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Geology and geochronometry of the Cogburn creek-settler creek area, northwest of Harrison lake, B.C. Gabites, Janet Elizabeth 1985

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GEOLOGY AND GEOCHRONOMETRY OF THE COGBURN CREEK-SETTLER CREEK AREA, NORTHEAST OF HARRISON LAKE, B.C. by JANET ELIZABETH GABITES B.Sc, V i c t o r i a University Of Wellington, New Zealand, 1973, B.Sc.(Honours), V i c t o r i a University Of Wellington, New Zealand, 1975 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department Of Geological Sciences We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1985 © Janet Elizabeth Gabites, 1985 in presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 n . t „ 8 October 1985 DE-6(3/81) i i Abstract Metamorphic supracrustal rocks in the Cogburn Creek area belong to the Cogburn Creek Group and the Settler Schist. These are separated by a melange zone, which has been correlated with the Shuksan thrust zone and contains Baird Metadiorite and ultramafic rocks, and intruded by the Spuzzum batholith and minor younger granodiorite. Three phases of folding are recognised in the schist units: f i is associated with contact metamorphism that preceded regional metamorphism, f 2 produced pervasive mica f o l i a t i o n and tight folds, and kinks and broad warps are associated with f 3 , which was l o c a l l y pervasive approaching pluton margins. Mineral assemblages indicate increasing metamorphic grade from west to east from garnet to garnet-staurolite, andesine-epidote amphibolite, s t a u r o l i t e - kyanite, f i b r o l i t e , and coarse s i l l i m a n i t e zones. Metamorphic conditions vary from 300 to 500 °C in Cogburn Creek Group rocks to 550 to 700 °C at 6 to 8 kbar for p e l i t e s in the Se t t l e r Schist. Conditions deduced for metamorphism of the ultramafic rocks are consistent with those for enclosing p e l i t i c s c h i s t s . Geochronometry indicates that the Baird Metadiorite i s probably Precambrian and equivalent to the Yellow Aster Complex of the North Cascade Mountains, Washington. The Cogburn Creek Group was dated as Late Paleozoic (296 ± 58 Ma, Rb-Sr WR isochron), and i s p r o v i s i o n a l l y correlated with the Bridge River Group. The p r o t o l i t h of the Settler Schist was deposited around 210 ± 27 Ma (Rb-Sr WR isochron), and i t contains 2450 ± 230 Ma d e t r i t a l zircon indicating p a r t i a l ultimate derivation from i i i Precambrian basement rocks. The Spuzzum batholith was intruded at 95 to 110 Ma, before the culmination of regional metamorphism. Rb-Sr b i o t i t e dates from a l l units and K-Ar Hb isochron dates in the range 66 to 88 Ma are metamorphic cooling dates. The youngest intrusive rocks, granodiorite dated at 32 ± 2 Ma to 42 ± 14 Ma, postdate the regional metamorphic and i n t r u s i v e event. Movement on the Shuksan Thrust is bracketed as Albian, after regional blueschist metamorphism of the Shuksan Suite in the North Cascade Mountains and before intrusion of Spuzzum batholith and regional metamorphism east of Harrison Lake. i v Table of Contents Abstract i i L i s t of figures v i i L i s t of plates xi L i s t of tables x i i Acknowledgements x i i i 1 Introduction 1 1.1 Geographic location and access 1 1.2 Previous work 1 1.3 Regional geology 4 2 Geology of the Cogburn Creek area 9 2.1 Baird Metadiorite 12 2.2 Ultramafic rocks 14 2.3 Cogburn Creek Group 15 2.4 Se t t l e r Schist 18 2.5 Premetamorphic intrusive rocks 21 2.6 Fol i a t e d d i o r i t e in melange zone 22 2.7 Spuzzum Batholith ' 23 2.8 Younger Intrusives 24 2.9 Breakenridge Formation gneiss 25 3 Metamorphism 27 3.1 Baird Metadiorite 27 3.2 Ultramafic rocks 27 3.3 Cogburn Creek Group 28 3.4 S e t t l e r Schist 31 3.5 Premetamorphic intrusive rocks 35 3.6 Discussion 36 V Ultramafic rocks 36 Contact metamorphism 36 Regional metamorphism 39 4 Structure 50 5 Geochronometry 59 5.1 Previous Geochronometry 59 5.2 Baird Metadiorite 64 5.3 Cogburn Creek Group 66 5.4 S e t t l e r Schist 67 5.5 Premetamorphic intrusive rocks 70 5.6 F o l i a t e d d i o r i t e in fault zone 72 5.7 Spuzzum batholith 72 5.8 Agmatised quartz diorite" 77 5.9 Breakenridge Formation 77 5.10 Discussion 78 6 Regional Synthesis 107 Plates 115 References 125 Appendix A. Isotopic dating methods. K-Ar, Rb-Sr, zircon U-Pb 134 Appendix B. Rb-Sr isotopic data ; 136 Appendix C. Dating sample descriptions, locations 147 Zircon descriptions 149 Appendix D. Thin section location map 150 Geochronometry sample location map 151 Appendix E. Rb-Sr data and isochron for Chilliwack batholith in the North Cascades Mountains 152 Maps • in pocket vi L i s t of Figures Figure 1.1 Area Location Map 2 Figure 1.2 Regional Geology of the North Cascade Mountains and southern Coast Mountains 5 Figure 1.3 Geographical location of the S e t t l e r Schist and Chiwaukum Schist 8 Figure 2.1 Generalised geology of the Cogburn Creek Area .. 10 Figure 3.1a Timing of mineral growth with respect to deformation, Cogburn Creek Group 29 Figure 3.1b Timing of mineral growth with respect to deformation, S e t t l e r Schist 29 Figure 3.2 Schematic T-X(C0 2) diagram for the system MgO- Si0 2-H 20-C0 2 at elevated pressures and temperatures, adapted from Johannes (1969) 37 Figure 3.3 Pressure-temperature conditions in the p e l i t i c assemblages, from Pigage (1973) 38 Figure 3.4 Metamorphic mineral assemblages in p e l i t e s and greenschist from the Cogburn Creek Group ... 40 Figure 3.5 Metamorphic mineral assemblages in p e l i t e s from the S e t t l e r Schist 41 Figure 3.6 E q u i l i b r i a calculated for P 2 at a(H 2O)=0.8l, plus the alumino-silicate e q u i l i b r i a , adapted from Bartholomew (1979) 44 Figure 3.7 Map of metamorphic mineral assemblages and isograds around Cogburn Creek 46 Figure 3.8 Regional isograds 47 Figure 4.1 Equal area projection for poles to compositional v i i layering in the Se t t l e r Schist 51 Figure 4.2 Equal area projection for poles to f 2 f o l i a t i o n for the S e t t l e r Schist south of Cogburn Creek .. 52 Figure 4.3 Equal area projection for poles to f 2 f o l i a t i o n in S e t t l e r Schist north of Cogburn Creek 54 Figure 4.4 Equal area projection for poles to f 2 f o l i a t i o n in Cogburn Creek Group 55 Figure 4.5 Equal area projection for poles to f o l i a t i o n and mineral lineations in Spuzzum batholith .... 58 Figure 5.1 Map showing geochronometry of the Harrison Lake - Fraser River region 63 Figure 5.2a Rb-Sr isochron plot for Baird Metadiorite 88 Figure 5.2b Rb-Sr isochron plot for Cogburn Creek Group .... 89 Figure 5.2c Rb-Sr isochron plot for Se t t l e r Schist 90 Figure 5.2d Se t t l e r Schist, expanded scale 91 Figure 5.2e Rb-Sr isochron plot for premetamorphic intrusive rocks 92 Figure 5.2f Rb-Sr isochron plot for small body of f o l i a t e d granodiorite in imbricate zone (SD92) 93 Figure 5.2g Rb-Sr isochron plot for Spuzzum batholith, Hut Creek body 94 Figure 5.2h Rb-Sr isochron plot for Spuzzum batholith, Settler Creek body 94 Figure 5.2i Rb-Sr isochron plot for agmatitic quartz d i o r i t e (SD14), marginal to Cogburn Granodiorite 96 Figure 5.2j Rb-Sr isochron plot for Breakenridge Formation gneiss 97 Figure 5.2k Rb-Sr isochron plot for Chilliwack Group 98 v i i i Figure 5.21 Rb-Sr isochron plot for Bridge River Group 99 Figure 5.2m Rb-Sr isochron plots for Shuksan Suite and Darrington P h y l l i t e 100 Figure 5.3 U-Pb concordia diagram for zircon dating 101 Figure 5.4a Plot of a o K / 3 6 A r v. 4 0 A r / 3 6 A r for hornblende separates from Spuzzum batholith 102 Figure 5.4b Plot of %K v. 4 0 A r nl/g for hornblende separates from Spuzzum batholith 102 Figure 5.5 U-Pb concordia diagram for Yellow Aster Complex, Skagit Gneiss and Swakane Gneiss, from Mattinson (1972) 103 Figure 5.6 Data-field diagram for Rb-Sr analyses from possible c o r r e l a t i v e s t ratigraphic units 104 Figure 5.7 Plot of %K v. *°Ar nl/g for regional data from Spuzzum batholith 105 Figure 5.8 Graph of eastward younging trend of K-Ar dates from Spuzzum batholith, from Bartholomew (1979).106 Figure 6.1 Tectono-stratigraphic terranes, modified from Monger and Berg (1984) 108 Figure 6.2 Reconstruction of Bridge River Group and Methow terrane, modified from Kleinspehn (1985) and Monger and Berg (1984) 109 Figure 6.3 Reconstructions of Settler Schist and Chiwaukum Schist, after Misch (1977) and Monger (1985) ...111 Figure 6.4 P a r t i a l reconstruction of the Methow-Tyaughton basin, modified from Kleinspehn (1985) 112 Figure D-1 Map of Cogburn Creek area showing locations of samples studied in thin section 150 ix Figure D-2 Figure E-1 Map 1 Map 2 Map 3 Map 4 Map showing locations of geochronometry samples, Cogburn Creek area 151 Rb-Sr isochron plot for Chilliwack Batholith ...153 Topographic map showing rock units -i-R—pee-ket Map showing planar structures y ^ ^ e ^ ^ A P©G-k-et Map showing li n e a r structures C a l U ^ * ^ Topographic map showing station numbers in-_paql.ei V_ in—pos-k-e-t X L i s t of Plates Frontispiece:01d Settler Mountain, looking south along S e t t l e r Creek xiv Plate 2.1 Imbricate zone: Baird Metadoirite, ultramafic rocks, Settler Schist 115 Plate 2.2 Mafic amphibolite pod in Set t l e r Schist 115 Plate 2.3 Photomicrograph of sample SD101 116 Plate 2.4 Photomicrograph of sample SD66 116 Plate 3.1 Photomicrograph of sample HL30 117 Plate 3.2 Photomicrograph of sample HL16 117 Plate 3.3 Photomicrograph of sample HL15 118 Plate 3.4 Photomicrograph of sample HL80 118 Plate 3.5 Photomicrograph of sample HL142 119 Plate 3.6 Photomicrograph of sample SS110 119 Plate 3.7 Photomicrograph of sample SSI 35 120 Plate 3.8 Photomicrograph of sample SS66 120 Plate 3.9 Twinned s t a u r o l i t e in SS57, Set t l e r Schist 121 Plate 3.10 Photomicrograph of sample SS114 121 Plate 3.11 Photomicrograph of sample SS53 122 Plate 3.12 S i l l i m a n i t e porphyroblasts from ridge north of Cogburn Creek 122 Plate 3.13 Photomicrograph of sample SSl82a 123 Plate 4.1 Fault contact at the base of the imbricate zone .124 x i L i s t of Tables Table 5.1 Table of dates and age estimates from previous studies between Harrison Lake and Fraser River . 60 Table 5.2 Summary of events in the Cogburn Creek area based on new analyses 79 Table B-1 Rb-Sr a n a l y t i c a l data 136 Table B-2 Rb-Sr isochron dates 140 Table B-3 K-Ar a n a l y t i c a l data, Spuzzum D i o r i t e 142 Table B-4 Sample weights for U-Pb analyses 143 Table B-5 U-Pb a n a l y t i c a l data including isotope ratios ...144 Table B-6 Calculated U-Pb dates 146 Table C-1 Rock descriptions and sample locations 147 Table C-2 Description of zircon samples 149 Table E-1 Rb-Sr a n a l y t i c a l data, Chilliwack batholith 152 Acknowledgements I would l i k e to thank Dr R.L. Armstrong for c r i t i c a l supervision of th i s project. Drs P.R. Bartholomew, J.S. Getsinger, L.C. Pigage, and J.W.H. Monger provided aid and discussions. K r i s t a Scott and Joe Harakal provided instruction and analyses for the geochronometry. Nel Grond and Lenore Shapka each survived a summer as f i e l d a ssistant. My housemates Cathy, Denise, Eddie, Gary, Ian, Joe and Nounou gave me support, encouragement, and assistance with babysitting. Margo McTaggart helped my daughter Charlotte through my last weeks of writing. F i e l d and laboratory expenses were funded through Dr Armstrong's National Research Council Grant 67-8841. Old S e t t l e r Mountain Looking south along Settler Creek. The front face i s a dip slope representing the upper contact of the Shuksan thrust imbricate zone. 1 1 . Introduction 1.1 Geographic Location and Access The Cogburn Creek area, which was mapped (Maps 1 to 4, in pocket) during the summers of 1981 and 1982, l i e s 20 km north of Harrison Hot Springs on the east side of Harrison Lake, B.C. (Fig. 1.1). The lake is north of the Fraser v a l l e y , 150 km east of Vancouver, in the southeastern Coast Mountains of B.C. Access i s by logging roads along the eastern shore of Harrison Lake and in the valley of Cogburn Creek; however, the ridge tops at 1800 to 1900 m are reached only by d i f f i c u l t climbs up steep, wooded h i l l s i d e s . The area mapped covers 90 km2. The lake shore is 10 m above mean sea l e v e l and the highest peak, Old Settler Mountain (2152 m), i s 8 km inland. A helicopter was used to place high camps during the 1982 f i e l d season. 1.2 Previous Work The area between Harrison Lake and the Fraser River has been studied by geologists since the 1920's. Monger (1970) compiled a l l work to 1969, including a review of the geology of the North Cascade Mountains by Misch (1966). Figure 1.1 shows areas mapped by some of the geologists mentioned below. Read (1960) studied schist similar to Settler Schist near the contact with b a t h o l i t h i c rocks on Zofka Ridge and along Stulkawhits Creek, and concluded that the metamorphic isograds were not related to the intrusions. Monger (1966) mapped the Chilliwack 2A Figure 1.1 Area Location Map Key to map areas 15. Gabites, t h i s study 14. Bartholomew 1979 13. Vining 1977 12. McLeod 1975 1 1 . Pigage 1973 10. Bremner 1973 9. Reamsbottom 1971, 1974 8. Lowes 1972 7. Richards 1971 6. Monger 1970 5. Roddick and Hutchison 1969 4. H o l l i s t e r 1969 3. McTaggart and Thompson 1967 2. Monger 1966 1 . Read 1960 2 122*00 USA 121*00 3 Group in the type area in the Chilliwack v a l l e y . McTaggart and Thompson (1967) mapped the region along the Fraser River Canyon and southward from Hope. A regional summary of the tectonic relationship between the southern Coast Ranges and North Cascade Mountains was published by McTaggart (1970). H o l l i s t e r (I969a,b) studied the relationships between metamorphic minerals in schists near Kwoiek Creek that are considered to be equivalent to S e t t l e r Schist. Roddick and Hutchison (1969) covered the northwestern part of the Hope map-area, including the Big S i l v e r River area north of Cogburn Creek. Richards (1971) mapped the Chilliwack batholith and Spuzzum batholith north and south of the Fraser River; Vining (1977) mapped the Spuzzum batholith further north in the v i c i n i t y of the Giant Mascot Nickel Mine. Lowes (1972) mapped the area from Harrison Lake east and south to the Fraser River; the ridges north of Cogburn Creek were his northern boundary. His northern edge overlapped the area around Big S i l v e r River mapped by Reamsbottom (1971 and 1974). Bremner (1973) studied metamorphism in a section of the Fraser Canyon north of Yale. Pigage (1973) mapped the Gordon Creek area south of Yale, with emphasis on metamorphism of the Settler Schist. Bartholomew (1975) extended the work of Pigage in the Yale Creek area, between Gordon Creek and Cogburn Creek. McLeod (1975) studied the geology of the Giant Mascot Nickel Mine in Stulkawits Creek. Previous geochronometry i s reviewed in Chapter 5. 4 1.3 Regional Geology The study area l i e s in a zone of complex geology at the south end of the Coast Plutonic Complex and the north end of the Cascade Mountains. Many of the structures are continuous with those of the North Cascade Mountains. Geology of the region has been summarised in Figure 1.2, after Monger (1970), Pigage (1973), Bartholomew (1979), and Haugerud (1979). The Coast Plutonic Complex consists of composite plutons of dominantly quartz d i o r i t e and granodiorite composition, mainly Cretaceous in age, in which l e n t i c u l a r bodies of metamorphosed sedimentary and volcanic rocks form pendants. Supracrustal rocks in the study area have been divided into two l i t h o l o g i c a l l y and s t r u c t u r a l l y d i s t i n c t units, the S e t t l e r Schist (Lowes 1972) and the Cogburn Creek Group (th i s study). The rocks of the Cogburn Creek Group were previously mapped as part of the Pennsylvanian - Permian Chilliwack Group of the North Cascades (Monger 1970, Lowes 1972), but as t h i s c o r r e l a t i o n i s uncertain a new name has been applied i n the Cogburn Creek area. Reamsbottom (1971 and 1974) included rocks of both Cogburn Creek Group and Settler Schist in his Cairn Needle Formation. Settler Schist was named and described by Lowes (1972) and dated as early Mesozoic (Bartholomew 1979). The two stratigraphic units are separated by a steeply dipping belt of imbricated tectonic s l i c e s of ultramafic and d i o r i t i c rocks that has been interpreted as a northern extension of the root zone of the Shuksan thrust of the North Cascades (Lowes 1972). Some of the units in t h i s zone resemble Yellow 5A Figure 1.2 Regional Geology of the North Cascade Mountains and South Coast Mountains Quaternary Miocene and 01igocene 23 22 21 Eocene Early T e r t i a r y 20 -Late Cretaceous Key for Regional Geology Map D r i f t and alluvium Granodiorite, quartz d i o r i t e ; Chilliwack Batholith, Cogburn Granodiorite, Needle Peak Pluton, S i l v e r Peak Pluton, Conglomerate D i o r i t e , granodiorite; Yale Intrusions Cretaceous 19 D i o r i t e ; Spuzzum Pluton and o u t l i e r s , Scuzzy Pluton 18 Gneiss; Custer, Breakenridge Formation 17 Jackass Mountain Group 16 Brokenback H i l l Formation 1 5 Peninsula Formation Late Jurassic 1 4 Granodiorite; -Cretaceous Upper Jurassic 13 Agassiz P r a i r i e Formation 1 2 Kent Formation Mid Jurassic 1 1 Billhook Creek Formation 10 Mysterious Creek Formation 9 Echo Island Formation 8 Harrison Lake Formation U-M-L Jurassic 7 Ladner Group Tr i a s s i c - J r 6 Cultus Formation Pm-Tr(?)-Jr 5 Hozameen Group T r i a s s i c or older 4 Settler Schist Tr or older 3 Cogburn Creek Group Penn.-Permian 2 Chilliwack Group Precambrian - 1 Yellow Aster Complex, Baird Metadiorite Early Paleozoic Age unknown A ultramafic rocks thrust f a u l t *"V steep f a u l t , downthrown side 5 3 0 ' 4 9 ° 1 5 ' 1 2 2 a 0 0 ' 4 5 ' 3 0 ' 1 2 1 ° 1 5 ' 6 Aster Complex (Misch 1966), which i s p a r t i a l l y of Precambrian age (Mattinson 1972). D i o r i t i c and granodioritic rocks of Cretaceous age have intruded the schists and gneisses. These are syn- to post- tectonic with respect to the regional metamorphism and deformation but predate movement on the Fraser Fault zone. Discordant mid- to l a t e - T e r t i a r y granodiorite bodies are the youngest i n t r u s i v e s . The youngest stratigraphic units are Eocene conglomerates that have been deformed along the Fraser Fault zone (McTaggart and Thompson 1967), and Pleistocene g l a c i a l deposits. McTaggart (1970) considered that the belt of c r y s t a l l i n e gneisses north of the International Border was the deeply eroded a x i a l zone of a Cretaceous orogen. The metamorphosed and deformed sediments of the North Cascade Mountains represented a shallower l e v e l . A l e n t i c u l a r body of gneiss (Custer Gneiss) bounded by f a u l t s of the Fraser Fault zone was interpreted as the core of t h i s orogen. The Custer Gneiss was f i r s t described by McTaggart and Thompson (1967) as migmatitic gneiss with small areas of s c h i s t . They correlated i t with gneisses mapped in the North Cascade Mountains by Misch (1966). Age estimates range from Early Paleozoic to Cretaceous (McTaggart 1970). McTaggart considers that the Custer Gneiss may be related to Spuzzum batholith. Reamsbottom (1971) correlated two domes of gr a n o d i o r i t i c gneiss in the Mount Breakenridge area with Custer Gneiss. Movement on the Fraser Fault zone was once thought to have 7 started in the Mesozoic and continued u n t i l Eocene times (Monger 1970). New data now indicates that most of the movement i s Terti a r y , mostly Eocene (Monger 1985). The fault zone i s sealed by the mid-Tertiary Chilliwack batholith (Appendix E) and related plutons. Major movement on the Straight Creek f a u l t in Washington ended by 33 Ma (Miller and Vance 1981). Correlation of the Settler Schist with the Chiwaukum Schist of the Skagit Metamorphic Suite in the Stevens Pass area led Misch (1977) to postulate 150 to 200 km right l a t e r a l offset on the Fraser- Straight Creek fault system (Figure 1.3). Displacement of the Hozameen and Bridge River Groups and Bridge River Metamorphic zone (Monger 1978, 1985) and Tyaughton-Methow Basins (Kleinsphen 1985) support that inference and may bracket the fault movement to within the Eocene. Kleinspehn (1985) restores the Tyaughton- Methow basin to a reasonable depositional environment by reversing 110 km of dextral motion on the Fraser - Straight Creek Fault. This was preceded by 150 km of Cenomanian to Turonian motion on the Yalakom - Ross Lake Fault. A more detailed discussion of fault movements i s in Chapter 6, with appropriate figures. 8 Figure 1.3 Geographical location of the S e t t l e r Schist and Chiwaukum Schist, modified from Evans and Berti (1985). Individual plutons intruding the two s c h i s t units are named. This map i s used as a base for Figure 6.2. 9 2. Geology of the Cogburn Creek area Rocks in the Cogburn Creek area are divided into f i v e main units, with areal d i s t r i b u t i o n as shown in Figure 2.1. The most extensive units are two l i t h o l o g i c a l l y and s t r u c t u r a l l y d i s t i n c t s c h i s t s , the Cogburn Creek Group and the S e t t l e r Schist, and the Spuzzum batholith that intrudes them. The main str u c t u r a l feature of the area i s a sinuous zone of almost v e r t i c a l tectonic s l i c e s of metadiorite and ultramafic rocks (Plate 2.1) that separates the Cogburn Creek Group and Settler Schist. This zone s t r i k e s east-west in the south part of the area around Old Se t t l e r Mountain, and turns to s t r i k e north-northwest in the northwestern part of the area, with a deflection around the Spuzzum bathol i t h , Hut -Creek body. Lowes ( 1972) correlated t h i s zone with the Shuksan thrust zone (Misch 1966) on the basis of structural p o s i t i o n , l i t h o l o g i c a l s i m i l a r i t i e s of rocks in the zone, and general continuity of the belt along s t r i k e with similar rocks and structures to the south. Both sc h i s t s have undergone at least three phases of deformation, mostly related to the Cretaceous episode of metamorphism and d i o r i t e intrusion. The dominant structures in the S e t t l e r Schist were formed by the second recognisable deformation (Pigage 1973, Bartholomew 1979). Although Pigage found graded bedding in the Settler Schist in the Gordon Creek area, no convincing evidence of bedding was found around Cogburn Creek. In the Cogburn Creek Group, metamorphosed cherts exhibit what appears to be o r i g i n a l bedding rather than metamorphic transposition. Schistosity i s subvertical over most of the 10A Figure 2.1 Generalised geology of the Cogburn Creek Area Key to map units. Q Pleistocene g l a c i a l debris 6 Granodiorite (Cogburn) 4-6 Agmatised d i o r i t e , mixture of Settler Schist and Cogburn Granodiorite 5 D i o r i t e (Spuzzum) 5a Fol i a t e d d i o r i t e in melange zone 5b Hornblende-hypersthene gabbro 4 Settler Schist Cogburn Creek Group 3a -grey p e l i t e 3b -greenschist 3c -metachert 3d -marble 2 Ultramafic rocks 1 Baird Metadiorite x ^ Shuksan thrust Cogburn Fault 49° 35 49° 31' 121° 45' 121° 40' 121° 35' 11 study area, but has variable s t r i k e . Regional metamorphic grade changes across the area from greenschist grade in the west (on the shore of Harrison Lake), through garnet-staurolite, kyanite, f i b r o l i t e ± kyanite, to s i l l i m a n i t e ± f i b r o l i t e in the northeastern corner. The increase i s consistent with proximity to the large mass of d i o r i t e that comprises the Spuzzum batholith. Within a kilometre of the two d i o r i t e bodies in the f i e l d area an e a r l i e r contact metamorphism i s indicated in the Settler Schist by r e l i c t c h i a s t o l i t i c andalusites. The latest event is post-metamorphic intrusion of small granodiorite plutons. An example i s the Cogburn Granodiorite stock (Bartholomew 1979) which occurs east of the forks of Cogburn Creek. Age relations among the non-intrusive rocks are not clear from l i t h o l o g i c , s t r a t i g r a p h i c , or structural evidence. Both schists are bounded by f a u l t s and intrusive contacts, and the metamorphic grade and degree of deformation are high enough to make i t unlikely that f o s s i l s would survive. The rock units are described below in order of decreasing age, as deduced from isotopic studies. 12 2.1 Unit 1. Baird Metadiorite The name Baird Metadiorite i s applied in t h i s study to a unit of variably metamorphosed mafic igneous rocks. Occurrence i s as lenses and tabular masses imbricated with ultramafic rocks, along the zone marking the northern extension of the Shuksan Thrust (Lowes 1972). The largest body forms the ridge which extends northwest from Old S e t t l e r Mountain to Cogburn Creek, and separates Talc and Settler Creeks (Figure 2.1). The mafic rocks are variable in composition from gabbro to quartz d i o r i t e , and have been metamorphosed to amphibolite f a c i e s . They are grey-green in colour, hard, and form massive blocky outcrops and steep c l i f f s with sparse vegetation. Texturally the rocks show weak to moderate f o l i a t i o n due to alignment of mafic minerals. Strain during metamorphism and deformation has been taken up along fine-grained, c h l o r i t i c shear zones through the otherwise equigranular rock. Compositionally the mafic rocks consist mainly of hornblende, plagioclase, quartz, c l i n o z o i s i t e , c h l o r i t e , and opaques, in varying proportions. Rutile i s a common accessory. The hornblende occurs both as porphyroblastic, probably r e l i c t , large c r y s t a l s and as small intergranular prisms. In the samples studied in thin section the plagioclase has been almost e n t i r e l y converted to epidote, a l b i t e , s e r i c i t e and c a l c i t e . R e l i c t plagioclase compositions, measured by a-normal methods, were similar to those reported by Lowes (1972), and are in the range An35 to An42. Chlorite occurs mainly in fine-grained shear zones. Minor r e l i c t pyroxene now overgrown by hornblende 13 (sample MD1) may be part of the o r i g i n a l mineral assemblage. Dynamothermal metamorphism to andesine - epidote amphibolite subfacies during the Cretaceous has reconstituted e a r l i e r mineral assemblages in the rocks, although there i s only minor internal deformation. Lowes (1972) reported limited evidence here for a previous amphibolite facies metamorphism observed by Misch (1966) in the North Cascade Mountains. Lowes (1972) correlated the rocks here c a l l e d Baird Metadiorite with the Yellow Aster Complex (Misch 1966) in the North Cascade Mountains, on the basis of st r u c t u r a l position with respect to the Shuksan Thrust. The Yellow Aster Complex occurs as tectonic s l i c e s with sheared ultramafic rocks in melange below the sole of the Shuksan thrust; in places i t i s imbricated with the underlying Chilliwack Group. Misch (1966) considered the Yellow Aster Complex to be "basement", and mapped i t in the type area as schistose and d i r e c t i o n l e s s metamorphic hornblendites and amphibolites, gneissose and d i r e c t i o n l e s s metagabbros, metadiorites, meta-quartz d i o r i t e s and metatrondhjemites, and migmatites of varied composition. He speculated that the schistose hornblendites and amphibolites are remnants of primary oceanic crust that was later converted to predominantly hornblende quartz d i o r i t i c continental crust. Zircons dated by Mattinson (1972) showed two age groups in the Yellow Aster Complex. Pyroxene gneiss was dated at 1400 to 1600 Ma and quartz d i o r i t e gneiss and pegmatite gneiss at 410 ± 85 Ma. Rock types in the Baird Metadiorite are similar to those comprising the Yellow Aster Complex. Rb-Sr and zircon U-Pb dating (Chapter 5) on the Baird Metadiorite suggest an old age similar to that measured by Mattinson on Yellow Aster pyroxene gneiss. Thus new isotopic evidence supports the correlation made by Misch (pers. comm. to Lowes 1972) based on l i t h o l o g i e s and other f i e l d aspects of the rocks. 2.2 Unit 2. Ultramafic Rocks Ultramafic rocks in the study area mainly occur as pods and elongate bodies of pe r i d o t i t e imbricated with the Baird Metadiorite along the root zone of the Shuksan thrust. The scale of imbrication varies from metres to kilometres. Some small bodies of pyroxenite and hornblendite are associated with the Spuzzum batholith, and occur within both the d i o r i t e and the surrounding schists. Outcrops of ultramafic rock are almost devoid of vegetation and weather orange, so they are recognisable from a distance. The largest body of ultramafic rock (mapped by Lowes 1972, outside the present study area) i s faulted into the Cogburn Creek Group along the west side of Talc Creek, giving the drainage i t s name. The peridotite composition i s variable, from serpentinite to unaltered dunite. The most common i s serpentinite, but metamorphic reconstitution of the o r i g i n a l dunite, harzburgite and pyroxenite i s quite variable within the rocks. Some of the serpentinite has been r e c r y s t a l l i s e d by metamorphism back to o l i v i n e and t a l c . 1 5 2.3 Unit 3. Cogburn Creek Group P e l i t i c schist, chert and metavolcanic rocks cropping out along the eastern shore of Harrison Lake and west of the Shuksan thrust zone have been given the informal name Cogburn Creek Group in thi s study. Because of their s i m i l a r i t y in str u c t u r a l position beneath the Shuksan thrust, Lowes (1972) had previously assigned them to the Chilliwack Group of Monger (1966); however, because of l i t h o l o g i c a l d i s s i m i l a r i t y and lack of f o s s i l s I have chosen to designate them as the Cogburn Creek Group. Crickmay (1930) used the name Slollicum Group for metavolcanic rocks and low grade p e l i t e s around Slollicum Peak that may be part of the same unit. Reamsbottom (1971 and 1974) mapped rocks in the same structural position in the Big Sil v e r River area to the north of Cogburn Creek as part of the Cairn Needle Formation, along with what i s herein c a l l e d Settler Schist. Monger (pers. comm. 1985) has followed the unit north at least as far as between the two gneiss domes in the Mount Breakenridge area. The stratigraphic top and bottom of the Cogburn Creek Group are not seen. The unit i s truncated on the northeast against Baird Metadiorite and ultramafic rocks in the melange zone below the Shuksan thrust. Since the rocks dip to the east, this i s the structural top. The southwest contact has not been studied; Lowes (1972) did not recognise any subdivision of his Chilliwack Group. Monger (pers. comm. 1985) has found Chilliwack Group north of the Fraser River, but considers that most of the rock along the eastern side of Harrison Lake does not belong to the Chilliwack Group. 16 Three main rock types of the Cogburn Creek Group have been mapped. They are, from west to east: grey p e l i t e , c h l o r i t e - a c t i n o l i t e greenschist, metamorphosed ribbon chert. A l l have been metamorphosed up to at least garnet grade during Cretaceous regional metamorphism. Grade increases eastward across s t r i k e . Rock Types 1) Grey P e l i t e The westernmost part of the Cogburn Creek Group at Cogburn Creek consists of l i g h t to dark grey p e l i t e . These rocks are very fine grained, micaceous, and may contain porphyroblasts, usually embayed garnet or skeletal ilmenite. The colour of the rock depends on graphite content, which may be up to 50% as in a black shale with magnetite porphyroblasts. Quartz and plagioclase are the main constituents. Plagioclase composition varies from oligoclase to andesine. B i o t i t e or muscovite are common primary metamorphic minerals, with l a t e r a l t e r a t i o n to c h l o r i t e . Minor a l t e r a t i o n and accessory minerals include c a l c i t e , orthoclase, epidote, ilmenite and tourmaline. 2) C h l o r i t e - a c t i n o l i t e greenschist Greenschist occurs both as layers in the ribbon chert and as a massive unit which contains bands of chert. Present mineralogy i s quartz, plagioclase (An8 to An28), green hornblende or a c t i n o l i t e , c h l o r i t e , c a l c i t e , epidote, sphene, and accessory mafics. Chlorite replaces hornblende in most of the thin sections studied. C a l c i t e i s common in parts of the unit. Grain size i s generally f i n e , although coarser, knobbly 17 greenschists were found. Mineralogy and association with ribbon cherts suggest that the greenschists were o r i g i n a l l y submarine volcanic rocks. Lowes (1972) found evidence of pillows south of Cogburn Creek. Calcareous rocks were probably marls interlayered with the volcanic rocks. 3) Metamorphosed ribbon chert A rock containing bands of f i n e l y to coarsely c r y s t a l l i n e quartz separated by fine grained b i o t i t e - r i c h layers has been interpreted as metamorphosed d i r t y ribbon chert. Opaques and accessory tourmaline and zircon are present in the b i o t i t e layers, and plagioclase i s found with quartz. The bands have been contorted into numerous minor folds. Chert on the ridge west of Old S e t t l e r Mountain i s interlayered and t i g h t l y folded with a 20 m thick band of marble. The marble i s coarsely c r y s t a l l i n e , and i s dark grey with white patches. The unit of ribbon chert contains layers of greenschist that become thinner and less numerous eastwards, away from the c h l o r i t e - a c t i n o l i t e greenschist. The association suggests a deep marine environment, mostly below the carbonate compensation depth and i s o l a t e d from c l a s t i c input. 18 2.4 Unit 4. Settler Schist The S e t t l e r Schist crops out east of the imbricate zone correlated with the Shuksan Thrust. The unit i s dominated by metasedimentary p e l i t i c s c h i s t s , with minor amphibolite and quartzite. In the Gordon Creek area Pigage (1973) mapped four d i s t i n c t rock types within the p e l i t i c s c h i st: layered p e l t i c schist, graphitic p e l i t i c s c h i s t , quartzofeldspathic schist, and micaceous quartzite. A similar subdivision was attempted in this study, but although d i f f e r e n t l i t h o l o g i e s could be i d e n t i f i e d they did not form mappable units. Two main episodes of metamorphism have r e c r y s t a l l i s e d the Settler Schist. Contact metamorphism by the Spuzzum batholith produced andalusite in rocks of suitable composition within a kilometre of the contact. Later regional (amphibolite facies) metamorphism caused an increase in grade up to s i l l i m a n i t e zone in the northeast corner of the study area. The grade increases from south to north across the Settler Schist in the study area, with garnet-staurolite grade rocks in the south and east along the Shuksan thrust, through kyanite grade to f i b r o l i t e ± kyanite to s i l l i m a n i t e ± f i b r o l i t e (Bartholomew 1979). This increase correlates with proximity to the main body of the Spuzzum batholith, although intrusion was before the peak of metamorphism (Bartholomew 1979). A general increase in grain size and the appearance of gneissic banding and thin leucocratic veinlets accompany the increase in grade. Chemical compositions of staurolite-bearing rocks are t y p i c a l l y r e s t r i c t e d to low Mg:Fe r a t i o s , high Al content, low 19 a l k a l i and Ca content (Hoschek 1967). Mineral assemblages in the S e t t l e r Schist suggest a stratigraphic sequence of dominantly Fe- and A l - r i c h shales with lesser amounts of volcanic and calcareous rocks (Pigage 1973). Rock Types 1) The p e l i t i c part of the Settler Schist i s fine-grained, and colour varies from black to medium grey depending on graphite content. The black a r g i l l i t e i s massive with poor cleavage, while more micaceous p e l i t e has good cleavage and variable metamorphic compositional layering. Some slate on the northern ridge and near d i o r i t e bodies has a spotted appearance due to porphyroblasts. The dominant mineral assemblage i s quartz ± plagioclase, b i o t i t e , graphite, garnet, s t a u r o l i t e , with minor and accessory muscovite, andalusite, kyanite, s i l l i m a n i t e , f i b r o l i t e , c h l o r i t e , magnetite, r u t i l e , epidote, tourmaline, or zircon ( d e t r i t a l ) . Garnet, s t a u r o l i t e , andalusite, kyanite and s i l l i m a n i t e occur as porphyroblasts in rocks with appropriate bulk compositions and metamorphic grade. Andalusite i s pseudomorphed by quartz and muscovite with kyanite or s i l l i m a n i t e . Both kyanite and s i l l i m a n i t e (and f i b r o l i t e ) can be found with garnet and s t a u r o l i t e . Tourmaline i s a common accessory. In rocks where i t i s present the b i o t i t e i s a d i s t i n c t i v e reddish brown. 2) The quartzofeldspathic rocks d i f f e r from the p e l i t e s mainly in grain size and graphite content. They are coarser than the p e l i t e s , and contain a lower percentage of graphite. Colour i s medium to l i g h t bluish grey. Mineralogy i s similar to the 20 p e l i t i c schists, but in d i f f e r e n t proportions. 3) The micaceous quartzite has a sandy appearance and is pink in colour from b i o t i t e . Mineralogy is quartz, plagioclase (An25 to An40), b i o t i t e , muscovite, with varying amounts of graphite, magnetite, garnet, s t a u r o l i t e , alumino-silicates, c h l o r i t e , tourmaline, epidote, r u t i l e , zircon and apatite. Two samples, SS66 and SS125, also contain pale green amphibole along the margins of quartz veins, indicating c a l c - s i 1 i c a t e assemblages. In both these rocks the groundmass around the euhedral hornblendes i s mylonitised. Large r e l i c t p o i k i l i t i c plagioclase phenocrysts were found in two samples, SS134 and SS143. 4) Amphibolites are a minor constituent in the S e t t l e r Schist, and have been divided into two types as in Pigage (1973) and Bartholomew (1979). Speckled amphibolites were found interlayered with other rock types. They contain plagioclase, blue-green hornblende, quartz, and b i o t i t e , with accessory magnetite, c h l o r i t e , c a l c i t e , r u t i l e , c l i n o z o i s i t e and apatite. The o r i g i n a l rock was probably marl or calcareous mudstone. The second type of amphibolite i s found in small pods and lenses within the s c h i s t . These pods contain large c r y s t a l s of dark green hornblende, and commonly have a very mafic rim around a l i g h t e r core (Plate 2.2). They are considered to have been mafic igneous rocks, perhaps dykes that were boudinaged during metamorphism and deformation. Age relations of the Settler Schist within the regional stratigraphy have not been deciphered as the unit i s bounded on a l l sides by faul t s and intrusive contacts. E a r l i e r workers 21 have based correlations on l i t h o l o g i c a l and chemical s i m i l a r i t i e s . The Settler Schist has been correlated previously with the Chiwaukum Schist of the Skagit Suite in the North Cascade Mountains (Lowes 1972). Lithologies and metamorphic history are remarkably s i m i l a r , and the two units can be brought together by removing 150 to 200 km of right l a t e r a l o f f s e t along the Straight Creek - Fraser fault system. Geochronometry (Chapter 5) indicates an old, possibly Precambrian provenance for at least part of the unit, which is similar to results of Mattinson (1972) for the Skagit Metamorphic Suite in Washington. Thus the l i t h o l o g i c a l c o r r e l a t i o n i s strengthened by isotopic evidence. The Rb-Sr whole rock isochron date of 210 ± 27 Ma (Bartholomew, 1979, and t h i s study) indicates that the time of deposition of the the p r o t o l i t h of the Settler Schist was probably early Mesozoic. 2.5 Premetamorphic intrusive rocks Some f o l i a t e d rocks are interpreted as pre-metamorphic intrusive rocks, for example, a large f o l i a t e d f e l s i c dyke on the ridge north of Cogburn Creek. HL 111 is a l i g h t grey f e l s i t e , which i s flecked with black b i o t i t e , and i s f o l i a t e d p a r a l l e l to surrounding chert. Mineralogy is plagioclase (oligoclase), quartz, muscovite, s i l l i m a n i t e , b i o t i t e , garnet, with minor zircon. Igneous textures are preserved in patches of large myrmekitic plagioclase grains, but most of the rock i s r e c r y s t a l l i s e d . Muscovite and s i l l i m a n i t e are aligned along the- f o l i a t i o n . 22 Garnet-cluster Dykes Metamorphosed f e l s i c dykes and s i l l s are common in the Settler Schist. The most common type contains d i s t i n c t i v e knobbly clu s t e r s of pink garnets up to 0.5 cm across with white depletion haloes. Pigage (1973) named them garnet-cluster dykes. The dated sample SS85 f a l l s in this category. The rock is l i g h t pinkish grey, fine grained quartz-biotite schist with garnet c l u s t e r s . Mineralogy i s quartz, plagioclase (andesine), b i o t i t e or muscovite, garnet and opaques, with minor apatite, zircon, and/or diopside. R e l i c t zoned plagioclase porphyroblasts indicate an igneous o r i g i n . Depletion haloes around the garnets contain only quartz and plagioclase. These are not necessarily p o s t - c r y s t a l l i n e reaction textures but may be a result of o r i g i n a l metamorphic growth of garnet. The growing garnet uses the mafic constituents within the surrounding area, leaving f e l s i c components behind. Two cleavages are present in the dykes, as in the surrounding sc h i s t , and are marked by alignment of b i o t i t e . The f 2 f o l i a t i o n i s deflected around both plagioclase porphyroblasts and garnet c l u s t e r s , indicating a pre-deformation timing for the halo reaction. 2.6 Foliated d i o r i t e in melange zone SD 92 was co l l e c t e d from a plug of f o l i a t e d d i o r i t e that l i e s within the thrust fault zone, on the western side of Settler Creek. The composition of t h i s body i s b i o t i t e , quartz, plagioclase, garnet, and epidote, with minor r u t i l e , and white 23 mica. Chlorite i s a late a l t e r a t i o n of b i o t i t e . Plagioclase can be separated into two generations based on texture and composition: 1) p o i k i l i t i c igneous andesine, An30, and 2) well- twinned metamorphic andesine, An35. Geochronometry suggests that i t i s part of the Spuzzum batholith; however, the presence of metamorphic plagioclase and f o l i a t i o n means that the body must be older than the Spuzzum batholith, or one of the e a r l i e s t phases. 2.7 Unit 5. Spuzzum Batholith Intrusive rocks belonging to the Spuzzum batholith form two discrete bodies c a l l e d S e t t l e r Creek body and Hut Creek body. The Hut Creek body i s near to but not continuous with the body of d i o r i t e to the north and east of Cogburn Creek. The Settler Creek body i s Unit 3d of Bartholomew (1979). Several small bodies of similar composition have been mapped as part of the same intrusive suite. Both large bodies intrude the S e t t l e r Schist and not the Cogburn Creek Group. Crosscutting relationships or otherwise with the thrust fault zone have not been determined, as c r i t i c a l areas are covered with g l a c i a l debris and/or thick vegetation. The fault zone appears to have been distorted during intrusion, implying that i t i s older than the Spuzzum batholith. Because the d i o r i t e i s more resistant to erosion than the supracrustal rocks, i t forms high ridges with steep c l i f f s into deeply incised v a l l e y s . The S e t t l e r Creek body forms part of the ridge north of Old Se t t l e r Mountain. The Hut Creek body 24 spans the Cogburn Creek va l l e y , up to 1000 m on either side. Hut Creek bisects a tongue of d i o r i t e that continues north into the area mapped by Reamsbottom (1971 and 1974). Most of the intrusive rocks are hornblende-quartz-diorite. Accessory b i o t i t e and garnet occur l o c a l l y either together or separately. Hornblende is generally brown, or has brown cores with green metamorphic overgrowths (Plate 2.3). Gabbro has been mapped west of the Cogburn Creek forks. The composition i s hypersthene - hornblende or clinopyroxene.with minor plagioclase and mafics, forming an equigranular rock that i s extremely hard, resulting in the d i s t i n c t i v e l y steep, blocky c l i f f s described above. Plate 2.4 shows replacement textures in hornblende in the gabbro. Fine grained opaque inclusions along cleavage planes are common in the r e l i c t hornblende cores. 2.8 Unit 6. Younger Intrusive Bodies 1) Cogburn Granodiorite i s a medium grained, leucocratic granodiorite containing b i o t i t e , plagioclase (zoned from An35 to An60), quartz, with minor hornblende. The boundaries of the pluton crosscut both Settler Schist f o l i a t i o n and margins of the Spuzzum batholith (Bartholomew 1979). 2) SD 14 comes from a small area of agmatised d i o r i t e in the south fork of Cogburn Creek. The composition suggests assimilation of a graphitic member of the Settler Schist that i t intruded. The rock can be divided into two types by difference in grain size and texture. The coarser part i s medium grey d i o r i t e , speckled by b i o t i t e and garnets; the finer i s darker 25 grey and f o l i a t e d , and looks l i k e S e t t l e r Schist. The grain size t r a n s i t i o n i s sharp but the major mineralogy does not change. It i s quartz, plagioclase (oligoclase), b i o t i t e , garnet, with minor magnetite, apatite and zircon. The finer part also contains minor graphite and green hornblende. The geochronometry indicates that t h i s rock is younger than Cretaceous, (see Chapter 5), and i s one of the younger intrusives rather than Spuzzum batholith. 3) A small body of granodiorite intrudes the Cogburn Creek Group southwest of Old Settler Mountain. The rock i s l i g h t grey, fine grained biotite-hornblende granodiorite. Mineralogy i s quartz, plagioclase, b i o t i t e , and hornblende. Igneous textures are preserved, and there is no f o l i a t i o n . 4) Basaltic dykes crosscut a l l units older than and including Spuzzum batholith. They are fine grained, grey to greenish grey, and undeformed. Mineralogy is quartz, plagioclase, hornblende, and opaques, with minor late c h l o r i t e a l t e r a t i o n . 2.9 Breakenridge Formation Gneiss Grey gneiss from the Breakenridge Formation mapped by Reamsbottom (1971,74) in the Big S i l v e r River valley was sampled for geochronometry, so a description i s included here. Roddick and Hutchison (1967) and Reamsbottom considered the grey gneiss coring the a n t i c l i n a l dome centred on Mount Breakenridge to be the oldest rock in the region. Dating car r i e d out in t h i s study yiel d s c o n f l i c t i n g results (Chapter 5). The following description i s from Reamsbottom (1974). The Breakenridge Formation i s composed mainly of homogeneous grey gneiss and amphibolite. Migmatitic, banded to i r r e g u l a r l y banded gneiss, p e l i t i c schist and skarn form heterogeneous zones between the grey gneisses and amphibolites. The grey.gneisses are medium-grained with an allotriomorphic granular texture. They consist of b i o t i t e , quartz and plagioclase (AnlO to An34) l o c a l l y with muscovite, garnet or microcline. Myrmekite commonly develops between grains of microcline and plagioclase. 27 3. Metamorphism Schists in the Cogburn Creek area have been subjected to contact metamorphism from the Spuzzum batholith followed by regional metamorphism during the Cretaceous. Lowes (1972) recognised an e a r l i e r metamorphism in the Precambrian or Paleozoic Baird Metadiorite. 3.1 Unit 1. Baird Metadiorite The present mineral assemblage of hornblende, plagioclase, quartz, c l i n o z o i s i t e , c h l o r i t e and opaques represents metamorphism of mafic intrusive rocks to andesine-epidote amphibolite subfacies. Hornblende i s pale green and a c t i n o l i t i c , and occurs as large ragged porphyroblasts. These appear to have been l e f t e s s e n t i a l l y undeformed while surrounding f e l s i c minerals were fragmented. Rare brown cores may be of primary igneous o r i g i n . In sample MD1 rare r e l i c t augite cores may be of primary igneous o r i g i n . They are now rimmed by pale green hornblende. Plagioclase compositions in cores of large porphyroblasts in MD1 reach An42, while rims and smaller grains have r e c r y s t a l l i s e d at around An35. Epidote and a l b i t e have replaced the plagioclase. 3.2 Unit 2. Ultramafic Rocks Both the o r i g i n a l composition of the ultramafic rocks and their degree of metamorphic reconstitution i s variable. Several samples of dunite were found, for example HL30, containing o l i v i n e and chromite with l a t e r a l t e r a t i o n to c a l c i t e and 28 tremolite (Plate 3.1). HL33 consists almost e n t i r e l y of t a l c ; HL31 and HL99 contain tremolite with minor late serpentine a l t e r a t i o n . The other rocks a l l contain varying proportions of o l i v i n e , chromite or magnetite, enstatite or clinopyroxene, c a l c i t e or magnesite, with later a l t e r a t i o n to t a l c , serpentine and c h l o r i t e . In HL54, consisting of c a l c i t e , serpentine, opaques and late c h l o r i t e in veins, some of the c a l c i t e exhibits deformed twin lamellae. This suggests that the rock was strained after i t r e c r y s t a l l i s e d . Its location, in the western tributary to Settler Creek (Appendix D), is very close to a v i s i b l e fault plane separating Settler Schist from ultramafic rock and Baird Metadiorite. Thus there may have been some continued movement on t h i s fault after metamorphism. 3.3 Unit 3. Cogburn Creek Group Lack of suitable l i t h o l o g i e s in the Cogburn Creek Group makes recognition of the metamorphic grade d i f f i c u l t . Mineral textures record three phases of deformation and folding, f,, f 2 and f 3 , with decreasing age. The timing of mineral growth with respect to these i s shown in Figure 3.1a. 1) In the grey p e l i t e , b i o t i t e ± muscovite belong to the equilibrium assemblage. Mica f o l i a t i o n developed during f 2 , and individual flakes have r e c r y s t a l l i s e d within t h i s f o l i a t i o n to p a r a l l e l f 3 . In sample HL16 (Plate 3.2) b i o t i t e i s found as large brown porphyroblasts resulting from early contact metamorphism by Spuzzum batholith. These appear to have been 29 Phase 1 Phase 2 Phase3 Post - tectonic Chlorite Biotite Muscovite Garnet Tourjn aline H o r n b l e n d e — - — Figure 3.1a Relative ages of metamorphic mineral growth with respect to deformation, Cogburn Creek Group. Phase 1 Phase 2 Phase 3 Po s t -tectonic Chlorite Biotite Muscovite Andalusite Garnet Staurolite Kyanite Fibrolite Sillimanite Tourmaline — Figure 3.1b Relative ages of metamorphic mineral growth with respect to deformation, Se t t l e r Schist. 30 rotated-during the formation of f 2 f o l i a t i o n , and are surrounded by quartz in pressure shadows. Large ske l e t a l ilmenite porphyroblasts are also found in t h i s sample. Textures in garnet porphyroblasts indicate that they formed during and aft e r f, and f 2 deformation. They contain inclusions and show evidence of rotation and synkinematic growth (e.g. HL15, Plate 3.3). A garnet in HL15 has a core with inclusions that mark f t and a s o l i d rim that grew post- kinematically to f,. Small euhedral garnets in HL16 are synkinematic to f 2 (Plate 3.2). Chlorite i s part of the main equilibrium assemblage in HL16, but in HL103 i t i s a late replacement of b i o t i t e and garnet. Tourmaline where present appears to have c r y s t a l l i s e d during f 2 . Alumino-silicates were not found, but that may be a factor of bulk composition rather than metamorphic conditions. Lowes (1972) found s t a u r o l i t e porphyroblasts in p e l i t i c schist in the upper part of Three Mile Creek, indicating that the higher range of epidote amphibolite facies was reached. 2) The main assemblage in the greenschist i s plagioclase, quartz, hornblende, epidote, c a l c i t e , magnetite, b i o t i t e , and c h l o r i t e . Two generations of plagioclase are present. Early phenocrysts with graphic intergrowths are c a l c i c oligoclase (An28), and igneous in o r i g i n . The later generation has a l b i t e composition (An8 to AnlO), and occurs aligned in f e l s i c segregations. Hornblende is pale blue-green. Rare brown cores (HL77) are found as r e l i c t s from the o r i g i n a l igneous assemblage. The grains are generally small, euhedral, and 31 aligned with the f o l i a t i o n around i t , suggesting c r y s t a l l i s a t i o n after f 2 . In HL80 (Plate 3.4) large hornblende grains have c r y s t a l l i s e d p a r a l l e l to an f 2 f o l d a x i a l plane, while smaller grains are randomised by r e c r y s t a l l i s a t i o n , suggesting c r y s t a l l i s a t i o n during and after f 2 . In t h i s same sample c a l c i t e f i l l s gashes that formed during late kink folding. HL142 (Plate 3.5) contains large porphyroblasts of dark brown- green to blue-green hornblende with inclusions of epidote, plagioclase and opaques. Although these porphyroblasts deflect the f o l i a t i o n , at least one contains i t and a t h i r d phase fo l d , indicating growth late in the t h i r d phase. Garnets also formed at t h i s time, and are now rounded. Epidote i s found both as sieve-textured porphyroblasts (HL77) and as fine grained replacement of plagioclase (HL80). Chlor i t e occurs as a late replacement of b i o t i t e , hornblende and garnet. 3) The ribbon chert has been r e c r y s t a l l i s e d during metamorphism to a coarse-grained aggregate of quartz and plagioclase. B i o t i t e r i c h layers are finer grained and represent more a r g i l l i c layers in the chert. A second cleavage is marked by b i o t i t e flakes oriented at an angle across the layers. No other metamorphic index minerals were recognised. 3.4 Unit 4, Settler Schist Metamorphism of the Set t l e r Schist was studied in d e t a i l by Pigage (1973) in the Gordon Creek area and by Bartholomew (1979) in the Yale Creek area. The metamorphic conditions in the Cogburn Creek area are similar, producing the same mineral 32 assemblages. As in the Cogburn Creek Group, three phases of folding are recognised. Figure 3.1b gives mineral growth age relationships for Settler Schist. The e a r l i e s t f o l i a t i o n i s seen only in inclusion t r a i l s in early garnets (Bartholomew 1979). Two episodes of metamorphism are recognised in these rocks. The f i r s t i s high temperature contact metamorphism due to the Spuzzum batholith. It produced c h i a s t o l i t i c andalusite in schists of suitable composition up to a kilometre from the pluton margins. Both the Hut Creek body and S e t t l e r Creek body produced th i s e f f e c t . Later upgrading due to regional metamorphism has caused r e c r y s t a l l i s a t i o n of the andalusite into aggregates of quartz, muscovite, s t a u r o l i t e , and kyanite or s i l l i m a n i t e . No andalusite remained in most of the specimens studied in thin section. Some small r e l i c t grains were found in SS43, SS80, SS96. Several c h i a s t o l i t e s were deformed during f 2 , before r e c r y s t a l l i s a t i o n (SS110, Plate 3.6). The lack of orientation of the minerals in the pseudomorphs indicates that r e c r y s t a l l i s a t i o n took place under s t a t i c conditions. In SS110 s t a u r o l i t e that has grown across the boundary of a pseudochiastolite contains graphitic inclusions of the external f o l i a t i o n where i t i s outside the pseudochiastolite. This gives the timing of pseudomorphing and s t a u r o l i t e growth as after f 2 and before f 3 deformation. Coexisting b i o t i t e and muscovite are ubiquitous throughout the s c h i s t . They define the f 2 f o l i a t i o n , and are r e c r y s t a l l i s e d around small scale f 3 kink folds. In SS135 33 (Plate 3.7) b i o t i t e has c r y s t a l l i s e d along the a x i a l plane of an f 2 f o l d . Garnet textures are indicative of growth from synkinematic to t, to post f 2 . Early garnets are now rounded and may have quartz pressure shadows or haloes. Garnets found in association with s i l l i m a n i t e are rounded and surrounded by quartz, muscovite and f i b r o l i t e (SS66, Plate 3.8). Staurolite c r y s t a l l i s e d a f t e r f 2 ; i t varies from synkinematic to f 3 to post-tectonic. Plate 3.9 shows post-tectonic twinned s t a u r o l i t e in p e l i t e . Many are con c e n t r i c a l l y zoned (SS114, Plate 3.10) and contain graphite or quartz inclusions. The inclusion trains mark f 2 f o l i a t i o n and f 2 and f 3 folds within the mineral grains (SS53, Plate 3.11). In many of the rocks containing both minerals, garnet i s usually e a r l i e r than s t a u r o l i t e . Kyanite, where present, is in pseudochiastolite, having replaced andalusite during thermal and pressure upgrading of metamorphism. Most of the alumino-silicates in rocks in the study area are s i l l i m a n i t e or f i b r o l i t e . C r y s t a l l i n e s i l l i m a n i t e porphyroblasts reach 10 to 20 cm in length on the north ridge of the area (Plate 3.13), in close proximity to the Spuzzum batholith to the north. The grade of regional metamorphism increases towards this area. In thin section these c r y s t a l s contain patches of f i b r o l i t e and inclusions of quartz, b i o t i t e and opaques, and have ragged outlines within a f a i r l y regular halo of quartz and b i o t i t e (SSl82a, Plate 3.14). Elsewhere, small c r y s t a l s produce a spotted or streaky appearance in the p e l i t e s . F i b r o l i t e occurs as mats with 34 b i o t i t e within the f o l i a t i o n but randomly oriented, and i s therefore post-kinematic. It appears to have formed at the same time as and s l i g h t l y before porphyroblastic s i l l i m a n i t e . Tourmaline is a minor but common constituent of the Settler Schist. It appears to have formed during f 2 and continued to grow pos t - t e c t o n i c a l l y . Some larger grains show concentric zoning with graphite inclusions, similar to s t a u r o l i t e grains in the same rocks. Tourmaline-bearing rocks also contain d i s t i n c t i v e reddish brown b i o t i t e . On the basis of the mineral associations four metamorphic zones of Barrovian series have been recognised in the Set t l e r Schist in the study area. They are as follows, in order of increasing grade: 1) Garnet-staurolite: b i o t i t e - garnet - muscovite - plagioclase - quartz - s t a u r o l i t e - (ilmenite - r u t i l e ) 2) Staurolite-kyanite: b i o t i t e - garnet - kyanite - muscovite - plagioclase - quartz - s t a u r o l i t e - (ilmenite - r u t i l e ) 3) F i b r o l i t e : b i o t i t e - garnet - muscovite - plagioclase - quartz - f i b r o l i t e ± kyanite ± s t a u r o l i t e - (ilmenite - r u t i l e ) 4) S i l l i m a n i t e : b i o t i t e - garnet - muscovite - plagioclase - quartz - f i b r o l i t e - s i l l i m a n i t e ± st a u r o l i t e The f i r s t s i l l i m a n i t e ( f i b r o l i t e ) isograd i s considered to represent equilibrium (Bartholomew 1979); i t marks the f i r s t appearance of f i b r o l i t e . The coarse s i l l i m a n i t e isograd marks the appearance of porphyroblastic s i l l i m a n i t e r e c r y s t a l l i s i n g from fine grained s i l l i m a n i t e ( f i b r o l i t e ) and disappearance of kyanite. Bartholomew (1979) considers that i t represents 35 overstepping of the kyanite to s i l l i m a n i t e reaction. Staurolite i s found in sillimanite-bearing rocks but is not necessarily associated with the f i b r o l i t e - b i o t i t e masses or s i l l i m a n i t e porphyroblasts. Although d e t a i l s of the relationships between metamorphic isograds and intrusive bodies are not clear, the grade increases towards the northeast. On a l o c a l scale, Bartholomew (1979) recognised a northwestern trend in Yale Creek, towards his Unit 3e, Fagervick body of the Spuzzum batholith. This was the last part of the Spuzzum batholith in his study area to intrude, and provided additional heat for the regional metamorphism to the northeast of Cogburn Creek. Metamorphic grade changes abruptly across Cogburn Creek, lending support to recognition of a fault along the south branch (Bartholomew 1979). The schist on the eastern side is in staurolite-kyanite zone, while rock d i r e c t l y west i s in garnet-staurolite zone. 3.5 Premetamorphic intrusive rocks The f e l s i c s i l l HL111 that intrudes Cogburn Creek Group contains s k e l e t a l garnet porphyroblasts and f i b r o l i t e intergrown with b i o t i t e and muscovite. Large mica grains define f 2 f o l i a t i o n . The rock was c o l l e c t e d from within 50 m of the contact with Spuzzum batholith Hut Creek body. The s i l l i m a n i t e i s most probably the result of l o c a l increase in temperature in the country rock by heat from the d i o r i t e during regional metamorphi sm.. 36 3.6 Discussion Ultramafic rocks Conditions of metamorphism in the ultramafic rocks appear consistent with those of the surrounding s c h i s t , indicating that the rocks were already juxtaposed before the culmination of regional metamorphism. The mineral assemblages can be used to place broad l i m i t s on metamorphic conditions across the ultramafic belt. Pigage (1973) notes that coexisting serpentine and t a l c indicate a temperature below 500°C with less than 0.1 mole fraction C0 2 in the f l u i d phase (Greenwood 1967, Johannes 1969). The association, seen here in HL55, HL56, HL65, provides a lower temperature l i m i t of 300°C (Figure 3.2). HL56 contains t a l c and magnesite, which i s in the same, s t a b i l i t y f i e l d . Tabular c r y s t a l s of o l i v i n e in HL30 (Plate 3.1) indicate metamorphic reconstitution of serpentinite back to o l i v i n e + t a l c , which requires a minimum temperature of 350°C. Contact Metamorphism Bartholomew (1979) considers that the andalusite formed under stable conditions at a pressure below the alumino-silicate t r i p l e point (Figure 3.3). H o l l i s t e r (1969b) suggested metastable c r y s t a l l i s a t i o n of andalusite in the kyanite s t a b i l i t y f i e l d for similar andalusite pseudomorphs in the Kwoiek area, with subsequent rapid conversion to kyanite and then s i l l i m a n i t e . However, th i s would require a steep Figure 3.2 Schematic T~X(C0 2) diagram for the system MgO-Si02- H20-CO2 at elevated pressures and temperatures. Adapted from Johannes (1969). 38 2 ' 1 1 1 1 400 600 800 T , % C 4 Pressure-Temperature gradient assuming / s t ab le formation of andalusite 4 P ressure-Temperature gradient assuming I metastable formation of andalusite in k y a n i t e f i e l d ( H o l l i s t e r 1 9 6 9 b ) Figure 3.3 Pressure-temperature conditions in the p e l i t i c assemblages, adapted from Pigage (1976). A A l 2 S i 0 5 System (Holdaway 1971) B C h l o r i t e + Muscovite=Staurolite + B i o t i t e + Quartz + vapour (Hoschek 1969) C St a u r o l i t e + Muscovite + Quartz=Al-silicate + B i o t i t e + vapour (Hoschek 1969) C Minimum temperature equilibrium position of curve C based on f i e l d evidence D Fe-staurolite + Quartz=Almandine +Sillimanite + Water (Pigage> and Greenwood 1982) E Muscovite + Quartz=Sanidine + S i l l i m a n i t e + Water (Chattergee and Johannes 1974). 39 temperature gradient with l i t t l e change in pressure associated with intrusion of d i o r i t e at depths of 17 to 21 km ( H o l l i s t e r 1969b). Subsequent studies have shown that H o l l i s t e r ' s interpretation i s u n l i k e l y . Bartholomew (1979) estimated pressure of regional metamorphism to be up to 3 kb above the alumino-silicate t r i p l e point, and noted that 14 km of burial would be required to produce the difference between contact and regional metamorphic conditions. He calculated that the Spuzzum batholith must have been emplaced at depths less than 14 km, and that a depth of b u r i a l of 27 ± 2 km (corresponding to a pressure of 7.6 ± 0.5 kb) must have been reached during regional metamorphism. Since andalusite formed before the most intense period of deformation, the time and pressure difference implied by stable formation i s quite reasonable (Bartholomew 1979). Regional Metamorphism Peli t e s of the Cogburn Creek Group belong to the greenschist facies of regional metamorphism, grading from the c h l o r i t e subfacies in the west on the shore of Harrison Lake (outside the map area) to the b i o t i t e subfacies. Temperature conditions range between 300°C and 450°C. Lowes (1972) recognised s t a u r o l i t e grade rocks in upper Three Mile Creek. An AFM diagram representing the metamorphic mineral assemblage found in the Cogburn Creek Group p e l i t e i s in Figure 3.4. Mineralogy in the greenschist suggests that the present metamorphic grade i s upper greenschist facies to kyanite zone of amphibolite f a c i e s . An ACF-AKF diagram of the mineral 40 K e y B i=B io t i te Cc*=Calcite ChfeChlorite Ep = E p i d o t e G a r - G a r n e t H b = H o r n b l e n d e l lm= l lmen i te M u = M u s c o v i t e P I = P l a g i o c l a s e R t l=Rut i l e Figure 3.4 AFM projection of metamorphic mineral assemblages found in p e l i t e s from the garnet-staurolite zone, and ACF-AKF projection of assemblages in greenschist, from the Cogburn Creek Group. 41 Gar Bi S T A U R O L I T E - G A R N E T Z O N E A A Fib S T A U R O L I T E - K Y A N I T E Z O N E A . Sil F IBROLITE Z O N E SILLIMANITE Z O N E Key Bi=Biotite FibsFibrollte Gar=Garnet llm=llmenite Ky=Kyanite Mu=Muscovite PUPIagioclase O O u a r t z Rtl-Rutile Sil=Sillimanite Figure 3.5 AFM projection of metamorphic mineral assemblages found in p e l i t e from the garnet-staurolite, staurolite-kyanite, f i b r o l i t e , and s i l l i m a n i t e zones, from the Set t l e r Schist. 42 assemblage found in the rocks i s in Figure 3.4. The temperature of metamorphism must have been between 300 and 525°C, probably in the higher part of the range. Sampling was not detailed enough to enable recognition of d i s c o n t i n u i t i e s in metamorphic grade. Mineral assemblages found in each of the metamorphic zones in the Settler Schist around Cogburn Creek are plotted on AFM diagrams in Figure 3.5. Pigage (1973) did regression analysis on p e l i t i c mineral assemblages from the Settler Schist to provide information on possible reactions based on mineral compositions. He found several reactions with too many co- existing phases, res u l t i n g from disequilibrium or a univariant reaction r e l a t i o n . Co-existing kyanite - s t a u r o l i t e - garnet - b i o t i t e - muscovite - quartz i s univariant on the AFM projection. This assemblage i s found in a 3 km wide zone in Gordon Creek (Pigage 1973), and i s recognised in Cogburn Creek (for example SS65, SS202). Bartholomew (1979) found that two reactions were involved in the replacement of andalusite. Some took place via d i r e c t polymorphic transformation to kyanite or s i l l i m a n i t e . Pseudomorphs containing s t a u r o l i t e imply a replacement reaction of the type: andalusite + b i o t i t e + H 20 = s t a u r o l i t e + muscovite + quartz. Regression analyses by Pigage (1973) suggest two s i l l i m a n i t e - forming reactions of the form: 1) s t a u r o l i t e + muscovite + quartz + r u t i l e = s i l l i m a n i t e + b i o t i t e + ilmenite +H20 43 2) garnet + muscovite + r u t i l e = s i l l i m a n i t e + quartz + b i o t i t e + ilmenite Since there are 8 phases in each reaction and the minerals represent 8 components, these reactions must have been continuous rather than discontinuous (Bartholomew 1979). A l l the phases co-exist in the s i l l i m a n i t e zone in Cogburn Creek. Bartholomew (1979) interprets the f i b r o l i t e and s i l l i m a n i t e isograds as representing equilibrium and overstepped kyanite- s i l l i m a n i t e transitions respectively. Estimates of metamorphic pressure and temperature for p e l i t i c rocks in the Settler Schist were obtained by Pigage (1973, 1976) and Bartholomew (1979) by means of mutual intersection of several calculated e q u i l i b r i a (Figure 3.6). The accuracy of the estimate i s based on the assumption of chemical equilibrium, accuracy and precision of microprobe analyses, v a l i d i t y of solid-so l u t i o n models, and the quality of thermodynamic data (Pigage 1976). Pigage notes that the presence of ubiquitous s t a u r o l i t e in the lowest grade rocks in the Settler Schist defines a minimum temperature for metamorphism of approximately 540°C. He concludes that regional metamorphism of the Settler Schist took place under conditions between 550 and 700°C, and 6 to 8 kbar. Bartholomew (1979) estimated conditions in the v i c i n i t y of the f i b r o l i t e (fine grained s i l l i m a n i t e ) isograd to have been 7.6 ± 0.5 kb and 705 ± 45°C, with an a c t i v i t y of water between 0.81 and 0.86 and log f ( 0 2 ) between -18.1 and -17.3. He was not able to determine upper l i m i t s on conditions in the p e l i t i c rocks of the highest, 44 400 500 600 700 800 Temperature, ° C Figure 3.6 E q u i l i b r i a c a l c u l a t e d f o r P 2 at a(H2O)=<0.81, plus the a l u m i n o - s i l i c a t e e q u i l i b r i a , adapted from Bartholomew (1979). I n t e r s e c t i o n s used to c a l c u l a t e metamorphic pressure and temperature c o n d i t i o n s i n the S e t t l e r S c h i s t . L i n e I I I ' i s scaled o f f from Pigage and Greenwood (1982), a recent r e c a l c u l a t i o n of the e q u i l i b r i u m of I I I . 45 coarse s i l l i m a n i t e grade. Recent recalculation of the equation: s t a u r o l i t e + quartz = almandine +Al 2Si0 5 + H 20 (Pigage and Greenwood 1982), narrows down the s t a b i l i t y f i e l d for s t a u r o l i t e + s i l l i m a n i t e , but does not s i g n i f i c a n t l y a l t e r Bartholomew's (1979) estimate of metamorphic conditions (Figure 3.6). Metamorphic conditions in the c o r r e l a t i v e Chiwaukum Schist (Lowes 1972) are sim i l a r . Getsinger (1978) found c h i a s o l i t i c andalusite predating kyanite and s i l l i m a n i t e , in Chiwaukum Schist on Nason Ridge, Stevens Pass. Plummer (1969, 1980) had found the same mineral assemblages and textures but misinterpreted the age relationships of the alumino-silicates (Evans and Berti 1985). Getsinger (1978) was unable to map clear isograds in the Chiwaukum Schist, because of overlap in assemblages. In the Stevens Pass area Berti (pers. comm. 1983) calculated a temperature of 550°C from garnet-biotite pairs and a pressure of 4 to 6 kbar. At Wenatchee Ridge, McLaughlin (pers. comm. 1985) estimates 550°C and 6 to 8 kbar using garnet- plagioclase geobarometry, suggesting a pressure gradient associated with c r u s t a l thickening after intrusion of the Mount Stuart batholith (Evans and Berti 1985). Isograds have been drawn for the Cogburn Creek area (Figure 3.7), based on mineral assemblages. The complex relationships between zones suggests post-metamorphic deformation, possibly related to the d i o r i t e bodies. In contrast to the relationships found by Bartholomew (1979) around Gordon Creek, the zone containing f i b r o l i t e only (no coarse s i l l i m a n i t e ) appears to be 46 to Figure 3.7 Map of metamorphic mineral assemblages and isograds for the Cogburn Creek area. See. Figure 2.1 for key to rock units. Abbreviations of mineral names are: f = f i b r o l i t e , g=garnet, k=kyanite, i = s i l l i m a n i t e , s=staurolite. 47 49*40' 49°30' C C h l o r i t Q Garnet 1 G-St Garnet- Staurolite p St-Ky Staurolit -Kyanite " Fib Fibrolit Sill Sillimanite \ Kyanite in > Fibrolite in * Sillimanite 121°45' 121°30' Figure 3.8 Regional isograds, modified from Bartholomew (1979), Lowes (1972), Pigage (1973), and Reamsbottom (1971, 1974). See Figure 1.2 for key to rock units. 48 d i r e c t l y r e l a t e d to contact metamorphism by the Cogburn Granodiorite. The presence of numerous small intrusive bodies throughout the region indicates large volumes of magma at f a i r l y shallow depths (Pigage 1973), which could have supplied some of the heat for the regional metamorphism. For comparison with surrounding areas a regional synthesis of isograds i s given in Figure 3.8. Information has been taken from Pigage (1973), Bartholomew (1979), Reamsbottom (1974) and Lowes (1972). Grade of regional metamorphism generally increases northeastward, toward the large northern pluton of Spuzzum batholith. However, the isograds are not p a r a l l e l to the pluton margins, and in places are truncated by the i n t r u s i v e s . Evans and Berti (1985) account for regional metamorphism of the Chiwaukum Schist as relaxation of isotherms related to the Mount Stuart batholith, during an episode of c r u s t a l thickening soon afte r intrusion. They propose that after dynamic contact metamorphism heat flowed from the b a t h o l i t h into the surrounding rocks, l e v e l l i n g out to about 600 °C. This corresponds to a continental-crust geothermal gradient of 30 °C/km and pressure of 6 kbar. Thus they consider that the regional metamorphism of the Mount Stuart batholith and nearby contact-metamorphosed rocks was retrograde, and that of the Chiwaukum Schist distant from the batholith was prograde. Since the Chiwaukum Schist and the S e t t l e r Schist are c o r r e l a t i v e and probably formed a continuous belt at the time of intrusion and metamorphism, the same interpretation can be applied to the area east of Harrison Lake. The Spuzzum batholith and Scuzzy D i o r i t e to the north form a much larger 49 body than the Mount Stuart batholith (Figure 2.2), so that the heat flow from i t would have been greater. This would account for the higher-grade metamorphic assemblages seen in the Settler Schist than in the Chiwaukum Schist. The metamorphism of the Cogburn Creek Group rocks would be prograde. Beck et a l . (1981) recognised a 30° t i l t down to the southeast from paleomagnetic evidence from the Mount Stuart b a t h o l i t h . If thi s were applied to the Spuzzum batholith, i t could also explain the northeastward increase of metamorphic grade in the surrounding s c h i s t . Recent paleomagnetic results of Irving et a l . (1985) on the Spuzzum Di o r i t e could be interpreted as indicating 28° t i l t down to the southeast, rather than l a t e r a l displacement (they consider that their data are i n s u f f i c i e n t to prove either a l t e r n a t i v e ) . 5 0 4. Structure Megascopic structure in the Cogburn creek area is expressed in the steep imbricate zone and deflections around intrusive bodies. Three outcrop-scale phases of folding have been recognised, consistent with observations of Lowes (1972) and Bartholomew (1979). The e a r l i e s t deformation (f,) i s recognised only from inclusion t r a i l s in porphyroblastic garnet and re- orientation of b i o t i t e porphyroblasts (HL16). Strong mica f o l i a t i o n in both the Cogburn Creek Group and Settler Schist i s assigned to f 2 , and i s p a r a l l e l to the a x i a l planes of i s o c l i n a l folds up to 1 m in amplitude. Compositional layering, where present, i s generally p a r a l l e l or subparallel to the f 2 f o l i a t i o n (Figure 4.1, 4.2). Pigage (1973) considered that compositional layering in S e t t l e r Schist in the Gordon Creek area was o r i g i n a l graded bedding and was able to determine a younging d i r e c t i o n . However in the area I have mapped i t may also be metamorphically transposed layering. Outcrops showing "bedding" were sparse around Cogburn Creek, and graded bedding or other facing c r i t e r i a were not seen, making i t impossible to specify a younging d i r e c t i o n . Fold style of minor folds associated with f 2 is i s o c l i n a l to t i g h t , rounded to chevron. On the map scale f o l i a t i o n orientation p a r a l l e l s the trend of the map units and the margins of plutons. In the S e t t l e r Schist f 2 f o l i a t i o n dips steeply northeast (Figure 4.2), except for one small area in the southeast corner of the map area, where i t dips steeply southwest. This may represent a fold hinge. 51 Figure 4.1 Equal area projection of poles' to compositional layering in the Settler Schist. 52 Figure 4.2 Equal area projection of poles to f 2 f o l i a t i o n for the Settler Schist south of Cogburn Creek. 53 The f 2 f o l i a t i o n has been rotated 40 to 50° westward on the north ridge (Figure 4.3). It may have been rotated by intrusion of the Spuzzum batholith, Hut Creek body. In the Cogburn Creek Group rocks around Three Mile Creek, bedding in chert and f o l i a t i o n in greenschist and p e l i t e i s p a r a l l e l to the melange zone. Dips are steep and to the northeast (Figure 4.4). Whereas the greenschist shows strong f o l i a t i o n , layering in the chert i s inferred to be bedding that has survived because of resistance of competent quartzose layers to transposition. The l a t e s t folding i s a result of both f 3 regional folding and intrusive a c t i v i t y . Folds of t h i s generation are c h a r a c t e r i s t i c a l l y broad warps and kinks. Close to pluton margins a penetrative f o l i a t i o n has developed with f 3 , producing mineral and cleavage-intersection l i n e a t i o n s . The f 3 f o l i a t i o n i s t y p i c a l l y at an angle of 25° to 40° to the f 2 f o l i a t i o n . Orientations of f o l d axes to both f 2 and f 3 folds are scattered, probably due to d u c t i l i t y contrasts causing l o c a l v a r i a t i o n (Bartholomew 1979). The melange zone containing imbricated tabular bodies of ultramafic rocks and Baird Metadiorite curves sinuously across the study area from southeast to northwest (Figure 2.1). It appears to have been pushed aside by l a t e r intrusion of Spuzzum batholith. It could also have bent around an already emplaced resistant core of d i o r i t e . Fine stringers of d i o r i t e intruding the ultramafic rocks in the thrust zone, on the ridge south of Cogburn Creek appear to indicate the former interpretation. 54 Figure 4.3 Equal area projection of poles to f 2 f o l i a t i o n in Settler Schist north of Cogburn Creek. Figure 4.4 Equal area projection of poles to f 2 f o l i a t i o n in Cogburn Creek Group, for the area north of 1 km south of Cogburn Creek. 56 In contrast to the Shuksan thrust farther south, the imbricate zone here was active before the culmination of regional metamorphism. This does not preclude c o r r e l a t i o n of the melange zone with the Shuksan thrust, as regional metamorphism of the Shuksan Suite was e a r l i e r than metamorphism in the Cogburn Creek area (Brown et a l . 1982). Metamorphic f o l i a t i o n i s found in places in the Spuzzum batholith. Poles to f o l i a t i o n and mineral lineations for the intrusions have been plotted in Figure 4.5. These appear to be randomly d i s t r i b u t e d . The main period of d i o r i t e intrusion postdates f 2 deformation and predates the culmination of regional metamorphism. Settler Schist i s s t r u c t u r a l l y above the imbricate zone, which forms the northeast boundary of the Cogburn Creek Group. Lowes (1972) mapped in d e t a i l the structure of the imbricate zone around Old Settler Mountain. The upper contact of the zone with the Se t t l e r Schist i s a steep (75°), north-dipping surface. The north face of Old Se t t l e r Mountain is a dip slope of Baird Metadiorite p a r a l l e l to th i s f a u l t , exposed by erosion of the narrow band of ultramafic rocks between that unit and the Se t t l e r Schist. F o l i a t i o n in the Baird Metadiorite i s generally p a r a l l e l to t h i s surface, which can be followed across the topography in both d i r e c t i o n s . The lower contact with the Cogburn Creek Group i s as sharp as the upper contact, where i t i s exposed west of Old Settler Mountain (Plate 4.1). However, just south of the gorge in Cogburn Creek the 2 km wide zone of metadiorite and ultramafic rocks becomes no more than 100 m 57 wide, giving way to Cogburn Creek Group chert. The nature of the contact i s unknown, as steep t e r r a i n precluded mapping. Macroscopic structure in the Cogburn Creek Group rocks in northern part of Three Mile Creek becomes complicated, but mapping was not continued far enough to unravel i t . A series of minor north-easterly oriented faul t s may have disrupted the strat igraphy. In spite of recent a l l u v i a l cover the Cogburn Fault was recognised by Bartholomew (1979) in the south branch of Cogburn Creek. He noted truncated geologic contacts (Figure 2.1) and contrasts in rock types, metamorphic grade, and structure across the v a l l e y . Alignment of the tributary stream to the north with the main valley of the south branch, and the occurrence of fault breccia on the ridge at i t s head, strengthen the evidence for the f a u l t . 58 N Figure 4.5 Equal area projection of poles to f o l i a t i o n and mineral lineations in Spuzzum bath o l i t h . 59 5. Geochronometry Dating of rock units in the study area was c a r r i e d out using Rb-Sr, K-Ar and U-Pb techniques. A n a l y t i c a l techniques are described in Appendix A; a n a l y t i c a l data, isotope ra t i o s and calcul a t e d dates are given in Appendix B; sample descriptions and locations in Appendix C, and locations plotted in Figure D- 2. Eighty-six whole rocks and mineral separates from 40 samples of Spuzzum batholith, Settler Schist, Cogburn Creek Group sch i s t , Breakenridge Formation gneiss, Baird Metadiorite and several pre- and syn-tectonic intrusive bodies were analysed for Rb-Sr and Sr isotopic composition. The data have been used to ca l c u l a t e isochron dates for the d i f f e r e n t rock units. The isochron plots are shown in Figures 5.2 a to h. Five hornblende separates from Spuzzum batholith were dated by K-Ar. Data are plo t t e d in Figure 5.4a and b. Zircons for U-Pb dating were separated from Baird Metadiorite, Breakenridge Formation gneiss, S e t t l e r Schist, Spuzzum batholith (Hut Creek and S e t t l e r Creek bodies), and one pre-metamorphic intrusive rock. The concordia diagram i s Figure 5.3. A l l figures are at the end of the Chapter. 5.1 Previous Geochronometry Although no one has undertaken an intensive dating study in the region between Harrison Lake and the Fraser Valley, a number of analyses have been car r i e d out by previous workers. Most of these have been on intrusive rocks, p a r t i c u l a r l y the Spuzzum 6 0 Table 5.1 Table of dates and age estimates from pre v i o u s s t u d i e s between H a r r i s o n Lake and F r a s e r R i v e r Reference Rock Date Ma Method B a i r d M e t a d i o r i t e (Unit 1)' and c o r r e l a t i v e s Lowes (1972) Ma t t i n s o n (1972) Yellow A s t e r pyroxene g n e i s s pegmatite g n e i s s q u a r t z d i o r i t e o r t h o g n e i s s Swakane g n e i s s b i o t i t e g n e i s s pegmatite g n e i s s p o s s i b l y Precambrian c o r r e l a t i o n with Yellow A s t e r Complex 711 912 1452 ± 20 64 75 368 375 427 ? 75 411 ? 15 U-Pb z i r c o n 2 3 U-Pb z i r c o n U-Pb z i r c o n 433 628 1419 ± 100 U-Pb z i r c o n 69 U-Pb z i r c o n Cogburn Creek Group (Unit 3) and c o r r e l a t i v e s Lowes (1972) Permian-Pennsylv- c o r r e l a t i o n w i t h anian C h i l l i w a c k Group Reamsbottom (1971) L. C r e t . and/or o l d e r c o r r e l a t i o n with C h i l l i w a c k Group Monger (1970) C h i l l i w a c k Group Permian-Pennsylvanian f o s s i l s Armstrong unpub. C h i l l i w a c k Group 191 ± 16 @ 0.70510 Rb-Sr WR" n=14. Armstrong unpub. Bridge R i v e r Group high grade s c h i s t s 104 ± 25 @ 0.70555 Rb-Sr WR n=14 low grade s c h i s t s 256 ± 35 @ 0.70394 Rb-Sr WR n=28 S e t t l e r S c h i s t (Unit 4) and c o r r e l a t i v e s Pigage (1973) S e t t l e r S c h i s t e a r l y - m i d P a l e o z o i c Bartholomew (1979) S e t t l e r S c h i s t M attinson (1972) S k a g i t g n e i s s b i o t i t e g n e i s s g n e i s s i c q t z d i pegmatite g n e i s s 214 ± 32 98 112 428 + 10 66 67 79 ± 10 90 57 c o r r e l a t i o n with Shuksan S u i t e Rb-Sr WR n=7 U-Pb z i r c o n U-Pb z i r c o n U-Pb z i r c o n U-Pb z i r c o n 61 Armstrong unpub. D a r r i n g t o n P h y l l i t e 132 ± 8 @ 0.70556 Rb-Sr WR n=22 Shuksan b l u e s c h i s t 172 ± 18 @ 0.70387 Rb-Sr WR n=l5 and g r e e n s c h i s t Brown et a l . Shuksan amphib. 148 ± 5 K-Ar Hb (1982) b a r r o i s i t e s c h i s t 164 ± 6 K-Ar Hb b l u e s c h i s t s 128 ± 5 Rb-Sr Mu 129 ± 5 . K-Ar Mu Spuzzum b a t h o l i t h (Unit 5) and c o r r e l a t i v e s McTaggart and t o n a l i t e 77 ± 4 K-Ar Bi Thompson ( 1 9 6 7 ) ( 6 ) t o n a l i t e 77 ± 4 K-Ar Hb Ri c h a r d s (1971) f o l i a t e d 80 ± 4 K-Ar Bi (1) d i o r i t e 82 ± 4 K-Ar Hb f o l . d i o r i t e 84 ± 4 K-Ar Bi f o l . d i o r i t e 105 ± 3 K-Ar Bi Wanless (1973)(4) quartz d i o r i t e 76 ± 4 K-Ar B i , Hb McLeod (1975) t o n a l i t e 80.7 ± 2.5 K-Ar Bi ( 5 ) t o n a l i t e 86.6 ± 2.8 K-Ar Hb d i o r i t e 91.5 ± 2.8 K-Ar Hb d i o r i t e 91.2 ± 3.1 K-Ar WR h o r n b l e n d i t e 96.4 ± 4 K-Ar Hb h o r n b l e n d i t e 121.6 ± 4 K-Ar Hb f e l s . h b i t e 106 ± 4 K-Ar Hb h o r n b l e n d i t e 112 ± 4 K-Ar Hb Richards and d i o r i t e 91.0 ± 2.8 K-Ar Hb McTaggart (1976) (1) Bartholomew (1979) u n i t 3b 77.3 ± 2.6 K-Ar Hb (2) u n i t 3e . 94.4 ± 3.2 K-Ar Hb Armstrong unpub. b i hb d i o r i t e 94.9 ± 7 K-Ar hb (3) 101 ± 3 K-Ar Bi b i hb d i o r i t e 91.3 ± 2.2 @ 0.7038 Rb-Sr WR- Pl-Hb-Bi 62 Armstrong and b i hb d i o r i t e 112 122 331 U-Pb z i r c o n Ryan unpub. (3) Younger i n t r u s i v e s (Unit 6) Bartholomew (1979) a p l i t e 34.4 ± 1.4 @ 0.7041 Rb-Sr WR (2) q u a r t z d i o r i t e 30.7 ± 0.6 @ 0.7041 Rb-Sr Bi-WR g r a n o d i o r i t e 33.6 ± 1 . 1 K-Ar Bi Woodsworth and granite(GEM-1) 35 ± 0.7 Rb-Sr WR Armstrong unpub. (3) 1 U n i t numbers are from t h i s study 2 A l l data r e c a l c u l a t e d with IUGS convention decay c o n s t a n t s ( S t e i g e r and Jager 1969), except U-Pb z i r c o n from Mattinson (1972). 3 Z i r c o n dates are r e p o r t e d as 2 0 6 P b / 2 3 ' U , 2 0 7 P b / 2 3 S U , 2 0 7 P b / 2 0 6 P b * I n i t i a l 8 7 S r / 8 6 S r r a t i o s are r e p o r t e d with Rb-Sr i s o c h r o n dates n= number of samples analysed f o r r e p o r t e d i s o c h r o n date. WR=whole rock, Hb=hornblende, B i = b o i t i t e , P l = p l a g i o c l a s e Numbers i n b r a c k e t s are r e f e r e n c e s f o r symbols on F i g u r e 5.1. 63 30' + . * + + + + + + + +|+ + + + + + + + + \\++J ++ \ . «+ + + + + + + + + + + +*70b+ + + +\ V + + + +' ' , \ + + + + + + + + + + + + + + + + + + \ \ + + + + H . r \ + + + + + + + + + + + + + + + + + +>+ + + + + - + + + + + + + + + + + + + + + + + ' + + + + H ' >+ + + + + + + + +..+ + + + + + + +,,\+ + +- a -.. \ + + + + Scuzzy Pluton + + + + + +̂+ + - \ IK \+ + + + + + + + + + + + + + + +* i + ^ + H * x Is. + + + + + + + + + + + + + + + + + r̂-'.-.i + + • I'-'i- + + 1 \ 1 0 5 Z ; 79Rr \ + + + + + + + + + + + + + 4l -+++++ + + + + + + + + n ,+ + Spuzzum Pluton + +-+ + -« ) + + + + + + + + + + + ' H + + + + + + + + + + + H\ + ' A + + + + + + + +76 bh**™- + (\+ + + + + + + + + + + +>\ i \ + + + + + + + + + + + /> + + + + + + + + + + + iS* N^/ > + + + + + + + + + + \ l r -\+ + + + + + + + + + + My V + + + + + + + + + + + + 30.7R+ 33.6b + 34.4r 77 70.11 391 .77. WW >1 i + i . 9 5 2 h 210i 794.4h Key k K-Ar WR b K-Ar Bi h K-Ar Hb r Rb-Sr WR R Rb-Sr minerals] z U-Pb zircon • this study *(1) • (2) u (3) References • (4) (5) f r o m (a\ Table 5.1 7 7Rv'<v; 78.9h _̂ 110z / + 96.4M LlXV0^ ++++ io6h+7l5. \ + -n v *. • 1 o 1 o K T i ^ A . 7b . r..6h \+ 12 1.6h + i + + + + i+ J - + + + i . + + k Spuzzum Pluton + + + + + + + + + + 1+ + + + + _+ <f + + + +91 + Hope j84. B2h AS ~r r r „ +*+iVA84b + + + + + + +> + + + + + + + +\ v-+ + + + + + +\_ *+ + + + + + +0 >+ + + + + + ( /+ + + + +.+ +' 4- + + + + + +' —-fe. + + + +_++> 49° 15'f Kilometres — 122°00' 45' 121° 30' Figure 5.1 Map showing geochronometry of the Harrison Lake - Fraser River region See Figure 1.2 for key to map units. Symbols indicate references l i s t e d in Table 5.1. 64 batholith. In the North Cascade Mountains, Washington, Mattinson (1972) dated by U-Pb zircons from units that are believed to be c o r r e l a t i v e s of rocks in the present study area. His r e s u l t s are tabulated in Table 5.1, along with other dating in the Harrison Lake - Fraser River region. The locations of these previously dated rocks have been plotted on a map (Figure 5.1). 5.2 Unit 1• Baird Metadiorite This unit has been correlated previously with the Yellow Aster Complex of the North Cascades, Washington (Lowes 1972), because of i t s appearance, and s t r u c t u r a l relationships within the Shuksan thrust imbricate zone. Mattinson (1972) dated zircons from pyroxene gneiss of the Yellow Aster Complex from the North Cascades as 1452 to 2000 Ma, and from the younger orthogneiss as 430 Ma. The Rb-Sr and U-Pb dating results reported here have large errors, but suggest a Precambrian age for the rocks. Figure 5.2a shows that 8 7 R b / 8 6 S r ratios of the samples are extremely small, a l l are < 0.02, and the differences hardly s i g n i f i c a n t . The r e s u l t i n g isochron age has a large error (3.4 ± 2.4 Ga), because i t i s a least squares f i t of a tight cluster of points. The c l u s t e r of points suggests that the strontium isotopic r a t i o s may have been homogenised toward a value of approximately 0.7043 by Cretaceous metamorphism, from an i n i t i a l l y lower value that would have defined a l i n e with even greater slope. If this i s the case, the calculated i n i t i a l value of 0.7038 i s only an 65 upper l i m i t for the true i n i t i a l r a t i o . The low rubidium values are consistent with a primitive ocean crust p r o t o l i t h . Mattinson (1972) noted that Rb-Sr analyses of the older orthogneiss in the Yellow Aster Complex had f a i l e d to y i e l d meaningful ages because although Sr concentrations are normal, the rocks are so depleted in Rb that Rb-Sr whole rock dating i s precluded (J.C. Engels, 1968, pers. comm. to Mattinson). The same comment holds for the Yellow Aster-type rocks of the Baird Metadiorite of this study. A highly discordant zircon U-Pb date (Table B-6) also suggests a pre-Mesozoic, perhaps Precambrian, age. Only one fraction was analysed (Table C-2), due to low zircon y i e l d , and the analysis could not be repeated although i t was of poor qual i t y . The 2 0 7 P b / 2 0 6 P b dates calculated for the natural and spiked runs are 704 ± 180 Ma and 1653 ± 175 Ma respectively, due to s i g n i f i c a n t differences in the measured 2 0 7 P b / 2 0 6 P b r a t i o s . The averaged 2 0 7 P b / 2 0 6 P b date i s 1251 ± 180 Ma. When plotted on concordia (Figure 5.3) the averaged value l i e s away from concordia but close to the l i n e defined by points from analyses of the S e t t l e r Schist. The single analysis for the metadiorite r e s t r i c t s the conclusions: either the rock i s old and the U-Pb clock has been reset by amphibolite-facies metamorphism during the Mesozoic, or old lead has been incorporated into Mesozoic zircons. The f i r s t interpretation, which i s reinforced by the Rb/Sr analyses, i s compatible with Mattinson's (1972) interpretation for rocks of the Yellow Aster Complex in the Cascade Mountains of Washington, namely intrusion around 1400 to 66 1600 Ma followed by metamorphism at 415 Ma and 90 Ma. His concordia for Yellow Aster Complex i s shown in Figure 5.5 for comparison with Baird Metadiorite. 5.3 Unit 3. Cogburn Creek Group Rb-Sr data from this group of rocks c l e a r l y define two events, the e a r l i e r one i s the approximate time of deposition and the later i s the Cretaceous metamorphism. The calculated isochron date when b i o t i t e and chert analyses are not included is 296 ± 58 Ma (Table B-2, Figure 5.2b), which could represent either a late Paleozoic age for the sequence of chert, volcanic rocks and p e l i t e s , or a reset caused by Mesozoic metamorphism of even older rocks. The f i r s t a l t e r n a t i v e i s favoured here, but no f o s s i l s have been found to define the stratigraphic age. A Rb-Sr whole rock isochron date for greenschist alone, 438 ± 68 Ma (n=2), i s probably not sensible geologically, as no similar dates have been found in the region. B i o t i t e dates from chert and greenschist are 81 i 5 Ma and 77 ± 1.6 Ma respectively. These dates represent the waning stages of the Cretaceous metamorphic episode, and are s i m i l a r to b i o t i t e dates from Settler Schist and Spuzzum b a t h o l i t h (see discussion below). On the Rb-Sr isochron diagram (Figure 5.2b) the point representing chert (HL37a) whole rock has a higher 8 7 R b / 8 6 S r and 8 7 S r / 8 6 S r value than the other whole rocks. S i m i l a r l y , the 8 7 S r / 8 6 S r i n i t i a l r a t i o , 0.7090, for the chert (HL37a)-Bi isochron i s higher than those for other rock-biotite p a i r s . The date i s , however, similar to other metamorphic dates. The 67 difference in 8 7 S r / 8 6 S r may be due to incorporation of 8 7 S r - r i c h seawater at the time the chert was deposited (Jager 1979). No zircons were dated from the chert and greenschist, although accessory zircon is present in sample HL37a. 5.4 Unit 4. S e t t l e r Schist The Settler Schist i s bounded by major fault s and plutons, so stratigraphic relations with other units are unclear. On the basis of s i m i l a r i t i e s in s t r u c t u r a l position with the Paleozoic (?) Shuksan Suite in the North Cascades, Pigage (1973) assigned an Early - Mid Paleozoic age to the Settler Schist. Reamsbottom (1971) suggested a late Paleozoic to Mesozoic age for similar schists in the Mount Breakenridge area north of t h i s study area because granitoid c l a s t s were found in conglomeratic horizons within the schists, and because there are no pre-Jurassic plutons. in the area (see also Pigage 1973). Both these age estimates have been invalidated by subsequent studies in the region. Shuksan Suite blueschists have been dated as Mesozoic (Armstrong et a l . 1982), and granite c l a s t s have been found in T r i a s s i c rocks in the Bridge River area, indicating the presence of early Mesozoic plutons. L i t h o l o g i c a l l y , the Settler Schist i s l i k e Chiwaukum Schist (Lowes 1972), which may be correlated with the Skagit Suite and i s d i f f e r e n t from the Shuksan Suite. The Shuksan Suite contains blueschist and greenschist that are metamorposed ocean fl o o r (Dungan et a l . 1981), and offshore sediments. Bartholomew (1979) obtained an Rb-Sr isochron date from 68 Se t t l e r Schist in the Yale Creek area of 214 ± 32 Ma. He interpreted t h i s as both the age of the source rock for the pre- metamorphic sediments and the time of their deposition because of the low i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.7043. Eleven Rb-Sr whole rock analyses from this study combined with data from seven rocks from Bartholomew (1979) ( a l l in Table B-1) produce an isochron age of 210 ± 27 Ma (Table B-2; Figure 5.2c and d ). This r e f l e c t s either T r i a s s i c - Jurassic deposition or p a r t i a l to complete resetting by Mesozoic metamorphism of pre-Jurassic rocks. If i t i s the age of deposition, then i t does not necessarily also represent the integrated age of the source rocks, as w i l l be discussed l a t e r . The mineral isochron date from a biotite-bearing graphitic p h y l l i t e (SS82) indicates either a s l i g h t l y l a ter end to the metamorphism, at 66 ± 1.6 Ma, or a p a r t i a l reset by the mid Te r t i a r y Cogburn Granodiorite which crops out within a kilometre, on the east side of Cogburn Creek. The youngest isochron drawn in Figure 5.2c and d i s from a quartz-biotite-garnet schist (SS128) from near the contact with Cogburn Granodiorite. Bartholomew (1979) dated the granodiorite by Rb-Sr WR isochron, Rb-Sr B i , and K-Ar Bi at 32 ± 4 Ma. The mineral isochron date for the schist SS128 of 39 ± 4 Ma i s due to resetting by t h i s young pluton. The i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.7066 for t h i s schist sample has been pulled up from a value around 0.704 by r e - e q u i l i b r a t i o n of Sr between the b i o t i t e and felspar components (Jager 1979). Zircon separated from two samples of Settler Schist is 69 small and subhedral, c e r t a i n l y d e t r i t a l . They are unusual in that they contain small opaque (graphite ?) inclusions. This may indicate metamorphic overgrowths on the d e t r i t a l cores, with inclusion of material from the sediments containing the zircons. The two samples yielded highly discordant U-Pb dates but similar 2 0 7 p b / 2 0 6 P b a g e S f 1183 ± 12 Ma and 1279 ± 32 Ma (Table B-5). Plotted on concordia (Figure 5.3), the l i n e joining the two points has a lower intercept of 211 ± 30 Ma, and an upper intercept of 2450 ± 230 Ma. The young lower intercept i s consistent with the Rb-Sr isochron age for the sc h i s t . The old upper intercept indicates a Precambrian source for the zircons in these sediments. The age obtained i s consistent with zircon ages obtained from schists and gneisses in the North Cascade Mountains of Washington by Mattinson (1972), as well as from the pyroxene gneisses of the Yellow Aster Complex. Zircons from the supracrustal Swakane Gneiss and Skagit Gneiss yielded highly discordant ages with 2 0 7 P b / 2 0 6 P b ages in the range 1400 to 1650 Ma (Figure 5.5). The concordia for zircons from the two units (Mattinson 1972) i s shown in Figure 5.5, for comparison with results from the Settler Schist. Mattinson considered those from the Swakane Gneiss to approximate stratigraphic age and those from the Skagit Gneiss to be d e t r i t a l and derived from a Precambrian source. Thus the stratigraphic age for the Skagit Gneiss could be younger than Precambrian. Misch (1966) considered that the pre-metamorphic equivalent of much of the Skagit Gneiss was immature greywacke. The isotopic s i m i l a r i t y between the S e t t l e r Schist and Skagit Gneiss i s not surprising 70 since the S e t t l e r Schist i s considered to be equivalent to the Chiwaukum Schist (Lowes 1972), which i s part of the Skagit Metamorphic Suite defined by Misch (1966). In summary, the zircon geochronometry suggests that the Settler Schist i s at least in part derived from an old source. The Rb/Sr date of 210 ± 27 Ma does not r e f l e c t t h i s old source; thus i t represents only the approximate time of deposition of Settler Schist and, because of the low i n i t i a l r a t i o , possibly the age of almost contemporaneous source rocks. From their l i t h o l o g i e s and chemistry, Pigage (1973) and Bartholomew (1979) stated that the sediments forming the schist were eugeosynclinal and derived from a r i s i n g volcanic arc of approximately the same age as those sediments. However, there also must have been old basement exposed nearby to supply the old zircon and mature fine-grained sediments that now form the p e l i t i c part of the Settler Schist. The time of deposition was most probably early Mesozoic, as indicated by the Rb-Sr whole rock isochron. Waning of the main Cretaceous metamorphic event i s shown by the mineral (biotite) ages obtained from both a p h y l l i t e and a syn-tectonic f e l s i c i n t r u s i v e . The l a s t intrusive event at 32 ± 4 Ma (Bartholomew 1979) reset b i o t i t e in the nearby s c h i s t . 5.5 Premetamorphic intrusive rocks 1) Zircon sample HL111, from a f e l s i c body (Figure D-2, Table B- 5), yielded U-Pb dates that are s l i g h t l y discordant; the 2 0 7 P b / 2 0 6 P b d a t e i s 3 7 6 ± 83 M a # Because of the proximity of the s i l l to the Spuzzum batholith, Hut Creek body, and lack of 71 similar old dates from other intrusive rocks in the region the s i l l i s probably Mesozoic. S i l l i m a n i t e i s intergrown with b i o t i t e and muscovite, indicating that the s i l l was in place before the height of regional metamorphism. The 2 0 6 P b / 2 3 8 U and 2 0 7 P b / 2 3 5 U ratios plot almost on concordia at 110 Ma (Figure 5.3). This Cretaceous date i s a reasonable estimate for time of s i l l emplacement, based on l o c a l geology. I believe the s i l l was emplaced early enough during the Cretaceous intrusive and metamorphic event to have been deformed with the schists i t intruded. The old 2 0 7 P b / 2 0 6 P b date could be due to contamination by country rock lead or zircons. The Rb-Sr whole rock - mineral isochron date of 74 ± 10 Ma for the s i l l i s similar to other dating that records the end of Cretaceous metamorphism. The f e l s i c composition ( b i o t i t e , no hornblende) and the high U and Pb contents compared with the Spuzzum batholith, Hut Creek body (Table B-4) suggests that t h i s s i l l i s not part of the main intrusive phase. 2) Mineral isochron dates from a f o l i a t e d , f e l s i c , garnet- cluster dyke (SS85) cutting the S e t t l e r Schist on the north ridge (Figure 5.2e), show the Cretaceous igneous and metamorphic event. Excluding Bi from the c a l c u l a t i o n y i e l d s a date of 105 ± 20 Ma, probably representing time of intrusion during the early part of the metamorphic cycle. The dyke became f o l i a t e d p a r a l l e l to the schist f o l i a t i o n , and cooled through the b i o t i t e blocking temperature of 300°C (Jager 1979) at 80 ± 6 Ma. 72 5.6 Foliated d i o r i t e in fault zone A small body of f o l i a t e d d i o r i t e crops out on the west side of Settler Creek, intruding the metamorphosed ultramafic rocks in the imbricate zone of the Shuksan thrust. F o l i a t i o n in the intrusive is p a r a l l e l to the fault zone. A Rb-Sr mineral isochron was obtained from this rock in the hope that i t might help date the thrusting event. The four point isochron gave a date of 77 ± 3 Ma (Figure 5.2f, Table 3-B). Since this i s in the range of metamorphic ages, the thrusting either took place during metamorphism or before i t so that a l l the rocks were equally affected. This small body was emplaced after the thrusting but before regional metamorphism, and the isotopic system has been reset. 5.7 Unit 5. Spuzzum batholith Two discrete bodies of Spuzzum batholith in the area were mapped, and dated (Figures 1.2, 5.2). The small body intruding only Settler Schist north of Old S e t t l e r Mountain is c a l l e d the Se t t l e r Creek body and is unit 3d of Bartholomew (1979). The Hut Creek body spans Cogburn Creek and extends north up Hut Creek (Figure 1.2). Five K-Ar dates were obtained on hornblende separates from the two bodies (Table B-3). Hornblende from gabbro of the Hut Creek body (SD66) with very low %K (0.046 ± 0.002%) yielded a conventional date of 162 ± 7 Ma, and the other samples gave dates between 100 ± 3 Ma and 77.5 ± 3 Ma. The dates from the gabbro may have large a n a l y t i c a l errors due to the very low K 73 content. The interpretation is further complicated by possible incorporation of i n i t i a l radiogenic argon. The measured K content has a re p r o d u c i b i l i t y of ± 4% (1a for 4 analyses), but such a low K means that the fraction of the t o t a l *°Ar that i s radiogenic is also low. For this sample, the radiogenic a o A r i s only 38% of t o t a l U 0 A r , leading to possible error in the age cal c u l a t i o n of up to 5 %. Apart from t h i s sample, ages obtained from the Hut Creek body were s l i g h t l y younger at 80.9 ± 3 and 77.5 ± 3 Ma than those from the Settler Creek body, 100 ± 3 and 94.5 ± 4 Ma. Figure 5.5a shows the data plotted on an isochron diagram of *°Ar/ 3 6Ar v. "°K/ 3 6Ar. If no excess i n i t i a l radiogenic argon were present the isochron lines would pass through an intercept of 295.5 on the argon axis, representing the atmospheric "°Ar/ 3 6Ar r a t i o . Whether the points are grouped together or s p l i t according to d i o r i t e body, the intercept i s s t i l l >295.5. The slopes of the l i n e s through the plo t t e d intercepts represent dates of 70.1 ± 5 Ma for the North body and 78.9'± 2 Ma for the Settler Creek body. The actual amount of excess argon can be found from Figure 5.4b, a concentration- isochron plot of , 0 A r v. %K. Lines on t h i s plot should pass through the o r i g i n ; a positive intercept on the 4 0 A r axis gives the amount of excess argon in nl/g. For these rocks the values are 0.19 ± 0.02 nl for the Hut Creek body and 0.14 ± 0.01 n l for the S e ttler Creek body. The slopes of the concentration- isochrons represent 65.7 ± 5 Ma and 79.7 ± 2 Ma respectively. The low date from the Hut Creek body r e f l e c t s the e f f e c t s of excess argon. The best minimum age estimates for the d i o r i t e 74 bodies are those calculated from the isochrons. Of the individual conventional dates, those for the Se t t l e r Creek body are closer to intrusive ages than those from the Hut Creek body, as the hornblendes contain less excess argon. Since the d i o r i t e was intruded before the peak of the high pressure and temperature regional metamorphism, a l l K-Ar dates must have been subject to some resetting. Pigage (1976) calculated the conditions of metamorphism of the Settler Schist to be 6 to 8 kbar and 550 to 700°C. The K-Ar blocking temperature for hornblende is 400°C. The d i o r i t e was subjected to temperatures high enough to reset the K-Ar clock of a l l the constituent minerals during metamorphism. In thin section, hornblendes in the d i o r i t e have brown cores and green rims that have r e c r y s t a l l i s e d during metamorphism. Rb-Sr dating of the Hut Creek body gives a WR-Pl-Hb isochron date of 127 ± 41 Ma and a b i o t i t e date of 88.3 ± 2 Ma (Figure 5.2g, Table B-2). One hornblende anomalously high in radiogenic strontium was not included in the WR-Pl-Hb isochron. The b i o t i t e date, similar to a l l other b i o t i t e dates from the area, indicates that the d i o r i t e cooled through the b i o t i t e blocking temperature of 300°C at the same time as the schists during the waning metamorphism. The WR-Pl-Hb isochron produces a date older than, but not in disagreement with, any date for d i o r i t e s in the region reported by previous workers (e.g. Richards 1971, McLeod 1975). K-Ar, Rb-Sr and U-Pb dating along Highway 1 west of Hope has produced dates in the range 70 to 110 Ma, and further north in the range 70 to 90 Ma. The 75 older date obtained in t h i s study may be due to movement of radiogenic strontium between mineral components during cooling (cf. Wasserburg et a l . 1964), especially as the d i o r i t e was subjected to high grade regional metamorphism along with the schists i t intrudes, or merely due to random a n a l y t i c a l error. Zircon from the Hut Creek body yielded a highly discordant date with large errors; the overlapping 2 0 7 P b / 2 0 6 P b dates calculated independently from unspiked and spiked runs were 778 ± 360 Ma and 1235 ± 600 Ma. The averaged value is 1023 ± 500 Ma. This date, which contradicts l o c a l geology, indicates the presence of old lead or old zircon. There seems no l i k e l i h o o d of physical contamination from the schist, as this sample was col l e c t e d well within the d i o r i t e body; however, the zircon y i e l d was extremely low, 210 zircons from 35 kg of rock. A n a l y t i c a l errors may be high, because of the small amount of Pb and U available for measurement. When the 2 0 6 P b / 2 3 8 U and 2 0 7 P b / 2 3 5 U r a t i o s are plotted the point l i e s off concordia. A mixing l i n e through t h i s point and those for Settler Schist intersects concordia at 110 ± 5 Ma. This i s a plausible intrusive age, and indicates that the discordance might be due to the presence of old zircon picked up from the country rock, such as Set t l e r Schist, at the time of intrusion ( c f . Gebauer and Grunenfelder 1979). Rb-Sr and U-Pb isotopic analysis of the Settler Creek body shows a pronounced effect of either assimilation of schist in the magma during intrusion or mobilisation of Sr and Pb by f l u i d s . Since the dating samples were c o l l e c t e d unintentionally 76 close to margins of the body the former i s quite l i k e l y . When an isochron i s calculated for WR analyses of a l l samples except SD97 a date of 274 ± 179 Ma is obtained. This date i s obviously too old, since i t i s greater than the isochron age obtained for the country rock, and most probably represents incorporation of Sr from the schists at the time of intrusion. A more r e a l i s t i c but s t i l l old date of 167 ± 46 Ma (Figure 5.2h) is obtained for Pl-Hb. The data for sample SD97 were l e f t out of the isochron calculations above as they have lower 8 7 S r / 8 6 S r values than the other samples for similar 8 7 R b / 8 6 S r values. The d i o r i t e body is apparently i s o t o p i c a l l y heterogeneous, as SD97 was co l l e c t e d close to SD96 and i s similar l i t h o l o g i c a l l y . The b i o t i t e - dominated mineral isochron date for sample SD97 i s 89 ± 7 Ma. The WR-Pl-Hb date i s 186 ± 49 Ma; thi s i s probably too old because the hornblende analysis i s abnormally high in 8 7 S r / 8 6 S r . A l l the hornblende samples analysed from the Set t l e r Creek body are high in radiogenic strontium; since they were a l l c o l l e c t e d near the margin of the pluton they may have been enriched with radiogenic strontium when they r e c r y s t a l l i s e d during metamorphism. Zircons from the Settler Creek body yielded 2 0 7 P b / 2 0 6 P b dates of 242 ± 194 Ma, 629 ± 88 Ma and 217 ± 138 Ma for the natural and two spiked runs. The spread i s due to the points l y i n g on a chord very close to concordia (Figure 5.3). On the plot of 2 0 6 P b / 2 3 8 U v. 2 0 7 P b / 2 3 5 U the points l i e near 95 ± 5 Ma on the concordia, so thi s i s taken as a minimum estimate of the age of the Settler Creek body. 77 5.8 Agmatised quartz d i o r i t e A d i o r i t e agmatite (SD 14), c o l l e c t e d on the east side of the south branch of Cogburn Creek, gave a three point isochron date of 42 ± 14 Ma with a high i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.7048, which indicates resetting by the 32 ± 4 Ma Cogburn Granodiorite. The WR-Pl date of 182 ± 33 Ma ( i n i t i a l 8 7 S r / 8 6 S r r a t i o of 0.7045) i s close to the WR date obtained for the Se t t l e r Schist. Re-equilibration of radiogenic strontium between b i o t i t e and some other mineral phase during resetting by the Cogburn Granodiorite has pivoted the isochron l i n e around the WR point to give a younger date. The data suggest that the body may have been intruded before the Spuzzum batholith, or they may be spurious due to 8 7 S r r e d i s t r i b u t i o n during resetting. 5. 9 Breakenridge Formation Roddick and Hutchison (1967) and Reamsbottom (1971) thought that t h i s unit contained the oldest rocks in the area based on structural p o s i t i o n in the cores of the domes, and the amount of deformation the rocks had undergone. Two samples of Breakenridge Formation gneiss were coll e c t e d from the western dome of Reamsbottom (1971) and dated by Rb-Sr and U-Pb. The composite Rb-Sr mineral isochron gave an age of 79 ± 2 Ma, so the metamorphism of the granodiorite that was deformed into gneiss (Reamsbottom 1971) and presumably the Cairn Needle Formation s c h i s t s surrounding i t ended simultaneously with metamorphism of the schists and d i o r i t e 10-15 km south in 78 the Cogburn Creek area. Zircon from CU2 was dated by U-Pb at 153 ± 87 Ma and 224 ± 180 Ma ( 2 0 7 P b / 2 0 6 P b ages). When the 2 0 6 P b / 2 3 8 U and 2 0 7 P b / 2 3 5 U ratios are plotted (Figure 5.3) the point l i e s very close to concordia at 105 ± 5 Ma. This i s a reasonable age for either intrusion of the granodiorite or i t s metamorphism to gneiss, and as with the Rb-Sr date i t i s similar to dates in the Cogburn Creek area. If i t represents metamorphism then the 2 0 7 P b / 2 0 6 P b date of 166 ± 115 Ma could represent intrusion of the granodiorite. There i s no supporting evidence for t h i s p o s s i b i l i t y , but i t i s feasible as the date i s younger than the Rb-Sr date for the Settler Schist surrounding the dome. The zircons are clear and euhedral and could have c r y s t a l l i s e d during either intrusion or high-grade regional metamorphism to s t a u r o l i t e - kyanite grade. 5.10 Discussion A summary of the timing of events based on geochronometry (this study) are given in Table 5.2. The oldest unit in the study area i s the Baird Metadiorite, which is equivalent to the Yellow Aster Complex of the North Cascade Mountains. A conclusive date was not obtained but both the Rb-Sr and zircon U-Pb (Table 5.2) suggest a Precambrian p r o t o l i t h . The rocks have been metamorphosed twice to amphibolite grade (Lowes 1972), which probably scrambled the geochronological clocks. Possible interpretations include intrusion during the Precambrian followed by metamorphism 79 Table 5.2 Summary of events i n the Cogburn Creek area based on new an a l y s e s Method Date Ma' I n t e r p r e t a t i o n B a i r d M e t a d i o r i t e (Unit 1) Rb-Sr 3.4 ± 2.4 Ga I n t r u s i o n d u r i n g Precambrian U-Pb 2 269 400 1251 ± 180 follo w e d by amp h i b o l i t e f a c i e s metamorphism at 415 and 90 Ma (Mattinson 1972) Cogburn Creek Group (Unit 3) Rb-Sr 296 ± 58 Rb-Sr 81 ± 5, 76.6 ± 1.6 S e t t l e r S c h i s t (Unit 4) U-Pb 2450 ± 230 upper i n t e r c e p t U-Pb 211 ± 3 0 lower i n t e r c e p t Rb-Sr 210 ± 27 Rb-Sr 66 ± 1.6 Rb-Sr 39 ± 4 Premetamorphic i n t r u s i v e rocks Rb-Sr 105 ± 20 Rb-Sr 80 ± 6, 74 ± 10 U-Pb 111 2 0 S P b / 2 3 8 U date Approx. time of d e p o s i t i o n End of Cretaceous metamorphism Precambrian source f o r z i r c o n s i n sediments D e p o s i t i o n of sediments n n Cretaceous metamorphism Reset by Cogburn G r a n o d i o r i t e I n t r u s i o n ? Cretaceous metamorphism Reset by Spuzzum b a t h o l i t h ? 80 Spuzzum b a t h o l i t h (Unit 5) a) Hut Creek body K-Ar 70.1 ± 5 isochron U-Pb 181 ± 7 Z 0 S P b / 2 3 a U date Rb-Sr 86 ± 4 b) S e t t l e r Creek body K-Ar 78.9 ± 2 isochron U-Pb 91.8 ± 1 2 0 S P b / 2 3 8 U date Rb-Sr 90 ± 8, 89 ± 7 Minimum i n t r u s i v e age, metamorphism Meaningless Minimum i n t r u s i v e age, metamorphism Minimum i n t r u s i v e age, metamorphism I n t r u s i o n ? Minimum i n t r u s i v e age,' metamorphism Breakenridge Formation Gneiss (Unit 7) U-Pb 105 ± 1 2 0 S P b / 2 3 8 U date E i t h e r i n t r u s i o n of g r a n o d i o r i t e or i t s metamorphism to gneiss Rb-Sr 79 ± 1.6 C o o l i n g a f t e r metamorphism to g n e i s s 1 A l l a n a l y t i c a l data and r e s u l t s are l i s t e d i n Appendix B. 2 U-Pb z i r c o n dates reported as 2 0 6 P b / 2 3 8 U , 2 0 7 P b / 2 3 5 U , 2 0 7 P b / 2 0 S P b 81 intrusion during the Precambrian followed by metamorphism at 415 and 90 Ma as proposed by Mattinson (1972) for rocks of the Yellow Aster Complex and Swakane Gneiss (see Table 5.1 for his dates). Cogburn Creek Group has been mapped previously as Chilliwack Group, based on st r u c t u r a l p o s i t i o n . The rocks at Cogburn Creek bear l i t t l e resemblance to the type section of the Chilliwack Group in the Chilliwack valle y and are u n f o s s i l i f e r o u s . L i t h o l o g i c a l c o r r e l a t i o n with Bridge River and Hozameen Groups would be more l o g i c a l but t h i s would require major re-interpretation of the regional geology. The whole rock isochron date from the Cogburn Creek Group of 296 ± 58 Ma at 0.7039 i n i t i a l 8 7 S r / 8 6 S r r a t i o represents late Paleozoic deposition of chert and p e l i t e s and extrusion of volcanic rocks. B i o t i t e ages of 74 to 89 Ma from a l l units represent the waning of the Cretaceous intrusive and metamorphic episode. The Chilliwack Group has been dated by Rb-Sr whole rock isochron at 191 ± 6 Ma at 0.7051 i n t i a l 8 7 S r / 8 6 S r r a t i o . This probably r e f l e c t s a metamorphic event, as f o s s i l s of Lower Pennsylvanian to Permian ages occur in a l l d i v i s i o n s of the Group (Monger 1970). A better c o r r e l a t i o n based on li t h o l o g y would be with the Bridge River Group, which also consists of chert and greenschist in greater abundance than p h y l l i t e and other c l a s t i c rocks. This c o r r e l a t i o n i s echoed i s o t o p i c a l l y , as can be seen" from the isotope c o r r e l a t i o n diagrams in Figure 5.6. Armstrong and others have dated the Bridge River Group (Rb-Sr isochron) at 256 ± 35 Ma at 0.7037 i n i t i a l 8 7 S r / B 6 S r r a t i o . This date i s 82 lower than expected from the spread of points, because the proportional errors assigned to the low 8 7 S r / 8 6 S r - r a t i o samples are high. Data boundaries have been drawn using the data points plotted in Figures 5.3b, d, j , k, and the d i f f e r e n t f i e l d s superimposed in Figure 5.6. The f i e l d s do overlap considerably, although isochrons for a l l three units have somewhat di f f e r e n t i n i t i a l 8 7 S r / 8 6 S r ratios and slopes. The Cogburn Creek Group and Bridge River Group data are very similar and d i f f e r e n t from Chilliwack Group. Calculations for the Cogburn Creek Group are based on a r e l a t i v e l y small number of samples, which may have biased the r e s u l t s . Since the date for the Chilliwack Group i s most probably metamorphic, the results presented here do not rule out the c o r r e l a t i o n of t h i s unit with the Cogburn Creek Group. Monger (1978) considers that the Bridge River Group is equivalent to the Hozameen Group on the east side of the Fraser Fault. No data are available from the Hozameen rocks to confirm t h i s i s o t o p i c a l l y . I propose that the Cogburn Creek Group i s c o r r e l a t i v e with the Bridge River Group, and therefore also the Hozameen Group (See Chapter 6). Settler Schist contains at least some detritus of Precambrian age, zircon 2450 ± 230 Ma old. The time of deposition was most probably around 210 ± 27 Ma (Rb-Sr WR isochron). The schist has undergone contact and then regional metamorphism. Contact metamorphism was related to intrusion of two bodies of Spuzzum batholith at around 95 to 110 Ma. B i o t i t e K-Ar and Rb-Sr dates from schists and d i o r i t e in the range 80-60 Ma indicate cooling after regional metamorphism. K-Ar isochron 83 dates from hornblende in the two d i o r i t e bodies, 70.1 t 5 Ma and 78.9 ± 2 Ma, are minimum values for time of intrusion. Rb-Sr WR dates and zircon upper intercept dates for Spuzzum batholith are too old and not v a l i d , probably due to assimilation of country rock during intrusion. Settler Schist has been correlated with Chiwaukum Schist of the Skagit Metamorphic Complex, based on s i m i l a r i t i e s in lithology and metamorphic hist o r y . Outcrops of Chiwaukum Schist that I have v i s i t e d in the Stevens Pass area look very similar to the Settler Schist. The same unusual rock types and metamorphic mineral assemblages are present. Foliated f e l s i c dykes and metatrondhjemites are common in both units, as are ultramafic pods. Both the Mount Stuart batholith and the Spuzzum batholith were intruded before the end of regional metamorphism. The Chiwaukum Schist and Shuksan Suite are not related, and are separated by faults and metamorphic facies differences. Figure 2.2 shows the present geographical locations of the Settler Schist and the Chiwaukum Schist. Chiwaukum Schist has not yet been dated. Babcock et a l . (1985) have found that Rb-Sr isotopic systematics of the Skagit Gneiss and Cascade River Schist show broad dispersion of data with no clear age, but a very strong overprint of Mesozoic and Early T e r t i a r y r e - e q u i l i b r a t i o n . Clear isotopic c o r r e l a t i o n i s not possible. Based on structural position with respect to the Shuksan thrust and comparison with the North Cascades in Washington, a possible alternative c o r r e l a t i v e of the Settler Schist i s the 84 Darrington P h y l l i t e of the Shuksan Metamorphic Suite. Where I have seen the Shuksan Suite in the North Cascade Mountains, the rock appears completely d i f f e r e n t from the S e t t l e r Schist. Their metamorphic history i s also d i f f e r e n t . Evidence points to the possible correlation of the two units being u n j u s t i f i e d . Geochronometry on the Darrington P h y l l i t e gives 132 ± 8 Ma (Rb- Sr WR, Figure 5.2m, Armstrong unpub.), which i s far younger than deposition of the p r o t o l i t h of the Set t l e r S c h i s t . Dates from blueschist and greenschist in the Shuksan Suite demonstrate a Late Jurassic to late Early Cretaceous metamorphic age (Figure 5.2m, Armstrong et a l . 1983), also d i s s i m i l a r . The data-fields in the c o r r e l a t i o n diagram (Figure 5.6) show an isotopic s i m i l a r i t y between the S e t t l e r Schist and Chilliwack Group, although that i s probably quite coincidental. Sampling of the Chilliwack Group may not have been representative, and could have included Mesozoic p e l i t e from a melange (Armstrong pers. comm. 1985). The Cogburn Creek Group and S e t t l e r Schist were juxtaposed before intrusion of Spuzzum batholith. A small body of d i o r i t e intruded into the imbricate zone before the end of metamorphism gave a mineral isochron date of 77 ± 3 Ma, a metamorphic cooling age. The imbricate zone appears to have been folded and possibly pushed aside at the western margin of the Spuzzum batholith in the Cogburn Creek area. Some l a t e s t r a i n is indicated by f o l i a t i o n of the small d i o r i t e body. Lowes (1972) correlated the imbricate zone north of the Fraser River with the Shuksan thrust of the North Cascades, 85 Washington. Misch (1966) considered the Shuksan thrust to be post-metamorphic and mid-Cretaceous in age. In the Cogburn Creek area the imbricate zone pre-dates the peak of regional metamorphism. In the Mount Watson area Armstrong et a l . (1980) dated metamorphism in blueschist facies rocks as 120 to 130 Ma, which i s e a r l i e r than regional metamorphism in Cogburn Creek. If the imbricate zone in Cogburn Creek i s the Shuksan thrust then the juxtaposition of the Cogburn Creek Group and Settler Schist can be bracketed between blueschist metamorphism of the Shuksan Suite at 120 to 130 Ma and d i o r i t e intrusion at 100 ± 10 Ma. Current studies in the North Cascades, Washington (Silverberg 1985, Leiggi and Brown 1985) show that the Shuksan thrust changes in style and amount of deformation along i t s length. Movement may die out northwards, as Reamsbottom (1974) recognised no major structural break across schists in the Mount Breakenridge area. The northern l i m i t of movement may in fact be at Cogburn Creek. This would explain the sudden change in li t h o l o g y across the Cogburn Creek v a l l e y , from ultramafic rocks to the south to Cogburn Creek Group chert to the north (Figure 2.1). A l l v a l i d dates from the Spuzzum batholith are Cretaceous (Table 5.2, Appendix B). The d i o r i t e intruded during regional metamorphism (Bartholomew 1979); thus the zircon U-Pb dates of 110 ± 5 Ma (Hut Creek body, lower intercept of chord through QDHC and S e t t l , Sett2) and 91.8 ± 0.5 Ma (Settler Creek body, 2 0 6 P b / 2 3 8 U date) are minimum intrusive ages (Table 5.2). Rb-Sr and K-Ar isochron dates of 66 Ma to 88 Ma r e f l e c t cooling 86 through blocking temperatures during waning of regional metamorphism. On a regional scale, dates from the southern part of the Spuzzum batholith around the Fraser Valley are older than those from in and north of the study area (Table 5.1, Figure 5.1). A concentration-isochron for a l l available Hb-Bi data from Spuzzum batholith has been plotted in Figure 5.7. Slope dates range from 75 to 97 Ma for individual sample pairs, with a calculated mean of 83 ± 5 Ma. Bartholomew (1979) found a trend of dates younging eastward towards the Fraser Fault zone (Figure 5.7). He considered that i t represented greater u p l i f t , erosion and later cooling through the argon blocking temperature approaching the f a u l t , due to v e r t i c a l movement on the f a u l t . However, t h i s implies t i l t i n g in the opposite sense to that suggested by paleomagnetic measurements. Evidence from the North Cascade Mountains indicates regional t i l t i n g down to the southeast of around 30° (Beck 1982), and Irving et a l . (1985) note that their data from the Spuzzum batholith could be interpreted as 28° t i l t down to the southeast. This would give r i s e to the northward younging trend of dates from the Spuzzum batholith that can be seen in Figure 5.1, and to the increase in metamorphic grade northwards. Breakenridge Formation gra n o d i o r i t i c gneiss gave a zircon 2 0 6 P b / 2 3 8 U date of 105 ± 1 Ma and Rb-Sr mineral isochron date of 79 ± 1.6 Ma, similar to dates from the Spuzzum batholith. The granodiorite i s highly deformed, so these dates may represent either intense metamorphism of an older rock or synkinematic intrusion. 87 The latest event was intrusion of small plutons of granodiorite. One of these, the Cogburn Granodiorite, has been dated at 32 ± 4 Ma (Bartholomew 1979) and l o c a l l y has i s o t o p i c a l l y reset nearby sc h i s t s . 88 0.7031 0.05 -I L 0.1 8 7 R b / 8 6 S r Figure 5.2a Rb-Sr isochron plot for Baird Metadiorite A l l analyses are WR (whole rock). 89 Rb/ 8 6 Sr Figure 5.2b Rb-Sr isochron plot for Cogburn Creek Group Lines represent 1) WR and Pi data excluding chert 2) chert HL37a WR-Bi pair 3) metavolcanics and p h y l l i t e Inset shows points near the intercepts, expanded scale, also i n i t i a l 8 7 S r / 8 6 S r r a t i o s . 90 Figure 5.2c Rb-Sr isochron plot for Settler Schist Lines represent 1) a l l data, not including Bi and Mu 2) WR-Pl-Bi isochron, p h y l l i t e SS82 3) WR-Pl-Bi isochron for quartz-biotite-garnet schist marginal to Cogburn Granodiorite (SS128). 91 0.709h 0.707^ co (O 00 CO oo 0.705r 0.703 8 7 R b / 8 6 S r Figure 5 . 2d S e t t l e r Schist, ..expanded scale Figure 5.2e Rb-Sr isochron plot for premetamorphic intrusive rocks. Lines represent 1) WR-Pl-Bi, f e l s i c s i l l HL111 intruding Cogburn Creek Group. 2 + 3 ) f o l i a t e d f e l s i c garnet-cluster dyke, SS85, intruding Settler Schist. 2) not including Bi 3) including Bi 93 0.715 FOLIATED DIORITE in fault zone 0.710 CO CO 1 1 £ CO CO CO 0.705h ^ 0.7040 0.700 8 7 R b / 8 6 S r + WR • PI A Bi T Mu 6 Figure 5.2f Rb-Sr isochron plot for small body of f o l i a t e d granodiorite in imbricate zone (SD92). • WR • PI • Hb Hut Creek Body A B j SPUZZUM B A T H O L I T H 0 0.2 0.4 10 8 7 R b / 8 6 S r 15 20 Figure 5.2g Rb-Sr isochron plot for Spuzzum batholith, Hut Creek body Lines represent 1) a l l data not including Bi 2) a l l data including Bi Inset shows points near intercepts on expanded scale. 95 0.730 0.720 0.7044 0.7042 0.7040 0.7031 CO CO co CO co <0.7041 >-0.7039 0.7038 0.710 SPUZZUM B A T H O L I T H Settler Creek Body + WR o PI • Bi I I Hb 0.1 0.2 10 15 20 8 7 R b / 8 6 S r Figure 5.2h Rb-Sr isochron plot for Spuzzum batholith, S e t t l e r Creek body Lines represent 1) WR-Pl isochron for a l l samples except SD97 2) WR-Pl-Hb isochron for a l l samples including SD97 3) isochron for a l l data including Bi Inset shows points near intercepts on expanded scale. 96 0.760K 0.740H 0 0.2 0.4 8 7 R b / 8 6 S r Figure 5.2i Rb-Sr isochron plot for agmatitic quartz d i o r i t e (SD14), marginal to Cogburn Granodiorite. Inset shows points near intercepts on expanded scale. 97 Figure 5.2j Rb-Sr isochron plot for Breakenridge Formation gneiss .98 0.730 0.720 CO CO 00 • 0.710 0.708 0.706 0.704 CHILLIWACK GROUP 0.700 expanded scale 8 7 R b / 8 6 S r Figure 5.2k Rb-Sr isochron plot for Chilliwack Group sediments. Data from Armstrong and others (unpub.). A l l analyses are WR Figure 5.21 Rb-Sr isochron plot for Bridge River Group. Data from Armstrong and others (unpub.). Lines represent 1) a l l data 2) high grade schists 100 Figure 5.2m Rb-Sr isochron plot for Shuksan Suite blueschist and greenschist, and Darrington P h y l l i t e . Data from Armstrong and others (unpub.). 101 0.4 2 0 7 P b / 2 3 5 U Figure 5.3 U-Pb concordia diagram for zircon dating. Plot of 2 0 6 P b / 2 3 8 U v. 2 0 7 P b / 2 3 5 U for a l l zircon samples. Line 1) i s for Settler Schist, 2) i s a l l samples combined. Sample numbers as l i s t e d in Appendix C. i r separates from Spuzzum batholith. Lines represent 1) Hut Creek body. Slope gives 70.1 ± 5 Ma Intercept 392 ± 25 2) Settler Creek body. Slope gives 78.9 ± 2 Ma. Intercept 388 ± 88 Figure 5.4b Plot of %K v. fl0Ar nl/g for hornblende separates from Spuzzum batholith. Lines represent 1) Hut Creek body. Slope gives 65.7 ± 5 Ma Intercept i n i t i a l a o A r 0.185 0.016 nl/g 2.) Settler Creek body. Slope gives 79.7 ± 2 Ma Intercept i n i t i a l 4 0 A r 0.139 0.010 nl/g 103 Figure 5.5 U-Pb concordia diagram for Yellow Aster Complex, Skagit Gneiss and Swakane Gneiss, from Mattinson (1972). Plot of 2 0 6 P b / 2 3 8 U v. 2 0 7 P b / 2 3 5 U for zircon samples. 1 04 0.710r 0.708r 8 7Sr/ 8 6Sr| 0.706 0.704k C o t t i e r C ^ m * » 210±27Ma i_ j s e t t l e r S c h i s t eo.7043 Chi l l iwack G r o u p Vô osi* l i ~ . . . 2 9 6 ± 5 8 M a i i C o g b u r n C k G r o u p •0.7039 • B r i d g e River G r o u p  24 6fg| M a S R I S O T O P I C C O M P A R I S O N S 87, R b / 8 6 S r F i g u r e 5 . 6 D a t a - f i e l d d i a g r a m f o r Rb-Sr a n a l y s e s f r o m p o s s i b l e c o r r e l a t i v e s t r a t i g r a p h i c u n i t s . 105" • . . i i i i %K Figure 5.7 Plot of %K v. 4 0 A r nl/g for regional data from Spuzzum batholith, not including t h i s study. Lines represent 1) lower l i m i t , slope gives 76.0 ± 5 Ma 2) calculated mean, slope gives 83.1 ± 5 Ma 3) upper l i m i t , slope gives 97.5 ± 5 Ma Lower and upper l i m i t isochron dates represent Hb- Bi pairs from single samples; mean i s for a l l data. Conventional K-Ar dates are l i s t e d in Table 5.1. 106 West 14 12 10 DISTANCE, km 8 East Figure 5.8 Graph of eastward younging trend of K-Ar dates from Spuzzum batholith, from Bartholomew (1979). K-Ar date versus perpendicular distance from a N-S g r i d l i n e on NTS 92/H, approximately 121° 20'. Boxes indicate age and distance error margins. Stippled boxes are averaged hornblende- b i o t i t e p a i r s . 1 07 6. Regional Synthesis The region between the Fraser River, Harrison Lake, and the Canada-United States Border i s geologically complex. Monger and Berg (1984) separate the Intermontane superterrane (comprising Quesnel, Cache Creek and Stikine terranes) and the Insular superterrane (Wrangellia and Alexander terranes) with f i v e small terranes (Figure 6.1). They are (from Monger 1985): 1) Methow; T r i a s s i c ( ? ) basalt overlain by Jurassic and Cretaceous c l a s t i c sediments, 2) Bridge River-Hozameen; Permian to mid-Jurassic disrupted basalt, chert, c l a s t i c and ultramafic rocks, 3) Cadwallader; T r i a s s i c basalt, c l a s t i c and carbonate rocks, Jurassic and Cretaceous c l a s t i c rocks, 4) Shuksan; Jurassic(?) basalt and c l a s t i c rocks, 5) Chilliwack-Nooksack; late Paleozoic volcanic, c l a s t i c and carbonate rocks, Mesozoic c l a s t i c and volcanic rocks. He has restored these terranes to their pre-Tertiary configuration by removing 80 km of dextral motion on the Fraser- Straight Creek fa u l t zone (Figure 6.2a). Timing of movement on these f a u l t s has been bracketed between Cretaceous and around 35 Ma (Monger 1985), and possibly a l l within the Eocene. Monger considers that the Skagit Metamorphic Suite may be derived largely from rocks of the Methow, Bridge River and Cadwallader terranes. This may be true for the Custer Gneiss but i t i s not for the S e t t l e r Schist and Chiwaukum Schist, which are l i t h o l o g i c a l l y d i f f e r e n t from the rocks in those terranes. 108 k i l o m e t r e s 0 5 0 ± 1 0 0 P r e s e n t d a y c o n f i g u r a t i o n B r i d g e R i v e r - H o z a m e e n t e r r a n e B R , H c C G ss&d." M e t h o w - T y a u g h t o n t e r r a n e M T • vw\ C a d w a l l a d e r t e r r a n e C • • S h u k s a n t e r r a n e S h h=Z-I-_i N o Q k s a c K t e r r a n e N S k a g i t S u i t e S k Figure 6.1 Present day configuration of tectono-stratigraphic terranes, modified from Monger and Berg (1984). 109 S h igure 6.2 Reconstruction of Bridge River and Methow terranes, using Monger and Berg (1984) and Monger (1985). 1 10 It i s also d i f f i c u l t to put them in a suitable time and space configuration for the derivation. Figures 6.3a and b show post- Cretaceous, pre-Eocene, restoration of the Settler Schist and Chiwaukum Schist based on a) 150 km offset on the Fraser- Straight Creek f a u l t s , from Misch (1977), and b) 80 km o f f s e t , from Monger (1985). Figure 6.3a restores the two units to a reasonable configuration, but Bridge River and Methow terranes are disrupted (Figure 6.4a). Figure 6.3b leaves the schists separated by about 80 km. If the 80 km offset i s correct, then there must have been at least 80 km of dextral movement on another, p a r a l l e l , fault system in order to separate the Settler Schist and Chiwaukum Schist by a t o t a l of 150 km. There i s a dilemma here, as the f a u l t reversal displacements proposed by Kleinspehn (1985) for the Tyaughton- Methow basin appear to be too great when applied to the most recent terrane map (Monger and Berg 1985). She proposed 110 km of o f f s e t on the Fraser-Straight Creek f a u l t s (Figure 6.4b) preceded by 150 km of movement on the Yalakom-Ross Lake Fault. More reasonable values for reconstruction of the Bridge River Group and Tyaughton-Methow basin are 80 km on the Fraser fault and around 100 km on the Yalakom-Ross Lake f a u l t . Movement on the Ross Lake fa u l t probably had a f a i r l y large orthogonal component as well as dextral s l i p , allowing for greater t o t a l displacement, but t h i s does not a f f e c t the r e l a t i v e positions of the S e t t l e r Schist and Chiwaukum Schist. Monger (pers. comm. 1985) surmises that the dilemma can be solved by having a stack of horizontal plates involved in the r i g h t - l a t e r a l 111 Figure 6.3 Reconstruction of Settle.r Schist-Chiwaukum Schist using a) Misch (1977); b) Monger (1985). Base map i s Figure 1.3. 1 12 Figure 6.4 Disrupted reconstruction of. Bridge River and Methow terranes using a) 150 km displacement, after Misch (1977); b) 110 km displacement, after Kleinspehn (1985). 1 13 displacement. By invoking d i f f e r e n t i a l movement down through the stack i t is possible to reconcile 150 km displacement for the Settler Schist-Chiwaukum Schist with 80 km displacement for the Bridge River-Kozameen Groups. Monger (1985) notes that post-Cretaceous deformations have masked the boundaries of the small terranes, so that their relationships are not clear. He considers that evolution of the small terranes records closure of oceanic and/or marginal basins between the superterranes. The closure may have been by orthogonal, transcurrent, or transpressive movement. Isotopic and geological evidence point to c o r r e l a t i o n of the Cogburn Creek Group with the Bridge River Group and Hozameen Group. Undoing documented fault movements (Figure 6.2 and 4) does not juxtapose them, but there are possible explanations. The f i r s t and simplest is that the Cogburn Creek Group never was continuous with the Bridge River Group, but is a sequence of similar sediments and volcanic rocks deposited at the same time from the same source in a nearby basin. The second i s that disruption of the Bridge River Group-Cogburn Creek Group Basin by f a u l t i n g such as the Shuksan thrust, combined with intrusion of voluminous d i o r i t e plutons, has separated the two units. The Shuksan thrust has brought in the Settler Schist and imbricate s l i c e s of older units and ultramafic rocks from the southeast to ove r l i e the Cogburn Creek Group. Timing of movement on the Shuksan Thrust i s bracketed to Albian by isotopic dating of regional blueschist metamorphism in the Shuksan Metamorphic Suite and intrusion of Spuzzum 1 1 4 batholith. Hence the Shuksan thrust was active before dextral motion on the other f a u l t s . Movement on the Shuksan thrust must have died out northwards, ending just north of Cogburn Creek, as Reamsbottom (1971, 1974) did not fi n d a major break between the "Chilliwack" Group (Cogburn Creek Group) and Cairn Needle Formation (Settler S c h i s t ) . The geometry involved in reconstruction of the Cogburn Creek Group-Bridge River Group as one continuous unit i s d i f f i c u l t , because the Cogburn Creek and Bridge River Groups now form p a r a l l e l belts (Figure 6.1). However, i t i s not impossible, because they are separated by a wide belt of late Cretaceous and Ter t i a r y i n t r u s i v e rocks that must have pushed aside the country rock. Not enough evidence is available to choose between the alternate relationships; however, the f i r s t (that the two units were never continuous) is simpler. Perhaps the Cogburn Creek Group was deposited in an intermediate position between the Bridge River Group and the Cadwallader terrane, which i s a volcanic arc. The s t r u c t u r a l position of the Se t t l e r Schist, overlying the Shuksan thrust, i s similar to that of the Darrington P h y l l i t e . However, the Darrington P h y l l i t e has been dated as 128 + 6 Ma (Armstrong unpub. data), thus at face value i s considerably younger than the Se t t l e r Schist. The s i m i l a r i t y in str u c t u r a l position may be purely coin c i d e n t a l , as a result of complex movements on various fault systems. 115 Imbricate zone, Shuksan Thrust, shown on saddle east of S e t t l e r Lake. Steeply dipping s l i c e s of ultramafic rock between Baird Metadiorite ( l e f t ) and S e t t l e r Schist (grassy slope on r i g h t ) . Mafic amphibolite pod in Settler Schist. Dark rim containing large hornblende c r y s t a l s , surrounding f e l s i c core. 116 Plate 2.3 Photomicrograph of sample SD101. Photo represents 10 mm in section. Hornblende with brown igneous cores and green metamorphic rims. Plate 2.4 Photomicrograph of sample SD66. Photo represents 10 mm in section. Alteration of hornblende during metamorphism, in the hornblende-hypersthene gabbro. 117 Plate 3.1 Photomicrograph of sample HL30. Photo represents 10 mm in section. Chromite and o l i v i n e r e c r y s t a l l i s e d during metamorphism, al t e r e d to c a l c i t e and tremolite. Crossed n i c o l s . Plate 3.2 Photomicrograph of sample HL16. Photo represents 10 mm in section. B i o t i t e porphyroblasts in quartz b i o t i t e schist. Crossed n i c o l s . P l a t e 3.3 P h o t o m i c r o g r a p h of sample HL15. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Garnet w i t h r o t a t e d c o r e and p o s t - t e c t o n i c r i m . C r o s s e d n i c o l s . P l a t e 3.4 P h o t o m i c r o g r a p h of sample HL80. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Coarse h o r n b l e n d e c r y s t a l l i s e d a l o n g an f 2 f o l d a x i s , f i n e h o r n b l e n d e randomly r e - o r i e n t e d . 119 Plate 3.5 Photomicrograph of sample HL142. Photo represents 10 mm in section. Porphyroblastic green hornblende (sides of photo) containing f 2 and f 3 . St a u r o l i t e porphyroblasts in centre, epidote with b i o t i t e . Plate 3.6 Photomicrograph of sample SS110. Photo represents 10 mm in section. Deformed pseudochiastolite with random internal texture and s t a u r o l i t e crossing the boundary. Note s i l l i m a n i t e in lower right corner. 120 . ̂, c o o s Plate 3.7 Photomicrograph of sample SS135. Photo represents 10 mm in section. B i o t i t e c r y s t a l l i s e d along the a x i a l plane of a tight f 2 f o l d , r e c r y s t a l l i s e d around the f 3 kink f o l d . Plate 3.8 Photomicrograph of sample SS66. Photo represents 10 mm in section. Rounded garnet with quartz, muscovite, f i b r o l i t e halo, from s i l l i m a n i t e zone. Cracks f i l l e d with b i o t i t e . 121 Plate 3.10 Photomicrograph of sample SS114. Photo represents 10 mm in section. Twinned s t a u r o l i t e , zoned with graphite. 122 Plate 3.12 S i l l i m a n i t e porphyroblasts from ridge north of Cogburn Creek. Location 49° 35.3'N 121° 39'W. 123 Plate 3.13 Photomicrograph of s i l l i m a n i t e porphyroblast. Photo represents 10 mm in section. F i b r o l i t e patches (fuzzy) and quartz inclusions, rounded garnet in quartz reaction rim. 124 Plate 4.1 Fault contact below the imbricate zone. Looking north, at 49° 38.5'N 121° 31'W. Cogburn Creek Group chert ( l e f t ) below Baird Metadiorite (forms peak). 1 25 REFERENCES Armstrong, R.L. 1980. Geochronometry of the Shuksan Metamorphic Suite, North Cascades, Washington. Geological Society of America, Abstracts with Programs, 12, p. 94. 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Geological Society of America, B u l l e t i n , 83, p. 3769-3784. McLeod, J.A. 1975. The Giant Mascot Ultramafite and i t s related ores. Unpub. M.Sc. thesis, University of B r i t i s h Columbia, Vancouver, 123 p. McTaggart, K.C. 1970. Tectonic history of the Northern Cascade Mountains. ijn Wheeler, J.O. (ed.). Structure of the Southern Canadian C o r d i l l e r a . Geological Association of Canada Special Paper 6, p. 137-148. McTaggart, K.C. and Thompson, R.M. 1967. Geology of part of the Northern Cascades in southern B r i t i s h Columbia. Canadian Journal of Earth Sciences, 4, p. 1199-1228. M i l l e r , R.B. and Vance, J.A. 1981. Movement history of the Straight Creek Fault. Geological Society of America, Cordilleran Section Abstracts with Program, p. 97. Misch, P. 1966. Tectonic evolution of the Northern Cascades of Washington State. Canadian Institute of Mining and Metallergy, Special Volume 8, p. 101-148. Misch, P. 1977. Dextral displacements at some major st r i k e f a u l t s in the North Cascades. Geological Association of Canada, Abstracts with Program, p. 37. 1 30 Monger, J.W.H. 1966. The stratigraphy and structure of the type area of the Chilliwack Group, southwestern B.C. Unpub. PhD thesis, University of B r i t i s h Columbia, Vancouver, 173 p. Monger, J.W.H. 1970. Hope map area, west half, B r i t i s h Columbia. Geological Association of Canada Paper 69-47, 75 p. Monger, J.W.H. 1985. Terranes in the southeastern Coast Plutonic complex and Cascade fold b e lt. Geological Society of America, Absracts with Programs, 17, p. 371. Monger, J.W.H. and Berg, H.C. 1984. Lithotectonic terrane maps of the North American C o r d i l l e r a . Part B-Lithotectonic terrane map of Western Canada and Southeastern Alaska. U.S. Geological Survey Open F i l e Report 84-523. Parrish, R. 1982. U-Pb zircon, common Pb and f i s s i o n track geochronology: Procedures at the Geochronology Laboratory, UBC. Part 1. U-Pb zircon dating. University of B r i t i s h Columbia, Vancouver, internal report. Pigage, L.C. 1973. Metamorphism southwest of Yale, B r i t i s h Columbia. Unpub. M.Sc. thesis, University of B r i t i s h Columbia, Vancouver, 95 p. Pigage, L.C. 1976. Metamorphism of the Settler Schist, southwest of Yale, B r i t i s h Columbia. Canadian Journal of Earth Sciences, 13, p. 405-421. Pigage, L.C. and Greenwood, H.J. 1982. Internally consistent estimates of pressure and temperature: the s t a u r o l i t e problem. American Journal of Science, 282, p. 943-969. 131 Plummer, C.C. 1969. Geology of the c r y s t a l l i n e rocks, Chiwaukum Mountains and v i c i n i t y . Unpub. M.Sc. thesis, University of Washington, Seattle, 137 p. Plummer, C.C. 1980. Dynamothermal contact metamorphism superposed on regional metamorphism in the p e l i t i c rocks of the Chiwaukum Mountains area, Washington Cascades. Geological Society of America, B u l l e t i n , 91, p. 1627-1668. Read, P.B. 1960. Geology of the Fraser Valley between Hope and Emory Creek, B r i t i s h Columbia. Unpub. MSc. thesis, University of B r i t i s h Columbia, Vancouver, 145 p. Reamsbottom, S.B. 1971. The geology of the Mount Breakenridge area, Harrison Lake, B.C. Unpub. M.Sc. thesis, University of B r i t i s h Columbia, Vancouver, 161 p. Reamsbottom, S.B. 1974. Geology and metamorphism of the Mount Breakenridge area, Harrison Lake, B.C. Unpub. PhD. thesis, University of B r i t i s h Columbia, Vancouver, 155 p. Richards, T. 1971. Plutonic rocks between Hope, B.C., and the 49th P a r a l l e l . Unpub. Ph.D. thesis, University of B r i t i s h Columbia, Vancouver, 178 p. Richardson, S.W. 1968. Staurolite s t a b i l i t y in a part of the system Fe-Al-Si-O-H. Journal of Petrology, 9, p. 467-488. Richardson, S.W., G i l b e r t , M.C. and B e l l , P.M. 1969. Experimental determination of kyanite-andalusite and an d a l u s i t e - s i l l i m a n i t e e q u i l i b r i a ; the aluminium s i l i c a t e t r i p l e point. American Journal of Science, 267, p. 259- 272. Roddick, J.A. and Hutchison, W.W. 1969. Northwestern part of 1 32 the Hope map-area, B.C. (92H W/2): in Report of a c t i v i t i e s , A p r i l to October 1968; Geological Survey of Canada Paper 69-1A, p. 29-38. Silverberg, D.S. 1985. The Shuksan Fault in the Whitechuck Mountain - Mount Pugh area, North Cascades, Washington. Geological Society of America, Abstracts with Programs, 17, p. 408. Stacey, S.J. and Kramers, J.D. 1975. Approximation of t e r r e s t r i a l lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, p. 207-221. Steiger, R.H. and Jager, E. 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, p. 359-362. Vining, M.R. 1977. The Spuzzum Pluton northwest of Hope, B.C. Unpub. M.Sc. thesis, University of B r i t i s h Columbia, Vancouver, 147 p. Wanless, R.K., Stevens, R.D., Lachance, G.R. and Delabio, R.N. 1973. Age determinations and geological studies: K- Ar isotopic ages, report 11. Geological Survey of Canada Paper 73-2. Wasserburg, G.J., Albee, A.L. and Lanphere, M.A. 1964. Migration of radiogenic strontium during metamorphism. Journal of Geophysical Research, 69, p. 4395-4401. York, D. 1967. The best isochron. Earth and Planetary Science Letters, 2, p. 479- York, D. 1969. Least squares f i t t i n g of a straight l i n e with 1 33 correlated errors. Earth and Planetary Science Letters 5, p. 320-324. 134 Appendix A. Isotopic Dating; A n a l y t i c a l Methods Isotopic dating using K-Ar, Rb-Sr and U-Pb zircon methods was car r i e d out at the University of B r i t i s h Columbia, with advice and assistance from R.L. Armstrong, Kr i s t a Scott, Joe Harakal, Randy Parrish and Peter Van der Heyden. An a l y t i c a l techniques are as follows: K-Ar Potassium analyses are carried out in duplicate by atomic absorption, using a Techtron AA4 spectrophotometer. Argon i s measured by isotopic d i l u t i o n with a high purity 3 B A r spike in an AEI MS-10 mass spectrometer. Reported errors are one standard deviation. The constants are l i s t e d in Table B-3. Rb-Sr Determination of Rb and Sr concentrations i s by replicate analysis of pressed powder p e l l e t s using X-Ray fluorescence. Mass absorption c o e f f i c i e n t s are obtained from Mo Ka Compton scattering measurements, and U.S. Geological Survey rock standards are used for c a l i b r a t i o n . Error on the 8 7 R b / 8 6 S r ratios i s 2 % (1 a) where Rb and Sr ppm are >50, and proportional to the reciprocal of the smaller value otherwise. Sr isotopic composition i s measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer i s a Micromass 54-R. Data ac q u i s i t i o n i s d i g i t i s e d and automated using a Hewlett Packard HP-85 computer. Experimental data have been normalised to an 8 7 S r / 8 6 S r r a t i o of 0.1194 and adjusted so that the NBS standard SrC03 (SRM 987) gives an 8 7 S r / 8 6 S r ratio of 0.71020 ± 2 and the Eimer and Amend 1 35 Sr a r a t i o of 0.70800 ± 2. The precision of a single 8 7 S r / 8 6 S r ra t i o i s 0.00010 (1 a). Decay constants for age calculations are in Table B-2. The regressions are calculated according to the technique of York (1967). U-Pb Zircon was separated from f i n e l y crushed 20 to 40 kg rock samples using wet shaking table, heavy l i q u i d , and a magnetic separator. They were acid washed in strong aqua regia and hand picked as required. Cleaned zircon separate was weighed, dissolved in acid, s p l i t for spiked and isotope composition runs, then put through ion exchange columns to separate the lead and uranium, using the procedure of Krogh (1973). We use a mixed 2 o s p j 3 _ 2 3 5 r j S p i k e . Samples were analysed using single Re filaments loaded with and s i l i c a gel on a V.G. Isomass 54R mass spectrometer. Data a c q u i s i t i o n and reduction are c a r r i e d out on a Hewlett Packard HP-85 computer. The mass spectrometer i s calibr a t e d using the NBS 981 standard. The system blank has composition: 6/4: 17.75, 7/4: 15.57, 8/4: 37.00. U-Pb date errors (la) are obtained by i n d i v i d u a l l y propagating a l l c a l i b r a t i o n and a n a l y t i c a l uncertainties through the entire date ca l c u l a t i o n program and summing a l l the individual contributions to the t o t a l variance. 1 36 A p p e n d i x B. I s o t o p i c D a t i n g : A n a l y t i c a l d a t a and d a t e s . Table B-1. Rb-Sr a n a l y t i c a l data Sample Rock D e s c r i p t i o n Sr Rb ' 7Rb l[Sr Number ppm ppm 8 6 S r 8 6 S r B a i r d M e t a d i o r i t e MD1 WR h i g h l y f o l i a t e d , green and 493 1 .4 0 .008 0 .7040 MD3 WR white, metamorphosed 1 35 0 .8 0 .016 0 .7045 MD6 WR d i o r i t e and gabbro 179 0 .3 0 .004 0 .7043 MD8 WR 145 0 .6 0 .012 0 .7045 MD9 WR 175 0 .4 0 .006 0 .7040 0.7043 Cogburn Creek Group HL37a WR miceous r e c r y s t a l l i s e d 18.1 24 .6 3 .94 0 .71 35 Bi r i b b o n c h e r t 16.7 252 43 .92 0 .7594 • 0 .7599 HL76 WR knobbly c h l o r i t i c green- 349 8 .4 0 .070 0 .7044 0 .7059 PI s c h i s t , coarse metavolc. 268 2 .4 0 .025 0 .7037 Hb 55 3 . 1 0 .161 0 .7041 HL97 WR micaceous q u a r t z i t e 141 58 .3 1 .20 0 .7082 HL103 WR grey p h y l l i t e 386 35 .2 0 .264 0 .7048 HL125 WR f i n e g r a i n e d b i o t i t e - 209 41 .7 0 .578 0 .7073 PI a c t i n o l i t e g r e e n s c h i s t , 483 1 .8 0 .011 0 .7068 12.5 139 32 .37 0 .7420 S e t t l e r S c h i s t S e t t l WR g r a p h i t i c p h y l l i t e 224 71 . 3 0.923 0.7074 Sett2 WR " 284 70. 7 0.720 0.7070 SS82 WR 314 76. 3 0.703 0.7071 PI 73.5 4.7 0.185 0.7066 1 37 • • • • B i 61 . 2 238 1 1 , .26 0. ,7170 SS109 WR g r a p h i t i c p h y l l i t e 1 50 52, .8 1 , .021 0. ,7072 SSI 14 WR 164 81 , .2 1 .436 0, .7088 SS128 WR q u a r t z - b i o t i t e - g a r n e t 1 72 73, .7 1 .24 0, .7075 PI s c h i s t , marginal to 285 4, .2 0 .043 0 .7065 Cogburn G r a n o d i o r i t e 23. 6 252 30 .95 0 .7234 SS130 WR g r a p h i t i c p h y l l i t e / b l a c k 356 28 .9 0 .235 0 .7065 s l a t e 0 .7062 SSI 32 WR bla c k s l a t e 1 57 98 .8 1 .824 0 .7090 SS135 WR g r a p h i t i c p h y l l i t e 331 65 .4 0 .571 0 .7070 SS143 WR coarse sandy b i s c h i s t 370 30 .4 0 .24 0 .7045 PRB 1 1 7 p e l i t i c s c h i s t 317 73 .5 0 .672 0 .7065 PRB 173 n n 1 44 41 .2 0 .830 0 .7064 PRB 72 n n 421 43 .8 0 .301 0 .7055 PRB 172 n n 276 46 .6 0 .489 0 .7054 PRB 122B q u a r t z o f e l s p a t h i c s c h i s t 640 5 .3 0 .024 0 .7038 PRB 90 p e l i t i c s c h i s t 204 80 .7 1 . 1 45 0 .7079 PRB 80 it i? 213 80 .2 1 .089 0 .7068 0 .7066 PRB samples from Bartholomew (1979) S i l l s i n S c h i s t s HL111 WR f o l i a t e d f e l s i c dyke 356 39 .3 0 .319 0 .7040 , . PI f l e c k e d with b i o t i t e , near 352 25 . 1 0 .206 0 .7043 Bi c o n t a c t with Spuzzum D. 22 .2 331 43 .3 0 .7493 SS85 WR f o l i a t e d f e l s i c garnet- 368 22 . 1 0 .174 0 .7041 PI c l u s t e r dyke i n t r u d i n g 367 2 .9 0 .023 0 .7036 .... Mu S e t t l e r S c h i s t 250 83 .0 0 .96 0 .7051 • • • • B l 1 20 209 5 .06 0 .7095 138 F o l i a t e d G r a n o d i o r i t e , small body i n imbricate zone SD92 WR PI .... Mu .... B i medium grained f o l i a t e d 390 gra n o d i o r i t e with b i o t i t e , 615 muscovite, garnets 155 94.4 26.9 5.0 108 210 0.199 0.024 2.02 6.43 0.7042 0.7040 0.7064 0.7109 Spuzzum D i o r i t e , Hut Creek body SD36 WR quartz d i o r i t e 724 1 . 4 0. 005 0.7037 PI 1 1 58 0. 9 0. 002 0.7036 SD66 WR hornblende-hypersthene gabbro 456 0. 4 0. 003 0.7036! SD67 WR 650 0. 8 0. 004 0.7038 Hb 1 1 1 0. 9 0. 024 0.7040 SD101WR quartz d i o r i t e 691 2. 3 0. 009 0.7036 SD103 WR quartz d i o r i t e 481 28. 5 0. 171 0.7039 710 2. 4 0. 010 0.7036 Hb 45.6 4. 3 0. 272 0.7042 26.4 213 23. 35 0.7330 SD110 WR f i n e grained d i o r i t e 412 5. 7 0. 040 0.7040 SD117 WR f i n e grained hornblende gabbro 668 10. 5 0. 046 0.7037 0.7035 1415 13. 7 0. 028 0.7037 Hb 104 4. 1 0. 1 15 0.7038 SD119 WR f i n e grained hornblende 422 1 . 8 0. 012 0.7036 gabbro Spuzzum D i o r i t e , S e t t l e r Creek Body SD71 WR quartz d i o r i t e PI Hb SD96 WR quartz d i o r i t e 385 5.6 0.042 0.7039 1 143 17.3 0.044 0.7039 65.3 1.2 0.054 0.7041 361 6.6 0.053 0.7039 1 39 PI 922 7.5 0.024 0.7038 .... Hb 71 . 7 2.5 0.101 0.70425 SD97 WR qu a r t z d i o r i t e with 354 16.1 0. 132 0.7039 PI b i o t i t e 496 7.3 0.043 0.7038 Hb 30. 2 2.7 0.256 0.70435 .... Bi 14. 9 127 24.71 0.7347 0.7343 SD98 WR qu a r t z d i o r i t e 358 12.0 0.097 0.7041 PI 1 148 29.4 0.074 0.7039 0.7037 Hb 53. 3 2.5 0. 134 0.7042 Aqmatised q u a r t z d i o r i t e SD14 WR agmatised d i o r i t e , near 509 56.0 0.318 0.7053 PI contact with Cogburn 705 2.11 0.009 0.7044E .... B1 G r a n o d i o r i t e 15, .9 250 45.6 0.7313 Breakenridqe Formation CU1 WR l e u c o c r a t i c g r a n o d i o r i t i c 21 1 33. 1 0.452 0.7041 ... PI gnei s s with b i o t i t e and 1 46 2.1 0.042 0.7038 ... Mu muscovite 83 .7 95.3 3.30 0.7075 ... Bi 50 .3 191 10.98 0.7157 CU2 WR l e u c o c r a t i c g r a n o d i o r i t i c 207 31.2 0.436 0.7042 ... PI gnei s s with b i o t i t e 139 5.0 0.105 0.7038 ... Bi 20 .7 231 32.4 0.7428 WR = whole rock, PI = p l a g i o c l a s e , Hb = hornblende, Bi = b i o t i t e , Mu = muscovite 1 40 Table B-2. Rb-Sr isochron dates Rock U n i t B a i r d M e t a d i o r i t e Cogburn Creek Group 1) a l l data l e s s B i , c h e r t , HL125 PI 2) c h e r t HL37a WR-Bi p a i r 3) metavolc. HL125 S e t t l e r S c h i s t 1) a l l WR data ( i n c l . PRB) 2) p h y l l i t e SS82 3) s c h i s t SS128 S i l l s i n S c h i s t s 1) f e l s i c s i l l HL111 2) F e l s i c dyke SS85 l e s s B i 3) F e l s i c dyke,SS85 F o l i a t e d g r a n o d i o r i t e , SD92 Spuzzum D i o r i t e , Hut Creek body 1) a l l data l e s s B i 2) a l l data Spuzzum D i o r i t e , S e t t l e r Creek b a) 1) WR, a l l l e s s SD97 2) Pl-Hb, a l l data 3) WR-Pl-Hb, a l l data 4) a l l data b) 1) SD97 WR-Pl-Hb 2) SD97 a l l data n 8 7 S r I n i t . 8 6 S r X10" Date s (Ma) , 6 0.70383 + 32 3.4 ± 2 .4 G 6 0.70390 + 38 296 + 58 2 0.70896 + 40 81 + 5 3 0.70673 + 7 77 + 1 .6 16 0.70429 + 32 210 + 27 3 0.70643 + 8 66 + 1 .6 3 0.70664 + 18 39 4 3 0.70388 + 21 74 + 10 3 0.70369 ± 16 105 + 20 4 0.70381 ± 15 80 + 6 < 4 0.70403 ± 9 77 + 3 14 0.70368 ± 4 127 + 41 15 0.70370 ± 4 86 + 4 3 0.70372 ± 17 274 + 179 8 0.70382 ± 7 167 + 46 12 0.70368 ± 6 285 + 97 13 0.70388 ± 4 90 + 8 3 0.70364 ± 12 186 + 49 4 0.70379 ± 10 89 + 7 141 Agmatite, SD14 3 0.70478 ± 34 42 ± 14 WR-P1 2 0.70448 ± 13 182 ± 33 Breakenridge Pm. g n e i s s 7 0.70369 ± 6 79 ± 1.6 Isochron dates c a l c u l a t e d u s i n g l e a s t squares f i t t i n g methods of York (1967). Programme w r i t t e n f o r HP-85 microcomputer by R.L. Armstrong. E r r o r i n 8 7 S r / 8 6 S r i s 0.0001; Rb/Sr r a t i o assigned 2% e r r o r when Rb and Sr c o n c e n t r a t i o n s over 50 ppm, otherwise p r o p o r t i o n a l e r r o r of r a t i o d i v i d e d by lowest ppm. The date f o r B a i r d M e t a d i o r i t e was c a l c u l a t e d using 2% e r r o r i n Rb/Sr because the York l e a s t squares c a l c u l a t i o n would not converge with p r o p o r t i o n a l e r r o r s a s s i g n e d . The slope obtained i s c l o s e to that obtained using a c o n v e n t i o n a l l e a s t squares f i t . Decay constant ( S t e i g e r and Jager 1977): X = 1.42 x 10- 11 y r " 1 E r r o r s are iff. / 142 Table B-3. K-Ar a n a l y t i c a l data, Spuzzum D i o r i t e Sample Rock Type %K , 0 K a o A r , 0 A r ( r a d . ) Date Number 3 6 A r 3 6 A r n l / g %~~tot Ma + a. X10 s x10 3 SD66 Hb hb-hyp gabbro, 0.046 0.178 0.470 0.310 37.5 162 ± 7 Hut Creek body SD103 Hb quartz d i o r i t e , 0.462 2.468 1.472 1.49 80.6 80.9 ± 3 Hut Creek body SD117 Hb hb-hyp gabbro, 0.391 1.388 0.929 1.20 68.8 77.5 ± 3 Hut Creek body SD71 Hb hb gabbro, 0.206 0.754 0.742 0.822 60.6 100 ± 3 S e t t l e r Creek body SD97 Hb quartz d i o r i t e , 0.308 1.025 0.869 1.16 66.5 94.5 ± 4 S e t t l e r Creek body Isotope r a t i o i sochron dates ( F i g u r e 5.5a) 1) Hut Creek body slope 70.1 ± 5 Ma n=3 i n i t i a l « ° A r / 3 6 A r 392 ± 25 2) S e t t l e r Creek Body slope 78.9 ± 2 Ma n=2 i n i t i a l " ° A r / 3 S A r 388 ± 88 Concen t r a t i o n isochrons ( F i g u r e 5.5b) 1) Hut Creek body slop e 65.7 ± 5 Ma n=3 i n i t i a l *°Ar 0.185 ± 0.016 n l / g 2) S e t t l e r Creek Body slope 79.7 ± Ma n=2 i n i t i a l , 0 A r 0.139 ± 0.010 n l / g Hb=hornblende, Hyp=hypersthene, rad.=radiogenic Decay constants ( S t e i g e r and Jager 1977): X 0 = 4.96 x 1 0 " 1 0 y r " 1 X e = 0.581 x 1 0 " 1 0 y r " 1 *°K/K = 0.01167 atom.% E r r o r s are 1a. 143 T a b l e B-4 Sample weights f o r U-Pb a n a l y s e s Sample No Wt s p i k e mg Wt d i s s o l v e d z i r c o n mg Pb b l a n k % t o t . P b Obs, 2 0 6 ] T U T j B a i r d MD 184 0.20 13 60 HL111 22 0.10 4 135 S e t t 1 13.3 1 .2 1 .4 950 S e t t 2 12 1.5 2.0 330 Qtz D i HC 26.2 0.010 27 40 Qtz D i SC 21 . 1 9.2 6 1300 3560 CU2/1 55.2 9.0 2.5 400 CU2/2 28.5 2.5 4 160 S o l u t i o n c o n t a i n i n g z i r c o n was s p l i t i n t o 2 a l i q u o t s , one of which was s p i k e d . 1 Table B-5, Sample No Baird MD U-Pb a n a l y t i c a l data including isotope ratios PB r U ppm 356.8 Pb ppm 29.3 29.9 PBc + r 0.52 0.53 2 0 6Pb T-nrjj 0.04258 0.04253 2 0 7 p b 2 3 5 u 0.3691 0.5956 2 0 7Pb 2 0 6Pb 0.06287 0.10157 HL1 1 1 5189 228.2 0.83 0.01729 0.1290 0.05411 227.3 0.84 0.01741 0.1230 0.05123 Sett 1 1117 53.4 0.95 0.04418 0.4820 0.07913 53.1 0.97 0.04436 0.4877 0.07973 Sett 2 493.5 27.5 0.88 0.04635 0.5262 0.08233 27.6 0.88 0.04637 0.5403 0.08450 Qtz Di HC 326.4 24.5 0.40 0.02865 0.2573 0.06512 24.7 0.39 0.02816 0.3166 0.08154 Qtz Di SC 945.6 944.5 13.3 13.4 14.0 0.99 1 .03 0.96 0.01398 0.01481 0.01453 0. 1170 0.1031 0. 1022 0.06071 0.05048 0.05103 CU 2/1 218.6 3.9 4.0 4.0 0.94 0.89 0.89 0.01654 0.01645 0.01649 0. 1273 0.1114 0. 1148 0.05583 0.04911 0.05046 CU 2/2 218.6 5.0 0.73 0.01641 0.1173 0.05187 4.9 0.73 0.01643 0.1119 0.04937 c=common Pb, r=radiogenic Pb. Isotopic abundance of common Pb based on 200 Ma Pb derived from the growth curve of Stacey and Kramers (1975), except for Baird MD and Settler Schist samples. 2000 Ma Pb was more appropriate for Baird Metadiorite, and 1000 Ma Pb for Set t l e r Schist. Calculated U-Pb ages are l i s t e d in Table B-6. Rock and zircon descriptions and sample locations are given in Appendix C. 1 46 Table B-6. Calculated U-Pb dates Sample Number 2 0 6 P b Ma 2 0 7 P b Ma 2 0 7 P b Ma 2 3 8TJ 2 3 5rj 2 0 6 p b Baird MD 269 ± 10 400 ± 34 1251 ± 180 Sett 1 279 ± 2 401 ± 3 1183 ± 12 Sett 2 292 ± 2 434 ± 7 1279 ± 32 HL111 111 ± 1.5 121 ± 13 315 ± 250 Qtz Di HC 181 ± 7 256 ± 50 1023 ± 500 Qtz Di SC 91.8 ± 0.9 97.0 ± 8 230 ± 180 CU 2/1 105 ± 1 107 ± 4 153 ± 87 CU 2/2 105 ± 1 110 ± 7 224 ± 180 Decay constants (Steiger and Jager 1977): X 2 3 8 = 1 . 551 25 x 1 0" 1 0 y r - 1 X 2 3 5 = 9.8485 x 10" 1 0 y r ' 1 X 2 3 2 = 4.9475 x 1 0" 11 y r " 1 2 3 8U/2 3 5 T J = 1 3 7 . 8 8 Errors (1a) are derived from spike c a l i b r a t i o n and fractionation uncertainty and mass spectrometer within-run isotope r a t i o measurement uncertainties. 1 47 A p p e n d i x C. I s o t o p i c D a t i n g : Sample d e s c r i p t i o n s and l o c a t i o n s , Table C-1. Rock d e s c r i p t i o n s and sample l o c a t i o n s Sample Rock D e s c r i p t i o n Number Baird M e t a d i o r i t e MD1,3, h i g h l y f o l i a t e d , green 6,8,9 and white, metamorphosed d i o r i t e and gabbro Cogburn Creek Group HL37a r e c r y s t . ribbon c h e r t , micaceous p a r t i n g s HL76 coarse knobbly c h l o r i t i c metavolcanic g r e e n s c h i s t HL97 grey micaceous q u a r t z i t e HL103 grey, p h y l l i t e HL125 f i n e grained b i o t i t e - a c t i n o l i t e g r e e n s c h i s t S e t t l e r S c h i s t Sett 1 g r a p h i t i c p h y l l i t e Sett 2 g r a p h i t i c p h y l l i t e SS82 g r a p h i t i c p h y l l i t e SS109 g r a p h i t i c p h y i l i t e SS114 g r a p h i t i c p h y l l i t e SS128 q t z - b i - g a r s c h i s t , near margin of Cogburn G r a n o d i o r i t e Location S e t t l e r Lake W shore f l o a t on beach L a t i t u d e N Longitude E 49° 31.4' 121° 37.6' Cogburn Creek road, cut 49° 33.4' above gorge 121° 43.2' Cogburn Creek road, cut 49° 32.85' before 1st t r i b u t a r y 121° 44.9* 1510', S side spur betw. 49° 32.45' Talc S. Cogburn Cks 121° 43.75' 750', S side spur betw. 49° 32.65' Talc & Cogburn Cks 121° 44.3' On W slope above upper 49° 34.6' fork i n 3-mile Creek 121° 44.5' Roadend, S fork Cogburn Creek, at 2500' S fork Cogburn Creek, bridge over N t r i b . , W side of v a l l e y 2995' S side 2nd t r i b W side S fork Cogburn 4150' on rid g e E of S e t t l e r Creek 2300', N of t r i b , W side S e t t l e r Creek In N fork Cogburn Ck, 0.5km up from j u n c t i o n 49° 121 ° 49° 121° 49° 121 ° 49° 121° 49° 121 ° 49° 121° 31.9' 35.15* 33.65' 36.4* 32.6' 35.8' 32.85' 38.9' 32.75' 39.9' 34.9' 37. 1 ' SSI 30 g r a p h i t i c p h y l l i t e - b l a c k s l a t e Halfway up stream from Cogburn Lake, S fork Cogburn Creek 49° 31.4' 121° 35.72' SS1 32 black s l a t e Halfway up stream from Cogburn Lake, S fork 49° 31.4* 121° 36.0' 1 48 SS1 35 g r a p h i t i c p h y l l i t e 2500' in stream from Cogburn Lake 49° 121° 31 . 35. 5' 3' SS143 coarse sandy b i o t i t e s c h i s t 5500', N end ri d g e N of Old S e t t l e r 49° 121° 32. 37. ,35' ,6' S i l l s i n S c h i s t s HL111 f o l i a t e d f e l s i c s i l l with b i , nr Spuzzum N Body Roadcut at 1000', N side Cogburn Creek 49° 121 ° 33. 42. ,5' .55' SS85 f o l i a t e d f e l s i c gar c l u s t e r dyke in S e t t l e r S c h i s t W si d e S fork Cogburn Ck, N of stm, at 3700' from Cogburn Lake 49° 121 0 31 , 35. .95' .7' F o l i a t e d g r a n o d i o r i t e , small body in imbricate zone SD92 medium grained g r a n o d i o r i t e with b i , mu, garnets Sidestm W of S e t t l e r Creek 49° 121 ° 32, 40, .2' .4' Spuzzum D i o r i t e , Hut Creek body SD36 f i n e grained l e u c o c r a t i c hb quartz d i o r i t e S s i d e Cogburn Ck, on W rd to S e t t l e r Lake 49° 121 ° 33 4-1 .45' .5' SD66 f i n e grained hornblende- hypersthene gabbro 1750' above road on N side Cogburn Creek 49° 121 ° 33 39 .95' .55' SD67 f i n e grained hornblende- hypersthene gabbro Rdcut at 1800' N sid e Cogburn Creek 49° 121° 34 38 .0' .8' SD1 01 f i n e quartz d i o r i t e 3600' on W ri d g e above Hut Ck, N of Cogburn Ck 49° 121 ° 34 42 .45' .25' SD103 coarse quartz d i o r i t e hb+bi On ridge c r e s t between 3-mi and Hut Cks, 4500' 49° 121° 34 43 .5' .2' SD1 10 f i n e quartz d i o r i t e hb+bi In r i v e r b e d , f o r k s of Cogburn Ck 49° 121 0 34 37 .45' .15' SD1 17 f i n e grained hornblende gabbro At 1850' on N s i d e of Cogburn Ck, at forks 49° 121 ° 34 38 .5' .25' SD1 19 f i n e g rained hornblende- hypersthene gabbro W of SD117, at 1850' • 49° 121 ° 34 38 .6* .2' QtzDiHC quartz d i o r i t e 2100' on N slope above Cogburn Creek 49° 121 ° 33 42 .93' .4' Spuzzum D i o r i t e , S e t t l e r Creek Body SD71 coarse gabbro 3800', W slopes above S branch Cogburn Creek 49° 121 0 32 36 .15' .2' SD96 hb-quartz d i o r i t e E side S e t t l e r Creek, roadcut at 2700' 49° 121 ° 31 39 .85' .3' SD97 f i n e l e u c o c r a t i c quartz d i o r i t e , hb+bi+ gar E side S e t t l e r Creek, roadcut at 2450' 49° 121° 32 39 .5' .3' SD98 coarse quartz'"diorite E side S e t t l e r Creek, . roadcut at 2500' 49° 121 ° 32 39 .5' .3' QtzDiSC Composite sample of SD71,96,98 149 Aqmatised quartz d i o r i t e SD14 f i n e f o l i a t e d quartz d i o r b i , g a r n e t s i t e 2000' beside N bridge W side S fork Cogburn Creek 49° 33.45' 121° 36.5' Breakenridge Formation CU 1 l e u c o c r a t i c g r a n o d i o r i t i c gneiss with b i and mu Big S i l v e r R i v e r , r d - cut i n 1st gorge 49° 38' 121° 49.5' CU 2 l e u c o c r a t i c g r a n o d i o r i t i c gneiss with b i o t i t e Big S i l v e r R i v e r , r d - cut in 1st gorge 49° 38' 121° 49.5' B a i r d MD S e t t 1 S e t t 2 HL1 1 1 Qtz Di NB Qtz Di SC CU 2 D e s c r i p t i o n of z i r c o n samples. C l e a r , c o l o u r l e s s , broken p i e c e s and complete euhedral g r a i n s . 5% contamination. Z i r c o n s are a l l s m a l l and s i m i l a r i n s i z e . C l e a r and c o l o u r l e s s . F a i r l y rounded, and many con t a i n opaque i n c l u s i o n s . Most probably d e t r i t a l . S i z e <325 mesh. Up to 20% contamination, mostly kyanite and g r a p h i t e . S i m i l a r to S e t t 1, but a few about 200 mesh s i z e . 5-10% contamination. Z i r c o n s mostly long and t h i n and c l e a r . S i z e 200- 325 mesh, <5% contamination. Some l a r g e r g r a i n s have opaque i n c l u s i o n s . Very small sample, 210 handpicked z i r c o n s and fragments. C l e a r and c o l o u r l e s s . S i z e 200-325 mesh. Z i r c o n s c l e a r , c o l o u r l e s s , e u h e d r a l . Most are long prisms, 200-325 mesh. About 7% contamination, an amber c o l o u r e d m i n e r a l ( r u t i l e ? ) . Z i r c o n s pink, 200-325 mesh. C l e a r , euhedral, no i n c l u s i o n s , <5% contamination. Some broken g r a i n s . Most are stubby. 150 Appendix D Figure D-1 Map of Cogburn. Creek area showing locations of samples studied in thin section. See Figure 2.1 for key to rock units. 151 Figure D - 2 Map showing locations Cogburn Creek area. of geochronometry samples, 1 52 Appendix E. A n a l y t i c a l data for Chilliwack batholith Table E-1. Rb-Sr analytical data Sample Number Sr ppm Rb ppm 8 7Rb BS S r B 7 S r a t S r ChilliWR main phase dio r i t e 372 33. 8 0.263 0.7040 PI 473 4. 7 0.029 0.7039 29. 0 10. 3 1 .03 0.7048 - 14. 3 232 47.2 0.7262 0.7278 JV216 WR sample from J. Vance 309 57. 3 0.537 0.7042 University of Washington, 31 . 5 51 . 1 4.70 0.7058 Bi Seattle 29. 2 130 12.8 0.7098 0.7097 WR = whole rock, Pi = plagioclase, Hb = hornblende, Bi = biotite 153 1 1 • 7 1 r CHILLIWACK BATHOLITH Figure E-1. Rb-Sr isochron plot for Chilliwack batholith, North Cascades Mountains, Washington.

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