mineralogically to the intrusive rocks of the northern Adel Mountains.

Notable differences are the presence of intermediate plagioclase

(la b ra d o rite ) and the absence in the rocks o f the Adel Mountains of

hornblende (except in a hornblende monzonite described by Lyons, 1944),

nepheline and ca n crin ite , and the accessory minerals sphene, r u t ile ,

zircon, and pyrochlore.

A closer examination of the non-carbonatite alkaline dikes intruding

many carbonatitic complexes reveals only a few instances of dikes extending

10 to 12 miles from the carbonatite center (McClure Mountain complex,

Colorado; see also Chishanya, Southern Rhodesia and Alnô, Sweden). Com­

positional ly, the dike rocks are often represented by larger rock units

of the carbonatitic "ring" (Heinrich, 1966, p. 56). Trachyte, the most

common light-colored, feldspathoid-free alkaline dike rock is probably

compositionally equivalent to the syenitic ring of the carbonatite complex.

Closely associated with the syenite ring are nepheline syenites and other

fo id a l rocks (fo r example Alno, Sweden; Fen, Norway; Bearpaw Mountains,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51

Montana). Rocks bearing nepheline or other foids are so far unknown

in the Adel Mountains and th is absence appears to support the claim

that these complexes rarely have associated carbonatites (Heinrich,

1966, p. 49).

The most serious doubt as to the likelihood of a carbonatite in the

Adel Mountains is cast by H einrich's (1966, p. 65) observation that

"generally, carbonatite complexes are distinguished by a near absence

of rocks that contain essential amounts of calcic and intermediate

plagioclase." The intrusive rocks of the northern Adel Mountains contain

10 to 25 percent plagioclase phenocrysts of intermediate composition

(Angg)- More calcic plagioclase varieties are present in several other

rock types described by Lyons (1944, p. 461).

Texture

The texture is dominated by clusters of coarse, euhedral pheno­

crysts of pyroxene. Biotite or olivine and relict plagioclase

phenocrysts are also locally included in the crystal clusters. Plagio­

clase phenocrysts are aligned if any flow lineation exists. Blocky

magnetite with apatite crystals occurs as inclusions in pyroxene

phenocrysts and as large descrete blebs.

Phenocrysts and magnetite/apatite "blebs" are surrounded by a

m icrolitic anorthoclase matrix. The euhedral to subhedral anorthoclase

laths are slightly coarser-grained in the intrusions of Haystack,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52

Fishback, and the s ill-lik e body south of Birdtail Butte. Also they

generally lack any indication of flow. A second generation of small

cubes of magnetite is nearly universal in the groundmass. Fine-grained

biotite, olivine, and pyroxene are common, but vary from intrusion to

intrusion. The fine-grained matrix minerals are often surrounded by

or have interstitial fillings of zeolites and aphanitic brownish glass.

Flow lineation, if present, is indicated in the groundmass by the

parallel arrangement of anorthoclase and plagioclase laths and ground­

mass pyroxene crystals. The lineation varies from vague to very definite.

Parallelism of the groundmass crystals characteristically is disturbed

around the large phenocrysts (Fig. 20). Sander (1970, p. 333) suggested

that a medium (in this case a crystal mush) carrying non-rounded rigid

grains (phenocrysts) is more susceptible to movement than the rigid

grains. With any magmatic disturbance (eg. injection pulsations) the

groundmass minerals would be more lik e ly to be forced around the la rg e r,

rigid phenocrysts, resulting in a disruption of the groundmass lineation.

Small scale convection-like deformation of the matrix flow lineation

suggests that final pulses of intrusion may have affected not only the

groundmass lineation, but also any pyroxene phenocryst lineation.

Problems Related to the Texture

Lyons (1944) and Beall (1973, p. 19) both mentioned the scarcity of

evidence of flow structures to indicate intrusion direction within the

dikes. The reasons for the lack of flow lineation and flow differentiation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53

b io tite

m

m

i %

s a lite

Figure 20. Groundmass anorthoclase and plagioclase m icro lite s show flow lin e a tio n that appears to separate around the s a lite phenocryst.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

the radial migration o f c rysta ls, especially of the elongate pyroxene

phenocrysts, away from the walls and toward the center of the dike during

flow, is not readily apparent and deserves some closer attention.

To verify Lyons' and Beall's observation, I spent considerable time

searching for megascopic evidence of flow lineation in the field. The

pyroxene phenocrysts with dimensions of 1 cm by .4 cm seemed a suitable,

somewhat rod-shaped form to show alignment (Shaw, 1965) and flowage

d iffe re n tia tio n (Bhattarchji and Smith, 1964). Alignment is , however,

very scarce. The phenocrysts could be described as evenly but randomly

distributed throughout the outcrops, with the exception of a few small

areas which show a vague flow orientation.

Why is there no alignment of pyroxene phenocrysts? This question

merits attention not only for problems related to the flow of phenocrysts

in the magma, but also to the crystallization site of the phenocrysts.

Several explanations are possible. One is that no lineation exists

because the pyroxene phenocrysts were not present when the magma was

injected. That is, the phenocrysts crystallized after the magma was

intruded into the dikes, and hence grew in random orientations. To

establish the validity of this possibility I sought evidence of finer-

grained phenocrysts near the contacts and deformation of the matrix by

growing phenocrysts. I found that phenocrysts existed not only to the

edge of the intrusion-country rock contact but locally appeared to be

partially incorporated into the country rock, especially into the shale.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

This suggests the phenocrysts existed during dike injection. The

enveloping o f exclusively early-formed minerals by euhedral pyroxene

phenocrysts suggests that the pyroxene crystallized later than most other

minerals but p rio r to the formation o f the groundmass, and hence, p rio r

to magma injection.

The problem then remains to explain why the early-formed pyroxene

phenocrysts do not show any evidence of flow during magma injection.

It is possible that under some circumstances of magma flow, orientation

and differentiation might not occur or evidence of flow might be

destroyed.

The few known examples o f intrusions showing flow d iffe re n tia tio n

include the sills of north Skye, Scotland (e.g. see Simkin, 1967), the

Muskox Intrusion (see Bhattacharji and Smith, 1964), and sills of the

Labrador Trough (Baragar, 1960). Komar (1972,1976) who has studied the

mechanical interactions of phenocrysts of dikes, noted that phenocrysts

of olivine demonstrate the mineralogical variations particularly well

and are most common. Plagioclase, as seen in the phenocrysts of the

Leopard Rocks of the Labrador Trough, is the only other mineral known to

demonstrate flow differentiation (Baragar, 1960).

The concept of flow differentiation has been applied to magmas as

an explanation of the mechanism for the formation of olivine-rich rocks

in a vertical or steeply-dipping position without prior accumulation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56

Such a mineral configuration could be explained by a process which

causes fractionation during and as a result of a single movement of

magma rather than multiple injection (Bhattacharji and Smith, 1964).

During flow olivine phenocrysts within the magma move away from the

dike walls and concentrate in the center of the dike. Such a process

also appears to explain the absence of chilled contacts between the

zones of high phenocryst concentration and low concentration and the

mineralogical symmetry (i.e. phenocryst concentration, olivine com­

position, etc.) noted through some dikes (Bhattacharji and Smith, 1964).

To verify the hypothesis that phenocryst-rich dike cores could be

formed by the concentration o f phenocrysts during flow, experimental

models were devised. Bhattacharji and Smith (1964) and Bhattacharji

(1965) experimented with scale models using solid-fluid mixtures. The

s ig n ific a n t observations from th e ir work are:

(1) During laminar flow solid particles move from walls to center.

(2) Spherical and rod shapes rotate as they move toward the center.

(3) The rate of concentration toward the center increases with

increased velocity or shear gradient (constrictions cause

concentration to accelerate).

(4) Rate of inward movement increases with particle size.

(5) Pulsations cause the solid particle filament to break up.

Experimental work and field observations also reveal conditions

under which flow differentiation would not be expected during the intrusions

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57

of a porphyritic magma. High viscosity caused by a high concentration

o f phenocrysts (Shaw, 1965) or cooling (Tweto, 1951) would cause an

absence o f axial migration during flow conditions. Above à volume con­

centration of 50 percent phenocrysts (Bhattacharji, 1965) and below a

volume concentration of 8 percent (Komar, 1972) flow differentiation

Is not expected to occur. Under low-veloclty conditions, axial migration

of rigid particles (phenocrysts) w ill not occur (Goldsmith and Mason,

1962). Conditions of both high viscosity and low velocity are not

favorable for flow differentiation; that Is, when the particle Reynolds

number Is less than 10“ ^ (Goldsmith and Mason, 1962).

One other condition which could destroy any evidence of flow

differentiation, or might never allow flow differentiation to occur.

Is turbulence. Turbulent flow may be caused by obstructions in the

dike, resulting in numerous "convection cells" (Shaw, 1965). Wall

roughness, sudden swelling of dike walls, or pulsations during magma

Injection may further add to Irregularities of phenocryst distribution.

While turbulent flow may disrupt or destroy flow differentiation, this

type o f flow can promote the creation o f permanent aggregates o f crystals

(glomerophenocrysts) so long as the turbulence Is not so vigorous as to

overcome the cohesive forces between crystals (Vance, 1969).

Low velocity of the flowing magma, possibly high viscosity, and

turbulence could all contribute to the lack of flow differentiation

and phenocryst orientation In the dikes of the northwestern Adel Mountains

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58

Turbulence created as the magma moved along the dike walls Is an

especially attractive possibility. The walls of the dikes are not smooth;

rather, the igneous dike rock often surrounds stoped blocks and wedges

of the sedimentary country rock. Pulsating magma injection may also

be responsible for the random phenocryst orientation. The flow orien­

tation of the fine-grained groundmass could be explained by the higher

susceptibility of the m icrolitic groundmass to movement than the

phenocrysts (Sander, 1970) during the final pulsations of magma injection

or final minor movement of magma during crystallization.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV

PETROGRAPHIC PROVINCE OF CENTRAL MONTANA

The T e rtia ry igneous rocks which comprise the Central Montana

Pétrographie Province are widely distributed along the eastern flanks

of the Rocky Mountains from the Canadian border on the north to

Yellowstone National Park on the south. The sim ilarity of the rocks

from the various igneous centers firs t gained recognition from

L. V. Pirsson's publication of "The Pétrographie Province of Central

Montana" (1905).

In his work Pirsson describes a group o f rocks genetically related

by the common features of mineralogy, chemistry and in some cases

pecularities of texture, but not then recognized as temporally related.

The province as Pirsson worked out, (Fig. 21) lies in the center of

Montana and includes the Castle Mountains, L ittle Belt Mountains,

Judith Mountains, Highwood Mountains, Bearpaw Mountains, and L ittle

Rocky Mountains. These igneous centers " lie roughly in an oval area

stretching from the northeast towards the southwest, about 150 miles

[240 km] long by 100 [160 km] broad in the middle o f Montana and shown

on the map by the [dashed] line" (Pirsson, 1905, p. 37).

Additional study of the rocks in Central Montana in the years

follow ing Pirsson's work prompted Esper Larsen to w rite "Pétrographie

59

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

SWEET GRASS HILLS « ^^P A W MTNS \ a _ 4 little rocky ^ ^ ^ ^ / MTNS

HIGHWOOD JUDITWTMTNS o Ma Î s s o u 1 a MOCASSIN MTNS • \ LITTLE BELT MTNS ^ V ftVCASTLE I V * o

a CRAZY B u tte BI I I Ings

ows National j Pafk j

Figure 21. Sketch map of the Cretaceous-Eocene igneous rock of Central Montana (modified from Larsen, 1940).

r ) Area enclosed is Pirsson's (1905) orig in a l Pétrographie Province

Subprovinces of the Central Montana Pétro­ graphie Province

Chadwick's (1972) lin e separating predominantly # alkaline rocks to the northeast from calc-alkaline rocks to the southwest

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61

Province of Central Montana" (1940). Larsen's work reflects his study

of the Highwood Mountains, a study of the literature, and the advance­

ment of the science of petrology in the intervening years and is generally

accepted as the most complete work so fa r w ritte n about the Central

Montana Pétrographie Province.

The greater scope of Larsen's work allowed some new observations.

The Central Montana Pétrographie Province is recognized as a fine

example of a group of rocks defined with respect to time and space

that can be divided into subprovinces. There is a close relation of

chemical, mineralogical and textural peculiarities of the rock as well

as relations of the rocks to structural features and the method of in­

trusion and extrusion characteristic to each subprovince. The rocks

among the subprovinces are less closely related.

The areas included within the Central Montana Pétrographie Province

are from north to south: Sweet Grass H ills (Weed and Pirsson, 1895;

Kemp and B illin g s le y , 1921); Bear Paw Mountains (Weed and Pirsson,

1896; Bryant and others, 1960; Schmidt and others, 1961; Pecora,

1962; Hearn, Pecora and Swadley, 1964; Schmidt and others, 1962); L it t le

Rocky Mountains (Weed and Pirsson, 1896; Emmons, 1908); Highwood

Mountains (Pirsson, 1905; Hurlbut and Griggs, 1939; Burgess, 1941;

Larsen and others, 1941; Woods, 1974); Northern end of the Big Belt

Mountains (Adel Mountain Volcanics) (Lyons, 1944; B eall, 1973); Moccasin

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62

Mountains ( B lix t, 1933; M ille r, 1959); Judith Mountains (Weed and

Pirsson, 1897; Goddard, 1950); L it t le B elt Mountains (Weed and Pirsson,

1895; W itkind, 1965, 1969, 1973); Castle Mountains (Weed and Pirsson,

1896; Winters, 1968); Crazy Mountains (W olff, 1892, 1938; Simms, 1966);

Yellowstone National Park and Absaroka Range (Hague, Iddings and Weed,

1899; Rouse, 1940; Parsons, 1958; Prostka and others, 1968; Chadwick,

1970). (For a brief description of each subprovince see Larsen, 1940).

The composition of rocks w ith in the province ranges from average

calc-alkalic to alkalic. Mafic rocks are characteristic of most sub­

provinces, though felsic and intermediate rocks are present in some

localities.

Laccoliths, stocks, plugs, s ills, dikes intruded at shallow levels

o f the crust are common forms o f igneous intrusions. Larsen (1940)

noted that calc-alkaline rocks form stocks with radiating dikes, but

few sills or laccoliths. Alkaline rocks form stocks with radiating dikes

and numerous s ills and la c c o lith s .

Relation of the Adel Mountain Volcanics to the Central Montana Pétrographie Province

In the earliest publication of the Central Montana Pétrographie

Province by Pirsson (1905), no mention of the north part of the Big Belt

Mountains (the Adel Mountains) was made. By the time of Larsen's work

(1940) sufficient study, especially by Lyons (1944), had been completed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

to recognize that the Adel Mountains should be included as part of the

Pétrographie Province.

A comparison of some aspects of the Adel Mountain Volcanics to the

remainder of the Central Montana Pétrographie Province w ill delineate

some differences. Areas of comparison are the age and location o f

volcanic activity and the petrology and chemistry of the igneous rocks.

The time of igneous activity is one of the most outstanding d if­

ferences between the Adel Mountains and other subprovinces. Volcanism

in the Adel Mountains is believed to have commenced near the end of

the late Cretaceous as evidenced by the rhyolitic tu ff layer near the

base of the Upper Cretaceous Two Medicine Formation, while igneous

activity within all other subprovinces occurred during Eocene time

(Chadwick, 1972). I t is s ig n ific a n t to note that the igneous a c tiv ity

in the Adel Mountains concluded before the end o f tectonism accompanying

the Laramide Orogeny based on the evidence of faulted syenogabbro and

hornblende monzonite dike and folded volcanic conglomerates which

contain fragments of all but the very latest intrusions (Lyons, 1944,

p. 455). The volcanism o f the other subprovinces followed or perhaps

accompanied the la te s t stages o f the Orogeny. The im plications o f a

re la tio n sh ip of the time o f igneous a c tiv ity to the Laramide Orogeny

are as yet unclear.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64

Igneous a c tiv ity o f the Adel Mountains is the western-most w ith in

the Central Montana Pétrographie Province. The Adel Mountains are the

only subprovince far enough west to be within the Disturbed Belt of

the Rocky Mountain Front. Deformation in the form of northwesterly

trending fold axes through the thick volcanic pile (Lyons, 1944), may

be the re s u lt o f Laramide orogenic a c tiv ity .

The rocks of the Adel Mountains are very similar to those seen in

other subprovinces. Lyons' (1944) reconnaissance work established

orothoclase basalts and analcime trachybasalts as the dominant types

o f extrusive rocks. Intrusive rocks range from gabbro to quartz

monzonite and megascopically resemble the dark-colored rocks with

pyroxene phenocrysts of the Highwood Mountains. D istinctly green

clinopyroxenes of the Adel Mountain Volcanics are characteristic of

clinopyroxenes in the igneous rocks o f central Montana, even i f the

titanium content is high, as in some shonkinites of the Highwood

Mountains (Pirsson, 1905, p. 39). The close sim ilarity of the

pyroxenes from the igneous centers of central Montana suggests a

genetic relationship of parent magmas.

The Adel Mountain Volcanics are included (Fig. 21) in the eastern,

alkalic side of the calc-alkalic-alkalic rock line drawn through Central

Montana (Chadwick, 1972). Chemically, the Adel Mountain Volcanics fa ll

within the lim its established by other subprovinces.

The results of chemical analysis by Beall (1973) are plotted on

variation diagrams (Fig. 22) with the results from some other subprovinces

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65

s.o,

CM

AV . — A*

•—

I#

it

M 3O

O'

c#o

N#,0 At

O*

$lO| ♦ KgO. F#0- MgO'CtO

Figure 22. Variation diagrams for the pétrographie province of Central Montana and the San Juan, Colorado province with the differentiation trends of the Adel Mountain Volcanics added. (From Larsen, 1940; modified by Beall, 1973).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66

The variation trend of the Adel Mountain Volcanics is generally parallel

to the trends of the other subprovinces and is similar to the trends

of the Crazy Mountains and to the high KgO trend of the Highwood Mountains,

The Adel Mountain Volcanics have a smaller range o f variatio n than other

subprovinces, only -5 to 18 (1/2 SiÛ 2 + KgO - MgO - CaO - to ta l Fe as

FeO) (B ea ll, 1973, p. 77) on the Larsen (1938) va ria tio n diagram.

A common origin is also supported by the sim ilarity of clinopyroxenes

in the igneous rocks o f central Montana. Larsen (1940) suggested that

the parental magma differentiated slowly at depth by crystal settling

to yield primary magmas to each subprovince. Within each subprovince,

d iffe re n tia tio n by crystal fra ctio n a tio n produced the va rie ty of rock

types now seen in the subprovinces.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER V

SUMMARY

The northern Adel Mountain Volcanics provide a unique view of

feeder dikes intersecting laccoliths. Exposure of these intrusions

prompted a study o f the formation o f these la c co lith s and a comparison

of these forms with G. K. Gilbert's (1877) conception of the ideal

laccolith. The steep-sided form of these small laccoliths in the

study area is probably the result of magma emplaced along the inter­

face between the Two Medicine Formation and the overlying Adel Mountain

Volcanics, a thick sequence of zeolitized alkali basaltic breccia flows,

A lk a li-ric h tra ch yd o le rite o f the northern Adel Mountains contains

conspicuous phenocrysts of diopsidic augite (salite). Phenocrysts of

olivine, biotite, and plagioclase are surrounded by a m icrolitic

groundmass of anorthoclase, plagioclase, pyroxene, and magnetite.

Blebs of magnetite and apatite suggest a possible genetic connection

with carbonatites. Zeolites and associated calcite are pervasive.

The presence of either olivine or biotite in the trachydolerite

is used to distinguish olivine-bearing varieties from biotite-bearing

varieties. Both rock types occur in the same dike, confirming the

composite origin of some dikes.

67

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68

In tru sive rock textures are dominated by elongate clinopyroxene

phenocrysts. Such phenocrysts should, according to experimental studies

(Bhattacharji and Smith, 1964), show some evidence of flow differentiation

The absence of flow features from the dikes of the northern Adel

Mountains, except in the groundmass anorthoclase, is thought to be

caused by turbulence during magmatic in je c tio n .

The Adel Mountain Volcanics are one of several early Tertiary

igneous centers distributed along the eastern side of the Rocky

Mountains that are included in the Central Montana Pétrographie Province.

The tectonic environment of the Adel Mountains and the remainder of

the Province shows characteristics of a foreland overlapping both a

subduction zone and a r i f t zone. The high potassium content conforms

to the pattern of increasing potassium content moving inland away from

the subduction zone (Lipman and others, 1971). But the Adel Mountains

also appear to be situated on deep northwest-trending crustal fractures

(Alpha, 1955; Smith, 1965; Chadwick, 1972). The close connection

between the alka lin e igneous a c tiv ity and major tectonic structures,

particularly fault zones, shown by other alkaline provinces such as

the East African R ift Zone, the Rhine-Oslo Graben and the Monterigian

Province of Quebec (Bailey, 1974) suggests that the igneous activity of

the Adel Mountains may be of similar origin.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES CITED

Alpha, Andrew G,, 1955, Tectonic History of North Central Montana: B illin g s Geol. Soc. Guidebook, Sixth Annual Field Conf., pp. 129-141,

Baragar, W. Robert A., 1960, Petrology of Basaltic Rocks in part of The Labrador Trough: Bull. Geol. Soc. Amer., v. 71, po. 1589- 1644.

Bailey, O.K., 1974, Continental Rifting and alkaline magmatism: In The Alkaline Rocks by H. Sorensen, John Wiley & Sons, London, pp. 148-159.

B eall, Joseph J . , 1973, Mechanics o f Intrusion, and Petrochemical Evo­ lution of the Adel Mountain Volcanics: Unpublished Ph.D. dissert., U. o f Montana, pp. 1-101.

, 1972, Pseudo-Rhythmic Layering in the Square Butte A lk a li- Gabbro Laccolith: Am. M ineral., v. 57, pp. 1294-1302.

Bhattcharji, S. and Smith, C.H., 1964, Flowage Differentiation: Science, 145, pp. 150-153.

______1967, Mechanics o f flow d iffe re n tia tio n in ultram afic and mafic s ills : Jour. Geol. 75, pp. 101-112.

B lix t, J .E ., 1933, Geology and Gold Deposits o f the North Moccasin Mountains, Fergus County, Montana: Mont. Bur. Mines and Geology, Mem. 8, 25 pp.

Bryant, B. and others, 1960, Geology o f the Maddux Quad., Bearpaw Mountains, Blaine County, Montana: U.S. Geol. Survey B u ll., 1081-C, pp. 91-116.

Burgess, C.H., 1941, Igneous Rocks o f the Highwood Mountains, Montana: B u ll. Geol. Soc. Amer., v. 52, pp. 1809-1828.

Chadwick, R.A., 1970, Beltsof Eruptive Centers in theAbsaroka- Gallatin Volcanic Province, Wyoming and Montana: Bull. Geol. Soc. Amer., v. 81, pp.267-274.

, 1972, Volcanism in Montana, Northwest Geology, v. 1, pp. 1-20.

Cobban, W.A., 1955, Cretaceous Rocks o f Northwestern Montana: B illin g s Geol. Soc. Guidebook, Sixth Annual Field Conf., pp. 107-119.

69

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70

Downs, M.O., 1974, Sector and O scilla to ry Zoning In Calcic Augites from Mt. Etna, Sicily: Contrib. Mineral. Petrol., v. 47', pp. 187-196.

Emmons, W.H., 1908, Gold deposits o f the L it t le Rocky Mountains, Mon­ tana: U.S. Geol. Survey B u ll. 340, pp. 96-116.

Erdmann, G.E., 1959, Supplemental Road Guide Simms to Bovmans Corner: B illin g s Geol. Soc. Guidebook, Tenth Annual Field Conf., pp. 188-191.

Gilbert, G.K., 1877, Report on the Geology of the Henry Mountains: U.S. Geog. and Geol. Survey o f the Rocky Mountain Region, 161 p.

Goddard, E.N., 1950, Structure o f the Judith Mountains, Montana (abs.): Geol. Soc. Amer. B u ll., v. 61, no. 12, pt. 2, p. 1465.

Goldsmith, H.L. and S.G. Mason, 1962, The Flow o f Suspensions Through Tubes. I. Single Spheres, Rods, and Discs: Jour, o f Colloid Science, v. 17, pp. 448-476.

Hague, A ., Iddings, J .P ., Weed, W.H., 1899, Geology o f the Yellowstone National Park, Part II: Descriptive Geology, Petrology and Paleontology: U.S. Geol. Monograph 32, pt. 2, pp. 269-325.

Hearn, B.C., W.T. Pecora, and W.C. Swadley, 1964, Geology o f the Rattlesnake Quad., Bearpaw Mountains, Blaine County, Montana: U.S. Geol. Survey B u ll. 1181-B.

Heinrich, E.W., 1965, Microscopic Identification of Minerals: McGraw- H ill Book Co., 216 pp.

______, 1966, The Geology o f Carbonatites: Rand McNally & Co., Chicago.

Hurlbut, C.S. J r ., and Griggs, D.T., 1939, Igneous Rocks of the High­ wood Mountains, Montana, pt. 1, The Laccoliths: B ull. Geol. Soc. Amer., v. 50, pp. 1043-1111.

Johnson, A.M., 1970, Physical Processes in Geology: Freeman, Cooper & Co., San Francisco, pp. 31-72.

and D.D. P ollard, 1973, Mechanics o f growth of some laccolith intrusion in the Henry Mountains, Utah; Tectonophy- s lcs, 18, no. 3/4, pp. 261-355.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71

Kemp, J,F. and B illin g s le y , P., 1921, Sweet Grass H ills , Montana: B u ll. Geol. Soc. Amer., v. 32, pp. 437-478.

Komar, Paul D ., 1972, Mechanical Interactions o f Phenocrysts and the flow d iffe re n tia tio n o f Igneous Dikes and S ills : B ull. Geol. Soc. Amer., v. 83, pp. 973-987.

, 1976, Phenocryst Interactions and the velocity profile of magma flowing through dikes or s ills : Bull. Geol. Soc. Amer., V. 87, pp. 1336-1342.

Larsen, E.S., 1938, Variation Diagrams fo r Groups o f Igneous Rocks: Jour. Geol., v. 46, pp. 506-570.

, 1940, Pétrographie Province o f Central Montana: B ull. Geol. Soc. Amer., v. 51, pp. 887-984.

, 1941, Igneous Rocks o f the Highwood Mountains, Montana: B u ll. Geol. Soc. Amer., v. 52, p. 1733-1752.

Lipman, P.W. and others, 1971, Cenozoic Volcanism and Plate Tectonic Evolution of the Western United States I: Early and Middle Cenozoic: Philosoph. Trans. Roy. Soc. London, sérias A, V. 271, no. 1213, pp. 217-248.

Lyons, J .B ., 1944, Igneous Rocks o f the Northern Big Belt Range, Montana: v. 55, pp. 445-472.

M ille r , R.N., 1959, Geology o f the South Moccasin Mountain, Fergus County, Montana: Mont. Bur. Mines & Geology, Mem. 37, 44 pp.

Parsons, W.H., 1958, Origin, Age and Tectonic Relationships of the Volcanic Rocks in the Absaroka-Yellowstone-Beartooth Region, Wyoming-Montana: B illin g s Geol. Soc. Guidebook, Ninth Annual Field Conf., pp. 36-43.

, 1969, Criteria for the recognition of Volcanic Breccias: Review in Igneous and Metamorphic Geology, L.H. Larsen (Ed.), Geol. Soc. Amer. Mem. 115, pp. 263-304.

Pecora, W.T., 1962, Carbonatite Problem in the Bearpaw Mountains, Montana: Geol. Soc. Amer., Buddington Volume, pp. 83-104.

Philpotts, A.R., 1967, Origin of Certain Iron-Titanium Oxide in A patite Rocks: Econ. Geol., v. 62, p. 303-315.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72

Philpotts, A.R., 1974, The Monteregian Province in : The Alkaline Rocks by H. Sorensen (e d .), John Wiley and Sons, London, pp. 293-310.

Pirsson, L.V., 1905, Pétrographie Province of Central Montana: Amer. Jour. Sci., V. 20, p. 35-49.

, 1905, Petrography and geology of the Highwood Mountains, Montana: U.S. Geol. Surv. B u ll. 237, 208 pp.

P o lla rd, D.D., 1973, Derivation and Evaluation o f a mechanical model fo r Sheet Intrusions: Tectonophysics, 19, pp. 233-269.

Prostka, H.J. and others, 1968, Igneous and Hydrothermal Geology o f Yellowstone National Park: Geol. Soc. Amer. Guidebook, Field T rip no. 6, Rocky Mountain Sect., 36 p.

Rouse, J.T., 1940, Structure and Volcanic Problems in the Southern Absaroka Mountains, Wyoming: B u ll. Geol. Soc. Amer., v. 51, pp. 1413-1428.

Sander, B., 1970, An Introduction to the Study of Fabric of Geological Bodies : Pergamon Press, Oxford, London, New York.

Schmidt, R.G. and others, 1961, Geology o f the Lloyd Quad., Bearpaw Mountains, Blaine County, Montana: U.S. Geol. Surv. B ull. 1081-E, pp. 159-188.

______and others, 1964, Geology o f the Cleveland Quad., Bearpaw Mountains, Blaine County, Montana: U.S. Geol. Surv. B ull. 1141-P, 26 pp.

Shaw, H.R., 1965, Comments on visco sity crystal s e ttlin g and convec­ tio n in g ra n itic magmas: Amer. Jour. Science, 263, pp. 120-151.

Simkin, T., 1967, Flow Differentiation in the Picrite Sills of North Skye: In P.J. Wiley (Ed.), Ultramafic & Related Rocks, New York.

Simms, F.E., 1966, The Igneous Petrology, Geochemistry and Structural Geology o f Part o f the Northern Crazy Mountains, Montana: Ph.D. Dissertation, Univ. of Cincinnati.

Smith, J.G., 1965, Fundamental Transcurrent Faulting in Northern Rocky Mountains: Bull. Amer. Assoc. Petrol. Geol., v. 49, pp. 1398- 1409.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73

Sorensen, H. (éd.), 1974, The Alkaline Rocks: John Wiley & Sons, London.

Strong, D.F., 1969, Formation of the Hour-glass Structure in Augite: M ineral. Mag., v. 37, pp. 472-479.

Tuttle, O.F., and J. Gittins, 1966, Carbonati tes : Interscience, John Wiley & Sons, New York.

Tweto, 0., 1951, Form and Structure of S ills near Pando, Colorado: B u ll. Geol. Soc. Amer., v. 62, pp. 507-531.

Vance, J.A., 1969, On Synneussis: Contr. Mineralogy and Petrology, V. 24, pp. 7-29.

Weed, W.H., and Pirsson, L .F ., 1895a, On the rocks o f the Sweet Grass H ills , Montana: Amer. Jour. S c i., vol. 50, pp. 309-313.

, and Pirsson, L.V ., 1895b, Igneous Rocks of Yogo Peak, Montana: Amer. Jour. S c i., v. 150, pp. 467-497.

, and Pirsson, L.V ., 1896a, Geology of the Castle Mountain mining d is t r ic t , Montana: U.S. Geol. Surv. B u ll. 139, 164 pp.

, and Pirsson, L.V ., 1896b, The Bearpaw Mountains, Montana; Amer. Jour. S c i., v. 151, pp. 283-301.

, and Pirsson, L.V ., 1896c, The Geology of the L itt le Rocky Mountains: Jour. Geol., v. 4, pp. 399-428.

, and Pirsson, L.V., 1897, Geology and Mineral Resources of the Judith Mountains o f Montana: U.S. Geol. Surv. 18th Ann. Rept., pt. 3, pp. 576-587, 445-616.

Wilkinson, J.F.G., 1974, The mineralogy and petrography of alkali b a sa ltic rocks in : The A lkaline Rocks by H. Sorensen (ed.), John Wiley and Sons, London, pp. 67-95.

W inters, A.S., 1968, Geology and ore deposits of the Castle Mountain Mining D is tr ic t, Meagher County, Montana: Mont. Bur. Mines and Geol. Bui 1. 64, 64 pp.

Witkind, Irving, J ., 1965, Relation of Laccolith Intrusion to Faulting in the Northern Part of the Baker Quadrangle, Little Belt Mountains, Montana: U.S. Geol. Surv. Prof. Paper 525-C, pp. 20-24.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74

Witkind, Irving J., 1969, Clinopyroxenes from Acid, Intermediate and Basic Rocks, L it t le B elt Mountains, Montana: Amer. M ineral., V. 54, pp. 1118-1137.

, 1973, Igneous Rocks and Related Mineral Deposits o f the Baker Quadrangle, L it t le Belt Mountains, Montana: U.S. Geol. Surv. Prof. Paper 752, 58 pp.

W olff, J. E ., 1892, The Geology of the Crazy Mountains, Montana: Bull. Geol. Soc. Amer., v. 3, pp. 445-452.

______, 1938, Igneous Rocks of the Crazy Mountains, Montana: B u ll. Geol. Soc. Amer., v. 49, pp. 1569-1626.

Woods, M.J., 1974, Textural and Geochemical Features of the Highwood Mountains Volcanics, Central Montana: Unpubl. Ph.D. dissert. Univ. o f Montana, 121 p.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.