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UBC Theses and Dissertations

Structure and deformation across the Quesnellia-Omineca terrane boundary, Mt. Perseus area, east-central… Elsby, Darren C. 1985

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S T R U C T U R E AND DEFORMATION ACROSS THE Q U E S N E L L I A - O M I N E C A TERRANE BOUNDARY, M T . PERSEUS A R E A , E A S T - C E N T R A L B R I T I S H COLUMBIA By DARREN C . E L S B Y B . A . POMONA C O L L E G E , 1981 S U B M I T T E D IN P A R T I A L F U L F I L L M E N T OF R E Q U I R E M E N T S FOR THE DEGREE OF MASTER OF S C I E N C E l n THE F A C U L T Y OF GRADUATE S T U D I E S D e p a r t m e n t o f G e o l o g i c a l S c i e n c e s We a c c e p t t h i s t h e s i s as c o n f o r m i n g t e the r e q m l f e d s t a n d a r d A T H E S I S THE THE U N I V E R S I T Y OF B R I T I S H OCTOBER 1985 © D a r r e n C . E l s b y , COLUMBIA 1985 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 0? 6>c>-/&<S#<r} /?SS i i ABSTRACT Deta i led s t ructura l mapping near Mt. Perseus, B r i t i s h Columbia, provides an overview of the nature of deformation across a portion of the Quesnellia-Omineca terrance boundary. Rocks within the Omineca Bel t are comprised of the Hadrym'an to mid-Paleozoic Snowshoe Group. These rocks are s t r u c t u r a l l y over la in by and act as basement to accreted rocks of the Intermontane Be l t (Quesnel l ia ) : the Upper Paleozoic S l ide Mountain Group (Antler Formation), Upper T r i a s s i c Black P h y l l i t e (unnamed), and Jurass ic volcanic rocks of Takla Group equivalence. Within the Snowshoe Group, four phases of reg ional ly s i g n i f i c a n t deformation have been es tab l i shed . Both basement and cover have common phases of deformation wherein the f i r s t phase of deformation present with in the cover sequence is equivalent to the second phase with in the basement. In general , deformation with in the cover i s less well developed with respect to the basement. E a r l i e s t s t ruc tures , only observed wi th in the Snowshoe Group are east -verg ing root less i s o c l i n a l fo lds accompanied by a transposed f o l i a t i o n of a regional nature. Associated with t h i s event i s the in t rus ion of a large tabular g r a n i t i c body, l a t e r metamorphosed into the Mt. Perseus Gneiss. Second phase structures are easter ly verging i i i and comprise large recumbent nappe s t ructures . Third phase westerly verging fo lds dip moderately to the northeast. It i s these large scale structures which control the present regional map pattern and loca l conf igurat ion of the Omineca-Quesnellia boundary, which in t h i s study, i s manifest in the Mt. Perseus antiformal culminat ion. Small scale crenulat ions and easter l y verging buckle fo lds comprise the fourth deformational phase and do not appreciably a f f e c t e a r l i e r geometries. Second phase deformation marks the obduction of the easter ly converging Quesnel l ia accret ionary package onto the Omineca ter rane. This tectonic contact i s f lanked by narrow long i tud ina l d u c t i l e shear zones containing myloni tes , which in Snowshoe rocks are often associated with i s o l a t e d f a u l t bounded s l i v e r s of oceanic cover rock ( o p h i o l i t e ) . These tectonic s l i v e r s are thought to be re lated to geometry resu l t ing from the eastward subduction of oceanic Quesnel l ia rocks beneath the Omineca craton during the t h i r d deformational phase. The development of the l a t e crenulat ion cleavage i s l i k e l y a consequence of l a t e eastward thrust ing of ear ly Jurass ic volcanics during the l a t e r deformation stages of the underlying p h y l l i t e s . Mineral assemblages describe a Barrovian metamorphic sequence which ranges from the middle to upper greenschist fac ies in cover rocks to the lower amphibolite in the Snowshoe basement. The e a r l i e s t recorded metamorphism i s associated with phase 1 deformation but d e t a i l s regarding t h i s event remain ambiguous as most textures have been destroyed by successive metamorphism. Microscopic textures iv indicate that the peak of metamorphism i s synchronous with phase 2 deformation followed by a reduction to the middle greenschist fac ies during the t h i r d deformational phase. Both obduction and subduction processes and the i r associated deformation and metamorphism were most l i k e l y the resu l t of mid Mesozoic tectonics related to the Columbian Orogeny. TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i LIST OF ILLUSTRATIONS • • • • • • • -ACKNOWLEDGEMENTS xv I. INTRODUCTION 1 Physiography 3 Regional Geology 3 II. STRATIGRAPHY 9 Snowshoe Group 11 Antler Formation 27 Upper Triassic Formation 35 - Lower Unit 36 - Middle Unit 37 - Upper Unit 37 III. STRUCTURE 39 Phase 1 (Dj) 43 Phase 2 (D2) 43 Phase 3 (D3) 61 Phase 4 (0U) 85 Phase 5 (D5) 86 Phase 6 (D6) 94 v i IV. MICROTEXTURES 95 Snowshoe Group 97 - Meta-pelitic - Psammitic Rocks 97 - Non-pelitic Rock Types I l l Antler Formation 124 Upper Triassic Formation 145 - Lower Unit 150 - Middle Unit 153 - Upper Unit 156 General Conclusions 158 V. SUMMARY AND DISCUSSION 161 Structure: General Conclusions 161 Regional Implications and Tectonic Conclusions . . . . . . 167 Bibliography 173 v i i LIST OF TABLES PAGE TABLE 3-1 Character is t ic Deformational Fabrics associated with the proposed deformation sequence 42 TABLE 4-1 Relevant strat igraphy, metamorphic rock types, and typical mineral constituents within the Snowshoe Group 96 4-2 Relation of mineral growth to the proposed deformation scheme in Snowshoe metapelites . . . 1 1 0 4-3 Relevant strat igraphy, metamorphic rock types, and typical mineral constituents within the Antler Formation 125 4-4 Relation of mineral growth to the proposed deformation scheme in the Antler Formation 144 4-5 Relevant st rat igraphy, metamorphic rock types, and typical mineral constituents within Upper T r iass ic Formation 146 4-6 Relation of mineral growth to the proposed deformation scheme in the Upper T r iass ic Formation 157 v i i i LIST OF FIGURES Figure 1-1 Location map of the Mt. Perseus area 2 1- 2 Regional geology of the Cariboo Mountains, Brit ish Columbia 5 Figure 2-1 Schematic structural succession across the Mt. Perseus area 10 2- 2 Regional geology of the Cariboo Moutains showing the spatial relations of the Snowshoe and Kaza Groups 12 2-3 Garnet-muscovite biotite schist of the , Snowshoe Group 16 2-4 Complexly folded calcareous metapelite of the upper Snowshoe Group 19 2-5 Micaceous quartzite of the Snowshoe Group 21 2-6 Xenoliths contained within the Mt. Perseus Gneiss 26 2- 7 Coarse pegmatite containing xenoliths of garnet-biotite schist 28 Figure 3-1 Map of the Quesnel Lake region showing the present configuration of the Quesnel1ia-Omineca terrane boundary 40 3- 2a F x isocl inal folds in Snowshoe Group garnet-biotite schist 44 3-2b F ? isocl inal folds in Snowshoe Group micaceous quartzite 45 3-3 Lower hemisphere stereo projections of the present geometry of phase 1 deformation as deformed by phase 2 and phase 3 folding 46 3-4a F 2 isocl inal folds in the Mt. Perseus Gneiss 48 ix 3-4b F 2 isocl inal folds in the Mt. Perseus Gneiss within the overturned limb of the Perseus antiform 49 3-5 Map view of Perseus Gneiss within the Snowshoe Group and F 2 fold vergence at various structural levels around the Perseus Anti form 50 3-6 Schematic cross-section through the phase 3 Perseus Anti form 52 3-7a Flattened F 2 folds within the Perseus Gneiss 53 3-7b F 2 isocl inal fold within Snowshoe garnet-biotite schist 54 3-8 Mesoscopic F 3 fold refolding an F 2 isocline 55 3-9 Mesoscopic F 2 fold contained within the hinge region of the Perseus Antiform 56 3-10 F 2 isocl inal folds contained within the Perseus Gneiss 57 3-11 Large F 2 folds within shallowly dipping Snowshoe metapelite in the southeastern portion of the Perseus Antiform hinge zone 58 3-12 Steeply inclined F 2 isoclinal folds overprinted by shallowly dipping open F^ folds 59 3-13 Schematic i l lus trat ing the unfolding of the superposed phase 3 Perseus Anti form 60 3-14 Map and schematic cross-section through the Perseus Antiform showing the nature and orientation of phase 2 and phase 3 axial surfaces and axial traces 62 3-15 Phase 2 isocl inal fold within a chlorite schist of the Antler Formation 63 X 3-16 Lower hemisphere stereo projections of F 2 minor fold geometry 64 3-17 Schematic cross-section i l lus trat ing the changing style of F 3 folding across the Perseus Antiform 67 3-18a Mesoscopic F 3 folds in micaceous quartzite of the Snowshoe Group 68 3-18b Phase 3 mesoscopic folds contained within the upright limb of the Perseus Antiform 69 3-19 Photograph of the superposition of phase 3 folds onto phase 2 geometry 70 3-20 Phase 3 folds characteristic of the Perseus Anti form core zone 71 3-21 F 3 folds contained within calcareous metapelites adjacent to the Perseus Anti form hinge zone 72 3-22 Superposition of phase 3 onto phase 2 structures within the antiform hinge zone 73 3-23 Small scale phase 3 disharmonic folds contained within calcareous metapelite 74 3-24 Isoclinal F , minor fold refolded by tight F 3 folds 75 3-25 Isoclinal phase 2 minor fold refolded around a phase 3 minor fold 77 3-26 Phase 3 disharmonic folding adjacent to a small shear zone in garnet-biotite schist of the Snowshoe Group 78 3-27 Isoclinal phase 3 minor fold adjacent to a minor shear zone 79 3-28a Mesoscopic phase 3 antiform in Snowshoe micaceous quartzite immediately adjacent to the Quesnel1ia-Omineca boundary 80 3-28b Phase 3 antiform situated immediately adjacent to Figure 3-28a 81 xi 3-28c Mesoscopic phase 3 antiform immediately adjacent to Figure 3-28b 82 3-28d Mesoscopic phase 3 isocline adjacent to Figure 3-28c 83 3-29 Talus blocks containing interfolded . Antler amphibolite and Snowshoe garnet metapelite 86 3-30 Schematic cross-section of phase 3 folding throughout the contact zone between the Black Phyll ite and Antler Formations and the Snowshoe Group 87 3-31 Lower hemisphere stereo projection of phase 3 minor fold geometry 88 3-32 Phase 4 recumbent folds refolding steeply inclined phase 2 isoclines 89 3-33 Nearly recumbent phase 4 folds deforming phase 2 isocl inal folds 90 3-34 Phase 4 minor folds contained within Snowshoe garnet-biotite schist 91 3-35 Lower hemisphere stereo projection of phase 4 fold geometry 92 3- 36 Large kink-band structure within the Perseus Gneiss 93 Figure 4-1 Photomicrograph of a Snowshoe Group 99 quartz-muscovite schist 4- 2 Photomicrograph of a Snowshoe mecaceous quartzite 100 4-3 Photomicrograph of a Snowshoe Group garnet-muscovite-biotite schist 102 4-4 Photomicrograph of a phase 2 microfold in Snowshoe micaceous quartzite 103 4-5 Photomicrograph of a h e l i c i t i c phase 1 type garnet in garnet-muscovite-biotite schist 104 x i i 4-6 Schematic i l l u s t r a t i o n of the timing of garnet growth with respect to deformation episodes and sty les in the Snowshoe Group 105 4-7 Photomicrograph of small i d i o b l a s t i c phase 2 type garnets in garnet -biot i te -muscovite sch is t 106 4-8 Photomicrograph of a l tered hornblende in Snowshoe amphibolite 107 4-9 Photograph of a thin section of Snowshoe augen gneiss 112 4-10a Photomicrograph of a K- fe ldspar -quartz augen in Figure 4-9 113 4-10b Cross -n ico ls view of Figure 4-10a 114 4 - 1la Photomicrograph of the Perseus Gneiss 118 4 - l l b P a r t i a l l y c ross -n i co l s view of Figure 4 - l l a showing detai l of i d i o b l a s t i c garnet form 119 4-12a Photomicrograph of the Perseus Gneiss 121 4-12b Photograph of the thin section appearing in Figure 4-12a 122 4-13a Photomicrograph of basal Antler Formation a c t i n o l i t e - t r e m o l i t e - t a l c schist 127 4-13b Cross -n ico ls view of Figure 4-13a 128 4-14a Photomicrograph of a c t i n o l i t e -t r e m o l i t e - a n t i g o r i t e - t a l c schist of the lowermost Ant ler unit 130 4-14b Tremolite-twins within Figure 4-14a 131 4-15a Photomicrograph of a rotated b i o t i t e porphyroblast in Antler c h l o r i t e -b i o t i t e schist 138 4-15b Cross -n ico ls view of Figure 4-15a 139 4-16 Sketch of a thin section of an Ant ler c h l o r i t e - b i o t i t e sch is t 140 x i i i 4-17a Photomicrograph of rootless i n t r a f o l i a l microfolds within micaceous quartz i te of the Antler Formation 141 4-17b Cross -n ico ls view of Figure 4-17a 142 4-18a Photomicrograph of basal micaceous quartz i te of the Upper T r iass ic Assemblage 148 4-18b Photomicrograph of basal micaceous quartz i te of the Upper T r iass ic Assemblage 149 4-18c Photomicrograph of basal micaceous quartz i te of the Upper T r iass ic Assemblage 149 4-19 Photomicrograph of an Upper T r iass i c graphi t ic phy l loni te 151 4-20a Photomicrograph of a typ ica l graphi t ic p h y l l i t e of the Upper T r iass ic assemblage 152 4-20b Photomicrograph of a typ ica l g raphi t ic p h y l l i t e of the Upper T r iass ic assemblage 152 4-21a Photomicrograph of a deformed quartz f i l l e d hydraulic fracture within graphit ic p h y l l i t e 154 4 - 21b Cross -n ico ls view of Figure 4-21a 155 Figure 5-1 Schematic cross -sect ions across the Quesnellia-Omineca boundary during the time of i n i t i a l convergence (obduction) 170 5 - 2 Schematic cross -sect ions across the Quesnellia-Omineca boundary af ter i n i t i a l convergence and during a reversal i n subduction 172 x i v LIST OF PLATES PLATE I Geologic map of the Mt. Perseus area (4n-poek-&t-) < PLATE II Structural Geology of the Mt. Perseus area Hn-pock-et-)— ^ C(pet-'.*i G)lletb> <^ PLATE III Cross-section A-A' across the Mt. Perseus area (-i-n-poek-et4~ PLATE IV Cross-section B-B' across the Mt. Perseus area (in-poek-etr X V ACKNOWLEDGEMENTS This study was done under the supervision of Dr. J . V . Ross who orig inal ly suggested the Crooked Lake area for a study of this kind. Technical assistance was provided by G.E. Montgomery and B. Cranston. Able f ie ld assistance was provided by W.H.G Matthews III and P. Baier. The structural presentation has been improved through discussions with Dr. J . V . Ross, J . Montgomery and J . F i l l ipone . Field and laboratory expenses were covered by NSERC 67-2134 awarded to Dr. J . V . Ross. F ina l ly , I wish to acknowledge and gratefully thank my wife Elisabeth for all typing and moral support given throughout the course of this study. 1 INTRODUCTION The southwestern margin of the Cariboo Mountains, situated in east central Br i t i sh Columbia, is located within a tectonic assemblage consisting of rocks of both the Omineca Crystall ine Belt and Intermontane Zone, which in this study is represented by the Quesnellia Terrane. A location map (Fig. 1-1) outlines the extent of the thesis area which includes the tectonic boundary. The area is characterized by polyphase deformation involving Late Proterozoic (Hadrynian?) metasediments and gneisses of the Snowshoe Group, Upper- Paleozoic amphibolites of the Slide Mountain Group, and un-named Upper Triassic phyll ites and Lower Jurassic metavolcanics of probable Takla Group equivalence (Struik, 1984). This study provides a detailed structural and metamorphic analysis immediately adjacent to this tectonic zone. Deformation styles, their trends, and metamorphic assemblages are compared, and the relations between deformation and metamorphism are presented across this marginal boundary. The specific area of study, the Mt. Perseus area, comprises some 66 square miles within the southern Cariboo Mountains, approximately 28 miles southeast of Quesnel Lake, Brit ish Columbia. The eastern extent of the area is contained within the Wells Gray Provincial Park Figure 1-1. Location map of the Mt. Perseus area. Inset map shows relation to the major structural elements of the Canadian Cordi l lera . Modified from Wheeler and Gabrielse (1972). 3 boundary. Detai led geologic mapping on the scale of 1:15,000 was completed during the summer of 1982. Physiography Total r e l i e f wi th in the area is approximately 4,400 feet (1,341 metres). The present physiography i s the resu l t of both s t ructura l control and g l a c i a t i o n . The ridges are often al igned with the metamorphic f o l i a t i o n . Abundant g l a c i a l s t r i a t i o n s , tarn l a k e s , and cirques ind icate that much of the present topography resulted from north westerly d i rected ice f low. Regional Geology The Cariboo Mountains form the northwesternmost part of the Columbia Mountains and l i e in the central part of the Omineca Geant ic l ine ; the core zone of the eastern fo ld be l t of the Canadian C o r d i l l e r a (Campbell, 1970). The r e l a t i v e l y unmetamorphosed and l i t t l e deformed rocks of the northern Cariboo Mountains l i e between and along s t r i k e with the i n t e n s e l y deformed, moderate to h i g h l y metamorphosed Shuswap metamorphic complex to the southeast and the Wolverine complex to the northwest. Northwest of the Shuswap metamorphic complex, a general though somewhat e r r a t i c decrease i n metamorphic grade i s accompanied by s i m p l i f i c a t i o n of fo ld patterns (Campbell, 1970, 1977). 4 The Shuswap metamorphic complex, as original ly defined, forms the core of the Omineca Crystall ine Belt and represents the highest grade zone of a wide zone of metamorphic rocks in southwestern Brit ish Columbia. The complex includes probable Hudsonian crystal l ine basement together with Proterozoic - early Paleozoic miogeoclinal strata and Mesozoic eugeoclinal rocks as young as Late Triassic (Ross, 1968; Campbell, 1977). Borders of the complex were original ly defined by the s i l l imanite isograd (Fig. 1-2). The position of this isograd has since changed as the result of detailed mapping in surrounding areas (Montgomery, 1985; F i l l ipone , 1985). Rocks of s i l l imanite grade have been found throughout the Quesnel Lake area, a fact which necessitates a re-definition of the present Shuswap metamorphic complex boundary. In central Brit ish Columbia, the Omineca Geanticline consists mainly of late Proterozoic metasediments of the Kaza Group and Cariboo Group, including the Hadrynian Snowshoe Formation (Sutherland-Brown 1957, 1963), most recently updated to Group status (Struik, 1984). Protolith rock types in both groups are of shallow water marine origin and were most l ike ly deposited in a basin west of the existing cratonic platform in the late Proterozoic to early Paleozoic (Campbell and Tipper, 1970). Monger and Price (1979) infer that these Western-most e last ics within the Omineca Belt are part of a continental margin terrace wedge that prograded into a marginal basin in which there was intermittant volcanic act iv i ty . In the southern Cariboo Mountains, Kaza Group rocks are in 5 120W JB UPPER TRIASSIC -LOWER JURASSIC TAKLA GROUP UTr UPPER TRIASSIC BLACK PHYLLITES SMG MISSISSIPPIAN-PEflMIAN SLIDE MOUNTAIN GROUP CO PALEOZOIC CARIBOO GROUP SG HADRYNIAN S N O W S H O E GROUP KG HADRYNIAN K A Z A GROUP Figure 1-2. Regional geology of the Cariboo Mountains, Brit ish Columbia. Modified from Wheeler, R.B. Campbell, Reesor, and Mountjoy (1972). 6 conformable contact with the overlying Snowshoe Group (F ig . 1-2) (Sutherland-Brown 1957; Campbell 1970; Struik 1981, 1982, 1984). The nature of th i s contact i s discussed by Fletcher and Greenwood (1978) and Pigage (1978) in which they conclude that the Snowshoe Group i s in part co r re la t i ve with the Kaza Group. Recent c l a s s i f i c a t i o n s by Struik (1984, 1985) divide the Omineca C r y s t a l l i n e Belt into two d i f fe rent zones termed the Barkerv i l l e and Cariboo Terranes. Hadrynian to Paleozoic Barkerv i l l e rocks, bounded on the west by the Quesnell ia Terrane, underly the Cariboo Terrane and are comprised sole ly of the Snowshoe Group. The overly ing Hadrynian to Paleozoic Cariboo Terrane i s separated from Barkerv i l l e rocks by the Pleasant Valley Thrust within the McBride map area, and by an unnamed southwest dipping duct i le f a u l t surface (Campbell, 1970; Fletcher and Greenwood, 1978; P igage , 1978) w i t h i n the Wel ls Gray a r e a . Strat igraphies within the Cariboo Terrane include the Jurassic Hobson Pluton and the Late Proterozoic Cariboo and Kaza Groups. The eastern boundary of t h i s terrane has not yet been f i rmly establ ished. Snowshoe rocks enclose several d i s t i n c t bodies of quartzo-fe ldspathic and augen gneiss. Okulitch (personal communication, 1983) interprets some of these rocks as orthogneiss of possible Devonian age. Campbell (1971), suggests that gneissic outcrops south of Quesnel Lake contain both orthogneiss and paragneiss components. Recent work by Montgomery (1985) and F i l l i p o n e (1985) in fers that these gneisses represent highly deformed mid-Paleozoic g ran i t i c bodies. 7 The Slide Mountain Group comprises rocks at least in part of Mississippian age and overlies with regional discordance older rocks in the Cariboo Mountains (Snowshoe Formation and Cariboo Group) (Sutherland-Brown, 1957). North of Quesnel Lake, the Guyet Formation, a basal conglomerate of the Slide Mountain Group, rests unconformably on the Cariboo Group. South of Quesnel Lake, including the study area, Antler Formation equivalents of the Slide Mountain Group, discordantly overlie Snowshoe rocks (Campbell, 1971; Campbell, 1973; Montgomery, 1978; Rees, 1981). Within the Intermontane B e l t , Mesozoic metasediments and metavolcanics are the oldest exposed rocks and are contained within the north-south aligned Quesnel Trough. These rocks consist of unnamed Upper Triassic phyllites and Lower Jurassic Takla Group volcaniclastics of basaltic to andesitic composition (Campbell, 1971; Bailey, 1976; Morton, 1976) which structurally overlie the Slide Mountain Group. Takla rocks conformably overlie the Upper Triassic phyll ites and in several loca l i t i e s the depositional record appears continuous between the two rock types (Campbell, 1971). Undeformed granitic rock, intrusive into the Takla Group, has been radiometrically dated at 100 m.y. (Campbell and Tipper, 1970) and is believed to postdate al l structures in the Quesnel and Omineca Geanticline, with the exception of Tertiary faults . Tertiary elastics and olivine basalts unconformably overlie Takla rocks and cover large areas within the region. The earl iest recorded deformation within Omineca rocks occurred during the Hadrynian East Kootenay orogeny, followed by additional 8 deformation in the late Devonian to Miss iss ippian Caribooan orogeny. The Omineca Geant ic l ine was the s i t e of renewed u p l i f t in the T r iass ic Tahltanian Orogeny and plutonism and metamorphism in the Columbian Orogeny (Wheeler, 1970; Campbell, 1971). This resulted in a regional map pattern characterized by structural culminations frequently associated with high grade metamorphism. Most structures are arched over regional northwest trending a n t i c l i n o r i a (Campbell, 1970) (F ig . 1 -2 ) . Structural s ty le ranges from early large scale duct i le shear zones and associated duct i le "flow" fo ld ing to la te stage buckle fo ld ing and b r i t t l e f rac tur ing . Within the Crooked Lake area of the southwest Cariboo Mountains, the present map pattern conf igurat ion is dominated by large scale southwest verging f o l d s , termed the Mt. Perseus Antiform, Eureka Synform, and Boss Mountain Antiform (F ig . 1 -2) . 9 STRATIGRAPHY The stratigraphic framework and regional correlation of units within the Mt. Perseus area are based on early studies by Holland (1954) and Sutherland-Brown (1957). Recent work within the Quesnel Lake area was performed by Campbell (1971), Campbell, (1973), Campbell (1978), Struik (1981, 82 , 83 , 84), and Rees (1982, 1983). A detailed schematic structural succession for the Mt. Perseus area is presented in Figure 2-1. The study area contains Late Proterozoic (Hadrynian?) meta-sediments and gneisses of the Snowshoe Group which are unconformably overlain by Mississippian to Permian amphibolites and metavolcanics of the Antler Formation. These rocks are in turn unconformably overlain by unnamed Upper Triass ic graphitic phyl l i tes , schists and Jurassic Takla volcaniclastics (outside of the f ie ld area). Within the study area all contacts are tectonic and metamorphic grade increases from lower greenschist in the southwest to at least lower amphibolite in the northeast part of the study area. Snowshoe rocks have been interpreted by Campbell (1971) to be miogeoclinal derivatives of the Omineca Geanticline. Textural evidence and facies relations within the Snowshoe metapelites in the Quesnel Lake and McBride map areas indicate a northern cratonic source for the original sediments (Campbell et. a l , 1973). 10 co UJ UPPER UNIT BLACK PHYLLITE.CHL-SCHIST. REEN3T0NE LENSES MIDDLE UNIT BLACK PHYLLITE:CALCAREOUS GRAPHITIC SCHIST LOWER UNIT BLACK PHYLLITE:GRAPHITIC SERICITE SCHIST. 'ARGILLITE MICACEOUS QUARTZITE ORNBLENDE-CHL SCHIST CHL-SCHIST.MICACEOUS QUARTZITE. REENSTONE LENSES MICACEOUS QUARTZITE CHL-SCHIST. PILLOW BASALT ACTINOLITE-HORNBLENDE EPIDOTE SCHIST ACTINOLITE-TREMOLfTE TALC SCHIST AUGEN GNEISS MYLONITE CALCAREOUS GARNET-BIOTITE SCHIST, ANDY MARBLE LAYERS. LENSES GARNET-BIOTITE SCHIST. INTERLAYERED MICACEOUS QUARTZITE QUARTZO-FELDSPATHIC GNEISS, MINOR PEGMATITE GARNET BIOTITE SCHIST. MICACEOUS QUARTZITE CALCAREOUS SCHIST. PEGMATITE Figure 2 - 1 . Schematic structural succession across the Mt. Perseus area. 11 Monger and Price (1979) in their discussion of the geodynamic evolution of the Canadian Cordil lera infer that western-most elastics within the Omineca Belt (which includes the Snowshoe Group) are part of a regional northeasterly tapering sedimentary wedge that overlaps precambrian structures of the cratonic basement. This continental margin terrace wedge (miogeoclinal wedge) is believed to have prograded into an intermittantly tectonically active back-arc type marginal basin based upon the stratigraphic incorporation of coarse feldspathic wackes and volcanics in a dominantly shale facies. The overlying Antler Formation of the Slide Mountain Group and Upper Triass ic assemblages show evidence of eugeoclinal a f f in i ty; their structural juxtaposition believed to be largely a result of regional wrench fault tectonics (Campbell, 1971; Monger and Price, 1979; Monger et a l . , 1982). All rock units within the study area have been multiply deformed and therefore, owing to the complex nature of the deformation, any original depositional contacts have in large part been completely transposed. Snowshoe Group The Snowshoe Group within the map area contains a sequence of pe l i t i c metasediments and quartzo-feldspathic gneisses of Late Proterozoic (Hadrynian?) age. Upper most units of the Snowshoe Group within the Quesnel Lake area are tectonically overlain by Antler Formation amphibolites. Lowermost units of the Snowshoe are in fault 12 120W JR U P P E R T R I A S S I C - L O W E R J U R A S S I C T A K L A G R O U P UTr U P P E R T R I A S S I C S L A C K P H Y L L I T E S IMQJ M I S S I S S I P P 1 A N - P E R M I A N S L I O E M O U N T A I N G R O U P CQ P A L E O Z O I C C A R I B O O G R O U P SG H A O R Y N I A N S N O W S H O E G R O U P KG H A O R Y N I A N KAZ* G R O U P Figure 2-2 Regional geology of the Cariboo Mountains showing the spatial relations between Snowshoe and Kaza Groups. 13 contact with the Kaza Group (Fletcher and Greenwood, 1978; Pigage 1978) (Fig. 2-2). The total present tectonic thickness of the Snowshoe Formation is believed to be in excess of 27,000 metres as original stratigraphic thickness has not yet been estimated. Exact ages and correlations of Snowshoe rocks with regional units are controversial and s t i l l under current investigation. Mapping performed by Struik (1982, 1983), northeast of the Quesnel Lake c lass i f ies rocks resembling the Snowshoe Group as part of the Cariboo Group which within the study area may be in part correlative with these strata but insuff icient mapping exists between the respective f ie ld areas to provide a conclusive correlation. Recent work by Struik (1984, 1985) reclassif ies and divides the Omineca Crystall ine Belt into two dist inct zones separated by a prominent fault zone. These zones are termed the Barkerville and Cariboo Terranes. Rocks of the Barkerville Terrane contain Hadrynian and Paleozoic metasediments and gneisses of the Snowshoe Group. The Cariboo Terrane is comprised of Hadrynian and Paleozoic rocks of the Cariboo and Kaza Groups and the Jurassic Hobson Pluton. Furthur correlations have been made between the Snowshoe Group and rocks of the Horsethief Creek and Miette Group based upon similar structural position and stratigraphy. Price and Douglas (1972) believe that Kaza Group rocks (including the Snowshoe Group) are in part a northern extension of the Horsethief Creek Group and a western continuation of the lower and middle Miette Group (of the Rocky Mountain fold belt ) . Work performed by Fletcher and Greenwood (1978) and Pigage (1978) 14 provides evidence that the Snowshoe Group may be a higher metamorphic grade equivalent of the Kaza Group which may imply that the Snowshoe is a westerly facies of the Kaza. The 5000 metre present tectonic thickness of exposed Snowshoe Formation within the study area is largely composed of garnetiferous quartz-mica schist, micaceous quartzites, calcareous schist, augen gneiss, and quartzo-feldspathic gneiss. A well exposed layer of this gneiss provides a continuous marker horizon throughout the f ie ld area. Present map pattern geometry indicates that a l l units have been folded into an overturned doubly-plunging, southwest verging antiform (Plates II and III) . The upper 2400 metres of Snowshoe rock'consist of carbonate horizons which contain dist inct layers of calcareous schist and numerous smaller intercalations of marble. Calcareous zones are repeated on both limbs of the antiform at similar structural levels. The appearance of garnet within Snowshoe rocks is controlled by the garnet isograd and in large part by compositional variation within the above l i thologies . The order and stratigraphic position of the following descriptions may be found in figure 2-1. Quartz-Mica Schist Light tan to dark brown weathering schists composed of quartz, plagioclase, K-feldspar, b iot i te , muscovite, hornblende, ca lc i te , and garnet (almandine) comprise approximately 60 percent of the exposed Snowshoe Formation. The development of garnet in these rocks is 15 strongly dependent upon composition and are therefore often confined to local layers. As a general rule , porphyroblast sizes increase in a northeasterly direction from the Snowshoe-Antler contact. Lithologies within this group include garnet-biotite schist, micaceous quartzite, calcareous mica schists, sandy marble, and amphibole bearing schists. All units contain a well developed penetrative fo l iat ion defined by mica layers and discontinuous quartz intercalations. Much evidence indicates that metamorphic crystal l izat ion outlasted deformation: randomly oriented biotite porphyroblasts, strongly annealed quartz, and h e l i c i t i c garnet. Present contacts between al l units are believed to be the result of multiple transposition. Garnet-Biotite Schist Schist composed dominantly of b iot i te , muscovite, garnet, and quartz occurs as an ubiquitous rock type throughout the study area. This rock type consists of alternating thin laminae (1 to 5 mm) of b io t i t e , muscovite, and quartz with garnet porphyroblasts ranging in diameter from 1 mm to 4 cm (Fig. 2-3). Adjacent to the Snowshoe-Antler contact schists become mylonitic and contain numerous randomly oriented biotite porphyrobl asts; evidence of post-deformational metamorphic overgrowth. Similar f ie ld textures are observed within the core region of the Perseus antiform such that schists of similar composition appear mylonitic and are often restricted to small tabular Figure 2-3. Photograph of a garnet-muscovite-biotite schist of the Snowshoe Group. Large early garnets, flanked by pressure shadows, are contained and have grown within the regional fo l ia t ion . 17 zones. Quartz grains within these rocks are entirely polygonized and thus one can infer that zones of concentrated shear and flattening strain exist within the Snowshoe Group. The nature and significance of this geometry wil l be discussed in later sections. Two generations of garnet are present within these rocks. An early phase of garnet displays h e l i c i t i c textures and rotation of up to 90 degrees. These garnets are contained and have grown within the regional fo l ia t ion . A later garnet phase confined to local layers is typical ly smaller than the large h e l i c i t i c garnets, contains random inclusions, and has overgrown the primary crysta l l izat ion fo l ia t ion . Rocks containing these two garnet growths crop out in random layers throughout the Snowshoe and l i t t l e can be said concerning their strati graphic/structural continuity. Calcareous Schist The upper 2400 metres of Snowshoe Group schists within the study area are composed dominantly of calcareous metapelite which contains dist inct layers of sandy marble and calcareous quartz-mica schist. These schists contain numerous minor lenses of amphibolite, calcareous gneiss and thin discontinuous quartz intercalations. Quartz-mica-schist containing calcite comprises the dominant rock type within this calcareous zone (Fig. 2-4). Mineral phases include quartz, biot i te , ca lc i te , muscovite and localized K-feldspar, 18 plagioclase and garnet. Contained within these schists are layers of sandy marble composed almost exclusively of quartz, ca lc i te , and muscovite. Layer thicknesses range from 10 to 120 metres. The sandy marble is l ight tan in color, high in outcrop re l i e f with respect to surrounding schists and is repeated in several loca l i t i es by isoclinal folding (Plates I, II) . Contacts with the enveloping schists are sharp and everywhere concordant. In most l o c a l i t i e s , lenses of amphibolite crop out adjacent to sandy marble layers. These rocks, composed of horn-blende, quartz, ca lc i te , and epidote attain a maximum thickness of 2 metres. Minor layers of calcareous gneiss, composed of alternating layers of hornblende, biot i te , K-feldspar, and quartz crop out both adjacent to amphibolite lenses and more randomly throughout the carbonate schists. Internally, all calcareous schists and gneisses are thinly laminated with individual laminae ranging from 5 to 60 mm in thickness. Calcareous schist proximal to the Antler-Snowshoe boundary shows evidence of strong mylonitization and annealing. Deformation textures become gradationally less mylonitic approximately 1.5 kilometres away from this boundary and throughout the remainder of this calcareous zone, carbonate units appear strongly folded with mylonites restricted to narrow zones which are subparallel to axial plane orientations of various fold geometries. Figure 2-4. Photograph of complexly folded calcareous metapelite of the upper Snowshoe. Compositional layers are outlined by numerous quartz and calcite layers. 20 Micaceous Quartzites L i tho log ies containing 70-90 percent quartz with minor phases of b i o t i t e , muscovite, p lag ioc lase , and garnet crop out in layers throughout a l l leve ls of the Snowshoe Group (F ig . 2 -5 ) . The frequency and thickness of the layers increase up-sect ion, with respect to the North-east study area boundary, where sequences in excess of 35 metres are exposed near the structural top of the Snowshoe. Layers below th is st ructural level range in thickness from 0.5 to 20 metres. Deformation textures are often poorly preserved in the quartz i tes due to strong polygonization within quartz grains . Several samples of quartz i te mylonite from the antiform core zone contain quartz ribbon grains showing elongation ra t ios in excess of 5 0 : 1 . I t i s uncertain as to the nature and degree of t ransposi t ion that has taken place in these un i ts . Amphibole-Bearing Schist Schists containing porphyrobl asts of amphibole, garnet, and b i o t i t e crop out as lenses within the upper 100 metres of the Snowshoe Group. Mineral phases within the groundmass incude quartz, c a l c i t e , b i o t i t e , epidote, and muscovite. Textures are generally coarse with hornblende crudely aligned para l le l with northwest-southeast trending fo ld axes. Post-kinematic b i o t i t e prophyroblasts are randomly oriented and often transverse to the f o l i a t i o n . 21 Figure 2-5. Photograph of Snowshoe micaceous quartzite. Compositional layers are parallel to the regional metamorphic fo l ia t ion . Light colored layers are composed of approximately 80% recrystal l ized quartz. 22 Alkal i c Feldspar Augen Gneiss Mylonite Light grey, rodded augen gneiss containing muscovite, quartz, and alkal i feldspar represents the upper-most unit within the exposed Snowshoe Group. The gneiss is in sharp tectonic contact and regionally discordant with lowermost amphibolites of the overlying Antler Formation. Structural thicknesses within this medium grained gneiss range from 50 to 180 metres in a southeasterly trend toward the hinge zone of the Eureka Synform. Texturally, the rock type varies irregularly in outcrop from a layered augen gneiss to schistose mylonite. Within the mylonitic layers, deformed alkal i feldspar and quartz augen appear flattened and elongate within the fo l ia t ion , thereby giving the gneiss a homogeneous leucocratic outcrop appearance. This textural var iabi l i ty is most l ike ly due to internal anisotropy and differential flattening active within the gneiss during deformation. Quartz grains within the matrix have been polygonized, which indicates that metamorphic recrystal l izat ion (annealing) outlasted deformation in this region. A strong pervasive northwest-southeast trending mineral l ineation contained within the fol iat ion has produced a rodded texture throughout the study area. Anhedral microcline, K-feldspar, and quartz augen, r e l i c t deformed porphyroclasts, range in diameter from 3 mm at the upper contact with Antler amphibolites to 15 mm at the lower contact zone with calcareous 23 Snowshoe metapel i tes . This lower contact can best be described as a ' zone as the contact i s d i f f u s e and often appears as a homogeneous mixture of gneiss and metapel i te . The nature and s ign i f i cance of t h i s gradational contact along with p r o t o l i t h considerat ions w i l l be discussed in l a t e r sect ions . Quartzo-Feldspathic Gneiss Within the exposed Snowshoe Group, quar tzo - fe ldspath ic gneiss forms a continuous marker horizon which out l ines the doubly plunging nature of the Perseus a n t i f o r m ( P l a t e I ) . Gneiss conta ined w i t h i n the overturned limb of the antiform crops out approximately 2200 metres below the upper contact of the Snowshoe and maintains a s t ructura l thickness of about 500 metres. Thickness with in the upright limb remains constant at approximately 200 metres. Textures with in t h i s u n i t vary from f ine -g ra ined leucocrat ic fe ldspath ic gneiss confined p r i n c i p a l l y wi th in the limbs of the ant i form, to a coarse grained m a f i c - r i c h gneiss exposed pr imar i ly w i th in the antiform core zone. Compositional layers composed of hornblende, epidote, b i o t i t e , muscovite, mic roc l ine , quartz , sphene, and loca l garnet are oriented subparal le l to the e a r l i e s t recognized metamorphic f o l i a t i o n . Indiv idual layer thickness range from 1 to 10 mm. Contacts between the gneiss and enclosing metapelite schists often 24 appear gradational over several metres and highly sheared but remain concordant with respect to the gneissic layer-parallel fo l ia t ion . Several loca l i t i e s adjacent to these contacts display annealed textures characterized by randomly oriented hornblende and biotite porphyro-blasts. In most other areas, both hornblende and biotite show preferred orientation in the form of a strong pervasive northwest trending mineral l ineat ion. Samples taken from within the antiform core zone contain quartz grains which have been completely polygonized. This evidence of annealing and recrystal l izat ion is most strongly developed adjacent to the schist-gneiss contact within the antiform core zone and in rocks proximal to the Anti er-Snowshoe contact. Throughout the rest of the f ie ld area, recrystal 1ized textures and mylonites are confined to zones subparallel to axial plane orientations of various fold geometries. In the region immediately southeast of the Mount Perseus summit, both gneiss and metapelitic schist are complexely interfolded. This area is characterized by inters l ic ing of both rock types in which minerals appear elongate and flattened within a mylonitic fo l ia t ion . This zone of shearing and flattening provides evidence for and in part documents the existence of early deformation geometries related to macroscopic folding. This and other factors concerning deformation will be discussed in the following chapter. Several loca l i t i es within the gneiss contain abundant xenoliths of melanocratic, fine grained amphibolite (Fig. 2-6). Xenolith diameters 25 range from 5 to 30 cm and do not def lect the metamorphic f o l i a t i o n present within the host rock though most display a moderately developed cleavage oriented para l le l to the gneissic f o l i a t i o n . Granitoid Pegmatite I r regular , scattered bodies of mi ldly to intensely deformed coarse and f ine grained granitoid pegmatite intrude a l l rock types within the Snowshoe Group exposed in the study area. Instrusives display both concordant and disconcordant contact r e l a t i o n s . Concordant pegmatites crop out mainly in deep structural leve ls within the core zone of the Perseus Antiform. Pegmatites with discordant contacts intrude shallower structural leve ls and within the limbs of the antiform. Within the deepest exposed leve ls of the antiform, pegmatites intrude along the metamorphic f o l i a t i o n present within sch is ts and gneisses. These int rus ives are generally f ine-grained and are often complexly folded along with the host rock. Subhedral microc l ine , K - fe ldspar , quartz , epidote, muscovite, and b i o t i t e comprise the mineralogy and are often aligned para l le l to the f o l i a t i o n within the host rock. In several l o c a l i t i e s , gneissic host rock and f ine grained pegmatite display a migmatite texture in which gneiss and int rus ive appear homogeneously mixed. i Pegmatites which crop out within shallower structural leve ls of the core zone and within the limb regions of the antiform structure are Figure 2-6. Photographs of the Mt. Perseus gneiss. Xenoliths of altered amphibolite are contained within the fo l ia t ion . 27 mostly coarse grained and discordant with respect to f o l i a t i o n s present in the host rock, are seldom involved in fo ld ing and display a very crudely defined f o l i a t i o n oriented subparallel to cleavages produced by l a t e stage fo ld geometries within host rocks. Mineral phases include 4-5 cm subhedral phenocrysts of b i o t i t e and muscovite and anhedral masses of K - fe ldspar , quartz, epidote, and garnet.. Abundant xenoliths of host rocks are v i s i b l e throughout the int rus ives (F ig . 2 -7 ) . From the above desc r ip t ion , one can in fer that int rus ives deep within the antiform core zone behaved p l a s t i c a l l y and were involved in fo ld ing along with the host rocks. Outcrops of migmatite may suggest that the intrus ives were in part generated from i n - s i t u par t ia l melting of host rock during peak metamorphism or that int rus ion from another source caused par t ia l melting within host rocks. As deformation continued, int rus ives penetrated shallower structural levels (antiform l imbs) , became increasingly contaminated by Snowshoe rocks, and suffered less intense deformation than s t ruc tu ra l l y deeper in t rus ives . Ant ler Formation (S l ide Mountain Group) Sutherland-Brown (1957, 1963) examined the Sl ide Mountain Group and divided i t into two formations: the basal Guyet Formation and the overly ing Antler Formation. The type section is loosely designated at S l ide Mountain (Sutherland-Brown, 1963), located in the northern Cariboo Mountains approximately 125 miles north of the Mt. Perseus area. S l ide Mountain l i t h o l o g i e s dominantly comprise basic volcanic 28 Figure 2-7. Photographs of an outcrop of coarse pegmatite containing xenoliths of highly foliated garnet-biotite schist. 29 rocks, ribbon chert, a r g i l l i t e , pillow basalts, aphanitic greenstone, limestone, fine grained tuffs, and coarse volcanic breccia (Sutherland-Brown, 1957). The Guyet Formation contains scant fossil evidence which has been dated as Early Mississippian and currently represents the lower age l imi t for the Slide Mountain Group (Sutherland-Brown, 1957). The base of the Antler Formation conformably overlies the Greenberry Limestone member of the Guyet Formation. Fossil evidence has not been found in the Antler, hence i ts upper age l imit can only be inferred from the presence of Upper Triassic (?) graphitic phyllites which conformably overlie the Antler Formation. Thus the age of the Antler is bracketed between the Early Mississippian and the Late Triassic (Campbell et. a l . , 1973). More recently, Campbell (1971), Montgomery (1978), and Rees (1981, 1983) have examined the Antler Formation at several locations along its trend. The Antler Formation in these areas often contains extensive outcrops of meta-ultramafic and mafic assemblages. Within the Dunford Lake area, Montgomery (1978) describes the Black Riders mafic-ultramafic complex as one of a series of alpine-type ultramafics which crop out adjacent to the Omineca - Quesnellia tectonic boundary. The complex tectonically truncates underlying Snowshoe rocks and is conformably overlain by greenstones, believed to be of Antler af f in i ty . The assemblage is composed of dunite, 30 peridoti te , gabbro and amphibole-bearing schists, a l l containing preserved cumulate structures. Similar outcrops have been found in Upper Triassic metavolcanics in the Spanish Lakes area (Ross, personal communication, 1984). Rees (1981, 1983) describes Antler l ithologies north of Quesnel Lake as being composed of banded greenstone and talc-antigorite bearing meta-ultramafic schist. Within the Mt. Perseus area, several trace outcrops and abundant talus of highly altered serpentinized dunites and talc-act inol i te schists were found adjacent to the Antler-Snowshoe contact. Mafic-ultramafic rocks are thus recorded as being restricted mainly to the Antler-Snowshoe contact together with minor outcrops s t r u c t u r a l l y contained within the over ly ing Upper T r i a s s i c metavolcanics. From the above information, i t is evident that ultramafic sl ices appear in both Antler rocks as well as in Upper Triass ic assemblages; and thus, l i t t l e can be said concerning the exact age of these rocks. It is possible that these slices are only associated with the Antler Formation but i t seems more plausible that they may represent successive oceanic floor rocks of both the Antler and Upper Triassic assemblages. Monger and Price (1979) attribute the regional presence of Miss iss ippian-Permian cherts , a r g i l l i t e s , and maf ic-ul tramaf ic volcanics to profound subsidence of miogeoclinal terrane in the Devonian, during which subsidence was followed by deposition of deep water sediments and mafic volcanics. This subsidence is inferred to 31 have taken place in a back-arc or marginal basin situated behind a volcanic arc in southern B.C. Later, in the Mesozoic, this assemblage is interpreted as having been thrust eastwards onto cratonic sediments. The Antler Formation within the Mt. Perseus area consists of discontinuous layered assemblages of coarse amphibole-talc-chlorite schist, chlorite-hornblende schist, chlorite schist, seric i te schist, and discontinuous lensoid outcrops of aphanitic pillowed greenstone and minor outcrops of grey micaceous quartzites. Mississippian-Permian (?) Ant ler rocks are of upper greenschist metamorphic grade and tectonically overlie late Proterozoic (Hadrynian?) Snowshoe Group units. Graphitic phyllites and arg i l l i t e s of an unnamed Upper Triass ic formation conformably and probably tectonically overlie Antler rocks. Contacts between the three formations (on outcrop scale) are often complexly interfolded and generally not well exposed. The tectonic thickness of the Antler Formation fluctuates along strike and compositional layering is internally discontinuous. Thicknesses of the Antler vary from a maximum of 800 metres to a minimum of approximately 150 metres. Discontinuities may be indicative of the paleo-environment of deposition or may be a function of post-depositional tectonics of both. Within the Antler, internal layering represents transposed original compositional layering. All units contain a layer parallel fo l iat ion that becomes progressively mylonitic and annealed adjacent to the contact with the Snowshoe Group. In general, metamorphism and porphyroblast size increase toward the Antler-Snowshoe contact. 32 Act i r to l i te -T remol i te -Ta lc Schist The lowermost units of the Antler Formation within the study area are composed of coarse grained magnesian schists containing assemblages of ac t ino l i te - t remol i t e , t a l c , c h l o r i t e , a n t i g o r i t e , and a l b i t i c p lag ioc lase . The s t ruc tura l l y lowest layers , in contact with augen gneiss mylonite of the Snowshoe Group, display a garbenschiefer texture in which large c rys ta ls of pale green act inol i te - t remol i t e are randomly oriented in a f ine grained annealed groundmass composed dominantly of t a l c , c h l o r i t e , and c a l c i t e . A poorly developed metamorphic f o l i a t i o n becomes recognizable approximately 75 metres s t ruc tu ra l l y above the lower contact. Adjacent to the Snowshoe Group-Antler Formation contact , small pods and lenses of extremely coarse grained a n t i g o r i t e -bearing a c t i n o l i t e - t r e m o l i t e - t a l c sch is t crop out randomly within the lowermost layer . Amphibole phases often occur as massive radiat ing splays with indiv idual c rys ta ls a t ta in ing lengths of up to 24 cm. The above stratigraphy i s often discontinuous along s t r ike and range in thickness from 50 to 100 metres in a southeasterly trend toward the hinge zone of the Eureka Synform. Amphibole-Chlorite Schist Dark and l i g h t green, medium to f ine grained f o l i a t e d a c t i n o l i t e -hornblende c h l o r i t e sch is t s t ruc tu ra l l y over l ies the lowermost 33 magnesian schists "garbenschiefer". This unit is recognized by a well developed l ight colored amphibole l ineation contained within the metamorphic fo l iat ion; the preferred orientation of which often gives the rock a strong rodded appearance. Texturally, porphyrobl asts of actinolite-hornblende are contained within a foliated groundmass of annealed plagioclase, epidote, ca lc i te , and quartz. Thicknesses range from 10 to 180 metres along strike. Chlorite - Hornblende Schist Dark green chlorite-hornblende schist structurally (and probably stratigraphically) overlies basal amphibole schists of the Antler Formation. Within this fine grained unit, the well developed mylonitic fo l ia t ion is parallel to compositional layering. Tectonic thicknesses range from 20 metres to 40 metres in a southeasterly trend along s tr ike . Contact relations with underlying mafic amphibolites are sharp and well defined whereas contacts with overlying metavolcanic rocks are gradational over several metres and generally not well exposed. Hornblende and chlorite porphyroblasts are elongate and flattened within the mylonitic fol iat ion and show preferred orientation in a northwest-southeastern trend. Muscovite-Sericite Schist, Micaceous Quartzite, Aphanitic Greenstone,  Pillowed Greenstone The remaining local Antler strata are composed of discontinuous layers of muscovite schist and lenses of micaceous quartzite, aphanitic 34 greenstone, and meta-pillow basalts. Within the f ie ld area, no lateral continuity was found to exist in the above strata. Coarse grained schist containing approximately 70% muscovite-seric i te occurs throughout this succession in layers up to 4 metres thick. Minor mineral phases include quartz, . chlorite and biot i te . This rock type has a well developed micaceous fol iat ion parallel to compositional layering and is typical ly somewhat mottled in appearance. Grey micaceous quartzites form discontinuous lensoid units within Antler stratigraphy. Compositionally, these rocks contain 80% quartz with more minor phases of muscovite and chlori te . Quartz appears as flattened stringers and ovoid porphyroclasts within the fo l ia t ion . Aphanitic greenstone displaying r e l i c t primary igneous pillow structure crops out adjacent to the contact between Upper Triassic graphitic phyllites and Antler rocks. Greenstone units are extemely fine grained and show evidence of intermittent compositional layering with a layer parallel fol iat ion outlined predominantly by chlorite . Pillow structures are ovoid and flattened within the fol iat ion and have semi-major el l ipse axes ranging from 6 to 15 inches in length. Outcrops of poorly exposed a r g i l l i t e , chlorite schist, and coarse amphibolite were found throughout the area. In general, Antler rocks become more fine-grained, micaceous, and less amphibole rich near to the contact with Upper Triassic assemblages and coarse annealed, and amphibole-rich at the Snowshoe-Antler contact. 35 Upper Triass ic Graphitic Phyll ites and Arg i l ! i t e s Within the Crooked Lake area, unnamed black graphitic phyl l i tes , pel i t ic shists and argil 1 ites unconformably overlie the Antler Formation and are conformably overlain by Upper Triassic - Lower Jurassic metavolcanics and volcaniclastics (Campbell, 1971). North of Quesnel Lake, similar graphitic assemblages are overlain by early Jurassic marine sediments and volcaniclastics (Tipper, I960; Rees, 1981, 1983). South of the Crooked Lake area, Campbell and Tipper (1970) mapped black phy l l i t i c rocks they thought to be stratigraphic-a l ly equivalent to the Slide Mountain Group. Scant fossil evidence found in these equivalent marine strata was dated to the early Jurassic . Therefore, the Upper Triassic age of this formation remains very tentative at this time. Within the Mount Perseus area, black graphite-bearing phyl l i t i c assemblages, graphitic schist, and a r g i l l i t e structurally overlie the Antler Formation. The contact between graphitic phyll i tes and overlying Takla Group volcaniclastics is exposed to the southwest, outside of the map area, and will therefore not be discussed. Structural thicknesses are estimated to be in excess of 1500 metres. Black graphitic phyll i te and schist comprise approximately 80 percent of the outcrop within this formation with the remaining strata composed mostly of one metre layers of chlorite schist, micaceous quartzite, and horizons of black calcareous schist. The Upper Triassic unit is divided into three sub-units based upon differences in rock type. The 36 sub-units are termed Lower, Middle and Upper based upon the order of structural position. Contacts between basal rocks of the Upper Triassic Lower Unit and the Antler Formation are poorly exposed. In areas where folding has not obscured the contacts, they appear sharp and local ly conformable. Lower Unit The lower unit represents a structurally complex zone which separates the Antler from Upper Triassic strata. Layers of well fol iated basal micaceous quartzite, dark grey to black a r g i l l i t e , chlorite schist, and quartz seric i te schist comprise the rock types within a 60 to 220 metre wide zone. Grey to tan micaceous quartzite mylonite forms the lowest exposed unit within the Upper Triassic assemblage, which, when exposed is in conformable tectonic contact with hornblende-chlorite schist of the underlying Antler Formation. Tectonic thickness of this unit varies from 20 to 150 metres. Muscovite outlines the earliest fol iat ion which i s parallel to compositional layering/bedding. This unit is c lass i f ied as a mylonite due to the presence of highly elongate, flattened quartz porphyroclasts contained within the fo l ia t ion . Remaining lower unit strata consist of complexly folded discontinuous layers and lenses of grey-black a r g i l l i t e , chlorite schist, and quartz sericite schist. Foliation within individual layers is often discordant with respect to fo l iat ion attitudes within underlying micaceous quartzite and overlying 37 black phyl l i tes . This discordancy is believed to be the result of local anisotropy within this unit as a whole during deformational shearing. Middle Unit The middle unit is characterized by grey to black lustrous phyl l i te with minor intercalations of limestone. Lenses, pods and irregular veins of translucent to milky white quartz outline the fo l iat ion in the phyl l i te . Porphyroblast phases of pyrite, magnetite and hematite occur infrequently. A prominant horizon exists within this middle unit that is dominated by boudinaged layers of s ider i te . Flattening effects have produced a "knotted" appearance in which individual "knots" are composed of soft, limonitic carbonate (calcite) and quartz. Upper Unit The upper unit, which consists of well fol iated black phyl l i te , chlorite schist and lenses of aphanitic greenstone probably forms a transition zone between the middle phyll ite unit and overlying mixed phyl l i te /volcanic last ic assemblages of the Takla Group. No lateral continuity of this sub-unit was found within the study area. The above sub units, as with most of the metavolcanic assemblages in the Mt. Perseus area, are often lateral ly discontinuous in both 38 composition and outcrop appearance. This discontinuous nature re f lec ts the environment of deposition as well as post -deposit ional tecton ics . 39 STRUCTURE All pre-Tertiary rocks within the Mt. Perseus area have been complexly deformed throughout the mid-Paleozoic to the late-Mesozoic. Early within the deformational history, two terranes of principally d i f f e r e n t o r i g i n and a f f i n i t y , termed the Intermontane zone (Quesnellia) and Omineca Crystal l ine Belt , were tectonically juxtaposed via low angle, east-directed thrusting (Monger and Price, 1979). Regional deformation associated with this convergence resulted in the formation of a ductile shear zone and large scale east-verging folds with associated amphibolite grade metamorphism. Protracted regional tectonism superposed upon earl ier folds produced nearly coaxial large scale southwest verging ductile folds which deform the convergent boundary and control the present map configuration within the Crooked Lake area (Fig. 3-1). Strain associated with this deformation was accommodated in part along the regional shear zone and in part in the formation of localized subsidary zones related to fold geometry. Successive superposed folding about similar axes and reactivation of high strain zones characterizes the present geometry within the Mt. Perseus area. The various structural distinguished in the f ie ld orientation. These elements elements associated with on the basis of their with their nomenclature folding were geometry and are l i s ted in 40 Figure 3-1. Map of the Quesnel Lake region showing the present configuration of Quesnel1ia-Omineca terrane boundary (modified from Campbell, 1978). 41 Table 3-1. Relative ages of the different phases of folding were established primarily from parallel or cross-cutting relationships between cleavages and fold axial surfaces and from refolding of earl ier cleavages and related linear structures. Slight differences in fold style of successive phases may also be distinguished but are much complicated by the varied response of different. 1 ithol ogies during any particular phase of folding. A sequence of six deformational phases has been established within the Snowshoe Formation of which only five are common to the overlying Antler and Upper-Triassic Formations. These overlying formations lack evidence of an early fold phase only observed within the Snowshoe Group (Table 3-1). 42 TABLE 3-1 CHARACTERISTIC DEFORMATIONAL FABRICS ASSOCIATED WITH THE PROPOSED DEFORMATION SEQUENCE EVENT DESCRIPTION NOMENCLATURE D 6 - open fracture set F g D 5 - symmetric vertical open buckle folds F 5 - locally developed fracture and crenulation S 5 cleavage; some kink banding D 4 - open to tight'NE-verging recumbent folds Fk - well developed SW-dipping crenulation cleavage S^  - poorly developed mica-edge lineations, L^ shallow NW plunge D 3 - disharmonic upright SW verging shear folds F 3 with associated high strain zones; production of Perseus Antiform and control of regional map pattern; NW-SE trend - strongly developed crenulation cleavage and S 3 local l ized axial planar shear zones - amphibole and mica-edge l ineations, shallow L 3 NW plunge D 2 - mesoscopic recumbent tight to isocl inal east F 2 verging folds; very appressed with sharp hinges and planar limbs; production of east-verging large scale folds and associated high strain zones - well developed axial planar fol iat ion associated S 2 with formation of regional ductile zone of convergence and subsidary zones - shallow plunging amphibole, quartz, flattened L 2 K-feldspar augen mineral lineation in gneiss; mica-edge l ineation in pe l i t i c schist; variable NW-SE trend. D 2 - mesocopic intrafol ia l isocl inal folds outlined F x by S 0 , flattened, appressed, with sharp hinges and planar limbs; evidence of east-vergence - primary schistosity/fol iation parallel to S x transposed compositional layering; development prior to formation of Quesnellia-Omineca convergent boundary - mica-edge lineation L 1 D 0 - compositional layering SQ 43 The earl iest recognized structures within the f ie ld area are only developed within the Snowshoe Group. These structures, designated phase 1, are rootless isocl inal intrafol ia l folds (Table 3 - 1 ) . Mesoscopic Fj folds deform and transpose SQ compositional layering and have a well developed axial plane fol iat ion (Fig. 3-2 a,b). This fo l ia t ion , the earl iest surface which outlines any . recognizable s t ruc ture , i s subparal le l in or ientat ion to S 0 and is hence considered to be transpositional in nature. Mica-edge l ineations, L l s associated with Fj folds plunge variably and shallowly to the northwest and southeast. Lower hemisphere stereographic projections of phase 1 fold elements are presented in Figure 3 - 3 . These diagrams i l lus trate the present geometry as deformed by successive deformation phases. L i t t l e can be inferred concerning the scope and original geometry of these folds due to the effects of superposed folding and metamorphism. As yet, these folds and their associated transposed fol iat ion have not been related to any large scale regional structure(s). The earl iest phase of deformation common to all formations within the study area is phase 2 folding. This deformation is easterly verging, recognizable on al l scales, and is associated with widespread ductile behaviour in all rock types. Associated with this folding is a Figure 3-2a. Snowshoe garnet-biotite schist intrafol ia l isoclinal folds. containing mesoscopic F Figure 3-2b. Snowshoe Group micaceous quartzite containing flattened mesoscopic F, isoclinal folds. Figure 3-3. Lower hemisphere stereo projections of the present geometry of phase 1 deformation structures as deformed by phase 2 and phase 3 folding. Solid dots = poles to S, foliation; circles = L1 lineations. a) 215 poles and 73 lineations taken from the northwestern region of the antiform. b) 178 poles and 87 lineations taken from the southeastern portion of the map area. Lx lineations are deformed along great circles and show girdle distribution around later fold axes. 47 well developed f o l i a t i o n that i s axial planar to both mesoscopic and larger fo ld st ructure . Mineral l ineat ions within the Perseus gneiss shows strong preferred or ientat ion para l le l to L 2 f o ld axes which plunge var iably to the northwest and southeast. Phase 2 represents the main penetrative phase of deformation with in the area and i s accompanied by synkinematic metamorphism within the lower amphibolite f a c i e s . Micro-textures and mineral assemblages indicate that metamorphism outlasted t h i s deformation and began to wane with the t h i r d deformation phase. Second phase deformation involves a l l rock types but does not deform the tectonic boundary separating them, and from t h i s , i t can be inferred that D2 i s associated with the formation of th i s convergent zone. Hence, th i s geometry most l i k e l y describes the nature of convergence between Intermontane and Omineca rocks. Further discussion of t h i s topic i s presented in the conclusion chapter. Minor folds within the Snowshoe (Table 3-1) show consistent vergence which out l ines a large scale second phase structure that closes to the southwest (F ig . 3 -4 ) . The o r ig ina l geometry of th i s s t ructure , best outl ined by the Perseus Gneiss, has been obscured and refolded by the nearly coaxial phase 3 Perseus antiform (Plates I I , I I I , IV). A composite diagram i l l u s t r a t i n g the geometry and sense of rotat ion of F 2 minor folds (as they re late to the larger polydeformed second phase structure) at various structural posit ions around th is phase 3 antiform i s presented in f igure 3 - 5 . From th is diagram, i t i s apparent that minor folds and the i r associated f o l i a t i o n within both Figure 3-4a. Mesoscopic F 2 isocl inal folds within the Perseus gneiss outlined by S, transpositional fo l ia t ion . 49 Figure 3-4b. Mesoscopic F 2 isocl inal folds, contained within gneiss in the overturned limb of the Perseus antiform (area IX, F ig . 3-5). 50 Figure 3-5. Map view of Perseus Gneiss within the Snowshoe Group. Circles represent close-up views of small cross-sections which illustrate the sense of rotation of F2 minor folds around the Perseus Antiform (arrows indicate plunge). F2 folds within both limbs of the antiform have similar vergence whereas F2 folds contained within the antiform core zone display an opposite sense of vergence. These opposing vergence senses outline the deformed limbs of a large phase 2 synform. See text for further explantlon. Refer to Plate II for additional nomenclature. 51 limb regions of the Perseus antiform share similar vergence and hence indicate closure to the southwest. Folds within the antiform core zone (in both schist and gneiss) display a consistent yet opposite sense of rotation with respect to minor folds contained within the limb regions. Therefore, from changing orientations of S 2 around the antiform and from variations in minor fold vergence, i t can be inferred that F 2 folds within both limb regions of the Perseus antiform outline the continuous deformed limb structure of an earl ier second phase fold. Minor folds within the phase 3 antiform core zone outline the folded complementary limb structure (Fig. 3-6). The convergence of these deformed limbs culminates in a re l i c t hinge zone (outlined by the Perseus Gneiss) located within the vic inity of the southeastern flank of Mt. Perseus (Plate I, III, IV). F 2 minor folds within this region show l i t t l e or no sense of rotation and are mostly isocl inal due to localized high flattening strains (Fig. 3-7). Based upon the geometry of minor folds (Figures 3-8,9,10,11,12) and the pattern of l i thologic contacts, a possible orientation of the F 2 structure prior to refolding can be approximated. By removing the effects of the third phase antiform, the phase 2 structure appears east-verging, synformal, and southeast plunging (Fig. 3-13). Phase 3 westerly verging folds refold pre-existing easterly verging phase 2 geometry and i t is this near coaxial superposition that gives rise to the curvil inear nature of phase 2 axial structures and results in regional elongate domes having the phase 2 trend. Axial 52 Figure 3-6. Schematic cross-section through the phase 3 Perseus Antiform (outlined by gneiss) which Illustrates the refolded nature of a large phase 2 fold. Circles represent close-up views of small cross-sections of F 2 minor folds observed at various structural positions within the folded Perseus Gneiss. F, fold vergence Is used to outline the limb regions of the larger deformed phase 2 fold. 53 Figure 3-7a. Mesoscopic flattened F 2 i socl inal folds within the Perseus Gneiss. Location corresponds to area I, Figure 3-5. 54 Figure 3-7b. Mesoscopic F , isocl inal fold within Snowshoe garnet-biotite schist. Location corresponds to area I, Figure 3-5. Figure 3-8. Mesoscopic open F 3 fold in gneiss refolding an F, isocline (lower le f t limb of F 3 structure). 56 NE SW Figure 3-9. Tight mesoscopic F 2 fold contained within the hinge region of the Perseus Antiform (between areas II and VI, Figure 3-5) in which Snowshoe metapelites occupy the core zones of folds within the Perseus Gneiss. Figure 3-10. Mesoscopic F 2 tight to isoclinal folds contained within the Perseus Gneiss. After the removal of l a t e r deformational e f fec t s , fo lds appear east-verging (location corresponds to area VI, Figure 3-5). 58 NE SW Figure 3-11 Large mesoscopic east-verging F , folds within shallowing dipping Snowshoe metapelite in the southeastern portion of the Perseus Antiform hinge zone (southeast plunging). Location corresponds to an area between areas VIII and IX, Figure 3-5. 59 SW NE Figure 3-12. Steeply inclined F 2 i socl inal folds overprinted by shallowing dipping open F^ folds. Compositional layers in this garnet schist are outlined by layers of quartz. Location corresponds to area II, Figure 3-5. 60 gure 3-13. Schematic illustating the unfolding of the superposed phase 3 Perseus Antiform. By unfolding the phase 3 limbs such that their dip equals the plunge of the phase 3 fold axis, the resultant phase 2 geometry appears synformal and east-verging. 61 traces of the deformed F 2 synform are presented in Figure 3-14. Phase-two minor fo ld structures within Antler and Upper-Triassic Formations are mostly i s o c l i n a l and i n t r a f o l i a l within the S 2 f o l i a t i o n (F ig . 3 -15) . S 2 fabr i cs within these rocks deform S Q compositional layer ing and thus can be considered transposit ional and s imi la r in nature to S x w i th in the Snowshoe. Minor folds display a sense of rotat ion that i s consistent with F 2 geometry in the Snowshoe within the overturned limb of the Perseus antiform. The f lat tened and often transposed nature of these folds is confined to a zone containing mylonites which at ta ins a maximum width of approximately 1 kilometre on e i ther side of the Snowshoe-Antler contact. Geometry within th is zone i s most l i k e l y the resu l t of concentrated high f la t ten ing s t ra in associated with regional deformation of the Intermontane-Omineca boundary. Transposit ion of ear ly fo ld structures i s more eas i l y accommodated in Antler and overlying rocks due to the strong d u c t i l i t y contrast present between them and the Snowshoe. Hence, only Snowshoe rocks immediately adjacent to th i s tectonic boundary have been subject to widespread myloni t i zat ion and t ranspos i t ion . Lower hemisphere stereographic projections of second phase structures are presented in Figure 3-16. P-3. Third phase geometry, unl ike phase 2, i s southwest-verging and i s associated with a well developed moderately to steeply northeast 62 Figure 3-14. Map and schematic cross-section through the Perseus Antiform showing the nature and orientation of phase 2 and phase 3 axial surfaces and axial traces. Figure 3-15. Photograph of a small phase 2 i socl inal fold within a chlorite schist of the Antler Formation. Figure 3-16. Lower hemisphere stereo projections of 101 poles to F» minor fold axial planes (solid dots), 41 L, mineral l i n e a t i o n s ( c i r c l e s ) , and 83 F 2 minor fold axes (triangles). Distributions outline the doubly plunging nature of the phase 3 antiform. Girdle distributions of deformed linear structures l i e along great c i r c l e s . 65 dipping axial planar f o l i a t i o n . Mineral assemblages and micro- textures ind icate that metamorphism associated with phase 3 was wi th in the middle greenschist fac ies and represents a reduction in metamorphic grade from the peak lower amphibolite fac ies act ive during second phase deformation. Adjacent to the Intermontane-Omineca boundary, phase 3 geometry i s character ized by large northwest plunging, upright antiformal c losures in the Snowshoe, separated by highly attenuated synforms. These synforms become transposed p a r a l l e l to the i r ax ial surfaces into small shear zones which extend down into the Snowshoe and are often cored by small sheared bodies of Ant ler amphibol i tes. Larger scale northwest plunging t h i r d phase folds define and control the present conf igurat ion of the Intermontane-Omineca convergent boundary wi th in the Crooked Lake area. The Perseus anti form, one example of these larger fo ld s t ruc tures , i s dome-like in outcrop with northwest trending fo ld axial traces and f o l d axes (Figure 3 -14; Plates I, I I I , IV). Geometry i s c l a s s i f i e d as moderately t i gh t and overturned to the northwest with a var iab le inter l imb angle averaging 50 degrees. Ax ia l planar f o l i a t i o n i s well developed throughout the st ructure wi th in Snowshoe and Ant ler rocks. Upper T r i a s s i c rocks c h a r a c t e r i s t i c a l l y contain a spaced cleavage and b u c k l e - f o l d s a s s o c i a t e d w i th phase 3 r a t h e r than p e n e t r a t i v e f o l i a t i o n . Third phase minor fo lds appear everywhere p a r a s i t i c to the la rger 66 antiform and change in both style and nature across the antiform as a function of l ithology and proximity to the Snowshoe-Antler tectonic boundary. (Fig. 3-17). Within the northeast limb of the antiform, minor folds vary as a function of lithology from moderately open structures with interlimb angles averaging 50 to 60 degrees in gneissic rocks to tight folds and associated small scale crenulations in metapelites (Fig. 3-18). Well developed mica-edge lineations oriented parallel to fold axes plunge shallowly to the northwest and southeast. Phase 3 structures within the core-zone of the antiform display l i t t l e sense of rotation and vary in style from tight folds with 30-40° interlimb angles to near isocl inal geometry (Figs. 3-19,20, 21,22). The more open structures show a characteristic progressive gradation into isoclinal form over the course of several metres. In several loca l i t i e s isocl inal folds become further flattened within small transposition (mylonite) zones oriented parallel to their respective axial planes (Fig. 3-17). Large volumes of metamorphic quartz "sweats" occupy the core zones within isoclinal folds and associated shear zones. These regions of high strain most l ike ly served as conduits for f l u i d transport during metamorphism. Disharmonic folding is observed within both gneiss and metapelite and increases in frequency toward the Intermontane- Omineca boundary. 67 SW NE SMALL X-SECT. Figure 3-17. Schematic cross-section through the Mt. Perseus area showing contacts between Upper Triass ic , Antler, and Snowshoe rocks. Small cross-sections, based upon f ie ld sketches, i l lus trate the changing style of F , folds across the antiform. Folds become progressively more flattened and sheared as the Quesnellia-Omineca boundary i s approached. Rocks adjacent to this boundary are penetrated by narrow longitudinal shear zones related to fold geometry. 68 Figure 3-18a. Mesoscopic F, folds in Snowshoe micaceous quartzite. The sense of rotation is directed to the southwest (southeast plunge) and is thus parasitic to the larger Perseus Antiform. Figure 3-18b. Phase 3 mesoscopic folds contained within the upright limb of the Perseus Antiform. Figure 3-19. Outcrop of garnet-biotite schist exposing the super-position of an upright phase 3 fold onto a phase 2 isocl inal fold (F 2 hinge zone located at lower lef t of photo). 71 Figure 3-20. Open style upright F 3 folds in metapelite contained within the antiform core zone. Most folds in this zone show l i t t l e or no sense of rotation. Compositional layers are composed of quartz and muscovite. Figure 3-21. Upright moderately tight F 3 fold in calcareous metapelite adjacent to the hinge area of the Perseus Antiform. 73 Figure 3-22. Outcrop of micaceous quartzite containing an F 2 isocline refolded by phase 3 minor folds. Location is within the antiform hinge zone and compositional layering is therefore shallowly inclined and is intersected at a high angle by S 3 . 74 Figure 3-23. Outcrop of calcareous garnet-bearing metapelite which contains small scale disharmonic F^ folds. Structural location corresponds to an area within the overturned limb of the Perseus Antiform in the southwestern portion of the f ie ld area. Figure 3-24. Close-up view of calcareous metapelite in which an F9 isocl ine (outlined by quartz) is refolded at a high angle by open to tight F 3 folds. Location is adjacent to Figure 3-23. 76 Within the deformed axial regions of the large scale phase 2 structure (Fig. 3-14), third phase minor folds are characterist ical ly flattened and transposed within their fol iation due to the convergence in orientation of second and third phase axial surfaces (Fig. 3-25). Strain associated with phase 3 in these regions was more easily accommodated through s l ip reactivation along the pre-existing S 2 fabric. Sequences of disharmonic folding accompanied by increased frequency of transposition (shear) zones characterize third phase minor structures within the southwest overturned limb region, and in particular, adjacent to the convergent boundary (Figs. 3-26,27,28). The tectonic contact zone or zone of convergence, contained within the southwest limb of the antiform, is characterized by a pronounced mylonite belt in which nearly a l l structures (phase 2 and phase 3) are highly sheared and often transposed and flattened within their respective nearly coplanar fol iat ions . Third phase minor folds within the Antler are mostly isocl inal and often sheared and transposed with zones along their respective axial planes. Phase 3 folds within the Upper Triass ic units are flattened and sheared within their respective fo l iat ion immediately adjacent to their lower contact with Antler rocks. Approximately 100 metres structurally higher within the phyl l i tes , F 3 folding is confined to zones averaging 3 to 5 metres in width. Third phase structures in the remainder of the formation are characterized by buckle folds associated with a well developed spaced 77 NE SW Figure 3-25. Mesoscopic phase 2 isocline (hinge at bottom right of photo) flattened and refolded by phase 3 folding. Location corresponds to areas I and II, Figure 3-5, within the deformed phase 2 axial region. 78 Figure 3-26. Phase 3 disharmonic folds in garnet-biotite schist adjacent to a small localized shear zone (far l e f t of photo). Fold geometry becomes more flattened as the zone is approached and again more open further from the zone. Location corresponds to an area within the overturned phase 3 limb in the vic inity of Mt. Perseus. 79 Figure 3-27. Mesoscopic phase 3 isoclinal fold adjacent to a small shear zone (left of photo) in which F 3 geometry becomes transposed parallel to S3 axial surfaces. These zones often contain large volumes of metamorphic quartz sweats. Figure 3-28a. Mesoscopic phase 3 antiform in Snowshoe micaceous quartzite immediately adjacent to the Quesnel1ia-Omineca boundary. This fold is one in a sequence of disharmonic fold sets that flank numerous ductile shear zones in this structural region. Microtextures indicate that rocks in these structures have undergone extensive mylonitization and recrystal l izat ion. Figure 3-28b. Phase 3 antiform situated immediately adjacent to photo (a). Fold geometry is becoming progressively tightened as a small shear zone is approached ( left side of photo). Note large volumes of quartz concentrated along the axial surface and limb of the fold. 82 Figure 3-28c. Mesoscopic F 3 antiform immediately adjacent to the previous photo (b). The intervening synform (far right of photo) has been nearly completely transposed within the fol iat ion and is generally unrecognizable. As a shear zone is approached (left of photo), fold geometry becomes isocl inal and rapidly transposed within the shear zone. 8 3 Figure 3-28d. Mesoscopic isoclinal fold adjacent to the far l e f t of photo (c). These phase 3 folds are flattened and part ia l ly transposed parallel to their axial surfaces and often have a rootless form. The remainder of the shear zone is characterized by abundant rootless isocl ines, mylonites, and large volumes of quartz sweats. 84 crenulation cleavage. Hence the wave length amplitude of F 3 folds increases away from the contact. Folds within Snowshoe rocks however, are nearly all transposed and where flattening strain was less intense near isoclinal geometry is cored by large volumes.of metamorphic quartz "sweats" (Figs. 3-28b,c,d). Northwest plunging mineral l ineations, most prominantly developed within Antler amphibolites and Snowshoe augen gneiss, are strongly elongate with the mylonitic fol iat ion parallel to L 3 fold axes. This geometry extends for approximately 1 kilometre on either side of the Snowshoe-Antler boundary. Mesoscopic fold structures within the Snowshoe outside of this 2 kilometre wide mylonitic zone are only transposed within isolated localized zones of high flattening strain averaging 5 to 10 metres in width. Adjacent to the tectonic boundary, south of Cayuse Creek, isolated elongate bodies of highly sheared Antler amphibolite, averaging 100 metres in length and 2 metres in width are structurally infolded within Snowshoe metapelites (Plate I). These infolds represent remnant, sheared out core zones of large scale attenuated phase 3 synforms developed within the Antler Formation that have been drawn-down and structurally isolated within the Snowshoe (Fig. 3-29). Large scale complimentary antiforms within the Snowshoe, adjacent to these sheared-out synforms are typically more open and less attenuated. From this geometry, i t can be inferred that the phase 3 structural style within the contact zone is characterized by large tight antiforms in 85 the Snowshoe separated by amphibolite-cored shear zones which represent attenuated synformal extensions of Antler rocks into the Snowshoe (Fig . 3-30). Smaller scale phase 3 shear zones throughout other portions of the Snowshoe most l ike ly represent structurally deeper extensions of these attenuated synformal closures. Lower hemisphere stereographic projections of phase 3 geometry are presented in Figure 3-31. PJ, Phase-4 deformation is character ized by non-penetrative east-verging folds. Axial cleavages which dip shallowly to the southwest, crenulate all pre-existing surfaces, and are associated with minor growth of b iot i te . Most often F 4 folds produce open recumbent warps within steeply inclined portions of the Snowshoe (Figs. 3-32,33 34) and are generally less well developed within Antler and Black Phyl l i te Formations. Fold styles vary from open to tight as a function of varied l i thologies . Lower hemisphere sterographic projection are presented in Figure 3-35. Phase-5 deformation is represented by non-penetrative small scale v e r t i c a l buckle folds accompanied by local crenulat ions and kink-banding (Fig. 3-36). Folds are typically symmetrical and open and are rarely associated with a measureable cleavage. Folding of this style is prominent within all units in the study area. Figure 3-29. Talus blocks containing interfolded Antler amphibolite and Snowshoe garnet metapelite. Location is adjacent to a narrow lense of isolated Antler rock in the Snowshoe Group approximately 200 metres structurally below the Antler-Snowshoe contact in the v ic in i ty of Cayuse Creek. Figure 3-30. Schematic cross-section of phase 3 folding throughout the contact zone between the Black Phyll ite and Antler Formations and the Snowshoe Group. F. synforms within the Snowshoe are local ly cored by Antler amphibol1tes which are manifest as shear zones at greater structural depths within the Snowshoe. Figure 3-31. Lower hemisphere stereo projections of 130 poles to F3 minor fold axial planes (solid dots), 96 L, lineations (circles), and 134 minor fold axes (triangles). Poles to minor fold axial planes outline the larger Perseus Antiform whose axial surface averages 125/54 NE. Linear structures appear doubly plunging due to their superposition on variably oriented earlier geometry. The majority of phase 3 linear structures plunge to the northwest and have an average orientation of 23/330. 89 Figure 3-32. Phase 4 recumbent open folds refolding steeply inclined phase 2 isoclines (outlined by compositional layers) . Photo location is within the phase 3 antiform core zone, adjacent to Mt. Perseus. Nearly recumbent phase 4 fold deforming phase 2 i socl inal folds contained within the Perseus Gneiss on the eastern flank of Mt. Perseus. Figure 3-34. Gently dipping phase 4 folds deforming S /S layering within Snowshoe garnet-biotite schist. Location is within the southeast flank of Mt. Perseus. 92 Figure 3-35. Lower hemisphere stereo projection of phase 4 fo ld geometry. So l id dots indicate poles to axial planes and t r iangles represent minor fo ld axes. Figure 3-36. Large kink-band structure This deformation overprints j o i n t s ) and is probably deformation. within the Perseus Gneiss, al l earl ier geometry (except associated with phase 5 94 The final phase of deformation consists of a pervasive open fracture set common within all rock units. Most fractures and joints have a common orientation of 032/81NW, and most l ike ly represent relaxation features associated with waning regional, deformation. 95 MICROTEXTURES The metamorphic character of rocks within the Mt. Perseus area has been described by Campbell (1971) who distinguished a Barrovian metamorphic sequence ranging from middle greenschist to the middle amphibolite facies. Isograds delineating biot i te , garnet, and kyanite/ staurolite zones were mapped on the basis of f i r s t appearance of the respective phases. In this study, characteristic textural and mineralogical features of relevant stratigraphy are described and related to deformational phases. Mineral assemblages within the study area describe a Barrovian sequence which increases in grade to the northeast from the middle greenschist facies in Upper Triassic phyllites to lower amphibolite (garnet grade) within portions of the Antler and in Hadrynian Snowshoe Group rocks. The peak metamorphic act ivity appears synkinematic to phase-two (D 2) deformation with a reduction to middle greenschist during the t h i r d deformational phase ( D 3 ) . No evidence of kyanite/staurolite zones were located. Post deformational annealing is stongly developed adjacent to the Snowshoe-Antler boundary and within narrow longitudinal subsidary shear zones throughout the Snowshoe. A summary of the relevant stratigraphy, metamorphic rock types and typical mineral constituents is presented in Table 4-1. 96 STRATIGRAPHIC UNIT TYPICAL MINERALOGY* PROTOLITH ROCK TYPE s N 0 w s H 0 AT kali-Feldspar Augen Gneiss (Mylonite) Microd ine + Quartz + Muscovite + Plag + Biotite (An 8 - 2 7 ) Feldspathic Sandstone, Arkose (?) or Granitic sheetlike instrusive (Quartz Monzonite?) Garnet-Biotite Schist Biotite + Quartz + Muscovite ± Garnet + Plagioclase (An 10-23) ± Hornblende Mixed assemblage of Sandstones, Mudstone and Greywacke E Micaceous Quartzite Quartz + Muscovite + Garnet + Plagioclase (An 15-27) Greywacke, Sandstone G R 0 U P Calcareous Garnet -Biotite Schist Quartz + Calcite + Dolomite + Biotite ± Muscovite + K-Spar + Plagioclase (An 13-23) + Garnet Impure calcareous Psammite, Greywacke Sandy Marble Quartz + Calcite + Dolomite + Muscovite Impure Limestone, Calcareous Psammite Quartzo - Feldspathic Gneiss Biotite + Muscovite + Quartz + Perthitic microcline + Hornblende + Plagioclase (An 14-30) ± Garnet ± Epidote ± Sphene Si 11-like Quartz -Dior i t ic Intrusives * Common Accessory Minerals Include: Tourmaline, Apatite, Zircon, and Opaque Dust TABLE 4-1 97 SNOWSHOE GROUP Mineral assemblages within the Snowshoe indicate that all rock types (with the exception of late stage pegmatites) have undergone peak metamorphism within the lower amphibolite facies (garnet grade) which gradually wained to middle greenschist grade. Micro-textures within coexisting garnet and biotite relate peak metamorphism to D 2 . Strong post-deformational annealing throughout the Snowshoe has rendered many early textures obscure and as yet, no evidence, either mineralogical or textural, was found to substantiate any metamorphism associated with METAPELITIC-PSAMMITIC ROCKS Garnet-Biotite Schist P e l i t i c schists containing coexisting garnet and biotite comprise the dominant lithology within the Snowshoe. The following text describes the mineral phases present within this rock type and their relation to the proposed deformation (refer to Table 4-1). 98 Chlorite Chlorite occurs in small amounts throughout the Snowshoe as fine laths and together with muscovite often defines the S 1 / S 2 fo l ia t ion . Chlorite is present as a retrogressive phase having formed from biotite and garnet. This retrogressive metamorphism was presumably associated with phase 3 and later deformation. Biot i te: Porphyroblastic brown biotite occurs throughout the Snowshoe and is most commonly recognized as small plates and laths within the matrix. Porphyroblastic phases crystal l ized at random throughout these rocks and are commonly associated with post-deformational annealing. Several samples yielded porphyroblasts which contain r e l i c t internal schistosity which has been rotated relative to the external Sl/S2 fo l ia t ion . Biotite of this type most l ike ly formed during D : and underwent rotational and flattening strain during D 2 and D 3 which caused the f o l i a t i o n to rotate with respect to the growing porphyroblast and hence "trap" earl ier fo l iat ion orientations. The matrix biotites formed before and after D 2 , and define, together with muscovite the schistosity (partial ly transposed folation) associated with F 2 folds. 99 Ihitwi Figure 4-1. Photomicrograph (x-nicols) of a Snowshoe Group quartz-muscovite schist. Muscovite laths outline S 2 and are deformed at a high angle by F3 crenulations. 100 I I Figure 4-2. Photomicrograph (x-n ico l s ) of a Snowshoe micaceous quartzite. Muscovite and biotite laths are aligned parallel to S, and are folded by F 3 crenulations. Note subgrain development at the grain boudaries of larger quartz grains. 101 Muscovite Muscovite occurs as small laths oriented parallel to the S^Sj fol iat ion and along S 3 cleavage planes (Fig. 4-1,2,3). Polygonal arcs defining F 2 and F 3 microfolds are seen throughout the Snowshoe (Fig. 4-4) except in the area adjacent to the Omineca-Intermontane boundary, where any polygonal arcs have been largely flattened and transposed. Garnet: Garnet (presumably almandine) development within the Snowshoe Group is largely dependent upon composition and hence is often confined to local layers. Two stages of prophyroblastic garnet growth are present within metapelitic and gneissic rocks. The f i r s t generation is characterized by large (up to 4 cm) xenoblasts which contain numerous inclusions that are generally recognizable as r e l i c t S 2 schistosity (Fig. 4-5). These inclusions are predominantly quartz with minor biot i te and muscovite. Substantial flattening has occurred around these garnets which has resulted in the formation of extensive quartz pressure shadows (Fig. 4-5). He l i c i t i c internal structures vary as a function of structural position within the Perseus antiform. Figure 4-6 presents a schematic intepretation of the timing of garnet growth. 102 Figure 4-3. Photomicrograph (plane-light) of a Snowshoe Group garnet-muscovite-biotite schist. Muscovite and biotite outline SQ/Sl/S2. Phase 3 crenulations are seen to deform mica laths. 103 Figure 4-4. Photomicrograph (plane-light) of a phase 2 outlined by fine grained muscovite and quartz (S t Snowshoe micaceous quartzite (hinge is located in lower right corner area of photo). The metamorphic growth of microfol d •Q/S,) in a muscovite throughout i n i t i a t e d early within D 2 and continued I I I mm Figure 4-5. Photomicrograph (x-nicols) of a h e l i c i t i c phase 1 type garnet in a Snowshoe garnet-muscovite-biotite schist . Rotated inclusion t ra i l s of quartz, muscovite, and biotite outline S 2 and have been rotated relative to the external S 2 schistosity through flattening processes active during D 3 . 105 EVENT DEFORMATION STYLE MINERAL GROWTH SYN -D2-POST ^^^^^^^^^ G A R N E T ( 1 ) ^s^^ S2*So/31 SYN -D3-POST S3. \VX\ V^<~ C^sT^ S2»So/S 1 GARNET(I) ^ZTT c^t=N. s z ^ o / s T ^ ^ ^ ^ ^ J j ^ ^ ^^^T ^ - —==- S3=S2-So/S1 : 5 ^ ^ ^ > ^ ^ = ^ - GARNET(2) gure 4-6. Schematic illustation of the timing of garnet growth with respect to deformation episodes and styles in the Snowshoe Group. 106 Figure 4-7. Photomicrograph (x-nicols) of small idioblast ic phase 2 type garnets in a Snowshoe Group garnet-biotite muscovite schist. Mica laths outline the Sl/S2 fo l iat ion which is deformed by F 3 crenulations. Phase 2 garnets grew during the latter stages of D 2 and during D 3 . 107 m rt Figure 4-8. Photomicrograph (x-nicols) of hornblende crystals which have been altered to epidote and cl inozoic i te in this calcareous Snowshoe amphibolite. Lenses of these rocks occur extensively throughout ca lc - s i l i ca te metapelites. 108 The second stage of garnet growth occurred after D 2 . Second generation garnets are characterized by their small size, idioblastic form, low content of random inclusions, and general lack of pressure shadows (Fig. 4-7). These garnets are most commonly found in metapelites adjacent to the Snowshoe-Antler contact and within gneisses in the antiform core zone. Hornblende: The crysta l l izat ion of dark green hornblende as a metamorphic phase in metapelites is generally restricted to small irregular pods situated adjacent to highly calcareous units in the Upper Snowshoe group. Hornblende shows preferred orientation within the S 2 fabric and is deformed by S 3 crenulation cleavages (Fig. 4-8). Hence this phase is considered synkinematic to D 2 deformation. Table 4-2 presents the re la t ionsh ip between the proposed deformation sequence and mineral paragenesis for Snowshoe Group pe l i t i c rock types. The following metamorphic reactions most l ike ly account for the formation of prominent phases observed in lower amphibolite-upper greenschist grade Snowshoe Group metapelites. 109 BIOTITE:* (1) stilpnomelane + phengite -»• b i o t i t e + c h l o r i t e + quartz + H20 (2) stilpnomelane + phengite + a c t i n o l i t e + b i o t i t e + ch lo r i te + epidote + H20 BIOTITE-MUSCOVITE:t (3) microcl ine + c h l o r i t e + muscovite (phengitic) -> b i o t i t e + muscovite ( less phengitic) + quartz (4) muscovite (phengitic) + c h l o r i t e ->• b i o t i t e + muscovite ( less phengitic) (5) muscovite + stilpnomelane ->• b i o t i t e (Fe-r ich) + epidote GARNET: (almandine)t (6) c h l o r i t e + b i o t i t e + quartz garnet + b i o t i t e + H 20 (7) c h l o r i t e + muscovite + quartz garnet + b i o t i t e + H20 * Winkler, 1979 t Turner, 1981 110 MINERAL PHASES Pi SYN POST ?2 SYN POST ?3 SYN POST SYN POST s N 0 W s H 0 E BIOTITE — — MUSCOVITE GARNET PLAGIOCLASE HORNBLENDE TABLE 4-2 Relation of mineral growth to proposed deformation scheme in Snowshoe metapelites, Mt. Perseus area, Crooked Lake, B.C I l l NON-PELITIC ROCK TYPES: Alkali-Feldspar Augen Gneiss S i l ic ious augen gneiss containing porphyroblasts of anhedral microcline and quartz defines a narrow layer which separates Antler and Snowshoe l i thologies . Small laths of muscovite together with quartz and plagioclase (An - 8-27) outline the well developed Sl/S2 fo l ia t ion . Quartz is generally strained such that larger grains display dist inct dimensional preferred orientation, strongly undulose extinction, subgrain development and serrate grain boundaries. These larger grains occur commonly as flattened augen or porphyroclasts in a matrix of polygonal quartz (Fig. 4-9) of finer grain size (and plagioclase (An xo-n) ) - Hence the term mylonite is used. The term porphyroclast i s used in describing the augen as these "clasts" appear to have undergone dynamic grain size reduction through shear and flattening processes while in a ductile environment (mylonitization) rather than metamorphic nucleation and growth within the fo l ia t ion . Flattening features are manifest in the formation of quartz pressure shadows around the augen (Fig. 4-9,10). Rotation of these porphyroclasts is generally small (less than 30° ) and is most l ike ly associated with flattening and shear synkinematic to D2 which continued well into and throughout D 3 . Textural and mineralogical features within the augen gneiss 112 l I Ironi Figure 4-9. Photograph (plane-light) of a thin section of Snowshoe augen gneiss. Augen are mainly composed of K-feldspar, microcline, and quartz and represent deformed r e l i c t porphyroclasts of an early granitic intrusive. 113 Figure 4-10a. Photomicrograph (plane-light) of figure 4-9 centered on a K-fe ldspar-quartz augen. The S 2 f o l i a t i o n i s flattened around the various porphyroclasts. Quartz grains throughout this rock type have undergone significant dynamic grain size reduction, as shown in figure 4-10b. 114 Figure 4-10b. Cross-nicols view of figure 4-10a which i l lustrates the highly recrystal l ized nature of quartz. This sample was subject to mylonitization (dynamic grain size reduction) during D 2 and recrystal l ization during and after D». Fine grained muscovite (sericite) and quartz outline the SJ/SJ^ fo l ia t ion . 115 present several poss ib i l i t i es for protolith rock types. From the high abundance of alkal i feldspar and quartz one could infer a sedimentary origin with protoliths of feldspathic sandstone or arkosic wacke. Most, i f any, stratigraphic indicators have been multiply transposed in the zone of convergence. A possible exception to this may exist in that augen or porphyrocl ast diameters range in diameter from 3 mm to 15 mm away from the Antler contact. This variance may be interpreted as r e l i c t graded bedding which would indicate that protol i thic rocks were stratigraphically deposited onto calcareous pe l i t i c rocks of the Upper Snowshoe Group. Problems within this assumption l i e in the fact that, f i r s t l y , material from presumably underlying calcareous sediments has not been incorporated within the augen gniess protolith and secondly, one would have to account for a rapid change in the depositional mechanism and source rock to obtain coarse feldspar rich e last ics . Similar assumptions would be required to account for carbonate deposition on top of pre-existing feldspathic sandstones. The contact between augen gneiss and calcareous metapelites may also represent a disconformity in which one or the other rock types were deposited at a much later time. Textural and mineralogical indicators may also support the poss ibi l i ty of an igneous protolith of quartz monzonitic composition which prior to the D 2 deformational event, penetrated Snowshoe Group meta-sediments as a sheet or s i l l - l i k e intrusive. Mylonitization and recrystal l izat ion processes have largely destroyed any evidence of igneous texture. Quartz and alkal i feldspar porphyroclasts may 116 represent r e l i c t igneous porphyritic phenocrysts which have been subsequently altered and reduced in grain size through mylonitization. Concordant contact relations with Upper Snowshoe Group meta-pelites may indicate that the proposed intrusive was s i l l - l i k e in structure as no evidence of discordant relations was found. The contact between the gneiss and underlying metapelites is best described as a zone in which gneiss is ductily intwined with calcareous garnet-biotite schist. This mixing may well represent simple tectonic intercalation of the two rock types or may be indicative of r e l i c t migmatite texture. This type of contact is not seen to exist with Antler amphibol ites and is hence interpreted as having possibly resulted from original igneous emplacement. Less deformed rocks of nearly identical composition and texture have been examined to the north of Quesnel Lake, in a similar structural stratigraphic setting adjacent to the Antler Formation. These rocks, interpreted as part of the Quesnel Lake Gneiss (Rees, 1983; Struik, 1984; Montgomery, 1985) have been designated as orthogneiss based upon r e l i c t igneous textures and detailed geochemical analysis. Observations within the Mt. Perseus area coupled with the above data most l i k e l y support an igneous o r i g i n for the alkali-feldspar augen gneiss. Quartzo-Feldspathic Gneiss Layered gniess containing porphyroblasts of garnet and hornblende and porphyroclasts of microcline and quartz defines a poly-deformed layer within the Snowshoe Group. This quartzo-feldspathic rock is 117 characterized by a granoblastic aggregate of quartz, microcline, plagioclase (An 15-23), hornblende, muscovite, b iot i te , epidote, chlori te , and sphene which are aligned along and define the Sl/S2 tectonic fabric (Fig. 4-11). Microcline and quartz porphyroclasts both show signs of recrystal l izat ion through grain size reduction of original ly larger single grains. Plagioclase porphyroclasts occur as small aggregates within the groundmass and may represent recrystal l ized remnants of once larger feldspar crystals . Quartz within the matrix is generally strained and exhibits subgrain development with sutured grain boundaries. Many show preferred orientation and optical continuity indicating that they were once part of larger grains. Larger quartz porphyroclasts are strained, show preferred orientation parallel to S 2 , and have serrate grain boundaries. All evidence indicates that microcline, quartz, and plagioclase represent porphyroclasts that have been deformed and recrystall ized in a ductile state into smaller grain aggregates. Porphyroblastic garnets are generally small (less than 1 mm in diameter), idioblast ic and contain random inclusions of quartz and biot i te (Fig. 4-11). Fol iat ion around these garnets is mildly displaced by signs of f lattening. Poorly developed quartz pressure shadows aligned with S 2 flank some garnets though most of the quartz has been highly recrystal l ized and now obscure most early textures. Crystal l izat ion most l ikely started during the later stages of D 2 and continued during D 3 . 118 Figure 4 - l l a . Photomicrograph (x-nicols) of the Perseus Gneiss. Location is within the hinge zone of a large phase 2 synform. The groundmass is composed of an equigranular aggregate of strained quartz and feldspar. Larger porphyroblast phases include green hornblende and idioblast ic garnet. Hornblende blasts show a preferred orientation within the Sl/S2 fol iat ion of 30/330. 119 Figure 4-llb. Part ia l ly cross-nicols view of figure 4-lla showing detail of the idioblastic garnet form and hornblende al ignment along Sl/S2. 120 Hornblende occurs as porphyroblastic laths aligned within the Sl/S2 fo l iat ion (Fig. 4-11,12). Some aggregates show signs of optical continuity through uniform extinction under cross-polarization and are hence interpreted as representing finer grained pseudomorphs after or ig inal ly larger hornblende crystals assocated with the protolith rock type. Porphyroc las t ic textures combined with the r e l a t i v e l y equigranular nature of this gneiss generate two main protolith poss ib i l i t i e s . Compositionally, the rock could be classifed as either a coarse arkosic c last ic or as a grano or quartz diorite intrusive. Problems associated with a sedimentary protolith involve source rock origin as the Snowshoe Group is largely composed of pe l i t i c and psammo-pelitic rocks. Rapid changes in sedimentation environment and source material would be required to account for this unique layer of feldspathic presumably arkosic rock. The presence of numerous hornblende porphyroclasts in an arkosic-type sediment are also d i f f i c u l t to explain. Hence, the igneous alternative seems more plausible in this area. Contact relations with the enveloping metapelites are mostly concordant but in several areas, contacts appear gradational and ambiguous possibly representing re l i c t migmatite texture, since 121 Figure 4-12a. Photomicrograph (plane-light) of the Perseus Gneiss. Garnet porphyrobl asts are small, id ioblas t ic , and contain random inclusions. The Sj/S^ fo l iat ion is outlined by green hornblende and biot i te and appears displaced around garnets. 122 Figure 4-12b. Photograph (p lane- l ight ) of the thin section appearinq in f igure 4-12a. The photo i l l u s t r a t e s the nature of the Sl/S2 f o l i a t i o n . 123 subjected to high degrees of shear and flattening. Xenoliths of highly altered country rock have been located in the gneiss adjacent to contact with meta-pelitic rocks. Composition and texture within the Perseus gneiss is nearly identical to features found in a large adjacent regional gneiss body termed the Quesnel Lake Gneiss (Montgomery, 1985). Structural and geochemical investigations (including trace-element studies) conducted on the Quesnel Lake Gneiss have concluded that the rock is orthogneissic and of grano-dioritic composition. (Rees, 1983; Montgomery, 1985). The Perseus gneiss, apparently located only several thousand feet structurally lower than the Quesnel Lake Gneiss (with respect to the Antler-Snowshoe contact), may represent a relict si l l - l ike off-shoot of this larger quartz-diorite intrusion that has since been structurally isolated through intense shear and flattening processes. The time of emplacement of the Perseus orthogneiss would have been prior to D, deformation. 124 Antler Formation Mineral assemblages within Antler rocks are indicative of the transition between the middle and upper-greenschist and lower amphibolite metamorphic facies. The contact area between Snowshoe Group and Ant ler Formations i s character ized by strong post deformational annealing and alteration which has rendered ambiguous textures within basal Antler rocks. A majority of the Antler stratigraphy is dominated by lower temperature mineral assemblages than in adjacent underlying Snowshoe metapelites. Pe l i t i c layers within the Ant ler show no evidence of garnet growth though, c h l o r i t e porphyroblasts within these layers may indicate retrograde metamorphic ac t iv i ty . Based upon the difference in phase assemblages between Antler and Snowshoe rocks of pe l i t ic composition and upon the diagnostic greenschist minerals within a majority of the Antler, i t seems l ike ly that an isograd separating upper-greenschist and lower amphibolite facies is present in the lower stratigraphy of the Antler. The existence of this isograd is based primarily upon the f i r s t appearance of respective mineral phases rather than on a particular metamorphic reaction ( i . e . t ie l ine f l i p ) . A summary of relevant stratigraphy, metamorphic rock types and typical mineral constituents are presented in Table 4-3. 125 STRATIGRAPHIC UNIT TYPICAL MINERALOGY PROTOLITH ROCK TYPE A N actinolite-hornblende chlorite schist actinolite-hornblende + chlorite + albite + sericite + quartz. + biotite ± magnetite ± calcite ± sphene diabase, basalt (tholeiite) ( s i l l s?) T L E R chlori te-biot i te schist chlorite + biotite + muscovite-sericite + quartz diabase, basalt (tholeiite) ( s i l l s? ) micaceous quartzite quartz + muscovite-sericite + chlorite + albite + sphene s i l i c ious sub-greywacke F 0 R M A T I acti noli te-hornbl ende-chlorite-al bite-epidote-schist actinolite-hornblende + chlorite + albite + epidote + calcite ± quartz + magnetite ± cl inozoicite diabase, basalt (tholeiite) ( s i l l s? ) actinol i te - ta lc schist actinol ite-tremol ite + talc + chlorite + calcite + a l b i t i c plagioclase + epidote ± hornblende gabbro 0 N actinolite-tremolite-antigorite-talc schist actinol ite-tremol ite + talc - antigorite + chlorite + epidote ± hornblende ultramafic peridotite (?) TABLE 4-3 126 A general l i thologic progression exists within the Antler Formation from coarse grained amphibole schists at the Snowshoe Group contact to finer grained chlorite schists and phyl l i t i c rocks adjacent to Upper-Triassic l i thologies . The coarse nature and magnesian composition of basal Antler units are interpreted as having resulted from post deformational annealing within the upper-greenschist facies and hydrothermal alteration of mafic-ultramafic igneous assemblages. The remaining Antler stratigraphy is composed of greenschist grade derivatives of calc-alkaline diabase, tho le i i t i c basalts, and minor intercalations of psammitic rocks. Basal Act inol i te - Tremolite - Talc Schist Basal or lowermost units of the Antler Formation within the Mt. Perseus area are composed of coarse-grained schists (almost gneissic) containing assemblages of actinol ite-tremol i te , ta lc , chlori te , ca lc i te , antigorite, and a lb i t i c plagioclase. The lowermost layer of the Antler, in tectonic contact with Snowshoe Group mylonites, consists of coarse schist containing large porphyroclasts of pale green actinol ite-tremolite randomly oriented in a groundmass of ta lc , chlori te , a lbi te , ca lc i te , sphene, quartz, and magnetite (Fig. 4-13). Externally this rock is best described as having garbenschiefer texture. Internally, post-deformational recrystal l izat ion (annealing) has resulted in an ambiguous texture 127 mm Figure 4-13a. Photomicrograph (plane-light) of basal Antler Formation actinolite-tremolite schist. Tremolite, act inol i te , and talc are coarse and randomly oriented throughout this unit. 128 Figure 4-13b- Photomicrograph ( x - m ' c o l s ) of F i g u r e 4 -13a . "Garbenschiefer" texture is the result of mylonitization and epidote-amphibolite metamorphism during the latter stages of D 2 and shear reactivation, recyrsta l l izat ion, and metasomatism during and after D 3 . Recrystall ization outlasted deformation. 129 showing l i t t l e or no preferred orientation of mineral constituents. Adjacent to the contact between this unit and Snowshoe Group augen gneiss, small minor pods of extremely coarse grained act inol i te-tremol i te-ant igorite-talc schists crop out randomly (Fig. 4-14). Compositionally, these rocks are similar to the enveloping schist with the exception of the presence of antigorite, a.metamorphic derivative of original o l iv ine . The amphibole phase occurs often as massive radiating splays with individual actinol ite-tremolite crystals attaining lengths_of up to 24 centimeters. Both rock types display magnesian compositions derived by metamorphism and hydrothermal alteration (carbon dioxide metasomatism) of mafic-ultramafic rocks with gabbroic-peridotitic compositions. Assemblages observed within these lowermost Antler units are indicative of metasomatic transformation involving large volumes of carbon dioxide (Turner, 1981). This would have most l ike ly occurred indirectly as a result of metamorphic dehydration of overlying Snowshoe carbonate metapelites in which carbonate-rich fluids may have been present in large quanities adjacent to lower Antler stratigraphy. During subsequent metamorphism these fluids would have been involved in Antler metamorphic act iv i ty . Amphibole phases are most l ike ly pseudomorphic after clinopyroxene and the presence of chlorite (prochlorite) and actinolite is probably controlled by the reaction: 130 Figure 4-14a. Photomicrograph (x-nicols) of actinolite-tremolite-antigorite-talc schist of the lowermost Antler unit. The groundmass is composed almost exclusively of talc and calcite and contains larger randomly oriented porphyroblasts of tremolitic and act ino l i t i c amphibole. Antigorite phases are less pronounced and constitute a small percentage. The "garbenschiefer" texture resulted from post-deformational annealing and alterat ion. 131 I I I mm Figure 4-14b. Magnified view of Figure 4-14a which shows an interesting "en-eschelon" growth of tremolite twins within the talc-carbonate groundmass. The present configuration of these twins is most l ike ly the result of flattening during the latter stages of metamorphism, which permitted twin gliding along their respective twin planes. Subsequent recrystal l ization and annealing has destroyed any earl ier textures. 132 Augite + Enstatite + H20 + Actinol H e + Chlorite The presence of calcite-dolomite probably resulted from the breakdown of c a l c i c p lag ioc lase components in the p r o t o l i t h . Metamorphic reactions (simplified) in the following table l i s t other possible assemblage combinations resulting from greenschist grade metamorphism of ultramafic minerals. REACTIONS'' 1) ol ivine + Sj0 2 + H20 antigorite (-MgO) 2) antigorite + Sn-02 * talc (-MgO) 3) olivine + Sn-02 + C0 2 + H20 ->• antigorite + talc + magnesite (-MgO) 4) 5) 6) 7) 8) antigorite + CO, > talc + magnesite (-H20) antigorite + CaO + C0 2 ^ talc + domomite (-MgO) talc + CO, magnesite + quartz (-S-j02) talc + CaO + CO2 -»• dolomite (-MgO - Si0 2 ) antigorite + C0 2 ->• magnesite + quartz (-Sn*02) RESULTING ASSEMBLAGE anti gor ite antigorite-talc antigorite-talc-carbonate talc-carbonate quartz-carbonate * Williams, Turner and Gilbert , 1954 133 Act inol i te - Hornblende - Chlorite - Albite - Epidote - Calcite Schist Green medium to fine grained foliated actinolite-hornblende chlorite schist overlies lowermost act inol i te- talc garbenschiefer of the Antler Formation. In outcrop, the schist is recognized by a well developed l ight colored mineral lineation outlined by chlorite and albite contained within the flattened S 2 metamorphic fo l ia t ion . Mineral assemblages consist of porphyroblastic actinol ite-hornblende contained within a well developed S 2 fo l iat ion outlined by chlori te , xenoblastic a lb i t i c plagioclase, epidote, xenoblastic ca lc i te , and quartz. Accessory minerals include sphene and magnetite. Actinolite porphyroblasts, flanked by chlorite-quartz-albite pressure shadows, show rotation within the flattened fol iat ion of up to 45 degrees. Small oriented laths of epidote occur throughout the fol iat ion and often appear concentrated around actinol ite-hornblende crystals . Quartz is mostly confined to small discontinuous layers and is generally strained displaying only moderate subgrain development. In thin sections oriented perpendicular to the fol iat ion and dominant mineral l ineation, S 2 is folded and crenulated by S 3 microfolds with minor chlorite crystal l izat ion along its cleavage trace. In several l o c a l i t i e s , L 3 intersection lineations overprint the main penetrative mineral l ineat ion. This would indicate that the mineral lineation was developed parallel to F 2 fold hinges, though no evidence of r e l i c t folds were seen within thin section. Compositionally, the typical mineral assemblage within this rock 134 type is most l ike ly the product of metamorphism of basic igneous rocks. Actinolite-hornblende are probably pseudomorphic after pyroxene and calcite from r e l i c t calcium feldspars. Reactions controlling the formation of actinol ite and hornblende are governed by the following (Winkler, 1976): ACTING-LITE: (1) augite + enstatite + H20 •* actinol ite + chlorite (2) pumpellyite + chlorite + quartz -v c l inozoicite + actinol i te HORNBLENDE: (3) actinolite + cl inozoicite + chlorite + quartz > hornblende The presence of coexisting actinolite-hornblende, epidote and calcite place this rock with the upper-greenschist facies and possibly within a zone of transition between the lower amphibolite and upper-greenschist facies (Turner, 1981; Mason, 1978). Composition may infer protoliths such as oversaturated calc-alkaline diabase and basalt, s p i l l i t i c diabase, gabbro, and poss ibly a t h o l e i i t e d e r i v a t i v e . The stratigraphic association with the underlying magnesian meta-ultramafic 135 indicates that this lower portion of the Antler is dominated by extremely mafic igneous stratigraphy similar to alpine-type ophiolite complexes such as those described within the Antler Formation further to the south adjacent to Dunford Lake (Montgomery, 1978). The remainder of the Antler Formation consists of irregular layers and lenses of chlorite schist, sericite-muscovite schist, hornblende-act inol i te-chlori te schist, chlorite biotite schist, micaceous quartzite, meta-pillow basalt (now greenstone), and a r g i l l i t e . Typical hornblende-actinolite-chlorite schists are fine grained and composed of porphyroblastic hornblende-actinolite aligned in a matrix of ch lor i te , a lb i t i c plagioclase (untwinned), ser ic i te , quartz, b io t i t e , magnetite, calcite and sphene. Amphibole, chlorite and seric i te are aligned and define the main S 2 fo l ia t ion . Intense flattening effects have displaced the fabric around amphiboles and some larger chlorite porphyroblasts. Quartz occurs as small strained polygonal grains in pressure shadows flanking both amphibole and chlorite porphyroblasts and in small discontinous layers parallel to S 2 . No optical continuity was observed in these grains. Microfabrics associated with F 3 folding are characterized by small warps and crenulations of the existing S 2 fo l ia t ion . Small laths of muscovite define S 3 though most often, l i t t l e mineral growth was observed along the S 3 plane. Rocks of the above composition most l ike ly represent middle greenschist facies equivalents of oversaturated calc-alkaline diabase and/or basalt. Amphibole phases are most l ike ly pseudomorphic 136 after pyroxene while chlorites represent pseudomorphs and alteration products after original amphibole and/or mica. Chi orite-biotite schist is characterized by porphyroblasts of brown biotite set in a fine grained assemblage of chlorite and muscovite-sericite which together with small layers of strained quartz, outline the prominant S 2 transpositional f o l i a t i o n . Accessory minerals include magnetite and sphene. Rocks of this composition within the central portion of the Antler contain chlorite altered biotite porphyroblasts (up to 3 mm) which contain r e l i c t S 2 internal schistosity that has been rotated up to 90° relative to the external S 3 f o l i a t i o n S 0/S 1=S 3) (Fig. 4-15). Thus peak metamorphism appears synkinematic to D2. Pressure shadows of fine grained strained quartz and muscovite-sericite flank biotite porphyrobl asts and are interpreted as having formed in response to flattening and shearing associated with D3. Chi orite-biotite schists adjacent to the Antier-phyllite contact contain chlorite-al tered biotite porphyrobl asts which have been shredded and drawn out along their respective cleavages within S 2 (Fig. 4-16). Relict elongate pressure shadows contain fine grained polygonized strain-free quartz which in places show optical continuity and have therefore recrystal 1 ized from once larger grains. Small layers and lenses of quartz not associated with pressure shadow development are characterized by small ribbon grains and aggregates of smaller polygonized nearly strain-free quartz, a l l showing optical continuity. 137 Pe l i t i c assemblages within the Antler consist mainly of small layers and lenses of fine grained micaceous quartzite and argil l i e phyllonite. Internally, layers of highly recrystallized-polygonized quartz are folded and often outline F 2 intrafo l ia l microfolds within S 2 (Fig. 4-17). Muscovite and sericite are aligned parallel to the fol iat ion and appear folded about F 2 . Small laths of pseudomorphic chlorite (after biotite) are similarly aligned within S 2 . Plagioclase of a lb i t i c composition ( A n 8 _ 1 5 ) occurs in minor amounts throughout this rock type with minor amounts of sphene, apatite, and carbonaceous material present as accessories. The presence of garnet was not detected anywhere throughout rocks of this composition in the Antler and based upon the quartz-muscovite-chlorite-albite assemblage, i t seems l ike ly that the metamorphic grade did not exceed the middle greenschist facies. Parent rock types for these pe l i t i c assemblages probably include s i l ic ious subgreywacke and argillaceous quartz sandstone. Relationships between mineral phases and the proposed deformation sequence are presented in Table 4-4. Rock types within the Antler, though varied in composition, outline a general metamorphic transition from the albite-epidote amphibolite facies (upper-greenschist) within rocks adjacent to the lower amphibolite grade Snowshoe to the middle-lower greenschist within structurally higher Antler l i thologies . Mineral assemblages also delineate a compositional change in the Antler from predominantly mafic-ultramafic (magnesian) rocks situated at the structural base of the exposed formation to a less mafic (F e -r ich) nature in central and 138 I I to w Figure 4-15a. Photomicrograph (plane-light) of a rotated biotite porphyroblast in an Antler Formation chlor i te-biot i te schist. Relict internal schistosity (S 0=S 2) has been rotated approximately 90° relative to the external S 3 fo l iat ion (S 2=S 3). 139 I WW Figure 4-15b. Cross-nicols view of 4-15a showing the highly recrystal-l ized nature of the groundmass (quartz, chlori te , muscovite-sericite). 140 Figure 4 -16. A sketch of a thin section of an Antler chlori te-biot i te schist. Porphyrobl asts of chlorite altered biotite have been shredded and drawn-out along their respective cleavages within the S 2 fo l iat ion. 141 I I I MM Figure 4-17a. Photomicrograph (p lane- l ight ) of folded root less compositional layers within a micaceous quartzite of the Antler Formation. Microfolds are of the F 2 generation and deform S 0 compositional layers composed of highly recrystal l ized quartz and muscovite-sericite. 143 upper stratigraphy. Microtextures indicate that peak metamorphism was synkinematic with D 2 and progressively waned during and after D 3 . Few post-deformational phases are present though post-deformational annealing is strongly developed in the lower portions of the Antler. The formation of mylonites is largely related to D 2 and is mostly restricted to narrow zones adjacent to the formation's tectonic contacts. Lower-most units of the Antler appear most strongly deformed and annealed and thus represent a zone in which high strain was accomodated during the convergence of Quesnellia on to the Omineca terrane. Further deformation (D 3) associated with the folding of this convergent zone, was accommodated in part along this tectonic boundary and in the formation of small widespread subsidiary mylonite zones related to F 3 fold geometry. This structural assemblage and succession of meta-igneous stratigraphy are similar in many respects to alpine-type ophiolite complexes observed in other orogenic belts . Interpretation of this nature would infer that the Antler Formation represents a s l ice of oceanic floor rock structurally emplaced (obducted) via low angle thrusting onto deformed continentally derived stratigraphy through the tectonic convergence of Omineca and Quesnellia terranes. Further discussion regarding this will be presented in the following chapter. 144 MINERAL PHASES 2 SYN POST SYN POST SYN POST SYN POST A N T L E R BIOTITE MUSCOVITE CHLORITE ACTINOLITE-TREMOLITE ACTINOLITE-HORNBLENDE CALCITE TALC ? ? TABLE 4-4 Relation of mineral growth to proposed deformation scheme 1n the Antler Formation, Mt. Perseus area, Crooked Lake, B.C. 145 Upper-Triassic Formation Un-named Upper-Triassic graphitic phyllites and psammitic schists comprise the structurally highest formation within the confines of the Mt. Perseus area. They appear mostly concordant in both contact and metamorphic fabric with underlying Antler Formation and Snowshoe Group rocks. The lowermost or basal unit of the Upper-Triassic assemblage consists of thin pe l i t i c layers of recrystal l ized quartzitic mylonite and graphitic phyllonite. Overlying pe l i t i c units are composed of calcareous graphitic schist and graphitic phyll i te which contain minor lenses of fine grained chlorite schist. Mylonitic versions of these rocks occur variably spaced in narrow shear zones related to D 3 folding. Mineral assemblages within these units are indicative of the lower greenschist facies. The primary metamorphic fabric observed within the phyll ites represents a transpositional fol iat ion oriented parallel to compositional layering, formed during phase-two regional deformation (equivalent to the f i r s t deformational phase within Antler and Upper-Triass ic rocks). Third phase deformation has produced an ubiquitious spaced cleavage related to disharmonic folds formed in both ductile and b r i t t l e environments. P h y l l i t i c rocks also contain numerous quartz-f i l led hydraulic fractures, now folded by F 2 and F 3 and probably represent features associated with i n i t i a l de-watering and pressure solution. Table 4-5 presents a summary of relevant stratigraphy, metamorphic rock types and typical mineral constituents. 146 STRATIGRAPHIC UNIT TYPICAL MINERALOGY PROTOLITH ROCK TYPE u p p E R T R I A S s I c F 0 R M A T I 0 N upper unit: graphitic phyllite muscovite-sericite + quartz + graphite + chlorite + plagioclase (albitlc) ± biotite + spinel argillaceous mudstone, fine grained sandstone chlorite schist greenstone chlorite + muscovite-sericite + quartz ± plagioclase + (An10_20) + sphene fine grained basalt middle unit: calcareous graphitic schist muscovite-sericite + calcite-dolomite + quartz + graphite + plagioclase (albltic) + biotite ± spinel calcareous mudstone lower unit: graphitic sericite schist muscovite-sericite + graphite + quartz + plagioclase (albltic) + calcite + biotite ± spinel argillaceous mudstone graphitic phyllonite muscovite + quartz + graphite + chlorite ± spinel argillaceous mudstone micaceous quartzite mylonite quartz + muscovite-sericite + calcite + chlorite t biotite ± opaques impure sandstone Table 4-5 147 Basal Quartzite Mylonite Golden tan to l ight grey fine grained micaceous quartzite mylonite and minor graphitic phyllonite form the lowermost layer within the Upper-Triassic formation. Quartz and fine grained muscovite-sericite outline the S 2 fo l ia t ion . Quartz occurs as highly strained individual grains exhibiting up to 100:1 e l l i p t i c a l elongation ratios parallel to the f o l i a t i o n ( F i g . 4-18). These ribbon grains and other porphyroclasts are commonly situated in a mass of more fine-grained essentially strain free quartz subgrains. Most subgrains exhibit a dimensional preferred orientation and therefore indicate that they are Muscovite-sericite occur as small laths and along with minor chlorite (after biotite) define S 2 . Fine grained laths of b iot i te , when not altered to chlorite are aligned primarily within S 2 though several samples contained large masses of biotite porphyroblasts which have overgrown muscovite within the S 3 cleavage plane. Plagioclase occurs as small irregular laths of A n 7 _ 1 5 composition. Poiki1i t ic K-feldspar and grains of calc i te are found as minor constituents throughout this rock type. Graphitic phyllonite is confined to small layers and lenses within the quartzite mylonite and is composed largely of quartz and muscovite/ ser ic i te with intercalations of carbonaceous material (Fig. 4-19). The 148 I I Figure 4-18a,b,c. Photomicrographs (plane-light) of basal micaceous quartzite of the Upper Triass ic assemblage. The prominent S 2 fol iat ion is mylonitic and is outlined by individual quartz layers and ribbon grains. Several ribbon grains have elongation ratios in excess of 100:1. Mylonitization processes were ini t iated during the lat ter stages of D 2 and continued during D 3 . Recrystall ization processes outlasted deformation. Figure 4-18c. I I mm 150 term phy l lon i t e is used in that th is rock type is large ly recrystal l ized and has a mylonite fabric related to D 2 similar to overlying phyl l i tes . Mylonitic fabrics within this unit indicate that strong ductile shearing was active during D 2 . This may indicate that the present stratigraphic position of the phyllites may in large be the result of tectonic emplacement onto Antler rocks via low angle thrusting synkinematic to D 2 . Mylonites may also infer that a stratigraphic contact existed between Antler and phy l l i t i c rocks prior to the tectonic convergence and was subjected to subsequent mylonitization during and after the convergence of the two terranes. Lower Unit Graphitic muscovite-sericite schist and minor a r g i l l i t e comprise the lower stratigraphic unit within the Upper Triassic formation. These rocks are typically dark grey to black and contain a well developed metamorphic fol iat ion subparallel in orientation to S 2 fo l iat ion attitudes present within underlying Antler rocks. Fine grained muscovite-sericite outline the S 2 fabric in the phyllites along with layers and lenses of graphitic material and fine grained pods of recrystal l ized quartz. Phase-3 microfolds (buckle folds) deform the muscovite-sericite, graphitic material and quartz lenses (Fig. 4-20), hence, primary metamorphic crystal l izat ion was active primarily during the latter stages of D 2 . Hydraulic fractures f i l l e d with quartz are 151 Figure 4 -19. Photomicrograph (x-nicols) of an Upper Triass ic graphitic phyllonite. The prominent fo l ia t ion , S 2 , is outlined by recrystal l ized quartz, muscovite-sericite, and graphite. 152 Figure 4-20a,b. Photomicrographs (plane-light) of typical graphitic p h y l l i t e s of the Upper T r i a s s i c assemblage. Compositional layers, which represent a transposed fo l ia t ion , are comprised of b iot i te , muscovite, s e r i c i t e , and g r a p h i t i c m a t e r i a l . Phase 3 crenulations deform S 2 layering and have produced a spaced cleavage in photo (b). The large white patches represent holes which were once f i l l e d by hematite p o r p h y r o b l a s t s , w h i c h have s i n c e b e e n re-precipitated. 153 often deformed by F 2 and F 3 and represent r e l i c t features associated with i n i t i a l de-watering and pressure solution phenomenon active during the early stages of deformation (D2) (Fig. 4-21). Middle Unit Rocks wihin the middle unit consist largely of mixed graphitic phyl l i te and carbonate. Carbonates (calcite , dolomite) occur as lenses and knots, remnants of boudinaged phase-2 isocl inal folds, intercalated within graphitic muscovite-sericite schist. Quartz, when present in layers appears strained and relatively fine grained though no appreciable grain boundary migration or subgrain development was detected. Muscovite-sericite define the S 2 fabric and are deformed by phase-3 open buckle-fold-microcrenulations. Several zones of t ightly folded (D3) phyllites revealed metamorphic textures in which biotite has crystal l ized along S 3 crenulation cleavage. These zones are analogous to transposition (shear) zones observed within Antler and Snowshoe rocks adjacent to their respective contact. The nature of transposition in these areas involved more ductile folding and has, local ly higher metamorphic grade, such that biot i te grew along third phase cleavage whereas metamorphism in rocks involved in more open phase-3 folding at this structural level did not support biotite growth. Figure 4-21a. Photomicrograph (plane-light) of graphite phyll i te which contains a deformed quartz f i l l e d hydraulic fracture. 155 156 Upper Uriit Upper-unit l ithologies within Upper-Triassic rocks are composed of graphitic phyll i te with intercalated layers of chlorite schist and greenstone lenses. Phyllites in this unit are similar in texture and mineralogy to underlying phyll i te with the exception of the presence of only very small amounts of calcareous material. Chlorite schists are composed of quartz, chlorite , muscovite and occasional porphyroblasts of brown biot i te . The primary fo l iat ion , S 2 , is outlined by and contains al l of these phases. Minor phases include magnetite and sphene. Finer grained rocks containing chlor i te-ser ic i te and quartz define massive lenses of greenstone. No recognizable fo l iat ion can be identif ied and i t is generally assumed that these "pods" may represent the stratigraphic incorporation of fine grained basaltic material, (most l ike ly associated with an arc environment) prior to the convergence of Quesnellia and Omineca terranes. Table 4-6 presents the relationship between the growth of mineral phases and the proposed deformation sequence. 157 MINERAL PHASES SYN | l POST SYN j 2 POST SYN | 3 POST SYN j1* POST U Tr BIOTITE P H Y L MUSCOVITE-SERICITE L I T E HEMATITE CHLORITE TABLE 4-6 Relation of mineral growth to proposed deformation scheme in the in the Upper Triassic Formation, Mt. Perseus area, Crooked Lake, B.C. 158 Microtextures: General Conclusions From the previous discussion of metamorphism and microtextures, one can infer that a Barrovian-type prograde metamorphic sequence exists within the Mt. Perseus area that ranges from the middle greenschist in structurally highest rocks to lower amphibolite at deeper structural levels. Snowshoe Group l i tho l ig ies are characterized by coexisting phases of garnet and biotite which indicate that metamorphism most l ike ly did not exceed the lower amphibolite facies (600°C) . Two generations of garnet growth have been re lated to phase-two and phase-three deformation and outline a continuous recrystal l izat ion sequence, the peak of which occurred sykinematically to D 2 . The presence of chlorite and post-kinematic biotite indicates that metamorphic activity within the Snowshoe Group was reduced to at least the upper greenschist after phase-three deformation and that metamorphism continued and outlasted deformation. Antler metavolcanic assemblages show a range in metamorphism from the epidote amphibolite (albite-epidote-hornblende facies, 500°C) to the middle greenschist facies. Structurally lowest rocks (mylonites), in contact with upper Snowshoe mylonites are typically magnesian, ultramafic and contain a mineralogy (antigorite-actinolite-carbonate; actinol ite-chlorite-talc-hornblende-epidote-albite) characteristic of 159 the transition between the upper-greenschist and epidote-amphibolite facies. Hence, an isograd based on the appearance-disappearance of these diagnostic phases exists within the lower units of the Antler Formation. Structurally higher lithologies appear less mafic more Fe enriched and contain a l b i t e , c h l o r i t e , epidote, c a l c i t e , and actinolite; phases more characteristic of the upper to middle greenschist facies (400°C). P e l i t i c compositions within the Antler contain albite, muscovite, and chlorite; minerals also diagnostic of the middle and upper greenschist facies. Microtextures indicate that peak metamorphic assemblages are a s s o c i a t e d with D 2 and recrystallization and annealing, during D3, was followed by further recrystal1ization which outlasted deformation. Lithologic compositions indicate that basal Antler stratigraphy was derived principally from mafic-ultramafic igneous rock types which grade s t r u c t u r a l l y upward into dominantly mafic meta-vol cam'c assemblages. This small cross-section is similar in many respects to ophiolite-type complexes associated with other convergent boundaries in the Cordillera. Upper-Triassic rocks reflect deep water anaerobic depositional environments and contain minerals diagnostic of the lower to middle greenschist facies. Minerals such as muscovite, quartz, and a l b i t i c plagioclase outline a transpositional metamorphic foliation formed as a result of phase 2 regional deformation. Metamorphic recrystal -1ization associated with phase 3 and later deformation (ductile and 160 bri t t l e ) is not widespread and is principally confined to narrow zones containing mylonites and ductile fold sets. Metamorphism within the Mt. Perseus area thus appears to represent a prograde sequence whose grade is a function of structural depth and proximity to the convergent boundary. The crystal l izat ion of peak metamorphic assemblages in a l l formations is related to phase 2 regional deformation and isograds delineating the f i r s t appearance of garnet parallel the tectonic boundary. Recrystallization associated with and subsequent to phase 3 folding is widespread throughout the Snowshoe but is restricted to narrow mylonite zones in Antler and Upper Triassic l i thologies . The timing of metamorphism can only be inferred as having been in i t iated in the post-Upper-Triassic and probably continued throughout the mid-Mesozoic in association with the Columbian Orogeny. 161 SUMMARY AND DISCUSSION Structural Conclusions The central conclusions arising from this study are concerned with a deta i led descr ipt ion of the s tructura l r e l a t i o n across the Intermontane (Quesnellia)-Omineca Belt boundary and in the presentation of a coherent picture of the tectonic evolution of part of the Quesnel Lake region. The Intermontane (Quesnellia)-Omineca boundary within the Mt. Perseus area is contained within an overturned fold limb associated with a series of regional northwesterly shallow plunging antiformal folds cored by metasediments of the westernmost extention of the Omineca Belt. These metasediments comprise members of the Snowshoe Group of rocks whose age may range from Hadrynian through lower Paleozoic (Campbell, 1978; Struik, 1984). Immediately structurally overlying the Snowshoe Group is an exposure of basic meta-volcanic and ultramafic rocks that may be the southern equivalents of the Slide Mountain Group composed of basic volcanics and chert of late Paleozoic age (Struik, 1982). This volcanic sequence is in turn structurally overlain by black phyllites of uncertain Upper Triassic age. 162 Each of these major groups i s separated from i t s neighbor by a we l l -de f ined f a u l t zone that i s usual ly out l ined by zones of myloni t ic rocks varying in width of up to 1 ki lometer . The Snowshoe Group i s thus considered to be basement to the over ly ing cover of younger la te Paleozoic and early Mesozoic volcanic and sedimentary rocks. Snowshoe Group metasediments within the f i e l d area cons is t la rge ly of m e t a - p e l i t i c rocks that envelope a layer of quartzo - fe ldspath ic orthogneiss. This g r a n i t i c gneiss may be re lated to the Quesnel Gniess which has y ie lded z i rcons of mid-Paleozoic age (Montgomery, 1985). A l l rock types have been var iably metamorphosed throughout the lower amphibolite fac ies and belong in part to the northernmost extremity of the Shuswap Complex. A sequence of 5 deformational phases has been establ ished wi th in the Snowshoe. The e a r l i e s t deformation phase is unique to the Snowshoe and consists of mesoscopic i s o c l i n a l fo lds that are commonly roo t less , east -verg ing , and contain mineral l i neat ions p a r a l l e l with t h e i r hinge l i n e s . A well developed axial f o l a t i o n , out l ined in part by compositional layer ing has not as yet been re lated to any large scale regional s t ruc tu re . Thus, the e a r l i e s t surface that - out l ines any recognizable regional structure i s a transposed f o l i a t i o n of regional nature. The e a r l i e s t deformation common to a l l formations in the study area, phase 2 deformation, i s recognizable on a l l scales and i s 163 associated with the widespread ductile behaviour in al l rock types. This deformation is probably the main phase of penetrative deformation everywhere recognized in Snowshoe Group rocks and associated gneisses and is responsible in the formation of a large east-verging synform, later deformed into the Perseus antiform. Pronounced metamorphic foliaton dips variably to the east and is axial planar to major and minor folds. Fold axes are curvilinear and are outlined by strongly developed mineral lineations throughout the area. Minor folds show consistent easterly vergence related to a major synformal closure outlined by the Perseus gneiss. Associated with this penetrative deformation is a metamorphism, whose mineral assemblages in the Snowshoe are characteristic of the lower amphibolite and epidote-amphibolite facies. Microtextures indicate that this metamorphism peaked and outlasted the second phase of deformation and began to wane well into the third phase. Nowhere within the f ie ld areas were phase-2 folds seen to deform the Intermontane (Quesnellia)-Omineca boundary. Thus, the formation of th i s boundary is re lated to the formation of large scale easterly-verging folds within the Snowshoe. Third phase deformation consists of meso-macroscopic folds having westerly vergence with an associated well developed axial planar fol iat ion which dips steeply to the northeast. Adjacent to the Intermontane (Quesnellia)-Omineca boundary phase 3 geometry consists of 164 large rounded antiformal closures separated by highly attenuated synforms into which overlying cover rocks have been drawn. Ductile shear zones of variable width and high flattening strain are located at and extend below the synform closures. Larger scale northwest plunging third phase folds define and control the present configuration of the convergent boundary. The Perseus antiform, an example of a large scale basement fold structure is dome-like in outcrop and overturned to the northeast. Phase 2 easterly verging folds are refolded by these almost coaxial westerly verging third folds and their associated fo l ia t ion . It is the superposition of this third phase stage on the pre-existing phase 2 folds that give rise to the curvilinear nature of these phase 2 axial structures and elongate dome geometry of the Perseus antiform. Phase 3 s l ip directions, determined from the locus of distorted phase 2 l inear features are seen to make a high angle with the third axial direction, almost parallel with the dip direction of i ts associated axial fo l ia t ion . Phase 4 deformation is characterized by. non-penetrative east verging folds and crenulations whose axial cleavages dip gently to the southwest across all previously developed surfaces. This deformation is particularly well developed with the core regions of large phase 3 structures and within cover rocks adjacent to the Intermontane-Omineca boundary. 165 Phase 5 deformation is represented by non-penetrative small scale vertical buckle folds whose axial surfaces dip steeply to the northwest. Folds of this generation are open and mildly deform al l existing structures throughout the study area. Thus, the Snowshoe Group, basement to the late-Paleozoic-early Mesozoic cover displays a deformation sequence whose vergence changes in direction with time, f i r s t l y and secondly to the northeast, then to the southwest and f ina l ly to the northeast again. Structurally above and in tectonic contact with Snowshoe Group rocks is a highly deformed package of meta-mafic-ultramafic rocks of volcanic af f in i ty believed to be of late Paleozoic age (Antler Formation). It has a variable thickness (150-800 metres) and has a well developed mylonitic lamination that is parallel with gross compositional layering which contain discrete rootless isocl inal folds. Metamorphism in these rocks varies between the middle and upper greenschist to the epidote-amphibolite facies. Immediately above and in tectonic contact with Antler rocks is a Mesozoic package of mixed sedimentary and basic volcanic rocks informally designated as Upper-Triass ic Black Phyl l i tes . This unit, metamorphosed within the middle greenschist facies, shows no gross structural inversion and near to i ts lower contact, the phyl l i te has a transposed fo l i a t ion , parallel with the transposed fo l ia t ion within the underlying volcanic assemblage. Both of these earl iest recognizable axial fol iat ions are parallel with the phase 2 axial fo l ia t ion recognized within Snowshoe basement rocks. 166 Approximately 200 metres above the phyll ite-volcanic contact, the degree of transposition becomes less so, and isocl inal folds gradually develop measurable angles between their limbs. Whereever present, these earliest cover folds - open or transposed - exhibit a north-easterly vergence. The earl iest recognizable cover folds, as described above, are refolded on al l scales by a westerly verging second fold set characterized by disharmonic geometry and associated small scale ductile shear zones. A well developed axial fo l iat ion parallel to third phase fol iat ion within the Snowshoe Group is characterized in large by a spaced crenulation cleavage. It is this fold set that controls the regional map pattern and gives the Intermontane (Quesnell ia)-0mineca convergent zone i t s present regional configuration. The last significant common deformation geometry consists of a mesoscopic crenulation and crenulation cleavage developed across a l l ear l ier surfaces. These small scale folds show consistent vergence to the northeast and become less well developed at higher structural levels within the cover rocks. 167 Regional Implications and Tectonic Conclusions It is apparent that all three major groups of rocks, Snowshoe Group, the Antler Formation and the Upper Triass ic Black Phyllites have common phases of deformation with the Snowshoe Group having an extra phase of deformation not present within the other two. Prior to phase 1 deformation within the Snowshoe Group, presumably within the early to mid-Paleozoic, cratonic sediments of pe l i t i c composition belonging to the leading edge of a westward prograding continental margin terrace wedge (Monger and Price, 1979; Monger et a l . , 1982) were intruded by large s i l l s of quartz diorite and quartz monzonite porphyry of uncertain af f ini ty . Zircons within the Quesnel Gneiss have placed the date of intrusion within the mid-Paleozoic (Montgomery, 1985). Phase 1 folding may be associated with mild deformation during the mid-Paleozoic Caribooan orogeny in which pe l i t i c rocks of the Snowshoe Group underwent i n i t i a l dewatering accompanied by pressure solution. Incipient widespread mesoscopic folding developed in both pelites and igneous instrusions which at the onslaught of later deformation associated with tectonic convergence became progressively flattened, transposed, and subsequently metamorphosed. As mentioned above, al l the major rock groups share common phases 168 of deformation, however, the conditions wherein the common phases are are accomplished are different for the Snowshoe Group (basement) and upper structural levels within the Antler and Upper-Triassic Formations (cover). At this time of the earl iest common phase (phase 2 within the Snowshoe Group) prograde metamorphism is synkinematic with the Snowshoe, peaking at the lower amphibolite facies adjacent to the convergent boundary and middle amphibolite at deeper structural levels. Metamorphism in the Snowshoe began to wane during phase-3, whereas the cover deformation during this earl iest time is accomplished through i n i t i a l dewatering accompanied by pressure solution and folding of the mixed cover assemblage. The junction between the cover and basement during this deformation episode is i t se l f not deformed but is outlined by mylonites. Thus one can infer that the cover assemblage was progressively thrust via low angle east-directed transport onto continental basement rocks. This thrusting occurred within a ductile regime and resulted in the formation of narrow mylonite zones adjacent to exisit ing contacts between the respective cover l i thologies . The junction between basement and cover becomes deformed during the second common phase of deformation and due to the extreme duct i l i ty contrast present during tectonic shortening, less competent cover rocks are drawn down into and occupy the cores of highly attenuated synforms between more open antiformal closures of the more competent basement. Below these nearly transposed synforms, basement shear zones contain isolated sl ivers of cover meta-volcanics. The following concept of the tectonic evolution of the Mt. Perseus 169 area and surrounding Quesnel Lake region is synthesized in combining the above geometry together with vergence directions of the different deformation phases to give one possible model. The f i r s t common phase of deformation within the basement and cover involves the development of easterly verging folds without any vis ible shortening of the basement-cover boundary. From this i t can be inferred that these two packages of rocks had l ikely undergone an i n i t i a l phase of convergence (Fig. 5-1). This convergence, involving the emplacement of a la te -Pa leozo ic maf ic-ul tramafic volcanic assemblage (ophiolite) together with early Mesozoic sediments and volcanics was l ike ly an obduction process whereby continental Hadrynain rocks were subducted westwards below the easter ly converging accretionary package. The association of deep water (anaerobic) argil 1ic sediments with underlying mafic volcanics and ultramafics most l ike ly infer that a subsiding marginal back-arc- type basin existed west of the continental margin prior to the mid- Mesozoic (Monger and Price, 1979). Subduction of continental rocks active during the mid-Mesozoic (Columbian Orogeny) resulted in the closing of this marginal basin and later obduction and accretion of sedimentary-volcanic assemblages of oceanic af f in i ty . A change in transport direction is inferred from the next common phase of deformation, wherein westerly verging folds are developed throughout both basement and cover rocks and the basement-cover 170 Figure 5-1. Schematic cross-section across the Quesnellia-Omineca boundary during the time of i n i t i a l convergence (obduction). 171 junction becomes markedly shortened. These westerly verging folds, shortening of the junction, and east-dipping shear zones within the basement and lower cover appear to indicate a reversal of the direction of subduction (Fig. 5-2). The development of the late crenulation cleavage is l i k e l y a consequence of late eastward thrusting of early Jurassic marine volcanics (outside of the f i e ld area) whichoverlie, with decollement, the Upper-Triassic phy l l i t e s . The tectonic model described above is but one interpretation of the data, is simple and involves only a change in transport direction. However, regional tectonic models proposed by Monger et. a l . (1982) involves not only convergence but also large scale lateral translation associated with the accretion process. In any convergent model the shortening direction observed in rocks is not directly related to the direction of convergence, rather i t is perpendicular to the zone of convergence even when the process is oblique. Often the component of convergence parallel with the zone is evidenced by large scale s t r ike - s l ip displacement in the arc region behind the subduction zone. No such s t r ike - s l ip displacement has been recognized within the region under discussion and no evidence of lateral regional extension parallel with the zone of convergence has been seen. Thus, i f large scale translat ion is involved in the accretion processes then the evidence for same has been destroyed during the convergence process or translation occurred before convergence began. 172 Figure 5-2. Schematic cross-sections across the Quesnellia-Omineca boundary after i n i t i a l convergence. A reversal in subduction direction produced a-shortening of the terrane boundary into the present day map configuration. 173 BIBLIOGRAPHY Archibald, D.A. , Glover, J . K . , Price, R .A. , Farrar, E. and Carmichael, D.M. , 1983. 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Journal of Petrology, 2, pp. 277-311. Monger, J.W.H. and Price, R .A. , 1979. Geodynamic evolution of the Canadian Cordil lera progress and problems. Canadian Journal of Earth Sciences, 16, pp. 770-791. Monger, J .W.H. , Price, R.A. and Tempelman-Kluit, D. , 1982. Tectonic accretion and the origin of two major metamorphic and plutonic welts .in the Canadian Cordi l lera . Geology, 10, pp. 70-75. Montgomery, J . R . , 1985. Structural relations of the southern Quesnel Lake Gneiss, Isoceles Mountain area, central Brit ish Columbia. M.Sc. thesis, University of Brit ish Columbia, Vancouver, Brit ish Columbia. Montgomery, S . L . , 1978, Structural and metamorphic history of the Dunford Lake map area, Cariboo Mountains, Brit ish Columbia. M.S. thesis, Cornell Unviersity, Ithaca, New York. Morton, R . L . , 1976. Alkalic volcanism and copper deposits of the Horsefly area, central Bri t i sh Columbia; unpublished Ph.D. thesis, Carleton University, 196 p. Murphy, D.C and Rees, C . J . , 1983. Structural transition and stratigraphy in the Cariboo Mountains, Brit ish Columbia. In Current Research, Part A . , Geological Survey of Canada, Paper 83-1A, pp. 245-252. Okulitch, A . V . , 1984. The role of the Shuswap Metamorphic Complex in Cordilleran tectonism. Canadian Journal of Earth Sciences, 21, pp. 1171-1193. Pigage, L . C . , 1977. Rb-Sr dats for granodiorite intrusions of the northeast margin of the Shuswap Metamorphic Complex, Cariboo Mountains, Brit ish Columbia. Canadian Journal of Earth Sciences, 14, pp. 1690-1695. Pigage, L . C . , 1978. Metamorphism and deformation on the northeast margin of the Shuswap Metamorphic Complex, Azure Lake, Bri t i sh 176 Columbia. Ph.D. thesis, University of Brit ish Columbia, Vancouver, Brit ish Columbia, 289 pages. Pigage, L . C . and Greenwood, H . J . , 1982. Internally consistent estimates of pressure and temperature: the staurolite problem. American Journal of Science, 282, pp. 943-969. Price, R.A. and Douglas, R.J.VI., 1972. 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