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Geology and geochronometry of the coast plutonic complex adjacent to Douglas, Sue and Loretta channels,… Runkle, Dita Elisabeth 1979

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GEOLOGY AND GEOCHRONOMETRY OF THE COAST PLUTONIC COMPLEX ADJACENT TO DOUGLAS, SUE AND LORETTA CHANNELS BRITISH COLUMBIA DITA ELISABETH RUNKLE B. A. Mount Holyoke C o l l e g e , 1974 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l Sciences We a c c e p t . t h i s t h e s i s as conforming to the req u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA October 1979 c) D i t a E l i s a b e t h Runkle, 1979 by MASTER OF SCIENCE i n In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa r t m e n t The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 D a t e / U n r . n 1 4 1 1 i F r o n t i s p i e c e i i ABSTRACT Fi v e major u n i t s were mapped i n s h o r e l i n e exposures of the Coast P l u -t o n i c Complex adjacent to Douglas, Sue and L o r e t t a Channels: 1) paragneiss and migmatite of the C e n t r a l Gneiss Complex, 2) q u a r t z - b i o t i t e s c h i s t , amphibolite, q u a r t z i t e , a n d marble of the Metasedimentary Rocks, 3) quartz d i o r i t e and quartz monzonite of the K i t k i a t a p l u t o n , 4) g r a n o d i o r i t e of the Quottoon pl u t o n and 5) Agmatite, composed of e a r l y x e n o l i t h s of banded metamorphic rock, middle stage i n t r u s i o n s of g r a n i t i c rock, and l a t e pegma-t i t e , a p l i t e and lamprophyre d i k e s . Pressure estimates of 7 - 1 kb f o r t h i s part of the Coast Mountains are a r r i v e d at from the s t a b l e assemblages i n a k y a n i t e - s t a u r o l i t e s c h i s t , and by comparison w i t h pressures published f o r areas along s t r i k e to the north. Amphibolites provide a low temperature estimate o f 55CTC. C a l c -s i l i c a t e assemblages place the high temperature l i m i t of metamorphism between 560 and 660°C at 5 kb. Elevated pressures would increase tempera-ture somewhat. The area i s s t r u c t u r a l l y c h a r a c t e r i z e d by a w e l l developed, s t e e p l y dipping f o l i a t i o n that s t r i k e s northwest, widespread i s o c l i n a l f o l d s w i t h a x i a l plane f o l i a t i o n p a r a l l e l to r e g i o n a l f o l i a t i o n , and a moderate to s t e e p l y plunging f o l d a x i s l i n e a t i o n . I n t e r f e r e n c e s t r u c t u r e s show that the abundant i s o c l i n a l f o l d s deform e a r l i e r approximately n o r t h e a s t - t r e n d i n g f o l d s . L a t e r t i g h t to i s o c l i n a l f o l d s deform the r e g i o n a l f o l i a t i o n . Meta-morphic r e c r y s t a l l i z a t i o n o u t l a s t e d deformation. The K i t k i a t a p l u t o n has an i n i t i a l ^ S r / ^ S r r a t i o of 0.7042 to 0,7043, depending on i t s age. One radiogenic S r - r i c h sample gives a model 87 86 date of 165 + 11 Ma, but the p o s s i b i l i t y of anomalous i n i t i a l Sr/ Sr i i i r a t i o makes t h i s r e s u l t h i g h l y u n c e r t a i n , + The Quottoon p l u t o n gives a wholer-rock isochron of 51 - 2 Ma w i t h 0.7045 i 1 i n i t i a l r a t i o . The low i n i t i a l r a t i o s i n d i c a t e a preponderance of mantle-derived magma of Mesozoic or e a r l y Cenozoic age i n the plutons s t u d i e d , Sr i s o t o p i c composition of the C e n t r a l Gneiss Complex i s compatible w i t h l a t e P a l e o z o i c - e a r l y Mesozoic ages f o r precursor d e t r i t a l and v o l c a n i c s t r a t a and l o c a l presence of marine carbonate w i t h moderately enriched 87 86 Sr/ Sr. The g e n e r a l l y low radiogenic Sr content of these core gneisses r u l e s out an o r i g i n by r e m o b i l i z a t i o n of g r e a t l y o l d er rocks. Plutons of the Coast P l u t o n i c Complex may have been generated by p a r t i a l m e l t i n g of the C e n t r a l Gneiss Complex, and emplaced not f a r from t h e i r source of o r i g i n during r e g i o n a l metamorphism, as the surrounding rocks cooled from maximum temperature and pressure of metamorphism. i v TABLE OF CONTENTS ABSTRACT i i GENERAL INTRODUCTION 1 PAPER 1 2 ABSTRACT 3 INTRODUCTION 4 PREVIOUS WORK IN AND NEAR STUDY AREA 6 REGIONAL GEOLOGY 7 St r u c t u r e 7 Me tamo rph i c Ro c ks 9 Metamorphic Grade 9 S t r a t i g r a p h i c Age and R e l a t i o n s h i p s of Metamorphic Rocks 10 Age of Regional Metamorphism 11 P l u t o n i c Rocks 13 P l u t o n i c S t r u c t u r a l S t y l e 14 Age of P l u t o n i c Rocks 16 GEOLOGY OF STUDY AREA 19 METAMORPHIC ROCKS 20 Ce n t r a l Gneiss U n i t 20 Metasedimentary Rock U n i t 22 Contact Between Metamorphic Rock Units 23 Precursors of Metamorphic Rocks 23 PETROGRAPHY OF METAMORPHIC ROCKS 24 Gneiss 24 Migmatite 25 Amphibolite 25 V Q u a r t z - B i o t i t e S c h i s t 26 C a l c - S i l i c a t e Rocks 26 METAMORPHIC CONDITIONS 28 Amphlbolite 28 Kyanite S c h i s t 28 C a l c - S i l i c a t e s 30 C a l c - S i l i c a t e s from C e n t r a l Gneiss 31 C a l c - S i l i c a t e s from Metasedimentary Rocks 31 Summary of Metamorphic Conditions 37 PLUTONIC ROCKS 38 K i t k i a t a P l u t o n 38 Quottoon P l u t o n 40 Agmatite 43 Lamprophyre Dikes 48 STRUCTURAL GEOLOGY 50 Planar Structures 50 Folds and L i n e a t i o n 53 Comparison w i t h S t r u c t u r e i n Nearby Areas 56 SUMMARY AND CONCLUSIONS 58 ACKNOWLEDGMENTS 60 REFERENCES 62 PAPER 2 66 ABSTRACT 67 INTRODUCTION 69 ECSTALL PLUTON 75 KITKIATA PLUTON 78 QUOTTOON PLUTON 79 v i SCHISTS NEAR PRINCE RUPERT 84 CENTRAL. GNEISS COMPLEX 85 IMPLICATIONS OF INITIAL 8 ? . S r / 8 5 S r RATIOS 90 ACKNOWLEDGMENTS 9 2 REFERENCES 93 v i i FIGURES f r o n t i s p i e c e i 1-1 Regional geology 5 1 - 2 Regional s t r u c t u r e 8 1 - 3 L o c a t i o n of map-areas 12 1-4 Regional K-Ar dates 17 1 -5 Geology of f i e l d area i n pocket 1-6 Banded migmatite 21 1-7 Metamorphic c o n d i t i o n s f o r k y a n i t e s c h i s t 29 1 - 8 T + X C 0 s t a b i l i t y of c a l c - s i l i c a t e s i n C e n t r a l Gneiss 34 1-9 Sample l o c a t i o n s i n pocket 1-10 T - X p n s t a b i l i t y of c a l c - s i l i c a t e s i n Metasedimentary Rocks 3 5 L U 2 1-11 Contact of K i t k i a t a p l u t o n w i t h Metasedimentary Rocks 39 1 - 1 2 Sheared K-feldspar veins i n Quottoon p l u t o n 4 2 1 - 1 3 Multiphase i n t r u s i o n i n agmatite 4 7 1-14 Planar s t r u c t u r e s 51 1 - 1 5 Fold s t y l e s i n C e n t r a l Gneiss rocks 54 1- 16 Fold s t y l e s i n Metasedimentary rocks 5 5 2 - 1. Rb-Sr sample l o c a t i o n s 7 0 2 - 2 Isochron diagram f o r the E c s t a l l p l u t o n 7 6 2 - 3 Isochron diagram f o r plutons sampled on Hawkesbury I s l a n d 8 0 2 - 4 Isochron diagram f o r s c h i s t s sampled near P r i n c e Rupert 8 6 2 - 5 Isochron diagram f o r metamorphic rocks of the C e n t r a l Gneiss Complex 88 v i i i TABLES 1- 1 C a l c - s i l i c a t e parageneses and r e l e v a n t r e a c t i o n s 32 2- 1 Sr isotope data, P r i n c e Rupert r e g i o n 71 2-2 U-Pb data f o r z i r c o n separates from the Quottoon p l u t o n 83 1 GENERAL INTRODUCTION This t h e s i s i s presented as two complete papers. The f i r s t describes the geology of the Coast P l u t o n i c Complex adjacent to Douglas, Sue and L o r e t t a Channels, B r i t i s h Columbia, based on f i e l d mapping and petrography of the area. Paper 2 i s a Rb-Sr i s o t o p i c study of rocks from the same area. Conclusions i n Paper 1 on the geology of the t h e s i s area are i n part drawn from r e s u l t s presented i n Paper 2. 2 Paper 1: GEOLOGY OF THE COAST PLUTONIC COMPLEX ADJACENT TO DOUGLAS, SUE AND LORETTA CHANNELS, B.C. by D i t a E l i s a b e t h Runkle 3 ABSTRACT Fiv e major u n i t s were mapped i n s h o r e l i n e exposures of the Coast P l u -t o n i c Complex adjacent to Douglas, Sue and L o r e t t a Channels: 1) paragneiss and migmatite of the C e n t r a l Gneiss Complex, 2) q u a r t z - b i o t i t e s c h i s t , amphibolite, q u a r t z i t e and marble of the Metasedimentary"Rocks, 3) quartz d i o r i t e and quartz monzonite of the K i t k i a t a p l u t o n , 4) g r a n o d i o r i t e of the Quottoon pl u t o n and 5) Agmatite, composed of e a r l y x e n o l i t h s of banded metamorphic rock, middle stage i n t r u s i o n s of g r a n i t i c rock, and l a t e peg-ma t i t e , a p l i t e and lamprophyre d i k e s . Pressure estimates of 7 - 1 kb f o r t h i s part of the Coast Mountains are a r r i v e d at from the s t a b l e assemblages i n a k y a n i t e - s t a u r o l i t e s c h i s t , and by comparison w i t h pressures published f o r areas along s t r i k e to the north. Amphibolites provide a low temperature estimate of 550°C. C a l c -s i l i c a t e assemblages place the high temperature l i m i t of metamorphism between 560 and 660°C at 5 kb. Elevated pressures would increase tempera-tures somewhat. The area i s s t r u c t u r a l l y c h a r a c t e r i z e d by a w e l l developed, s t e e p l y dipping f o l i a t i o n that s t r i k e s northwest, widespread i s o c l i n a l f o l d s w i t h a x i a l plane f o l i a t i o n p a r a l l e l to r e g i o n a l f o l i a t i o n , and a moderate to s t e e p l y plunging f o l d a x i s l i n e a t i o n . I n t e r f e r e n c e s t r u c t u r e s show that the abundant i s o c l i n a l f o l d s deform e a r l i e r ^ a p p r o x i m a t e l y n o r t h e a s t - t r e n d i n g f o l d s . L a t e r t i g h t to i s o c l i n a l f o l d s deform the r e g i o n a l f o l i a t i o n . Meta-morphic r e c r y s t a l l i z a t i o n o u t l a s t e d deformation. 4 INTRODUCTION Gneiss and migmatite of the Central Gneiss Complex, undated meta-sedimentary and metavolcanic rocks and plutonic igneous rocks ranging in age from Jurassic to Eocene comprise the Coast Plutonic Complex that forms the Coast Mountains of British Columbia between latitudes 5 3 ° and 5 5 ° north. The area of this study l i e s 40 km south-southeast of Kitimat (Fig. 1 - 1 ) where Douglas, Sue and Loretta Channels expose a cross section of the major rock types in the Coast Plutonic Complex. This part of the Coast Mountains is unique because of the abundance of metasedimentary rocks. Shoreline exposures were mapped in detail from a 2 . 5 meter inflatable rubber boat. Exposures above treeline were not visited. Field observations, microscopic petrography, and Rb — Sr geochronometry are the basis of this report. The following topics are examined: 1 ) structure and petrology of the metasedimentary rocks, gneisses, migmatites and plutonic rocks, and 2)timing and physical conditions of metamorphism, intrusion and deformation. 55° TOON L AKE TSIMPSE AN PENNINSUL A * PRINCE VRUPE--Plutonic rocks Quartz monzonite Granodiorite Quartz diorite Dior il-e Gabbro - diorite Metasedimentary and metavolcanic rocks ^ Central Gneiss Complex i — 20 4 0 k m 53 Figure 1-1. Gen e r a l i z e d geologic map of the Coast P l u t o n i c Complex between l a t i t u d e s 53 and 55 no r t h . From Hutchison (1967) and Roddick (1970). Area of t h i s study i s o u t l i n e d . K = K i t k i a t a p l u t o n ; KL=Khtada Lake. 6 PREVIOUS WORK IN AND NEAR STUDY AREA Dolmage (1922, 1923) was the f i r s t to map the s h o r e l i n e of the C e n t r a l Coast Mountains. The region north of Douglas Channel was mapped by Padgham (1958) and Hawkesbury I s l a n d was mapped by Money (1959), both f o r M.Sc. theses at U.B.C. From 1962 to 1970, the Coast P l u t o n i c Complex was the subject of a four m i l e to one inch mapping p r o j e c t by the Ge o l o g i c a l Survey of Canada (Baer, 1967, 1968, 1973; D u f f e l l , 1959; D u f f e l l and Souther, 1964; Hutchison, 1967, 1970; Roddick, 1965, 1970; Souther, 1971; Tipper, 1969; Woodsworth, 1978, 1979). T o p i c a l studies along the Douglas Channel cross s e c t i o n have been made by Symons (1974, 1977a,b), Magaritz and Taylor (1976) and Smith and others (1979). 7 REGIONAL GEOLOGY The Coast Mountains are an 80 to 190 km wide physiographic zone that extends the length of B r i t i s h Columbia. The mountains are u n d e r l a i n by p l u t o n i c and metamorphic rocks, which i n t h e i r t o t a l extent (from the Cascades i n Washington to southeast Alaska and the Yukon) comprise the l a r g e s t known Phanerozoic b a t h o l i t h i c complex i n l t h e world. The Coast P l u t o n i c Complex (also c a l l e d the Coast C r y s t a l l i n e Complex) i s flanked by Mesozoic and P a l e o z o i c sedimentary and v o l c a n i c rocks of the I n s u l a r and Intermontane B e l t s . Between 53° and 55° N, ..the Complex i s c h a r a c t e r -i z e d by numerous elongate northwest-trending, c o a l e s c i n g plutons, l a r g e areas of migmatite, and subordinate metamorphic pendants and screens. S t r u c t u r e The gross s t r u c t u r a l trend of the c e n t r a l Coast C r y s t a l l i n e Complex, defined by g n e i s s i c l a y e r i n g , s t e e p l y dipping f o l i a t i o n s , axes of minor f o l d s and elongate plutons, i s northwest-southeast. Major s t r u c t u r e s and trends are shown on F i g u r e 1-2. Metasedimentary rocks north of the Skeena Ri v e r and northeast of the Quottoon p l u t o n are i s o c l i n a l l y f o l d e d i n t o an asymmetric, northwest plunging synform, i t s a x i a l s u rface d i p p i n g northeast. The Foch Antiform i n the Douglas Channel area i s s l i g h t l y more open and l i k e w i s e overturned southwest, w i t h the Quottoon emplaced i n t o the f o l d core. Padgham (1958) shows a synform i n metasedimentary rocks southwest of the Foch Antiform and north of Douglas Channel. The major northwest trend i s superimposed on an o l d e r northeast f o l d trend, overturned both to the north and south (Hutchison, 1967; Baer, 1967). Metamorphic s t r u c -tures are o v e r p r i n t e d by the Hawkesbury Warp, a counterclockwise r o t a t i o n , 8 Figure 1-2. Major s t r u c t u r e s i n the Coast P l u t o n i c Complex between l a t i t u d e s 53° and 55 no r t h . From Hutchison (1967), Roddick (1970) and Padgham (1958) . 9 v e r t i c a l a x i s f l e x u r e i n s t r u c t u r a l trends across Douglas Channel. Steeply dipping l a y e r i n g and f o l i a t i o n are d e f l e c t e d to almost east-west s t r i k e across Hawkesbury I s l a n d . Previous workers i n the Coast P l u t o n i c Complex i n d i c a t e that the s t r u c t u r e i s m u l t i - e p i s o d i c , but the g e n e r a l l y l a r g e g r a i n s i z e , and completeness of r e c r y s t a l l i z a t i o n has made i t d i f f i c u l t to document a d e f i n i t i v e sequence of s t r u c t u r a l events. Metamorphic R.ocks Metamorphic rocks achieve highest grade and have g r e a t e s t a r e a l extent i n the a x i a l core zone, forming the C e n t r a l Gneiss Complex. In P r i n c e Rupert — Skeena R i v e r map-area (Hutchison, 1967; see F i g . 1-3), the Cen-t r a l Gneiss Complex i s comprised of leucogneiss, s i l l i m a n i t e paragneiss, a s s o c i a t e d migmatite and some amphibolite. Douglas Channel — Hecate S t r a i t map-area (Roddick, 1970) contains paragneiss, h o r n b l e n d e - b i o t i t e gneisses, amphibolites and a s s o c i a t e d migmatites. Mapped se p a r a t e l y from the C e n t r a l Gneiss Complex are Metasedimentary Rocks that i n c l u d e hornblende s c h i s t s and impure q u a r t z i t e s derived from a r g i l l i t e , graywacke and t u f f , w i t h i n t e r c a l a t i o n s of limestone and conglomerate. Metamorphic Grade In the Coast Mountains, metamorphism of the B a r r o v i a n type increases from greenschist f a c i e s i n the western Tsimpsean Peninsula through epidote-amphibolite and lower to upper amphibolite f a c i e s f u r t h e r east. East of the Ponder and A l a s t a i r Lake pl u t o n s , metamorphic grade decreases a b r u p t l y . H o l l i s t e r (1975) described l o c a l occurrences of g r a n u l i t e f a c i e s metamor-phism, p a r t i c u l a r l y i n the r e g i o n near Khtada Lake. From phase e q u i l i b r i a and f l u i d i n c l u s i o n s t u d i e s , H o l l i s t e r and others (1975) defined two p o i n t s 10 on a metamorphic gradient. " The f i r s t p o i n t i s 10 km east of P r i n c e Rupert, at 8 ± 1 kbars and 600 + 50°C. The second, 50 km east of P r i n c e Rupert, i s at 6 to 8 kbars and 700 to 800°C ( H o l l i s t e r and others, 1975; Lappin, 1976). Migmatite appears midway between the two p o i n t s , i n the k y a n i t e s t a b i l i t y f i e l d . M o b i l i z a t i o n of g r a n i t i c m a t e r i a l begins near the high temperature p o i n t . H o l l i s t e r concluded that the g r a n u l i t e s were formed a t 15 to 25 km depth, w i t h temperatures high enough to p a r t i a l l y melt rocks, even w i t h PTT . below P The C e n t r a l Gneiss Complex i s not p a r t of a p a i r e d H2O t o t a l metamorphic b e l t . S t r a t i g r a p h i c Age and R e l a t i o n s h i p s of Metamorphic Rocks The r e l a t i o n s h i p between C e n t r a l Gneiss and Metasedimentary Rocks i s not c e r t a i n , i n p a r t , because contacts are obscured by migmatite. In the P r i n c e Rupert area, Hutchison (1967) suggests a basement-cover r e l a -t i o n s h i p between the two, based on marked d i f f e r e n c e s i n metamorphic character and s t r u c t u r a l s t y l e . In Douglas Channel, Roddick (1970) recognized no c l e a r d i f f e r e n c e i n metamorphic grade, and described grada-t i o n between the two sequences, but does not discount the p o s s i b l e e x i s -tence of a major s t r a t i g r a p h i c h i a t u s . Baer (1968) reports a s t r u c t u r a l and metamorphic unconformity between s u p r a c r u s t a l rocks and o l d e r gneisses i n B e l l a Coola — Laredo Sound map-area south of Douglas Channel. In W h i t e s a i l Lake map-area, east of Douglas Channel, Woodsworth (1978, 1979) recognizes the Gamsby Group, a northwest trending b e l t of volcanogenic sedimentary rocks that grades by i n c r e a s i n g metamorphism and metasomatism southwest i n t o the C e n t r a l Gneiss Complex. Hutchison (1967) favors a pre-Permian o r i g i n f o r the C e n t r a l Gneiss Complex, although f o s s i l evidence i s l a c k i n g i n metasedimentary rocks of 11 the P r i n c e Rupert — Skeena River and Douglas Channel — Hecate S t r a i t map-areas. The p r o x i m i t y of unmetamorphosed, f o s s i l i f e r o u s Permian s t r a t a i n the adjacent Terrace map-area ( D u f f e l l and Souther, 1964) and the l a c k of a metamorphic equivalent w i t h i n the C e n t r a l Gneiss Complex are given as evidence that the gneisses predate the Permian (Roddick and Hutchison, 1972). F o s s i l s are not present i n the Gamsby metasedimentary rocks, but Woodsworth assumed a l a t e P a l e o z o i c s t r a t i g r a p h i c age. Roddick (1970) considers the metasedimentary rocks i n Douglas Channel to be Permian, c o r r e l a t i n g them / w i t h the s i m i l a r but unmetamorphosed limestone-bearing s t r a t a i n the W h i t e s a i l Lake area. He a l s o notes the s i m i l a r i t y of the.banded q u a r t z i t e s to ribbon cherts of the Permo-Carboniferous Cache Creek group. This s i m i l a r i t y was noted by Souther and Armstrong (.1966) working f u r t h e r north i n the S t i k i n e r e g i o n . Rocks i n the C e n t r a l Gneiss Complex are probably l a t e P a l e o z o i c and perhaps o l d e r , and the s t r u c t u r a l l y higher metasedimen-tary rocks may be l a t e P a l e o z o i c or younger. Age of Regional Metamorphism Information a v a i l a b l e from the f o l l o w i n g areas i n the Coast Mountains i n d i c a t e s that r e g i o n a l metamorphism occurred during the e a r l y Mesozoic, perhaps from E a r l y T r i a s s i c to Middle J u r a s s i c : 1) West of A t l i n Lake, Werner (1978) obtained a 235 Ma Rb-Sr is o c h r o n f o r a p l u t o n that cuts metamorphic terrane. Also near A t l i n Lake, Bultman (1979) reports Upper T r i a s s i c s t r a t a r e s t i n g unconformably on metamorphosed rocks of the Coast P l u t o n i c Complex, i n d i c a t i n g pre-Upper T r i a s s i c metamorphism. 2) In Tulsequah map-area, Souther (1971) r e p o r t s i n t e n s e f o l d i n g and r e g i o n a l metamorphism of Permian to 12 THIS STUDY MASS RIVER PRINCE RUPERT-SKEENA RIVER TERRACE DOUGLAS CHANNEL-HECATE STRAIT jWHITESAIL LAKE (BELLA COOLA-LAREDO SOUND JMOUNT 5 I WADDINGTON ANCOUVER 48° L Figure 1 - 3 . Locations of map-areas referred to in text. 13 Middle T r i a s s i c s t r a t a , during the pre-Upper T r i a s s i c Tahltanian orogeny. 3) In Nass R i v e r map-area, (Monger and Hutchison, 1971), Upper J u r a s s i s rocks are unmetamorphosed, so meta-morphism was pre-Upper J u r a s s i c , 4) Arguments f o r post-Permian metamorphism i n the P r i n c e Rupert and Douglas Channel regions have already been mentioned. 5) In B e l l a Coola — Laredo Sound map-area, Baer suggests a T r i a s s i c to Middle J u r a s s i c metamorphism. 6) In Mount Waddington map-area (Tipper, 1971), unmeta-morphosed H a u t e r i v i a n rocks ( e a r l y Cretaceous) o v e r l i e metamorphosed Upper T r i a s s i c s t r a t a . Discrepancies i n the timing of metamorphic cul m i n a t i o n i n these various areas may r e f l e c t a c t u a l m i g r a t i o n of the focus of metamorphism w i t h time, or the spotty and sometimes i n c o n c l u s i v e nature of the evidence. P l u t o n i c Rocks In the Coast P l u t o n i c Complex, quartz d i o r i t e and g r a n o d i o r i t e are the predominant p l u t o n i c igneous rock types, d i o r i t e and quartz monzonite are more abundant than gabbro, and t r u e g r a n i t e i s r a r e . Roddick (1965) estimated f o r the Vancouver map-area that an average p l u t o n i c rock would be a h o r n b l e n d e - b i o t i t e quartz d i o r i t e w i t h 5% K-feldspar. Quartz d i o r i t e i s more abundant towards the west, and K-feldspar increases towards the east, but a quartz d i o r i t e l i n e (Moore and others, 1962) i s not sharply defined. 14 P l u t o n i c S t r u c t u r a l S t y l e Hutchison (1970) recognized s e v e r a l s t r u c t u r a l s t y l e s of plutonism f o r the c e n t r a l Coast Mountains. He proposed that the plutons were generated i n the C e n t r a l Gneiss Complex, and.that p l u t o n i c s t y l e s represent an evolu-t i o n a r y sequence of the va r i o u s stages of emplacement. Autochthonous, m i g m a t i t i c plutons represent plutons a t t h e i r source. Homogeneous p l u t o n i c rock and and migmatite i n t e r f i n g e r w i t h the e n c l o s i n g gneisses. Therefore, p l u t o n contacts are g r a d a t i o n a l , and mapped according to predominance of rock type. At the margins, the composition i s mafic-r i c h d i o r i t e and quartz d i o r i t e which grades to more l e u c o c r a t i c quartz d i o r i t e and g r a n o d i o r i t e i n the i n t e r i o r . This s t y l e i s poorly represented i n the P r i n c e Rupert r e g i o n , the Kasiks p l u t o n being the best example. Best developed i n the c e n t r a l Coast Mountains are the steep-sided para-autochthonous plutons. These are tadpole-shaped i n p l a n , w i t h a round i n t r u s i v e head i n the northwest and a mig m a t i t i c t a i l to the south-east. The E c s t a l l and Quottoon plutons are two examples. They have been f o r c i b l y emplaced, as the heads, p a r t i c u l a r l y of the E c s t a l l p l u t o n , have deformed metamorphic isograds and s t r a t a on the Tsimpsean Pen i n s u l a . The outer zones of the heads are d i o r i t e or quartz d i o r i t e . F o l i a t i o n s are steep and more s t r o n g l y developed on the margins. Towards the center, the rock i s more homogeneous and l e u c o c r a t i c g r a n o d i o r i t e and quartz monzonite. The t a i l s of these plutons are s i m i l a r to the autochthonous s t y l e ; a l t e r -n a t i n g zones of gneiss, migmatite and g n e i s s i c d i o r i t e and quartz d i o r i t e . The t a i l of the Quottoon i s much longer and wider than the E c s t a l l . In a d d i t i o n , the contact zone i s broader and more g r a d a t i o n a l . Hutchison b e l i e v e s that t h i s i s because the E c s t a l l i s at a higher s t r u c t u r a l l e v e l and has t r a v e l l e d f a r t h e r . 15 Recumbent, tongue-shaped, para-autochthonous plutons a r e represented by parts of the Ponder and A l a s t a i r Lake plutons. They appear to occupy the cores of l a r g e nappe s t r u c t u r e s and are s i m i l a r to migmatite tongues i n East Greenland ( H a l l e r , 1955). The tongues are west-vergent and concordantly o v e r l i e f l a t to gently eastward-dipping gneisses. The under-l y i n g contact i s a gradual t r a n s i t i o n from quartz d i o r i t i c gneiss upwards to g n e i s s i c quartz d i o r i t e , and f i n a l l y to homogeneous quartz d i o r i t e . The c e n t r a l part of A l a s t a i r Lake p l u t o n i s dominated by quartz d i o r i t e and low K g r a n o d i o r i t e . The Ponder p l u t o n i s g r a n o d i o r i t e and quartz monzonite. In both p l u t o n s , f o l i a t i o n i s more s t r o n g l y developed and migmatite more abundant to the northwest and southeast. The eastern contacts are sharp and d i s c o r d a n t , w i t h f o l i a t i o n only weakly developed. Small p o s t - t e c t o n i c allochthonous plutons are mainly g r a n o d i o r i t e and quartz monzonite w i t h l i t t l e i n t e r n a l s t r u c t u r e . The Toon Lake p l u t o n i s one example. In the autochthonous and para-autochthonous p l u t o n i c s t y l e s , contacts w i t h i n the plutons are sharper between phases of markedly d i f f e r i n g compo-s i t i o n , and g r a d a t i o n a l between s i m i l a r types. I n c l u s i o n s of country rock are abundant. They commonly occur as screens o r i e n t e d p a r a l l e l to the f o l i a t i o n , and are f l a t t e n e d near the margins of the plutons. I n c l u s i o n s i n the allochthonous plutons are l i m i t e d to the margins. Several trends are noted f o r the plutons i n going from deep to shallow emplacement. They become l e s s f o l i a t e d and c o n t a i n fewer i n c l u s i o n s , mig-matite gives way to more homogeneous p l u t o n i c igneous rocks, they become more a c i d i c , b i o t i t e replaces hornblende as the common mafic m i n e r a l , and the p l a g i o c l a s e has a decreasing a n o r t h i t e content. In general, p l a g i o c l a s e i n the p l u t o n i c rocks d i s p l a y s o s c i l l a t o r y zoning, i n d i c a t i n g a complex and 16 f a i r l y r a p i d h i s t o r y of c r y s t a l l i z a t i o n and emplacement. Age of P l u t o n i c Rocks Timing of plutonism i n the Coast P l u t o n i c Complex can be p a r t i a l l y understood through s t r a t i g r a p h i c r e l a t i o n s h i p s . G r a n i t i c d ebris i n Upper J u r a s s i c rocks i n Mount Waddington map-area (Tipper, 1969) and a l s o i n a metamorphic sequence l y i n g unconformably below E a r l y to Middle J u r a s s i c Hazelton Group i n W h i t e s a i l Lake map-area ( D u f f e l l , 1959) i n d i c a t e s that plutonism commenced p r e - E a r l y J u r a s s i c . I n t r u s i v e r e l a t i o n s h i p s provide evidence f o r l a t e r plutonism. The western contact of the southern Coast P l u t o n i c Complex i s i n t r u s i v e i n t o Upper T r i a s s i c Karmutsen Formation and Lower J u r a s s i c Bonanza Group (Nelson, 1979). On the east, i n W h i t e s a i l Lake map-area, plutons i n t r u d e Middle J u r a s s i c Hazelton Group ( D u f f e l , 1959). The youngest s t r a t a i ntruded i n P r i n c e Rupert map-area are Bowser Group sedimentary rocks east of the Ponder P l u t o n (Hutchison, 1967) that c o n t a i n Late J u r a s s i c and E a r l y Cretaceous f o s s i l s . Therefore, plutonism continued at l e a s t from p r e - E a r l y J u r a s s i c to p o s t - E a r l y Cretaceous. Published K-Ar dates f o r the Coast P l u t o n i c Complex are p l o t t e d on Figure 1-4. Hutchison (1967) grouped the K-Ar dates f o r p l u t o n i c igneous rocks i n the P r i n c e Rupert r e g i o n i n t o three zones that p a r a l l e l the r e g i o n a l s t r u c t u r e . They are the Eastern (40-50 Ma), C e n t r a l (64-80 Ma), and Western (84-140 Ma) Zones. The C e n t r a l and Western Zones are separated by the G r e n v i l l e Channel lineament. Hutchison (1967) suggested three p o s s i b l e explanations f o r the zones: 1) Sequential p l u t o n i c emplacement and r a p i d c o o l i n g from west to east, 2). A metamorphic f r o n t passing from west to east, or 3) Sequential block u p l i f t and c o o l i n g from west to east. Paleomagnetic s t u d i e s i n the P r i n c e Rupert r e g i o n by Symons (1974, 17 Figure 1-4. Locations of K-Ar dates f o r the Coast P l u t o n i c Complex between l a t i t u d e s 53 and 55 n o r t h . Compiled by G. Woodsworth from Wanless and others (1967, 1968) and in-house data. 18 1977a, b) c o n t r i b u t e to i n t e r p r e t a t i o n of the K-Ar dates. Paleopoles f o r the Quottoon and Kasiks plutons are concordant w i t h other Eocene poles f o r North America. Although the two plutons are not comagmatic, paleomagnetic and K-Ar r e s u l t s show that they have s i m i l a r c o o l i n g times. The remanent magnetism (RM) d i r e c t i o n s f o r both have normal p o l a r i t y , which suggests that they cooled through the Curie temperature (320-580°C; Haggarty, 1978) during a s i n g l e p o l a r i t y i n t e r v a l . P o l a r i t y i n t e r v a l s f o r the Cenozoic average about \ Ma, maximum about 1 Ma. Thus Symons concluded that the Quottoon and Kasiks plutons cooled through the Curie temperature i n l e s s than about % Ma. Results from the E c s t a l l p l u t o n show that some phases have reverse p o l a r i t y , t h e r e f o r e c o o l i n g took place over a t l e a s t two magnetic i n t e r v a l s ; probably i n l e s s than 1 Ma, according to Symons. Increasing metamorphic grade and decreasing K-Ar ages from west to east r e q u i r e that r e g i o n a l isotherms migrate down and eastward w i t h time. The paleomagnetic r e s u l t s show c l u s t e r s of c o n s i s t e n t p o l a r i t y w i t h i n plutons r a t h e r than an a l t e r n a t i n g p a t t e r n , and t h i s l e d Symons to conclude that the K-Ar ages record time of emplacement as a consequence of r a p i d c o o l i n g . 19 GEOLOGY OF STUDY AREA The study area is located on the southern limb of the Foch Antiform, and the central, east-west segment of the Hawkesbury Warp (Figure 1-2). Five units were mapped (Figure 1-5). Oldest are the polydeformed gneiss and migmatite of the Central Gneiss Complex. They are structurally over-lain by metavolcanic rocks and semi-pelitic metasedimentary rocks in the ~ov. south. On Hawkesbury Island, the southern part of the Central Gneiss at i t s contact with Metasedimentary Rocks, i s intruded by a stockwork of irregular dikes, to form a complex agmatite. To the northeast, Central Gneiss rapidly grades by increasing migmatization into rocks of the Quottoon pluton. The Kitkiata pluton intrudes the Metasedimentary Rocks in the southwest. The entire metamorphic and plutonic assemblage is cut by much younger lampro-phyre dikes, too small and numerous to map. Planar structures in the rocks are steeply dipping, several fold styles and trends are evident, and the ov overall fabric i s one of extreme flattening concordant to the regional foliation. 20 METAMORPHIC ROCKS Ce n t r a l Gneiss U n i t Banded quartz d i o r i t i c paragneiss and amphibolite are the most abun-dant rock types of the C e n t r a l Gneiss Complex (also r e f e r r e d to as " C e n t r a l Gneiss" and as "Gneiss and Migmatite"). Migmatite i s a l s o abundant. I n t e r -banded c a l c - s i l i c a t e rocks are r a r e and meta p e l i t e was not seen. In the f i e l d , f i n e - g r a i n e d paragneiss occurs as a l t e r n a t i n g dark and l i g h t gray to greenish bands (Figure l-14a). Width of the bands ranges from l e s s than a centimeter to n e a r l y a meter. There i s a s l i g h t v a r i a t i o n i n g r a i n s i z e between bands, but f o r the most p a r t , banding i s compositional and probably of metamorphic o r i g i n . Composition ranges from quartz d i o r i t i c to a m p h i b o l i t i c . In a d d i t i o n tb banding, mineral f o l i a t i o n i s w e l l developed. The rocks e x h i b i t both c h a o t i c and r e g u l a r f o l d s . Petrographic d e s c r i p t i o n s f o r t h i s and other rock types f o l l o w i n a l a t e r s e c t i o n . The percentage of migmatite increases toward the Quottoon p l u t o n . Migmatite looks s i m i l a r to gn e i s s , but i s g e n e r a l l y more l e u c o c r a t i c and coarser grained (Figure l - 1 4 c ) . Bands of l e u c o c r a t i c migmatite o f t e n a l t e r n a t e subconcordantly w i t h banded gneiss (Figure 1-6), and w i t h i n wider bands of more homogeneous gn e i s s , narrow coarse-grained l e u c o c r a t i c bands occur. M i g m a t i t i c areas a l s o c o n t a i n f o l d s that do not p a r a l l e l f o l d s i n nearby paragneiss. "•-Scattered through the g n e i s s i c rock are s e v e r a l i s o l a t e d bands con-t a i n i n g c a l c - s i l i c a t e m i n e r a l s ; u s u a l l y boudins of di o p s i d e w i t h a s s o c i -ated garnet, or small epidote pockets. Another c a l c - s i l i c a t e occurrence c o n s i s t s of 3 to 10 cm wide bands of a l t e r n a t i n g f i n e - and coarse-grained quartz-, epidote-, garnet- or d i o p s i d e - r i c h rock which repeat to a t o t a l 21 Figure 1-6. Banded migmatite, subconcordant to f o l i a t i o n i n Central Gneiss. 22 thickness of almost 5 meters. Metasedimentary Rock Uni t The Metasedimentary Rocks i n the southwest p o r t i o n of the map area s t r u c t u r a l l y o v e r l i e metamorphic rocks of the C e n t r a l Gneiss Complex. The most abundant rock type i n t h i s u n i t i s low alumina s e m i - p e l i t i c quartz-b i o t i t e s c h i s t . Other rock types i n order of abundance, are non-garnet-i f e r o u s and g a r n e t i f e r o u s amphibolite, limy q u a r t z i t e , s i l i c e o u s marble, and one area of muscovite s c h i s t . Q u a r t z - b i o t i t e s c h i s t i s f i n e - g r a i n e d (^lcm) and medium to dark gray. I t s appearance i n the f i e l d i s homogeneous, with a f a i n t b i o t i t e f o l i a t i o n which i s f u r t h e r obscured by post-kinematic b i o t i t e growth. On c l o s e r i n -s p e c t i o n , t i n y epidote granules give the rock a greenish appearance. Com-p o s i t i o n a l l a y e r s are u s u a l l y s e v e r a l meters t h i c k ; t h i c k e r than i n the C e n t r a l Gneiss; Folds are defined by weathered out l a y e r s . Amphibolite looks s i m i l a r to q u a r t z - b i o t i t e s c h i s t . Grain s i z e i s small enough that t h i n s e c t i o n i n s p e c t i o n i s o f t e n necessary to determine rock type. In t h i s d i s c u s s i o n , a rock w i l l be c a l l e d a m p h i b o l i t i c i f horn-blende i s more abundant than b i o t i t e , and i f p l a g i o c l a s e i s more abundant v than quartz. F i v e centimeter, rounded and f r a c t u r e d garnet porphyroblasts occur i n some amphibolite l o c a l i t i e s . Q u a r t z i t e s are f i n e - g r a i n e d and rusty-weathering (Figure l-14b) or gray to p i n k i s h i n c o l o r , sometimes w i t h a f a i n t s c h i s t o s i t y , depending on the amount of muscovite present. D i f f e r e n c e s i n c o l o r and composition are r e f l e c t e d as bands, approximately 10 cm wide. One l o c a l i t y of q u a r t z i t e contains dark streaks which could e i t h e r be o r i g i n a l crossbeds or transposed l a y e r i n g . 23 Marble bands tens of centimeters wide are i n t e r l a y e r e d w i t h quartz-b i o t i t e s c h i s t and amphibolite. One l o c a l i t y of marble i s white-.and cry-s t a l l i n e , the r e s t are gray. Muscovite s c h i s t i s rusty weathering, w e l l - f o l i a t e d and crenulated, wit h s m a l l garnet porphyroblasts. Contact Between Metamorphic Map Units On Hawkesbury I s l a n d , the contact between C e n t r a l Gneiss and Metasedi-mentary rocks i s obscured by agmatite. On the mainland northwest of Douglas Channel, agmatite i s not developed:, and medium-grained, banded f e l d s p a t h i c gneiss i s i n contact w i t h garnetiferous amphibolite that i s f i n e - g r a i n e d and lack s obvious f o l i a t i o n . The contact i s discordant to the f o l i a t i o n i n gneiss by almost 90°. I t may represent e i t h e r an o r i g i n a l sedimentary i n t e r f a c e between gneiss and the amphibolite parent, or e l s e a f a u l t . Precursors of Metamorphic Rocks Metamorphic rocks i n the study area represent two o r i g i n a l l y hetero-geneous assemblages of rocks. In the C e n t r a l Gneiss, the abundance of hornblende and p l a g i o c l a s e leads to the conclusion that these rocks could have once been b a s a l t - a n d e s i t e v o l c a n i c / v o l c a n i c l a s t i c rocks or graywackes. Greater amounts of b i o t i t e and quartz i n the Metasedimentary Rocks, and the i n t e r l a y e r i n g of q u a r t z i t e and marble i n d i c a t e that they were more l i k e l y derived from a v a r i e t y of more mature sediments. 24 PETROGRAPHY OF METAMORPHIC ROCKS S i m i l a r rock types from both the metamorphic map u n i t s have s i m i l a r mineralogy and petrography, so they w i l l be discussed together. Gneiss Gneisses are c h a r a c t e r i z e d by s m a l l g r a i n s i z e (—1mm) and by the gen-e r a l l y equidimensional nature of the g r a i n s . Mineralogy i s simple, and there i s a compositional v a r i a t i o n from g r a n o d i o r i t i c to d i o r i t i c leuco-gneiss. P l a g i o c l a s e and quartz are present i n subequal amounts i n gneiss, and together comprise up to 70% of the mode, by es t i m a t i o n . P l a g i o c l a s e ( A n 0 0 „ c) i s u s u a l l y unzoned. Rarely, normal zoning occurs at g r a i n edges. P o l y s e n t h e t i c twins and bent lamellae are a l s o r a r e . Grains are anhedral w i t h angular boundaries, except f o r some s m a l l rounded grains which resemble b a l l bearings, and appear along w i t h quartz at t r i p l e j u n c t i o n s . A few samples have s a u s s u r i t i z e d p l a g i o c l a s e . Wherever K- f e l d s p a r i s i n contact wit h p l a g i o c l a s e , myrmekitic quartz i s developed i n the p l a g i o c l a s e . Small polygonized quartz grains occur at t r i p l e j u n c t i o n s , and along e n t i r e g r a i n boundaries. Larger quartz grains are c l e a r and unstrained. K-feldspar i s not abundant. I t may comprise up to 20% of the t o t a l mode, but g e n e r a l l y accounts f o r < 5 % . In a few samples, K-feldspar i s l a r g e r augen-like g r a i n s , but the usual h a b i t i s small i n t e r s t i t i a l grains w i t h and without m i c r o c l i n e twinning. Where quartz i s abundant, b i o t i t e i s the major mafic m i n e r a l . I f amphibole i s al s o present, i t i s u s u a l l y broken and corroded, and as s o c i a t e d w i t h epidote and sphene. Baer (1973) n o t i c e d t h i s same a s s o c i a t i o n i n 25 rocks from the B e l l a Coola — Laredo Sound area. He a t t r i b u t e d i t to K metasomatism. Migmatite Petrography of migmatite i s not markedly d i f f e r e n t from the gneiss, B i o t i t e i s the only mafic mineral and comprises about 5% of the rock. P l a g i o c l a s e (An2^) and quartz are anhedral and occur i n a range of g r a i n s i z e s . M i c r o c l i n e comprises l e s s than 5% of the rock, o c c u r r i n g i n t e r s t i - : t i a l l y . Amphibolite Amphibolite i s common to both metamorphic rock u n i t s . Amphibolites range from 98% hornblende s c h i s t s , to q u a r t z - b i o t i t e - p l a g i o c l a s e - h o r n b l e n d e s c h i s t s . Hornblende-rich amphibolites have hornblende w i t h o l i v e brown-green pleochroism. When p l a g i o c l a s e makes up an appreciable percent of the mode, the hornblende pleochroism i s more blue-green. Hornblende grains are gen e r a l l y s u b i d i o b l a s t i c . They are o c c a s i o n a l l y f r a c t u r e d and embayed, but the o r i g i n a l c r y s t a l shape i s s t i l l preserved. P l a g i o c l a s e grains are subhedral to anhedral. P l a g i o c l a s e compositions from amphibolites w i t h i n the C e n t r a l Gneiss are An„~ c „ , and from the-. 30-500 Metasedimentary Rocks, range from An^^_^^. P l a g i o c l a s e i s g e n e r a l l y free from i n c l u s i o n s , and i n some s e c t i o n s only f a i n t l y s a u s s u r i t i z e d . B i o t i t e occurs both as ragged ended f l a k e s , and t a b u l a r euhedral g r a i n s . Quartz grains are u s u a l l y s m a l l and polygonal w i t h sharp g r a i n boundaries. Garnet occurs i n some samples as subhedral p o i k i l o b l a s t s w i t h abundant quartz, z o i s i t e and opaque ( p y r i t e / p y r r h o t i t e ) i n c l u s i o n s . 26 Microcline was recognized as an accessory in one sample. Others con-tain small amounts of anhedral diopside. Primary chlorite appears in one sample from the Metasedimentary Rocks. Common accessory minerals are rounded green sphene, irregular, clinozoisite/epidote and zoisite, tiny apatite needles, and rounded zircoh. Calcite i s present only in the Metasedimentary Rocks: ; in:'veinlets, i n reaction rimming garnet, and also rimming opaque inclusions in garnet. A mafic calc-silicate from a screen within the Kitkiata pluton is more similar to diopside bearing amphibolites in the gneiss unit. It contains poikiloblastic diopside in a pc(An )-qtz-bi-hb amphibolite. Clinozoisite JO' is an accessory. Several small scattered garnet grains appeared in hand specimen but not in the thin section. Quartz-Biotite Schist Quartz-biotite schists contain subequal amounts of quartz and biotite, with accessory plagioclase (An^^) and epidote. Schistosity is defined by parallelism of biotite. Grain boundaries are smooth, and quartz grain shapes are irregular and lobate'; to polygonal. Tiny f i b r o l i t e hairs also occur in a sample from a screen within the Kitkiata pluton. Calc-Silicate Rocks In the Central Gneiss, calc-silicates occur concentrated into iso-lated bands; in the Metasedimentary Rocks, they occur as minor phases in quartzite and marble. Quartz-epidote rock is the most common calc-silicate inc.the Central Gneiss. It occurs both as fine and coarse grained rocks. The grain boundaries are lobate and serrated, and grains of epidote and quartz have 27 d i s t i n c t s t r a i n shadows. Other c a l c - s i l i c a t e bands are e i t h e r quartz-, garnet-, or d i o p s i d e - r i c h . Quartz i s s t r a i n e d , garnet and diopside are anhedral and f r a c t u r e d . Z o i s i t e and c a l c i t e occur only as i n c l u s i o n s i n garnet. Other phases present are t r e m o l i t e , sphene, and aggregates of q u a r t z - e p i d o t e / z o i s i t e a f t e r p l a g i o c l a s e . In the Metasedimentary Rocks, d i o p s i d e , t r e m o l i t e , p l a g i o c l a s e , mus-c o v i t e , w o l l a s t o n i t e , c l i n o z o i s i t e / e p i d o t e - z o i s i t e , s c a p o l i t e and sphene occur as minor minerals i n s i l i c e o u s marble and limy q u a r t z i t e . In marble, c a l c i t e grains are large and polygonized. No c a l c i t e / dolomite t e s t s were made, aside from HC1 on the handspecimen, which efferT vesced r e a d i l y . Quartz, u s u a l l y i n l e n t i c u l a r concentrations, i s amoeboid and p a r t i a l l y polygonized. Tremolite occurs as r e l a t i v e l y l a r g e p o i k i l o -b l a s t s . Tiny t a l c f l a k e s and i r r e g u l a r s c a p o l i t e are acc e s s o r i e s s c a t t e r e d throughout some samples. In q u a r t z i t e , quartz grains are i n t e r l o c k i n g , w i t h 120° g r a i n bounda-r i e s . I r r e g u l a r blobs of c a l c i t e are u s u a l l y i n clos e p r o x i m i t y to musco-v i t e , which i s a l i g n e d i n the s c h i s t o s i t y plane. Muscovite u s u a l l y occurs as t h i n f l a k e s , but one sample contains l a r g e muscovite p o i k i l o b l a s t s around quartz. (Because of t h e i r s i m i l a r o p t i c a l p r o p e r t i e s , i t i s p o s s i b l e that t a l c has been m i s i d e n t i f i e d as muscovite. This would have minor i m p l i c a t i o n s i n the d i s c u s s i o n of metamorphic grade.) P l a g i o c l a s e ( A n ^ ) i s i n t e r s t i t i a l and occurs i n only a few samples. M i c r o c l i n e was seen i n j u s t one. I r r e g u l a r w o l l a s t o n i t e grains occur i n samples near the agmatite contact. Other minerals that occur, but not i n a l l q u a r t z i t e s examined, are b i o t i t e f l a k e s , s c a p o l i t e , anhedral sphene and z o i s i t e . 28 METAMORPHIC CONDITIONS Amphibolite C o e x i s t i n g phases i n the amphibolites from Hawkesbury I s l a n d are t y p i -c a l of Winkler's (1975) medium grade metamorphism of metamorphic rocks which i s equivalent to the a l b i t e - e p i d o t e amphibolite and amphibolite f a c i e s of r e g i o n a l metamorphism (Miyashiro, 1973). In p e l i t i c r ocks, medium grade begins w i t h the f i r s t appearance of s t a u r o l i t e ( r e a c t i o n 1, Figure 1-7). The p l a g i o c l a s e composition and blue-green amphibole i n rocks from Hawkes-bury I s l a n d are c h a r a c t e r i s t i c of lower temperatures of the "medium grade". Engel and Engel (1962) observed the change from blue-green hornblende to the more o l i v e hornblende to take place w i t h i n the amphibolite grade of metamorphism i n the Adirondacks. With f u r t h e r increase of metamorphic grade to high grade metamorphism or upper amphibolite f a c i e s , there i s a decrease i n quartz and hornblende i n favor of clinopyroxene and more c a l c i c p l a g i o c l a s e . On Hawkesbury I s l a n d , the changes described by Engel and Engel are present, but not s p a t i a l l y systematic, so they are a t t r i b u t e d to l o c a l d i f f e r e n c e s i n metamorphic f l u i d and rock composition. Grains of o l i v e - g r e e n hornblende, although more f r a c t u r e d and embayed, have the most i d i o b l a s t i c o r i g i n a l shape. They appear to be of metamorphic o r i g i n , but quenched i n d i s e q u i l i b r i u m at a subsequent lower grade that does not de-grade the blue-green hornblende. Temperature estimates f o r metamorphism of the amphibolites are T>550°C. Kyanite S c h i s t A sample of k y a n i t e s c h i s t (UBC c o l l e c t i o n , S-20157) c o l l e c t e d by P. B. Read from the Metasedimentary Rocks on the c e n t r a l mountain r i d g e of Figure 1-7. Estimated pressure-temperature c o n d i t i o n s of metamorphic assemblages from k y a n i t e s c h i s t , U.B.C. c o l l e c t i o n S-20157 (shaded a r e a ) . L i g h t s t i p p l e represents r e g i o n of m e l t i n g . 30 Hawkesbury I s l a n d provides important i n f o r m a t i o n . P e r t i n e n t r e a c t i o n s are presented i n Figure 1-7. Giant p o i l k i l o b l a s t s of k y a n i t e enclose s t a u r o l i t e , muscovite and quartz. S t a u r o l i t e i s present as l a r g e porphyro-b l a s t s c o n t a i n i n g quartz and muscovite i n c l u s i o n s . S t a u r o l i t e i n places appears to be replaced by k y a n i t e . Muscovite snd p l a g i o c l a s e ( A n ^ ) are abundant. B i o t i t e i s not present. A l l phases seem to be i n e q u i l i b r i u m w i t h eachother except 1 ) s s t a u r o l i t e , which i s everywhere separated from p l a g i o c l a s e by small grains of quartz, and 2) f e l t y mats of muscovite, p l a g i o c l a s e and quartz which may be the consequence of l o c a l m e l ting. The presence of s t a u r o l i t e i n t h i s sample i n d i c a t e s the attainment of medium grade metamorphism, which occurs w i t h i n a narrow temperature range, near 525°C ( r e a c t i o n 1, Figure 1-7). The presence of k y a n i t e as the alumino-s i l i c a t e phase l i m i t s the pressure to greater than 4 kb (Holdaway, 1971). P l a g i o c l a s e i n e q u i l i b r i u m w i t h k y a n i t e f u r t h e r l i m i t s the s t a b i l i t y f i e l d to temperatures higher than a r e a c t i o n intermediate to r e a c t i o n s 2 and 3, depending on An content. I t i s not known whether migmatites occur near the sample l o c a t i o n , but since muscovite, p l a g i o c l a s e and quartz e x h i b i t a p o s s i b l e quenched melt r e l a t i o n s h i p , t h i s places the s t a b i l i t y range f o r t h i s sample i n the v i c i n i t y of r e a c t i o n 5. A reasonable estimate of pressures and temperatures of r e g i o n a l metamorphism based on t h i s s i n g l e sample i s 6 to 8 kb and 600 to 650°C. C a l c - S i l i c a t e s I s o b a r i c T-X diagrams are used i n the f o l l o w i n g d i s c u s s i o n because CU 2 e q u i l i b r i a i n v o l v i n g c a l c - s i l i c a t e minerals are s t r o n g l y dependent on f l u i d composition. The diagram has been drawn f o r 5 kb pressure because published data f o r the c a l c - s i l i c a t e r e a c t i o n s are a v a i l a b l e f o r only a l i m i t e d 31 number of pressures; 5 kb being one of them. Data f o r 7 kb would be more appropriate, based on the s t a b i l i t y of the k y a n i t e s c h i s t p r e v i o u s l y dee. s c r i b e d , and with estimates of 6 to 8 kb by H o l l i s t e r (1977)' along s t r i k e to the northwest. At 7 kb, temperatures would not be markedly d i f f e r e n t than at 5 kb. For s i m p l i f i c a t i o n , epidote and z o i s i t e w i l l be considered to respond to metamorphic c o n d i t i o n s i n the same manner. Reactions i n v o l -v i n g p l a g i o c l a s e are p l o t t e d f o r A n ^ ^ . For An content l e s s than 100%, r e a c t i o n s take place at lower temperatures. C a l c - S i l i c a t e s from C e n t r a l Gneiss Stable assemblages of c a l c - s i l i c a t e s f o r three samples from the C e n t r a l Gneiss are l i s t e d i n Table 1-1, along w i t h p e r t i n e n t r e a c t i o n s . S t a b i l i t y f i e l d s are p l o t t e d on Figure 1-8. The presence of z o i s i t e i n the sample r e s t r i c t s the s t a b i l i t y f i e l d to low CO /HO r a t i o s (X <0.5), depending on temperature, l i m i t e d by the r e a c t i o n zo + CO2 = an + ca. An upper temperature l i m i t i s set by the s t a b i l i t y of quartz and z o i s i t e , a t 665°C. A p o s s i b l e lower temperature l i m i t and upper X l i m i t i s the r e a c t i o n ca + zo + qtz = gr + CX^ + ^ 0 based on the presence of garnet at the expense of ca + zo + qtz i n sample 88-1. Ca + czo/ep + qtz (no garnet) i s s t a b l e i n sample 92-3, however, which g r e a t l y extends the s t a b i l i t y f i e l d f o r the C e n t r a l Gneiss c a l c - s i l i c a t e s . C a l c - S i l i c a t e s from Metasedimentary Rocks The ubiquitous paragenesis i n c a l c - s i l i c a t e s from the Metasedimentary Rocks i s ca + qtz + ms (samples 30-1, 39-1, 59-4, 16-2). This assemblage i s s t a b l e f o r any f l u i d composition, and temperatures up to a maximum of 565°C (Figure 1-10) at 5 kb. Above t h a t , ca + qtz + ms breaks down to 32 Table 1-1. C a l c - s i l i c a t e parageneses and r e l e v a n t r e a c t i o n s Sample no, Parageneses Reactions Metamorphism of C e n t r a l Gneiss Rocks 92-4 zo - qtz - d i t r + ca + qtz = d i + vapor ca + an + R^ O = zo + C0 2  zo + qtz = gr + an + wo 88-1 zo - ca - gr - d i zo - qtz - d i (no ca) ca - qtz (no zo) t r + ca - qtz = d i + vapor zo + ca + qtz = gr + vapor  zo + qtz = gr + an + R^O zo + C0 2 = an + ca + HO ca + qtz = wo + CO2 (zo + wo = gr + qtz + H^O) 92-3 czo/ep - ca - qtz zo + ca + qtz = gr + vapor zo + CO, an + ca + H 20 ca + qtz = wo + CO^ zo + q t z = an + gr + H 20 Metamorphism of Metasedimentary Rocks 30-1 ca - qtz - ta/ms - scap no zo ca + qtz + ms = ksp + an + vapor 39-1 ca - qtz - ms no zo ca + qtz + ms = ksp + an + vapor 59-4 ca - qtz - ms wo - b i - qtz ca + qtz + ms = ksp + an + vapor ca + qtz - wo + C0„ 59-3 zo - qtz - pc wo - qtz - pc zo + qtz = gr + an + H 20 zo + ca + qtz = gr + vapor 33 Table 1-1. (continued) 59-3 ca - qtz - zo ca + qtz = wo + C0 o (contd.) 2 ca - qtz - wo 16-1 ksp - ms - qtz qtz - ms - pc ksp - pc(An 3 g) no ca ,ca + qtz + ms = ksp + an + vapor 16-2 ca - qtz - ms ms + ca + qtz = ksp + an + vapor ca - pc - ms ca - qtz - pc ms an + ca + H^ O = zo + CO^ 16-3 ca - qtz - gr pc - ca - qtz no gr - pc 26-2 ta - t r - qtz - pc 26-1 qtz - ca - ms/ta - t r - zo 25-4 d i - czo - qtz - pc(An^g) 25-5 b i - qtz - ms - s i l l ca + an + qtz = gr + CO. tr + ca + qtz d i + z vapor ta + ca + qtz = tr + vapor ms + ca + qtz _ ksp + an + vapor t r + ca + qtz = d i + vapor zo + c o 2 = : ca + an zo + qtz = : gr + an tr + ca + qtz = d i + vapor zo + qtz = + an zo + c o 2 = ca + an ky - s i l l Notes: Underlined portion of reaction i s st a b l e . an=anorthite, bi= b i o t i t e , ca=calcite, czo=clinozoisite, di=diopside, ep=epidote, gr=grossular-i t e , ksp=K-feldspar, ms=muscovite, pc=plagioclase, qtz=quartz, scap=scapolite, s i l l = s i l l i m a n i t e , ta=talc, tr=tremolite, wo=wollastonite, zo=zoisite, Sample locations are plotted on Figure 1-9. 700 H 400 Holdaway (1971) Hewitt (1973) Slaughter, Kerrick & Wall (1975) T.H. Brown (pers. comm., 1979) Schreinemaker construction 600 - i 500 H x c o 2 Figure 1-8. I s o b a r i c T - X diagram of c a l c - s i l i c a t e assemblages i n Central Gneiss. V e r t i c a l l i n e s are s t a b i l i t y f i e l d for sample 92-3, h o r i z o n t a l l i n e s for sample 92-4, diagonal l i n e s for sample 88-1. 700 H Aft+Wb —V • 600 H 500 H .//( ***** . 0 * . 1 1 J - * " 400 o.o *> >,° 0.1 Holdaway (1971) Hewitt (1973) Slaughter, Kerrick & Wall (1975) T.H. Brown (pers. comm., 1979) Schreinemaker construction — jillimnitfc-s 0.2 0.3 0.4 0.5 0.6 0.7 0.8 5 kb 0.9 1.0 x c o 2 F i g u r e 1-10. Isobaric, T-X diagram of c a l c - s i l i c a t e assemblages i n Metasedimentary Rocks. V e r t i c a l CO2 l i n e s represent s t a b i l i t y f i e l d f o r r e g i o n a l metamorphism of u n i t ; h o r i z o n t a l l i n e s are p o s s i b l e contact thermal e f f e c t of K i t k i a t a p l u t o n (samples 25-4 and 25-5); diagonal l i n e s are p o s s i b l e contact thermal e f f e c t of agmatite (sample 16-3). 36 ksp + an + vapor, the actual temperature depending on f l u i d composition: at X < 0.3 and below 550°C, the breakdown products are ksp + zo + vapor. The parageneses i n sample 16-1 indi c a t e that the reaction ca + ms + qtz = ksp + an + vapor may have occurred. Sample 59-3 records comparable temp-eratures, but the presence of z o i s i t e indicates a water-rich f l u i d ( X ^ Q < 0.5 according to reaction zo + CO^ = an..+ ca). Samples containing t a l c and tremolite are more magnesium-rich, and provide a lower temperature l i m i t C0„ of metamorphism that varies with f l u i d composition, from T = 400 C at X , 0.08, to T = 450°C at X =0.2. M i s i d e n t i f i c a t i o n of dolomite as c a l c i t e C U 2 would extend the f i e l d to higher X N N , with T = 550°C at X = 0.6. Mis-i d e n t i f i c a t i o n of t a l c for muscovite would a l t e r temperature estimates by less than 15°C. Several samples contain parageneses i n d i c a t i v e of s l i g h t l y higher temperatures, and possible contact metamorphism with nearby i n t r u s i v e rocks. Sample 25-4 i s from a screen within the contact zone of the K i t k i a t a pluton. In t h i s sample, the presence of diopside indicates equilibrium on the high temperature side of reaction t r + ca + qtz = d i + CT^ + H^O. The presence of zo + qtz l i m i t s the temperatures to below 660°C and indicates a water-r i c h f l u i d . S i l l i m a n i t e , . present at the same l o c a l i t y i n sample 25-5 indicates temperature above 560°C, and above 660°C i f P = 7 kb. Both products and reactants of reaction ca + an + qtz = gr + CO^ are stable i n the th i n section of sample 16-3. Limits f or this reation are 640 to 710°C, and 0.22 to 0.31 X . These temperatures are s l i g h t l y higher than regional Lt>2 metamorphic temperatures previously discussed f or the Metasedimentary Rocks (Figure 1-10) , and possibly r e f l e c t a thermal e f f e c t related to the devel-opment of agmatite nearby. The s t a b i l i t y of the assemblage more l i k e l y r e f l e c t s an e f f e c t of s o l i d s o l u t i o n series i n garnet and plagioclase, be-37 cause other samples from the same l o c a t i o n are compatible w i t h p r e v i o u s l y i n f e r r e d r e g i o n a l metamorphic c o n d i t i o n s . Summary of Metamorphic Conditions 4-Pressure estimates of 7 - 1 kb f o r r e g i o n a l metamorphism f o r t h i s part of the Coast Mountains are a r r i v e d at from the s t a b i l i t y assemblages i n a k y a n i t e - s t a u r o l i t e s c h i s t , and by comparison w i t h pressures published f o r areas along s t r i k e to the north. Amphibolites provide a low temperature estimate of 550°C. C a l c - s i l i c a t e assemblages place the high temperature l i m i t of metamorphism between 560 and 660°C at 5 kb. Elevated pressure would increase the temperature somewhat. There i s s l i g h t contact meta-morphism near the agmatite, and i n screens w i t h i n the contact zone of the K i t k i a t a p l u t o n . 3 8 PLUTONIC ROCKS P l u t o n i c igneous rocks i n the study area are the Quottoon p l u t o n on the north side of the d e t a i l e d map, and a small p l u t o n that s t r a d d l e s Douglas Channel on the south s i d e of the map, that i s h e r e i n r e f e r r e d to as the K i t k i a t a p l u t o n , named a f t e r the adjacent i n l e t that opens i n t o Douglas Channel. K i t k i a t a P l u t o n The K i t k i a t a p l u t o n i s emplaced i n t o metasedimentary and metavol-canic rocks. I t has been p r e v i o u s l y mapped as quartz monzonite, w i t h quartz d i o r i t e along the shore of Hawkesbury I s l a n d , (P.oddick, 1970) . Where sam-pled f o r t h i s study, i t i s predominantly quartz d i o r i t e as f a r south as mapped, and quartz monzonite to the north. P l u t o n i c rock extends approx-imately 5 kil o m e t e r s f u r t h e r north than o r i g i n a l l y mapped, and contains l a r g e (10-50 m wide) screens of metasedimentary rock. The nature of the contact i s a l a r g e s c a l e i n t e r f i n g e r i n g s u b p a r a l l e l to the r e g i o n a l f o l i -a t i o n (Figure 1-11). The southern contact was not i n v e s t i g a t e d , but was reported by Roddick (1970) to be a 0.1 m b r e c c i a t e d zone against quartz monzonite which i n tu r n i s l i t - p a r - l i t w i t h Metasedimentary Rocks to the south. The rocks are medium to coarse grained, and a l l e x h i b i t some degree of f o l i a t i o n p a r a l l e l or s u b p a r a l l e l to the r e g i o n a l trend. In t h i n s e c t i o n , they have t y p i c a l a l l o t r i o m o r p h i c to hypidiomorphic granular t e x t u r e s . P l a g i o c l a s e composition ranges from A n ^ i n quartz mon-zo n i t e to An^Q i n d i o r i t e . P l a g i o c l a s e i n quartz d i o r i t e and d i o r i t e i s normally zoned, with l a r g e c a l c i c (about A n ^ ) cores, and a r a p i d An decrease near the rims to about An~_. This suggests a pe r i o d of slow, then 39 F i gure 1—11. L i t — p a r — l i t i n t e r f i n g e r i n g of K i t k i a t a pluton w i t h Metasedimentary Rocks. 40 more rapid cooling, perhaps following u p l i f t . S e r i c i t i z a t i o n i s minor. Mafic minerals are blue-green hornblende and brown b i o t i t e i n varying pro-portions. The hornblende i s very i r r e g u l a r , broken and embayed. Small flakes of b i o t i t e are sometimes a l t e r e d to c h l o r i t e . Quartz forms i n t e r -s t i t i a l strained grains and also small rounded grains, suggesting the onset of r e c r y s t a l l i z a t i o n as a r e s u l t of s t r a i n . Microcline and untwinned K-feldspar occur i n t e r s t i t i a l l y and also as s l i g h t l y larger p e r t h i t i c grains i n the quartz monzonite. Accessory minerals are abundant fractured and anhedral sphene and e p i d o t e / c l i n o z o i s i t e , and also tiny apatite needles. Small garnets i n the quartz monzonite are rounded and free of i n c l u s i o n s . The minerals and textures are t y p i c a l of g r a n i t i c plutonic complexes. Quottoon Pluton The Quottoon pluton appears to be emplaced into the core of the Foch antiform; metamorphic rocks of the Central Gneiss Complex wrap around the i n t r u s i v e body, and i n Loretta Channel, plutonic f o l i a t i o n s are more gently dipping, p a r a l l e l i n g the h o r i z i n t a l metamorphic f o l i a t i o n s reported at higher elevations north of Douglas Channel. B i o t i t e > hornblende grano-d i o r i t e predominates, but rock type varies from quartz monzonite to quartz d i o r i t e . The structure of the plutonic rock i s v a r i a b l e . Areas of unfoliated, t e x t u r a l l y homogeneous plutonic rock are mixed with areas that display ghost fol d s , highly contorted banded rock, and multiphase i n t r u s i o n s . Faint compo-s i t i o n a l bands outline the ghost structures, which for the most part are disharmonic and chaotic. Swirls of l e u c o c r a t i c plutonic rock contain i n -clusions of more mafic and g n e i s s i c rock, which become less abundant and then absent, north of the Central Gneiss contact. Ghost structures also give way to more homogeneous p l u t o n i c rock i n the northern part of the area. The contact between p l u t o n i c rock and gneiss i s a gradation from homogeneous, or c h a o t i c a l l y f o l d e d p l u t o n i c rock, to banded and f o l i a t e d p l u t o n i c rock, and f i n a l l y to f o l d e d g r a n o d i o f i t i c gneiss and migmatite. G r a n i t i c rocks are cut by contorted, as w e l l as s t r a i g h t pegmatite and a p l i t e d i kes. With the exception of s e v e r a l l e n s o i d c l o t s of K-feldspar which are not t e c h n i c a l l y d i k e s , dikes and veins have p a r a l l e l margins. Contortions i n the dikes are g e n e r a l l y random and meandering. At s t a t i o n 20 however, narrow K-feldspar v e i n l e t s appear to have been sheared w h i l e the complex was s t i l l p l a s t i c , and the present appearance i s of r e g u l a r f o l d s w i t h gently dipping a x i a l planes, and one sheared limb (Figure 1-12). A l -though there may be exceptions, g r a i n s i z e i n the dikes is<. r e l a t e d to com-p o s i t i o n ; K - f eldspar dikes are u s u a l l y p e g m a t i t i c , and p l a g i o c l a s e - q u a r t z dikes are more a p l i t i c . Wider pegmatite dikes are zoned, w i t h a p l i t i c cores. With the exception of a p l i t e , Quottoon rocks are medium to coarse grained, w i t h hypidiomorphic granular t e x t u r e . P l a g i o c l a s e compositions range between samples, from A ^ Q to An^^, w i t h some normal and reverse zonf1 i n g w i t h i n 5% An. P l a g i o c l a s e a l s o has deformation twins and low to moderate s a u s s u r i t i z a t i o n . Hornblende and b i o t i t e are present in" subequal amounts i n g r a n o d i o r i t e and quartz d i o r i t e , t o t a l l i n g 20-30%. B i o t i t e i s the mafic mineral i n quartz monzonite, comprising about 10% of the mode. There i s a r e l a t i o n s h i p between the b i o t i t e / h o r n b l e n d e r a t i o , and the degree of f r a c -t u r i n g and embayment i n the hornblende. For hornblende b i o t i t e , the horn-blende grains are s t i l l i n t a c t , but they d e t e r i o r a t e s t e a d i l y u n t i l b i o t i t e i s the s o l e mafic m i n e r a l . Quartz forms c l e a r i n t e r s t i t i a l grains that are s t r a i n e d and f r a c t u r e d . K-feldspar i s u s u a l l y twinned and forms as l a r g e r anhedral phenocrysts i n quartz monzonite, and a l s o smaller i n t e r s t i t i a l grains i n quartz monzonite and g r a n o d i o r i t e . Accessory minerals are l a r g e Figure 1 - 1 2 . Sheared K-feldspar veins i n Quottoon pluton. 43 euhedral sphene ( v i s i b l e i n hand specimen), large anhedral epidote, and sm a l l rounded a p a t i t e and z i r c o n . A p l i t e dikes have s i m i l a r textures and mineralogy to the other g r a n i t i c rocks. The are not, however, f o l i a t e d . Average g r a i n s i z e i s 0.5 mm. P l a g i o c l a s e (An,,) i s the most abundant m i n e r a l , t o t a l l i n g 60% of the mode. 16 M i c r o c l i n e and quartz are. present In subequal amounts. A l l three minerals are subequal and equigranular. Quartz grains are s t r a i n e d . Tiny f l a k e s of b i o t i t e , the s o l e mafic mineral, make up about 1% of the mode. Muscovite i s present as t i n y f l a k e s intergfown w i t h and rimming b i o t i t e , and as i n c l u -sions i n p l a g i o c l a s e . Rounded, corroded garnets are an accessory. Agmatite Agmatite has developed at the southern edge of the Gneiss and Migmatite.. u n i t . Here, the Gneiss has been invaded by s e v e r a l i n t r u s i v e phases, i n ad-d i t i o n to i t s involvement i n s e v e r a l deformational events. The general ap^r pearance i s that of a stockwork of d i k e s , some of which w e l l out i n t o l a r g e r bodies and i n c l u d e r o t a t e d subangular to angular blocks of g n e i s s i c country rock. P r o p o r t i o n of i n t r u s i v e rock to country rock v a r i e s from place to place. Nowhere do a l l p e t r o l o g i c phases c o e x i s t , but observation of d i f f e r e n t areas allows c o r r e l a t i o n of phases to e a r l y , intermediate and l a t e stages. There are three e a r l y , p r e - i n t r u s i v e phases whose mutual c r o s s c u t t i n g r e l a t i o n s h i p s were not observed. These are: 1) banded gneiss, 2) f i n e grained gneiss, and 3) f l a s e r gneiss. 2 Angular 1 to 5 m x e n o l i t h s of hornblende-plagioclase gneiss have 3 to 5 cm wide bands of coarse and f i n e grained m a t e r i a l . These x e n o l i t h s are cut by t h i n a p l i t i c veins w i t h i n d i s t i n c t margins. The gneiss (73-1) i s banded but not s c h i s t o s e . In the coarse grained 44 bands, average g r a i n s i z e of the groundmass i s 0.5 mm, w i t h 2 to 5 mm p o i -k i l o b l a s t s of e p i d o t e / c l i n o z o i s i t e . P l a g i o c l a s e (An^y) grains are anhedral, sometimes twinned, normally zoned ( A ^ g i n the core, A ^ ^ r i m ) , and f r e s h except along f r a c t u r e s where i t has been s e r i c i t i z e d . Blue-green hornblende occurs as ragged, f r a c t u r e d , anhedral, o f t e n rounded grains overgrown by b i o t i t e and e p i d o t e / c l i n o z o i s i t e . B i o t i t e f l a k e s seem to have no p r e f e r r e d o r i e n t a t i o n and may be post-kinematic. E p i d o t e / c l i n o z o i s i t e p o i k i l o b l a s t s contain up to 30% i n c l u s i o n s of rounded p l a g i o c l a s e and hornblende. Quartz i s minor and sphene i s an accessory m i n e r a l . An o r i g i n a l hornblende-plagio-clase rock, t h i s u n i t was subsequently retrograded to produce e p i d o t e / c l i -n o z o i s i t e and b i o t i t e from p l a g i o c l a s e and hornblende. One to three meter long rectangular x e n o l i t h s of f i n e grained gneiss (105-1) are s i m i l a r to the f i n e grained bands i n the hornblende-plagioclase gneiss ( e a r l y u n i t 1). Their o r i e n t a t i o n and pinched ends suggest that they are d i s j o i n t e d e a r l y d ikes. This q u a r t z - p l a g i o c l a s e - e p i d o t e / c l i n o z o i -s i t e - ( h o r n b l e n d e ) - b i o t i t e rock has a l l o t r i o m o r p h i c granular t e x t u r e . L i k e e a r l y u n i t 1, these x e n o l i t h s are a l s o veined by a p l i t e w i t h i n d i s t i n c t margins. The most abundant country rock i s a garnet-bearing quartz d i o r i t i c f l a s e r gneiss (72-1, 105-2, 112-1). The major f o l i a t i o n i n blocks of hneiss i n the agmatite has been ro t a t e d w i t h respect to the a t t i t u d e i n surrounding rocks. E a r l y a p l i t e dveins w i t h i n d i s t i n c t margins were not observed i n the f l a s e r gneiss. Quartz d i o r i t i c f l a s e r gneiss has up to 10% K - f e l d s p a r (making i t grano-d i o r i t i c i n some l o c a t i o n s ) , 10-30% quartz, 20-30% mafic minerals ( v a r y i n g proportions of b i o t i t e , hornblende and e p i d o t e / c l i n o z o i s i t e ) , and minor sphene, c a l c i t e and garnet. Average g r a i n s i z e of major mineral phases i s 45 one to f i v e millimeters. F o l i a t i o n i s defined by mineral segregation and by p a r a l l e l i s m of b i o t i t e f l a k e s . Most plagioclase grains ( A ^ ^ to An^g, aver-age ^n20-35^ a r e S U D n e d r a l a n < i blocky. Untwinned grains have more i r r e g u l a r convolute boundaries than twinned ones. Myrmekite occurs at contacts with K-feldspar. Plagioclase i s also present as blocky p e r t h i t i c intergrowths with K-feldspar which i s otherwise large untwinned grains. Quartz i s generally fi n e grained, anhedral and r e l a t i v e l y unstrained. I t contains tiny euhedral apatite i n c l u s i o n s . Blue-green pleochroic hornblende i s heavily embayed, always accompanied by b i o t i t e , often by e p i d o t e / c l i n o z o i s i t e , and r a r e l y by small rounded c a l c i t e rhombs or blobs. Where hornblende makes up a s u b s t a n t i a l percentage of the rock, however, i t i s clean and nearly euhedral. In mafic c l o t s , e p i d o t e / c l i n o z o i s i t e grains are large and anhed-r a l . In the groundmass, they are smaller and more rounded. Where epidote/ c l i n o z o i s i t e i s absent, plagioclase i s pervasively s e r i c i t i z e d ; b i o t i t e , where present, i s extensively altered to c h l o r i t e . These v a r i a t i o n s take place even within a si n g l e hand specimen. More amphibolitic f l a s e r gneiss (111-1, 1-1, 109-1, 14-2) i s f i n e r grained, and contains l i t t l e quartz. Hornblende pleochroism i s more o l i v e -green, and larger grains are p o i k i l o b l a s t i c around pl a g i o c l a s e . Plagioclase ( t y p i c a l l y An^Q^^) i s reverse zoned ( A ^ ^ core, A n ^ rim) . Small rounded c a l c i t e grains are scattered along grain boundaries. The intermediate agmatite phases, forming the bulk of the i n t r u s i v e rock, are i r r e g u l a r g r a n o d i o r i t i c dikes that swell out into larger bodies and contain xenoliths of the several early phases. Two phases are pre-dominant and t y p i c a l : 1) f i n e grained quartz monzonite (8-2, 72-2, 7-2, 1-2, 12-1), and 2) coarser grained quartz d i o r i t e (74-1, 72-3, 12-2). The f i n e grained (< 1 mm) l e u c o c r a t i c garnetiferous quartz monzonite-46 g r a n o d i o r i t e has a weak f o l i a t i o n defined by s u b t l e gneissose mineral segre-gations, s u b p a r a l l e l to the r e g i o n a l trend. Coarser grained (1-2 mm) grano-d i o r i t e - quartz d i o r i t e i s more mafic and has g r a d a t i o n a l contacts w i t h the quartz monzonite. I t appears as dikes w i t h l e u c o c r a t i c and s l i g h t l y f i n e r grained margins w i t h i n quartz monzonite. I n t r u s i v e rocks of the middle phases have t y p i c a l g r a n i t i c textures and mineralogy. B i o t i t e ( p a r t l y c h l o r i t i z e d ) i s the main mafic m i n e r a l , com-p r i s i n g up to 20% of the rock, but u s u a l l y l e s s than 5 or 10%. P l a g i o c l a s e (An^g to An^g) i s continuously normally zoned. S e r i c i t i z a t i o n i s minor. P l a g i o c l a s e i n contact w i t h K-feldspar has i n c l u s i o n s of myrmekitic quartz. There i s some m i c r o c l i n e , but most K-feldspar i s untwinned. In quartz d i o r i t e , K-feldspar i s rare and i n t e r s t i t i a l ; i n g r a n o d i o r i t e , s m a l l grains of i t occur i n c l o t s w i t h quartz, and i n quartz monzonite, i t occurs as l a r g e r grains w i t h anhedral boundaries. Anhedral quartz grains are s t r a i n e d and broken. Small, very i r r e g u l a r sphene; elongate broken and embayed gar-net w i t h i n c l u s i o n s of quartz; and epidote, both as t i n y rounded g r a i n s , and as l a r g e r embayed and broken grains w i t h quartz i n c l u s i o n s are present. The l a t e phase of the agmatite i s a network of l e u c o c r a t i c d i k e s , r e -sembling i n some areas, fancy basket weaving (Figure 1-13). The f o l l o w i n g sequence was observed i n one. 1) A K-feldspar-quartz pegmatite dike w i t h p a r a l l e l margins meanders across e a r l y and intermediate phases. 2) Garnet bearing a p l i t e , more planar than the pegmatite, cuts the pegmatite. 3) An 8 cm wide mafic dike crosscuts a l l previous phases and o f f s e t s the l a t e a p l i t e . The b a s i c dike i t s e l f i s broken and s l i g h t l y o f f s e t . 4) A massive quartz v e i n cuts a l l e a r l i e r phases. 47 48 Other areas show mafic dikes cut by pegmatite-aplite. The mafic dike may be older than the pegmatite generation already discussed, or an addition-a l pegmatite i n j e c t i o n follows a si n g l e episode of basic dike emplacement. F o l i a t i o n and folds i n rocks of the early stage are s i m i l a r i n appear-ance to those i n the adjacent gneisses. Within the agmatite, older gneisses have a w e l l developed f o l i a t i o n or banding which has subsequently been cr i n k l e d , folded and refolded. Some layers have been stretched and boudined, with quartz-rich material f i l l i n g i n the boudin necks. Other layers are coarse grained and contorted. Flaser gneiss was not observed outside the agmatite. Even i n t h i n sections that display f o l d s , mineral grains are not strained, i n d i c a t i n g that metamorphism accompanied deformation and was i n part, post-kinematic. The older structures have been disrupted and rotated by the intermediate stage i n t r u s i o n s . Intrusion occurred p r i o r to the end of regional deformation, as the i n t r u s i v e rock displays a weak f o l i a t i o n sub-p a r a l l e l to the regional trend. The whole complex was s t i l l behaving p l a s t i c a l l y during emplacement of the f i r s t l a t e stage dikes. The f i r s t l a t e stage dikes are folded and r e c r y s t a l l i z e d . Later dikes are more planar, but have r e c r y s t a l l i z e d mineral grains aligned oblique to t h e i r mar-gins. Agmatite contact with the Gneiss to the north i s gradational, and with Metasedimentary Rock to the south, more abrupt but s t i l l gradational. Late planar dikes occur throughout the area, and so contacts were mapped where country rock exceeded i n t r u s i v e rock i n abundance and l a t e r dikes were not folded. Lamprophyre Dikes Fine grained lamprophyre dikes crosscut the e n t i r e f i e l d area. The dikes are s t r a i g h t and have p a r a l l e l margins which are often c h i l l e d . One 49 m i l l i m e t e r long brown hornblende phenocrysts occur i n a very f i n e grained a l t e r e d matrix c o n s i s t i n g of c a l c i t e , c h l o r i t e , epidote and hematite. Amygdules are c o n c e n t r i c a l l y l a y e r e d , w i t h c h l o r i t e on the o u t s i d e , c a l c i t e intermediate and hematite i n the center. S i m i l a r dikes i n the P r i n c e Rupert area are of Miocene age (R. L. Armstrong, o r a l comm., 1976). 50 STRUCTURAL GEOLOGY S t r u c t u r a l s t y l e and h i s t o r y of the two metamorphic rock u n i t s i n the study area are s i m i l a r . These rocks are c h a r a c t e r i z e d by a w e l l developed f o l i a t i o n and t i g h t to i s o c l i n a l f o l d s w i t h an associated mineral l i n e a t i o n . Minor d i f f e r e n c e s i n f o l d s t y l e are most l i k e l y expressions of d u c t i l i t y c o ntrast i n d i f f e r e n t rock types. S t r u c t u r e s i n the p l u t o n i c rocks are discussed elsewhere. In plutons, ghost f o l d s are c h a o t i c , but near the contact,with metamorphic rocks, f o l i a t i o n and banding c l o s e l y p a r a l l e l s i m i l a r s t r u c t u r e s i n the metamorphic rocks. S t r u c t u r a l data i s p l o t t e d on the geologic map (Figure 1-5) and on equal area stereonets ( i n s e t s , Figure l-5a to h ) . Planar S t r u c t u r e s The most obvious s t r u c t u r a l elements i n the study area are s t e e p l y dipping f o l i a t i o n and compositional banding (Figure 1-14). More b r i t t l e l a y e r s are s t r e t c h e d i n t o boudins i n the plane of f o l i a t i o n w h i l e more duct t i l e l a y e r s f i l l i n boudin necks w i t h small p a r a s i t i c f o l d s . In the C e n t r a l Gneiss, metamorphic compositional banding and b i o t i t e - h o r n b l e n d e f o l i a t i o n s t r i k e northwest and dip s t e e p l y to the north and south. Compositional banding i s not common i n the Metasedimentary Rocks; where present i t i s l i k e l y r e l i c t or transposed sedimentary l a y e r i n g . From north to south, b i o t i t e f o l i a t i o n i n the Metasedimentary Rocks swings through 25°, from steep south d i p s , to v e r t i c a l , then steep north d i p s . Aside from that p a r t i c u l a r systemmatic v a r i a t i o n , f o l i a t i o n s throughout the f i e l d area are s u b p a r a l l e l , as evident from the s t e r e o p l o t s of poles to f o l i a t i o n i n Figures l-5b and d. Figure l-14a. Banding in f i n e grained Central Gneiss (looking NW). Figure l-14b. Banding i n interbedded qtz-bi s c h i s t and rusty weathering qua r t z i t e , Metasedimentary Rocks (looking NW). Figure l-14d. Banding i n contact zone of Quottoon pluton ( l o o k i n g NW). 53 Folds and L i n e a t i o n The most abundant f o l d s are t i g h t to i s o c l i n a l w i t h thickened hinges (some r o o t l e s s ) , and a x i a l plane f o l i a t i o n that i s p a r a l l e l to the r e g i o n a l f o l i a t i o n (Figures l-15a, b and l-16a, b and c ) • In the Metasedimentary Rocks, t r a n s p o s i t i o n of o r i g i n a l sedimentary l a y e r i n g i s seen (Figure l-16a). On a l a r g e r s c a l e , l a y e r s are i s o c l i m a l l y f o l d e d , w i t h p a r a s i t i c f o l d s developed along the limbs. Where there i s a d u c t i l i t y c o n t r a s t between l a y e r s , a more co n c e n t r i c f o l d s t y l e occurs, but the a x i a l plane f o l i a t i o n remains c h a r a c t e r i s t i c . I n t e r f e r e n c e s t r u c t u r e s at one l o c a l i t y (Figure l-16c) i n d i c a t e that the abundant i s o c l i n a l f o l d s do not represent the e a r l i e s t phase of deformation; they f o l d even e a r l i e r approximately northeast-trending f o l d s . F o l d axes and f o l d - a x i s - p a r a l l e l l i n e a t i o n s i n C e n t r a l Gneiss rocks (Figure l-5d) show a strong p a r a l l e l i s m , t r e n d i n g west-northwest and plunging 20-40°. In the Metasedimentary Rocks, the l i n e a r s t r u c t u r e s are roughly coplanar w i t h r e g i o n a l f o l i a t i o n , but v a r i a b l e i n plunge f o r about 75° through v e r t i c a l . The v a r i a b l e plunge of i s o c l i n a l f o l d s i n the Meta-sedimentary Rocks i s an enigma. I f a l l the i s o c l i n a l f o l d s i n the C e n t r a l Gneiss and Metasedimentary u n i t s were p a r a l l e l when formed, subsequent deformation p r e f e c e n t i a l l y steepened, by as much as 50°, the f o l d axes i n the Metasedimentary Rocks. Folds that deform the f o l i a t i o n ace g e n e r a l l y oblique to the e a r l i e r f o l d s and d i s p l a y both harmonic (Figures l-16d and l-15d,e) and disharmonic (Figure l-16e,f) f o l d s t y l e s . These f o l d s are normal to t i g h t , w i t h rounded hinges. The t r a n s p o s i t i o n l a y e r i n g p r e v i o u s l y mentioned i s r e f o l d e d (Figure l-16d), and micaceous l a y e r s are crenulated and polygonized. Data on these f o l d s are sparse and s c a t t e r e d (Figure l - 5 c , e ) . 54 a. i s o c l i n a l f o l d s w i t h a x i a l plane f o l i a t i o n ; q u a r t z o f e l d s p a t h l c l a y e r i n a m p h i b o l i t e . ( S t a t i o n b. I s o c l i n a l f o l d w i t h a x i a l plane f o l i a t i o n ; q u a r t z o f c l d s p a t h l c l a y e r In g r a n o d t o r l t l c g n e i s s . ( S t a t i o n k) d. F i n e g r a i n e d g r a n o d l o r l t l c g n e i s s w i t h C. Refolded q u a r t z o f e l d s p a t h l c l a y e r In f o l d e d f o l i a t i o n . ( S t a t i o n 4)0) f i n e g r a i n e d q u a r t z d l o r l t l c g n e i s s ; both f o l d s have a x i a l p l a n e f o l i a t i o n . ( S t a t i o n 80) Figure 1-15. F o l d s t y l e s i n C e n t r a l Gneiss roclcs. 55 Figure 1-16. F o l d s t y l e s i n Metasedimentary Rocks. 56 Comparison w i t h S t r u c t u r e i n Nearby Areas I n the B e l l a Coola — Laredo Sound area, Baer (1973) describes planar s t r u c t u r e s that may or may not accompany f o l d s . Oldest i s r e l i c t north-e a s t - t r e n d i n g f o l i a t i o n that he c a l l s S^. I t i s d i s p l a c e d by the dominant northwest-trending r e g i o n a l f o l i a t i o n (S^) , and accompanying i s o c l i n a l f o l d s . Both phases are a l s o present i n p l u t o n i c rocks. L o c a l l y developed are n o r t h e a s t - t r e n d i n g S^ cleavage f r a c t u r e s . In the P r i n c e Rupert area.(Hutchison, 1967), e a r l y s t r u c t u r e s are east-west-trending, north-dipping bands, and recumbent f o l d s w i t h east-west axes. The e a r l y trends are cut by p l u t o n i c rock and deformed by the domi-nant northwest-trending s t r u c t u r e s . Dips are g e n e r a l l y steep, except f o r some shallow east dips i n the metamorphic rocks between Quottoon and A l a s t a i r Lake plutons. Fold axes commonly plunge s h a l l o w l y to the north. On Hawkesbury I s l a n d , the north-northwest r e g i o n a l s t r u c t u r e trend i s d e f l e c t e d to west-northwest by the Hawkesbury Warp, but nevertheless c o r r e l a t e s w i t h Baer's f o l i a t i o n , and Hutchison's north to northwest dominant s t r u c t u r a l trend. The r e f o l d e d approximately n o r t h e a s t - t r e n d i n g f o l d axes p r e v i o u s l y desvribed on Hawkesbury I s l a n d can be t e n t a t i v e l y c o r r e l a t e d w i t h Baer's n o r t h e a s t - t r e n d i n g and Hutchison's e a r l y east-west s t r u c t u r e s . Hutchison and Baer do not describe s t e e p l y - p l u n g i n g f o l d axes, but Kenah (1979) reports f o l d axes i n metamorphic rocks northeast of the Quottoon pluton j u s t south of the Skeena R i v e r , w i t h s i m i l a r geometry to those found on Hawkesbury I s l a n d . In the area mapped by Kenah, i s o c l i n a l f o l d a x i s l i n e a t i o n s ( h i s L^) l i e on a great c i r c l e , whose pole i s the same as the pole to the prominant r e g i o n a l a x i a l plane f o l i a t i o n ( h i s F^). He suggests r e f o l d i n g of the i s o c l i n a l f o l d a x i s about a f o l d a x i s ( L 0 ) perpendicular to 57 the F^ f o l i a t i o n . This would have l i t t l e e f f e c t on the f o l i a t i o n , but would rotate l i n e a t i o n s to produce the great c i r c l e d i s t r i b u t i o n . Although the perpendicular r e l a t i o n s h i p between the F^ f o l i a t i o n and l a t e r f o l d axis must be f o r t u i t o u s , Kenah argues that the f o l i a t i o n constrained the r e f o l d i n g process. He considers the deformation associated with r o t a t i o n of f o l d axes to be related to p a r t i a l melting rather than the r e s u l t of regional compression. 58 SUMMARY AND CONCLUSIONS Metamorphic rocks from the study area represent two d i f f e r e n t composi-t i o n a l u n i t s . The Ce n t r a l Gneiss was o r i g i n a l l y v o l c a n i c rocks, v o l c a n i -c l a s t i c sediments and immature graywackes. In c o n t r a s t , the psammitic Meta-sedimentary Rocks c o n t a i n a p p r e c i a b l e amounts of q u a r t z i t e and marble and 87 86 are t h e r e f o r e a more mature sediment a s s o c i a t i o n . Low i n i t i a l Sr/ Sr r a t i o s from the metamorphic rocks e l i m i n a t e the l i k l i h o o d that they c o n t a i n a s i g n i f i c a n t amount of pre-Mesozoic c r a t o n i c m a t e r i a l (Armstrong and Runkle, 1979). Culmination of metamorphism was at 6 to 8 kb, and 550 to 660°C. Planar s t r u c t u r e s s t r i k e northwest and are concordant to the r e g i o n a l trend. Tight to i s o c l i n a l f o l d s occur as two s t y l e s : one has an accompanying a x i a l plane f o l i a t i o n p a r a l l e l to the r e g i o n a l trend, and the other deforms the f o l i a t i o n . The s t r u c t u r a l discordance between the two metamorphic rock u n i t s north of Douglas Channel, and the development of agmatite on Hawkesbury and Ma i t l a n d I s l a n d s , suggests a f a u l t contact of C e n t r a l Gneiss w i t h Metasedi-mentary Rocks. The j u x t a p o s i t i o n predates deformation and metamorphism because f o l d s t y l e s i n the two u n i t s are s i m i l a r , and metamorphic pressure and temperature are comparable. Metamorphic c u l m i n a t i o n occurred when these rocks were a t about 25 km depth at 65 to 97 Ma, based on concordant U-Pb dates on z i r c o n s from g r a n u l i t e s nearby (R. Wanless and G. Woodsworth, personal Communication to R. L. Armstrong, 1977). There appears to be a s l i g h t U thermal c o n t r a s t between metamorphic and p l u t o n i c rocks. 87 86 P l u t o n i c tfocks have low i n i t i a l Sr/ Sr r a t i o s , i n d i c a t i n g a mantle-derived magma w i t h l i t t l e contamination from o l d c r u s t a l rocks. This does not e l i m i n a t e the p o s s i b i l i t y that p l u t o n i c rocks were, i n p a r t , derived from p a r t i a l m e l t i n g of the C e n t r a l Gneiss and Metasedimentary Rocks during c u l m i n a t i o n of metamorphism, and deformation. The K i t k i a t a p l u t o n was 59 probably emplaced 165±11 Ma but i t s age i s s t i l l h i g h l y u n c e r t a i n . I t s contact i s l i t - p a r - l i t i n t o Metasedimentary Rocks. The extremely g r a d a t i o n a l and mi g m a t i t i c character of the contact between C e n t r a l Gneisses and Quottoon g r a n o d i o r i t e show that i n t h i s area, emplacement of the Quottoon was not f a r from i t s s i t e of generation. The Quottoon p l u t o n was emplaced during u p l i f t of the Coast P l u t o n i c Complex, at about 13 km depth, and 51±2 Ma. I n t r u s i o n of pegmatitic dikes continued u n t i l a f t e r p l a s t i c deformation ceased. Subsequent u p l i f t brought the rocks to w i t h i n 4 km of the surface by about 46 Ma, where 35 Ma of i s o s t a t i c s t a -b i l i t y allowed the rocks to c o o l ( H a r r i s o n et a l . , 1979). Miocene lampro-phyre dikes completed the i n t r u s i v e sequence. Miocene and l a t e r r e g i o n a l u p l i f t (Mathews, 1974) exposed the rocks of t h i s study area. 60 ACKNOWLEDGMENTS When a thesis takes as long as-this one to produce, the author i s sure to run into many people along the way who contribute i n some manner or other, be i t with expertise or q u a l i t y of l i f e . Glenn Woodsworth suggested this project, provided h e l p f u l l o g i s t i c advice p r i o r to the f i e l d work, v i s i t e d us on Hawkesbury Island, and when our attempts at mapping were thwarted (not i n midstream, but on the other side of Douglas Channel), he ch e e r f u l l y endured calloused palms and the long watch for another boat. Other v i s i t o r s to the f i e l d were supervisors Dick Armstrong, who found us i n the dark and l e f t us with a radio and extra t o i l e t paper; and Hugh Wynne-Edwards who had a f i r e going and a steak and salad dinner waiting for us one rainy evening. A l l a n Runkle provided f i e l d assistance with a ready g r i n and a smart remark i n what was h i s wettest summer ever; including a free ride down a w a t e r f a l l and another rid e i n a w a t e r f i l l e d boat with waves breaking into h i s lap. We benefitted from the watchful eye of F i s h e r i e s Service employees John MacDonald and Bob Majewski, who served as our connection with the outside world and brought us aboard t h e i r boats to go f i s h i n g when i t was r a i n i n g too hard to do anything else. Stan Johnson encouraged us to use his cabin and loaned us a boat which handled the barnacles better than our rubber one. Several other Kitimat fishermen and loggers are g r a t e f u l l y remembered. At U. B. C , Ed Montgomery and h i s f l e e t of technicians were always ready to help with anything; K r i s t a Scott was invaluable i n the lab; and Randy P a r r i s h , Ian Duncan, Joan Grette and Lee Pigage among others c o n t r i -buted t h e i r knowledge through discussion and comment. Gwen Ditson, F e l i c i e Chronic, Joan Came and Rob Carne helped to prepare the f i n a l copy. Dick 61 Armstrong was extremely p a t i e n t w i t h me and spent many hours h e l p i n g to r e v i s e the manuscript, f o r which I am very g t a t e f u l . Most s p e c i a l thanks go to unnamed f r i e n d s who had unflagging love and understanding through the pendulum swing of emotions experienced at a l l stages of t h i s p r o j e c t . F i n a n c i a l support was provided through N. R. C. grants to Drs. R. L. Armstrong and H. R. Wynne-Edwards, and U. B. C. Teaching A s s i s t a n t s h i p s i n the Department of Geology. 62 REFERENCES Armstrong, R. L., and Runkle, D. 1979. 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Geotectonics of Cretaceous and Eocene plutons i n B r i t i s h Columbia: a paleomagnetic f o l d t e s t . Canadian J o u r n a l of Earth Sciences, 1A, pp. 1246-1262. 1977b. Paleomagnetism of Mesozoic plutons i n the westernmost Coast Complex of B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences, 24, PP- 2127-2139. Tipper, H. W. 1969. Mesozoic and Cenozoic geology of the northeast part of Mount Waddington map-area (92N), Coast D i s t r i c t , B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 68-33. 103 p. Wanless, R. K., Stevens, R. D., Lachance, G. R., and Edmonds, C. M. 1967. Age determinations and g e o l o g i c a l s t u d i e s : K-Ar i s o t o p i c ages, report 7. G e o l o g i c a l Survey of Canada, Paper 66-17. 120 p. — 1968. Age determinations and g e o l o g i c a l s t u d i e s : K-Ar i s o t o p i c ages, report 8. G e o l o g i c a l Survey Of Canada, Paper 67-2. 141 p. Werner, L. J . 1978. Metamorphic t e r r a n e , northern Coast Mountains west of A t l i n Lake, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 78-1A, pp. 69-70. W i l l i a m s , H., Turner, F. J . , and G i l b e r t , C. M. 1954. Petrography: An I n t r o d u c t i o n to the Study of Rocks i n Thin Sections. W. H. Freeman and Company, San F r a n c i s c o . 406 p. Winkler, H. G. F. 1974. Petrogenesis of Metamorphic Rocks. Springer-V e r l a g , New York. 320 p. Woodsworth, G. J . 1978. Eastern margin of the Coast P l u t o n i c Complex i n W h i t e s a i l Lake map-area, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 78-1A, pp. 71-75. 1979. Geology of W h i t e s a i l Lake map-area. G e o l o g i c a l Survey of Canada, Paper 79-1A, pp. 25-29. 66 Paper 2: RB-SR GEOCHRONOMETRY OF THE ECSTALL, KITKIATA, AND QUOTTOON PLUTONS AND THEIR COUNTRY ROCKS, PRINCE RUPERT REGION, COAST PLUTONIC COMPLEX, BRITISH COLUMBIA by Richard Lee Armstrong and Di t a E l i s a b e t h Runkle 67 ABSTRACT The minimum age of the E c s t a l l p l u t o n i s 78-79 Ma, based on Rb-Sr mineral isochrons f o r two pegmatites. At that time the e n t i r e body of the p l u t o n , zoned c o n c e n t r i c a l l y from quartz monzonite to d i o r i t e , was Sr rich-Rb poor, r e l a t i v e l y homogeneous i n Rb/Sr r a t i o (0.024-0.061), and somewhat v a r i a b l e i n i n i t i a l Sr i s o t o p i c composition ( 8 7 S r / 8 6 S r = 0.7044 to 0.7048). Late-stage l e u c o c r a t i c dikes c o n t a i n r a d i o g e n i c Sr that i s probably derived from o l d and R b - r l c h country rocks. Pegmatite dikes from two d i f f e r e n t l o c a l i t i e s are s i m i l a r to one another i n Rb/Sr and 8 7 S r / 8 6 S r and e i t h e r have high i n i t i a l r a t i o s (0.7055 and 0.7058), l i k e the l e u c o c r a t i c dikes (0.7051 and 0.7059), or the p l u t o n and i t s pegmatite dikes are very much older (^190 Ma) than the 78-79 Ma pegmatite mineral isochrons. Sr i s o t o p i c data do not r e s o l v e t h i s ambiguity. The K i t k i a t a p l u t o n has an i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.7042-0.7043, depending on i t s age. One radiogenic S r - r i c h sample gives a model date of 165±11 Ma, but the p o s s i b i l i t y of anomalous i n i t i a l r a t i o makes t h i s r e s u l t h i g h l y u n c e r t a i n . The Quottoon p l u t o n , which l i e s a b i t f a r t h e r east of the E c s t a l l p l u t o n , gives a whole-rock isochron of 51±2 Ma w i t h 0.7045±1 i n i t i a l r a t i o and somewhat o l d e r , near concordant, U-Pb z i r c o n dates of 60 Ma. The low i n i t i a l r a t i o s i n d i c a t e a preponderance of mantle-derived magma of Mesozoic or e a r l y Cenozoic age i n the plutons s t u d i e d . Radiogenic Sr i n s c h i s t s near P r i n c e Rupert suggests an age on the order of 524 Ma, and c o r r e l a t i o n w i t h upper Precambrian and lower P a l e o z o i c rocks on the Alexander Terrane of the Alaska Panhandle. 68 Sr i s o t o p i c composition of the Central Geniss Complex i s compatible with l a t e Paleozoic- early Mesozoic ages for precursor d e t r i t a l and volcanic s t r a t a and l o c a l presence of marine carbonate with moderately enriched 8 7 S r / 8 6 S r . The generally low radiogenic Sr content of these core gneisses rules out an o r i g i n by remobilization of greatly older rocks. INTRODUCTION Among the most a c c e s s i b l e p a r t s of the Coast P l u t o n i c Complex of B r i t i s h Columbia are the cross s e c t i o n along Highway 16 between P r i n c e Rupert and Terrace and the cross s e c t i o n along Douglas Channel southwest of K i t i m a t . The e n t i r e complex has been mapped at a s c a l e of 1 i n c h to 4 m i l e s by the G e o l o g i c a l Survey of Canada (Roddick and Hutchison, 1974) and t o p i c a l s t u d i e s have concentrated on the more a c c e s s i b l e areas. For example, paleomagnetic (Symons, 1974, 1977a, 1977b), s t a b l e i s o t o p e (Margaritz and T a y l o r , 1976), p e t r o l o g i c ( H o l l i s t e r , 1975; H o l l i s t e r and Burruss, 1976), and z i r c o n d a t i n g s t u d i e s (R. Wanless, o r a l communication) have been concentrated i n the same re g i o n from which we have c o l l e c t e d samples f o r the Sr isotope and c o o l i n g h i s t o r y s t u d i e s described i n t h i s paper and the ones that f o l l o w (Harrison, et a l . , 1978; H a r r i s o n and C l a r k e , 1978). Hutchison (1970), Roddick and Hutchison (1974), and Baer (1968) have published general d e s c r i p t i o n s of the Coast Complex. The P r i n c e Rupert r e g i o n (Figure 2-1) i s t y p i c a l and c h a r a c t e r i s t i c . A c e n t r a l core of amphibolite f a c i e s and m i g m a t i t i c gneisses and f l a n k i n g b e l t s of lower grade metasedimentary and metavolcanic rocks i s invaded by numerous g r a n i t i c p l u t ons. The p l u t o n s , l a r g e l y quartz d i o r i t e and g r a n o d i o r i t e i n composition, vary i n s t y l e from m i g m a t i t i c autochthonous masses through para-autochthonous tadpole-shaped p l u t o n s , w i t h discordant heads to the northwest and f o l i a t e d t a i l s to the southwest, to discordant allochthonous stocks and b a t h o l i t h s . Our study concentrates on two of the most notable tadpole-shaped p l u t o n s , the E c s t a l l , which l i e s west of the high-grade core gneisses, and the Quottoon, which i s enclosed w i t h i n the core gneisses (Figure 2-1), and on the country rocks. Sample data are reported i n Table 70 Figure 2-1. Generalized g e o l o g i c map of the P r i n c e RupertHDouglas Channel r e g i o n showing sample l o c a l i t i e s f o r Sr i s o t o p i c analyses. L a r g e l y from Hutchison (1967) and Roddick (1970). Samples discussed as s c h i s t s near P r i n c e Rupert are shown as t r i a n g l e s ; samples of Mesozoic plutons as con-c e n t r i c c i r c l e s ; C e n t r a l Gneiss Complex samples as square o r , near Hawkesbury I s l a n d , as round symbols; and Quottoon p l u t o n sample l o c a l i t i e s as s t a r s . Table 2 - 1 . Sr isotope data, Prince Rupert region Sample No. Description l.ocnlitv Latitude Longitude ppmSr ppm Kb Rh/Sr "'Sr/"''Sr PR 2 mar PR 2 peg PR3 maf PR3 peg l'R2 A PR3 A PR 2 peg, Q + IMag l>R2 peg, K PR3 peg, 0 + IMag PR 3 peg, K PR4 Biotite quartz diorite, map unit lUa (Hutchison 1967) Pegmatite, plagioclasc-quartz-K-feldspar-hiotile Epidotc-biotitc-hornblcndc quartz diorite, map unit 10a, plagioclase zoning almost obliterated Pegmatite, K-feldspar-plagioclase-quarlz-hiotilc Lcuco-quartz diorite dike with minor biotite, plagioclase shows relict igneous zoning Leuco-granodiorite dike with minor biotite Quartz-plagioclase concentrate from PR2 pegmatite K-feldspar concentrate from PR2 pegmatite Quartz-plagioclase concentrate from PR3 pegmatite K-feldspar concentrate from PR3 pegmatite Diorite, map unit 9a (Hutchison I9(>7), malic-rich hornblende diorite RD-65-10217 Biotite granodiorite RD-65-IO204 Hornblende granodiorite RD-65-10212 Hornblende quartz monzonite W V 77-253 Quartz monzonite core of Lcstail pluton, map unit 12 (Hutchison 1967) W V 77-252-1 Quartz monzonite core of iicstall pluton, map unit 12 W V 77-252-2 Quartz monzonite core of Lcslall pluton, map unit 12 I-cstall Pluton Quarry along Highway 16 541415' Quarry along Highway 16 54 1415" Road cut along Highway 16 54 I3'50" Road cut along Highway 16 54 13'50" Quarry along Highway 16 541415" Road cut along Highway 16 54 I.V50" Quarry along Highway 16 54 1415" 54 1415" Road cut along Highway 16 54' I.V50" 54 13'50" Quarry along Highway 16 Ridge - 3 km SSI- of Herman Island on Lotall Ri\er Ridge ~ 3 km north of Madeline Lake Ridge ~3 km SSVV of Madeline Lake 130 S O " 130 8'0" 130 3'29' 130 3'29' 130'SO" 130 3 '29' 130 NO' 130 S O " 130 3 '29" 130 3 '29" 54 I.V54" 129 52 41 53 56.0' 129 41.5' 53'59.3' 129 49.7' 5.V54.4' 129 52.4' Ridge north of Drown Lake 54' 2.0' 129 56.9' Ridge north of Brown Lake 542.15' 129 55.0' Ridge north of Brown Lake 54 2.15' 129 55.0' S53 . 423 1009 355 926 1067 I9S 612 2S5 467 1030 I 143 1212 939 950 1326 1250 52.4 0.061 0.7047 131 0.30S 0.7068 33.6 0.033 0.7047 112 0.316 0.7067 75.9 0.0S2 3.4 o . o i : 0.7055 30.2 0.02S 0.7060 5.7 0.029 0.7059 217 0.355 0.7069 0.7055 230 0.492 0.7070 24.4 0.024 0.7047 40.0 0.035 0.704S 30.5 0.025 0.704X 52.,S 0.056 0.7049 34.5 0.036 0.7049 40.4 0.030 0.7046 31.5 0.025 0.7045 Table 2-1. (continued) Sample No. Description Locality Latitude Longitude ppmSr ppm Rb Rb/Sr ,7Sr/,8Sr Kitkiata Pluton 25-2 Diorite Hawkesbury Island, Douglas Channel 53°37'10" 129"10'46" 1030 19.7 0.0191 0.7046 27-1 Quartz monzonite Hawkesbury Island, Douglas Channel 53°38'2I" 129"09'03" 64.0 153 2.38 0.7204 37-1 Quartz diorite Hawkesbury Island, Douglas Channel Quottoon. PlutQn 53°38'43" 129"08'I4" 736 38.2 0.052 0.7042 21 Quartz diorite Maitland Island, Loretta Channel 53"44'19" 128°54'01" 825 39.0 0.047 0.7046 23-2 Pegmatite Maitland Island, Loretta Channel 53°43'25" 128"54'25" 328 116 0.354 0.7051 48-1 Aplite Loretta Island, Sue Channel 53"42'56" I28°54'43" 43.7 132 3.03 0.7109 20-1 Granodiorite Loretta Channel 53"45'06" I28"53'33" 546 65.7 0.120 0.7049 Metamorphic Rocks Near Prince Rupert PR1 Biotite-plagioclase-quartz schist Road cut along Highway 16 54°I4'27" I30"9'17" 332 77.2 0.233 0.7102 101 Kyanite-muscovite-biotite-plagioclasc-quartz schist Junction of Highway 16 and Port Edward Road 54°I5'14" I30"I5'I5" 90.7 70.0 0.772 0.7101 102 Muscovite-garnet-biotite-quartz-feldspar schist Junction or Highway 16 and Port Edward Road 54°I5'14" 130°I5'I5" 182 44.8 0.246 0.7084 103 Schist interlayered with hornblende-plagioclase amphibolite Junction or Highway 16 and Port Edward Road 54°15'14" I30'I5'I5" 188 70.4 0.375 0.7072 345 Pelitic chlorite-muscovite schist Tsimpsean Peninsula 54°27.0' I30"28.5' 229 23.8 0.104 0.7073 533 Graphitic schist Digby Island 54°I8.0' I30"'23' 386 56.7 0.147 0.7058 598 Metarhyolite Digby Island 54°I8.4' I30"26.0' 42.8 8.7 0.204 0.7092 221 Chlorite schist Gneiss Digby Island Between Ecstall and Quottoon 54°I5.5' Plutons I30°25.0' 214 5.7 0.027 0.7056 PR-6 A Amphibolitic migmatite-gneiss, 1.5 cm thick leucocratic band like 6C Quarry along Highway 16 54°13'17" 129"43'I0" 1118 9.8 0.009 0.7052 PR-6B Mafic-rich dark middle band, 3 cm thick (hornblende-biotite-quartz-plagioclase schist) Quarry along Highway 16 54°I3'17" 129"43'I0" 787 21.3 0.027 0.7053 PR-6C Leucocratic band, 2 cm thick (plagioclase gneiss with minor biotite and hornblende) Quarry along Highway 16 54"13'17" I29°43'I0" 1075 6.1 0.006 0.7053 Table 2-1. (continued) Sample N o . Descr ip t ion Loca l i ty Latitude Longitude p p m S r ppm R b R b / S r " ' S r / ^ S r K w i n i t s a Gneiss P R - 5 a A Banded garnctifcrous granulitc gneiss, 5 c m thick ( h o r n b l c n d c - b i o t i t c - K - f c l d s p a r - q u a r t z -plagioclasc gneiss) P R - 5 a B Biot i te r ich band o f garnctifcrous granulitc gneiss, ~ 2 c m thick P R - 5 b C E n d slab o f banded granulitc" gneiss, ~ 2 c m thick, mafic-r ich band (garnct-cummingtoni te-b io l i tc -quar tz-p lag ioc la sc gneiss) P R - 5 b D Slab o f banded granul i tc gneiss, 3 c m thick, d iops idc-r ich calc-silicate band (sphenc-c a l c i l e - q u a r l z - h o r n b l c n d c - d i o p s i d c - p l a g i o -clasc gneiss) P R - 5 b E Slab o f banded granul i tc gneiss, 2 c m thick, amphibo lc - r i ch calc-silicate band (sphe'ne-d i o p s i d c - h i o t i t c - c u m m i n g t o n i t c - h o r n b l c n d c -quartz-plagioclase gneiss) P R - 5 b I- S lab o f banded granul i tc gneiss, 2 c m thick, calc-silicate and schist band ( K - f c l d s p a r - b i o t i t c -garnct -cummingtoni tc-quar tz-p lag ioc la sc gneiss) P R - 5 b G E n d s lab o f banded granuli tc gneiss, > 4 c m thick, garnctifcrous leucognciss (coarse garnct-plagio-c lasc-quartz gneiss wi th m i n o r biotite) Wn-34-75 Leucognciss Wn-35-75 Coarse gneiss C G I B io l i t c -garnct -pIag ioc la sc -K- fc ldspar -quar tz gneiss C G 2 B io t i t c -p lag ioc la sc -garnct -hornhlcnde gneiss C G 3 Diops idc-ca l c i t c -quar tz - f c ld spar gneiss 72-1 Gnc i s s i c quartz monzoni te o f agmatite K w i n i t s a 4900' elevation, ridge 10 k m south of Skecna River 4900' e levat ion, o n ridge 10 k m south or Skecna River 4900' elevation, on ridge 10 k m south of Skecna River G r a n t Point , M a i t l a n d Island 54 I .V50" Centra l Gneiss C o m p l e x 8 k m south of A m o l h Lake ( U T M 9 461500 605900) 54 4 0 . 7 4 ' M l . C a m p a g n o l o ( U T M 9 452900 6041400) 54 31.OX' 54' 1 2 1 0 " 54' I 2 ' 1 0 " 5 4 " I 2 ' I O " 5341 ' 3 0 " I29 32 9 " X0X 44.X 0 .055 0.7056 X3X X5.1 0 .102 0.7057 914 52 .2 0 .057 0 .7056 616 0 . 7 0.001 0.7055 6X2 7.9 0 .012 0.7054 606 12.2 0 .020 0 .7056 747 5.1 0 .007 0 .7057 129 3 5 . 6 1 ' 101 6X.4 0 .679 0.7103 129 4 3 . 4 4 ' 223 3X.6 0 .173 0.7057 129 1 9 1 5 " 320 141 0.441 0 .7067 129 | 9 | 5 " 5|() 4.4 0 .009 0 .7042 1 2 9 1 9 1 5 " 211 5 .0 0 .024 0.7058 129 05 0 0 " 526 71 .0 0 .135 0 .7050 --4 O J Table 2-1. (continued) Sample N o . Descr ipt ion Loca l i ty Lati tude Longi tude p p m S r ppm R b R b / S r " 7 S r / " \ S r 72-2 Quartz monzoni te o f agmatite G r a n t Po int , M a i t l a n d Island •53"4I '30" 129 0 5 ' 0 0 " 2X2 103 0 .0364 0.7058 72-3 Quartz d ior i te o r granodiori te o f agmatite G r a n t Po int , M a i t l a n d Island 53 4 T 3 0 " 129 05 ' 0 0 " 576 32.5 0 .056 0.7041 15-3 Biotite schist Sue C h a n n e l , Hawkesbury Island 53 4 2 ' 3 3 " 128 5 4 ' 0 8 " 108 . 112 1.036 0 .7089 85-1 Lcucognciss Sue C h a n n e l , Hawkesbury Island 53"42 '37" 128 "53 ' 0 6 " 154 98 .0 0 .637 0.7073 5-2 H o r n b l e n d e schist Sue C h a n n e l , Hawkesbury Island 5 3 " 4 I ' 4 8 " I28"56 '06" 59.7 15.1 0 .252 0 .7052 5-3 Hornblende-p lag ioc la se schist Sue C h a n n e l , Hawkesbury Island 5 3 ' 4 I ' 4 8 " I28"56 '06" 386 6 .6 0 .017 0 .7046 14-2a Hornb lendc-p lag ioc l a sc gneiss o f agmatite Sue C h a n n e l , Hawkesbury Island 5 3 " 4 I ' 4 I " I28 5 9 ' 5 3 " 654 16.4 0 .025 0.7041 74-3 Garnet amphibo l i t e o f agmatite M a i t l a n d Island 53 "41 1 0 " 129 :0414 ' ' 135 44 .0 0 .325 0.7054 60-2 A m p h i b o l i t e Hawkesbury Island, Douglas C h a n n e l 5 3 " 4 0 ' I 0 " 129 0 5 ' 2 4 " 448 38.0 0 .085 0 .7047 59-2 Garnet amphibo l i t e Hawkesbury Island, Douglas C h a n n e l 53"40 '04" I29"05 '42" 479 15.8 0 .033 0.7061 3 8 - l b Q u a r t z - m i c a schist Hawkesbury Island, Douglas C h a n n e l 53"39"27" 129' 06 ' 4 2 " 138 143 1.035 0.7113 37-2 B io t i t e -hornblcndc-quar tz -p lag ioc la sc schist Hawkesbury Island, Douglas Channe l 53 3 K ' 4 3 " 129 0 8 1 4 " 171 53.6 0 .314 0.7058 N e > T f » : A n a l y t i c a l t e c h n i q u e s : R b a n d S r c o n c e n t r a t i o n s were de te rmined by repl icate ana lys is o r pressed p o w d e r pellets us ing X - r a y f luorescence. U n i t e d Stales ( i c o l o g i c a l Survey rock s tandards were used l o r c a l i b r a t i o n ; m a s s a b s o r p t i o n c o e l l i u c n l s were o b t a i n e d f r o m M o Ky C 'o ropton scat ter ing measurements . R b / S r rat ios have a prec is ion o r 2 - ; U n ) a n d c o n c e n t r a t i o n s a prec is ion o l s- ( lo> S r i s o t o n i c c o m p o s i t i o n was measured o n u n s p i k c d s a m p l e s p r e p a r e d us ing s t a n d a r d ion exchange techniques . T h e mass spect rometer (Ml sector . .10 c m rad ius , so l id s o u r c e ! is o l Un i ted Sta les N i ' l i o n i i l l u rc ' iu o l ' S t a n d a r d s ( N B S ) des ign m n d l l t t d by II. l a u l . D a t a a c q u i s i t i o n is d ig i t ized a n d a u t o m a t e d us ing a N O V A c o m p u t e r , l i s p e r i n i e n l a l data have been n o r m a l i z e d l o a " ' S r / " » S r ratio o l t) I I'M a n d adjusted s o that the N B S s t a n d a r d S r C O , ( S R M W ) gives a " ' S r / « " S r rat io o r 0.71022 i 2 a n d the E i m c r a n d A m e n d S r a rat io o l 0.7OH0O i 2. T h e prec is ion o l a single " ' S r / " \ S r ra l io is O.IMKII 1 ( l o ) R b -Sr da les a rc based o n a R b d e c a y c o n s t a n t o f 1.42 s. 1 0 - 1 1 y e a r - ' . T h e regressions a rc c a l c u l a t e d a c c o r d i n g to (he technique » r Y o r k (l"f>7). 75 ECSTALL PLUTON This zoned, tadpole-shaped p l u t o n has been described by Hutchison (1970). Previous K-Ar da t i n g (64+8 Ma on b i o t i t e - G.S.C. 65-31, 70+4 Ma on b i o t i t e and 87±15 Ma on hornblende G.S.C. 66-13 and G.S.C. 66-12, l i s t e d i n Wanless and others, 1967 and 1968) suggested a Cretaceous age but the p o s s i b i l i t y of an o l d e r age was not r u l e d out. Hutchison (1970) recognized the p o s s i b i l i t y that the dates r e f l e c t e d u p l i f t and c o o l i n g that progressed eastward across the Coast P l u t o n i c Complex s i g n i f i c a n t l y a f t e r p l u t o n emplacement. Our i n i t i a l sample c o l l e c t i o n from l o c a t i o n s along the P r i n c e Rupert-Terrace highway included t y p i c a l p l u t o n i c phases, pegmatite, and l i g h t - c o l o r e d d i k e s . Considerable e f f o r t was expended i n t r y i n g to o b t a i n a sample s u i t e that v a r i e d i n Rb/Sr r a t i o from the d i o r i t e to quartz monzonite zoned pl u t o n . We found that the d i f f e r e n t petrographic phases are a l l S r - r i c h , Rb-poor, and f a i r l y uniform i n Rb/Sr r a t i o (0.024 to 0.061). A d d i t i o n a l samples s u p p l i e d by J . Roddick and G. Woodsworth of the G e o l o g i c a l Survey of Canada d i d not widen the spread i n Rb/Sr r a t i o . Present-day Sr i s o t o p i c composition ( 8 7 S r / 8 6 S r ) of the major petrographic phases v a r i e s from 0.7045 to 0.7049, only somewhat more than our a n a l y t i c a l u n c e r t a i n t y (+0.00015 f o r one standard d e v i a t i o n ) . These data do not define a meaningful isochron but do weakly suggest a small spread i n i n i t i a l i s o t o p i c composition (Figure 2-2). The two E c s t a l l pegmatites, both f a i r l y r i c h i n K-feldspar are s i g n i -f i c a n t l y enriched i n Rb r e l a t i v e to e n c l o s i n g rocks and both pegmatites are s i m i l a r i n Rb/Sr and Sr i s o t o p i c composition. I f the pegmatites repre-sent l a t e - s t a g e l i q u i d s e q u i l i b r a t e d i s o t o p i c a l l y w i t h t h e i r e n c l o s i n g 76 707 h .706 h 8 7 S r / 8 6 S r .705 h .704 Figure 2-2. Isochron diagram f o r the E c s t a l l pluton. Whole-rock pluton and pegmatite samples are shown by black dots; l e u c o c r a t i c dikes with anomalous radiogenic Sr by t r i a n g u l a r symbols; and mineral g^pargges from the from the pegmatites by s t a r s . One standard deviation errors f o r Sr/ Sr are shown with each symbol. For reference, the mineral isochron f o r sample PR3 maf (Harrison et a l . 1979) and the isochron f o r the country rock s c h i s t s with greatest model ages (from F i g . 2-4, t h i s paper) are shown as dotted l i n e s . The pegmatite mineral isochrons, s o l i d l i n e s , are minimum ages f o r the pluton; the pegmatite-country rock isochron may be an a r t i f a c t due to enrichment of the l a t e stage l i q u i d s i n radio-genic Sr, presumably derived from the country rocks. rocks, then a date of about 190+15 Ma i s suggested. This cannot be a unique i n t e r p r e t a t i o n because other data i n d i c a t e contamination of l a t e -stage l i q u i d s w i t h r a d i o g e n i c Sr. Two l i g h t - c o l o r e d medium- to f i n e - g r a i n e d dikes of quartz d i o r i t e and g r a n o d i o r i t e from the same two pegmatite l o c a l i t i e s were analysed and found to co n t a i n radiogenic Sr that cannot be explained by i n s i t u gener-a t i o n from R b — t h e c a l c u l a t e d dates are unreasonably l a r g e and i n the case of l o c a l i t y PR3 even negative. These rocks cannot be derived i n a closed system from the major g r a n i t i c phases of the E c s t a l l p l u t o n , but they may be reasonably i n t e r p r e t e d as melts of more rad i o g e n i c country rock (as w i l l be described i n a l a t e r s e c t i o n of t h i s paper) or melts contaminated by country rock and introduced l a t e i n the s o l i d i f i c a t i o n h i s t o r y of the plut o n . To i l l u s t r a t e t h i s on Figure 2-2, the E c s t a l l country rock isochron from Figure 2-4 i s shown—only modest amounts of the radi o g e n i c Sr of the s c h i s t s near P r i n c e Rupert would be needed to contaminate E c s t a l l magmas to y i e l d the l a t e - s t a g e l i q u i d s . A s i m i l a r contamination may a f f e c t the peg-matites s t u d i e d . A f u r t h e r attempt to define an age f o r the pegmatites was made by anal y s i n g f e l d s p a r separates. These y i e l d e d two mi n e r a l isochrons of ne a r l y i d e n t i c a l slope (corresponding to 78 and 79 Ma dates) and d i f f e r e n t i n i t i a l r a t i o s (0.07055 f o r PR2 and 0.7058 f o r PR3, r e s p e c t i v e l y ) as shown on Figure 2-2. These are minimum dates f o r the emplacement of the pegma-t i t e s and p l u t o n , but the p o s s i b i l i t y of r e s e t t i n g by intense post-emplace-ment metamorphism cannot be r u l e d out by the Sr data. T e x t u r a l l y the rocks at l o c a l i t y PR2 do not appear to be metamorphosed, but at l o c a l i t y PR3, f a r t h e r east, p l a g i o c l a s e f e l d s p a r zonation has faded s i g n i f i c a n t l y . No mineral dates were obtained by H a r r i s o n , et^ a l . , (1978) that exceeded the 78 l i m i t provided by the pegmatite mineral isochrons. I f the isochrons are accepted as time of emplacement, then a l l l a t e - s t a g e l i q u i d s show s i g n i f i -cant and v a r i a b l e r a d i o g e n i c Sr contamination. At the time of pegmatite c r y s t a l l i z a t i o n the main body of the p l u t o n had i n i t i a l Sr i s o t o p i c com-p o s i t i o n s of 0.7044-0.7048, w h i l e the l e u c o c r a t i c dikes were 0.7051 and 0.7059—bracketing the two pegmatite values of 0.7055 and 0.7058. A Middle J u r a s s i c age would imply very gradual c o o l i n g or a complicated c o o l i n g h i s t o r y as discussed i n the companion paper by H a r r i s o n , et a l . , (1978). Middle J u r a s s i c plutons are known both east and west of the Coast P l u t o n i c Complex i n B r i t i s h Columbia (Roddick and Hutchison, 1974). They are p a r t i c u l a r l y extensive i n the I n s u l a r B e l t on Vancouver I s l a n d and are a l s o known from the western s i d e of the Coast Complex northwest of Vancouver. Baer (1968, 1973), working near B e l l a Coola about 300 km south-east of P r i n c e Rupert, argued f o r two periods of p l u t o n emplacement—the o l d e r one of T r i a s s i c to Middle J u r a s s i c age, the younger one e a r l y Cenozoic. The o l d e r pegmatite whole-rock dates would support h i s model. On the other hand, a l l our mineral dates and the pegmatite f e l d s p a r isochrons support an Upper Cretaceous age as o r i g i n a l l y suggested by the G e o l o g i c a l Survey of Canada K-Ar dates. KITKIATA PLUTON A small discordant quartz d i o r i t e p l u t o n s t r a d d l e s Douglas Channel j u s t east of the E c s t a l l p l u t o n (Roddick, 1970). This p l u t o n , i n t r u s i v e i n t o metasedimentary and metavolcanic r o c k s , i s named f o r the adjacent i n l e t that opens i n t o Douglas Channel. On Hawkesbury I s l a n d , g r a n i t i c rock extends approximately 5 km f u r t h e r n o r t h than mapped by Roddick (1970). The southern p a r t of the p l u t o n , where sampled, i s l a r g e l y quartz d i o r i t e . 79 The northern part i s quartz monzonite with large (10-50m) screens of meta-sedimentary rock. Along the northern contact, g r a n i t i c and metamorphic rocks are inter l a y e r e d on a large scale. Roddick (1970) reports the southern contact to be a 0.1 m brecciated zone against quartz monzonite which i s l i t -p a r - l i t mixed with metasedimentary rocks far t h e r south. The g r a n i t i c rocks are medium- to coarse-grained and moderately f o l i a t e d , with t y p i c a l g r a n i t i c textures, normally zoned pla g i o c l a s e , horn-blende, and b i o t i t e as p r i n c i p a l mafics, and n e g l i g i b l e a l t e r a t i o n . A s u i t e of samples (25-2, 27-1, 37-1) c o l l e c t e d on Hawkesbury Island contained only one Rb-enriched quartz monzonite that together with whole-rock samples of quartz d i o r i t e and d i o r i t e gives a model date of 165i"ll Ma (Figure 2-3). The other two samples only define an i n i t i a l r a t i o (0.7042-0.7043 depending on t h e i r age). The p o s s i b i l i t y of radiogenic Sr contamination from the intimately intermixed metamorphic rocks casts considerable doubt on the calculated model date. No mineral dates are a v a i l a b l e f o r these samples. QUOTTOON PLUTON In t h i s paper we r e f e r to a long narrow b e l t of g r a n i t i c and migmatitic rock that p a r a l l e l s the regional s t r i k e as the Quottoon pluton because no formal segmentation has been proposed. The portion north of 54° was named by Hutchison (1967, 1970) and described as tadpole-shaped. The southern continuation, mapped by Roddick (1970), i s more i r r e g u l a r or d i k e - l i k e , with steep and crudely p a r a l l e l contacts. Later workers may wish to d i s t i n g u i s h several d i s c r e t e plutons linked with migmatite zones and i n t e r f angering contacts. Hornblende and b i o t i t e K-Ar dates, and the s t r u c t u r a l and metamorphic s e t t i n g f o r both the Skeena River and Douglas Channel sections of t h i s long g r a n i t i c b e l t are v i r t u a l l y F i g u r e 2-3. Isochron diagram f o r plutons sampled on or near Hawkesbury I s l a n d . The K i t k i a t a p l u t o n i s shown w i t h c i r c u l a r symbols; the younger Quottoon p l u t o n i s shown w i t h t r i a n g u l a r symbols. Note the reduced s c a l e of t h i s p l o t i n c o n t r a s t w i t h Figures 2-2, 2-4, and 2 The age of the K i t k i a t a p l u t o n i s q u i t e u n c e r t a i n because the i n i t i a l i s o t o p i c homogeneity has not been e s t a b l i s h e d . The number given i s a c a l c u l a t e d model date, assuming i n i t i a l homogeneity. I n i t i a l h e t e r o -geneity i s shown by the r e s u l t s f o r the nearby E c s t a l l p l u t o n . 81 i d e n t i c a l , so we w i l l use the name Quottoon f o r our sample s u i t e . Samples 2 0 - 1 , 2 1 , 2 3 - 2 , and 4 8 - 1 were c o l l e c t e d from the northern end of Sue Channel and i n L o r e t t a Channel. Our observations are thus s t r i c t l y r e l a t e d to the Douglas Channel cross s e c t i o n and apply elsewhere only by p r o j e c t i o n along s t r i k e . Where the Quottoon pluton crosses Douglas Channel, i t appears to have been emplaced i n t o the core of the Foch An t i f o r m composed of f o l i a t e d rocks of the C e n t r a l Gneiss Complex. The Quottoon rocks are h o r n b l e n d e - b i o t i t e g r a n o d i o r i t e w i t h v a r i a t i o n to quartz monzonite and quartz d i o r i t e . The g r a n i t i c rocks are cut by contorted as w e l l as s t r a i g h t pegmatite and a p l i t e d i k e s . Wider pegmatite dikes are zoned, w i t h f i n e r - g r a i n e d a p l i t e centers. v U n f o l i a t e d , t e x t u r a l l y homogeneous g r a n i t i c rocks are mixed w i t h areas t h a t d i s p l a y ghost f o l d s , h i g h l y contorted banded rock, and multiphase i n t r u s i o n s . G r a n i t i c rock grades i n t o banded and f o l i a t e d rock and i n t o gneisses w i t h a reg u l a r f o l d p a t t e r n . T y p i c a l g r a n i t i c rocks are medium- to coarse-grained, w i t h normally and r e v e r s e l y zoned p l a g i o c l a s e (An2rj-An35), and b i o t i t e and l e s s e r hornblende as mafic minerals. B i o t i t e appears to be r e p l a c i n g horn-blende. K-feldspar occurs as l a r g e phenocrysts and together w i t h quartz as i n t e r s t i t i a l g r a i n s . Sphene and epidote are v i s i b l e i n hand specimens, a p a t i t e and z i r c o n are present as microscopic a c c e s s o r i e s . P l a g i o c l a s e i s l i t t l e to moderately s a u s s u r i t i z e d , but otherwise a l t e r a t i o n i s not evident. A p l i t e contains small ragged muscovite f l a k e s and rounded, corroded garnets. K-Ar dates p r e v i o u s l y reported by Wanless, et a l . , ( 1 9 6 8 ) f o r the Quottoon p l u t o n (50+5 Ma f o r b i o t i t e , G.S.C. 6 6 - 6 ; 4 8 ± 9 Ma f o r hornblende, G.S.C. 6 6 - 7 ; 44+5 Ma f o r b i o t i t e , G.S.C. 6 6 - 9 ; and 4 9 ± 7 Ma f o r hornblende, G.S.C. 6 6 - 8 ) suggest an Eocene age or time of c o o l i n g . Four whole-rock samples, i n c l u d i n g Rb-enriched a p l i t e and pegmatite, give an isochron date 82 of 51-2 Ma (Figure 2-3), i n good agreement w i t h the K-Ar dates. This sug-g e s t i o n of r a p i d c o o l i n g f o l l o w i n g emplacement was the reason sample 20 was chosen f o r the f i s s i o n t r a c k c o o l i n g h i s t o r y s t u d i e s of H a r r i s o n , et a l . , (1978). To complete the geochronometry of t h i s e x t e n s i v e l y s t u d i e d sample, s e v e r a l grams of z i r c o n were separated at the U n i v e r s i t y of B r i t i s h Columbia and dated i n the G e o l o g i c a l Survey of Canada l a b o r a t o r y i n Ottawa. The r e s u l t s are shown i n Table 2-2. The U-Pb dates f o r two z i r c o n f r a c t i o n s are n e a r l y concordant at 60 Ma—somewhat greater than our whole-rock isochron date of 51 Ma, but s t i l l e a r l y Cenozoic. Several i n t e r p r e t a t i o n s of the d i f f e r e n c e are p o s s i b l e . The Quottoon date i s the youngest U-Pb date yet obtained i n t h i s part of the Coast P l u t o n i c Complex. P r e v i o u s l y the G e o l o g i c a l Survey of Canada l a b o r a t o r y has obtained concordant U-Pb dates of 66-97 Ma f o r high-grade c e n t r a l gneisses near K w i n i t s a (PR5 on Figure 2-1) and o l d e r , discordant dates f o r gneiss samples f a r t h e r n o r t h (Wn 34-75 and Wn 35-75 on Figure 2-1) (R. Wanless, unpublished data). The z i r c o n dates reported here may date formation of the g r a n o d i o r i t e at 60 Ma w h i l e the a p l i t e and pegmatite were s t i l l open systems at high temperature. In t h i s i n t e r p r e t a t i o n the a p l i t e and pegmatite became closed systems, perhaps being formed as much as 10 Ma l a t e r . A l t e r n a t i v e l y the g r a n o d i o r i t e formed 51 Ma ago w i t h some x e n o c r y s t i c z i r c o n from the surrounding country rock. Zircons are known to be r e s e t by g r a n u l i t e f a c i e s metamorphism (Montgomery, ejt a l . , 1977; Grauert and Wagner, 1975) or even lower grade metamorphism i f they are U - r i c h or o l d and metamict ( A l l e g r e , et^ a l _ . , 1974; Gebauer and Grllnenfelder, 1976). On the other hand, f r e s h l y c r y s t a l l i z e d and low-U z i r c o n s have been reported to s u r v i v e above g r a n i t e s o l i d u s temperatures (Koppel and Sommerauer, 1973; Gulson and Krogh, 1973; H i g g i n s , et a l . , 1977). Table 2-2. U-Pb data for zircon separates from sample DR-20 from the Quottoon pluton Weight Unspiked Pb isotopic Composition Calculated dates* dissolved 206 p b / 207 p b / 207 p b / Sample (mg) ppm U ppm Pb 208 : 207 : 206 : 204 238TJ 2 3 5u 2 06pb - 230 + 325 mesh magnetic fraction 14.97 864. .8 7.94 14.20 : 5.689 : 79.98 : 0.1258 58.6 59.1 85.1 - 230 + 325 mesh nonmagnetic fraction 18.35 806. ,3 7.48 13.23 : 6.543 : 80.04 : 0.1836 59.5 60.0 83.0 Notes: The G e o l o g i c a l Survey of Canada a n a l y t i c a l procedures are s l i g h t l y modified from Krogh (1973). I s o t o p i c analyses are done on a 15 i n . (38 cm) r a d i u s , 90 d e f l e c t i o n , s o l i d source mass spectrometer equipped w i t h an e l e c t r o n m u l t i p l i e r system. Samples analyzed by J.-C. Bisson and W.D. Loveridge i n the lab o r a t o r y of the G e o l o g i c a l Survey of Canada i n Ottawa. * Assuming a contamination Pb composition of 52.21:21.63:24.78:1.374 and common Pb composition of 38.48:15.62£l8.57:1^00 and a 5.15 ng blank. Decay constants 2 3 8 U = 1.55126 X 10 1 0 y e a r " 1 , 2 3 5 U = 9.8485 X 10 1 0 year The sample c o n s i s t s of c l e a r rounded to subhedral z i r c o n s w i t h many i n t e r n a l blebs but no v i s i b l e zoning. 84 Contamination w i t h the near-concordant and o l d e r country rock z i r c o n s might thus be r e s p o n s i b l e f o r the U-Pb vs. Rb-Sr date discordance. U-Pb systematics cannot yet r e s o l v e the a l t e r n a t e i n t e r p r e t a t i o n s , but we are confident that the d i f f e r e n t i a t i o n and s o l i d i f i c a t i o n of t h i s part of Quottoon p l u t o n must l i e between 60 and 51 Ma. The r a p i d c o o l i n g of the plu t o n that followed 50 Ma (Harrison, et a l . , 1978) leads us to p r e f e r the Rb-Sr whole-rock isochron date of 51 Ma as the time of pl u t o n emplacement. I t i s d i f f i c u l t to generate a p h y s i c a l l y r e a l i s t i c c o o l i n g h i s t o r y that proceeds s l o w l y f o r the f i r s t 10 Ma and then r a p i d l y , as i s documented by the mineral dates. The Quottoon and other plutons f a r t h e r east a l l have n e a r l y concordant Eocene K-Ar dates and some are observed to crosscut Upper J u r a s s i c s t r a t a (Hutchison, 1970; Roddick and Hutchison, 1974). A l l are thought to be Eocene i n age. This i s Baer's (1968, 1973) second major p l u t o n i c episode i n the Coast Complex near B e l l a Coola. SCHISTS NEAR PRINCE RUPERT Medium- and low-grade metasedimentary and metavolcanic rocks enclose the northern end of the E c s t a l l p l u t o n . Hutchison (1967, 1970) suspected a T r i a s s i c or upper P a l e o z o i c age on the b a s i s of l i t h o l o g i c s i m i l a r i t y w i t h dated rocks i n the I n s u l a r B e l t to the west and along s t r i k e i n southeastern Alaska. Lack of d i a g n o s t i c f o s s i l s , s c a r c i t y of d i s t i n c t i v e rock u n i t s , poor exposure, and complex s t r u c t u r e f r u s t r a t e any p r e c i s e age assignment f o r these rocks. One sample of s c h i s t (PR1) from near the E c s t a l l contact contained very radiogenic Sr, suggesting a much older age than i n f e r r e d by Hutchison. A d d i t i o n a l samples of s c h i s t and p h y l l i t i c s c h i s t were obtained from Dingee 85 (G. Dingee, M.Sc. thesi s , i n progress) and R. R. P a r r i s h . The r e s u l t s are plotted as c i r c l e s i n Figure 2-4. Four whole-rock s c h i s t s l i e on a 524+21 Ma isochron, but other samples f a l l o f f towards younger model dates. Even specimens from the same large road cut give a discordant pattern. Meta-morphism of p e l i t i c rocks usually produces discordant patterns and reset isochrons, giving some time between the l i m i t s of age of provenance and age of metamorphism (Dasch, 1969; Compston and Pidgeon, 1962; Hofmann and Grauert, 1973; Clauer, 1973, 1974; Gebauer and Grunenfelder, 1974). Our r e s u l t s are not conclusive but suggest that the rocks near Prince Rupert are at le a s t p a r t l y of lower Paleozoic age, possibly even older. The implication i s that the Alexander Terrane of the Alaska Panhandle extends at le a s t to Prince Rupert. In Alaska, the Alexander Terrane i s composed of lower Paleozoic and l a t e Precambrian eugeosynclinal s t r a t a and includes dated lower Paleozoic and la t e Precambrian g r a n i t i c rocks (Brew, et a l . , 1966; Berg, et a l . , 1972; Churkin and Eberlein, 1977). One block of migmatitic d i o r i t e from a quarry along the Prince Rupert-Terrace highway ( l o c a l i t y PR6) was cut into slabs before Rb-Sr analysis. The three slabs are v i r t u a l l y i d e n t i c a l i n Sr i s o t o p i c composition and th i s i s interpreted as i s o t o p i c homogenization during metamorphism about 50 Ma ago. The composite sample l i e s on the 524 Ma isochron near i t s o r i g i n , but because of i t s generally low Rb-Sr r a t i o i t s p o s i t i o n on the isochron i s not diagnostic. I t may be part of the old rock s u i t e but could quite w e l l be much younger. CENTRAL GNEISS COMPLEX The sample su i t e of core gneisses and s c h i s t s a v a i l a b l e f o r the present 86 .710 1 8 7 S r / 8 6 S r i / i i T " ~ - 1 1 .709 \ / V -.708 * / 7 -.707 O ~ 47 Ma SLAB IS0CHR0N -.706 • / ® S C H I S T S . N E A R .705 P R i i i I N C E R U P E R T Rb / Sr . ... i i i i 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Figure 2 - 4 . Isochron diagram for s c h i s t s c o l l e c t e d near Prince Rupert and the slab isochron f o r migmatitic d i o r i t e from between the E c s t a l l and Quottoon plutons along the Prince Rupert highway. 87 Rb-Sr reconnaissance included a v a r i e t y of l i t h o l o g i e s from Hawkesbury I s l a n d and v i c i n i t y , a c o l l e c t i o n made by R. R. P a r r i s h on a r i d g e south of the Skeena R i v e r (CGI,2,3), blocks of banded C a - r i c h gneiss from K w i n i t s a (PR-5), and two samples from which the G e o l o g i c a l Survey of Canada had separated discordant z i r c o n s (Wn 34-75 and Wn 35-75). The analyses, p l o t t e d i n F i g ure 2-5, s c a t t e r widely so that no simple i n t e r p r e t a t i o n i s p o s s i b l e . One purpose of t h i s i n i t i a l study was to seek evidence of r e m o b i l i z e d pre-P a l e o z o i c m a t e r i a l i n the high-grade gneisses. None of the samples shows any s i g n of great age. In f a c t , none need be any o l d e r than upper P a l e o z o i c and most may be e i t h e r Mesozoic or have been i s o t o p i c a l l y r e c o n s t i t u t e d during the Mesozoic. The s u i t e from Hawkesbury I s l a n d , shown as hexagons on Figure 2-5, form, w i t h one exception, an elongate c l u s t e r between isochrons of 115 and 171 Ma that have f a i r l y low i n i t i a l r a t i o s so i t i s u n l i k e l y that any s i g n i f i c a n t amount of pre-Mesozoic rock i s present. The e x c e p t i o n a l p o i n t i s f o r a g a r n e t - b i o t i t e - p l a g i o c l a s e - a m p h i b o l i t e . There are s e v e r a l p o s s i b l e explanations that cannot be d i s t i n g u i s h e d . This may be a Mesozoic rock that has reacted w i t h sea water and exchanged w i t h , or acquired, oceanic Sr w i t h a 8 7 S r / 8 6 S r r a t i o of 0.707-0.708. Such a process has been observed to occur i n greenschist-grade Mesozoic v o l c a n i c rocks i n B r i t i s h Columbia. A second p o s s i b i l i t y i s that severe Rb l o s s during p a r t i a l m e l t i n g has occurred. This would s h i f t a p o i n t from i t s closed-system p o s i t i o n on the Mesozoic isochron to near the 8 7 S r / 8 6 S r a x i s . A f i n a l p o s s i b i l i t y i s that the e n r i c h -ment i n 8 7 S r i s evidence of a pre-Mesozoic age. The f i r s t i n t e r p r e t a t i o n i s p r e f e r r e d because c l o s e l y a s s o c i a t e d rocks are c a l c - s i l i c a t e s and c a l c i t e -bearing s c h i s t s . The banded gneisses near K w i n i t s a have a t t r a c t e d a t t e n t i o n because of t h e i r g r a n u l i t e f a c i e s metamorphic mineral assemblages ( H o l l i s t e r , 1975). 88 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Figure 2-5. Isochron diagram f o r metamorphic rocks of the Central Gneiss Complex. Hawkesbury Island and v i c i n i t y samples shown as hexagons; Kwinitsa slab isochron samples shown as v e r t i c a l bars; and samples from other l o c a l i t i e s shown as t r i a n g l e s . Phanerozoic seawater compositions (Peterman and others ^ 7 0 ) g ^ e i z e r and Compston 1974) l i e between 0.7070 and 0.7090 along the :. Sr/ Sr a x i s . The s e v e r a l samples above 0.705, along t h i s same ax i s , are a l l associated with c a l c i t e porphyroblasts or c a l c -s i l i c a t e lenses, suggesting admixture of seawater Sr to d e t r i t a l and v o l -canic precursor rocks. 89. Two blocks from that l o c a l i t y were cut i n t o s l a b s and analysed f o r Rb/Sr and 8 7 S r / 8 6 S r . On a s c a l e of 10's of cm, the rocks were i s o t o p i c a l l y homogenized at the time of r e g i o n a l metamorphism. The 36±40 Ma sla b isochron (Figure 2-5) i s comparable to the 66-97 Ma concordant z i r c o n s from the same l o c a l i t y (R. Wanless, unpublished data) and 40-50 Ma K-Ar dates f o r the surrounding r e g i o n . The Rb-poor K w i n i t s a gneisses have a f a i r l y r a diogenic Sr i s o t o p i c composition (^0.7056). The p o s s i b l e explanations f o r t h i s have already been l i s t e d i n the d i s c u s s i o n of the Hawkesbury I s l a n d sample s u i t e . The presence of f r e e c a l c i t e and g e n e r a l l y C a - r i c h character of the K w i n i t s a rocks i s support f o r the hypothesis that these are Mesozoic or upper P a l e o z o i c rocks c o n t a i n i n g a component of oceanic Sr. The two samples of core gneiss from w e l l north of the Skeena R i v e r (Wn 35-75 and Wn 34-74 on Figure 2-1) are the rocks which have y i e l d e d d i s -cordant z i r c o n s (R. Wanless, unpublished data). Their Sr i s o t o p i c composition i s c o n s i s t e n t w i t h a l a t e P a l e o z o i c or e a r l y Mesozoic age i f they are i s o -chemically r e c o n s t i t u t e d greywacke or arkose. The z i r c o n data might then be i n t e r p r e t e d as r e f l e c t i n g a d e t r i t a l component i n gneisses derived from sediments by intense Mesozoic metamorphism. The a l t e r n a t e i n t e r p r e t a t i o n i s that o l d e r rocks have been completely r e c o n s t i t u t e d w i t h admixture of a l a r g e (>50%) amount of Mesozoic or younger g r a n i t i c m a t e r i a l . Near Amoth Lake the core gneisses appear to u n d e r l i e s c h i s t s l i k e those around P r i n c e Rupert (Hutchison, 1967). This c o n t r a d i c t i o n of Sr i s o t o p i c data and ap-parent s u p e r p o s i t i o n can only be r e s o l v e d by f u r t h e r d e t a i l e d s t u d i e s . The f i n a l s u i t e of c e n t r a l gneiss samples was c o l l e c t e d by R. R. P a r r i s h on a r i d g e south of the Skeena R i v e r (CGI, 2, and 3) i n an area under study by L. H o l l i s t e r and students from P r i n c e t o n U n i v e r s i t y . The three samples are widely s c a t t e r e d on Figure 2-5. Their o v e r a l l average 90 composition i s c l o s e to the l i n e defined by the G e o l o g i c a l Survey of Canada core gneiss samples. Two of the three f a l l w i t h i n the Hawkesbury c l u s t e r , suggesting a Mesozoic age. The t h i r d shows anomalous 8 7 S r / 8 6 S r enrichment i n a s s o c i a t i o n w i t h the presence of c a l c i t e — a g a i n arousing the s u s p i c i o n of contamination w i t h oceanic Sr. Regional geologic r e l a t i o n s and l i t h o l o g i c s i m i l a r i t i e s have l e d to the c o r r e l a t i o n of the core gneisses w i t h upper P a l e o z o i c or lower Mesozoic s t r a t a (Baer, 1968; Roddick, 1970; Woodsworth, 1978) or pre-Permian rocks (Hutchisonj 1967). Extreme metamorphism has r a i s e d a s u s p i c i o n of remobil -i z e d or t e c t o n i c a l l y incorporated Precambrian rocks (Roddick and Hutchison, 1974). The Sr i s o t o p i c r e s u l t s are most compatible w i t h the f i r s t a l t e r n a -t i v e and o f f e r no support f o r the t h i r d . There i s l o c a l evidence f o r contamination w i t h marine carbonate. The g e n e r a l l y low i n i t i a l r a t i o s i n d i c a t e a fresh-from-the-mantle v o l c a n i c - p l u t o n i c source f o r most of the core gneiss m a t e r i a l and not e r o s i o n of an ancient borderland or metamorphic r e c o n s t i t u t i o n of g r e a t l y o l d e r rocks. Culbert (1972) analysed one sample of C e n t r a l Gneiss from the Nass Ri v e r (along the northern edge of Figure 2-1 near the f i r s t 0 i n Toon). The r e s u l t l i e s near the 200 Ma isochron i n Figure 2-5 ( 8 7Sr/ 8 6Sr=0.7068 at Rb/Sr=0.026) but i t i s not p l o t t e d because i t s u n c e r t a i n t y (la=±0.0015) i s very l a r g e . IMPLICATIONS OF INITIAL 8 7 S r / 8 6 S r RATIOS The Sr isotope i n i t i a l r a t i o s of plutons provide a clue to t h e i r o r i g i n . The r a t i o s 0.7042-0.7048 are r e l a t i v e l y low (Faure and P o w e l l , 1972) but not as low as f o r plutons i n the southern Coast P l u t o n i c Complex 9 1 (^0.7037), plutons of the Intermontane B e l t (^0.7038), or Cenozolc v o l c a n i c rocks of the Coast Mountains (0.7029-0.7035) (R. L. Armstrong, unpublished data). The increase of ^ 0.0005-0.0010 can be explained by i n c o r p o r a t i o n of a considerable amount of core gneiss or a very small amount of the s c h i s t s near P r i n c e Rupert i n t o a mantle-derived magma. Low r a t i o s are not com-p a t i b l e w i t h an i n t r a c r u s t a l o r i g i n by m o b i l i z a t i o n of Precambrian or lower P a l e o z o i c c r u s t . Much of the Sr i n the Coast- Complex plutons must have come from the mantle during the Mesozoic, e i t h e r i n magmas that evolved d i r e c t l y through a s s i m i l a t i o n , c r y s t a l l i z a t i o n , and f r a c t i o n a t i o n to give the observed plutons or o r i g i n a t e d i n d i r e c t l y by ultrametamorphic m e l t i n g of s u p r a c r u s t a l rocks that were themselves the descendents of e a r l i e r Mesozoic or l a t e P a l e o z o i c mantle-derived magmas. ACKNOWLEDGMENTS We thank G. J. Woodsworth f o r help i n the f i e l d , extensive d i s c u s s i o n and c r i t i c a l comments. R. Wanless made the U-Pb datin g f a c i l i t y of the G e o l o g i c a l Survey of Canada a v a i l a b l e f o r a n a l y s i s of the Quottoon z i r c o n s G. Woodsworth, R. Wanless, and L. H o l l i s t e r reviewed the manuscript. K. L. Scott a s s i s t e d i n the l a b o r a t o r y . This research was funded by a N a t i o n a l Research C o u n c i l of Canada operating grant to R. L. Armstrong. 93 REFERENCES A l l e g r e , C. J . , Albarede, F., Grunenfelder, M., and Kbppel, V. 1974. 2 3 8 u / 2 0 6 p b _ 2 3 5 u / 2 0 7 p b _ 2 3 2 T h / / 2 0 8 p b z i r c o n geochronology i n a l p i n e and non-alpine environment. C o n t r i b u t i o n s to Mineralogy and P e t r o l o g y , 43, pp. 163-194. Baer, A. J . 1968. Model of e v o l u t i o n of the B e l l a Coola - Ocean F a l l s r e g i o n , Coast Mountains, B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences, 5_, pp. 1429-1441. 1973. B e l l a Coola - Laredo Sound map-areas, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Memoir 372. 122p. Berg, H. C., Jones, D. L., and R i c h t e r , D. H. 1972. Gravina - N u t z o t i n B e l t — t e c t o n i c s i g n i f i c a n c e of an upper Mesozoic sedimentary and v o l c a n i c sequence i n southern and southeastern Alaska. United States G e o l o g i c a l Survey, Paper 800D, pp. 1-24. Brew, D. A., Loney, R. A., and M u f f l e r , L. J . P. 1966. Tectonic h i s t o r y of southeastern Alaska. Canadian I n s t i t u t e of Mining and M e t a l l u r g y , S p e c i a l Volume 8, pp. 149-170. Churkin, M. J r . , and E b e r l e i n , G. D. 1977. Ancient borderland terranes of the North American C o r d i l l e r a . C o r r e l a t i o n and m i c r o p l a t e t e c -t o n i c s . G e o l o g i c a l Society of America B u l l e t i n , 88^, pp. 769-786. Clauer, N. 1973. U t i l i s a t i o n de l a methode rubidium — strontium pour l a d a t a t i o n de niveaux sedimentaires du Precambrien superieur de l'Adrar mauritanien (Sahara o c c i d e n t a l ) et l a mise en evidence de t r a n s f o r -mations precoces des mineraux a r g i l e u x . Geochimica et Cosmochimica Acta, 37, pp. 2243-2255. 1974. U t i l i s a t i o n de l a methode rubidium — strontium pour l a d a t a t i o n d'une s c h i s t o s i t e de sediments peu metamorphises: a p p l i -c a t i o n au Precambrien I I de l a boutonniere de Bou Azzer — E l Graara ( A n t i - A t l a s , Maroc). Earth and Planetary Science L e t t e r s , 22_, pp. 404-412. Compston, W. , and Pidgeon, R. T. 1962. Rubidium — strontium d a t i n g of shales by the t o t a l - r o c k method. J o u r n a l of Geophysical Research, 67, pp. 3493-3502. Cu l b e r t , R. R. 1972. Abnormalities i n the d i s t r i b u t i o n of K, Rb and Sr i n the Coast Mountains b a t h o l i t h , B r i t i s h Columbia. Geochimica et Cosmochimica Acta, 36^, pp. 1091-1100. Dasch, E. 1969. Strontium isotopes i n weathering p r o f i l e s , deep sea sediments and sedimentary rocks. Geochimica et Cosmochimica A c t a , 33, pp. 1521-1552. 94 Faure, G., and P o w e l l , J . L. 1972. Strontium isotope geology. Springer, New York, N. Y. 188 p. Gebauer, D., and Griinenfelder, M. 1974. Rb-Sr whole-rock dat i n g of l a t e d i a g e n e t i c to anchimetamorphic, P a l e o z o i c sediments i n southern France (Montagne N o i r e ) . C o n t r i b u t i o n s to Mineralogy and P e t r o l o g y , 47, pp. 113-130. 1976. U-Pb z i r c o n and Rb-Sr whole-rock,dating of low grade sediments. Example: Montagne Noire (southern France). C o n t r i b u t i o n s to Mineralogy and P e t r o l o g y , _59_, pp. 13-32. Grauert, B. and Wagner, M. E. 1975. Age of the g r a n u l i t e f a c i e s meta-morphism of the Wilmington Complex, Delaware — Pennsylvania Piedmont. American J o u r n a l of Science, 275, pp. 683-691. Gulson, B. L., and Krogh, T. E. 1973. Old lead components i n the young B e r g e l l M a s s i f , southeast Swiss A l p s . C o n t r i b u t i o n s to Mineralogy and Pe t r o l o g y , 40, pp. 239-252. Ha r r i s o n , T. M., and C l a r k e , G. K. C. 1979. A model of the thermal e f f e c t s of igneous i n t r u s i o n and u p l i f t as a p p l i e d to the Quottoon p l u t o n , B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences, pp. 411-420. H a r r i s o n , T. M., Armstrong, R. L., Naeser, C. W., and Harakal, J . E. 1979. Geochronology and thermal h i s t o r y of the Coast P l u t o n i c Complex, near P r i n c e Rupert, B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences, 16, pp. 400-410. Higgins, M. W., Sinha, A. K., Zartman, R. E., and K i r k , W. S. 1977. U-Pb z i r c o n dates from the Appalachian Piedmont. A p o s s i b l e case of i n h e r -i t e d r a diogenic l e a d . G e o l o g i c a l Society of America B u l l e t i n , 88, pp. 125-132. Hofmann, A. W., and Grauert, B.. 1973. E f f e c t of r e g i o n a l metamorphism on whole-rock:.Rb-Sr systems i n sediments. Carnegie I n s t i t u t e of Washing-ton, Yearbook 72, pp. 299-302. H o l l i s t e r , L. S. 1975. G r a n u l i t e f a c i e s metamorphism i n the Coast Range c r y s t a l l i n e b e l t . Canadian J o u r n a l of E a r t h Sciences, J^2, pp. 1953-1955. H o l l i s t e r , L. S., and Burruss, R. C. 1976. Phase e q u i l i b r i a i n f l u i d i n c l u s i o n s from the Khtada Lake metamorphic complex. Geochimica et Cosmochimica Acta, 4f), PP- 163-175. Hutchison, W. W. 1967. P r i n c e Rupert and Skeena map-area, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 66-33. 27 p. 1970. Metamorphic framework and p l u t o n i c s t y l e s i n the P r i n c e Rupert region of the c e n t r a l Coast Mountains, B r i t i s h Columbia. Cana-dian J o u r n a l of Earth Sciences, 1_, pp. 376-405. Kbppel, V., and Sommerauer, J . 1973. Trace elements and the behavior of the U-Pb system i n i n h e r i t e d and newly formed z i r c o n s . C o n t r i b u t i o n s to Mineralogy and Petr o l o g y , A3, pp. 71-82. Krogh, T. E. 1973. A low contamination method f o r hydrothermal decompo-s i t i o n of z i r c o n and e x t r a c t i o n of U and Pb f o r i s o t o p i c age deter-minations. Geochimica et Cosmochimica Acta, 37, pp. 485-494. Magaritz, M., and Tayl o r , H. P., J r . 1976. 1 8 0 / 1 6 0 and D/H s t u d i e s along a 500 km t r a v e r s e across the Coast Range b a t h o l i t h and i t s country rocks, c e n t r a l B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences, 13, PP- 1514-1536. Montgomery, C. W., Hurley, P. M., F a i r b a i r n , H. W., and Gaudette, H. E. 1977. E q u i l i b r a t e d domains and combined Rb-Sr and U-Pb systematics i n the h i s t o r y of a g r a n u l i t e . Massachusetts I n s t i t u t e of Technology Geochronology Laboratory, 21st Progress Report f o r 1974-1976, pp. 1-25 Peterman, Z. E., Hedge, C. E., and T o u r t e l o t , H. A. 1970. I s o t o p i c compo-s i t i o n of strontium i n sea water throughout Phanerozoic time. Geochim i c a et Cosmochimica Acta, J34_, pp. 105-120. Roddick, J . A. 1970. Douglas. Channel — Hecate S t r a i t map-area, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 70-41. 56 p. Roddick, J . A., and Hutchison, W. W. 1974. S e t t i n g of the Coast P l u t o n i c Complex, B r i t i s h Columbia. P a c i f i c Geology, j5, pp. 91-108. Symons, D. T. A. 1974. Age and t e c t o n i c i m p l i c a t i o n s of paleomagnetic r e s u l t s from plutons near P r i n c e Rupert, B r i t i s h Columbia. J o u r n a l of Geophysical Research, _79, pp. 2690-2697. 1977a. Geotectonics of Cretaceous and Eocene plutons i n B r i t i s h Columbia: a paleomagnetic f o l d t e s t . Canadian J o u r n a l of Earth Sciences, 14., pp. 1246-1262. 1977b. Paleomagnetism of Mesozoic plutons i n the westernmost Coast Complex of B r i t i s h Columbia. Canadian J o u r n a l of Earth Sciences pp. 2127-2139. V e i z e r , J . , and Compston, W. 1974. 8 7 S r / 8 6 S r composition of seawater during the Phanerozoic. Geochimica et Cosmochemica Act a , 38.> PP- 1461 1484. Wanless, R. K., Stevens, R. D., Lachance, G. R., and Edmonds, C. M. 1967. Age determinations and g e o l o g i c a l s t u d i e s : K-Ar i s o t o p i c ages, report 7. G e o l o g i c a l Survey of Canada, Paper 66-17. 120 p. 1968. Age determinations and g e o l o g i c a l s t u d i e s — K-Ar isotope ages, re p o r t 8. G e o l o g i c a l Survey of Canada, Paper 67-2. 141 p. Woodsworth, G. J . 1978. Eastern margin of the Coast P l u t o n i c Complex i n W h i t e s a i l Lake map-area, B r i t i s h Columbia. G e o l o g i c a l Survey of Canada, Paper 78-1A, pp. 71-75. York, D. 1967, The best isochron. Earth and Planetary Science L e t t e r s , pp, 479-482. LEGEND TO ACCOMPANY FIGURE 1-5 [t<\ peseta^ Eocene Q u o t t o o n P l u t o n b l > hb g r a n o d i o r i t e X X X * X P r e - E o c e n e v Agmat i t e J u r a s s l c V L > A K i t k i a t a P l u t o n hb - b i q u a r t z d i o r i t e , q u a r t z m o n z o n i t e P r e - J u r a s s i c M e t a s e d i m e n t a r y R o c k s : q t z - b i s c h i s t , a m p h i b o l i t e , q u a r t z i t e and m a r b l e G n e i s s and M i g m a t i t e : g r a n o d l o r i t i c g n e i s s , a m p h i b o l i t e , m i g m a t i t e and m i n o r c a l c - s i l i c a t e \ r ^ g e o l o g i c c o n t a c t w i t h d i p , g r a d a t i o n a l \ \ ^ f o l i a t i o n , a x i a l p l a n e ; i n c l i n e d , v e r t i c a l J ^ l i n e a t i o n , f o l d a x i s ; p l u n g i n g , v e r t i c a l m i n o r f o l d a x i a l p l a n e ; i n c l i n e d , v e r t i c a l J i/Xl m ' n o r f o l d a x i s ; p l u n g i n g , v e r t i c a l m a f i c d i k e ; i n c l i n e d , v e r t i c a l jt j 1 j o i n t ; i n c l i n e d , v e r t i c a l s h e a r ; i n c l i n e d , v e r t i c a l * g e o c h r o n o l o g y s a m p l e l o c a t i o n a nd number J 1 

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