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Structure, metamorphism, and geochronology of the Northern Wolverine complex near Chase Mountain, Aiken… Parrish, Randall Richardson 1976

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STRUCTURE, METAMORPHISM, AND GEOCHRONOLOGY OF THE NORTHERN WOLVERINE COMPLEX NEAR CHASE MOUNTAIN, AIKEN LAKE MAP-AREA, BRLTISH COLUMBIA by RANDALL RICHARDSON PARRISH B.A., Middlebury College, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE xn THE FACULTY OF GRADUATE STUDIES (Department of Geological Sciences) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1976 © Randall Richardson.Parrish, 1976 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t ha t t he L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V 6 T 1 W 5 Date 3qyhua/i^ I f / y / 7 ? ? i a ABSTRACT The Wolverine Complex (Armstrong, 1949; Roots, 1954) and s i m i l a r rocks to the northwest are of Precambrian age, c o r r e l a t e with Wlnderemere-type stratigraphy, and are poly-metamorphic and poly-deformational. Wolverine rocks near Chase Mountain have experienced two periods of t i g h t to i s o c l i n a l f o l d i n g ( F l , F2) overprinted by one or more periods of northwest-trending large-scale open f o l d i n g (F3), small-scale crenulation f o l d i n g of various o r i e n t a t i o n s , and minor f a u l t i n g . The two e a r l i e r periods of f o l d i n g were accompanied by metamorphism culminating i n amphibolite f a c i e s at the close of F2. These early folds are recumbent to gently i n c l i n e d , and the geometry of F2 i s consistent with north-eastward transport of rocks i n nappe-like fashion. An F3 large-scale f o l d deformed e a r l i e r f o l i a t i o n s into an upright to steeply eastward i n c l i n e d antiform which correlates with structures mapped to the north by Mansy (1972, 1974). Geochronometric data strongly suggest that metamorphic culmination occured i n mid-Cretaceous or e a r l i e r time, and that many rocks i n wide-spread areas south of 56^°N have experienced r e s e t t i n g of K-Ar and Rb-Sr dates during the Eocene. A stock of b i o t i t e quartz monzonite, termed the Blackpine Lake g r a n i t i c stock, has a Rb-Sr whole rock isochron age of 62+7 m.y. @ 0.7052+.0002 Sr 87/86 i and a mineral isochron age of 44.7+2 m.y., and i t intrudes the Wolverine Complex. In surrounding s c h i s t s , gneisses, pegmatites, and muscovite-bearing g r a n i t i c rocks r e l a t e d to the metamorphism, Rb-Sr mineral dates (muscovite, plagioclase, K-feldspar, whole rock) range from 52-84 m.y. and r e f l e c t p a r t i a l to complete r e s e t t i n g , whereas K-Ar dates are e n t i r e l y reset to 43-47 m.y.. Rb-Sr mineral dates on b i o t i t e from metamorphic rocks are anomalously younger than K-Ar dates, a problem which i s not understood. Though Eocene volcanic rocks and sediments i n d i c a t i v e of rapid up-l i f t occur within or flanking the Omineca C r y s t a l l i n e B e l t , t h e i r s p a t i a l d i s t r i b u t i o n bears no r e l a t i o n s h i p to the areas of r e s e t t i n g of K-Ar and Rb-Sr dates. Though an e n t i r e l y s a t i s f a c t o r y explanation remains e l u s i v e , the r e s e t t i n g of dates must i n part be due to the thermal e f f e c t of i n -trusions of g r a n i t i c rock s i m i l a r to the Blackpine Lake g r a n i t i c stock which i s shown to have disturbed dates i n surrounding metamorphic rocks. ic T A B L E O F C O N T E N T S PART I Structure and Metamorphism Page Introduction 1 Previous Work 4 Geologic Setting 5 Stratigraphy 7 Str u c t u r a l Geology 13 Chase Mountain Area 13 Phase 1 13 Phase 2 16 Phase 3 22 Other Minor Structures 25 Stereographic Analysis 25 Domains 1,2 29 Domains 3, 4, 5 32 Domain 6 34 Summary 34 Blackpine Lake Area 35 Phases 1, 2 35 St r u c t u r a l Relations of the Blackpine Lake Stock 38 Relations between C r y s t a l Growth and Deformation 38 Metamorphic Petrology 43 Introduction 43 Chase Mountain Area 43 Blackpine Lake area 51 Summary of P - T - f l u i d conditions 58 Gr a n i t i c Rocks 58 PART II Geochronology and the Eocene Resetting Event Introduction 61 Geologic Setting of the Wolverine Complex 63 Geochronology of the Wolverine Complex and Related Rocks 69 Review of Previous Work 69 Rb-Sr and K-Ar Data of t h i s Study 69 Discussion and Problems of Interpretation 77 Early T e r t i a r y Tectonic Setting 81 Summary 85 Bibliography 86 LIST OF FIGURES i i 1. Geologic Map of Wolverine Complex and adjacent areas of north-c e n t r a l B r i t i s h Columbia. 2. a,b St r u c t u r a l map of thesis area showing orientations of compo-s i t i o n a l l a y e r i n g . 3. Correlation chart of uppermost Precumbrian and lowermost Cambrian rocks i n north- and east-central B r i t i s h Columbia. 4. St r u c t u r a l map of thesis area showing phase 1 data. 5. Photographs of phase 1 folds and rela t e d s t r u c t u r a l features. 6. Str u c t u r a l map of thesis area showing phase 2 data. 7. Photograph of phase 2 folds showing crenulated s c h i s t o s i t y . 8. Photograph of phase 2 f o l d r e f o l d i n g phase 1 f o l d . 9. Photograph of phase 2 folds depicting general form. 10. Sketches of east-verging phase 2 f o l d s . 11a. Sketch of s t r u c t u r a l cross-section east of Chase Mountain, l i b . Photograph of d i s l o c a t i o n zone. 12. Schematic and somewhat conjectural tectonic p r o f i l e near Chase Mountain. 13. Sketch of "phase 3b f o l d s . 14. Cross-sections of Mansy across Wrede and Russell Ranges. 15a. Sketches of l a t e r , b r i t t l e minor structures. 15b. Equal-area stereonet diagram of l a t e minor structures. 16. Map of the Chase Mountain area showing domain boundaries. 17a. Equal-area stereonets of s t r u c t u r a l features, Domain 1-5. 17b. Equal-area stereonet of structures, Domain 6. 18. St r u c t u r a l Map'of SQ, S p and S2 orientations across f a u l t east of Chase Mountain. 19. Photograph of refolded l i n e a t i o n s . 20. Photograph of phase 1 f o l d s , Blackpine Lake area. 21. Photograph and sketch of fo l d s and re l a t e d pegmatites, Black-pine Lake area. 22. Equal-area steronet of s t r u c t u r a l data, Blackpine Lake area. 23. Photomicrographs of deformational textures within and surround-ing the Blackpine Lake G r a n i t i c Stock. 24. Photomicrographs of metamorphic textures i n s c h i s t s . 25. Photograph and photomicrograph of muscovite pseudbmorphs of kyanite. 26. Diagram depicting i n t e r r e l a t i o n s of deformation, metamorphism, and i n t r u s i o n . 27. Map of the s i s area showing d i s t r i b u t i o n of samples referred to i n text. 28. Photomicrograph of muscovite pseudormorphs a f t e r kyanite. 29. PH2O-T diagram of general metamorphic conditions. 30. T-X diagrams of Pf=6 kbars f o r the system S1O2 - CaO - MgO -K 20 - AI2O3 - CO2 - H 20 31. T-X diagram of Pf=6 kbars for samples 17, 18, and 271-2. 32. T-X diagram of Pf=6 kbars f o r sample 87-2. 33a. T-X-..diagram of Pf=6 kbars for Fe - free, Na - free system 33b. T-X diagram of Pf=6 kbars f o r system containing b i o t i t e , plag-iodase, and actinojjjtic amphibole. 34. T-X diagram of Pf=6 kbars for Blackpine Lake area. 35. Photomicrographs of Rock 304B showing r e l a t i o n s of garnet, s t a u r o l i t e , s i l l i m a n i t e , and muscovite. 36. PH2O - T diagram of conditions of Sample 304b near Blackpine Lake. 37. Map of Wolverine Complex and related metamorphic rocks showing a l l a v a i l a b l e geocronologicdata. TABLE OF FIGURES cont. 38. Rb-Sr evolution diagram for Schist of sample 134. 39. Rb-Sr evolution diagram for "Wolverine" G r a n i t i c rocks. 40. Rb-Sr evolution diagram for Blackpine Lake G r a n i t i c Stock. 41. Rb-Sr diagram showing a l l whole rock determinations. 42. Sketch map showing the tectonic s e t t i n g during Eocene time North-central B r i t i s h Columbia. XV LIST OF TABLES Page I. Representative assemblages from the Chase Mountain Area 46 I l a . K-Ar data from the Geological Survey of Canada. 70 l i b . K-Ar a n a l y t i c a l data from t h i s study. 71 I I I . Rb-Sr a n a l y t i c a l data from t h i s study. 74 V ACKNOWLEDGEMENTS F i e l d and a n a l y t i c a l expenses were defrayed by NRC grant.678841 awarded to R.L. Armstrong and by the Department of Labor, Province of B r i t i s h Columbia under the Careers '75 program. L o g i s t i c a l support from the Geological Survey of Canada i s also g r a t e f u l l y acknowledged. During the tenure of t h i s study the author was supported on a Graduate Fellow-ship at the Univ e r s i t y of B r i t i s h Columbia. The manuscript has been greatly improved by the comments and suggestions of H. Gabrielse, R.L. Armstrong, and Ian Duncan. Competent and cheerful f i e l d assistance was provided by C. Parrish, and the help of K. Scott and J . Harakal during the c o l l e c t i o n of a n a l y t i c a l data i s also g r a t e f u l l y acknowledged. 1 PART I. STRUCTURE, STRATIGRAPHY, AND METAMORPHISM OF THE CHASE MOUNTAIN-BLACKPINE LAKE AREA INTRODUCTION The thesis area i s situated at the northwestern end of the Wolverine Complex as defined by Armstrong (1949) and Roots (1954), as shown i n figu r e 1. Rocks include metamorphosed c l a s t i c sediments, now mica s c h i s t , micaceous and f e l d s p a t h i c q u a r t z i t e , biotite-muscovite gneiss, and rare, d i s c o n t i n u -ous layers of c a l c - s i l i c a t e marble, amphibolite, tourmaline s c h i s t , and quartz-pebble conglomerate. Metamorphic grade i s amphibolite f a c i e s , i n some areas lower s i l l i m a n i t e grade. The rocks have been involved i n poly-phase deformation. Broad open f o l d i n g and l a t e r f a u l t i n g are superimposed on e a r l i e r syn-metamorphic n e a r - i s o c l i n a l f o l d i n g and f l a t t e n i n g . The geochronologic evolution i s complex and indicates a long and complicated thermal h i s t o r y . This study was undertaken because i t was f e l t that the s t r u c t u r a l , metamorphic, and geochronologic evolution of the Wolverine Complex was poorly known and warranted d e t a i l e d study. The s p e c i f i c area near Chase Mountain was chosen because i t i s located at the northwestern end of the Complex where the dominant s t r u c t u r a l a n t i c l i n o r i u m plunges northwesterly beneath lower grade and s t r u c t u r a l l y higher rocks. I t was hoped that metamorphic isograds and a change i n s t r u c t u r a l s t y l e might be documented. The Blackpine Lake g r a n i t i c stock was examined f or deformational and geochronologic h i s t o r y i n hopes that the absolute ages of s t r u c t u r a l events might be determined. The area was also selected because of the proximity of Eocene K-Ar dates, so t y p i c a l of the Wolverine and Shuswap Complexes, and nearby Early Cretaceous dates on s i m i l a r 'Wolverine' rock 2 types (Wanless et a l , 1971). I t was f e l t that a combined K-Ar and Rb-Sr approach to the 'Eocene r e s e t t i n g ' problem on both metamorphic and igneous rocks might shed some i n s i g h t into the thermal h i s t o r y . The s i g n i f i c a n c e of the Wolverine Complex to the evolution of t h i s part of the! B r i t i s h Columbia c o r d i l l e r a i s poorly known, and i t s r e l a t i o n to plate tectonics even more obscure; a regional synthesis i s presented to provide a us e f u l reference frame i n which to view the accumulated data pertinent to these problems. 3 STRATIGR APHIC ROCKS QUATERNARY I^ V] Drift 0 » « Al lor iMt MID-LATE TERTIARY [»3 " '•' t i tO-MocOr-O' X U l l Oteooeooo-Miooooo' noMly keiot l EOCENE | ; ; | O g t n Loko Volcomct' tfacito, rhyoiito LATE CRETACEOUS-EOCENE [ :| Soohrl-SiHeo Awo*b>oeO' conflorooroto, SQotfotOAO LATE JURASSIC -EARLY CRETACEOUS | ' ' | BoMOr AtMffiblogO1 ClCSbC l t d i A * l l l EARLY JURASSIC || | | H O » l t M Group' bOMrr, OfttfoUtO, >«4tRHHlT* LATE TRIASSIC | | Toklo Groap' botolt, broccio, s o d M n o n t f PENNSYLVANIAN AND PERMIAN Aiitko Group |Pe>—M bowlt , cbort. Cocli* Crook Grout > linootooo, Lo> R a n g o - N i M Crook * » * - y orgiaito SILURIAN AND DEVONIAN |»-Dw| Umostono, do*om*»», quortzito, obolo LATEST PRECAMBRIAN | pC | Ptlyllito, limefttono, quortzito, ocMst, g o o i » » INTRUSIVE ROCKS GRANITIC ROCKS [N/-—[ Jurostic and Crotacoooo (I,K)= oorly (o), n a o l o ( »>} . l o t . (I) GABBROIC AND ULTRAMAFIC ROCKS IIHlJ Crotocoou* Axriooid Goborotc Complox [igg Zoned Ultranvr ic Caoplox Alpioo-typo U l t r o M f i c Rocks CONTACT \ INFERRED \ \ F A U L T THRUST F A U L T \ANT1F0RMAL T R A C E SYNFORMAL T R A C E , G A R N E T ISOGRAD tick! on kigk graoo * * S0_ k i l o m e t e r s too Figure 1. Geologic map of Wolverine Complex and adjacent areas of north-ce n t r a l B r i t i s h Columbia. 4 PREVIOUS WORK Armstrong (1949) defined the Wolverine Complex as the metamorphic and igneous rocks c o n s t i t u t i n g the Wolverine Range of the Fort St. James map-area. He also mapped portions of the Aiken Lake map-area with Roots (Arm-strong and Roots, 1948) extending the'northwestern end of the Complex. Roots (1954) defined and examined the uppermost Proterozoic and lowest Cam-bria n Tenakihi and Ingenika groups i n the northeastern h a l f of the Aiken Lake map-area i n greater d e t a i l , b r i e f l y subdividing the stratigraphy and crudely documenting metamorphic zonations and major structures. Roots also mapped the remainder of the Aiken Lake sheet on a scale of 1" = 4 miles, i n which mainly Paleozoic and Mesozoic rocks are found. More recent work i n the Aiken Lake map-area has been done by the Geo-l o g i c a l Survey of Canada. Mansy (1971, 1972, 1974, 1976) has mapped the northern h a l f i n greater detail,-, and has delineated d e t a i l e d stratigraphy and structure. Paleozoic and Mesozoic sedimentary and igneous rocks to the west have been recently studied by Irvine (1975, 1976), Monger (1974, 1976), Woodsworth (1976), Garnett (1974), Richards (1976), and e a r l i e r by Lord (1948). Wanless et a l (1967, 1971, 1973, 1974) i n ass o c i a t i o n with workers from the Geological Survey of Canada have been the p r i n c i p l e creators and Wolverine Metamorphic Complex of K-Ar data on plutonic rocks of the Hogem composite b a t h o l i t h v . E i s -bacher (1974a) has studied the Sustut and S i f t o n Basins, o u t l i e r s of which occur i n the Aiken Lake map-area. A regional tectonic synthesis has been prepared by Gabrielse (1967) for the northern Canadian C o r d i l l e r a . GEOLOGIC SETTING 5 The area studied (figure 1) l i e s within the extensive uppermost Pre-cambrian metamorphosed c l a s t i c sequence broadly c o r r e l a t i v e with the Kaza-Miette (Winderemere) assemblage of rocks farther south i n the McBride map-area (Campbell et a l , 1973; Gabrielse, 1972) and with the Missinchinka Group to the northeast across the Rocky Mountain Trench ( I r i s h , 1970) . This group of rocks forms the backbone of the Omineca C r y s t a l l i n e Belt and comprises a deformed, metamorphosed, u p l i f t e d block nearly 60 km. wide that i s fault-bounded on the east adjacent to the Rocky Mountain Trench and Rocky Mountains and on the west against the northern extension of the Quesnel Trough. The Quesnel Trough can be divided into an eastern group of f a i r l y continuous M i s s i s s i p p i a n - Permian (±Upper T r i a s s i c ?) c l a s t i c sediments rocks with conglomerate and a western group of Upper T r i a s s i c volcanicY, the Takla Group. The eastern upper Paleozoic rocks are intruded by gabbro s i l l s and alpine and zoned ultramafic rocks, and are associated with abun-dant basalts. The western Takla rocks are intruded by various phases of the Hogem composite b a t h o l i t h of Early Jurassic (175 my.) and Early Cre-taceous (120 my.) ages (Eadie, 1976; Garnett, 1974; Woodsworth, 1976). The contact between these two terranes i s generally fau l t e d , although depos i t i o n a l contacts have been shown i n some places. The Hogem ba t h o l i t h i s composite and consists of early mafic phases followed by more c a l c - a l k a l i n e and s y e n i t i c p l utonic rocks. Much of the older Hogem terrane i s deformed, e s p e c i a l l y near i t s western margin with the Pinchi f a u l t system, which separates i t from an assemblage of upper Paleozoic and Upper T r i a s s i c eugeoclinal sedimentary, mafic and u l t r a -mafic plutonic, and mafic volcanic rocks farther west including both the Stuart Lake b e l t of Monger and Patterson (1974) and the Asitka Group of Lord (1948) and Richards (1976). The Hazleton Group, consisting of v o l -6 canics and sediments of Early J u r a s s i c age, rests upon these upper Pale-ozoic and Upper T r i a s s i c rocks, and they i n turn are faulted and folded during l a t e r Mesozoic and Cenozoic deformation (Richards, 1976). The boundary between the Precambrian rocks and the upper Paleozoic rocks to the west i s a f a u l t . Roots (1954) concluded that i t was a steep normal f a u l t , whereas Mansy (1974) and Gabrielse et a l (1976) provide evidence that the contact i s i n places e i t h e r a steep thrust or a reverse f a u l t . Several periods of movement are probable. To the east, normal f a u l t s separate the Rocky Mountain Trench f l o o r from the highlands on e i t h e r side. Across the Trench to the east, east-verging foreland t h r u s t - f o l d b e l t geometry i n general p r e v a i l s i n l a t e Precambrian - Mesozoic miogeoclinal rocks ( I r i s h , 1970), although to the north,, the s t y l e of f o l d i n g becomes . more reminiscent of the Mac-kenzie Mountains where older Proterozoic s t r a t a are present. These older Proterozoic rocks ( P u r c e l l - Belt equivalent strata) are not exposed be-neath the Ingenika - Tenakihi Group rocks i n the Aiken Lake map-area, and they may be absent altogether. Within the Rocky Mountain Trench region ( S i f t o n Basin), d e t r i t a l sediments of Upper Cretaceous to Eocene age are present and record high-r e l i e f sedimentary and metamorphic source terrane (Eisbacher, 1974a). K-Ar dates from d e t r i t a l mica and g r a n i t i c boulders indicate u p l i f t and cooling of metamorphic core zone rocks by mid-Cretaceous time, although the erosional h i s t o r y i s no doubt more complex. The geology of t h i s general area i s dominated by the e f f e c t s of Mesozoic and Cenozoic plutonic and deformational events, thereby obscuring possible Paleozoic a c t i v i t y , and the g e o l o g i c a l evolution i s i n many ways analogous to that of the Shuswap complex of southern B r i t i s h Columbia where T r i a s s i c and Paleozoic rocks are juxtaposed against higher grade metamorphic rocks of complex h i s t o r y . 7 STRATIGRAPHY Strat i g r a p h i c r e l a t i o n s within the study area are considerably d i s -rupted because of obvious l o c a l r e p e t i t i o n and s t r u c t u r a l complexity. Rock types that are most abundant consist of thin-layered coarse quartz-mica s c h i s t s and micaceous, f e l d s p a t h i c , and schistose quartzites with com-plete gradation between these l i t h o l o g i e s . Many rocks show porphyroblas-t i c garnet and pl a g i o c l a s e , and mineral segregations are w e l l developed i n places. Quartzo-feldspathic mica gneisses which appear to be f e l d s p a r - r i c h equivalents of the s c h i s t s and quartzites are also abundant. C a l c - s i l i c a t e marbles, amphibolites, tourmaline s c h i s t s , and quartz pebble-conglomerates are also present but are quite rare and t h i n ; consequently, they were d i f -f i c u l t to map for any great distance. Nearly a l l conspicuous marker layers were found to pinch out into the surrounding s c h i s t s within several hundred meters. With the exception of a few t h i c k q u a r t z i t e s , the abundant schistose l i t h o l o g y proved to be discouraging during mapping because of i t s discontinu-ous and gradational nature caused by boudinage, repeated f o l d i n g , and possibly discontinuous i n h e r i t e d d e p o s i t i o n a l c h a r a c t e r i s t i c s . Because of these s t r a t i g r a p h i c complexities, i n t e r p r e t a t i o n of the s t r u c t u r a l evolution r e l i e s upon minor structures; the large scale e f f e c t of early syn-metamorphic deformation i s l a r g e l y unknown. The monotonous nature of the l i t h o l o g y was occasionally interrupted by pegmatite s i l l s and bodies, both concordant and discordant. These igneous rocks are minor constituents i n the area near Chase Mountain and south of Ravenal Creek (figure 2a), but become very numerous i n the high grade rocks near Blackpine Lake (figure 2b). These rocks w i l l be dealt with i n a l a t e r section. R e c r y s t a l l i z e d mylonitic rocks are present south and east of Ravenal Creek; they occur i n a small area as a 10 meter t h i c k l a y e r , dipping 20°-30° to the southwest. This mylonitic zone seperates rocks of s i m i l a r 8 Figure 2a. S t r u c t u r a l and geologic map of thesis area near Chase Mountain showing a t t i t u d e of compositional laye r i n g . kilometers Fold axis or l i n e a t i o n A t t i t u d e of compositional l a y e r i n g F l a x i a l plane a t t i t u d e F2 a x i a l plane a t t i t u d e F l ( ? ) or F2(?) a x i a l plane a t t i t u d e Trend of compositional l a y e r i n g Figure 2b. Geologic and s t r u c t u r a l map of thesis area near Blackpine Lake. 10 schistose l i t h o l o g y and i t s r e l a t i o n to the structural sequence i s unclear. The grain s i z e of the mica s c h i s t s increases i n a very general way to the southeast and correlates with an increase i n quantity of pegmatites. There i s probably a corresponding increase i n the metamorphic grade although t h i s i s d i f f i c u l t to prove due to s c a r c i t y of aluminous s c h i s t s or other grade i n d i c a t o r s . O r i g i n a l bedding i n most cases has been transposed into the a x i a l plane of the e a r l i e s t f o l d s ; however, recognizable load structures, graded bedding, and s l i g h t l y deformed cross-bedding were observed i n a few places. The o r i g -i n a l sediments may have been d i s t a l parts of t u r b i d i t e sequences. With i n -creasing deformation and f l a t t e n i n g , these structures become unrecognizable. Despite the rare deviations from the dominant l i t h o l o g y , the l a s t i n g impres-sion of these rocks i s one of a uniform, monotonous sequence of metamorphosed d i r t y sandstones, s i l t s t o n e s , and qua r t z - r i c h s e m i - p e l i t i c sediments. No estimate'of s t r a t i g r a p h i c thickness can be made because of s t r u c t u r a l compli-cations that render the s t r a t i g r a p h i c sections of Roots (1954) f o r Tenakihi Group rocks of dubious value. The l i t h o l o g y of the Tenakihi Group mapped i n t h i s study i n many ways resembles a more highly metamorphosed s t r a t i g r a p h i c c o r r e l a t i v e of the Middle Miette and Kaza Groups as described i n the McBride map-area by Campbell and others (1973) and Sutherland Brown (1963). There, the Middle Miette consists of coarse sandstone, pebbly sandstone ' g r i t ' , quartz s i l t -stone and a r g i l l i t e , the coarser units being very immature both t e x t u r a l l y and chemically, i n places containing up to 25% feldspar grains. Overlying the Middle Miette, the Upper Miette group consists of mudstones, s i l t y a r g i l l i t e s , a zone of coarse c l a s t i c rocks, and a th i c k limestone c o r r e l a t i v e with the Cunningham Formation which rests on the Kaza Group west of the Rocky Mountain Trench. The Middle Miette i s c o r r e l a t i v e with the Kaza Group west of the Trench which;lies below a thick, recessive a r g i l l a c e o u s unit known as the Isaac Formation, which i n turn forms the base of the Cariboo Group. These same rocks (Tenakihi-Kaza-Middle Miette) are probably also broadly c o r r e l a t i v e with the lower part of the Missinchinka Group des-cribed by I r i s h (1970) i n Halfway River map-area, and correspond i n general to the lower portion of the Windermere Series exposed through the e n t i r e length of the B r i t i s h Columbia C o r d i l l e r a (Gabrielse, 1972). Figure 3 shows the r e l a t i o n s h i p s between these rock groups. Similar l i t h o l o g i c c o r r e l a t i v e s to the Upper Miette group are present i n the Aiken Lake area; these rocks have been recently described and cor-related by Mansy (1972).' B r i e f l y , from oldest to youngest, they are as follows: 1) g r i t t y c l a s t i c u n i t : shale, s i l t s t o n e , quartzo-feldspathic sand-stone and conglomerate (Tenakihi and lower Ingenika groups). 2) thin-bedded calcareous p h y l l i t e , equivalent to the Isaac Formation. 3) r e s i s t a n t , well-bedded limestone, equivalent to Cunningham For-mation. 4) ' diverse unit of limestone, p h y l l i t e , impure qu a r t z i t e , sandy limestone, dark green shales, and impure sandstone, equivalent to the Yankee Belle Formation. 5) pure white q u a r t z i t e , equivalent to the Yanks Peak Formation. 6) impure quartzite interbedded with shale, equivalent to the Midas Formation. 7) blue-gray limestone with s i l t s t o n e i n i t s middle part, equiva-lent to the Mural Formation of lowest Cambrian age. The above units 3 and 4 correlate with the Good Hope Group and units 5 to 7 are equivalent to the Atan Group i n the Cassiar Mountains (Mansy, 1972) . In general, the d i s t i n c t i o n made by Roots (1954) between the Tenakihi Group and the lower Ingenika Group has been found d i f f i c u l t to apply, both i n the study area where the s t r a t a at Blackpine Lake and near Chase Moun-t a i n are i d e n t i c a l (but belong to Ingenika and Tenakihi Groups, respec-12 Cassiar Omineca Cariboo Rocky Mountains age Mountains Mountains Mountains G a b r i e l s e , 1963 M a n s y , 1972 C a m p b e l l e t a l , 1973'. M c B r i d e I r i s h , 1970 i.e 7 M u r a l F m . M u r a l F m . A t a r i G r o u p a. 3 6 M i d a s F m . M c N a u g h t o n F m . O w CD 5 Y a n k s P e a k F m . ( G o g G r o u p ) pe G o o d H o p e O 4 Y a n k e e B e l l e F m . U p p e r M i e t t e F m . M i s i n c h i n k a G r o u p Ihg eni 3 C u n n i n g h a m F m . G r o u p b a s e not exposed Ihg eni 2 I s a a c F m . b a s e not exposed 1 K a z a F m . M i d d l e M i e t t e F m . bas e not exposed L o w e r M i e t t e F m . b a s e not exposed Figure 3. Cor r e l a t i o n chart of uppermost Precambrian and lowermost Cambrian rocks i n north and east-central B r i t i s h Columbia. t i v e l y ) and elsewhere as noted by Mansy (1976). In many places, the apparent l i t h o l o g i c contrast may be a function of metamorphic grade and s t r u c t u r a l s t y l e rather than rock type. The rocks i n the study area near Chase Moun-ta i n are located near the 'apparent' base of the exposed section, and they l i e several thousands of feet s t r u c t u r a l l y below the thicker quartzites and limestones i n the Finlay and Swannell Ranges near P e l l y Lake where the s t r u c t u r a l h i s t o r y appears much simpler (Mansey, 1972, 1974). 14 Figure 4. S t r u c t u r a l map of thesis area showing phase 1 data. STRUCTURAL GEOLOGY There are at least four phases of deformation recognized within the Chase Mountain area (Parrish, 1976). Phases 1 and 2 ( F l , F2) appear to be relat e d to d u c t i l e rock conditions associated with metamorphism; they are characterized by N to NW-trending, t i g h t to i s o c l i n a l , gently i n c l i n e d to recumbent folds with abundant mineral r e c r y s t a l l i z a t i o n and development of a x i a l plane f o l i a t i o n . Later phases 3a and 3b trend NW and N re s p e c t i v e l y , are open upright f o l d s , and appear to be post-metamorphic. These l a t e r phases have produced an elongate antiformal dome or culmination i n the e a r l i e r f o l i a t i o n s and are probably related to the large scale structures i n the Ingenika Group rocks to the north i n Wrede Range and near P e l l y Lake i n the Russ e l l Range. The study area has been divided into s i x domains possessing moderately homogeneous geometry for stereographic s t r u c t u r a l a n a l y s i s . Phase 1 (Figure 4) F l folds are approximately s i m i l a r i s o c l i n a l f o lds trending from N-S to NW-SE, and are characterized by a gently dipping a x i a l plane s c h i s t o s i t y . Amplitudes vary from a meter or so to several tens of meters, and the folds are most commonly rootless and extremely fla t t e n e d (figure 5 ). F l f o l d i n g has resulted i n transposition of the bedding into the a x i a l plane, so that the two are su b p a r a l l e l . The planar and l i n e a r elements of t h i s event have been refolded both by F2 and F3 f o l d s ; F l orientations therefore vary across the major antiform i n a systematic way (figure 4). The extreme f l a t t e n i n g of F l hinges i s no doubt due to renewed closure during F2 f o l d i n g so that the o r i g i n a l form of F l folds p r i o r to F2 i s not known. Features seen i n the f i e l d associated with F l are i s o l a t e d f o l d hinges (figure 5a), spec-tacular boudins (figure 5b), discontinuous and transposed l a y e r i n g , and an extremely fl a t t e n e d , flaggy appearance of compositional l a y e r i n g . When 15 Figure 5a. F.. f o l d hinges within a quartzo-feldspathic layer, looking northwest. Figure 5b. I s o c l i n a l F ^ f o l d with a x i a l plane s c h i s t o s i t y i n quartz-mica s c h i s t , looking NW. Figure 5c. Photograph of hinge of F^ f o l d , looking NW, showing the development of a x i a l plane s c h i s t s i t y . Figure 5d. Photograph of large boudin of q u a r t z i t e i n flaggy s c h i s t , looking NW. quartz pebble conglomerate i s seen, a marked f l a t t e n i n g and an ' elongation p a r a l l e l to f o l d axes i s p a r t i c u l a r l y evident with aspect r a t i o s of the pebbles t y p i c a l l y 1:4:15. It i s not known whether t h i s s t r a i n i s r e g i o n a l l y representative or not. Phase 2 (Figure 6) F2 folds are distinguished from F l folds by the presence of a folded and crenulated s c h i s t o s i t y i n the cores of F2 folds (figure 7), r e f o l d i n g of F l hinges (figure 8), l o c a l divergence of a x i a l plane o r i e n t a t i o n ( f i -gure 8), and by t h e i r t i g h t (as opposed to su b - i s o c l i n a l ) f o r m with rounded hinges (figure 9). A near h o r i z o n t a l to moderately dipping a x i a l plane f o l i a t i o n i s often associated with these f o l d s , but i t i s only r a r e l y developed as penetratively as F l s c h i s t o s i t y , and r e l i c t s of a folded e a r l i e r f a b r i c are generally present. Many exposures (figure 10 and Par-r i s h , 1976") i l l u s t r a t e these folds 'cascading' or verging to the east-northeast, and these vergences have been shown i n the stereographic diagrams. No s i g n i f i c a n t r e v e r s a l of vergence was recognized throughout the area, and i t i s not known whether there i s a corresponding lower limb of a very large structure i n which vergences would be reversed. Several large exposures were observed where there i s a progressive transposition of the compositional layering into the F2 a x i a l plane ( f i -gure 11a). This t r a n s p o s i t i o n i s accomplished by increased f l a t t e n i n g and tightening of hinges, accompanied by high s t r a i n and/or s l i p along d i s c r e e t zones of s t r u c t u r a l d i s l o c a t i o n r e s u l t i n g i n the shearing off and los s of one limb of a f o l d . M y l o n i t i c rocks are sometimes associated. This struc-t u r a l s t y l e (figure 11a) i s recognizable on both small and large (over 100 meters) scales. Many d i s l o c a t i o n or f a u l t zones are present throughout the study area (figure l i b ) , and these are l i k e l y F2 structures; t h e i r displacements are 18 Figure 7. Photograph of folded s c h i s t o s i t y i n hinge region of F f o l d , looking S. Figure 8. Photograph of refolded F^ hinge on the limb of l a t e r F^ f o l d , looking S. The F r i n g e was tightened during F_ deformation. 17 N 1 Chase 56°35' Mountain \5£ 10 Quartzite Flattened conglomerate Recrystalized mylonite D i s l o c a t i o n zone Geologic contact Antiformal trace & plunge A x i a l plane, axis of F2 f o l d Limit of outcrop 125°10* 2 r-M w- ••' ; i-B..^! • 8 ( J f * ^ Figure 6. S t r u c t u r a l map of Chase Mountain area showing phase 2 f o l d o r i e n t a t i o n . 19 Figure 10. Sketches of east-verging phase 2 f o l d s . A l l of the loca-tions exhibit very strong development of rodding or l i n e a t i o n p a r a l -l e l to the f o l d axes. 21 Figure l i b . Photograph of d i s l o c a t i o n zone i n mica s c h i s t s , looking W. 22 Ch-«e Kr\. Q0fl«TZlT6 0 J Z I I I ^fif C O I ^ O l L O M E R A T E Figure 12. Schematic tectonic profile near Chase Mountain. not known but are not thought to be great as no exotic rocks were found. In spite of the lack of mappable stratigraphic units, an attempt was made to construct a tectonic profile across the area showing gross geometry (figure 12). The section indicates that existing data are consistent with eastward transport of rocks in nappe-like fashion where the lower limbs of large recumbent folds are either sheared off or not exposed. Lack of stratigraphic control renders the section somewhat conjectural. Phase 3 Two additional distinct sets of folds are present within the area, but their relative ages are indeterminate. Phase 3a is the major northwest trending antiformal fold which culminates in the thesis area and deforms earlier Fl and F2 fabrics (figures 4, 6). This i s a very large feature, being an antiform at least 15 km. in width. It is an open upright fold, the limb dips being 30°-40°, and i t does not appear to be associated with minor structures which reflect i t s geometry and orientation. This structure causes the variable orientation of Fl and F2 planar and linear structures as well as the regional changes in attitude of the compositional layering. The antiform plunges both NW and SE away from the Chase Mountain area, and the a x i a l trace i s not o f f s e t by l a t e r f a u l t i n g or f o l d i n g within the study area. In addition, a minor culmination - depression p a i r i s present 3 km. southeast of the Summit of Chase Mountain and near Ravenal Creek, respec-t i v e t i v e l y (figure 2). Metamorphism does not appear to vaccompanied t h i s f o l d i n g . Phase 3b i s an a r e a l l y r e s t r i c t e d set of f o l d s of v a r i a b l e but con-siderable s i z e and wavelength that i s present i n a northwest trending zone-near the crest of the F3a antiform southeast of Ravenal Creek. The folds themselves are north-trending, upright to steeply i n c l i n e d to the east, and have va r i a b l e form. They possess f l e x u r a l s l i p to f l e x u r a l flow geo-metry, and t h e i r form varies from open at higher s t r u c t u r a l l e v e l s to t i g h t at depth (figure 13). These structures deform both F l and F2 folds and are en echelon near the F3a antiformal crest. These folds are.:colinear (but not coplanar ) with F l and F2 f o l d s found i n the immediate v i c i n i t y , and i t appears lifcely that the older s t r u c t u r a l f a b r i c has been reactivated upon compression i n the core of the F3a antiform. This explanation implies that they r e s u l t from a 'room' problem i n the core of the larger structure. This i n t e r p r e t a t i o n i s supported by the increasing closure of the f o l d limbs at depth as w e l l as t h e i r en echelon and s p a t i a l l y r e s t r i c t e d nature. The large antiform i s most probably rel a t e d to the large f o l d s and associated thrusts mapped by Roots (1954) and Mansy (1972, 19 74) i n the cen t r a l Swannell Ranges underlain by the Ingenika Group rocks, e s p e c i a l l y near the Wrede Range, the major structure of which i s shown i n fig u r e 14. The geometry of the large antiforms are reasonably compatible, although i n the ranges that are at low metamorphic grade ( c h l o r i t e ) , there appears to be a lack of the e a r l i e r deformations present near Chase Mountain. More work i s obviously needed to substantiate or r e j e c t t h i s view. Recent work by Mansy (1976) i n greenschist and lower amphibolite f a c i e s rocks i n ce n t r a l Swannell Ranges indicate the presence of e a r l i e r i s o c l i n a l f o l d s and a more complex h i s t o r y than previously recognized, i n accord with the 24 10 nn. 5 m. c) Figure 13. Sketches of F „ f o l d s , looking S. These folds deform F^ f o l d s , as shown i n c ) , as we l l as structures. Figure 14. Structure sections across Wrede and Russ e l l Ranges from J . L. Mansy WR60£WW** B GARNET BI0TIT6 INGENIKA Riven B RAKS NUUWTOW . RAVENAL PEAK POLY CREEK \ \ * L R.M.TRJWCH sr. ^ MTNS. ( F0W£5 MTN. 126° /Z5° 157' SIUTY <AA&lUACeooS Mtt«TOI\lE, CALCAREOUS SHAU: fc V i ' j q DARK WEATHEAlNS SHALE, QOARTnTE 9 O O LOWER CAMBRIAN Q0AATHT6 , SHALE SHAte, QUAATZITE, UMe»ToN6 LIMESTONE, D0L0ST0NE CALCAREOUS PHfLUTK GiKIT, SILTST01M6, SHALE ( tote not e«f»«d) 25 data presented i n t h i s paper. However, the structure i n the very low grade Ingenika Group i n Russell Range near P e l l y Lake i s f a i r l y simple (Mansy, 1972, 1974); an e a r l i e r , i s o c l i n a l f o l d i n g i s missing there. It i s clear that differences i n pressure and temperature conditions a f f e c t the way that rocks behave under s t r e s s , and caution must be used when c o r r e l a t i n g s t r u c -tures i n rocks of v a r i a b l e l i t h o l o g y and d i f f e r e n t metamorphic grade. In-deed, structures developed at higher pressure and temperature conditions may not be present i n rocks that a r e ' s t r u c t u r a l l y higher and at d i f f e r e n t grade. This has been shown by Campbell (1970) i n the Cariboo Mountains. F3a structures i n the study area and'those c o r r e l a t i v e structures to occurred during the north i n the Wrede Range V e i t h e r non- or only low-grade metamorphism, and the author believes that these structures f o l d metamorphic isograds. If so, the metamorphic conditions during F3a f o l d i n g near Chase Mountain and during analagous f o l d i n g i n Wrede Range were probably not s i g n i f i c a n t l y d i f f e r e n t , and the s t r u c t u r a l c o r r e l a t i o n i s reasonably j u s t i f i e d . Other Minor Structures Later folds of more b r i t t l e deformational s t y l e are l o c a l l y present throughout the area. These include small kink and crenulation structures and associated l i n e a t i o n s , f l e x u r a l s l i p f o l d s of small amplitude (figure 15a), f a u l t s , and j o i n t s . No convincingly consistent orientations were found i n these l a t e structures, (figure 15b) and t h e i r s i g n i f i c a n c e i s not known. Stereographic Analysis The o r i e n t a t i o n of F l , F2, F3a, and F3b structures i s summarized i n stereographic fashion i n figures 16 and 17. Domains 1 and 2 define the northwest plunging region of the F3a antiform near Chase mountain, and do-mains 3, 4, and 5 define the southeast plunging hinge southeast of Ravenal 26 (NW) Figure 15a. Sketches of l a t e structures, viewed towards d i r e c t i o n i n parentheses. Figure 15b. Equal-area stereonet p l o t of orientations of l a t e structures; s o l i d c i r c l e represents l i n e a -t i o n or f o l d axis, open c i r c l e denotes a x i a l plane or cleavage of l a t e structure, t r i a n g l e denotes att i t u d e of dike. 0 _ | _axial plane of l a t e f o l d 4_,dike • f o l d axis of l a t e structure 28 Figure 16. S t r u c t u r a l map of Chase Mountain area showing domain boundaries. Creek. Domain 6 i s overwhelmed by F3b folds and neatly depicts F3b geo-metry (figure 17b). Domain 1, 2 (figure 17a-l,2) On a s t a t i s t i c a l b a s i s , F l and F2 f o l d axes (LI, L2) near Chase Mountain are co a x i a l , trending 324-10° and plunging l e s s than 20°. F l a x i a l planes (SI) i n domain 1 cl u s t e r about 105°/24°NNE at the hinge but are very scattered along a p a r t i a l great c i r c l e g i r d l e roughly perpendi-cular to the concentration of LI. The concentration of SI coincides with poles to compositional layering, confirming the n e a r - i s o c l i n a l nature of F l f o l d s . When SI and the compositional l a y e r i n g , S q are compared f o r domains 1 and 2, i t i s clear that SI forms a scattered array of points that l i e along a p a r t i a l small c i r c l e g i r d l e approximately 20 -30 about a non-cylindroidal v e r t i c a l f o l d axis. This indicates the presence of a dome-shaped culmin-ation defined by the planar elements So and SI. In domain 2, fo l d axes of F l and F2 vary from 145*/0<> to 182 e/35°, most points being from the southward-plunging part of t h i s dome. In general SI and S Q dip gently to the south and southwest i n domain 2, and to the north i n domain 1. Phase 2 structures are very common i n domain 1, but less so i n domain 2. Despite the fact that great care was taken to appropriately i d e n t i f y F l and F2 f o l d s , i t i s possible that some folds were mistakenly i d e n t i f i e d i n the f i e l d due to t h e i r s i m i l a r appearance and lack of other diagnostic c r i -t e r i a . In domain 1, f o l d axes L2 trend from 291°/10° to 340°/10° with most data c l u s t e r i n g near 325°±10°/13°±11°; they are characterized by a strong sense of assymetry or vergence toward the northeast as indicated i n figure 17a. S2 data are scattered along a great c i r c l e g i r d l e with a calculated f o l d axis of 322°/19*, which defines the F3a f o l d and v e r i f i e s that F3a de-Figure 17a. Equal-area stereonets of s t r u c t u r a l features, domains 1-5. compositional layering ° F l a x i a l planes • F2 a x i a l planes 0 F l f o l d axis 9 F2 f o l d axis A unspecified f o l d axis or l i n e a t i o n (~* sense of vergence 30 Figure 17b. Equal-area stereonet of orientations of structures i n domain 6 which i s dominated by phase 3b f o l d s . • j. compositional layering •1F2 a x i a l planes AJ. F3b a x i a l planes x mineral l i n e a t i o n © F2 f o l d axes F3b f o l d axes 32 forms and i s alsoi.colinear with F l and F2 on a s t a t i s t i c a l b a s i s . The fact that S2 planes do not f a l l on a small c i r c l e locus i s due to non-random d i s t r i b u t i o n of data and outcrop. Fold F3a, the regional large scale a n t i -form, i s a post-F2 event that deforms e a r l i e r structures. In domain 1 below an important zone of d i s l o c a t i o n or f a u l t i n g , F2 s t r u c t u r a l data are markedly d i s t i n c t from F2 orientations above t h i s f a u l t , as shown i n fi g u r e 18. Below the f a u l t , S2 s t r i k e s SSE and dips 20°-40° southwest. The rocks below are characterized by extreme f l a t t e n i n g and strong t r a n s p o s i t i o n of f o l i a t i o n i n t o the F2 a x i a l plane. This f a u l t i s thought to be analogous or continuous to that shown i n figurer.,6 and i t i s probably of l a t e F2 age. Domains 3,4, and 5 ( f i g u r e 17a - 3,4,5) Domains 3, 4, and 5 r e s p e c t i v e l y occupy the SE-plunging hinge region and adjacent limbs of the major F3a antiform (figure 16). In domain 5, the dominant f o l i a t i o n s t r i k e s northwest and the dips steepen northeasterly from 18°, 500 meters east of the crest, to about 50°, at a distance of 3 km. from the crest. Axes of F2 folds plunge to the north near the crest but plunge southwest farther away. Phase 1 f o l d s where seen are i s o c l i n a l and trend NW-SE. Domains 3 and 4 were studied i n greater d e t a i l and reveal the dominance of F2 structures that overwhelm F l f o l d s . Generally, LI and L2 are roughly colinear and plunge 10°-25° to 176°+ 5°. Because most data l i e on the r: south to southwesterly dipping limb of the F3a antiform, poles to SI, SO, and S2 do not f a l l on a well-defined g i r d l e . However, combined with poles to data from domain 5, t h e i r d i s t r i b u t i o n can_.be considered consis-tent with a near-great c i r c l e d i s t r i b u t i o n about the F3a f o l d a xis, Due to considerable s t r u c t u r a l complexity, the s c a t t e r i s great. 33 Figure 18. S i m p l i f i e d s t r u c t u r a l map of change i n orien t a t i o n s of S n, S.. , and S„ surfaces across major f a u l t , domain 1. Figure 19. Photograph of refolded l i n e a t i o n s . The deformed l i n e a r structure i s probably r e l a t e d to F 2, which i s i n turn refolded by a l a t e r F,, (?) f o l d . 34 Where seen, F2 folds r e t a i n the strong east-northeast vergence recorded farther to the northwest i n domains 1 and 2. Domain 6 (figure 17b) Domain 6 contains F3b folds that are numberous and that overwhelm e a r l i e r F l and F2 structures. Poles to SO,, the compositional layering, and SI are scattered about a well defined great c i r c l e whose calculated pole or f o l d axis i s due north and h o r i z o n t a l (±5*). This o r i e n t a t i o n i s subpar-a l l e l to minor F3b folds seen i n the f i e l d as well as s u b p a r a l l e l to older F2 f o l d axes and regional l i n e a t i o n . Refolded folds and refolded l i n e a t i o n s are l o c a l l y present here (figure 19). F3b a x i a l planes, S3b, c l u s t e r about 176°/60°E and b i s e c t a nearly v e r t i c a l limb and a moderately eastward dip-ping limb. I n c l i n a t i o n of these folds i s therefore westward. As stated previously, these folds are t i g h t e s t at lowest s t r u c t u r a l l e v e l s and more open at higher l e v e l s and are interpeted as folds developed en echelon as a 'room' problem i n the core region of the large F3a antiform. Summary The following observations are pertinent: 1) S t r u c t u r a l overprinting has caused considerable scat t e r i n the data. The d i s t r i b u t i o n of poles to SO, SI, and i n most cases S2 c l e a r l y defines the geometry of F3a and F3b folds which post-date these e a r l i e r events. 2) I t i s c l e a r that previously developed s t r u c t u r a l f a b r i c and aniso-tropy provide continued influence on the o r i e n t a t i o n of l a t e r struc-t u r a l features. The c o l i n e a r i t y of f o l d s i n any region or domain attests to t h i s . 3) The major F3a antiform i s d i v i s i b l e into two sub-culminations, cen-tered 3 km. and 8 km. r e s p e c t i v e l y southeast of the crest of Chase Mountain, as defined by the orientations of SO, SI, and S2 planar and L l and L2 l i n e a r structures. 4) In at l e a s t one area east of Chase Mountain, a gently dipping zone of s t r u c t u r a l d i s l o c a t i o n separates rocks having markedly d i f f e r e n t f a -b r i c orientations and s t y l e s , but the regional importance of t h i s type of feature i s not c l e a r . 35 5) F2 deformation has consistent northeastward vergence, and no rev e r s a l of t h i s vergence was recognized. I t i s possible that the e n t i r e se-quence observed l i e s on the upper limb of a large recumbent east-facing antiform that may have suffered detachment along i t s lower s y n c l i n a l hinge, i n a nappe-like structure. This i n t e r p r e t a t i o n , although consistent with mesoscopic geometry, i s e n t i r e l y conjectural and cannot be proved without knowledge of structures at depth. Blackpine Lake Area (Figures 20-22) Phases 1, 2 The s t r u c t u r a l evolution of the Blackpine Lake area i s s i m i l a r i n i t s e a r l i e r stages to that of the Chase Mountain area. F l fo l d s are present which are s u b - i s o c l i n a l and possess a x i a l plane s c h i s t o s i t y (figure 20); F2 folds are present that f o l d t h i s s c h i s t o s i t y and re l a t e d t r a n s p o s i t i o n f o l i a t i o n . Both generations of folds are t i g h t to s u b - i s o c l i n a l , and both seem to be syn-metamorphic and formed i n a d u c t i l e rock regime. Pegmatitic bodies and s i l l s are intruded before, during, and a f t e r f o l d i n g and bear corresponding deformational f a b r i c (figure 21); many synkinematic g r a n i t i c s i l l s are intruded p a r a l l e l to the a x i a l planes of F2 f o l d s . These early folds c l u s t e r about northwest trending (323°±5°), gently plunging (10°-20°) axis o r i e n t a t i o n s , and F l and F2 a x i a l planes dip at moderate angles (30°-50°) to the north and northwest (figure 22). In many cases, i t was not possible to c l a s s i f y a l l early folds as e i t h e r F l or F2, and consequently, the s t r u c t u r a l h i s t o r y i s less c l e a r . The grade of metamorphism i s higher here and may account for increased development of F2 a x i a l plane s c h i s t o -s i t y , making i t more d i f f i c u l t to d i s t i n g u i s h these f o l d sets. Poles to SO, however, are scattered about a f a i r l y w ell defined great c i r c l e whose calculated f o l d axis i s 340°/0°-30°, and t h i s may r e f l e c t l a t e r open f o l d i n g c o r r e l a t i v e with Phase 3 near Chase Mountain. Figure 21. Photograph of folded and deformed g r a n i t i c s i l l s i n the Blackpine Lake area, looking S. Black l i n e s o u t l i n e f o l d s . 37 N S Figure 22. Equal-area stereonet plot of orientations of structural data in the Blackpine Lake area. • 1compositional layering o 1F1 axial planes • J.F2 axial planes o 1 foliation in Blackpine Lake stock x lineation or fold axis of unknown generation 0 Fl fold axis ® F2 fold axis 38 S t r u c t u r a l Relations of the Blackpine Lake G r a n i t i c Stock The Blackpine Lake g r a n i t i c stock i s f o l i a t e d , and i n thi n section ap-pears strongly deformed, possessing mortar f a b r i c (figure 23a). The surround-ing country rocks possess a s i m i l a r texture that can be interpreted to have been caused by f l a t t e n i n g approximately normal to the f o l i a t i o n ( f i g u r e 23b, c,d). Roots (1954) stated that f o l i a t i o n i n the g r a n i t i c stock was crudely p a r a l l e l with the margin of the i n t r u s i o n , and he a t t r i b u t e d t h i s to igneous flow processes. He also considered t h i s body to be part of the Wolverine Complex and rela t e d to the 1 g r a n i t i z i n g ' a c t i v i t y . However, undulatory e x t i n c t i o n and strong r e c r y s t a l l i z a t i o n i n quartz i n d i c a t e s i g n i f i c a n t post-c r y s t a l l i z a t i o n deformation, at l e a s t near the margins. The outcrop pattern and domal f o l i a t i o n (Roots, 1954) are interpreted i n t h i s study to be due to f o r c e f u l , possibly d i a p i r i c emplacement with corresponding deformation of surrounding country rock. These processes c l e a r l y post-date early f o l d i n g as well as the peak of regional metamorphism. Mortar f a b r i c s are best developed i n the qua r t z - r i c h country rock sur-rounding the stock; however, undulose e x t i n c t i o n and sutured quartz grain boundaries are present i n many rocks throughout the Chase Mountain-Black-pine Lake area and in d i c a t e that deformation has continued (or resumed) a f t e r most r e c r y s t a l l i z a t i o n has ceased. Relations between C r y s t a l Growth and Deformation In general, the metasediments have acquired t h e i r mineralogy and tex-ture by synkinematic r e c r y s t a l l i z a t i o n . The s c h i s t o s i t y defined by the pre-ferred o r i e n t a t i o n of b i o t i t e and muscovite was o r i g i n a l l y formed during F l fo l d i n g . F2 f o l d i n g deformed t h i s f o l i a t i o n and i n many rocks, a new generation of micas has grown p a r a l l e l to S2 (figure 24 a). 39 Figure 23a. Photomicrograph of Black-pine g r a n i t i c stock showing mortar f a b r i c and r e c r y s t a l l i z e d quartz grains. X-nichols, 25X. Figure 23b. Photomicrograph of pegmatite of country rock showing kink bands i n mica and r e c r y s t a l l i z e d quartz. X-nichols, 25X. Figure 23c. Photomicrograph of peg-matite showing kinked micas and mylonitic texture of ribbon quartz. X-nichols, 25X. Figure 23d. Photomicrograph of pegmatite showing broken f e l d -spar grain with quartz fr a c t u r e f i l l i n g and m y l o n i t i c f a b r i c . X-nichols, 25X. 40 Figure 24a. Photomicrograph of r e l i c t F_ fold with some growth of new micas. Plane-polarized light, 25X. Figure 24b. Photomicrograph of r e l i c t fold with well-developed polygonization of micas i n d i -cating that recrystallization outlasted deformation. Plane-polarized light, 25 X. Figure 24c. Photomicrograph of s e r i -c i t e + c h l o r i t e pseudomorph a f t e r synkinematic garnet. X-nichols, 25X. Figure 24d. Photomicrograph of chlorite + muscovite + quartz pseudomorph after synkinematic garnet. Plane light, 25X. 41 During t h i s F2 f o l d i n g , micas were strongly kinked, bent, and crenulated i n the cores of F2 folds (figure 24b), i n d i c a t i n g a pre-F2 metamorphism producing the s c h i s t o s i t y . Where u n c h l o r i t i z e d garnets r e t a i n t h e i r form, i t i s clear that the f o l i a t i o n wraps around porphyroblasts of garnet as w e l l as plagioclase (figure 24c, d); ' s p i r a l ' or 'snowball' garnets are rare to absent and t h i s observation supports a f l a t t e n i n g as opposed to shear o r i g i n f o r the f o l i a t i o n . Following F2 deformation, post-kinematic poly-gonization of micas and r e c r y s t a l l i z a t i o n of quartz have given r i s e to present f a b r i c s . The metamorphism accompanying F2 probably outlasted deformation i n most areas. Late b r i t t l e f o l d s , kinks, and fractures that have deformed the rocks are probably re l a t e d to the development of undu-lose e x t i n c t i o n and somewhat sutured grain boundaries i n quartz throughout the area. The l a t e (post F2) development of c a t a c l a s t i c and mortar f a b r i c surrounding the Blackpine stock may be an e n t i r e l y separate and l o c a l event. Retrograde a l t e r a t i o n of garnet to c h l o r i t e and kyanite to s e r i c i t e i s widespread (figure 25), and t h i s i s most l i k e l y a post-F2 low grade metamor-phic event which developed l a t e r than the polygonization of micas. Rela-tionships are i l l u s t r a t e d i n fi g u r e 26. Figure 25a,b. Photomicrographs of s e r i c i t e replacement of kyanite. 25X. • • u £ t e ! t + - - -Eocene W i n e , -k-Ar « Rb-Sr Systems o i ^ e s I I pre-rnid-Cretaceoos- mid-la+e Crelaceoos 55mu 4-5 Figure 26. Schematic diagram showing r e l a t i o n s between deformation, metamorphism, g r a n i t i c i n t r u s i o n , geochronologic disturbance, and time. 43 METAMORPHIC PETROLOGY Introduction The Chase Mountain and Blackpine Lake areas have been subjected to both amphibolite f a c i e s metamorphism (medium grade of Winkler, 1974) and l a t e r retrogression. This statement i s supported by the following data: 1) presence of s t a u r o l i t e , kyanite, and/or s i l l i m a n i t e i n aluminous s c h i s t s ; 2) absence of the equilibrium assemblage c h l o r i t e + muscovite + quartz throughout the study area; 3) s t a b i l i t y of muscovite + quartz into the s i l l i m a n i t e - f i b r o l i t e f i e l d ; 4) lack of i n s i t u melting of p e l i t i c compositions; 5) a l t e r a t i o n of kyanite, s t a u r o l i t e , and garnet to muscovite + c h l o r i t e . Because of the low alumina content of the metasediments, diagnostic mineral assemblages are only r a r e l y developed i n p e l i t i c rocks. C a l c - s i l i -cate marbles, however, host a v a r i e t y of mineral associations, some of which provide data on temperature and composition of the f l u i d phase. Rocks that do provide c r i t i c a l P-T-X information are scattered, and s p e c i f i c isograds could not be mapped (figure 27). Chase Mountain Area Because d e t a i l e d compositional data on coexisting mineral phases was not obtained, and because aluminous s c h i s t s were so rare, the pressure of metamorphism i s not w e l l determined. Muscovite pseudomorphs a f t e r kyanite were found at several l o c a l i t i e s (figure 28), and the assumption w i l l be made that kyanite represented the stable a l u m i n o s i l i c a t e polymorph during the p r i n c i p l e metamorphic episode. Using the alu m i n o s i l i c a t e t r i p l e point of Richardson, G i l b e r t , and B e l l (1969) of 5.5 kbars at 622° C, the pressure of metamorphism w i l l be assumed to have been about 6 kbars. 44 INGENIKA RANGE / / < + CHAS6 MTN. / <?<> + T lTLf l r ' j 1?,ISO * /R/We^Au tMCK s \ LAKE \ /A 0 1 7 3 ^ CARINA LAKE S\ N 1 Z 3 f|ILE6 KYANITE. (?) SlLWIMANITE Figure 27. Map of area showing d i s t r i b u t i o n of samples referred to i n text. Near Blackpine Lake, s t a u r o l i t e breakdown occurs i n the s i l l i m a n i t e f i e l d , which places an upper l i m i t of pressure at about 7-8 kbars. In the Chase Mountain area, no s t a u r o l i t e or f i b r o l i t e were found, but the absence of the equilibrium assemblage c h l o r i t e + muscovite + quartz i n -dicates s t a b i l i t y i n the amphibolite f a c i e s or medium grade of Winkler (1974). Retrograde metamorphismhas produced c h l o r i t e ± quartz ± s e r i c i t e pseudomorphs a f t e r garnet (figure 24c) and muscovite pseudomorphs a f t e r kyanite (figure 28). P e l i t e s and carbonates are characterized by the relevant assemblages l i s t e d i n table I. * The author i s aware of the experimental and thermodynamic problems with the determination of .kyahite-sillimanite-andalusite e q u i l i b r i a , 1 .but the data of Richardson, G i l b e r t , and B e l l i s thermodynamically con-s i s t e n t and seems as good as data from other workers. Figure 29« General pressure-temperature conditions of metamorphism. TABLE I. Representative assemblages from the Chase Mountain area. P e l i t e s and semi-pelites:* qtz + mus + b i o t + ox ± plag (oligoclase) ± gnt ± cc qtz + mus + b i o t + plag + cc + czo ± sph ± apa ± ox qtz + mus + ky (pseudomorphs) + b i o t ± gnt ± plag ± ox qtz + plag + kf + b i o t + mus + ox qtz + b i o t + gnt + Mg-chl qtz + tour + ep + mus Amphibole-bearing rock: qtz + hbl ± gnt + plag ± czo ± sph ± b i o t hbl + gnt + plag + b i o t + qtz + cc + kf hbl + gnt + plag + mus + b i o t + czo + sph C a l c - s i l i c a t e s : czo + act + b i o t + qtz + plag + kf ± mus ± Mg-chl ± sph cc + trem + qtz + plag + kf + b i o t + sph cc + czo + qtz + diop + Mg-chl + kf + trem qtz + cc + mus ± b i o t ± czo ± sph ± c h l * Retrograde c h l o r i t e and muscovite are present i n many of these rocks, but have not been included i n equilibrium assemblages. Mineral abbreviations qtz - quartz, mus - muscovite, b i o t - b i o t i t e , plag - p l a g i o c l a s e , gnt - garnet, c h l - c h l o r i t e , ky - kyanite, kf - K-feldspar, ox -oxides, ep - epidote, czo - c l i n o z o i s i t e , cc - c a l c i t e , h b l - horn-blende, trem - tremolite, act - a c t i n o l i t e , diop - diopside, sph -sphene, apa - apatite, tour - tourmaline, ctd- c h l o r o t o i d , aim -almandine. ; 47 A l l p e l i t e s appear to have e q u i l i b r a t e d above the s t a b i l i t y of quartz + muscovite + F e - c h l o r i t e and within the s t a b i l i t y f i e l d of quartz + muscovite + plagioclase due to the lack of primary c h l o r i t e (except Mg-rich variety) and the lack of A^SiO,. + K-feldspar and i n s i t u migmatitic rocks that have experienced melting according to the reaction, muscovite + quartz + a l b i t e + I^O = melt + A^SiO,. (Winkler, 1974). Assuming P = 6 kbars, these l i m i t s r e s t r i c t the P-T environment to the shaded region of fig u r e 29 assuming P = P t o t a l H20" The composition of the f l u i d phase i n a l l rock types was H^O-rich because of the ubiquitous presence of c l i n o z o i s i t e (or z o i s i t e ) i n c a l c -s i l i c a t e s and most amphibolites. Grossular garnet i s not present i n any c a l c - s i l i c a t e s and c l i n o z o i s i t e ( z o i s i t e ) + quartz remains a stable assem-blage. At 6 kbars, t h i s demands that X be less than 0.1 (Storre, 1973; Hewitt, 1975). This i s consistent with observations by Ghent and Devries (1972) i n s i m i l a r types of rocks. The s t a b i l i t y of z o i s i t e + quartz i s li m i t e d by the reaction, z o i s i t e + quartz = grossular 4- anorthite + ^ 0 , and the s t a b i l i t y of z o i s i t e alone by the reaction, z o i s i t e + CO^ = c a l c i t e + anorthite + ^ 0 , as shown i n figu r e 30a. D i l u t i o n of the anorthite content of p l a g i c l a s e s h i f t s the i s o b a r i c , univariant curve toward lower temperatures and more H^O-rich f l u i d compo-s i t i o n s as depicted schematically i n fig u r e 30b. Hewitt (1973) has calcu-lated t h i s displacement for the reactions. z o i s i t e + CC>2 = c a l c i t e + anorthite + ^ 0 , and muscovite + c a l c i t e + quartz = anorthite + K-feldspar + vapor at 6 kbars f l u i d pressure, as shown i n figu r e 30a. Plagioclase compositions i n rocks of t h i s study are less than A n ^ , generally about An,-Q> which l i m i t X to less than .05 or so, varying s l i g h t l y with temperature. L.U2 J J I L_ .1 .2. .3 A X c o z Figure 30a,b. T-X diagrams for Si0 2-CaO-Al 20 3-K„0-MgO-H 20-C0 2; a) c a l i b r a t e d diagram b) schematic diagram snowing displacement. Rocks 17, 18, and 271-2 contain the assemblage, quartz + muscovite + c a l c i t e + ' c l i n o z o i s i t e + p l a g i o c l a s e , which r e s t r i c t s them to the region below the s t a b i l i t y of An^Q + K-feldspar and on the reaction of z o i s i t e breakdown (figure 31). Using the displaced curves of Hewitt (1973), conditions for these rocks with oligoclase-andesine o plagioclase are approximately T i 510 , = .04, and P f - ^ ^ = 6 kbars. The f l u i d composition of the rocks that contain z o i s i t e + c a l c i t e + p l a g i -oclase i s being buffered by the reaction, and the rock i s constrained to remain on t h i s reaction u n t i l one of the components i s used up. Sample 87-2 contains the assemblage, kf + trem + diop + cc + czo + Mg-chl + qtz, which l i e s on the reaction trem + cc + qtz = diop + vapor, on the ^ O - r i c h side of z o i s i t e + CC^ = cc + anorthite + H^O, below z o i s i t e + cc + qtz = grossular + vapor, and above b i o t + cc + qtz = amphibole + kf + H^ O (figure 32). The f l u i d composition i s less than .08 and probably le s s than .05 X , and without any further constraints, the temperature at 6 kbars may L U 2 vary greatly (less than 550°) while r e t a i n i n g the same mineralogy. The s t a b i l i t y of phlogopite + cc + quartz i s enhanced with increasing s u b s t i -t u t i o n of FeO, T i 0 2 , and F into phlogopite and with a r e a l i s t i c composition, t h i s curve probably would r e s t r i c t the temperature to greater than 500 or so. Unless X were l e s s than .02, the temperature must be greater than about 2 510 Q. Sample 273 contains the assemblage, kf + b i o t + a c t i n o l i t e + cc + czo + An^ 2 + qtz. This assemblage and i t s s t a b i l i t y has been discussed by Hewitt (1975); i t i s usually found i n amphibolite grade rocks i n regional metamorphic terrane. This assemblage must l i e on the curves z o i s i t e + CO,, = cc + anorthite (An^ 2) + *Curve for trem + cc + qtz = diop + vapor i s extrapolated from 5kb curve of Slaughter, Kerrick and Wall (1975). Figure 32. T-X diagram showing conditions of metamorphism for sample 87-2 for the same pressure conditions. 51 CO^ and b l o t + cc + qtz = kf + amphibole + vapor. It must have already crossed the reaction mus + cc + qtz = anorthite + kf + vapor to produce a bulk composition suitable to place i t on the b i o t + cc + qtz reaction. This condition presents problems i f phases are pure as i l l u s t r a t e d i n figure 33a. However, as noted by Hewitt (1975), FeO, TiC^, and F s u b s t i t u t i o n i n phlo-gopite enhances the thermal s t a b i l i t y of t h i s reaction. Na, however, reduces the s t a b i l i t y of the mus + cc + qtz reaction due to s u b s t i t u t i o n i n plag-i o c l a s e ; these two e f f e c t s must bring these curves together so a reasonable topology i s produced to account for the natural occurence of t h i s assemblage (figure 33b). There i s i n s u f f i c i e n t data to determine the quantitative e f -fe c t of such s o l i d s o l u t i o n upon s t a b i l i t y r e l a t i o n s , so that the phlogo-p i t e + cc + qtz reaction i s of l i t t l e value. However, the presence of A n ^ and the absence of diopside indicate conditions approximating X ^ .04 and T = 510 at 6 kbars P r i .,, as shown i n figure 33b. f l u i d These c a l c - s i l i c a t e s are d i s t r i b u t e d over a large area near Chase Moun-t a i n and t h e i r mineralogies are a l l compatible with ^ O - r i c h f l u i d condi-o o tions and temperatureSjat 6 kbars f l u i d pressure^of about 510 -530 , or lower amphibolite f a c i e s . Given the l i m i t e d compositional range of rocks studied, i t i s not possible to argue that metamorphic conditions vary i n any syste-matic way across the f i e l d area. Blackpine Lake Area The Blackpine Lake area appears to have been metamorphosed to higher temperatures than the Chase Mountain area; t h i s i s concluded by the following data: 1) both f i b r o l i t e and coarse s i l l i m a n i t e have been observed i n ad-d i t i o n to muscovite pseudomorphs a f t e r kyanite; 2) the reaction trem + cc + qtz = diop + vapor has occurred i n c a l c -s i l i c a t e marbles; T : ( 1 T Figure 33a. T-X diagram of Fe and Na-free system at 6 kb. Figure 33b. Schematic T-X diagram showing a possible topology i n a system with Na and Fe that could explain the mineralogy of sample 273.-53 3) s t a u r o l i t e bearing p e l i t e s also contain s i l l i m a n i t e + garnet + b i o t i t e ; 4) muscovite + garnet bearing g r a n i t i c rocks and pegmatites are very common; 5) muscovite + quartz remains stable. In p e l i t e s , the following assemblages have been observed: qtz + mus + bi o t + f i b + plag (An^) + op qtz + mus + b i o t + gnt + staur + r i b + plag + op + chl (retrograde) qtz + mus + b i o t + gnt + plag + op + ky (pseudomorph). And, of s p e c i a l s i g n i f i c a n c e , s i l l + gnt + bi o t and staur + gnt form i n -clusions i n a mus + b i o t + qtz + plag + f i b + staur + and s c h i s t . In c a l c -s i l i c a t e s , the assemblage cc + diop + trem + czo + plag (An^) + sph + apa i s common. C a l c - s i l i c a t e s probably remain on the curve zo + CC^ = plag + cc + H^ O and above the s t a b i l i t y of trem + cc + qtz. Assuming ^ £ - ^ u ^ = 6 kbars once again, X = .05 and T = 530 , but less than 640 because zo + cc + qtz 2 remains stable (figure 34). Presence of f i b r o l i t e and s i l l i m a n i t e i n ad-jacent rocks indicates T £• 622 based on the Richardson, G i l b e r t , and B e l l t r i p l e point. P e l i t e s contain f i b r o l i t e (300, 304, 305) that i s usually associated with muscovite or less commonly b i o t i t e , but the reaction(s) leading to i t s formation i s not cl e a r . Probably Na-muscovite + qtz = f i b + a l b i t e + K-ri c h e r muse + HgO i s occurring, forming s l i g h t l y more sodic rims on plagi o -clase, analogous to:that observed by Gu i d o t t i (1970). When s t a u r o l i t e i s present, i t i s usually surrounded by a sheath of coarse and fine-grained muscovite without f i b r o l i t e (figure 35a), and no obvious re a c t i o n r e l a t i o n s can be t e x t u r a l l y ascertained. However, i n 304b, the presence of both s i l l + and s t a u r o l i t e b i o t + mus (figure 3 5 c ) v i n c l u s i o n s i n garnet seems s i g n i f i c a n t and points towards eit h e r more than one period of metamorphic e q u i l i b r a t i o n , d i s e q u i l i -brium, or toward a buffered reaction. In the matrix of t h i s rock s t a u r o l i t e Figure 34. T-X diagram of conditions of metamor-phism f or c a l c - s i l i c a t e marbles near Blackpine Lake. Hatched l i n e i n d i c a t e s possible conditions. Because s i l l i m a n i t e i s found i n p e l i t e s , tem-perature are nearer the high temperature l i m i t . 55 Figure 35a. Photomicrograph of stauro- Figure 35b. Photomicrograph of l i t e with halo of s e r i c i t e near larger s i l l i m a n i t e + b i o t i t e + mus-garnet. S t a u r o l i t e + quartz are not covite i n c l u s i o n i n garnet, i n contact. Plane l i g h t , 25X. X-nichols, 160X. Figure 35c. Photomicrograph of Figure 35d. Photomicrograph of s t a u r o l i t e + s e r i c i t e ( a l t e r a - andalusite + muscovite + b i o -tion) i n c l u s i o n i n garnet. t i t e + f i b r o l i t i c s i l l i m a n i t e Note absence of quartz with i n matrix of rock. X-nichols, s t a u r o l i t e . X-nichols, 40X. 32X. i s surrounded by pseudomorphous coarse-and fine-grained muscovite and andal-u s i t e + mus + biot (figure 35d) and may be involved i n reaction r e l a t i o n s . F i b r o l i t e grows i n and appears to be replacing muscovite and andalusite of the matrix, and andalusite porphyroblasts are p o i k i l i t i c about muscovite, b i o t i t e , quartz, oxides, and pl a g i o c l a s e , but not s t a u r o l i t e . Andalusite i s i t s e l f being replaced by f i b r o l i t e , muscovite, and c h l o r i t e , and may have been l a t e synkinematic. Reaction r e l a t i o n s of s t a u r o l i t e and e s p e c i a l l y muscovite pseudomorphs a f t e r s t a u r o l i t e have been dealt with by Albee (1972) and G u i d o t t i (1970). Where pseudomorphs are of coarse muscovite without obvious retrogressive textures and where chemical equilibrium between phases can be shown, the pseudomorphs are l i k e l y prograde (Guidotti, 1970). S t a u r o l i t e c r y s t a l s i n thi s study are not large and are surrounded by coarse and fine-grained mus-covite generally without f i b r o l i t e . There i s evidence f or retrogression of garnets and kyanite, for instance, and most l i k e l y there i s prograde and retrograde muscovite growth .after s t a u r o l i t e , and perhaps more than one generation of s t a u r o l i t e . The following reactions are probably involved: Na-mus + qtz = f i b + a l b i t e + K-richer mus + R^O staur + Na-mus + qtz = f i b + b i o t + K-richer mus + a l b i t e + H^ O ± gnt low Zn-staur + mus + qtz = f i b + b i o t + Zn-richer staur + ± gnt. Rock 304b i s interpreted to have formed s t a u r o l i t e and upon further heating, to have crossed into s i l l i m a n i t e f i e l d and probably crossed the reaction staur + mus + qtz = gnt + s i l l + b i o t + 1^0 (figure 36). The fuga-c i t y of water was probably buffered by t h i s reaction while performing s o l i d s o l u t i o n s u b s t i t u t i o n i n the manner described i n the above reactions. Upon lowering of pressure, probably r e l a t e d to contact metamorphism by- the g r a n i t i c stock, andalusite porphyroblasts c r y s t a l l i z e d and were p a r t i a l l y replaced by f i b r o l i t e and l a t e r retrograde muscovite and c h l o r i t e . Ambi-Figure 36. Generalized pressure-temperature diagram of path of sample 304b i n the Blackpine Lake region, assuming an H„0 pres-sure of 6000 bars p r i o r to the formation of post-kinematic andalusite. 58 g u i t i e s r e s u l t i n g from a l t e r a t i o n and lack of compositional data for coexisting phases precludes making more d e f i n i t e statements concerning equilibrium conditions. Summary Though chemical analyses of mineral phases were not done, reasonable estimates can be obtained f o r the conditions of metamorphism. The f l u i d phase has been shown to be IL^O-rich with no more than 10% CO^ and probably no CH^as graphite i s absent. F l u i d compositions may have been r e l a t i v e l y constant throughout the area. Given these conditions, temperatures varied from 510-530 i n the Chase Mountain area to nearly 650 i n the Blackpine Lake area. I f P = P .. , then melting would have begun to occur i n place at s l i g h t l y higher temperatures. S t a u r o l i t e r e l a t i o n s i n d i c a t e temperatures up to 680° and t h i s may i n d i c a t e that ^ w a s greater than P , or that other elements (Zn?) enhance the s t a b i l i t y of s t a u r o l i t e . Later lower pressure contact (?) metamorphism i n the area near the Black-pine Lake g r a n i t i c stock i s probably responsible f o r the c r y s t a l l i z a t i o n of andalusite i n some rocks. Retrograde metamorphism(s) has v a r i a b l y affected most rocks studied. G r a n i t i c Rocks As noted previously, pegmatites and small g r a n i t i c s i l l s and bodies are present throughout the Chase Mountain - Blackpine Lake area, and the quantity of g r a n i t i c material bears a d i r e c t r e l a t i o n to the grade of regional metamorphism. In the Chase Mountain area, pegmatites form le s s than 1% of the rock volume and increase i n quantity with lower s t r u c t u r a l l e v e l . In the Blackpine Lake area where s i l l i m a n i t e occurs, g r a n i t i c rocks are very common and constitute greater than 10% of the rock volume 59 and i n places much more. F i e l d evidence suggests that these g r a n i t i c rocks were not melted i n place, but represent material i n j e c t e d from elsewhere. The mineralogy of these rocks i s simple with the following assemblages: qtz + plag + mus + op + kf + gnt + b i o t + zi r c o n . Most tabular g r a n i t i c rocks are folded or f l a t t e n e d and nearly a l l ... have a deformational f o l i a t i o n defined by the preferred o r i e n t a t i o n of mica and the mortar, or ribbon structure of quartz which occurs as elongate fla t t e n e d and r e c r y s t a l l i z e d grains (figure 23b). Some, however, are cross-cutting and undeformed, showing only undulatory e x t i n c t i o n i n quartz grains. Most g r a n i t i c rocks were emplaced synkinematically with probably F l f o lds (figure 21). These rocks probably c r y s t a l l i z e d at pressures greater than about 3 kbars due to the coexistence of muscovite + quartz. The Blackpine Lake g r a n i t i c stock, as shown i n figu r e 2, i s a b i o t i t e quartz monzonite i n t r u s i o n that contains no muscovite or garnet and i s probably post-kinematic with respect to F2. I t i s semi-discordant and dome-shaped (Roots, 1954), s l i g h t l y f o l i a t e , and ranges i n composition from granodiorite to quartz monzonite. I t i s , however, deformed and contains abundant evidence of r e c r y s t a l l i z a t i o n of quartz grains. Elongate quartz grains usually have mortar structure and are separated from smaller, poly-gonal, s t r a i n - f r e e r e c r y s t a l l i z e d grains by sutured grain boundaries. B i o t i t e c r y s t a l s are v a r i a b l y bent and rotated into the f o l i a t i o n plane of the i n t r u s i o n . Roots (1954) considered both the pegmatites and the Blackpine Lake g r a n i t i c stock as part of the Wolverine Metamorphic Complex and consequently rel a t e d to the metamorphic and ' g r a n i t i z i n g ' a c t i v i t y . He also considered 60 the stock of muscovite-biotite granodiorite 5 .km. northeast of Black-pine Lake (figure 2) to be s i m i l a r to the Blackpine stock, contrary to the conclusions of t h i s study. The evidence presented here and i n Part II suggests that the f o l i a t e d muscovite + garnet-bearing g r a n i t i c rocks and pegmatites are d i s t i n c t i n age, mineralogy, i n t r u s i v e r e l a t i o n s , and o r i g i n from the coarse-grained Blackpine Lake g r a n i t i c stock. Further-more, i t i s suspected that the pegmatites and muscovite-bearing rocks are p a r t i a l l y a product of anatexis at greater depths and bear a genetic r e l a t i o n s h i p to the regional metamorphism of the country rocks. 61 PART I I . Geochronology and the Eocene Resetting Event INTRODUCTION Throughout extensive regions of southern B r i t i s h Columbia, adja-cent northeastern Washington, and northern and c e n t r a l Idaho, K-Ar dates on pre-Eocene igneous and metamorphic rocks have been reset to 45-55 m.y. Many of the metamorphic rocks are part of or c o r r e l a t i v e with the Shuswap Metamorphic Complex of southern B r i t i s h Columbia. Most workers a t t r i b u t e t h i s to an Eocene thermal event accompanied by intense volcanism, plutonism, and hydrothermal a l t e r a t i o n (Ross, 1974; Armstrong, 1974a; M i l l e r and Engels, 1975; Medford, 1975). In many places Eocene volcanic rocks l i e upon an erosion surface above g r a n i t i c and g n e i s s i c rocks that i n turn give Eocene apparent ages. Mathews (1976) has shown that apparent ages of gneisses decrease with increasing depth below t h i s unconformity, l e v e l i n g o ff at about 45 m.y. An a d d i t i o n a l explanation of Eocene r e s e t t i n g i s rapid u p l i f t and unroofing, and t h i s has been invoked to explain Eocene dates i n some areas of Idaho (Ferguson, 1975). In north-central B r i t i s h Columbia, extensive Eocene r e s e t t i n g of K-Ar dates occurs i n medium to high grade metamorphic rocks of the Wolverine Complex between l a t i t u d e s 54°N and about 56%°N. The Wolverine Complex Armstrong, 1949), s i m i l a r to the Shuswap Complex i n many ways, i s a b e l t of l a t e s t Precambrian (Winderemere) sediments that were r e g i o n a l l y meta-morphosed by middle Cretaceous time and perhaps e a r l i e r i n the Mesozoic or Paleozoic. The explanation f or the r e s e t t i n g of dates i n these rocks i s l e s s obvious because volcanic rocks of Eocene age, where present, do not o v e r l i e the metamorphic complex. The Eocene r e s e t t i n g of K-Ar dates i n the Omineca C r y s t a l l i n e Belt of the B r i t i s h Columbia c o r d i l l e r a i s one manifestation of a profound disturbance during the early T e r t i a r y , and i t coincides with the abrupt tectonic t r a n s i t i o n from compressional continental arc tectonics to r e l a t i v e quiescence i n the Oligocene and Miocene (Coney, 1972; Wheeler et a l 1972; Souther, 1976): This paper presents new Rb-Sr and K-Ar data and discusses both the Wolverine Complex and the Eocene 'event' i n r e l a t i o n to the early T e r t i a r y tectonic s e t t i n g . 63 GEOLOGIC SETTING OF THE WOLVERINE COMPLEX The Wolverine Complex and re l a t e d lower grade rocks to the north (Figure 1) consist of l a t e s t Precambrian Winderemere-equivalentstrata (Mansy, 1972; Gabrielse, 1972) metamorphosed i n middle Cretaceous or e a r l i e r time. Higher grade rocks of the Wolverine Complex are s c h i s t s , q u artzites, gneisses, amphibolites, and c a l c - s i l i c a t e marbles intruded by g r a n i t i c s i l l s , dykes, small stocks, and pegmatities (Armstrong, 1949; Roots, 1954; P a r r i s h , 1976 ). These medium to high grade rocks are involved i n polyphase deformation including two generations of early synmetamorphic northwest-trending i s o c l i n a l folds and the development of nearly f l a t l y i n g metamorphic f o l i a t i o n . These structures are overprinted by open to t i g h t northwest-trending large scale f o l d i n g and thrusting, p r i n c i p a l l y southwest-directed (Mansy, 1971, 1972, 1974; P a r r i s h , 1976..). Later structures i n -clude b r i t t l e f o l d s , j o i n t s , or f a u l t s , some of which are probably related to Eocene(?) f a u l t i n g and deformation i n the Rocky Mountain Trench region (Eisbacher, 1972). Medium to high grade metamorphism accompained the early phase(s) of t i g h t to i s o c l i n a l f o l d i n g which led to the development of the metamorphic f o l i a t i o n (Parrish, 1976 ). Kyanite, s t a u r o l i t e , and s i l l i m a n i t e bearing rocks are now exposed mainly i n a n t i c l i n o r i a , i n d i c a t i n g that t h i s e a r l i e r metamorphic f o l i a t i o n and related gently dipping isogradic surfaces are probably folded by the l a t e r large scale northwest-trending f o l d s , many of which are southwest vergent. A l a t e r post-kinematic prograde metamorphism has been noted by Mansy (1976) i n some areas, and a s t i l l l a t e r retrograde metamorphism i s present throughout extensive regions, e s p e c i a l l y near Chase Mountain. The age of the high grade metamorphism and intense deformation i s not w e l l determined • Because medium grade rocks near and to the north and 64 northwest of Chase Mountain y i e l d K-Ar ages of 87-128 my., (figure 37), t h i s metamorphic period must be middle Cretaceous or e a r l i e r . Roots (1954) f e l t that t h i s period of metamorphism was accomplished i n post-Lower Cam-brian, pre-Mississippian time because 1) Lower Cambrian rocks are involved and 2) sedimentary and volcanic rocks of the Lay Range of M i s s i s s i p p i a n to Permian (possibly including T r i a s s i c ) age do not appear to be affected pre-Mesozoic metamorphism (though no unconformity. > was observed). Further j u s t i f i c a t i o n f o r v i s the presence of granite, q u a r t z i t e , and schist-bearing micaceous conglomerate i n the Lay Range assemblage 20 miles west of Chase Mountain (Roots, 1954). A K-Ar determination on d e t r i t a l white mica by the Geological Survey of Canada y i e l d s an age of 246 my. or Permo-Triassic (based on the revised time scale of Armstrong, 1974b). This conglomerate i s a r e a l l y very r e s t r i c t e d and apparently u n f o s s i l i f e r o u s , but to date i n t h i s region i s the only sub-s t a n t i a l evidence for pre-Mesozoic metamorphic and plutonic a c t i v i t y . Pene-t r a t i v e deformation of rocks of the Omineca C r y s t a l l i n e B e l t occured i n pre-middle Cretaceous time because g r a n i t i c rocks of the Early Cretaceous Cassiar b a t h o l i t h cut penetrative structures and metamorphic isograds (Gabrielse and Reesor, 1974). In summary, i t i s known that many of the rocks of the Omineca C r y s t a l -l i n e B e l t are polymetamorphic and most are polydeformational. Thus Paleozoic a c t i v i t y i n t h i s area i s possible but not proved; much of the deformation however, i s probably J u r a s s i c or Cretaceous i n age based on s t r a t i g r a p h i c , radiometric, and s t r u c t u r a l arguments. No unconformities separating lower Mesozoic or l a t e Paleozoic rocks from metamorphic "basement" have been d i s -covered, and where metamorphic and nonmetamorphic rocks are juxtaposed, several a l t e r n a t i v e explanations are possible and i n fa c t documented i n t h i s type of metamorphic terrane (see Campbell, 1970; Coney, 1974; Davis, 1975). + + + - 4 - + +• + •f-0 4 7 ' K-Ar date 0 76R ' R b - S r date b> biotite TO' muscovite h' hornblende \t' whole rock sample number (this study only) See figure I for geologic legend. Figure 37. Map of Wolverine Complex and r e l a t e d metamorphic rocks showing the geochronologic data, i n c l u d i n g dates of t h i s study. 66 The Wolverine Complex i s flanked to the northeast by the northern Rocky Mountain Trench which contains Upper Cretaceous to Eocene c l a s t i c sedimentary rocks (Sifton Basin) o v e r l a i n by Quaternary g l a c i a l and post-g l a c i a l deposits (Eisbacher, 1974a). The o r i g i n of the Rocky Mountain Trench and i t s sediments has been discussed by Leech (1966) and Eisbacher (1972), and i t i s f a i r l y c l e a r that fanglomerates derived from the east and relat e d sediments of the S i f t o n formation are the r e s u l t of normal f a u l t i n g very near the present Trench and the D a l l Lake lineament (H. Gabrielse, per-sonal communication, 1976). These rocks of Late Cretaceous and early Ter-t i a r y age have been affected by a l a t e stage (probably Eocene) deformation involving kink f o l d i n g , f a u l t i n g , and possibly r i g h t l a t e r a l s t r i k e - s l i p f a u l t i n g (Eisbacher, 1972). Volcanic rocks of probably Eocene age are present i n the Rocky Moun-t a i n Trench north of 56°, i n part of the D a l l Lake lineament, and as nu-merous f e l s i c : dikes i n the P e l l y lineament (H. Gabrielse, personal com-munication, 1976). Because t h e i r extent does not coincide with the area of Eocene dates, the s i g n i f i c a n c e of these volcanic rocks for r e s o l u t i o n of the r e s e t t i n g problem i s not c l e a r . The Rocky Mountains l i e to the northeast of the Rocky Mountain Trench, and t h e i r stratigraphy and structure have been described by many workers (Bally, Gordy, and Stewart, 1966; I r i s h , 1970; Pr i c e and Mountjoy, 1970). The p r i n c i p l e deformational period i n the Rockies i s thought to be Late Cretaceous and early T e r t i a r y , but may extend back into the Early Creta-ceous. Late Precambrian rocks c o r r e l a t i v e with those west of the Trench are present i n the western ranges of the Rocky Mountains where metamorphic . grade i s very low. In addition, a small wedge of g r a n i t i c gneiss with a kyanite-bearing aureole i s present on the east side of the Trench near Fort Grahame (Gabrielse, 1971). 67 To the west of the Wolverine Complex and rela t e d Precambrian rocks l i e s a complex sequence of low grade upper Paleozoic and lower Mesozoic volcanic and sedimentary rocks of both "oceanic" and "volcanic arc" af-f i n i t y (Monger, 1975; Richards, 1976) that have been intruded by a v a r i e t y of igneous rocks ranging from ultramafic, both zoned Alaskan type (Irvine, 1975, 1976) and alpine type (Monger and Patterson, 1974; Monger, 1975) to gabbroic (Irvine, 1975, 1976) to g r a n i t i c (Garnett, 1974; Gabrielse and Reesor, 1974; Woodsworth, 1976). The bulk of the g r a n i t i c rock i s Early J u r a s s i c and l a t e - E a r l y Cretaceous i n age. The Wolverine Complex i s faulted against and i n extensive areas thrust westward over these younger rocks (Mansy, 1974). The large scale southwest-vergent structures i n the meta-morphic terrane are probably associated with t h i s f a u l t i n g , but i t s age i s not well known though probably Jurassic or Cretaceous. Recurrent movement during l a t e r times i s also probable. To the west of t h i s l a t e Paleozoic and early Mesozoic terrane l i e c l a s t i c deposits of the Middle Jurassic to Early Cretaceous Bowser and Late Cretaceous to early T e r t i a r y Sustut Basins. Both sequences have been folded and faulted and much of the deformation i n the Sustut Basin involving com-pre s s i o n a l f o l d i n g and thrusting i s dated as e a r l y T e r t i a r y (Eisbacher, 1974a). The Wolverine Complex and related l a t e s t Precambrian rocks to the north (Ingenika group) thus form the westernmost exposures of Precambrian rocks believed to have been deposited along the cratonic continental margin (Ga-b r i e l s e , 1972). These rocks are juxtaposed against younger upper Paleozoic and lower Mesozoic rocks to the west that record an e n t i r e l y d i f f e r e n t set of s t r u c t u r a l and metamorphic conditions. The Wolverine Complex i s part of the C o r d i l l e r a n metamorphic core zone traceable southward i n t o the Shuswap Complex, northeast Washington, parts of c e n t r a l and southeast Idaho, and northeast and eastern Nevada (Armstrong and Hansen, 1966; Snoke, 1975; 68 Coney, 1974). Though t h e i r h i s t o r i e s are not i d e n t i c a l , these metamorphic be l t s form a fundamental s t r u c t u r a l element of the C o r d i l l e r a whose s i g n i -ficance i n plate tectonic terms i s neither f u l l y understood nor appreciated. 69 Geochronology of the Wolverine Complex and Related Rocks The Geological Survey of Canada has been the p r i n c i p l e c o l l e c t o r of K-Ar data i n t h i s region (Wanless et a l , 1971, 1973, 1974), and these data are compiled i n figure 37 and table I l a . These compilations c l e a r l y show the occurence of Eocene dates c l u s t e r i n g about 45-50 my. from 55 N to about 56^°N, including some dates east of the Rocky Mountain Trench. Though data are incomplete, the evidence suggests that northwest from the Chase Mountain region, the apparent K-Ar ages abruptly increase from 45-55 my. to 87-128 my. Where mineral pa i r s have been analysed, the determinations are concordant with only a few exceptions d i f f e r i n g by 10% or so. Eocene dates i n the Wolverine Complex, however, are also r e l a t i v e l y concordant but much younger. The coincidence of higher grade 'Wolverine' rocks with Eocene K-Ar ages at f i r s t would seem to suggest that the l a s t important high grade meta-morphism was during the Eocene; however, present evidence indicates that t h i s i s not the case. The key to the Eocene r e s e t t i n g problem may l i e i n the t r a n s i t i o n zone from young to older dates and i n areas surrounding Eocene plutons. Rb-Sr and K-Ar Data of t h i s study Near Chase Mountain a sample (no. 134) of muscovite-biotite s c h i s t y i e l d s a concordant K-Ar p a i r of 89 my. ( b i o t i t e ) and 92 my. (muscovite) as shown i n table l i b . Less than 1 km. away a s t r u c t u r a l l y discordant hornblende porphyry dike y i e l d s a hornblende K-Ar date of 42.6 my. This 42.6 my. date i s i d e n t i c a l to s i m i l a r dated dikes i n the Rocky Mountain Trench region (table I l a , samples GSC 73-55, 73-56). Rb-Sr analyses of muscovite, p l a g i o -clase, b i o t i t e , and whole rock from sample 134 y i e l d comparable r e s u l t s ( f i -gure 38) and indicate that excess Ar retention i s not a problem. Individual mineral isochron dates are discordant, and the b i o t i t e - p l a g i o c l a s e and b i o t i t e -70 Table H a . Geological Survey of Canada K-Ar data on rocks r e l a t e d to the Wolverine Complex analysis # rock type mineral age(m.y.) latitude(N) longitude(w' 70-1 2 schist muscovite 128+6 56 ° 3 1 . 5' 125°36' 70-13 schist biotite 124+6 56 ° 3 1 . 5' 125°36' 70-14 granite muscovite 47+3 56°23' 125°21.5' 125°21 .5' 70-15 granite biotite 43+3 56°23' 70-37 schist muscovite 45+3 55°23' 30" 123°39'50" 70-38 pegmatitic granite muscovite 50+6 55°23' 30" 1 23°39'50" 70-39 gneiss muscovite 47+3 55°19' 123°30' 70-40 gneiss biotite 44+4 55°07*35" 123°29'15" 70-41 gneiss muscovite 46+3 55 071 35" 1 2 3 ° 2 9 ' 1 5 " 70-42 gneiss biotite 43+4 55°32' 10" 123°52'40" 70-43 granite greisen muscovite 40+2 55°07' 35" 1 2 3°29 ' 1 5 " 70-44 amphibolite biotite 45+2 55°32- 1 0" 1 2 3 ° 5 2'40" 72-27 micaceous sandstone detrital muscovite 117+5 56°02' 124°U' 72-28 pegmatite muscovite 49+4 56°29" 124°44' 72-29 pegmatite muscovite 45+4 56°03' 30" 124°29' 73-46 garnet amphibolite hornblende 64.8+2 56°22' 124°39' 73-47 amphibolite hornblende 53.8+3 56°22' 124°39' 73-48 amphibolite biotite " . 40.5+2 56°22' 124°39' 73-49 amphibolite hornblende 45.4+2, 47.9+2 56°52'30" 124°48* 73-50 amphibolite biotite 40.8+2 56 u 5 2 ' '30" 124°48' 73-51 granodiorite biotite 86.8+3 58°00' 126°48* 73-52 granodiorite muscovite 88.4+4 58°00' 1 26°48' 73-53 qtz. monzonite boulder muscovite 94.9+4 57°27' i 125°47' 73-54 qtz. monzonite boulder muscovite 104+4 57°27' i 125°47' 73-55 lamprophyre biotite 49+2 57°25 ' i 125°47' 73-56 lamprophyre biotite 37+2 57°27 ' i 125°52' 71 T a b l e n a ( c o n t i n u e d ) . U n p u b l i s h e d G e o l o g i c a l S u r v e y o f C a n a d a d a t e s a n a l y s i s # r o c k t y p e m i n e r a l a g e ( m . y . ) latitude(N) 1 o n g i t u d e ( w ) g n e i s s / s c h i s t b i o t i t e 4 2 5 6 ° 4 7 ' 1 2 4 ° 4 6 ' s c h i s t s c h i s t m u s c o v i t e b i o t i t e 4 7 4-1-5 6 ° 3 2 ' 5 6 ° 3 2 ' 1 2 4 ° 4 2 ' 1 2 4 ° 4 2 ' m i c a c e o u s c o n g l o m e r a t e m u s c o v i t e 2 4 6 5 6 ° 3 2 ' 1 2 5 ° 4 8 * g n e i s s / s c h i s t g n e i s s / s c h i s t b i o t i t e m u s c o v i t e 1 0 6 1 2 0 5 7 ° 0 3 ' 5 7 ° 0 3 ' 1 2 6 ° 1 4 ' 126°14' g r a n i t e g r a n i t e m u s c o v i t e b i o t i t e 9 9 . 7 9 4 . 2 5 7 ° 1 8 ' 5 7 ° 1 8 ' 1 2 6 ° 2 8 ' 1 2 6 ° 2 8 ' g r a n i t e g r a n i t e m u s c o v i t e b i o t i t e 9 9 . 5 9 0 . 9 5 7 ° 2 2 ' 5 7 ° 2 2 ' 1 2 6 ° 2 9 ' 1 2 6 ° 2 9 ! T a b l e l i b . K - A r d a t a f r o m t h i s s t u d y r o c k # r o c k t y p e m i n e r a l A r ( r a d ) / A r ( t o t a l ) * * A r 4 0 (rad)/K 4 0 1 3 4 s c h i s t m u s c o v i t e 8 . 0 1 0 . 9 1 5 5 . 5 2 8 x 1 0 ~ 5 1 3 4 s c h i s t b i o t i t e 7 . 5 5 0 . 9 5 4 5 . 3 2 4 x 1 0 ~ 3 1 4 4 h o r n b l e n d e p o r p h y r y h o r n b l e n d e 0 . 7 7 0 . 6 3 2 2 . 5 2 x 1 0 ~ 3 r o c k # a p p a r e n t a g e ( m . y . ) l a t i t u d e ( N ) l o n g i t u d e ( w ) 1 3 4 m u s c o v i t e 9 2 . 2 + 3 - 3 5 6 ° 3 2 ' 1 2 5 ° 1 8 ' 1 3 4 b i o t i t e 8 8 . 9 + 3 . 2 5 6 ° 3 2 ' 125°18" 1 4 4 h o r n b l e n d e 4 2 . 6 + 2 5 6 ° 3 2 ' 1 2 5 ° 1 9 ' * K a n a l y s e s p e r f o r m e d b y K . S c o t t o f U n i v e r s i t y o f B r i t i s h C o l u m b i a * * A r m a s s s p e c t r o m e t r y p e r f o r m e d b y J . H a r a k a l o f U n i v e r s i t y o f B r i t i s h C o l u m b i a ; A K 4 0 t o A r 4 0 = 0 . 5 8 5 x 1 0 ~ 1 0 y r , = 4.72. * JO"10^"* , 4 O K / K = 1.181 * \0'\ . 8 0 0 H . 7 9 0 -\ Sr87 Sr86 , 7 8 0 1 . 7 7 0 i 134 SCHIST MUSCOVITE ^ T O . ^ ^ ^ ^ ^ " ^134 WHOLE ROCK ^ ^ ^ ^ ^ ""PLAGIOCLASE 4 6 8 Rb 87 / Sr 86 10 12 Figure 38. Rb-Sr evolution diagram for s c h i s t of sample 134. Figure 39. Rb-Sr evolution diagram for "Wolverine" g r a n i t i c rocks. . 7 2 5 . 7 2 0 1 Sr87 Sr86 .715 i . 7 1 0 1 7 0 5 "WOLVERINE" GRANITIC ROCKS 79 + 10 X-"1' TO SC0V]I! PLAGIOCLASE 52 M.Y PLAGIOCLASE 308 PEGMATITE WHOLE ROCK 0 R T H 0 C L A S E _ 7 \ - M ^ 31 M.Y-308 WHOLE ROCK 1 1 1 -2 3 Rb 87 / Sr 86 4 73 whole rock dates are anomalously younger (77 m.y.) than the K-Ar date (89 * m.y.), and thus g e o l o g i c a l l y unreasonable.- The muscovite-plagioclase-whole rock date i s 106 + 6 m.y. and confirms a Late Cretaceous or e a r l i e r age for the metamorphism. m The rocks near sample^lie at the northwestern end of the Wolverine Complex, and they are metamorphosed to lower amphibolite f a c i e s but have suffered extensive retrogression of kyanite and garnet. Farther south near Blackpine Lake (figure 37), K-Ar dates from two-mica g r a n i t i c rocks are 47 and 43 m.y. r e s p e c t i v e l y f or a muscovite-biotite p a i r (table I l a ; Wanless and others, 1971). F i e l d i n v e s t i g a t i o n indicates that these muscovite + garnet-bearing g r a n i t i c rocks are c l o s e l y r e l a t e d to the regional metamorphism which i n t h i s s p e c i f i c area i s s i l l i m a n i t e grade. Here, where the t y p i c a l occurrence of Wolverine-type g r a n i t o i d rocks i n t e r -spersed with s c h i s t s and gneisses i s well-developed, Rb-Sr analysis from these muscovite-bearing g r a n i t i c rocks y i e l d s a composite mineral isochron of 79 + 10 m.y. and plagioclase-mineral isochrons of 52-84 m.y. (figure 39, table III) and are thus somewhat older than the K-Ar dates. The area 10-15 km. south of Chase Mountain has been intruded by a small (8 sq. km.) stock of coarse-grained b i o t i t e quartz monzonite (figure 2), termed the Blackpine Lake g r a n i t i c stock. This stock was o r i g i n a l l y mapped by Roots (1954) as part of the Wolverine Complex and y i e l d s a Rb-Sr 87 whole rock isochron age of 62 + 7 m.y. at S r ^ - ^ of .7052 + .0002 and a miner-a l isochron age of 44.7 + 2 m.y. @ .7058 (figure 40, table I I I ) . This stock i s deformed by p a s t - c r y s t a l l i z a t i o n deformation r e l a t e d to i t s d i a p i r l c (?) em-placement, but i t i s post-tectonic with respect to the e a r l i e r synmetamorphic In a l l 'Wolverine' Complex b i o t i t e s analyzed i n t h i s study, t h i s anomaly was encountered, and neither i t s cause nor i t s s i g n i f i c a n c e i s understood. Similar r e s u l t s , however, have been encountered elsewhere i n terrane of old metamorphic rock reset at a l a t e r date ( S a t i r , 1974; R. L. Armstrong, per-sonal communication, 1976). T a b l e ' I I I . R b - S r a n a l y t i c a l d a t a s a m p l e # r o c k / m i n e r a l p p m R b p p m S r R b 8 7 S r 8 6 S r 8 7 * S r 8 6 - e r r o r 1 3 4 s c h i s t 2 0 9 8 4 7 . 2 0 . 7 8 6 4 . 0 0 0 1 134 b i o t i t e 491 1 0 . 5 135.5 . 9 2 4 7 . 0 0 0 3 1 3 4 m u s c o v i t e 252 8 5 8 . 5 8 . 7 8 9 7 . 0 0 0 2 1 3 4 p l a g i o c l a s e 3 4 2 3 3 0 . 4 2 . 7 7 6 7 . 0 0 0 1 3 0 0 s c h i s t 1 9 6 1 6 2 3 . 5 0 . 7 6 0 4 . 0 0 0 1 3 0 8 q t z . m o n z o n i t e 1 4 0 254 1 . 6 0 . 7 1 1 3 . 0 0 0 1 3 0 8 m u s c o v i t e 3 3 3 3 2 3 0 . 1 . 7 4 0 8 . 0 0 0 1 3 0 8 b i o t i t e 6 0 5 7 1 2 4 . 7 . 7 2 1 1 . 0 0 0 1 3 0 8 o r t h o c l a s e 2 4 2 3 8 0 1 . 8 4 . 7 1 1 5 . 0 0 0 1 3 0 8 p l a g i o c l a s e 2 6 1 9 3 0 . 3 9 . 7 1 0 5 . 0 0 0 1 3 0 8 p e g p e g m a t i t e 8 8 2 2 7 1 . 1 2 . 7 1 8 3 . 0 0 0 1 3 0 8 p e g m u s c o v i t e 3 6 3 4 1 2 5.6 . 7 4 7 5 . 0 0 0 1 3 0 8 p e g p l a g i o c l a s e 3 7 3 5 8 0 . 3 0 . 7 1 7 6 . 0 0 0 1 3 1 8 g r a n o d i o r i t e 123 4 4 6 0 . 8 0 . 7 0 9 8 . 0 0 0 1 3 2 4 a p l i t e 2 5 4 6 3 1 1 . 7 . 7 3 6 7 . 0 0 0 1 B l a c k p i n e : L a k e g r a n i t i c s t o c k 323 q t z . m o n z o n i t e 2 0 4 3 0 7 1 .92 . 7 0 6 9 . 0 0 0 1 323 b i o t i t e 5 7 1 8 9 1 8 . 6 . 7 1 8 0 . 0 0 0 1 3 2 3 o r t h o c l a s e 552 4 0 2 3 . 9 7 . 7 0 8 2 . 0 0 0 1 3 2 3 p l a g i o c l a s e 3 7 3 7 1 0 . 2 9 . 7 0 6 0 . 0 0 0 1 323 2 q t z . m o n z o n i t e 234 2 7 2 . 2 . 4 9 . 7 0 7 3 . 0 0 0 1 325 g r a n o d i o r i t e 1 4 8 3 1 7 1 .35 . 7 0 6 3 . 0 0 0 1 * S r 8 7 / 8 6 r a t i o s h a v e b e e n a d j u s t e d t o c o n f o r m t o t h e N B S s t a n d a r d S r C O . ( 9 8 7 ) a c t u a l v a l u e o f . 7 1 0 2 2 . **1<r R b a n d S r c o n c e n t r a t i o n s a r e a c c u r a t e t o w i t h i n 2$>, or 1 ppm. , a s * , determined by XRF. A R b 8 7 = 1 . 4 2 x 1 0 ~ 1 1 y r ; Rb 87/Sr 86 = 2.894 x Rb/Sr . 7 0 9 . 7 0 8 i S r 8 7 S r 8 6 . 7 0 7 . 7 0 6 H . 7 0 5 BLACKPINE LAKE GRANITIC STOCK O R T H O C L A S E ( 3 2 3 ) * \ 3 2 3 ? W H O L E R O C K ^ > ^ - ^ 3 2 3 W H O L E R O C K I 3 2 5 W H O L E R O C K feT- ^ - ~ | T INITIAL= .7052' ± .0002 -I 1 1 r 2 3 Rb 8 7 / S r 8 6 Figure 40. Rb-Sr evolution diagram for Blackpine Lake g r a n i t i c stock. Figure 41. . 8 0 0 Rb-Sr diagram showing a l l whole rock determinations. . 7 8 0 -. 7 6 0 - 1 Sr87 S r 8 6 . 7 4 0 . 7 2 0 . 7 0 0 5 0 r n ^ 3 0 0 S C H I S T * 134 S C H I S T 32 4-A P L I T E 3 0 8 P E G M A T I T E 318 . 3 0 8 } T W O - M I C A G R A N I T I C R O C K S 3 2 5 3 2 3 3 2 3 z } B L A C K P I N E L A K E GRANIT IC R O C K S —r~ 2 —r— 4 -r-8 i— 1 0 — r — 12 Rb 8 7 / S r 8 6 76 structures i n the country rock (Parrish, 1976). I t s f o l i a t i o n i s not a primary igneous feature as thought by Roots (1954) , and i s best developed near contacts. 87 The whole rock Rb-Sr and Sr^-r values of the syndeformational, syn-oo metamorphic muscovite-bearing g r a n i t i c rocks do not define an isochron (figure 41), and i t i s not possible to date these igneous rocks with res-pect to the early t i g h t to i s o c l i n a l f o l ds that they are involved i n ; hence, the age of early structures i s not known. The Sr i s o t o p i c compositions of the country rock s c h i s t s and gneisses were not systematically determined, but two determinations i n d i c a t e that t h e i r r a t i o s are much higher than the igneous rocks that intrude them (figure 41). This implies that the o r i g i n of the 'Wolverine' g r a n i t i c rocks i s both anatectic as we l l as magmatic and probably involves con-tamination of a le s s radiogenic source magma by highly radiogenic country rock. 87 The i n i t i a l Sr r a t i o of the Blackpine Lake stock i s .7052 + .0002 and i s much lower than both country rock s c h i s t s (>.740) and muscovite-bearing 'Wolverine' g r a n i t i c rocks (.710-.737). A d i f f e r e n t , l e s s radio-genic magma i s thus c l e a r l y d istinguishable both i n age, s t r u c t u r a l r e l a -t i o n s , and Sr i s o t o p i c composition from the synmetamorphic type. I n i t i a l Sr i s o t o p i c compositions from the Hogem composite b a t h o l i t h to the west (figure 1) are very low, generally l e s s than .704 (Eadie, 1976), and i t i s postulated that ancient Precambrian basement rock may extend beneath but e s s e n t i a l l y no farther west than the Wolverine Complex and relat e d Precambrian metasediments. Obviously more information i s needed to extend t h i s boundary. 77 Discussion and Problems of Interpretation Wolverine-type rocks are metasediments with g r a n i t i c s i l l s , dikes, and pegmatites present i n abundance and with the metamorphic grade roughly corresponding to the s i l l i m a n i t e zone (see Part I ) . This rock as s o c i a t i o n i s now exposed i n several a n t i c l i n o r i a throughout the e n t i r e length of the metamorphic b e l t from 54°N to 58°N where s t r u c t u r a l u p l i f t i s great. Though det a i l e d s t r u c t u r a l and metamorphic data are lacking i n most areas, regional studies (Mansy, 1976; Gabrielse, 1976) i n d i c a t e that the high grade rocks throughout the b e l t have experienced a reasonably s i m i l a r s t r u c t u r a l evolu-t i o n to that near Chase Mountain, whereas i n low grade areas 20 km. to the north of Chase Mounatin, the s t r u c t u r a l h i s t o r y appears simpler and apparently does not involve early synmetamorphic i s o c l i n a l f o l d i n g (Mansy, 1972, 1974). These s t r u c t u r a l and metamorphic considerations demonstrate that s i m i l a r rocks both north and south of about 56%°N have experienced s i m i l a r metamor-phic and s t r u c t u r a l conditions, but they have very d i f f e r e n t geochronologic h i s t o r i e s . The Eocene r e s e t t i n g event observed south of 56^° cannot be r e -sponsible for the dominant metamorphic f a b r i c of the higher grade rocks since rocks with the same f a b r i c possess Early Cretaceous dates north of the reset region. The main period of metamorphism of these rocks must be l a t e E a r l y Cretaceous or older. This i s supported by the presence of d e t r i t a l mica and granite boulders y i e l d i n g K-Ar dates of 95-117 m.y. i n coarse c l a s t i c sediments found with the S i f t o n formation of Late Cretaceous to e a r l y Ter-t i a r y age i n the Rocky Mountain Trench (Eisbacher, 1974a). When viewed i n t h i s structural-temporal context, the Wolverine Complex (Armstrong, 1949) i s not unique because i d e n t i c a l rocks are found i n many a n t i c l i n o r i a where the s t r u c t u r a l u p l i f t i s s u f f i c i e n t l y great (fi g u r e 1). • 78 The p r o b l e m o f r e s e t K - A r d a t e s i s n o t e a s i l y e x p l a i n e d . The e x -p l a n a t i o n s f o r r e s e t ages i n t h e Shuswap Complex o f s o u t h e r n B r i t i s h C o l u m b i a ( see I n t r o d u c t i o n ) have s e r i o u s s h o r t c o m i n g s f a r t h e r n o r t h . F o r i n s t a n c e , o n l y a few s c a t t e r e d s m a l l T e r i a r y i n t r u s i o n s i n t o W o l v e r i n e -t y p e r o c k s a r e known, and Eocene v o l c a n i c s , t h o u g h l o c a l l y abundan t i n R o c k y M o u n t a i n T r e n c h , D a l l L a k e l i n e a m e n t , and P e l l y l i n e a m e n t ( H . G a b r i e l s e , p e r s o n a l c o m m u n i c a t i o n , 1 9 7 6 ) , a r e n o t w i d e s p r e a d and b e a r no s p a t i a l c o r r e l a t i o n w i t h K - A r r e s e t d a t e s . These v o l c a n i c s may have once b e e n much more e x t e n s i v e , b u t t h i s i s n o t s u p p o r t e d b y p r e s e n t d a t a . R a p i d b l o c k u p l i f t c a n n o t y e t s a t i s f a c t o r i l y e x p l a i n t h e o b s e r v e d d a t e s b e c a u s e t h e s e d i m e n t a r y e v i d e n c e o f Eocene u p l i f t and f a u l t i n g p r e s e n t i n f a n g l o m e r a t e s i n R o c k y M o u n t a i n T r e n c h and e l s e w h e r e does n o t b e a r a s p a t i a l r e l a t i o n s h i p w i t h t he r e s e t a r e a . H o w e v e r , more d a t a a r e needed t o f u l l y e v a l u a t e t h i s s t a t e m e n t . U n u s u a l l y h i g h h e a t f l o w may a l s o be a p p e a l e d t o ; i n d e e d , one m i g h t be t empted t o r e l a t e s u c h h e a t f l o w ( w i t h p o s s i b l e h y d r o t h e r m a l i n t e r a c t i o n s ) t o t h e r e t r o g r a d e metamorph ism o b s e r v e d t h r o u g h o u t e x t e n s i v e a r e a s . H o w e v e r , r e t r o g r a d e e f f e c t s a r e p r e s e n t w i t h i n b o t h r e s e t r o c k s and r o c k s p r e s e r v i n g C r e t a c e o u s d a t e s , and s u c h h i g h h e a t f l o w w o u l d have no o t h e r p r e s e n t l y r e c o g n i z e d m a n i f e s t a t i o n e x c e p t p e r h a p s o x y g e n i s o t o p e v a r i a t i o n s . A f u r t h e r a p p e a l c a n be made t o t h e h e a t f l u x i n t r o d u c e d upon t h e i n j e c t i o n o f v a r i o u s p o s t - t e c t o n i c d i k e s and s i l l s o f f e l d s p a r and h o r n b l e n d e p o r p h y r y ( R o o t s , 1954) and m i n e t t e ( E i s b a c h e r , 1972) t h a t a r e s p o r a d i c a l l y p r e s e n t i n t h e m e t a m o r p h i c t e r r a n e as w e l l as t h e R o c k y M o u n t a i n T r e n c h and P e l l y l i n e a m e n t . B u t a g a i n , a l l d a t e d d i k e s a r e w i t h i n t h e ' o l d e r ' t e r r a n e and i n d i c a t e no a p p r e c i a b l e a f f e c t o f t h e d i k e s on t h e K - A r ages o f m e t a m o r p h i c r o c k s ; a n d , i n a d d i t i o n , n e a r l y a l l d e t e r m i n a t i o n s on d i k e s ( t a b l e I l a ) f a l l b e t w e e n 37 and 43 m . y . w h i c h i s s l i g h t l y y o u n g e r and p e r h a p s u n r e l a t e d t o Eocene r e s e t t i n g . 79 Several of these dikes were observed to occupy f a u l t s or j o i n t s which may be r e l a t e d to possible Eocene f a u l t i n g i n the Rocky Mountain Trench; si m i -l a r r e l a t i o n s have been observed by Eisbacher (1972) near Ware, B r i t i s h Columbia. A workable explanation of the r e s e t t i n g phenomena i n t h i s region must take into account the following: 1) Pervasive r e s e t t i n g of K-Ar dates between 55°N and roughly 56Jg°N during the Eocene; 2) Occurrence of young K-Ar dates i n some metamorphic rocks d i r e c t l y east of the Rocky Mountain Trench as w e l l ; 3) Occurrence of older Rb-Sr mineral dates (52-84 m.y.) within the 45-50 m.y. K-Ar area near Blackpine Lake; 4) Absence of s p a t i a l r e l a t i o n s h i p between Eocene volcanic rocks and reset metamorphic rocks; 5) Presence of sedimentary record of intense Eocene (?) u p l i f t , but lack of s p a t i a l c o r r e l a t i o n with the reset area, given present data; 6) Occurrence of Early Cretaceous K-Ar dates i n d e t r i t a l rocks of the S i f t o n formation d i r e c t l y east of as w e l l as far t h e r north of the reset terrane; 7) Emplacement of dikes throughout the region i n the Eocene, perhaps accompanied by f a u l t i n g i n the Rocky Mountain Trench; 8) Presence of a few small but important Eocene g r a n i t i c stocks gen-e r a l l y within the reset area (several more no doubt remain undiscov-ered) . Intrusion of small Eocene g r a n i t i c plutons such as the Blackpine Lake stock accompanied by high heat flow, hydrothermal c i r c u l a t i o n , and perhaps oxygen isotope r e e q u i l i b r a t i o n i s probably important i n explaining the reset dates. The Blackpine Lake stock i s l i k e l y responsible for the young K-Ar dates surrounging i t and for the p a r t i a l r e s e t t i n g of Rb-Sr dates. F a u l t i n g , doming, and rapid u p l i f t may also be important f a c t o r s , but the s i g n i f i c a n c e of these processes i n explaining r e s e t t i n g of dates remains unclear and e l u s i v e . Based upon s p a t i a l arguments, block f a u l t i n g , r e l a t e d (?) dike emplacement, and Eocene (?) volcanism are probably not r e l a t e d to the r e s e t t i n g event. Rather, a l l of these events-deformation, u p l i f t , erosion, volcanism, and i n t r u s i o n may have been occuring simultaneously as part of an intense pulse of early T e r t i a r y tectonism. The only clear c o r r e l a t i o n with the re-se t t i n g of K-Ar and Rb-Sr mineral dates i s the e f f e c t of small Eocene gran-i t i c i n t r u s i v e rocks which have caused geochronologic disturbance i n pre-v i o u s l y metamorphosed country rock. Early T e r t i a r y Tectonic Setting In order to evaluate the s i g n i f i c a n c e of the Eocene K-Ar r e s e t t i n g event, i t i s necessary to view the event i n terms of the early T e r t i a r y tectonic s e t t i n g i n which volcanism, plutonism, deformation, and sedimen-t a t i o n are interwoven. During the Eocene, volcanism was ex p e c i a l l y widespread and intense i n B r i t i s h Columbia as shown i n figure 42. In the Intermontane zone of ce n t r a l B r i t i s h Columbia, the Odtsa Lake volcanics occupy large regions from the Skeena Arch southward and are c o r r e l a t i v e with the Princeton and Marron volcanics of southern B r i t i s h Columbia and rela t e d volcanics i n Washington and Idaho. These Eocene volcanics are rela t e d to small i n t r u -sive bodies of Eocene age i n the Intermontane zone, the Katzberg i n t r u -sions (Richards, 1974), which extend without t h e i r volcanic equivalent into the eastern Bowser Basin and elsewhere including a few bodies i n the metamorphic terrane. Eocene volcanic rocks are lacking i n the Bowser Basin, but the Sloko volcanics l i e farther to the north (Souther, 1972, 1976). Much larger bodies of i n t r u s i v e rock are associated.iwith the Sloko Group near the eastern margin of the Coast Plutonic Complex where many large discordant plutons y i e l d Eocene radiometric dates. Though large regions of the cen t r a l and eastern Coast Mountains have also been reset to young Eocene ages (Hutchinson, 1970), many of the ages on eastern border plutons are probably emplacement ages (Smith, 1975). During the intense volcanism and plutonism i n the Intermontane zone and eastern Coast Plutonic Complex, metamorphic rocks of the Wolverine Complex were being reset. Sedimentation during t h i s igneous episode was confined mainly to the Sustut and S i f t o n basins where coarse c l a s t i c sediments were shed from high r e l i e f source terrane composed of g r a n i t i c , metamorphic, sedimentary and 8 2 "jv\ tywNmc INTRUSION / TOLDIM* Vv SILICIC VOLCANISM THRUSTING W teotmmm^ DISTDWWCE NCWIAL FAULTS = /\ft£A OF 5ePIHEWTflT|0rJ 0 50 ico l 5 0 200 K f t ] Figure.'42. ... Sketch map i l l u s t r a t i n g tectohi'c-event's during' the Eocene. volcanic rocks. The present form of these basins i s a r e s u l t of both or-i g i n a l basin extent as well as l a t e r erosional modification, and i t i s doubtful that the Sustut and S i f t o n basins were ever connected (Eisebacher, 1974a). It i s l i k e l y that the Bowser and western part of the Sustut basins were source areas during the Eocene i n addition to parts of the Omineca C r y s t a l l i n e Belt and the Rocky Mountains. Deformation during the Eocene was considerable and extended across the C o r d i l l e r a . The f i n a l stages of c r u s t a l shortening i n the Rocky Moun-tains f o o t h i l l s took place i n Paleocene and Eocene time ( B a l l y , Gordy, and Stewart, 1966; Pric e and Mountjoy, 1970). Probable contemporaneous com-pressional deformation occurred i n the S i f t o n and Sustut Basins deforming Upper Cretaceous, Paleocene(?), and Eocene s t r a t a by east-vergent f o l d i n g and thrusting i n the Sustut Basin and by kink f o l d i n g and f a u l t i n g i n the S i f t o n Basin (Eisbacher, 1972, 1974a). Folds and thrusts present on the western part of the<upper Paleozoic and lower Mesozoic terrane that d i s -place Permian, Upper T r i a s s i c , and Lower Jurassic rocks westward over Cretaceous and T e r t i a r y sediments and older rocks (Richards^ 1976) are pro-bably of more than one age, and l o c a l l y may be early T e r t i a r y structures. Deformation of the Bowser and Sustut Basins continued from post-Lower Cre-taceous to post-45 my. time c o n t r o l l i n g paleocurrent trends and sedimenta-t i o n patterns (Eisbacher, 1974a). Early T e r t i a r y u p l i f t of the Coast P l u -tonic Complex has s t r u c t u r a l l y impinged on the western Bowser Basin causing decollement s t y l e f o l d i n g and thrusting (Eisbacher, 1974b). Deformation, metamorphism, plutonism, and volcanism are widespread i n the Mesozoic and e a r l y T e r t i a r y h i s t o r y of the C o r d i l l e r a but are e s p e c i a l l y intense i n the Eocene. This Eocene episode was the concluding stage of an episodic, but c o n s i s t e n t l y compressional regime extending from e a r l i e r i n 84 the Mesozoic. This s i t u a t i o n was followed by an abrupt t r a n s i t i o n to a quiescent tensional regiime f o r much of the remaining Cenozoic (Wheeler et a l , 1972), characterized by absence of a c i d i c volcanism, plutonism, and f o l d i n g , and by the presence of r e s t r i c t e d block f a u l t i n g , extrusion of b a s a l t i c plateau lavas, and intermittent sedimentation and peneplana-t i o n u n t i l the Pliocene rejeuvenation. Summary In t h i s study Rb-Sr whole rock isochron data show; that .a small Eo-cene plutonic body intrudes rocks grouped i n the Wolverine Complex. Rb-Sr mineral isochrons on medium to high grade metamorphic and synmetamorphic gran i t o i d rocks within the Eocene K-Ar reset terrane near the Blackpine Lake stock are p a r t i a l l y reset to 52-84 my. from probable middle Cretaceous or older o r i g i n a l dates. Metamorphic rocks to the north that are s t r u c t u r a l l y continuous with the Wolverine Complex and with the Eocene reset province have K-Ar and Rb-Sr mineral ages of 89-1P6 my. i n d i c a t i n g that the medium to high grade metamorphism and i t s accompanying s t r u c t u r a l disturbance are middle Cretaceous or older i n age. Although much work needs to be done to f u l l y explain the phenomena, the Eocene r e s e t t i n g event i s l i k e l y the r e s u l t of both the i n t r u s i o n of small bodies of g r a n i t i c rock and the accompanying high heat flow. F a u l t -ing and u p l i f t may be responsible for some of the pattern i n K-Ar date d i s t r i b u t i o n , but t h e i r importance i s d i f f i c u l t to evaluate because of lack of d e t a i l e d data i n surrounding regions. The Eocene event was accompanied by intense volcanism, plutonism, and deformation i n other areas of northern and c e n t r a l B r i t i s h Columbia. I t coincides with the cessation of compressional tectonics i n the C o r d i l l e r a and i s abruptly followed by a t e c t o n i c a l l y quiescent regime u n t i l the P l i -ocene rejeuvenation which has resulted i n much of the present physiography. 86 BIBLIOGRAPHY Albee, A., 1972, Metamorphism of p e l i t i c s c h i s t s : Reaction r e l a t i o n s of c h l o r o t o i d and s t a u r o l i t e : Geol. Soc. Amer. B u l l . 83, 3249-3268. Armstrong, J.E., 1949, Fort St. James map-area, Cassiar and Coast d i s t r i c t s , B r i t i s h Columbia: Geol. Surv. Can. Mem. 252, 210 p. and Roots, E.F., 1948, Aiken Lake map-area, B r i t i s h Columbia: Geol. Surv. Can. Paper 48-5. Armstrong, R.L., 1974a, Geochronometry of the Eocene v o l c a n i c - p l u t o n i c episode i n Idaho: Northwest Geology, v.3, p. 1-15. , and McDowall, W.G., 1974b, Proposed refinement of the Phanerozoic time scale (abs.): I n t ' l . Meeting for Geochron., P a r i s , Abs. Vol. and Hansen, E., 1966, C o r d i l l e r a n i n f r a s t r u c t u r e i n the eastern Great Basin: Am. Jour. S c i . 264, 112-127. B a l l y , A.W., Gordy, P.L., Stewart, G.A., 1966, Structure, seismic data, and orogenic evolution of southern Canadian Rocky Mountains: B u l l . Can. Pet. Geol., 14, 337-381. Campbell, R.B., 1970, S t r u c t u r a l and metamorphic t r a n s i t i o n s from i n f r a -structure to suprastructure, Cariboo Mountains, B r i t i s h Columbia: Geol. Assn. Can. Sp. Paper 6, 67-72. , Mountjoy, E.W., and Young, F.G., 1973, Geology of McBride map-area, B r i t i s h Columbia: Geol. Surv. Can. Paper 72-35. Coney, P.J., 1972, C o r d i l l e r a n tectonics and North America p l a t e motion: Am. Jour. S c i . 272, 603-628. , 1974, S t r u c t u r a l analysis of the Snake Range "decollement", east-central Nevada: Geol. Soc. Amer. B u l l . 85, 973-978. Davis, G., 1975, Gravity-induced f o l d i n g o ff a gneiss dome complex, Rincon Mountains, Arizona: Geol. Soc. Amer. B u l l . 86, 979-990. Eadie, E.T., 1976, K-Ar and Rb-Sr geochronology of the northern Hogem b a t h o l i t h , B r i t i s h Columbia: unpub. B.Sc. t h e s i s , Univ. of B r i t i s h Columbia. Eisbacher, G.H., 1972, Tectonic overprinting near Ware, northern Rocky Mountain Trench: Can. Jour. Earth S c i . 9, 903-913. , 1974a, Sedimentary h i s t o r y and tectonic evolution of the Sustut and S i f t o n basins, north-central B r i t i s h Columbia: Geol. Surv. Can. Paper 73-31. , 1974b, Evolution of successor basins i n the Canadian C o r d i l l e r a : Soc. Econ. P a l . and Miner. Sp. Pub. 19, 274-291. 87 Ferguson, J.A., 1975, Tectonic implications of some geochronometric data from the northeastern border zone of the Idaho b a t h o l i t h : Northwest Geology 4, 53-58. Gabrielse, H., 1967, Tectonic framework of the northern Canadian C o r d i l l e r a : Can. Jour. Earth. S c i . 4, 271-298. , 1971, Operation F i n l a y : i n Rept. of Act., part A, Geol. Surv. Can. Paper 71-1A. , 1972, Younger Precambrian of the Canadian C o r d i l l e r a : Am. Jour. S c i . 272, 521-536. , Dodds, C.J., and Mansy, J.L., 1976, Operation Fin l a y : i n Rept. of Act., part A, Geol. Surv. Can. Paper 76-1A, 87-90. and Reesor, J.E., 1974, The nature and s e t t i n g of g r a n i t i c plutons i n the c e n t r a l and eastern parts of the Canadian C o r d i l l e r a : P a c i f i c Geology 8, 109-138. Garnett, J.A., 1974, Geology and copper-molybdenum mine r a l i z a t i o n i n the southern Hogem b a t h o l i t h , north-central B r i t i s h Columbia: Can. Inst. Min. and Metal. B u l l . 749,101-106. Ghent, E. and Devries, .C.D.S., 1972, Plagioclase-garnet-hornblende e q u i l i b r i a i n hornblende-bearing rocks from the Esplanade Range, B r i t i s h Columbia: Can. Jour. Earth. S c i . 9, 618-6 Gu i d o t t i , C.V., 1970, The mineralogy and petrology of the t r a n s i t i o n from lower to upper s i l l i m a n i t e zone i n the Oquossoc area, Maine: Jour. Petrology 11, 277-336. Hewitt, D.A., 1973, S t a b i l i t y of the assemblage muscovite-calcite-quartz: Amer. Mineral. 58, 785-791. , 1975, S t a b i l i t y of the assemblage phlogopite-calcite-quartz: Amer. Mineral. 60, 391-397. Hutchison, W.W., 1970, Metamorphic framework and plutonic s t y l e s i n the Prince Rupert region of the c e n t r a l Coast Mountains, B r i t i s h Columbia: Can. Jour. Earth S c i . 7, 376-405. I r i s h , E.J.W., 1970, Halfway River map-area, B r i t i s h Columbia: Geol. Surv. Can. Paper 69-11. Irvine, T.N., 1975, Axelgold layered gabbro i n t r u s i o n , McConnell Creek map-area, B r i t i s h Columbia: i n Rept. of Act. part B, Geol. Surv. Can. Paper 75-1B, 81-88. , 1976, Studies of C o r d i l l e r a n gabbroic and ultramafic i n t r u s i o n s , B r i t i s h Columbia,•part 1 and part 2: i n Rept. of Act. part A, Geol. Surv. Can. Paper 76-IA, 75-81. 88 Leech, G.B., 1966, The Rocky Mountain Trench: i n Irvine, T.N., ed., The world r i f t system, Geol. Surv. Can. Paper 66-14, 307-330. Lord, C.S., 1948, McConnell Creek map-area, B r i t i s h Columbia: Geol. Surv. Can. Memoir 251. Mansy, J.L., 1971, The Ingenika Group: i n Rept. of Act. part A, Geol. Surv. Can. Paper 71-1A, 26-28. , 1972, Stratigraphy and structure of the Ingenika Group i n Finlay and Swannell Ranges, B r i t i s h Columbia: i b i d . , Paper 72-1A, 29-32. , 1974, Operation F i n l a y : i b i d . , Paper 74-1A, 17-18. , and Dodds, C.J., 1976, Stratigraphy, structure, and metamorphism i n northern and c e n t r a l Swannell Ranges, B r i t i s h Columbia: i b i d . , Paper 76-1A, 91-92. Mathews, W.H., 1976, Anomalous K-Ar dates from gneisses of the T r i n i t y H i l l s , B r i t i s h Columbia: Geol. Ass. Can. Prog, with Abs. 1, p.47. Medford, G., 1975, K-Ar and f i s s i o n track geochronometry of an Eocene thermal event i n the K e t t l e River (west/2) map-area, southern B r i t i s h Columbia: Can.Jour. Earth S c i . , 12, 836-843. M i l l e r , F.K., and Engels, J.C., 1975, D i s t r i b u t i o n and trends of d i s -cordant ages of the p l u t o n i c rocks of northeastern Washington and northern Idaho: Geol. Soc. Amer. B u l l . , 86, 517-528. Monger, J.W.H., 1975, C o r r e l a t i o n of eugeosynclinal t e c t a n o - s t r a t i g r a -phic b e l t s i n the North American C o r d i l l e r a : Geosciences Canada 2, 4-9. , 1976, Lower Mesozoic rocks i n McConnell Creek map-area: i n Rept. of Act. part A, Geol. Surv. Can. Paper 76-1A, 51-55. and Patterson, I.A., 1974, Upper Paleozoic and lower Mesozoic rocks of the Omineca Mountains: i b i d . , Paper 74-IA, 19-20. P a r r i s h , R.R., 1976, Structure and metamorphism i n southern Swannell Ranges, B r i t i s h Columbia: i b i d . , Paper 76-1A, 83-86. Pr i c e , R.A., and Mountjoy, E.W., 1970, Geologic structure of the Canadian Rocky Mountains between Bow and Athabasca Rivers-a progress report: Geol. Assn. Can. Sp. Paper 6, 7-26. Richards, T.A., 1974, Hazleton east h a l f : i n Rept. of Act. part A, Geol. Surv. Can. Paper 74-1A, 35-38. , 1976, McConnell Creek map-area (94 D east/2): i b i d . , Paper 76-1A, 43-50. 89 Richardson, S.W., G i l b e r t , M.C., and B e l l , P.M., 1969, Experimental determination of kyanite-andalusite and a n d a l u s i t e - s i l l i m a n i t e e q u i l i b r i a , the aluminum s i l i c a t e t r i p l e point: Amer. Jour. S c i . 267, 259-272. Roots, E.F., 1954, Geology and mineral deposits of Aiken Lake map-area, B r i t i s h Columbia: Geol. Surv. Can. Memoir 274. Ross, J.V., 1974, A T e r t i a r y thermal event i n south-central B r i t i s h Columbia: Can. Jour. Earth S c i . 11,1116-1122. S a t i r , M., 1975, Die entwicklungsgeschichte der westlichen Hohen Tauern und der suedlichen Oetztalmasse auf grund radiometrischer a l t e r s b e s -timmungen: Inauguraldissertation der Uni v e r s i t a t Bern. Slaughter, J . , Kerrick, D.M., and Wall, V.J., 1975, Experimental and thermodynamic study of e q u i l i b r i a i n the system CaO-MgO-SiC^-t^O-CC^: Amer. Jour. S c i . 275, 143-162. Smith, J.G., 1975, K-Ar evidence for timing of metamorphism and plutonism i n the Coast Mountains near Ketchikan, Alaska: Geol. Assn. Can. C o r d i l l e r a n Sect., Prog, with Abs., p. 21. Snoke, A.W., 1975, A s t r u c t u r a l and geochronologic puzzle: Secret Creek Gorge area, northern Ruby Mountains, Nevada: Geol. Soc. Amer. Abs. with Prog. 7, no. 7, p. 78. Souther, J.G., 1972, Telegraph Creek map-area, B r i t i s h Columbia: Geol. Surv. Can. Paper 71-44. , 1976, Volcanism and tectonic environments i n the Canadian C o r d i l l e r a - a second look: ms. i n prep. Storre, B. and Nitsch, K., 1972, Die reaktron 2 z o i s i t + 1 C0 2 = 3 anorthit + 1 c a l c i t + H^O: Cont. Mineral. Petr. 35, 1-10. Sutherland Brown, A., 1963, Geology of the Cariboo River area, B r i t i s h Columbia: B.C. Dept. of Mines and Pet. Res. B u l l e t i n 47. Wanless, R.K., Stevens, R.D., Lachance, G.R., and Delabio, R.N., 1967, 1971, 1973, 1974, Age determinations and geologic studies: Geol. Surv. Can. Papers 67-2, 71-2, 73-2, 74-2. Wheeler, J.O., Aitken, J.D., Berry, M.J., Gabrielse, H., Hutchison, W.W., Jacoby, W.R., Monger, J.W.H., N i b l e t t , E.R., N o r r i s , D.K., P r i c e , R.A., and Stacey, R.A., The C o r d i l l e r a n s t r u c t u r a l province: Geol. Assn. Can. Sp. Paper 11, 1-82. Winkler, H.G.F., 1974, Petrogenesis of Metamorphic Rocks, 3rd ed., Springer-Verlag, New York. Woodsworth, G.J., 1976, Plutonic rocks of McConnell Creek and Aiken Lake map-areas: i n Rept. of Act. part A, Geol. Surv. Can. Paper 76-1A, 69-73. 

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