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U-pb and K-Ar geochronometry of the coast plutonic complex, 53°N to 54° N, British Columbia, and implications.. van der Heyden, Peter 1989

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U-Pb AND K-Ar GEOCHRONOMETRY OF THE COAST PLUTONIC COMPLEX, 53°N TO 54°N, BRITISH COLUMBIA, AND IMPLICATIONS FOR THE INSULAR-INTERMONTANE SUPERTERRANE BOUNDARY by PETER VAN DER HEYDEN B.Sc, University of Leiden, Netherlands, 19 77 M.Sc, University of Br i t i s h Columbia, 19 82 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF BRITISH COLUMBIA We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 1989 (8) Peter van der Heyden> 19 89 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 of %60tOQi\ZhL £c\&JCE& The University of British Columbia Vancouver, Canada DE-6 (2/88) i i ABSTRACT This study presents 48 new U-Pb and 35 new K-Ar dates for magmatic rocks of the Coast Plutonic Complex (CPC) between 53°N and 54°N. The eastern flank of the CPC is underlain by the Gamsby Complex, an early Late J u r a s s i c (160-155 Ma) magmatic, metamorphic and ductile compressional belt superimposed on Ear l y (197-190 Ma) and Middle (178 Ma) J u r a s s i c magmatic rocks of the Intermontane superterrane. The Gamsby Complex was uplifted and cooled in l a t e s t J u r a s s i c - E a r l y Cretaceous time (145-132 Ma). Small Early Cretaceous intrusions (132- 120 Ma) in the Gamsby Complex are c l e a r l y post-kinematic, and no evidence was found f o r a major middle Cretaceous orogenic episode in the eastern flank of the CPC. The Central Gneiss Complex in the core of the CPC contains large Late Cretaceous (80 Ma) and Eocene plutons that are commonly s i l l - l i k e . The Central Gneiss Complex gives Eocene K-Ar dates, and is proposed to be an Eocene (51 Ma) metamorphic core complex, which underlies the eastern flank of the CPC beneath a gently to moderately dipping ductile extenslonal shear zone; the tectonic boundary with the western flank of the CPC is gently to steeply dipping. B i o t i t e s from positions s t r u c t u r a l l y low in the Gamsby Complex give K-Ar dates (51- 50 Ma) which appear to r e f l e c t r e s e t t i n g above a rapidly rising hot Central Gneiss Complex. The Eocene extenslonal boundary between the Central Gneiss Complex and the Gamsby Complex was disrupted by steep b r i t t l e f a u l t s of the Sandifer Lake fau l t zone, which localize Oligocene (33 Ma) lamprophyre dyke swarms. I l l The west flank o£ the CPC contains three major tectonic belts, from west to east: 1) The Banks Island belt, composed of large, 160-155 Ma pre-, syn-, and post-kinematic plutons intrusive Into s t r a t a of the Insular superterrane; this belt cooled in l a t e s t Late J u r a s s i c time (148-143 Ma). 2) The McCauley Island belt, containing Early Cretaceous (131-123 Ma) plutons which cooled in middle Cretaceous time (109-97 Ma). 3) The E c s t a l l belt, representing a major middle Cretaceous (110-94 Ma) magmatic pulse; this belt cooled by Late Cretaceous time (67 Ma). The new geochronometric r e s u l t s are integrated into a regional tectonic model in which a l l Jurassic-Eocene components of the CPC and adjacent belts are related to changing patterns of east-dipping subduction beneath a single allochthonous Alexander-Wrangellia-Stikinia (AWS) megaterrane, which was emplaced against the western margin of North America in Middle J u r a s s i c time, closing off the Cache Creek- Bridge River ocean. Following accretion of the AWS megaterrane an early Late J u r a s s i c magmatic arc and associated s t r u c t u r e s were built mainly along the new outboard margin of North America, but they also overprinted the Cache Creek suture and eastern terranes, including rocks of North American a f f i n i t y . Early in i t s h i s t o r y the Washington to Alaska c e n t r a l part of the Late J u r a s s i c arc was r i f t e d (and perhaps displaced longitudinally as well) causing the formation of i n t r a - a r c flysch basins and in e f f e c t breaking up the AWS megaterrane into two fragments corresponding to the Insular and Intermontane superterranes. These were subsequently overprinted by magmatic belts that define the present CPC. A major middle Cretaceous magmatic pulse i v was accompanied by widespread c r u s t a l shortening across the CPC, and Eocene magmatism was accompanied by c r u s t a l extension, which may have continued into late T e r t i a r y time. Successive episodes are presumably related to changes in r e l a t i v e plate motions and v e l o c i t i e s between the North American continent and Pacific oceanic lithosphere, within a primary setting of long-lived, obliquely east-dipping subduction beneath a single terrane. This i n t r a - t e r r a n e model for the evolution of the CPC d i f f e r s from the superterrane c o l l i s i o n model of Monger et a l . (1982), and is compatible with the scenario suggested by Brew and Ford (1983), who f i r s t suggested that the CPC may be an intra-terrane feature. There are no sutures in the CPC, and the CPC is not the r e s u l t of a c o l l i s i o n between discrete superterranes. V TABLE OF CONTENTS ABSTRACT 11 TABLE OF CONTENTS V LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS X CHAPTER 1: INTRODUCTION 1 CHAPTER 2: U-PB AND K-AR GEOCHRONOMETRY OF THE COAST PLUTONIC COMPLEX, WHITESAIL LAKE AREA, BRITISH COLUMBIA .. 11 I n t r o d u c t i o n and g e o l o g i c a l s e t t i n g 11 Present work 25 A n a l y t i c a l techniques 26 A n a l y t i c a l r e s u l t s and i n t e r p r e t a t i o n s 28 Gamsby Complex U-Pb dates 29 C e n t r a l Gneiss Complex U-Pb dates 52 Intermontane B e l t U-Pb dates 56 Gamsby Complex K-Ar dates 64 C e n t r a l Gneiss Complex K-Ar dates 70 Intermontane B e l t K-Ar dates 71 D i s c u s s i o n 71 Late T r i a s s i c - M i d d l e J u r a s s i c , pre-kinematic u n i t s ... 71 Late J u r a s s i c , syn- to l a t e - k i n e m a t i c u n i t s 76 E a r l y Cretaceous, p o s t - k i n e m a t i c u n i t s 78 Middle and Late Cretaceous u n i t s 80 The Gamsby t h r u s t and a s s o c i a t e d s t r u c t u r e s 82 E a r l y T e r t i a r y u n i t s 83 Con c l u s i o n s 93 CHAPTER 3: U-PB AND K-AR GEOCHRONOMETRY OF THE COAST PLUTONIC COMPLEX, DOUGLAS CHANNEL AND HECATE STRAIT AREAS, BRITISH COLUMBIA 98 I n t r o d u c t i o n 98 G e o l o g i c a l s e t t i n g 99 Present work 115 A n a l y t i c a l techniques 116 A n a l y t i c a l r e s u l t s and i n t e r p r e t a t i o n s 116 U-Pb dates of o l d e r p l u t o n s 117 U-Pb dates f o r the Banks I s l a n d B e l t : Late J u r a s s i c p l u t o n s 124 U-Pb dates f o r the McCauley I s l a n d B e l t : E a r l y Cretaceous p l u t o n s 135 U-Pb dates f o r the E c s t a l l B e l t : middle Cretaceous p l u t o n s 149 U-Pb date f o r the Quottoon p l u t o n , C e n t r a l Gneiss Complex 158 Hornblende and b i o t i t e K-Ar dates 160 Whole-rock K-Ar date: E a r l y Miocene b a s a l t i n the west f l a n k of the CPC ; 165 D i s c u s s i o n 165 Older p l u t o n s 165 Banks I s l a n d B e l t 169 McCauley I s l a n d B e l t 170 v i E c s t a l l B e l t 172 C e n t r a l Gneiss Complex 173 T e r t i a r y b a s a l t s on the west s i d e o£ the CPC 177 Co n c l u s i o n s 178 CHAPTER 4: CORDILLERAN SETTING AND TECTONIC EVOLUTION OF THE COAST PLUTONIC COMPLEX AND THE INSULAR-INTERMONTANE SUPERTERRANE BOUNDARY, BRITISH COLUMBIA 181 I n t r o d u c t i o n 181 Terrane s e t t i n g 183 E a r l y Cretaceous 186 Middle Cretaceous 187 Late Cretaceous 193 D i s c u s s i o n of Cretaceous f e a t u r e s 195 E a r l y T e r t i a r y 201 Late J u r a s s i c .. 207 Middle J u r a s s i c and o l d e r p l u t o n s 212 Late J u r a s s i c c o n f i g u r a t i o n of the western Canadian C o r d i l l e r a 216 A model f o r the e v o l u t i o n of the CPC 221 C o n c l u s i o n 231 REFERENCES 233 APPENDIX 1: SAMPLE LOCATIONS AND DESCRIPTIONS 254 W h i t e s a i l Lake area 254 Douglas.Channel-Hecate S t r a i t area • 261 APPENDIX 2: U-PB PROCEDURES OF THE GEOCHRONOMETRY LABORATORY, UNIVERSITY OF BRITISH COLUMBIA 269 APPENDIX 3: PREPARATION, CALIBRATION, AND GUIDELINES FOR THE USE OF THE * » P B - 2 3 S U - 2 3 3 U MIXED SPIKE AND ASSOCIATED MIXED REFERENCE STANDARDS IN THE UBC GEOCHRONOMETRY LABORATORY 332 APPENDIX 4: STANDARDS/ BLANKS, AND ANALYTICAL QUALITY IN THE UBC U-PB GEOCHRONOMETRY LABORATORY, 1983-1988 ... 354 APPENDIX 5: "UBCUPB" HP-85 COMPUTER PROGRAM FOR CALCULATING PB/U AND PB/PB DATES 386 APPENDIX 6 ( i n bound l i b r a r y copy o n l y ) : U-PB AND K-AR DATA SHEETS 393 v i i TABLE 1: U-Pb A n a l y t i c a l Data, W h i t e s a i l Lake area . ... 48 TABLE 2: K-Ar A n a l y t i c a l Data, W h i t e s a i l Lake area 69 TABLE 3: U-Pb A n a l y t i c a l Data, Douglas-Channel-Hecate S t r a i t Area 139 TABLE 4: K-Ar A n a l y t i c a l Data, Douglas-Channel-Hecate S t r a i t Area 164 v i i i L I S T OF FIGURES Figure 1: Early tectonic model for the Coast Plutonic Complex 2 Figure 2: Inter-terrane c o l l i s i o n model for the Coast Plutonic Complex .... 3 Figure 3: Location of study area 7 Figure 4: Major subdivisions of the Whitesall Lake area 12 Figure 5: Geology and sample locations, western whitesall Lake area 13 Figure 6: Gently dipping Eocene tonalite s i l l s , Central Gneiss Complex ... 16 Figure 7: Gently dipping paragneiss capping the Tsaytis pluton, Central Gneiss Complex 17 Figure 8: Northeast dipping contact of the Tsaytis pluton with overlying paragneiss, Central Gneiss Complex 18 Figure 9: Geology of the upper Tsaytis River area 19 Figure 10: View of the Coast Plutonic Complex from the Intermontane Belt, upper Tsaytis River area 20 Figure 11: U-Pb and K-Ar dates, western Whitesall Lake area 31 Figure 12: Foliated metavolcanic rocks of the Gamsby Complex 32 Figure 13: Concordia diagram, f e l s i c metavolcanic rocks, Gamsby Complex ... 33 Figure 14: Quartz diorite dyke intruding diorite, Black Dome pluton, Gamsby Complex 35 Figure 15: Concordia diagram, Black Dome pluton, Gamsby Complex 37 Figure 16: Concordia diagram, pre-kinematic intrusions, Gamsby Complex .... 39 Figure 17: Syn-kinematic quartz diorite intrusion, Gamsby Complex 42 Figure 18: Concordia diagram, syn-kinematic intrusions, Gamsby Complex 43 Figure 19: Concordia diagram, syn-kinematic intrusion in Gamsby Complex gneiss 45 Figure 20: Concordia diagram, Whitecone pluton, Gamsby Complex 46 Figure 21: Concordia diagram, post-kinematic intrusions, Gamsby Complex ... 51 Figure 22: Leucogranite intruding orthogneiss, Gamsby Complex 53 Figure 23: Concordia diagram, plutons in the Central Gneiss Complex 55 Figure 24: Concordia diagram, plutons of the Intermontane Belt, upper Tsaytis River area 57 Figure 25: Concordia diagram, Trapper pluton, Intermontane Belt 60 Figure 26: Concordia diagram, volcanic rocks, Intermontane Belt 61 Figure 27: Concordia diagram, porphyritic granite dyke, Intermontane Belt 63 Figure 28a & b: Post-kinematic pegmatite intruding metasomatized gneiss, Gamsby Complex 68 Figure 29: Foliated quartz diorite, Tahtsa Complex 75 Figure 30: Cross-section illustrating hypothetical structures across the Central Gneiss Complex-Gamsby Complex-Intermontane Belt boundaries 87 Figure 31: K-Ar dates and elevations of biotites, Gamsby Complex and Central Gneiss Complex 88 Figure 32: Section along the Kemano fault 89 Figure 33: Subdivisions of the Coast Plutonic Complex, Douglas Channel- Hecate Strait area 102 Figure 34: Geology and sample locations, Douglas Channel- Hecate Strait area 103 Figure 34a: Sections across the Coast Plutonic Complex, Douglas Channel- Hecate Strait area 105 Figure 34b: Plutonic rocks and sample locations, west-central Banks Island 106 Figure 35: Deformed banded quartzite and marble, Banks Island 109 Figure 36: U-Pb and K-Ar dates, Douglas Channel- Hecate Strait area 118 ix Figure 37: Concordia diagram, 85V-17 120 Figure 38: Concordia diagram, 78WV-Dala 122 Figure 39: Concordia diagram, 85V-16 123 Figure 40: Concordia diagram, sample 86V-1 125 Figure 41: View of western Banks Island 126 Figure 42: Concordia diagram, sample 86V-2 127 Figure 43: Concordia diagram, sample 87-YGTL 130 Figure 44: Concordia diagram, sample DY-3065 131 Figure 45: Concordia diagram, sample 86V-3 132 Figure 46: Concordia diagram, sample 85V-27 133 Figure 47: Concordia diagram, samples 85V-28-1 and 85V-28-2 136 Figure 48: Concordia diagram, sample 85V-29 137 Figure 49: Concordia diagram, sample 85V-33 138 Figure 50: Concordia diagram, sample 86-YGTL 145 Figure 51: Concordia diagram, sample 85V-10 146 Figure 52: Concordia diagram, sample 85V-13 147 Figure 53: Concordia diagram, sample 87V-12-2 148 Figure 54: Concordia diagram, sample 85V-15 153 Figure 55: Concordia diagram, sample 85V-18 154 Figure 56: Concordia diagram, samples 85V-23 and 85V-30 155 Figure 57: Concordia diagram, sample 85V-19 156 Figure 58: Concordia diagram, sample 85V-21 157 Figure 59: Concordia diagram, sample 85V-24 159 Figure 60: Plutons, metamorphic rocks and major structures, 53°-55°N 182 Figure 61: Terrane a f f i n i t y of plutons and supracrustal rocks, 53°-55°N . 184 Figure 62: Early Cretaceous features, 53°-55°N 188 Figure 63: Middle Cretaceous features, 53°-55°N 189 Figure 64: Biotite K-Ar cooling dates, 53°-55°N 192 Figure 65: Late Cretaceous-Paleocene features, 53°-55°N 194 Figure 66: Early Cretaceous magmatic rocks, Canadian Cordillera 199 Figure 67: Cretaceous plutons and structures, Canadian Cordillera 200 Figure 68: Early Tertiary features, 53°-55°N 202 Figure 69: Early Tertiary plutons and Eocene rest terranes, Canadian Cordillera 204 Figure 70: Late Jurassic features, 53°N-55°N 208 Figure 71: Middle Jurassic and older plutons, 53°-55°N 213 Figure 72: Late Jurassic features, Canadian Cordillera 217 Figure 73: Late Middle Jurassic features of the Canadian Cordillera 223 Figure 74: Early Late Jurassic features of the Canadian Cordillera 228 Figure 75: Late Late Jurassic features of the Canadian Cordillera 230 IN APPENDIX 4: Figure 76: Pb content of 2B IN HBr 359 Figure 77: Pb content of 2B 6.2N HC1 359 Figure 78: Pb content of 2B H20 360 Figure 79: Pb content of 2B 8N HNOa 360 Figure 80: Pb content of 2B 3.IN HCl 361 Figure 81a: Pb total procedural blanks, Nov.'81-Feb.'84 365 Figure 81b: Pb total procedural blanks, Feb.'84-Jun.'88 365 Figure 82: Pb total procedural blank, isotope composition 366 Figure 83: Concordia diagram for GSC-TMH zircon solution, Faraday measurements 376 Figure 84: Concordia diagram for GSC-TMH zircon solution, Daly measurements 377 X ACKNQWL^PGEJMSNTS My work on the geology of the c e n t r a l Coast Plutonic Complex over the past 10 years has benefitted immensely from the help, care and i n t e r e s t of G.J. Woodsworth of the Geological Survey of Canada, who f i r s t introduced me to the areas discussed in this thesis. I thank him and my thesis supervisor R.L. Armstrong for their generous support and supervision of this project and f o r t h e i r reviews of e a r l i e r versions of this manuscript. My i n t e r e s t in the Coast Plutonic Complex and the tectonics of the Insular-Intermontane boundary has been stimulated by discussions with M.L. Hill, R. Friedman, C. Greig, D. Murphy, M. Journeay, R. Haugerud, G. Gehrels, L. Hollister, D. Brew, and p a r t i c u l a r l y J. Monger, who's ideas stimulated the concluding tectonic synthesis. R. Parrish Is thanked for introducing me to the promises and p i t f a l l s of U-Pb geochronology and f o r much technical advice along the way, R. Kelly for f i e l d assistance, D. Woodsworth for his excellent navigation s k i l l s , J. Beekman for help in the lab, and D. Runkle and J. Harakal f o r doing the K and Ar analyses, respectively. I thank M. McLaren, J. Shearer and S. Crawford, formerly of Trader Resources Ltd., for t h e i r h o s p i t a l i t y and permission to do work in the Yellow Giant gold camp during the 1986 and 1987 f i e l d season. D. Lefebure of the B.C. Ministry of Energy, Mines and Petroleum Resources provided l o g i s t i c a l support and f i e l d assistance during the 1986 f i e l d season. F. Letz of Oona River provided occasional lodging, and R. Gamble and members of the Kitkatla Indian band generously helped with transportation of outboard f u e l in a remote area. L o g i s t i c a l f i e l d support was provided by the Geological Survey of Canada, the B r i t i s h Columbia Ministry of Energy, Mines and Petroleum Resources, and TRM Engineering Ltd.. Field c o s t s and the laboratory part of this study were funded by NSERC grant A-8841 to R.L. Armstrong. 1 CHAPTER 1; INTRODUCTION The Coast Plutonic Complex (CPC) Is one of five major geological belts of the Canadian Cordillera and has long been recognized as one of the worlds larg e s t batholiths (e.g. McConnell, 1913). In the f i r s t decade of plate tectonic theory the CPC was interpreted as the deeply eroded root of a major Cretaceous and T e r t i a r y magmatic arc (Fig. 1), s i t u a t e d above an east-dipping subduction zone, with the associated subduction complex represented by the Chugach terrane and the Pacific Rim Complex (Monger et al., 1972; Muller, 1977; Monger and Price, 1979). More recently, the origin of the CPC and i t s s t r u c t u r a l extension in the North Cascades has been explained as the r e s u l t of Mesozoic interactions between discrete tectonostratigraphic terranes (e.g. Godwin, 1975; Davis et al., 1978; Monger and Price, 1979; Monger et al., 1982; Monger, 1984; Price et al., 1985). These departures from the Andean-Sierran subduction model view the CPC as a metamorphic and s t r u c t u r a l welt that formed in middle Cretaceous time 1 as a r e s u l t of col l i s i o n of a f a r - t r a v e l l e d allochthonous terrane, here r e f e r r e d to as the Insular superterrane, to the Intermontane superterrane, which was by then attached t o North America (Fig. 2). This model f o r the evolution of the CPC is here r e f e r r e d to as the in t e r - t e r r a n e c o l l i s i o n model. Implicit in th i s c o l l i s i o n model and i t s concept of discrete superterranes i s the former presence of subducting oceanic lithosphere between the Insular and Intermontane superterranes, closure of the associated ocean basin, and the development of a 1 throughout the text the DNAG time-scale (Palmer, 1983) i s used to rela t e isotopic dates and ages to the geologic time-scale; middle Cretaceous will r e f e r in a general way to an informal Epoch comprising Barremian to Turonian time (124-89 Ma). 2 FIGURE 1: E a r l y model f o r the e v o l u t i o n of the Coast P l u t o n i c Complex: Cretaceous magmatic a r c , with the a s s o c i a t e d s u b d u c t i o n complex to the west. 3 FIGURE 2: The i n t e r - t e r r a n e t e c t o n i c model f o r the Coast P l u t o n i c Complex: magmatic and s t r u c t u r a l o v e r p r i n t on a mid-Cretaceous c o l l i s i o n a l suture (adapted from Monger, 1984). 4 c o l l i s l o n a l suture zone. Suggested remnants of the intervening ocean basin are the Bridge River complex and the Tyaughton trough east of the southern CPC, and the Gravina-Nutzotin belt west of the northern CPC. Elsewhere the suture zone i s in f e r r e d to l i e somewhere in the high grade core of the CPC, obliquely overprinted by Late Cretaceous to e a r l y T e r t i a r y p o s t - c o l l i s i o n plutons, which welded the two terranes together. The p o s t - c o l l i s i o n magmatic arc, in Monger et al.'s (1982) in t e r - t e r r a n e model, is re l a t e d to a new east-dipping subduction zone beneath the accreted superterranes, and is e s s e n t i a l l y i d e n t i c a l to the simple magmatic arc of the older subduction model. The CPC i s s t i l l a r e l a t i v e l y poorly understood c r y s t a l l i n e belt, and the enormous volume of middle Cretaceous to early T e r t i a r y plutons, coupled with the high grade metamorphism and complex deformation in much of the CPC, makes i t d i f f i c u l t to decipher the ear l y tectonic configuration, and has allowed much unsupported geologic speculation. Studies in areas where plutonic rocks are a r e l a t i v e l y minor component unambiguously indicate a middle Cretaceous age for major compression across the CPC and i t s s t r u c t u r a l extension into the high grade core of the North Cascades (e.g. Misch, 1966; Berg et al., 1972; Crawford et al., 1987; Glover and Schiarizza, 1987; Rusmore and Woodsworth, 1988; Brandon et al., 1988). However, the external tectonic causes and paleogeographlc details of the Cretaceous compressional event are by no means cle a r from the published data. In addition, the timing of Insular superterrane co l l i s i o n i s disputed. For instance, Monger et al. (1982; also Monger, 1984) and Price et al. (1985) favor a middle Cretaceous c o l l i s i o n , Brown (1987) suggests an Ear l y Cretaceous age, Armstrong (1988) and 5 Armstrong and Ward (in press) prefer a Late J u r a s s i c to very Ear l y Cretaceous age, and Brandon et a l . (1988) and Potter (1986) suggest Middle or Late J u r a s s i c ages for accretion. In spite of these differences, there appears to be widespread acceptance of a c o l l i s i o n model involving the existence of discrete terranes, separated by subducting oceanic lithosphere prior to accretion. However, no compelling evidence has been published that supports the previous existence of an intervening ocean basin and subduction assemblage along the length of the CPC, east of the Insular superterrane. Taking issue with Monger et al.'s (1982) in t e r - t e r r a n e c o l l i s i o n model, Brew and Ford (1983; see also Brew, 1983) argued in a formal Comment that the Insular superterrane and Intermontane superterrane in fact belonged to the same terrane, based on lit h o s t r a t i g r a p h i c , biostratigraphic and paleomagnetic si m i l a r i t i e s . They proposed that the marine basins between the Insular and Intermontane superterranes represent a i n t r a - t e r r a n e r i f t zone, which closed in Late Cretaceous time and was p a r t l y overprinted by the CPC. Their objections were countered in a formal Reply by Monger et a l . (1983). At issue in this Comment and Reply were the similarities and differences between the Insular and Intermontane superterranes; the problem of intervening oceanic lithosphere and subduction assemblages was not discussed. Based in part on the s c a r c i t y of Late J u r a s s i c - E a r l y Cretaceous subduction assemblages, Gehrels and Saleeby (1985; and pers. comm.) proposed emplacement of a discrete and f a r - t r a v e l l e d Insular superterrane (refered to by them as the Alexander-Wrangellia- Peninsular superterrane) by s t r i k e - s l i p faulting along a complex system of Late J u r a s s i c - E a r l y Cretaceous transtensional f l y s c h 6 basins. In this model, subsequent collapse o£ the flysch basins and accretion of the superterrane were caused by transpression r e l a t e d to major changes in Pacific and North American plate vectors in middle and Late Cretaceous time. In recent years the Insular-Intermontane superterrane boundary problem has been a subject of much informal debate, including a t o p i c a l workshop (2 n d Southern Cordilleran Workshop, Pacific Geoscience Centre at P a t r i c i a Bay, B.C., A p r i l *88), but i t has not yet been s p e c i f i c a l l y addressed in the l i t e r a t u r e . A rapidly expanding database, with considerable implications for the evolution of the CPC and the Insular-Intermontane superterrane boundary, has only recently been finding i t s way into print. New models based on this information should therefore s t a r t appearing in the near future. This thesis i s a contribution to the geochronological database for the CPC, and the f i n a l chapter in this t hesis presents a model for the evolution of the CPC based in part on this new information. The main part of this thesis is a study of temporal and s p a t i a l distribution of magmatic, metamorphic and s t r u c t u r a l episodes in the flanks of the CPC between 53° and 54°N latitude, in the Whitesail Lake and Douglas Channel-Hecate S t r a i t map-areas (Fig. 3). Combined U-Pb and K-Ar geochronometry, with a resolving power about 5 to 10 Ma, were used to establish broad patterns in age distribution in this transect. For Mesozoic and e a r l y Cenozoic rocks, dating resolution was limited by the medium precision 1 U-Pb techniques in use at the UBC lab during the past four years, and by interpretive limitations where the young isotopic system has not remained closed (common in the CPC). 1 For a description of what i s meant by medium precision, see the section on a n a l y t i c a l e r r o r s in Appendix 4. FIGURE 3: L o c a t i o n of study area, i n c l u d i n g other map-areas mentioned i n t e x t . 8 The observed distribution of major components of the CPC since 200 Ma is compatible with a succession of magmatic, metamorphic and s t r u c t u r a l episodes related t o east dipping subduction of Pacific oceanic lithosphere beneath a single terrane, which was part of the western margin of Mesozoic and Cenozoic North America. It is incompatible with the middle Cretaceous origin f o r the CPC by c o l l i s i o n of discrete superterranes proposed by Monger et a l . (1982). Chapter 2 presents new isotopic data f o r the western Whitesail Lake area, focusing on chronological aspects of previously documented stratigraphic and s t r u c t u r a l relations between the CPC and the Intermontane Belt (Intermontane Belt; van der Heyden, 1982). Important observations from this area include: 1) The presence in the eastern flank of the CPC (part of the Intermontane superterrane) of e a r l y Late J u r a s s i c magmatism, metamorphism and deformation, which c o l l e c t i v e l y created the Gamsby Complex. No evidence was found in the Gamsby Complex for a middle Cretaceous orogenic episode, suggesting that the c o l l i s i o n model of Monger et a l . (1982) for the Insular-Intermontane superterrane boundary does not apply to this part of the CPC. 2) The observation of younger thermal and s t r u c t u r a l overprints on the Gamsby Complex, which are considered to be p a r t l y the r e s u l t of heating above an Eocene metamorphic core complex represented by the Central Gneiss Complex, and of l a t e r b r i t t l e faulting, respectively. Chapter 3 presents new isotopic data for the Douglas Channel- Hecate S t r a i t area of the western CPC and l o c a l i t i e s distributed across the CPC at this latitude. Emphasis i s on dating the major plutons and on refining the previously recognized regional cooling 9 pattern (Hutchison, 1970, 1982; Roddick, 1970). New observations include: 1) The discovery of two mid-Paleozoic plutons in the east and west flanks of the CPC respectively, and of a T r i a s s i c pluton in the west flank. 2) The i d e n t i f i c a t i o n of a suite of early Late J u r a s s i c plutons and associated metamorphism and deformation along the western edge of the CPC. 3) The demonstration that Cretaceous and e a r l y Cenozoic plutons l i e in eastward younging belts, each with i t s own metamorphic and s t r u c t u r a l development. 4) K-Ar geochronometry has f i l l e d in the previously known, eastwardly younging cooling pattern, and shows that cooling through the b i o t i t e closure temperature lagged behind the migrating magmatic front by about 15-20 Ma. Chapter 4 is a regional synthesis that places the observations of the previous chapters in a larger, Cordilleran tectonic framework. The s p a t i a l and temporal distributions of major magmatic, metamorphic and s t r u c t u r a l features of the larger region are summarized and the tectonic implications are deduced. The J u r a s s i c through T e r t i a r y record of the CPC is interpreted as the integrated r e s u l t of magmatic, metamorphic and s t r u c t u r a l processes due primarily to east dipping subduction beneath the western edge of a single terrane, which was part of the western margin of North America. This model, here r e f e r r e d to as the i n t r a - t e r r a n e model (name proposed by J. Monger), Invokes c o l l i s i o n only in the sense of collapse of i n t r a - a r c r i f t or wrench fault basins. Successive tectonic overprints are Identified and explained as r e s u l t s of changes in the convergence vectors of the 10 North American continental and Farallon/Kula oceanic plates: the CPC records these externally controlled changes. Appendix 1 contains sample locations and descriptions for samples analysed in this study. Appendix 2 is a handbook of current procedures and a n a l y t i c a l techniques used in the UBC U-Pb laboratory. Details of support and development work ( a o sPb spike preparation, calibrations, standard and blank runs, etc.) performed during the course of t h i s study are reported in Appendix 3 and 4. Included in Appendix 4 is a discussion ("Comments on a n a l y t i c a l e r r o r s f o r data reported in this thesis") of the nature, causes and c o r r e c t i v e measures taken with regard to the rather large U-Pb a n a l y t i c a l e r r o r s reported for some samples in this thesis. This discussion, in conjunction with the r e s u l t s of analyses of a GSC intra-lab zircon standard (Appendix 4 : "GSC-TMH zircon solution"), will provide the facts by which to judge the quality of a n a l y t i c a l data reported in this thesis. The quoted 2cr e r r o r s given with each a n a l y t i c a l r e s u l t are accurate guides to the individual a n a l y t i c a l uncertainties. Appendix 5 is a current l i s t i n g of computer program "UBCUPB", which was used to calculate Pb/U and Pb/Pb dates from raw mass spectrometer data, and which includes complete e r r o r propagation. Appendix 6 (in the bound thesis copy only) contains the complete U-Pb and K-Ar a n a l y t i c a l data set, including sample locations and brief sample descriptions, recorded on UBC Geochron Lab data sheets. Note that a l l relevant U-Pb and K-Ar a n a l y t i c a l data are also reported in Chapter 2 (Tables 1 and 2) and in Chapter 3 (Tables 3 and 4). 11 CHAPTER 2; U-Pb AND K-Ar GEOCHRONOMETRY OF THE COAST PLUTONIC COMPLEX. WHITESAIL LAKE AREA. B.C. INTRODUCTION AND GEOLOGICAL SETTING The Whitesall Lake area, located in west-central B r i t i s h Columbia (Fig. 3), straddles the boundary between the Coast Plutonic Complex (CPC) and the Intermontane Belt, two of the five major geological belts of the Canadian Cordillera. A large part of the eastern flank of the CPC in the Whitesall Lake area has not been overprinted by young plutons or deformation, and preserves what is herein interpreted to be a record of pre-Cretaceous terrane interactions. Contrasting with this i s the T e r t i a r y high grade core of the CPC, which has been extensively reworked during l a t e r , post-middle Cretaceous events. This chapter presents new U-Pb and K-Ar isotopic data from both the older eastern flank and the adjacent younger core of the CPC. The Whitesail Lake area can be divided into three major domains: the Central Gneiss Complex (CGC), the Gamsby Complex, and the Intermontane Belt (Fig. 4). The CGC and Gamsby Complex are subdomains of the CPC, forming the core and eastern flank respectively. The geology of the western Whitesail Lake area (Fig. 5) is taken from previous work of Woodsworth (1979, 1980) and van der Heyden (1982, 1983, 1985). Geological mapping at 1:250,000 and 1:25,000 scales and preliminary U-Pb geochronometry indicated that the Whitesail Lake area is an integral part of the Intermontane superterrane, as i t is underlain e n t i r e l y by Stikinia, successor deposits, intruding magmatic elements, and metamorphic equivalents. In simple terms the CGC is composed of high grade, commonly migmatitic para- and orthognelss and thick, fo l i a t e d plutonic s i l l s , 12 F i g u r e 4: Major s u b d i v i s i o n s and t h e i r bounding s t r u c t u r e s , W h i t e s a i l Lake a r e a . 13 126°45' * o FIGURE 5: S i m p l i f i e d geology, western W h i t e s a i l Lake area, showing geochronometry sample l o c a t i o n s (adapted from Woodsworth, 1980). Legend on next page. H y p o t h e t i c a l c r o s s - s e c t i o n along A-B on F i g . 30. LEGEND FOR FIGURE 5: STRATIFIED ROCKS | I K A S A L K A . O O T S A L A K E , AND ENDAKO GROUPS I 8 | MIDDLE CRETACEOUS TO EARLY TERTIARY CONTINENTAL VOLCANICS SKEENA GROUP MIDDLE C R E T A C E O U S CALBIAN) MARINE SEDIMENTS GAMBIER GROUP EARLY C R E T A C E O U S (VALANGINIANt?) - HAUTERIVIANJ VOLCANICS I I LATE JURASSIC OR EARLY C R E T A C E O U S (??) SUBAERIAL VOLCANICS I 5 I AND SEDIMENTS I I BOWSER L A K E GROUP (ASHMAN FORMATION) I 4 I MIDDLE JURASSIC CBATHONIAN TO CALLOVIAN) MARINE SEDIMENTS | I H A Z E L T O N GROUP | 3 I LATE TRIASSIC TO MIDDLE JURASSIC (BAJOCIAN), MARINE AND NON-MARINE VOLCANICS AND SEDIMENTS PLUTONIC ROCKS :-;;i3 2 ;:. EARLY TERTIARY (PALEOCENE & EOCENE) QUARTZDIORITE. GRANODIORITE AND GRANITE, INCLUDES OUANCHUS, BOLOM. NANIKA & G O O S L E Y L A K E INTRUSIONS LATE CRETACEOUS QUARTZDIORITE AND GRANODIORITE, INCLUDES BULKLEY & K A S A L K A INTRUSIONS LATE JURASSIC COXFORDIAN-KIMMERIDGIAN) FOLIATED QUARTZDIORITE" SYN-KINEMATIC PLUTONIC ROCKS IN THE GAMSBY COMPLEX MIDDLE JURASSIC (BAJOCIAN) QUARTZDIORITE. GRANODIORITE AND GRANITE EARLY JURASSIC DIORITE AND QUARTZDIORITE: PRE-KINEMATIC PLUTONIC ROCKS IN THE GAMSBY COMPLEX METAMORPHIC & PLUTONIC COMPLEXES '• w j*"- GAMSBY COMPLEX LATE JURASSIC COXFOROIAN) GREENSCHIST AND AMPHIBOLITE FACIES TECTONITES C E N T R A L GNEISS COMPLEX LATE CRETACEOUS AND EARLY TERTIARY(?) MIGMATITIC ORTHO- AND PARAGNEISS HIGH ANGLE FAULT LOW ANGLE NORMAL FAULT THRUST FAULT GEOLOGIC CONTACT • GEOCHRONOMETRY SAMPLE LOCATION 15 which together are l o c a l l y complexly folded, but which on a regional scale form an e s s e n t i a l l y homoclinal slab with gently to moderately northeast-dipping foliations (Figs. 6, 7 and 8). Contacts between plutons and gneisses range from simple crosscutting relations t o exceedingly complex plutonic and s t r u c t u r a l interdigltation. The CGC is separated from the Gamsby Complex mainly by the steeply dipping Kemano fault (Fig. 5). The Gamsby Complex is the oldest s t r u c t u r a l element in the eastern CPC at this latitude. Excellent exposures of this unit occur in the upper Tsaytis r i v e r area (Figs. 9 and 10). There, and elsewhere, i t i s a more than 3 km thick, gently west- to south-dipping homoclinal s t r u c t u r a l sequence, composed of highly deformed ductile L-S t e c t o n i t e s with an inverted metamorphic gradient. The t e c t o n i t e s range from low grade volcaniclastic rocks and small plutonic bodies at low s t r u c t u r a l levels, with recognizable primary fabrics, through greenschist facies metavolcanic rocks and mylonitic granitoid rock, to amphibolite facies, migmatitic para- and orthogneiss at high s t r u c t u r a l levels. Paragneiss and greenschist facies metavolcanic rocks of the Gamsby Complex are inferred to be s t r a t i g r a p h i c a l l y equivalent units. Major and trace element chemistry of metavolcanic rocks indicated that protollths were t h o l e i i t i c and calc-alkaline basalt-andesite and calc-alkaline dacite-rhyolite which were erupted in a mature island arc setting (van der Heyden, 1982). A provisional U-Pb zircon date of 210 Ma, together with a 230±39 ( l a error) Rb-Sr whole rock isochron date suggested a Late T r i a s s i c or older p r o t o l i t h age for these rocks, and they were t e n t a t i v e l y c o r r e l a t e d with T r i a s s i c or l a t e Paleozoic rocks of the Intermontane Belt. FIGURE 6 : V i e w n o r t h a l o n g G a r d n e r C a n a l a t g e n t l y d i p p i n g E o c e n e t o n a l i t e s i l l s i n t h e C e n t r a l G n e i s s C o m p l e x . FIGURE 7: View l o o k i n g n o r t h e a s t at g e n t l y e a s t t o n o r t h e a s t d i p p i n g p a r a g n e i s s c a p p i n g t h e n o r t h e r n p a r t o f the T s a y t i s p l u t o n . R i d g e s i n background on r i g h t are u n d e r l a i n by r o c k s o f t h e Gamsby Complex and Intermontane B e l t . 18 FIGURE 8 : D e f o r m e d , m o d e r a t e l y n o r t h e a s t d i p p i n g c o n t a c t o f t h e n o r t h e r n p a r t o f t h e T s a y t i s p l u t o n w i t h o v e r l y i n g p a r a g n e i s s o f t h e C e n t r a l G n e i s s Complex. G n e i s s i c l a y e r i n g and p e n e t r a t i v e f o l i a t i o n s p r o j e c t b e l o w Gamsby Complex i n b a c k g r o u n d . 53 24'48" LEGEND STRATIFIED ROCKS INTERMONTANE BELT LATE TRIASSIC(?)-MIDDLE JURASSIC HAZELTON GROUP f7~] Snate, sandstone, volcanc oouuer congiomeraie unci, granitoid ciaslsj Basaltic to rnyoktic tlows. Breccia, lull, lanar KICI. muastone. siiisione ana o lymiciic conglomerate at Dase LATE TRIASSIC snaie. sitlsione, tossiliterous tonesione ana vocanc Oreccia w. Permian 1st. clasts COAST PLUTONIC COMPLEX GAMSBY COMPLEX : LATE JURASSIC METAMORPHIC TECTONITES Banded green sctiist tacies ma tic-1 else metavocanc myionitas. scnisis and phyites mnor caroonate Banded amohfiokte ana leucognetss. maroie. and unditlerenualea auartzaioniic to granodtontc ortnognetss PLUTONIC ROCKS MDDLE JURASSIC (BAJOC1AN) Altered g/anodonte and Quanzdionie . mnor Degmame; localy strongly cataclasic LATE CRETACEOUS post-Kinematic muscovue garnet teucogramte EARLY CRETACEOUS Dost-Knematc bioDie granootonie AGE UNKNOWN my torn tc bioDte Quart zoo me LATE JURASSIC COXFORDIAN) syn-nnematc oorphyroctastc auartzaome igreenscnist tacies j ana auartzaioniic ortnogneiss lamoncolte tacies! MIDDLE JURASSIC (BAJOCIAN) Dre-Knematm, mylonitic granite; cataciasnc auartzdionte ai Dase 01 Gam so y CorriDiex BEDDING FOLIATION THRUST FAULT HIGH ANGLE FAULT GEOLOGIC C O N T A C T FOLD AXIS. LINEATION ANTICLINE/SYNCLINE LIMESTONE. MARBLE FIGURE 9: S i m p l i f i e d geology of the upper T s a y t i s River area, •  1 \ showing l o c a t i o n s of gedchronometry samples. •k a E O C H H O N O M E T R Y S A M P L E L O C A T I O N 20 Pk. 2088m FIGURE 10: V i e w l o o k i n g w e s t i n t o t h e C o a s t P l u t o n i c Complex f r o m n o r t h e a s t c o r n e r o f a r e a shown i n F i g . 9. S t r a t a o f t h e I n t e r m o n t a n e B e l t ( H a z e l t o n G r oup) i n f o r e g r o u n d . A m p h i b o l i t e f a c i e s g n e i s s e s s t r u c t u r a l l y o v e r l i e g r e e n s c h i s t f a c i e s m e t a v o l c a n i c and m e t a p l u t o n i c r o c k s o f t h e Gamsby C o m p l e x , i n l e f t and c e n t e r o f p h o t o . P e a k s i n b a c k g r o u n d a r e C e n t r a l G n e i s s C o m p l e x , i n c l u d i n g t h e T s a y t i s p l u t o n , w h i c h a p p e a r t o d i p e a s t w a r d b e l o w t h e Gamsby Co m p l e x . 21 The low grade rocks In this complex were previously known as the Gamsby Group, and the gneissic part of the complex was included in the CGC (Woodsworth, 1978; van der Heyden, 1982, 1983); they are here both included in the Gamsby Complex, and the name Gamsby Group i s discontinued. The gneisses and associated intrusive magmatic rocks in the Gamsby Complex are not readily distinguishable in outcrop or in thin section from those in the core of the CPC (west of the Kemano fault), but the new K-Ar dates reported in this study indicate that these two gneissic domains have different thermal and s t r u c t u r a l h i s t o r i e s . For this reason the name CGC is herein r e s t r i c t e d to the core gneisses and their younger intruding plutons. The s t r u c t u r a l boundary between the CPC and Intermontane Belt is defined largely by the eastern limit of the Gamsby Complex (Figs. 4, 5, and 9). Plutons east of this boundary are here considered to be part of the Intermontane Belt, and the CPC-Intermontane Belt t r a n s i t i o n i s therefore e s s e n t i a l l y a s t r u c t u r a l and metamorphic break. Along much of i t s length the boundary is marked by steeply dipping, NNW-trending f a u l t s , which belong to a family of steep fa u l t s named the Sandifer Lake Fault Zone (SLFZ, Fig. 4). This steep fault zone i s superimposed on a older, NE vergent imbricate f a u l t zone that includes a major thrust, here r e f e r r e d to as the Gamsby thrust, along which the Gamsby Complex was emplaced over volcanic, sedimentary and intrusive rocks of the Intermontane Belt. For instance, approximately 4 km northeast of the head of the Tsa y t i s River (Fig. 9), the homoclinal succession of tect o n i t e s in the Gamsby Complex i s juxtaposed against unmetamorphosed volcanic rocks and sediments of the Intermontane Belt along a pronounced b r i t t l e f a u l t (Fig. 29 in van der Heyden, 1982). This fa u l t , strongly folded and l a t e r disrupted 22 by the Tsaytis fault (a member of the SLFZ), is interpreted as the remnant of a major thr u s t fault, with the exception of this major st r u c t u r e and the other b r i t t l e t h r u s t s in the upper Tsaytis River area, the imbricate fault zone is not shown in Fig. 5. The Intermontane Belt is underlain mainly by unmetamorphosed, marine to non-marine volcanic s t r a t a and sediments of the E a r l y to Middle J u r a s s i c Hazelton Group, unnamed subaerial volcanic rocks and sediments of Late J u r a s s i c or E a r l y Cretaceous age, and non-marine volcanic rocks of the middle-Late Cretaceous and E a r l y T e r t i a r y Kasalka, Ootsa Lake and Endako Groups (Fig. 5). These three dominantly volcanic units are separated, respectively, by marine sediments of the Middle J u r a s s i c Bowser Lake Group (Ashman Formation) and the E a r l y Cretaceous Skeena Group. Near the southern border of the Whitesail Lake area are outcrops of volcanic rocks and minor sediments c o r r e l a t i v e with the Early Cretaceous Gambler Group (Armstrong, 1953; Roddick, 19 66) of the southern Coast Mountains. Miocene to Pliocene plateau basalts underlie parts of the eastern whitesall Lake area, which is not included in Fig. 5. A small fault-bounded area, about 6 km east of the Gamsby Complex in the upper Tsaytis River area, is underlain by a complex association of Permian limestone and Late T r i a s s i c to E a r l y Jurassic(?) sedimentary and volcanic rocks (Fig. 9). Here, folded Permian limestone, containing fusullnids c h a r a c t e r i s t i c of Permian carbonate of the Stiklne Assemblage (Monger, 1977), appears to be unconformably overlain by Late T r i a s s i c shale, s i l t s t o n e , limestone and volcanic breccia, which grades into a distinctive polymictic conglomerate of the basal Telkwa Formation (Hazelton Group). Field evidence suggests considerable r e l i e f f o r the Permo-Triassic erosion 23 surface, and large o l i s t o l i t h s of Permian limestone are present in the basal volcaniclastic succession of the Telkwa Formation. Volcanic s t r a t a of the Hazelton Group are l o c a l l y overlain by fine grained marine e l a s t i c s of the Middle J u r a s s i c Ashman Formation. These are the f i r s t successor basin sediments of the Bowser Lake Group. Late J u r a s s i c coarse e l a s t i c s of the Bowser Lake Group, such as those of the Smithers and Hazelton areas (Tipper and Richards, 19 76), are unknown in the Whitesail Lake area, and this part of the Intermontane Belt was probably emergent at that time. The hiatus above the Ashman Formation is occupied in s e v e r a l places by boulder conglomerate (locally with granitic c l a s t s ) and overlying volcanic rocks, which apparently underlie marine e l a s t i c s of the Ear l y Cretaceous Skeena Group (Maclntyre, 1985; Diakow and Koyanagi, 1988). Very l i t t l e i s known about this potentially important Late J u r a s s i c or Early Cretaceous nonmarine(?) unit. The Skeena Group represents a f i n a l marine transgression in the Intermontane Belt, and consists of fine c l a s t i c d e t r i t u s shed to the west from the rising Omineca Belt. Overlying the Skeena Group is a distinctive boulder conglomerate which forms the base to the continental volcanic rocks of the Kasalka Group. The conglomerate thickens to the west and l o c a l l y contains granitoid boulders derived from the CPC. This i s the only significant evidence in the Whitesail Lake area for middle Cretaceous tectonism or uplift in the CPC; this study has not uncovered evidence f o r Cretaceous tectonism from the eastern CPC i t s e l f . From Late Cretaceous through Early T e r t i a r y time the Intermontane Belt was the s i t e of Andean type volcanism, and in Late T e r t i a r y time the eastern part of the Whitesail Lake area was partly(?) covered by plateau basalts. 24 Plutons In the Intermontane Belt are mostly high l e v e l intrusions r e l a t e d to the various volcanic units. Some of these plutons can be distinguished from plutons in the CPC by their lack of metamorphic fabrics and mineral assemblages, but massive, unfoliated plutons occur in both belts and some cut across the s t r u c t u r a l boundary between the CPC and Intermontane Belt. In contrast to the s t r u c t u r a l and metamorphic boundary, the magmatic boundary between the CPC and Intermontane Belt i s diffuse. S t r u c t u r a l l y the Intermontane Belt i s characterized by a network of steeply dipping f a u l t s superimposed on broad, commonly east-west trending open folds and variably oriented gentle warps of s t r a t a older than Miocene. The exact ages of these s t r u c t u r e s are unknown; as discussed l a t e r , the steeply dipping f a u l t s are probably Early T e r t i a r y in age. The stratigraphic and s t r u c t u r a l relations between the CPC and Intermontane Belt at this latitude were reasonably well understood at the onset of the present study, but the timing and kinematics of tectonic events responsible f o r the development and emplacement of the CPC remained largely unresolved. Combined f i e l d observations and Rb-Sr and minor K-Ar geochronometry of metavolcanic and metaplutonic rocks suggested, but did not prove, that metamorphism and ductile deformation of the Gamsby Complex were Middle to Late J u r a s s i c in age (van der Heyden, 1982, 1983). A Rb-Sr isochron fo r a suite of tectonized, greenschist facies metavolcanic rocks gave an age of 160±24 Ma, which was interpreted as the time of r e s e t t i n g during a major metamorphic and s t r u c t u r a l event. A 159 Ma Rb-Sr age for a mylonitic granite r e s u l t e d from a isochron forced through a reasonable i n i t i a l Sr r a t i o and the mean of a close c l u s t e r of points on a Sr evolution diagram. This age was also thought to r e f l e c t 25 isotopic resetting. Finally, metamorphic hornblende from a deformed contact agmatite bordering the mylonitic granite gave a 145±5 Ma K-Ar date, which suggested cooling of the tectonites through the hornblende blocking temperature at the end of Late J u r a s s i c time. These pre-Cretaceous dates were i n i t i a l l y interpreted (van der Heyden, 19 82, 19 83) to r e f l e c t Middle or Late J u r a s s i c c o l l i s i o n a l suturing of the Insular and Intermontane superterranes. B r i t t l e t h r u s t s were postulated to have been superimposed in Cretaceous time on the e a r l i e r , Middle or Late J u r a s s i c ductile s t r u c t u r e s , as a r e s u l t of continuing compression a f t e r the co l l i s i o n . For instance, the age of the Gamsby thru s t was inferred to be middle Cretaceous, because i t is cut by a Late Cretaceous pluton (the Horetzky dyke) about 12 km east of Kemano (Fig. 5), and because the rocks in the footwall, near the headwaters of the Tsaytis River (Fig. 9), were previously c o r r e l a t e d on l i t h o l o g i c a l grounds with Early Cretaceous s t r a t a in the southern Whitesail Lake area (Woodsworth, 19 79). PRESENT WORK During July and August of 19 83 and 198 4 s e v e r a l days were spent collecting U-Pb and K-Ar geochronometry samples from the major magmatic and metamorphic units within and near the Gamsby Complex. Twenty three samples were selected for U-Pb analysis; four of these were also processed for K-Ar dating. Thirteen other samples were selected f o r K-Ar analysis. Several of the samples were from previous collections (MSc thesis, van der Heyden, 1982; PhD t h e s i s , Parrish, 1982), and a few were collected by G. Woodsworth between 1978 and 1985. 26 F i f t y one zircon and two monazite fractions were dated; these represent twelve rock samples from the Gamsby Complex, three from the CGC, and nine from the Intermontane Belt. Nineteen hornblende and biotite separates were dated; fourteen of these are from the Gamsby Complex, four from the CGC, and one from the Intermontane Belt. The a n a l y t i c a l r e s u l t s confirm the preliminary isotoplc data of the preceding study (van der Heyden, 1982) and provide new insight into the evolution of the CPC. I n i t i a l r e s u l t s of this restudy of the Whitesail Lake area were reported by van der Heyden (1983, 1985) and by van der Heyden and Woodsworth (19 88). Sample locations are shown in Figures 5 and 9. Exact locations, together with brief rock and mineral separate descriptions, are l i s t e d in Appendix 1. ANALYTICAL TECHWXQVES U-Pfr Zircon concentrates were recovered from 10-50 kg samples of clean, fresh rock using standard crushing, grinding, wet shaking (Wilfley table), magnetic and heavy liquid (methylene iodide) separation techniques. E s s e n t i a l l y 100% pure zircon populations were handpicked in ethanol from a range of magnetic (Frantz isodynamic separator) and size fractions (nylon mesh sieve, range -44 to +210 )im). Following f i n a l cleaning in warm, dilute n i t r i c acid, zircons were dissolved in acid digestion vessels similar to, but smaller than, those of Krogh (1973), following modifications by J.M. Mattinson (pers. commun., 1984) and R.W. Sullivan (pers. commun., 1985). During the l a t t e r phases of the study zircon dissolution was done in microcapsules using the technique of Parrish (1987). U and Pb chemistry procedures were modified from 27 the technique developed by Krogh (19 73). Where necessary, a i r abrasion of zircon fractions was done with pyrite or pyrrhotite, using the procedure of Krogh (1982). U and Pb concentrations were determined using either a 2 o a P b - 2 3=U (prior to 1987) or a 2° sPb- 2 : a : 3U- 2 : 3 SU (1987 onward; Parrish and Krogh, 1987) mixed spike. Prior t o 1986 U and Pb were loaded separately on rhenium filaments. U was run as the oxide and loaded with n i t r i c acid on tantalum oxide, Pb was run as metal and loaded with HaPO* on s i l i c a gel. From 1986 onward, U and Pb were loaded together on the same rhenium filament using HaPCU and s i l i c a gel. Mass spectrometry was done on a VG Isomass 54R sol i d source mass spectrometer in single c o l l e c t o r mode (Faraday cup). A few f i n a l runs were done with a more sensitive photomultiplier (Daly collector) which was in s t a l l e d in Feb. 1988. For most analyses the precision, at the l a le v e l , was be t t e r than 0.1% for 2 0 7 P b / 2 o e P b and 2 0 a P b / 2 0 e P b , and bett e r than 0.5% for ̂ P b / ^ P b (in runs using the 2 0 S P b spike). ao'*Pb/20:i'Pb precisions ranged up to 21% with the Faraday cup, due to d i f f i c u l t i e s in measuring very small 2C"*Pb signals (2C"*Pb ion beam currents were commonly in the 10~ 1 S Ampere range, subequal to noise in the c o l l e c t o r system; see Appendix 4 f o r additional comments), but most were better than 3%. T o t a l procedural blanks ranged between 50 picograms and 4.6 nanograms Pb and 30-40 picograms U, based on repeated procedural blank runs during the period in which the analyses were c a r r i e d out. Pb/U and Pb/Pb e r r o r s f o r individual zircon fractions were obtained by individually propagating a l l calibration and a n a l y t i c a l uncertainties through the entire date calculation program and summing the individual contributions to give the t o t a l variance. Where 28 possible, ages for discordant fractions were determined by f i t t i n g a straight line to data points using a computer routine based on York (1969), and extrapolating to concordia using the algorithm of Ludwig (1980). E r r o r s f o r individual dates and f o r calculated intercept ages are quoted at the 2 sigma l e v e l (95% confidence interval). For additional information on an a l y t i c a l technique see Table 1 and Appendices 2 to 4. K-Ar Hornblende and b i o t i t e separates were prepared by standard rock crushing and grinding techniques, sieving to 60-80 and 80-100 mesh fractions, concentration using heavy liquids (bromoform and methylene iodide-acetone mixtures), vibrating table, and Frantz isodynamic separator, and f i n a l rinsing in acetone, d i s t i l l e d water and ethanol. Remaining impurities were largely removed by handpicking. Potassium concentrations were determined by replicate atomic absorption analysis of mineral solutions, using a Techtron AA4 spectrophotometer. Argon was measured by isotope dilution using a high p u r i t y 3 B A r spike in an AEI MS-10 mass spectrometer (White et al., 1967). Unless otherwise indicated the l i s t e d precision of the K-Ar dates in the text and in Table 2 is the estimated a n a l y t i c a l uncertainty at one standard deviation. ANALYTICAL RESULTS AND INTERPRETATIONS In the following sections the r e s u l t s and brief i nterpretations of U-Pb and K-Ar geochronometry are presented mainly in order of decreasing age, for each of the major map subdivisions. A l l relevant U-Pb and K-Ar an a l y t i c a l data for the dated samples are l i s t e d in 29 Tables 1 and 2, in the same order as they are discussed in the text. Complete a n a l y t i c a l data (see notes with Table 1), recorded on UBC Geochron Lab data sheets, are included in the bound copy of this t hesis (Appendix 6), and are also stored in the Canadian Cordillera Geochron File (CCGF) in the UBC Geochronometry Laboratory. The U-Pb data are plotted on =°*pb/ 2 : 3 , au vs. '"'Pb/ 2 3 8!] concordia diagrams (Wetherill, 19 56), which were generated with a modified plotting program of Ludwig (1983). Figures 5 and 9 are simplified and l o c a l l y i n t e r p r e t i v e versions of the 1:250,000 scale Whitesail Lake map (Woodsworth, 1980) and the 1:25,000 scale upper Tsaytis River geology map (van der Heyden, 1982). For additional details the Interested reader is r e f e r r e d to these references, with the cautionary note that s e v e r a l age assignments in this study d i f f e r from those in the references. New and previous U-Pb and K-Ar dates are shown together on Fig. 11, using the same base as Fig. 5, but without labels or patterns. GAMSBY COMPLEX U-Pb DATES Metavolcanic rocks (Fig. 9, unit 2) Sample 79V-MFP is a mylonitic, l o c a l l y porphyroclastic meta- rhyolite from one of s e v e r a l lenticular bodies of f e l s i c metavolcanic rocks in the upper part of the greenschist succession. The larger of these bodies are mappable at 1:2 5,000 scale, but are not distinguished in Fig. 9. They are thought to be deformed and metamorphosed porphyritic flows. In outcrop they may be d i f f i c u l t to distinguish from tectonic s i l l s of younger mylonitic granite (Fig. 9, unit 6), such as sample 79V-GMG, described l a t e r . Sample 79V-174a Is representative of 30 the f e l s i c component of the t y p i c a l l y banded, interdigltated f e l s i c - mafic metavolcanic succession of the Gamsby Complex (Fig. 12). Single fractions from each of these rocks were analysed f o r U- Pb. The data points plot on concordia (Fig. 13), but the large e r r o r s don't preclude the p o s s i b i l i t y of lead loss from either fraction. This s, is p a r t i c u l a r l y true f o r 79V-174a, which contained about 26% common lead ( i n i t i a l + blank) and did not yield stable ion beams, explaining the very large uncertainty f o r this sample. Additional analyses of coarse, non-magnetic, abraded fractions for each sample would probably be su f f i c i e n t t o answer this question, but no sample remains f o r 79V- 174a, and overlap of the e r r o r envelopes of both points suggests that the U-Pb dates are close to the c r y s t a l l i z a t i o n age. Based on the intersections with concordia of the e r r o r ellipse for 79V-174a, a 197±4 Ma minimum age i s here assigned to the banded metavolcanic succession of the Gamsby Complex. A previously reported 210 Ma U-Pb approximate age f o r a bulk single zircon f r a c t i o n from 79V-174a (van der Heyden, 1982) is of lower r e l i a b i l i t y . The analysis was c a r r i e d out on a previous generation, 90°, 12 inch radius mass spectrometer, and procedural blanks were extremely high (approximately 10 ng Pb). Re-processing of the original isotopic data through a more rigorous reduction routine in 1983, using b e t t e r spike calibrations than were available in 19 79, resulted in U-Pb dates of 187±4 and 191 + 6 Ma (Fig. 13), reasonably close to the Early J u r a s s i c age suggested by this study. 31 FIGURE 11: U - P b and K-Ar dates, western W h i t e s a i l Lake area ( r e f e r e n c e s i n p a r e n t h e s e s ) . 32 FIGURE 12: S t r o n g l y f o l i a t e d , g r e e n s c h i s t f a c i e s f e l s i c a n d m a f i c m e t a v o l c a n i c r o c k s o f t h e Gamsby Complex. W h i t e l a y e r s , f r o m w h i c h s a m p l e 79V-174a was c o l l e c t e d , a r e i n f e r r e d t o be m e t a r h y o l i t e . 33 0.020' • I • I i I i I i 1 1 1— 0.15 0.17 0.19 0.21 0.23 0.25 0.27 207pb/235u FIGURE 13: U-Pb concor d i a diagram f o r z i r c o n s from f e l s i c metavolcanic rocks of the Gamsby Complex. 34 Earlv and Middle Jurassic, preklnematic plutonic rocks and c l o s e l y intermingled younger phases (Fig. 5. unit 10 and Fig. 9~ unit 6) Large, f o l i a t e d plutons and smaller tectonized intrusions are present at s e v e r a l locations throughout the Gamsby Complex. In this report they are separated into two groups of differing age and s t r u c t u r a l character: a pre-kinematic group (Fig. 5, unit 10 and Fig. 9, unit 6) and a syn-kinematic group (Fig. 5, unit 12 and Fig. 9, unit 7). The Black Dome pluton (Fig. 5, unit 10), a large, pre-kinematic intrusion in the eastern flank of the CPC, was previously known as the Black Dome Complex (Woodsworth, 1979) and i s here considered to be part of the Gamsby Complex. It is a heterogeneous unit composed of massive t o well layered dio r i t e and l e s s e r gabbro, quartz dio r i t e and mafic dykes, a l l penetratively deformed and metamorphosed to greenschist facies (Woodsworth, 19 79). Locally the pluton i s cut by post-kinematic quartz diorite dykes (Fig. 14). Although the distribution of phases in the pluton was not mapped in detail, the layered diorite and gabbro undoubtedly are magmatic rocks that intruded volcanic and sedimentary rocks of the Gamsby Complex prior t o regional deformation and metamorphism. Penetrative deformation is not homogeneous throughout the pluton; locally, massive, unfoliated diorite (sample 84V-96) and quartz dio r i t e (sample 81WV-971) are present. Irregular masses as well as large, xenolithic screens of metavolcanic rocks are enclosed in fo l i a t e d diorite. The screens and the enclosing dio r i t e have been deformed and metamorphosed together. Some of these screens, in the extreme northwestern margin of the pluton, contain f e l s i c and mafic metavolcanic rocks similar to the banded volcanic succession of the Gamsby Complex. Also present here are irregu l a r masses of fo l i a t e d quartz diorite (sample 84V-97), which intrude 35 FIGURE 14: F o l i a t e d d i o r i t e o f t h e B l a c k Dome p l u t o n i n t r u d e d by p o s t - k i n e m a t i c q u a r t z d i o r i t e d y k e on l e f t . P h o t o (G. Woodsworth) was t a k e n n e a r l o c a t i o n o f s a m p l e 84V-96. 36 dior i t i z e d mafic volcanic rocks, but their relationship with the layered diorite and the f e l s i c metavolcanic rocks is unclear from f i e l d relations alone. The Black Dome pluton bears some resemblance to the Tahtsa Complex (Stuart, 1960; Woodsworth, 1978) at the west end of Tahtsa Lake, p a r t i c u l a r l y to i t s western side, which is penetratively deformed in the v i c i n i t y of the Gamsby thrust. Two zircon fractions from f o l i a t e d quartz dio r i t e sample 84V-9 7 plot on concordia with overlapping e r r o r envelopes, with the coarsest, non-magnetic fr a c t i o n giving U-Pb dates of 210.5 Ma (Fig. 15). This is interpreted as an approximate age f o r c r y s t a l l i z a t i o n of the zircons; i t may be a minimum age due to the p o s s i b i l i t y of small amounts of lead l o s s , indicated by the somewhat younger U-Pb dates of the finer fraction. Unfoliated diorite sample 84V-9 6 is from a small sample collected for K-Ar dating, and therefore yielded very l i t t l e zircon. A single bulk f r a c t i o n was abraded in an attempt to minimize components with lead loss and common lead contamination. The resulting point (Fig. 15) plots on concordia at 190+1.4 Ma, which is taken to indicate an Ear l y J u r a s s i c age for the diorite/gabbro phase of the Black Dome pluton. Unfoliated quartz diorite sample 81WV-971 yielded abundant zircon. Three fractions are a l l concordant (inset, Fig. 15), but small amounts of lead loss are indicated by the spread in Pb/U r a t i o s . The most abraded fraction (-149 + 74 u.m, NM2/1 ABR) has concordant U-Pb dates of 129 Ma, with a Pb/Pb date of 137±8 Ma. A precise age cannot be assigned, but the zircons must in any case be Early Cretaceous. Samples 84V-96 and 81WV-971 were i n i t i a l l y thought to be from the same intrusion, and the discrepancy in their ages therefore 37 0.040 0.03G 0.032 0 . 01G h 0.012 0. GRMSBY COMPLEX BLRCK DOME PLUTON 84V-97 -218+149 -149+74 B4V-96 NM RBR . 0207 B1HV-971 0203F 0199 0195h 0191 1 > 1 > 1 > 1 a 131cT 8 1 WV -971 J/ 8 129jSy 1 Zy-nf^ -149+74 NMZ^l RBR ' 125^ -149+74 M2X3 NMZ/5 +149 NHZ/l RBR 128 .130 .132 .134 .135 .138 ERRORS RRE 2 SIGMR J i I i i 10 0. 14 0.1B 0.22 2 0 7 P b / 2 3 5 u 0.26 FIGURE 15: U-Pb concordi a diagram f o r z i r c o n s from the Black Dome plut o n and a s s o c i a t e d i n t r u s i v e rocks, Gamsby Complex. 38 came as a surprise; the meaning of this age difference is discussed in a l a t e r section. The smaller, pre-kinematic intrusions in the Gamsby Complex (Fig. 9, unit 6) are represented by a dis t i n c t i v e tabular sheet of mylonitic granite (sample 79V-GMG), which occupies a c e n t r a l s t r u c t u r a l l e v e l within the metavolcanic sequence at the headwaters of the Tsaytis River. A megascopically folded, similarly sheet-like body of cataclastic* quartz diorite to granodiorite and quartz-feldspar pegmatite (sample 83V-5; Fig. 29 in van der Heyden, 1982), r e s t r i c t e d to a single area at the base of the Gamsby Complex, also belongs to this group. In this area original s t r u c t u r a l relations between metamorphosed and unmetamorphosed rocks have been preserved to some extent from l a t e r disruption in the SLFZ. The c a t a c l a s t i c body occupies a folded b r i t t l e shear zone, which is thought to be the remnant of a major, basal thr u s t (the Gamsby thrust) upon which the overlying tectonite sequence was emplaced over the Intermontane Belt. Two zircon fractions from mylonitic granite sample 79V-GMG are concordant at 179 Ma and very near concordant at approximately 175 Ma re s p e c t i v e l y (Fig. 16). A very precise age for this rock cannot be derived from the data. The fine f r a c t i o n may have l o s t some lead, and the 179±3 Ma ^ P b / ^ U age of the bulk f r a c t i o n i s here interpreted to be the emplacement age. Five zircon fractions of the pink, stubby population from quartz- feldspar pegmatite sample 83V-5 show varying amounts of discordance, defining a rather imprecise, linear lead-loss t r a j e c t o r y (Fig. 16). Degree of discordance does not appear to be cor r e l a t e d with U content or grain size. The regression line through these 5 points gives an upper intercept age of 178.4 + 8.6 and a lower intercept 39 0.029h 0.027h 0.025H ID CD m OJ \ J3 Q_ CD O OJ 0.023K 0.021 0.019K 0.017h 0.015 0. GRMSBY COMPLEX PRE-KINEMRTIC INTRUSIONS -74+44 NM2.1/1 -149+74 M2.1/2 -74+44 NM2.1/1 (alsndar prtini) 149+74 NM2.1/2 /' -74+44 M2.1/2 B3V-5 INTERCEPTS RT 17B.4+B.G & 4+32 MSWD=0.295 ERRORS RRE 2 SIGMR _ i 1 1 i i _ 10 0. 12 0.14 0.1G 207p b /235u O. 18 0.20 FIGURE 16: U-Pb c o n c o r d i a diagram f o r z i r c o n s from pre- kinematic i n t r u s i o n s i n the Gamsby Complex. 40 e s s e n t i a l l y at 0 Ma. The upper intercept age is interpreted as the time of emplacement of the granodiorite/pegmatite body. The single, more discordant f r a c t i o n from the c o l o r l e s s , slender zircon population i s distinguished by i t s much lower U and Pb concentration (see Table 1). Although this population has igneous morphology, i t s isotopic character and the rounded zircon terminations and edges suggest i t may be a xenocrystic component, possibly Early J u r a s s i c or Late T r i a s s i c in age, as suggested by i t s higher ^^Pb/^^Pb date. It is of i n t e r e s t to note that the zircons from the mylonitic granite, which must have deformed at much higher temperatures than the c a t a c l a s t i c quartz diorite at the base of the Gamsby Complex, have apparently not l o s t significant amounts of radiogenic lead. Most of the zircons in the severely a l t e r e d c a t a c l a s t i c unit are microfractured, whereas those in the mylonite are not, suggesting that low temperature, hydrothermal leaching of fractured zircons is more e f f e c t i v e at removing lead than high temperature so l i d s t a t e diffusion. Late J u r a s s i c , syn- to late-kinematic plutonic rocks (Fig. 5, unit 12 and Fig. 9, unit 7) A group of s i l l - l i k e , syn-kinematic intrusions (Fig. 9, unit 7) occupies the higher s t r u c t u r a l levels of the greenschist succession near the headwaters of the Tsaytis River (samples 83V-8 and 83V-12). Correlative orthogneiss bodies (sample 83V-10) are common In the high grade gneissic part of the Gamsby Complex. A large f o l i a t e d pluton now known to be c o r r e l a t i v e with these smaller bodies (sample 85WV-WHIT) is exposed in the southern part of the SLFZ (Fig. 5, unit 12). Syn-kinematic quartz diorite samples 83V-8 and 83V-12 are from plutonic injection complexes at and near the greenschist-amphibolite 41 f a d e s t r a n s i t i o n (Fig. 17). These units occur as lenses and l a y e r s of d i s t i n c t l y megacrystic to porphyroclastic, proto-mylonitic quartz diorite s t r u c t u r a l l y interlayered with, but l o c a l l y c r o s s c u t t i n g the metavolcanic t e c t o n i t e s at low angles. Dykes of similar material lo c a l l y branch off the concordant s i l l s and c l e a r l y cut across the f o l i a t i o n in the metavolcanic rocks, but are themselves folded and l o c a l l y interdigitated with the country rocks. These relations argue for a syn- to late-kinematic emplacement of the quartz di o r i t e . Six zircon fractions from sample 83V-8 yield an upper concordia intercept of 156 Ma (Fig. 18). This intercept has very large e r r o r s and can therefore only be considered to represent an approximate emplacement age. An abraded f r a c t i o n (-74 + 44 NM2/1°) has small Pb/U e r r o r s and plots almost on concordia with a 2oepb/ 2 : 3 eu date of 14 5 Ma, which provides a reliable minimum age. Two fractions from sample 83V-12 plot on concordia within an a l y t i c a l e r r o r (Fig. 18), but t h e i r different U-Pb dates indicate some Pb loss. The coarse, magnetic fr a c t i o n has large e r r o r s due to imprecise measurement of 2 C , , 4Pb (see discussion in Appendix 4). The concordant 160±1.5 Ma U-Pb date for the non-magnetic fr a c t i o n is here interpreted as the emplacement age of the granodiorite. This age i s close to the upper intercept derived for sample 83V-8. Irregularly layered quartz d i o r i t i c to granodioritic orthogneiss, from which quartz diorite orthogneiss sample 83V-10 was co l l e c t e d (Fig. 9), becomes increasingly abundant upward from the greenschist- amphibolite facies transition. It occurs as bands up to 2 meters in thickness interlayered with paragneiss, and as much larger s i l l - l i k e bodies up to s e v e r a l hundred meters thick. Many of these might be mappable at 1:50,000 scale, but apart from the orthogneiss from which 42 FIGURE 17: V i e w s o u t h e a s t t o s y n - k l n e m a t l c s h e e t o f q u a r t z d i o r i t e , s a n d w i c h e d b e t w e e n g r e e n s c h i s t f a c i e s ( l o w e r l e f t ) and a m p h i b o l i t e f a c i e s m e t a v o l c a n i c r o c k s ( u p p e r r i g h t ) , Gamsby C o m p l e x , s o u t h o f t h e head o f t h e T s a y t i s R i v e r ( F i g . 9). 43 0.025 0.023 ZD CD m CM \ -Q 0.021 CL- IO O (XI 0.019 1 1 1 -74+44 NM1.7/2 GRM5BY COMPLEX SYN-KINEMRTIC INTRUSIONS. II B3V-12 -7 4+4 4 NM2/1 •149+74 Ml.7/2 -74+44 NM2/1 flBR -149+44 NM1.5/3 \^ 149+74 M+NM 83V-8 74+44 M2/1 -149+74 NM2/1 83V-8 INTERCEPTS ARE 156132 &. 461186 MSWD-2.6 ERRORS PRE 2 SIGMR 0.017 0.125 0.135 0.145 0.155 0.165 207 P b /235|j 0. 175 FIGURE 18: U-Pb concord i a diagram f o r z i r c o n s from syn- kinematic i n t r u s i o n s i n the Gamsby Complex. 44 sample 83V-10 was collected, they are here included in undifferentiated gneiss (Fig. 9, unit 1). Although original contact relations with neighbouring amphibolite facies metavolcanic and metasedimenty rocks have been for the most part obliterated, t r a n s i t i o n s through deformed agmatite and l i t - p a r - l i t injection zones are l o c a l l y preserved. A syn-tectonic emplacement h i s t o r y i s indicated by the l o c a l presence of rotated amphibolite xenoliths in these agmatites, with foliations discordant to those in the orthogneiss. Four zircon fractions from quartz diorite orthogneiss sample 83V-10 were dated (Fig. 19), including a coarse, non-magnetic, abraded fraction. A regression line through the data points has a negative lower intercept. Forcing a line through the origin r a i s e s the upper intercept by only 1.5 Ma, to 156 + 3.5 Ma. This is e s s e n t i a l l y identical to the age of 83V-12 and 83V-8. Sample 8 5WV-WHIT is from the Whitecone pluton, a large, mostly faultbounded, f o l i a t e d quartz diorite body (Fig. 5, unit 12) situated within the SLFZ in the southern part of the study area. Foliation in the pluton i s generally steep and is p a r a l l e l to major f a u l t s . The Whitecone pluton contains large, 1:250,000 map-scale tectonic screens of f o l i a t e d metavolcanic and metasedimentary rocks (unit PMs in Woodsworth, 1980) which resemble su p r a c r u s t a l units of the Gamsby Complex. It i s not c l e a r from f i e l d observations whether the pluton is syn- or pre-kinematic, but because of i t s general character and similar age i t is here included in the Gamsby Complex. Three fractions, including a coarse, non-magnetic, abraded fr a c t i o n plot on concordia within a n a l y t i c a l e r r o r s (Fig. 20). The large e r r o r s for the unabraded fractions are due to large uncertainty in the a o , 4Pb measurement (see Appendix 4). Nevertheless, the e r r o r 45 0.025 - 0.023h =) CO m OJ \ -Q 0.021 Q_ CO O OJ 0.019 GRMSBY COMPLEX QUARTZDIORITE ORTHOGNEISS 83V-10 130, V •149+74 NM2/1 BBR 140, -149+44 NM1.5/3 -74+44 M2/3 •149+74 NM1.5/3 UPPER INTERCEPT IS 156+3.5 ( l i n e f o r c e d through o r i g i n ) 0.017 _L ERRORS RRE 2 SIGMR J. 0. 125 0. 135 0. 145 0. 155 0. 165 0. 175 207 P b /235u FIGURE 19: U-Pb c o n c o r d i a diagram f o r z i r c o n s from a syn- kinematic i n t r u s i o n i n the g n e i s s i c p a r t of the Gamsby Complex. 46 FIGURE 20: U-Pb c o n c o r d i a diagram f o r z i r c o n s from the syn- k i n e m a t i c ( ? ) Whitecone p l u t o n , Gamsby Complex. 47 envelopes for a l l three fractions overlap concordia between 150 and 156 Ma. The abraded fraction, which has the smallest uncertainty, is concordant at 156 + 1 Ma, which is interpreted as the emplacement age of the Whitecone pluton. In summary, the ages of syn-kinematic intrusions in the Gamsby Complex f a l l in the 153 to 162 Ma range. Ductile deformation of the Gamsby Complex and coeval intrusion of quartz diorite and granodiorite are therefore interpreted to be Oxfordian or Kimmeridglan in age. Cretaceous post-kinematic plutonic rocks (Fig. 9. units 8 and 10) Sample 84V-99 was collected from a small but important b i o t i t e granodiorite intrusion exposed only at the foot of a glacier feeding a t r i b u t a r y to the Tsaytis River (Fig. 9, unit 8). In previous work this unit was included as i r r e g u l a r l y banded orthogneiss (van der Heyden, 19 82), but during the collecting phase of the present study i t was found to be a unfoliated, discordant intrusion, post-kinematic with respect to ductile deformation in the enclosing paragneiss. The exposed area is probably the roof of a small stock or pluton, as the contact with neighbouring gneisses is generally p a r a l l e l t o gently dipping compositional banding, and large, rotated xenoliths of gneiss occur in a megascopic injection breccia in the contact zone. These rocks are cut by chilled dykes and small stocks of pink granodiorite, which were the original collecting target in this study; no further work has been done on those intrusions. Four zircon fractions from granodiorite 84V-99 plot on concordia within a n a l y t i c a l uncertainty but c l e a r l y show the e f f e c t s of Pb loss (Fig. 21). Two fine, non-abraded, U-rich fractions show that Pb-loss is mainly r e s t r i c t e d to U-rich outer rims. A fine, abraded fraction has 48 TABLE X; U-Pb ANALYTICAL DATA 1 r WHITESAIL LAKE AREA Sample | t U Pb 3 Isotonic abundance4 6/4s Isotopic r a t i o s 6 t 2a errors F r a c t i o n 2 (nig) (ppn) 2 O 6Pb=100 2 0 S P b ' / 2 3 B » 2 0 7 P b * / 2 3 3 U 2 0 7 P b , / 2 0 6 P b ' 208 207 204 Dates (Ma) 7 i 2or errors GAMSBY COMPLEX 79V-MFP -149+44 12.5 819 25.5 11.83 5.15 0.0098 3371 0.03090154 0.2132i42 0.05004i28 NHL 5/3 196.2*3.4 196.2*3.8 197*13 7 9 V - 1 7 4 a -149+44 8.1 287 9.2 11.97 5.91 0.0619 204 0.03106t66 0.21421186 0.05002i410 HH1.5/3 - 197.2t4.2 197.1*15.6 196* 1 B 5/-, 9 6 -149+44 89.1 361 13.4 24.7 11.10 0.4084 240 0.02937*62 0.2066*66 0.05102*126 old analysis 186.6*3.8 190.7*5.6 241.6* 3 6/- 37 & 4 Y - 9 7 -149+74 2.2 587 23.7 22.47 10.52 0.3730 213 0.03272*36 0.2276*56 0.05044*118 NM2/1 207.5t2.2 208.2*4.6 2 1 5 + " / - s a -210+149 8.3 350 11.6 9.66 5.75 0.0483 1939 0.03318*36 0.2304i30 0.05037i38 HH2/1 210.4*2.2 210.5*2.6 212*17 8 4 V - 9 6 -149+44 1.8 502 16.1 14.04 7.17 0.1487 610 0.02993*20 0.2059120 0.04990*38 HH1.5/3 ABR 190.1*1.2 190.hi.6 190*17 81WV-971 +149 7.4 331 6.5 10.23 5.09 0.0165 4309 0.01967*8 0.1314i6 0.04845*10 HH2/1 ABB 125.6*0.6 125.410.6 122t5 -149+74 4.4 393 7.8 10.38 5.21 0.0244 2962 0.01984*8 0.1328110 0.04855.*30 M2/3 8H2/5 \ 126.6*0.6 126.6*0.8 126*14 -149+74 1.5 386 7.7 10.17 4.98 0.0067 2916 0.02017*12 0.1357*8 0.04878*18 NN2/1 ABR 128.7*0.8 129.2*0.8 137*8 79V-GMG -149+44 18.2 908 25.1 8.64 5.12 0.0105 2495 0.0282h49 0.1931i38 0.04964124 SHI.5/3 179.3i3.2 179.3*3.4 179*11 -74 2.2 801 21.8 9.00 5.25 0.0185 3723 0.02757il2 0.1891*8 0.0497616 HM1.5/3 175.3i0.8 175.9i0.8 184*3 83 V - 5 -74+44 1.1 1814 32.4 12.49 5.43 0.0313 2424 0.01748122 0.1197il8 0.04968*40 M2.1/2 111.7*1.4 U4.8 i l . 6 180*19 -149+44 21.6 2419 48.6 12.05 5.45 0.0334 2331 0.01973*78 0.1348158 0.04956.*70 HM1.5/3 126.0i5.0 128.4*5.2 175*33 -149+74 0.9 1634 36.5 12.69 5.33 0.0270 2513 0.02189*28 0.1487126 0.04928158 HM2.1/2 139.6il.8 140.8*2.2 161*28 -74+44 0.6 481 12.2 17.92 7.99 0.2013 460 0.02255il0 0.1564tl8 0.05030*52 DM.1/1 colorless, slender prisns 143.7i0.6 147.5il.6 209*24 -149+74 0.8 1987 55.2 23.51 9.72 0.3234 307 0.02273*14 0.1555120 0.04962*52 M2.1/2 144.9*0.8 146.7il.8 177*24 -74+44 1.2 1690 45.7 16.25 6.53 0.1067 919 0.02509i8 0.1718i8 0.04965il4 NM2.1/1 159.810.6 161.0*0.6 179*6 8 3 V - 8 -149+74 0.4 611 12.2 6.51 5.20 0.0188 1407 0.02078126 0.1412i28 0.04927+74 112/1 132.6il.6 134.h2.6 161* 3 3/- 3 6 1,2,3,4,3,6,7 p 0 0tnotes are on page 50. 49 83V-8 (cont.) -74+44 0.7 476 9.7 7.52 5.00 0.0046 1635 0.02113*22 0.1437i42 0.0493hl30 M2/1 134.8*1.4 136.3*3.8 163* 6 l/-63 -74+44 2.9 509 10.4 6.55 5.17 0.0199 3623 0.02114*10 0.1420il0 0.04874t28 HH2/1 134.8*0.6 134.8*1.0 135*13 -149+74 0.7 416 9.0 7.95 5.51 0.0378 1181 0.02194*8 0.1498*10 0.04952.+28 M+HH 139.9*0.6 141.8+1.0 173+13 -149+44 21.2 597 12.6 6.16 5.16 0.0145 5203 0.02195*22 0.1496124 0.04943+56 NH1.5/3 139.9*1.4 141.512.0 168126 -74+44 7.5 586 12.8 6.60 5.03 0.0085 8504 0.02275*10 0.1539+8 0.0490616 NM2/1 ABR 145.0*0.6 145.3*0.6 151+3 83Y-12 -149+74 0.5 190 4.9 14.77 6.74 0.1212 409 0.02417*16 0.1653198 0.04961t274 HI.7/2 153.9il.O 155.3i8.4 177* l 2 6/-, a, -74+44 2.7 176 4.8 16.12 7.34 0.1656 542 0.02516il6 0.1703*16 0.04909138 HH1.7/2 160.2*1.0 159.7tl.4 152+18 8 3 V - 1 0 -149+74 9.4 1059 22.6 9.29 5.01 0.0052 9338 0.02172124 0.1478il6 0.04934i24 ML 5/3 138.511.6 139.9*1.4 164+11 -74+44 0.8 696 15.5 10.44 5.17 0.0175 3757 0.02229110 0.1508110 0.04909+22 N2/3 142.1t0.6 142.7+1.0 152+11 -149+74 25.2 627 14.1 9.66 5.02 0.0070 9479 0.02281*26 0.1546116 0.04916114 M l . 5/3 145.411.6 146.0+1.4 155+6 -149+74 2.4 1248 29.3 9.14 4.96 0.0026 20386 0.02394*38 0.1623i26 0.0491616 NH2/1 ABB 152.5*2.4 152.7+2.4 156*3 85WV-WHIT -74+44 2.1 396 9.3 7.51 5.62 0.0559 1424 0.02388130 0.1578188 0.04793+248 Hl.8/1 152.2.*2.0 148.8i7.8 9 6 * t 2 0 / - i 2 S -149+74 3.1 271 6.4 8.84 5.33 0.0346 2261 0.02404*30 0.1598*54 0.048211146 IM1.8/1 153.1+2.0 150.5*4.8 110 + 7 ,/-7 2 -149+74 13.6 240 5.7 8.32 5.06 0.0087 6786 0.02444il6 0.1662110 0.04933122 111.1/1 ABB 155.611.0 156.1*0.8 1 6 4 i l l 8 4 V - 9 9 -74 1.3 2294 36.6 11.65 5.16 0.0207 3243 0.01582*20 0.1059+14 0.04855136 H+HH 101.2+1.2 102.2+1.4 126il8 -74 1.1 2235 40.3 19.00 7.91 0.2077 456 0.01587120 0.1061+16 0.0485h50 81 101.5*1.2 102.4il.4 124125 -149+74 0.5 1672 27.9 9.10 5.03 0.0140 1700 0.01695i22 0.1127i24 0.0482h76 108.3il.4 108.4*2.2 110 + 3 7/-3B -74+44 2.3 932 18.2 13.40 5.66 0.0555 1629 0.01884*8 0.125816 0.0484218 HH1.8/2.5 A BB 120.310.6 120.3i0.6 120i4 8 3 V - 9 -74+44 0.3 11450 182.0 38.52 9.95 0.3451 281 0.01161+40 0.0779146 0.048671230 Ml.2/5 Honazite 74.412.6 76.2*4.4 1 3 2 + l 0 9 / - , i 3 -74+44 0.3 13764 182.9 22.46 11.56 0.4567 215 0.01050*18 0.0700i28 0.048361158 Ml.2/5 Monazite 67.411.2 68.7*2.6 117* 7 7/-7 B CENTRAL msm COMPLEX 78WV-344 -149+44 26.6 240 2.8 9.18 5.86 0.0696 964 0.01181*21 0.0788.+20 0.04835+694 HH1.5/3 75.7*1.3 77.0t2.0 117+34 79WV-347 -149+44 12.2 219 2.6 8.26 5.68 0.0591 1452 0.01208*82 0.0802162 0.04813il56 SMI.5/3 ABB 77.4t5.2 78.3+5.8 106 + 7 6/-77 50 -149+74 mi. 7/2 83V-3 4.4 815 7.0 9.06 5.71 0.067 1299 I.WTPRMQMTJfflE BJjLT 0.00856*4 54.9*0.2 0.0557H 55.1i0.4 0.04722+22 6 0 t l l -149+44 27.2 244 7.3 18.50 5.77 0.0536 1622 0.02760t32 0.1897180 0.049841192 INI.5/3 175.5*2.0 176.4+6.8 188* B B/-9o +74 3.1 209 6.1 18.15 5.87 0.0617 1188 0.02707114 0.1853114 0.04964122 M l . 5/3 172.2il.O 172.6il.2 178110 m-i •74 0.8 1219 30.3 9.58 5.14 0.0121 3506 0.02515+12 0.1721+10 0.04964il0 mi. 5/3 I6O.I1O.8 161.3s0.8 178+4 85WV-TRAP -74 1.2 284 7.8 12.47 5.26 0.0206 1281 0.02699i36 0.1845i28 0.04960146 H2/1 171.7*2-2 172.0+2.4 176+21 -149+74 2.6 313 8.8 12.97 5.16 0.0079 3195 0.02753134 0.1914136 0.05041172 IH2/1 175.h2.2 177.813.2 214i33 -149*74 12.2 268 7.8 15.38 5.61 0.0439 2033 0.02789+10 0.1910+8 0.04969+14 1N2/1 MR 177.3i0.6 177.510.8 180+6 -149+44 21.6 172 6.5 30.52 10.68 0.3786 200 0.02888156 0.20421106 0.051281238 M l . 5/3 183.613.6 188.619.8 254*' 0 6/-ito 83V-3 -149+44 0.5 333 8.6 18.09 6.78 0.0931 311 0.02353+104 0.1760*236 0.05423*668 M l . 5/3. 150.016.4 164.6120.2 3 8 1 * 2 6 6 / - „ o -74+44 0.9 117 2.7 26.55 5.17 0.0235 305 0.01997+44 0.13431114 0.04878+374 Ml.8/2 127.512.8 128.0il0.2 1 3 7 * 1 7 8 / - 1 8 S -149+74 2.6 204 4.7 21.13 5.23 0.0219 1993 0.02123+12 0.1437i38 0.04908+120 111.172 mm) 135.4i0.8 136.3+3.4 152* S 7/- a 8 -149+74 3.2 182 4.2 19.71 5.46 0.0359 1189 0.02138+26 0.1454156 0.049321172 IH1.8/3 ABR(M) 136.3il.6 137.8i5.0 163* 0 l/-M 8 3 V - 1 ( p r e l i m i n a r y ) -74+44 1.7 789 93.8 0.11 49.35 3.0241 33 0.02283i88 0.1512t860 0.0480212614 HN2/1 ABR{ l i g h t l y ) 145.515.6 143.0*74.4 1 0 0 + i o a 8 / _ i 6 . -149+74 5.0 706 106.8 0.13 54.20 3.3147 30 0.02397i70 0.2070i770 0.0626512186 M2/1 152.7+4.4 191.1+63.8 696 + 6 6 7/-«M7 1 Coiplete a n a l y t i c a l data, including the leasoied 2 0 6 p b / 2 0 4 P b errors, the l o l e \ blank Pb and the Pb7(Pb*+Pbeo..on) ratios in the analyses, the asseied Stacey-Iraaers couon Pb ages and their errors, and the correlation c o e f f i c i e n t s for the Pb/U r a t i o s , are recorded on UBC Geochroo Lab data sheets i n Appendix 6 of the bound thesis copy. 2 -149+74 = size range in Ra; H,IH 3 u g n e t i c , non-iagnetic on Prantz isodynaiic separator at indicated amperage and side t i l t (e.g. 1.5/5 * 1.5 aips at 5° side t i l t ) ; ABR = abraded. 3 radiogenic + couon Pb. 4 radiogenic + couon Pb, corrected for 0.15Vain fractionation and for 0.07-4.5 ng Pb blank v l t h composition 208:207:206:204=37.30+0.75:15.50+0.34:17.75t0.19:1. s 2 0 6 p b / 2 0 4 p b I e a s n r e a # corrected for 0.15t/aao fractionation. 6 corrected for fractionation (0.12Vaaa for I), 0.15\/aia for Pb), blank Pb (see note 4 above), and for couon Pb nsing the Stacey and Kraaers (1975) growth curve; errors are 2 sigaa, only l a s t d i g i t s are shown. 7 decay constants used in age calculation : A 2 3 SO=1.55125xl0- 1°, A a 3 8 0 = 9 . 8 4 8 5 x l 0 - l ° ; 2 3 eU' 2 3 0B=137.88 (Steiger and Jager, 1977). Errors are 2 sigaa. 51 0.020 - 0.018 0.016 Z) 00 £ 0.014 \ _Q Q_ CO O OJ 0.012 0.010 0.008 0.006 0. T T T GRMSBY COMPLEX POST-KINEMATIC INTRUSIONS l 70 90, B3V-9 Monazite -74+44 Ml.2/5 ERRORS ARE 2 SIGMA i I i L_ B4V-99 •149+74 -74 NM -74 M+NM .0191 .0189 .018? .0185 .0183 122p . B4V-99 1 2 1 . 1 I**' -74+44 NM1 8/2.5' flBRHDED 123 . 124 .125 .126 .127 .128 J_ 05 0.07 0.09 0. 11 0. 13 207 P b/235u FIGURE 21: U-Pb co n c o r d i a diagram f o r z i r c o n s and monazite from postkinematic i n t r u s i o n s i n the Gamsby Complex. 52 significantly less U and is concordant within small a n a l y t i c a l uncertainty, at 120.3±0.6 Ma, which is interpreted as the emplacement age. The south side of the orthogneiss body (Fig. 9, unit 7) from which the ea r l y Late J u r a s s i c orthogneiss sample 8 3V-10 was collected, i s cut by thick dykes and i r r e g u l a r pods of heterogeneous, medium to coarse grained muscovite leucogranite (Fig. 9, unit 10; Fig. 22), which is l o c a l l y garnetiferous. The garnet bearing v a r i e t y (83V-9), in c ontrast to the non-garnetiferous granite, contains monazite in excess of zircon and has yielded some free gold in the heavy mineral separate. Analysis of two separately handpicked monazite fractions from the same magnetic sp l i t (Fig. 21) yielded sli g h t l y different U-Pb and Pb/Pb dates. Of the two analyses, the younger one is considered more reliable because of somewhat b e t t e r a n a l y t i c a l quality. The approximately 6 8 Ma U-Pb age for this fraction is here interpreted as the emplacement age f o r the leucogranite. This i n t e r p r e t a t i o n i s supported by a 64.4 ± 2.3 Ma b i o t i t e K-Ar date from the immediately adjacent orthogneiss (sample 83V-10, see K-Ar, below), which almost c e r t a i n l y r e f l e c t s resetting during intrusion of the leucogranite. Also, other post-kinematic dykes of this age were previously known from the Gamsby Complex (e.g. sample 80V-27 in van der Heyden, 1982; see K-Ar, below). CENTRAL GNEISS COMPLEX U-Pb DATES Kemano pluton (Fig. 5 r unit 13) Quartz diorite samples 78WV-344 and -347 from the Kemano pluton (Fig. 5) were collected at different elevations from the steep 53 FIGURE 22: P o s t k i n e m a t i c l e u c o g r a n i t e ( f o r e g r o u n d ; l o c a t i o n o£ s a m p l e 83V-9) i n t r u d i n g s y n - k i n e m a t i c o r t h o g n e i s s ( s a m p l e 8 3 V - 1 0 ) , Gamsby C o m p l e x . 54 slopes west of the village of Kemano by G, Woodsworth, as part of a study of f i s s i o n track geochronology of the CPC (Parrish, 1983). The east side of this pluton was previously described by Stuart (I960), who r e f e r r e d to i t as the Dubose Stock. The Kemano pluton i s a medium to coarse grained granodiorite to quartz diorite. It is l o c a l l y weakly foliat e d , but contact relations with the enclosing gneisses are commonly agmatitic and are c l e a r l y post-kinematic. Single zircon fractions from each sample (Fig. 23) plot on concordia, with overlapping e r r o r envelopes. The large a n a l y t i c a l uncertainty f o r 78WV-347 is due to unstable ion-beams (see Appendix 4 for more details) but the absolute value .is considered accurate. The 77.4±5.2 Ma 2 0 GPb/* 3 eU date fo r 78WV-347 provides a minimum age of emplacement. Tsa y t i s pluton (Fig. 5 r unit 14) The T s a y t i s pluton (sample 84V-98; Figs. 7 and 8) is the s t r u c t u r a l l y highest member of a set of f o l i a t e d tonalite s i l l s exposed along the spectacular ridges and slopes flanking Gardner Canal (Fig. 6). These s i l l s , which range in thickness from tens of meters to s e v e r a l hundred meters, are concordant to the o v e r a l l gently to moderately northeast dipping f o l i a t i o n and layering in the neighbouring paragneisses. Foliation is defined by aligned mafic minerals and by stretched xenoliths in the margins of the plutons, and decreases toward th e i r i n t e r i o r . Contacts between the pluton and the gneisses are l o c a l l y strongly folded. A coarse, non-magnetic zircon fraction is concordant at 55.0 + 0.3 Ma (Fig. 23), sl i g h t l y older than previously published K-Ar dates from this same l o c a l i t y (see K-Ar section); this is interpreted as the emplacement age. 55 0.016 0.014 Z) CD 0.012 CO \ -Q 0_ L0 0.010 O OJ 0.008 0.006 - i 1 1 1 1 1 . [— CENTRAL GNEISS COMPLEX KEMANO PLUTON 78WV-344/347 TSAYTIS PLUTON B4V-98 7BWV-347 78WV-344 0089 0087 0085 0083 84V-9B 56 149+74 NM1.7/2 ERRORS HRE 2 SIGMfl i i i i i _ .0081 .0535 .0545 .0555 .0565 .0575 JL 0.05 0.06 0.07 0.08 2 0 7 P b / 2 3 5 | j 0.09 0. 10 FIGURE 23: U-Pb co n c o r d i a diagram f o r z i r c o n s from plutons i n the C e n t r a l Gneiss Complex. 56 INTERMONTANE BELT U-Pb DATES Middle J u r a s s i c plutonic rocks (Fig. 5, unit 11 and Fig. 9, unit 5) Samples 8 3V-2 and 8 3V-4 (Fig. 9, unit 5) were collected from outcrops of altered, c a t a c l a s t i c , coarse grained granite and granodiorite adjacent to the eastern flank of the Gamsby Complex (Fig. 9). The granodiorite from which 8 3V-4 was collected l o c a l l y i s indistinguishable from the c a t a c l a s t i c quartz dio r i t e at the base of the Gamsby Complex (83V-5), but has been disrupted by a younger steep fault of the SLFZ. The granite of 83V-2 is s t r u c t u r a l l y overlain by volcanic rocks and sediments (Fig. 9, units 3 and 4) along a northwest dipping thrust fault of unknown age. Primary contacts with the neighbouring volcanic rocks and sediments have mostly been obscured by these f a u l t s , but in a few places Intrusive relations with volcanic rocks of unit 3 (Fig. 9) were documented. Based on lithologic similarities these volcanic and sedimentary rocks were previously c o r r e l a t e d with Ear l y Cretaceous s t r a t a in the southern Whitesail Lake and Bella Coola areas (Woodsworth, 1980), and the intruding granitoid rocks were therefore thought to be Cretaceous or younger. Analytical r e s u l t s for granite 83V-2 and granodiorite 83V-4 (Fig. 24) indicate a Middle J u r a s s i c emplacement age. One f r a c t i o n from 83V- 2 has a very large 2 ° 7 P b / 2 3 S U e r r o r due to large 2c"*Pb uncertainty; the other f r a c t i o n is concordant at about 172 Ma within a n a l y t i c a l error, but may have l o s t some lead. This fraction has a ^''Pb/^^Pb date of 178 + 10 Ma. A single f r a c t i o n from 83V-4 i s discordant and must have experienced lead l o s s , but the 2 0 7 ,Pb/ 2 0 SPb date is 178±4.4 Ma. The 178±10 ^ P b / ^ P b date is a conservative estimate of the emplacement age, and is identical to the age of Middle J u r a s s i c quartz-feldspar pegmatite 83V-5 at the base of the Gamsby Complex, 57 0. 0221 1 I 1 I ' • ' 1 I 0.155 0.165 0.175 0.165 0.195 0.205 207p b /235u FIGURE 24: U-Pb c o n c o r d i a diagram f o r z i r c o n s from p l u t o n i c rocks i n the Intermontane B e l t , upper T s a y t i s R i v e r area. 58 which was previously believed to be a related body based on l o c a l lithologic similarities (van der Heyden, 1982). The Trapper pluton (sample 8 5WV-TRAP), at the southern edge of the study area (Fig. 5), i s a coarse grained, unfoliated granodiorite. This pluton is fault bounded, except in relation to overlying volcanic rocks (Fig. 5, unit 6), which are either intruded by the pluton, or which l i e above i t on a thrust fault or an erosional unconformity. The l a t t e r p o s s i b i l i t y , which is discussed in a following section, i s considered most likely. Three fractions from granodiorite 85WV-TRAP were analysed, one of which was abraded. The e r r o r ellipses of a l l three points overlap concordia (Fig. 25), but i t i s evident that small amounts of lead loss must have a f f e c t e d the unabraded fractions. A f t e r abrasion the coarse non-magnetic f r a c t i o n i s concordant at 177.4 + 0.7 Ma. This i s Interpreted as the emplacement age of the Trapper pluton. Volcanic rocks (Fig. 5, units 3 and 6. Fig. 9 r unit 3). Sample 83WV-1038 is a rhy o l i t i c or dacitic t u f f from the informally named Whitesail formation of the Hazelton Group (Fig. 5, unit 3, undivided). This unit overlies the Telkwa Formation, l o c a l l y with erosional unconformity, and is composed of cream colored, reddish and dark grey r h y o l i t i c flows and pyroclastic rocks. Paleontologic evidence indicates a Toarcian(?) to lower Bajocian age (ca.180 Ma) f o r marine parts of the formation (Tipper, 1979). A single f r a c t i o n plots on the Early to Middle J u r a s s i c portion of concordia, but has very large e r r o r s (Fig. 26). The 2 ° s P b / 2 3 S U date of 184±4 Ma i s in accord with the paleontologic age of the Whitesail 59 Formation, but should be considered no more than a provisional isotopic age for this sample u n t i l more an a l y t i c a l work has been done. Sample 83V-3 is- an andesitic or dacitic flow collected from the mixed volcanic-sedimentary succession (Fig. 9, unit 3), which l i e s east of and s t r u c t u r a l l y below the Gamsby Complex in the upper Ts a y t i s River area. As mentioned above, this succession was previously thought to be Early Cretaceous in age, but i t is intruded by Middle J u r a s s i c plutons (samples 83V-2 and 83V-4). A large (80 kg) sample yielded barely enough zircon for a single analysis. Unfortunately, the resulting point (Fig. 26), with a minimum U-Pb date of about 150 Ma, is very discordant and because of 2C"*Pb e r r o r has large a n a l y t i c a l uncertainty, precluding meaningful interpretation. A large sample of this unit will have to be recollected in the future; c a r e f u l sample preparation and analysis should enable a zircon age determination. The only constraint available at this time is a minimimum age of about 178 Ma, based on the age of cross cutting granodiorite (Fig. 9, unit 5, sample 83V-4). A Cretaceous age, as originally inferred, is ruled out. Sample 85WV-SAL is a rhyodacite t u f f breccia from volcanic s t r a t a near the southern border of the Whitesail Lake area (Fig. 5, unit 6), which appear to unconformably overly the Trapper pluton. Hornblende from an andesitic flow in the volcanic succession had previously given a K-Ar date of 124±4 Ma (Stevens et.al., 1982). This, coupled with t h e i r lithologic similarity to paleontologically dated Early Cretaceous volcanic rocks in the Bella Coola area, suggested an Early Cretaceous age. Three zircon fractions from 85WV-SAL plot on concordia within a n a l y t i c a l uncertainty (Fig. 26). Two coarse fractions, abraded by different amounts, have overlapping e r r o r envelopes, and give a mean 60 FIGURE 25: U-Pb c o n c o r d i a diagram f o r z i r c o n s from the Trapper p l u t o n , Intermontane B e l t . 61 0.034, 0.0301- Z) GO 0.026 CO OJ \ J2 tS" 0.022 O OJ 0.018h 1 1 1 1 1 1 r INTERMONTANE BELT VOLCRNIC ROCKS 83HV-1B38 B5HV-5RL, _L30 B3V-3 -149+74 NM1.8/2 RBR 10X -143+74 NM1.8/2 HBR 50X -74+44 Ml.B/H ERRORS RRE 2 SIGMR 0.0141 _L X X X 0.11 0.13 0.15 0.17 0.19 207p b/235u 0.21 0.23 FIGURE 26: U-Pb co n c o r d i a diagram f o r z i r c o n s from v o l c a n i c r o c k s , Intermontane B e l t . 62 2 ° e P b / 2 3 B U date of about 136 Ma. The fine, magnetic f r a c t i o n is concordant at about 128 Ma, but has large ^''Pb/^^U e r r o r s on account of imprecise 2 0 4Pb/ 2 0 ,'Pb measurement. Two interpretations are possible. The younger fr a c t i o n could represent the true age of th i s rock, with small amounts of Ju r a s s i c inheritance explaining the older dates of the abraded fractions. More likely the fine magnetic f r a c t i o n has l o s t some Pb and the K-Ar date is somewhat low, making the true age about 136 Ma. Either way, the volcanic rocks are Early Cretaceous in age. \ Late Jurassic(?) dyke (Fig. 9, sample 83V-1, marked 'granite dyke') An attempt was made to date zircons from a porphyritic granite dyke (sample 83V-1) which cuts across a northwest dipping b r i t t l e thrust, near the north end of the northerly course of the Tsaytis River (Fig. 9). This t h r u s t fault belongs to a set of deformed imbricate t h r u s t s in the Intermontane Belt, which are believed to be related to the Gamsby thrust. Two zircon fractions were dated, from one of two populations (see Appendix 1). The resulting points have extremely large e r r o r s (Fig. 27), which are related to the observed presence of very large amounts (about 80%) of common lead in the zircons. Nevertheless, the 146±6 Ma and 153 + 4 Ma ^ ^ b / 2 3 ^ dates hint at an early Late J u r a s s i c minimum age f o r emplacement of this dyke. Neither of these fractions contained opaque or unusual inclusions or other obvious signs of contamination. The individual fractions were run in different batches, about 6 months apart (in Jan. '88 and June '88), and none of the other samples in these batches had unusually high common or blank lead. The amount of blank lead required to give the observed r e s u l t s i s ca. 128 ng for the -74 + 44 )im 63 FIGURE 27: U-Pb c o n c o r d i a diagram f o r z i r c o n s from a p o r p h y r i t i c g r a n i t e dyke, Intermontane B e l t . 64 fraction, and ca. 4 45 ng for the -149 + 7 4 um fraction. These observations suggest that the observed common lead contents are not due to anomalously high procedural blanks, but that the common lead resides within the zircons. Such a large amount of common lead i s r a r e l y observed, and i t s presence in these zircons is enigmatic. Therefore, the : 2 0 Gpb/ 2 3 BU dates f o r this sample are here reported as preliminary r e s u l t s only. GAMSBY COMPLEX K-Ar DATES Prior to the present study, only two K-Ar determinations had been done on rocks of the Gamsby Complex. Analytical r e s u l t s for these rocks (samples 80V-66 and 80V-27) were reported in van der Heyden (1982). Sample 80V-66 is a dior i t i z e d mafic xenolith from a duc t i l e l y sheared injection agmatite (unit Jg2 in van der Heyden, 19 82), which forms the hanging wall contact of the mylonitic granite from which sample 79WV-GMG (Fig. 9, unit 6) was collected. The hornblende K-Ar date of 145 + 5 Ma was interpreted as the time of cooling of the Gamsby Complex below the hornblende argon retention temperature (approximately 530°C, Harrison and McDougall, 19 80) following regional metamorphism and ductile deformation. Sample 80V-2 7 is a fine grained diorite from a postkinematic dyke that intrudes the deformed injection agmatite. This dyke has di s t i n c t i v e , zoned chilled margins against i s o l a t e d wallrock xenoliths and against the neighbouring agmatite. It has a hornblende K-Ar date of 65.7 + 2.3 Ma, which indicated that this part of the Gamsby Complex was quite cool by l a t e s t Cretaceous time. K-Ar dates f o r hornblendes in this study are Late J u r a s s i c and Early Cretaceous, ranging from 14 4 to 108 Ma (Table 2 and Fig. 11). In 65 contrast, b i o t i t e s are considerably younger, ranging from 50 to 6 5 Ma. The range in hornblende dates and the discordance between hornblende and bioti t e dates are believed to r e f l e c t a complex, multi-phase thermal and s t r u c t u r a l evolution. Pre- and syn-metamorphic/kinematic hornblende Hornblende from 79V-331A i s dated at 143±5 Ma, e s s e n t i a l l y identical to the previously determined age for hornblende from sample 80V-66, which was collected from the same unit (mylonitized agmatitic contact of unit 6 with overlying metavolcanic rocks, Fig. 9 ; see above) about 2 km to the northeast. Sample 80V-20A, also from this unit, gives a hornblende K-Ar date of 122±4 Ma. These hornblendes are thought to be contact-metasomatic in origin, having formed at the time of granite intrusion, through d i o r i t i z a t i o n of mafic volcanic rocks. Two interpretations are possible: 1) Ductile deformation and regional metamorphism, which induced t o t a l degassing of older hornblende, pre-date l a t e s t J u r a s s i c time (approximately 144 Ma), when the greenschist component of the Gamsby Complex cooled below 530°C. Additional, p a r t i a l degassing caused by localized p o s t - J u r a s s i c thermal disturbances explains the younger K-Ar date of 80V-2OA and the younger dates of metamorphic hornblendes in the Gamsby Complex. 2) Regional metamorphism and ductile deformation postdate l a t e s t J u r a s s i c time. The K-Ar dates are p a r t i a l l y r e s e t from a primary metasomatic age, and have no meaning beyond indicating the variable degassing e f f e c t of post - J u r a s s i c metamorphism. The f i r s t i n terpretation is favored because V80-20A is from an area with pervasive b r i t t l e f r a c t u r e s and abundant younger mafic 66 dykes, which may explain i t s younger hornblende date. The other two rocks are from unfractured outcrops with no post-kinematic intrusions in t h e i r near vicinity. The identical hornblende dates from these l a s t two rocks suggest synchronous setting of K-Ar clocks sometime a f t e r complete degassing during metamorphism. That temperatures high enough to degas hornblende were reached is substantiated by the t y p i c a l greenschist metamorphic assemblages of the surrounding mafic metavolcanic rocks (van der Heyden, 19 82). Samples 80V-30 and 83V-10 are syn-kinematic granodiorite and quartz dio r i t e (Fig. 9, unit 7) from the greenschist-amphibolite t r a n s i t i o n and from the gneissic succession respectively. Hornblendes in these rocks are either r e l i c s of primary magmatic c r y s t a l s , or they could be r e c r y s t a l l i z e d , metamorphic grains. Their K-Ar dates are 116±4 Ma (80V-30) and 124±4 (83V-10). These are interpreted to r e f l e c t Cretaceous or younger p a r t i a l argon loss. Metamorphic hornblende was separated from three amphibolltes in the gneissic part of the Gamsby Complex (Fig. 9, unit 1). A l l three are believed to be metabasalts c o r r e l a t i v e with metavolcanic rocks from the greenschist succession. Their K-Ar dates are 133 + 5 Ma (80V-60), 132±4 Ma (83V-11) and 108±4 Ma (80V-34). The older dates are interpreted to approximate the time of f i n a l cooling of the gneissic part of the Gamsby Complex below 530°C, and in any case are minimum cooling ages. The younger date for sample 80V-34 is probably due to l a t e r , more complete argon loss. This sample was collected about 200 m from a small, post-kinematic diorite intrusion (not shown on Fig. 9, but see Fig. 2 in van der Heyden, 19 82). 67 Post-metamorphic/kinematic hornblende Sample 84V-9 8-1 is a very coarse grained hornblendite from the gneissic succession in the Gamsby Complex (Fig. 5, unit 1). It forms angular, xenolithic blocks in a post-kinematic, agmatitic injection breccia (Figs. 28a and 28b). The intrusive phase is a coarse grained quartzo-feldspathic pegmatite re l a t e d to nearby granitic dykes and s i l l s , which are post-kinematic with respect to gneissic foliation. The hornblendites were c l e a r l y derived by contact metasomatism of mafic gneiss. They vary t y p i c a l l y in grain size; l o c a l l y hornblende forms c r y s t a l s up to 30 cm in length. The K-Ar date f o r these hornblendes is 132±4 Ma. This date is identical to that of two metamorphic hornblendes from the gneissic succession mentioned above. Taken together these dates suggest that a large part of the Gamsby Complex, mainly consisting of i t s gneissic component, did not cool below the hornblende argon retention temperature until Early Cretaceous time. Two massive hornblende dio r i t e samples from unfoliated parts of the Black Dome pluton (Fig. 5, unit 10; samples 81WV-971 and 84V-96) are similar in outer appearance, but look d i s t i n c t l y d i f f e r e n t in thin section (see Appendix 1). The texture of 81WV-971 is magmatic, whereas 84V-9 6 is r e c r y s t a l l i z e d . The hornblendes analysed from both rocks are magmatic (relict in the case of 8 4V-9 6), but they have different K-Ar dates: 144±5 Ma for 81WV-971, and 127±4 Ma f o r 84V-96. The zircon dates for these same rocks (see U-Pb section above) are Early Cretaceous and Early J u r a s s i c respectively. The significance of these age contradictions is discussed in a l a t e r section. 68 FIGURE 28a: Gamsby Complex ( g e n t l y s o u t h d i p p i n g g n e i s s e s i n b a c k g r o u n d ) , i n t r u d e d by p o s t k i n e m a t i c p e g m a t i t e . FIGURE 28b: C o a r s e h o r n b l e n d i t e a g m a t i t e b l o c k i n p e g m a t i t e , Gamsby C o m p l e x . TABLE 2; K-Ar ANALYTICAL DATA. WHITESAIL LAKE AREA Sample Mineral %K ^ A r * 1 V">Ar t 2 Date 3, e r r o r 4 (xlO _ 6cc/gm) Ma GAMSBY COMPLEX 79V-331A Hb 0.346 2.006 72.4 143 + 5 80V-20A Hb 0.409 2.011 77.8 122 + 4 80V-30 Hb 0.797 3.716 91.6 116 + 4 83V-10 Hb 1.150 5.715 94.5 124 + 4 Bi 3.52 8.968 75.0 64.4 + 2.3 80V-60 Hb 0.221 1.184 66.4 133 + 5 83V-11 Hb 0.794 4.216 79.5 132 + 4 80V-34 Hb 0.839 3.633 87.6 108 + 4 84V-98-1 Hb 0.683 3.645 88.0 132 + 4 81WV-971 Hb 0.381 2.223 49.2 144 ± 5 84V-96 Hb 0.264 1.355 79.7 127 + 4 80V-65 Bi 6.80 13.690 90.4 51.1 + 1.8 84V-99 Bi 6.50 12.776 80.9 49.9 + 1.7 79V-HOR Bi 6.82 14.625 88.0 54.4 + 2.0 CENTRAL GNEISS COMPLEX 79WV-654 Hb 0.942 1.968 82.7 53.0 + 1.9 Bi 7.16 14.079 89.1 49.9 + 1.7 78WV-348 Bi 6.99 14.329 89.8 52.0 ± 1.8 78WV-342 Bi 7.08 14.315 87.2 51.3 ± 1.8 INTERMONTANE BELT 79V-228 Hb 0.394 0.5045 72.7 32.7 ± 1.1 Botes; 1 Radiogenic 4 0 A r . 2 Radiogenic as percentage of t o t a l 4 0 A r . 3 Decay constants used in age calculation : ^ b=4.96xlO- 1 0, X o=0.581xl0- l°; 4OK/K=0.01167 atom \ (Steiger and Jager, 1977). 4 Errors are 1 s i g i a . 70 Blotlte dates Four b i o t i t e s from granitoid intrusions in the Gamsby Complex gave dates between 50 Ma and 65 Ma. Two of these intrusions (Fig. 9, unit 7 and 9; samples 8 3V-10 and 80V-65) have been involved in ductile deformation and metamorphism, the other two (Fig. 9, unit 8 and Fig. 5, unit 13; samples 84V-99 and 79V-HOR) are post-kinematic. The b i o t i t e s are much younger than either emplacement of the intrusions or metamorphism of the Gamsby Complex. In view of the low argon blocking temperature for bioti t e (approximately 280°C / Harrison and McDougall, 1980), the young dates are here interpreted to r e f l e c t r e s e t t i n g of the b i o t i t e K-Ar clock during a heating event, which will be discussed l a t e r . CENTRAL GNEISS COMPLEX K-Ar DATES Sample 79WV-654 is a f a i n t l y f o l i a t e d b i o t i t e hornblende tonalite (Fig. 5, unit 14) from a s i l l along the east shore of Gardner Canal. Similar s i l l s are common in this part of the CGC (Fig. 6). The larg e s t of these is the T s a y t i s pluton, for which a hornblende K-Ar date of 50.0±1.5 Ma and a b i o t i t e K-Ar date of 51.4±1.2 Ma had been published (Wanless et.al., 1979; from same location as zircon sample 84V-98). 79WV-654 has a hornblende K-Ar date of 53 + 1.9 Ma. Biotite i s somewhat younger at 49.9±1.7 Ma. Biotite from Kemano pluton quartz diorite samples 78WV-342 and 78WV-348 (Fig. 5, unit 13) gave 51.3 + 1.8 and 52.0±1.8 Ma K-Ar dates respectively, similar to the b i o t i t e K-Ar dates from the Tsaytis pluton and the aforementioned s i l l s . 71 INTERMONTANE BELT K-Ar DATES Sample 79V-228 (Fig. 9) is from one of many hornblende lamprophyre dykes which intrude the Gamsby Complex and rocks of the Intermontane Belt, t y p i c a l l y as long, continuous features which can be followed for some distance. They are brown to r u s t y weathering, commonly they have chilled margins, and l o c a l l y they occur in conspicuous dyke swarms, p a r t i c u l a r l y along faults and splays of the SLFZ (van der Heyden, 1982). Sample 79V-228 has a hornblende K-Ar date of 32.7±1.1 Ma, which represents the emplacement age of the dykes into cold country rocks. DISCUSSION LATE TRIAS SIC-MIDDLE JURASSIC. PRE-KINEMATIC UNITS. The new zircon dates reported in this study for pre-kinematic units in the Gamsby Complex, coupled with previously reported s t r u c t u r e , petrography and whole rock chemistry (van der Heyden, 1982), indicate that this part of the CPC is a metamorphosed and strongly deformed equivalent of Early and Middle J u r a s s i c volcanic and intrusive rocks in the western Intermontane Belt. The 19 7±4 Ma concordant U-Pb age obtained from f e l s i c metavolcanic rocks in the Gamsby Complex (Fig. 9, unit 2; samples 79V- 174A and 78WV-MFP) c o r r e l a t e s well with the Sinemurian to ea r l y Pliensbachian stratigraphic age of the Telkwa Formation (Tipper and Richards, 19 7 6), which underlies the area d i r e c t l y east of the Gamsby Complex. This c o r r e l a t i o n i s supported by lithologic resemblance of s t r u c t u r a l l y lowest volcaniclastic rocks of the Gamsby Complex, deformed but s t i l l recognizable, to e s s e n t i a l l y unmetamorphosed volcanic rocks of the Hazelton Group. Also, major and trace element 72 chemistry of metavolcanic rocks in the Gamsby Complex (van der Heyden, 19 82) i s similar to that of the Howson facies of the Telkwa Formation (Tipper and Richards, 19 76). Zircons from volcanic rocks of the Hazelton Group in the Stewart area, approximately 24 0 km NNW of the Whitesail Lake area, were previously dated between 190 and 210 Ma (D.A. Brown, 19 87). The pre-kinematic Black Dome pluton (Fig. 5, unit 10) was probably emplaced in Early J u r a s s i c time, and may be cogenetic with the Telkwa Formation. Diorite sample 84V-96 has a concordant U-Pb zircon age of 190 + 1.4 Ma, sli g h t l y younger than the ages obtained from f e l s i c metavolcanic rocks 79V-174A and 78WV-MFP. Material similar to these metavolcanic rocks occurs as screens in the Black Dome pluton. Plutons of this age are unknown elsewhere in the Whitesail Lake area, but they occur further north and northeast as the Topley Intrusions (Woodsworth et al., in press). The Texas Creek pluton near Stewart (Alldrick et al., 19 86) is the same age as the Black Dome pluton, and is probably the best known Early J u r a s s i c pluton in c e n t r a l B r i t i s h Columbia. The much younger, Early Cretaceous age of quartz diorite sample 81WV-971 can only be explained one way: this sample must be from a post-kinematic intrusion which has not been previously distinguished. This suggestion is supported by the primary igneous texture observed in thin section, which contrasts with the thoroughly r e c r y s t a l l i z e d nature of sample 84V-96. Also, although 81WV-971 has been a f f e c t e d by hydrothermal a l t e r a t i o n , i t does not have the metamorphic assemblage seen in 8 4V-9 6. Finally, i t is significant that the diorite from which 84V-96 was collected, about 2.5 km west of sample 81WV-971, i s intruded by post-kinematic quartz diorite dykes (Fig. 14). Its 127±4 Ma 73 hornblende K-Ar date is within e r r o r of the probable emplacement age of 81WV-9 71. This suggests that the quartz diorite dykes and the intrusion from which 81WV-9 71 was collected are related, and that older hornblende from 84V-9 6 was completely r e s e t at that time. The 14 4 Ma hornblende date for 81WV-9 71 is enigmatic; i t is surprisingly similar to the dates f o r two pre-metamorphic hornblendes from deformed agmatite in the Gamsby Complex. However, the zircon age indicates that this rock i s much younger, and the anomalous K-Ar date is here interpreted as the res u l t of excess radiogenic argon derived by degassing from r e c r y s t a l l i z i n g hornblende in the enclosing diorite. A previous hornblende K-Ar determination by the Geological Survey of Canada for dio r i t e from the Black Dome pluton had given a unreasonably old date of 34 4 + 88 Ma (Woodsworth, unpubl. data), which must likewise be explained as the e f f e c t of excess argon. Several irre g u l a r masses of fo l i a t e d quartz dio r i t e (sample 84V- 9 7) in the Black Dome pluton are probably Late T r i a s s i c in age, and therefore most likely represent foreign material brought in from different s t r u c t u r a l levels. This older quartz diorite r a i s e s the pos s i b i l i t y that older intrusions, possibly cogenetic with late Carnian and Norian(?) volcanic rocks exposed about 20 km to the north (Woodsworth, 1980; van der Heyden, 1982), may be present in the Gamsby Complex. It i s also possible that c o r r e l a t i v e s of these Late T r i a s s i c s t r a t a , together with their underlying Permian carbonates, may themselves form l o c a l elements in the Gamsby Complex. Of particular i n t e r e s t in this respect is the occurrence in the Gamsby Complex of dist i n c t i v e carbonate units (Fig. 9), which form good marker horizons (Woodsworth, 1979; van der Heyden, 1982). These carbonates 74 could be highly deformed and metamorphosed Permian limestone of the Stikine Assemblage. The U-Pb data f o r pre-kinematic granites in the Gamsby Complex (Fig. 9, unit 6; samples 78V-GMG and 8 3V-5) suggest a probable emplacement age of 178-179 Ma. A previously reported Rb-Sr whole rock age of approximately 160 Ma for mylonitic granite was interpreted to r e f l e c t the timing of ductile deformation and metamorphism (van der Heyden, 1982). These granites are probably mylonitized equivalents of granitoid rocks of the Tahtsa Complex (Fig. 5, unit 11), which was previously dated by hornblende K-Ar at 175±5 Ma (Stevens et. al., 19 82), and which intrudes the Telkwa Formation near the west end of Tahtsa Lake (Stuart, 1960). The west side of the Tahtsa Complex has been involved in ductile deformation (Fig. 29) together with metavolcanic rocks of the Gamsby Complex. The Tahtsa Complex i s very heterogeneous and contains xenoliths and very large, i r r e g u l a r masses of dior i t i z e d volcanic rocks, suggesting that exposures at present erosion levels are in the roof of a large pluton. A deformed, dioritized injection agmatite in the hanging wall of the mylonitic granite could likewise represent the roof of a intrusion. Quartz diorite sample 83V-5 from the s t r u c t u r a l base of the Gamsby Complex, which was involved in polyphase b r i t t l e deformation of uncertain age, is also coeval with the Tahtsa Complex. Samples 83V-4, 83V-2 and 85WV-TRAP are from Middle J u r a s s i c plutons (Fig. 5, unit 11 and Fig. 9, unit 5) which intrude or underly volcanic rocks of the Intermontane Belt. They a l l have U-Pb ages suggesting emplacement at about 178 Ma. Other large plutons from the western Whitesail Lake area have previously yielded K-Ar dates of 176±20 Ma (Morice Lake pluton, Stevens et al., 1982) and 181 + 4 Ma (the 75 76 smaller pluton northeast of the Morice Lake pluton, Carter, 1974). The Sias Creek pluton (Fig. 5) is similar in many respects to the other Middle J u r a s s i c intrusives and is inferred to be of the same age. Together with similar plutons in the Smithers and Terrace areas (Woodsworth et al., in press), these form part of a larger belt of coeval plutons along the east flank of the CPC. They may be oogenetic with the Whitesail Formation (informal name) of the Hazelton Group, which has yielded Toarcian(?) to Bajocian f o s s i l s (Tipper, 19 79). LATE JURASSIC. SYN- TO LATE-KINEMATIC UNITS. Early hints of Late J u r a s s i c tectonism and metamorphism in the Whitesail Lake area, which were based on Rb-Sr and K-Ar geochronometry of metavolcanic and metaplutonic rocks in the Gamsby Complex, are corroborated by the new U-Pb and K-Ar r e s u l t s of this study. The data reported here indicate that early Late J u r a s s i c ductile deformation and regional metamorphism were accompanied by intrusion of quartz diorite and granodiorite magmas: syn-kinematic plutonic rocks in the Gamsby Complex (Fig. 9, unit 7; Fig. 17; samples 83V-8, 83V-10, and 83V-12) give U-Pb ages clustering in the 155-160 Ma range. Orthogneiss is a major component of the gneissic part of the Gamsby Complex, and most i f not a l l of this material could well be Late J u r a s s i c in age. These orthogneisses are very similar in appearance to sample 83V-10, and they are cut in many places by postkinematic intrusions, some of which have been dated as Early Cretaceous (see below). The large, f o l i a t e d Whitecone pluton at the south end of the study area (Fig. 5, unit 12; sample 85WV-WHIT), is here included in the Gamsby Complex because of i t s penetrative deformation and because of 77 Its early Late J u r a s s i c U-Pb age, identical to that of the smaller bodies to the northwest. Primary relations with metamorphosed s t r a t a in enclosed tectonic screens are not clear, nor are relations with the Black Dome pluton to the northwest; based on the ages for these rocks, they are probably deformed and disrupted intrusive contacts. The inverted metamorphic gradient of the Gamsby Complex and retrograde metamorphic assemblages in the greenschist-amphibolite t r a n s i t i o n zone in the upper Tsaytis River area indicate that hot, ductile material was rapidly emplaced above cool rocks at high c r u s t a l levels (van der Heyden, 1982). Pre- and syn-kinematic/metamorphic hornblendes in the Gamsby Complex give dates ranging between 14 5 Ma and 108 Ma. The younger dates are probably due to localized p a r t i a l r e s e t t i n g in Cretaceous or l a t e r time, and the oldest K-Ar dates (sample 79V-331A; also sample 80V-66 from van der Heyden, 1982) are interpreted to mark the end of regional metamorphism and f i r s t cooling of the greenschist succession of the Gamsby Complex below 530°C at the end of Late J u r a s s i c time. The amphibolite facies gneissic component of the Gamsby Complex had cooled below 530°C by early Early Cretaceous time. Volcanic rocks cogenetic with plutons of Late J u r a s s i c age are not known from the Intermontane Belt in the Whitesail Lake area. A conglomerate and volcanic unit, about which very l i t t l e i s known, apparently is situated between the Middle J u r a s s i c Ashman Formation and the Early Cretaceous Skeena Group (Maclntyre, 19 85; Diakow and Koyanagi, 1988). These rocks (Fig. 5, unit 5) could be c o r r e l a t i v e with the Oxfordian Trout Creek assemblage and Netalzul volcanic rocks of the Bowser Lake Group in the Smithers and Hazelton map areas (Tipper and Richards, 19 76), which are the same age as the syn-kinematic 78 plutons in the Whitesail Lake area. The volcanic rocks in the Whitesail Lake area contain r h y o l i t i c members, which makes them a candidate f o r future U-Pb dating. EARLY CRETACEOUS. POST-KINEMATIC UNITS Regional metamorphism and ductile deformation had defin i t e l y ceased by Early Cretaceous time, because the Gamsby Complex i s cut by post-kinematic intrusions of that age. Granodiorite sample 84V-99 (Fig. 9, unit 8), the f i r s t sample to be dated proving this relation, was emplaced 120 Ma ago. Evidence presented here indicates that the Black Dome pluton, which was involved in regional deformation and metamorphism, is cut by 127 Ma old post-kinematic quartz diorite (sample 81WV-9 71). Finally, migmatitic gneiss in the higher s t r u c t u r a l levels of the Gamsby Complex is intruded by post-kinematic, pegmatitic granite dykes and s i l l s (Fig. 28a and 28b) which are at l e a s t 132 Ma old, based on the age of contact-metasomatic hornblende (sample 8 4V- 98-1). This is es s e n t i a l l y identical to the oldest dates f o r metamorphic hornblendes (samples 80V-60, 83V-11), suggesting that the gneisses forming the higher s t r u c t u r a l levels of the Gamsby Complex did not cool below 530°C unt i l Early Cretaceous time. Pre-metamorphic hornblendes from lower, greenschist facies s t r u c t u r a l l e v e l s , on the other hand, give dates as old as 145 Ma. This contrast in age may indicate that the higher grade gneisses cooled below 530°C about 12 Ma l a t e r than the underlying greenschists. That the gneisses indeed remained at high temperatures much longer than the underlying greenschists is c l e a r l y implied by the i r s t a t i c a l l y annealed fabrics, which contrast with the primary tectonite fabrics in the greenschist 79 succession (van der Heyden, 1982). A complex cooling h i s t o r y is envisioned, in which upward r e t r e a t of the 530°C isotherm through the greenschist package occurred at about 145 Ma, following inversion of isotherms during early Late J u r a s s i c tectonism. Final cooling of annealed gneisses at higher s t r u c t u r a l levels, perhaps during uplift, did not occur u n t i l about 12 Ma l a t e r , in Early Cretaceous time. This fi n a l cooling of the Gamsby Complex appears to have coincided with the pronounced Late J u r a s s i c - E a r l y Cretaceous Cordilleran magmatic l u l l reported by Armstrong (1988). The younger, middle Cretaceous hornblende K-Ar dates from the Gamsby Complex (samples 83V-10, 80V-20A, 80V-30 and 80V-34) are interpreted to r e f l e c t variable p a r t i a l argon l o s s due to lo c a l , post- kinematic thermal overprint, either in the aureoles of middle Cretaceous and younger plutons or in the v i c i n i t y of hot c r u s t of the Eocene CGC. One middle Cretaceous intrusion (Fig. 9, unit 8; sample 84V-9 9) and s e v e r a l large, younger plutons (see below) intrude the Gamsby Complex; others may not yet have been exposed by erosion. The Gamsby Complex was in direct contact with the rising, hot CGC during the Eocene upli f t , and the CGC is inferred to underlie the Gamsby Complex at f a i r l y shallow depth (see ea r l y T e r t i a r y units, below). The Early Cretaceous, post-kinematic intrusive rocks of the Gamsby Complex are perhaps cogenetic with roughly coeval volcanic rocks in the southern Whitesail Lake area (Fig. 5, unit 8; sample 85WV-SAL). These appear to unconformably overlie the Late J u r a s s i c Trapper pluton and Middle J u r a s s i c s t r a t a of the Hazelton Group, or they are t h r u s t over these rocks. Similar Early Cretaceous volcanic rocks to the south, the Gambier Group, occur in a broad, discontinuous belt between the Whitesail Lake and Vancouver areas 80 (Wheeler and McFeely, 19 87). North of Vancouver these volcanic rocks overlie older, deformed, Late Jurassic(?) plutons at s e v e r a l locations (Roddick, 1966; Caron, 1974; Mathews, 1972; McKillop, 1973), and they include conglomerates with c l a s t s derived from these plutons, suggesting that Late J u r a s s i c tectonism and Late J u r a s s i c - E a r l y Cretaceous upl i f t and erosion were widespread in the CPC. The Ear l y Cretaceous plutons and volcanic rocks of the CPC represent the f i r s t igneous a c t i v i t y following the Late J u r a s s i c - E a r l y Cretaceous Cordilleran magmatic l u l l . MIDDLE AND LATE CRETACEOUS UNITS Late Cretaceous plutonic and hypabyssal rocks of the Bulkley and Kasalka intrusions (Fig. 5, unit 13) are common in the c e n t r a l part of the Whitesail Lake area (Woodsworth, 1980), where they are associated with important ore deposits (Maclntyre, 1985). Several of these intrusions were previously dated by K-Ar geochronometry, giving biot i t e and whole rock ages between 75.7 + 2.3 and 87±4 Ma (summarized in Maclntyre, 19 8 5). Late Cretaceous intrusive a c t i v i t y was not r e s t r i c t e d to the Intermontane Belt as indicated by the ca. 68 Ma U- Pb date for leucogranite (sample 83V-9) in the Gamsby Complex (Fig. 22), and the 77.4±5.2 Ma U-Pb date for the Kemano pluton (samples 78WV-344 and -347) in the CGC. The Horetzky Dyke, previously dated by bioti t e K-Ar at 73.5 + 1.2 Ma (Wanless et. al., 1979), appears to be a high l e v e l discordant extension of the Kemano pluton into the Gamsby Complex. Late Cretaceous plutons are also known from the CPC in the Prince Rupert and Douglas Channel-Hecate S t r a i t areas (Hill, 19 84; Chapter 2, this study). 81 The Late Cretaceous plutons in the Intermontane Belt intrude continental volcanic rocks of the Kasalka Group, which unconformably overlie marine sediments of the middle Albian Skeena Group (Woodsworth, pers. comm.) and which are therefore constrained to be middle Cretaceous (late E a r l y - e a r l y Late Cretaceous) in age. A dacitic t u f f from the near the base of the Kasalka Group previously gave a 105±5 Ma whole rock K-Ar date, and an andesite dyke which cuts immediately underlying Skeena Group, and which may be a feeder to the overlying volcanic rocks, gave a 104±8 Ma whole rock K-Ar date (Maclntyre, 19 76). A hornblende K-Ar date of 9 3.4±2.4 Ma was recently obtained by the Geological Survey of Canada from a porphyritic flow in the upper part of the Kasalka Group (Woodsworth, unpubl. data). Collectively, these dates indicate that the Kasalka Group i s late Albian and/or Cenomanian in age. The Kasalka Group is. therefore time- c o r r e l a t i v e with the Albian-Cenomanian Spences Bridge Group of southern B r i t i s h Columbia, the upper part of which was dated by whole rock K-Ar at 94.4±3.4 Ma by Thorkelson (1985). This i s an important observation. The base of the Kasalka Group, an e a s t e r l y tapering wedge of coarse c l a s t i c material which contains granitoid c l a s t s derived from the CPC (Woodsworth, 1979), provides the only unambiguous evidence in the Whitesail Lake area for middle Cretaceous tectonism in the CPC. Cretaceous tectonic a c t i v i t y in the eastern CPC is also suggested by the l o c a l s t r u c t u r a l involvement of the Skeena Group in apparent northeast vergent b r i t t l e thrusting of the Gamsby Complex on the the Gamsby thrust, near the head of Whitesail Lake (Fig. 5, unit 7; van der Heyden, 1982; Diakow and Koyanagi, 19 88). Major northeast vergent thrusting in middle Cretaceous time has been documented elsewhere along the eastern 82 flank of the CPC in the Terrace area (Woodsworth et al., 19 85), in the Mt. Waddington area (Tipper, 1969, 1978; Rusmore and Woodsworth, 1988) and in the northeast Cascades (Misch, 1966; Coates, 1974). The evidence is scanty, but these observations for the Whitesail Lake area indicate a brief episode, in Albian time, of northeast vergent b r i t t l e thrusting of the CPC over the Intermontane belt. The basal conglomerate of the Kasalka Group, which was probably shed off t h r u s t - f a u l t escarpments bounding the CPC, formed a small c l a s t i c wedge, which covered the Skeena Group and which was s h o r t l y afterward blanketed by continental volcanic rocks. THE GAMSBY THRUST AND ASSOCIATED STRUCTURES The observations made above for the Intermontane Belt imply middle Cretaceous tectonic a c t i v i t y in the western Whitesail Lake area, but supporting evidence from the CPC i t s e l f is rare. The Gamsby thrust appears to have been at l e a s t l o c a l l y active in Cretaceous time, as indicated above by the involvement of the Skeena Group. Also, some b r i t t l e t h r u s t s within the Gamsby Complex, which overprint and li e sub-parallel to the ductile fabrics, must be p o s t - E a r l y Cretaceous, because they occur within the gneissic succession, which did not cool below 530°C unt i l E a r l y Cretaceous time. One of these thr u s t s (near sample 80V-60) has northeast trending slickensides, similar to lineations observed in deformed sediments of the Skeena Group near the head of Whitesail Lake. However, along most of i t s length the age of the Gamsby thru s t is poorly constrained. It cuts the Middle J u r a s s i c (ca. 175 Ma) Tahtsa Complex and is intruded by the Late Cretaceous Horetzky Dyke (ca. 73.5 Ma, see below). The close s p a t i a l and s t r u c t u r a l association of 83 the Gamsby thru s t with the overlying inverted metamorphic tectonite zone suggests a Late J u r a s s i c age f o r this b r i t t l e thrust. The preliminary 146±6 Ma and 153±4 Ma 2 C , ePb/ 2 : a aU dates from a granite dyke (sample 83V-1) which cuts across a thrust believed to be r e l a t e d to the Gamsby thrust, also suggest a Late J u r a s s i c age for t h r u s t motion. I therefore t e n t a t i v e l y i n f e r an e a r l y Late J u r a s s i c age f o r the Gamsby thrust, and suggest that some of the younger b r i t t l e t h r u s t s within the Gamsby Complex, as well as the l o c a l s t r u c t u r a l involvement of the Skeena Group in the footwall, were associated with l o c a l r e a c t i v a t i o n of the Gamsby thru s t in middle Cretaceous time. EARLY TERTIARY UNITS. Plutons Paleocene and Eocene plutons (Fig. 5, unit 14 in part; Nanika, Goosly Lake and Quanchus intrusions; Carter, 19 81; Church, 19 71; Duffel, 19 59) and associated volcanic rocks (Fig. 5, unit 8 in part; Ootsa Lake and Endako Groups; Duffel, 19 59; Armstrong, 19 49) are major components of the Intermontane Belt in the c e n t r a l and eastern parts of the Whitesail Lake area. Many were previously dated by hornblende and biot i t e K-Ar (e.g. Carter, 1981; Wanless at al., 1979; Stevens et al., 1982). The Gamsby pluton (48.9 + 1.3 Ma, biotite) and the Ear Creek pluton (58.8±0.9 Ma, b i o t i t e ; Fig. 5), are two of these plutons, which intrude the Gamsby Complex in the SLFZ. The Gamsby pluton has a horizontal, intrusive contact with the overly