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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 . S c , V i c t o r i a U n i v e r s i t y Of W e l l i n g t o n , New Zealand, 1973, B.Sc.(Honours), V i c t o r i a U n i v e r s i t y Of W e l l i n g t o n , 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 G e o l o g i c a l Sciences  We accept t h i s t h e s i s as  conforming  to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA September 1985  ©  Janet E l i z a b e t h G a b i t e s , 1985  in  presenting  degree  this  at the  thesis  in  partial  fulfilment  University of  British  Columbia,  freely available for reference and study. copying  of  department  this or  thesis by  his  for scholarly or  her  of  the  I agree  requirements  for  may  representatives.  It  be is  granted  by the  understood  for extensive head  that  publication of this thesis for financial gain shall not be allowed without permission.  Department of G e o l o g i c a l  Sciences  The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 n  . „ t  DE-6(3/81)  8 October 1985  advanced  that the Library shall make it  I further agree that permission  purposes  an  of  my  copying  or  my written  ii  Abstract Metamorphic  s u p r a c r u s t a l rocks i n the Cogburn Creek area  belong to the Cogburn Creek Group and the S e t t l e r S c h i s t . are separated by a melange  These  zone, which has been c o r r e l a t e d with  the Shuksan t h r u s t zone and c o n t a i n s B a i r d M e t a d i o r i t e and u l t r a m a f i c rocks, and i n t r u d e d by the Spuzzum b a t h o l i t h and minor younger g r a n o d i o r i t e .  Three phases of f o l d i n g are  recognised in the s c h i s t u n i t s : f i i s a s s o c i a t e d with contact metamorphism that preceded r e g i o n a l metamorphism, f p e r v a s i v e mica f o l i a t i o n and t i g h t  approaching p l u t o n margins. i n c r e a s i n g metamorphic  produced  f o l d s , and kinks and broad  warps are a s s o c i a t e d with f , which was 3  2  locally  pervasive  M i n e r a l assemblages  indicate  grade from west to east from garnet to  g a r n e t - s t a u r o l i t e , andesine-epidote a m p h i b o l i t e ,  staurolite-  k y a n i t e , f i b r o l i t e , and coarse s i l l i m a n i t e zones.  Metamorphic  c o n d i t i o n s vary from 300 to 500 °C i n Cogburn Creek Group  rocks  to 550 to 700 °C at 6 to 8 kbar f o r p e l i t e s i n the S e t t l e r Schist.  C o n d i t i o n s deduced f o r metamorphism of the u l t r a m a f i c  rocks are c o n s i s t e n t with those f o r e n c l o s i n g p e l i t i c Geochronometry  schists.  i n d i c a t e s that the B a i r d M e t a d i o r i t e i s  probably Precambrian and e q u i v a l e n t to the Yellow A s t e r Complex of the North Cascade Mountains, Washington. Group was dated as Late P a l e o z o i c  The Cogburn Creek  (296 ± 58 Ma,  Rb-Sr  WR  i s o c h r o n ) , and i s p r o v i s i o n a l l y c o r r e l a t e d with the Bridge River Group.  The p r o t o l i t h of the S e t t l e r S c h i s t was d e p o s i t e d around  210 ± 27 Ma  (Rb-Sr WR  d e t r i t a l zircon  i s o c h r o n ) , and i t c o n t a i n s 2450 ± 230 Ma  i n d i c a t i n g p a r t i a l u l t i m a t e d e r i v a t i o n from  iii  Precambrian basement rocks. at  95 to 110  Ma,  metamorphism. isochron dates. 2 Ma  Spuzzum b a t h o l i t h was  before the c u l m i n a t i o n  of  range 66 to 88 Ma  youngest i n t r u s i v e rocks,  to 42 ± 14 Ma,  i n t r u s i v e event.  postdate the  K-Ar  granodiorite  dated at 32  r e g i o n a l metamorphic  Movement on the Shuksan Thrust  i n the North Cascade Mountains and  Spuzzum b a t h o l i t h and Lake.  Hb  are metamorphic c o o l i n g  Shuksan  before i n t r u s i o n of  r e g i o n a l metamorphism east  of  ±  and  i s bracketed  A l b i a n , a f t e r r e g i o n a l b l u e s c h i s t metamorphism of the Suite  intruded  regional  Rb-Sr b i o t i t e dates from a l l u n i t s and  dates i n the The  The  Harrison  as  iv  Table  of Contents  Abstract List  i i  of f i g u r e s  vii  L i s t of p l a t e s List  xi  of t a b l e s  xii  Acknowledgements 1  xiii  Introduction  1  1.1 Geographic l o c a t i o n and access  1  1.2 Previous  work  1  1.3 Regional  geology  4  2  Geology of the Cogburn Creek area 2.1  Baird Metadiorite  2.2 U l t r a m a f i c  9 12  rocks  14  2.3 Cogburn Creek Group  15  2.4 S e t t l e r S c h i s t  18  2.5 Premetamorphic i n t r u s i v e rocks  21  2.6 F o l i a t e d d i o r i t e  22  i n melange zone  2.7 Spuzzum B a t h o l i t h  ' 23  2.8 Younger I n t r u s i v e s  24  2.9 Breakenridge Formation gneiss  25  3  Metamorphism 3.1  27  Baird Metadiorite  3.2 U l t r a m a f i c  rocks  27 27  3.3 Cogburn Creek Group  28  3.4 S e t t l e r S c h i s t  31  3.5 Premetamorphic i n t r u s i v e rocks  35  3.6 D i s c u s s i o n  36  V  Ultramafic  rocks  36  Contact metamorphism  36  Regional metamorphism  39  4  Structure  50  5  Geochronometry  59  5.1  P r e v i o u s Geochronometry  59  5.2 B a i r d M e t a d i o r i t e  64  5.3 Cogburn Creek Group  66  5.4 S e t t l e r S c h i s t  67  5.5 Premetamorphic  i n t r u s i v e rocks  5.6 F o l i a t e d d i o r i t e  70  i n f a u l t zone  72  5.7 Spuzzum b a t h o l i t h  72  5.8 Agmatised quartz d i o r i t e "  77  5.9 Breakenridge Formation  77  5.10 D i s c u s s i o n  78  6  Regional Synthesis  107  Plates  115  References  125  Appendix A. I s o t o p i c d a t i n g methods. K-Ar,  Rb-Sr,  z i r c o n U-Pb Appendix B. Rb-Sr  134  i s o t o p i c data  ;  136  Appendix C. Dating sample d e s c r i p t i o n s , l o c a t i o n s  147  Zircon d e s c r i p t i o n s Appendix D. Thin  149  s e c t i o n l o c a t i o n map  Geochronometry  150  sample l o c a t i o n map  Appendix E. Rb-Sr data and isochron  151  for Chilliwack  b a t h o l i t h i n the North Cascades Mountains Maps  152  • i n pocket  vi  L i s t of  Figure  1.1  Area L o c a t i o n Map  Figure  1.2  Regional and  Figure  1.3  Figures  2  Geology of the North Cascade Mountains  southern  Coast Mountains  Geographical  5  l o c a t i o n of the S e t t l e r S c h i s t  and  Chiwaukum S c h i s t  8  F i g u r e 2.1  G e n e r a l i s e d geology of the Cogburn Creek Area  Figure  Timing of m i n e r a l  3.1a  deformation, F i g u r e 3.1b  29  Timing of m i n e r a l growth with respect  to  Settler Schist  29  Schematic T-X(C0 ) diagram f o r the system 2  Si0 -H 0-C0 2  2  2  at e l e v a t e d pressures  MgO-  and  temperatures, adapted from Johannes (1969) F i g u r e 3.3  Pressure-temperature c o n d i t i o n s i n the  38  Metamorphic m i n e r a l assemblages i n p e l i t e s and g r e e n s c h i s t  F i g u r e 3.5  from the Cogburn Creek Group ...  40  Metamorphic m i n e r a l assemblages i n p e l i t e s from the S e t t l e r S c h i s t  F i g u r e 3.6  37  pelitic  assemblages, from Pigage (1973) F i g u r e 3.4  10  to  Cogburn Creek Group  deformation, F i g u r e 3.2  growth with respect  ..  Equilibria calculated  for P  41 2  at a(H O)=0.8l, p l u s 2  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) F i g u r e 3.7  Map  44  of metamorphic mineral assemblages  and  i s o g r a d s around Cogburn Creek  46  F i g u r e 3.8  Regional  47  F i g u r e 4.1  Equal area p r o j e c t i o n f o r p o l e s to  isograds compositional  vii  layering F i g u r e 4.2  i n the S e t t l e r S c h i s t  Equal area p r o j e c t i o n f o r p o l e s t o f  51 2  foliation  f o r the S e t t l e r S c h i s t south of Cogburn Creek .. 52 F i g u r e 4.3  Equal area p r o j e c t i o n f o r p o l e s t o f  2  foliation  in S e t t l e r S c h i s t north of Cogburn Creek F i g u r e 4.4  Equal  area p r o j e c t i o n f o r poles to f  2  54  foliation  in Cogburn Creek Group Figure  4.5  Equal area p r o j e c t i o n f o r p o l e s to f o l i a t i o n and m i n e r a l  Figure  5.1  55  l i n e a t i o n s i n Spuzzum b a t h o l i t h .... 58  Map showing geochronometry of the H a r r i s o n Lake - F r a s e r R i v e r region  63  F i g u r e 5.2a  Rb-Sr i s o c h r o n p l o t  for Baird Metadiorite  88  F i g u r e 5.2b  Rb-Sr i s o c h r o n p l o t  f o r Cogburn Creek Group .... 89  F i g u r e 5.2c  Rb-Sr isochron p l o t  for Settler Schist  Figure  S e t t l e r S c h i s t , expanded s c a l e  5.2d  F i g u r e 5.2e  Rb-Sr i s o c h r o n p l o t  90 91  f o r premetamorphic i n t r u s i v e  rocks F i g u r e 5.2f  92  Rb-Sr isochron p l o t  f o r small body of f o l i a t e d  g r a n o d i o r i t e i n i m b r i c a t e zone (SD92) F i g u r e 5.2g  Rb-Sr i s o c h r o n p l o t  93  f o r Spuzzum b a t h o l i t h ,  Hut Creek body F i g u r e 5.2h  94  Rb-Sr i s o c h r o n p l o t  f o r Spuzzum b a t h o l i t h , S e t t l e r  Creek body F i g u r e 5.2i  Rb-Sr i s o c h r o n p l o t (SD14), marginal  Figure  5.2j  94 f o r a g m a t i t i c quartz  to Cogburn G r a n o d i o r i t e  Rb-Sr i s o c h r o n p l o t  Rb-Sr i s o c h r o n p l o t  96  f o r Breakenridge Formation  gneiss F i g u r e 5.2k  diorite  97 f o r C h i l l i w a c k Group  98  viii  F i g u r e 5.21  Rb-Sr i s o c h r o n p l o t  F i g u r e 5.2m  Rb-Sr isochron p l o t s f o r Shuksan S u i t e Darrington  F i g u r e 5.3  U-Pb  F i g u r e 5.4a  P l o t of  99  and  Phyllite  100  c o n c o r d i a diagram for z i r c o n d a t i n g a o  K/  separates F i g u r e 5.4b  f o r Bridge River Group  3 6  Ar  v.  4 0  Ar/  3 6  Ar  101  f o r hornblende  from Spuzzum b a t h o l i t h  P l o t of %K v.  4 0  Ar  102  n l / g f o r hornblende  separates  from Spuzzum b a t h o l i t h F i g u r e 5.5  U-Pb  102  c o n c o r d i a diagram for Yellow Aster Complex,  Skagit Gneiss  and  Swakane Gneiss,  from  Mattinson  (1972) F i g u r e 5.6  103  D a t a - f i e l d diagram for Rb-Sr analyses  from  possible correlative stratigraphic units F i g u r e 5.7  P l o t of %K v. *°Ar n l / g for r e g i o n a l data  104 from  Spuzzum b a t h o l i t h F i g u r e 5.8  105  Graph of eastward younging trend of K-Ar  dates  from Spuzzum b a t h o l i t h , from Bartholomew (1979).106 F i g u r e 6.1  Tectono-stratigraphic terranes, modified Monger and  F i g u r e 6.2  Berg  Reconstruction  (1984)  F i g u r e 6.3  Berg  Reconstructions  108  of Bridge River Group and Methow  terrane, modified Monger and  from  from Kleinspehn  (1985) and  (1984)  109  of S e t t l e r S c h i s t and Chiwaukum  S c h i s t , a f t e r Misch (1977) and Monger (1985) ...111 F i g u r e 6.4  Partial  r e c o n s t r u c t i o n of the Methow-Tyaughton  basin, modified F i g u r e D-1  Map  from Kleinspehn  (1985)  112  of Cogburn Creek area showing l o c a t i o n s of  samples s t u d i e d i n t h i n s e c t i o n  150  ix  Figure  D-2  Map  showing l o c a t i o n s of geochronometry samples,  Cogburn Creek area  151  F i g u r e E-1  Rb-Sr  Map 1  Topographic map  Map  2  Map  showing p l a n a r  Map 3  Map  showing l i n e a r s t r u c t u r e s  Map 4  Topographic map  isochron plot  f o r C h i l l i w a c k B a t h o l i t h ...153  showing rock u n i t s structures  showing s t a t i o n  -i-R—pee-ket  y^^e^^A CalU^*^ numbers  P©G-k-et in-_paql.ei in—pos-k-e-t  V_  X  L i s t of P l a t e s  Frontispiece:01d  S e t t l e r Mountain,  looking  south along  S e t t l e r Creek P l a t e 2.1  xiv  Imbricate zone: B a i r d M e t a d o i r i t e , rocks,  ultramafic  Settler Schist  115  P l a t e 2.2  M a f i c amphibolite pod i n S e t t l e r S c h i s t  115  P l a t e 2.3  Photomicrograph of sample SD101  116  P l a t e 2.4  Photomicrograph of sample SD66  116  P l a t e 3.1  Photomicrograph  of sample HL30  117  P l a t e 3.2  Photomicrograph of sample HL16  117  P l a t e 3.3  Photomicrograph of sample HL15  118  P l a t e 3.4  Photomicrograph of sample HL80  118  P l a t e 3.5  Photomicrograph of sample HL142  119  P l a t e 3.6  Photomicrograph of sample SS110  119  P l a t e 3.7  Photomicrograph of sample SSI 35  120  P l a t e 3.8  Photomicrograph of sample SS66  120  P l a t e 3.9  Twinned s t a u r o l i t e i n SS57, S e t t l e r S c h i s t  121  P l a t e 3.10  Photomicrograph of sample SS114  121  P l a t e 3.11  Photomicrograph of sample SS53  122  P l a t e 3.12  S i l l i m a n i t e p o r p h y r o b l a s t s from r i d g e north of Cogburn  Creek  122  P l a t e 3.13  Photomicrograph of sample SSl82a  P l a t e 4.1  F a u l t contact  a t the base of the i m b r i c a t e  123 zone .124  xi  L i s t of Tables  Table 5.1  Table of dates and age estimates from previous s t u d i e s between H a r r i s o n Lake and F r a s e r River . 60  Table 5.2  Summary of events  i n 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 f o r U-Pb analyses  143  Table B-5  U-Pb a n a l y t i c a l data i n c l u d i n g  Table B-6  C a l c u l a t e d U-Pb dates  146  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  147  Table C-2  D e s c r i p t i o n of z i r c o n samples  149  Table E-1  Rb-Sr a n a l y t i c a l data, C h i l l i w a c k b a t h o l i t h  152  isotope r a t i o s ...144  Acknowledgements I would l i k e to thank Dr R.L. Armstrong f o r c r i t i c a l s u p e r v i s i o n of t h i s p r o j e c t . Drs P.R. Bartholomew, J.S. G e t s i n g e r , L.C. Pigage, and J.W.H. Monger p r o v i d e d a i d and discussions. K r i s t a Scott and Joe Harakal p r o v i d e d i n s t r u c t i o n and a n a l y s e s f o r the geochronometry. Nel Grond and Lenore Shapka each s u r v i v e d a summer as f i e l d a s s i s t a n t . My housemates Cathy, Denise, Eddie, Gary, Ian, Joe and Nounou gave me support, encouragement, and a s s i s t a n c e with b a b y s i t t i n g . Margo McTaggart helped my daughter C h a r l o t t e through my l a s t weeks of w r i t i n g . F i e l d and l a b o r a t o r y expenses were funded through Dr Armstrong's N a t i o n a l Research C o u n c i l Grant 67-8841.  Old S e t t l e r  Mountain  Looking south along S e t t l e r Creek. The f r o n t f a c e i s a d i p slope r e p r e s e n t i n g the upper c o n t a c t of the Shuksan t h r u s t imbricate zone.  1  1.  1.1  Geographic L o c a t i o n and The  Access  Cogburn Creek area, which was  pocket) d u r i n g H a r r i s o n Hot B.C.  Introduction  the summers of  Springs  ( F i g . 1.1).  The  1981  mapped (Maps 1 to 4, i n  and  1982,  climbs km . 2  Coast Mountains of the eastern  km  B.C.  shore of  1800  to 1900  m are reached only by The  difficult  area mapped covers  lake shore i s 10 m above mean sea l e v e l and  highest peak, Old S e t t l e r Mountain h e l i c o p t e r was  150  in the v a l l e y of Cogburn Creek; however, the  up steep, wooded h i l l s i d e s . The  north of  lake i s north of the F r a s e r v a l l e y ,  Access i s by logging roads along  ridge tops at  20 km  on the east s i d e of H a r r i s o n Lake,  east of Vancouver, in the southeastern  H a r r i s o n Lake and  lies  (2152  m),  i s 8 km  used to p l a c e high camps d u r i n g the  90  the inland.  1982  A  field  season.  1.2  Previous Work The  area between H a r r i s o n Lake and  been s t u d i e d by g e o l o g i s t s s i n c e the compiled  a l l work to 1969,  the F r a s e r R i v e r  1920's.  has  Monger (1970)  i n c l u d i n g a review of the geology of  the North Cascade Mountains by Misch  (1966).  Figure  1.1  shows  areas mapped by some of the g e o l o g i s t s mentioned below.  Read  (1960) s t u d i e d s c h i s t s i m i l a r to S e t t l e r S c h i s t near the  contact  with b a t h o l i t h i c  rocks on Zofka Ridge and along  Creek, and concluded r e l a t e d to the  Stulkawhits  that the metamorphic isograds were not  intrusions.  Monger (1966) mapped the C h i l l i w a c k  2A  Figure  1.1  Area L o c a t i o n Map Key t o map  areas  15.  Gabites, t h i s  14.  Bartholomew  13.  Vining  1977  12.  McLeod  1975  1 1 . Pigage  1973  10.  Bremner  study  1979  1973  9.  Reamsbottom  8.  Lowes  7.  Richards  6.  Monger  5.  Roddick and Hutchison  4.  Hollister  3.  McTaggart and Thompson  2.  Monger  1 . Read  1971, 1974  1972 1971  1970  1969  1966  1960  1969  1967  2  122*00  USA  121*00  3  Group i n the type area in the C h i l l i w a c k v a l l e y .  McTaggart  and  Thompson (1967) mapped the region along the F r a s e r River Canyon and  southward from Hope.  A r e g i o n a l summary of the  tectonic  r e l a t i o n s h i p between the southern Coast Ranges and North Cascade Mountains was  p u b l i s h e d by McTaggart  (1970).  Hollister  (I969a,b) s t u d i e d the r e l a t i o n s h i p s between metamorphic minerals in s c h i s t s near Kwoiek Creek that are c o n s i d e r e d to be e q u i v a l e n t to S e t t l e r S c h i s t . covered  the northwestern  Roddick and Hutchison  (1969)  part of the Hope map-area, i n c l u d i n g  the Big S i l v e r R i v e r area north of Cogburn Creek. (1971) mapped the C h i l l i w a c k b a t h o l i t h and  Richards  Spuzzum b a t h o l i t h  north and south of the F r a s e r R i v e r ; V i n i n g (1977) mapped the Spuzzum b a t h o l i t h f u r t h e r north in the v i c i n i t y of the Giant Mascot N i c k e l Mine. Lake east and  Lowes (1972) mapped the area from H a r r i s o n  south to the F r a s e r R i v e r ; the r i d g e s north of  Cogburn Creek were h i s northern boundary. overlapped  His northern edge  the area around Big S i l v e r R i v e r mapped by  Reamsbottom (1971  and  1974).  Bremner (1973) s t u d i e d  metamorphism i n a s e c t i o n of the F r a s e r Canyon north of Y a l e . Pigage  (1973) mapped the Gordon Creek area south of Yale, with  emphasis on metamorphism of the S e t t l e r S c h i s t . (1975) extended the work of Pigage  i n the Yale Creek area,  between Gordon Creek and Cogburn Creek. the geology  Bartholomew  McLeod (1975) s t u d i e d  of the Giant Mascot N i c k e l Mine i n S t u l k a w i t s Creek.  Previous geochronometry i s reviewed  i n Chapter  5.  4  1.3 Regional The  Geology  study area l i e s  i n a zone of complex geology a t the  south end of the Coast P l u t o n i c Complex and the n o r t h end of the Cascade Mountains.  Many of the s t r u c t u r e s are c o n t i n u o u s with  those of the North Cascade Mountains. been summarised i n Figure 1.2,  Geology  of the r e g i o n has  a f t e r Monger (1970),  (1973), Bartholomew (1979), and Haugerud  Pigage  (1979).  The Coast P l u t o n i c Complex c o n s i s t s of composite  p l u t o n s of  dominantly q u a r t z d i o r i t e and g r a n o d i o r i t e composition, mainly Cretaceous i n age,  i n which l e n t i c u l a r bodies of metamorphosed  sedimentary and v o l c a n i c rocks form  pendants.  S u p r a c r u s t a l rocks i n the study area have been d i v i d e d 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 u n i t s , the  Schist The  (Lowes 1972)  into  Settler  and the Cogburn Creek Group ( t h i s s t u d y ) .  rocks of the Cogburn Creek Group were p r e v i o u s l y mapped as  p a r t of the Pennsylvanian - Permian C h i l l i w a c k Group of the North Cascades correlation  (Monger 1970,  Lowes 1972), but as  i s u n c e r t a i n a new  Cogburn Creek a r e a .  name has been a p p l i e d i n the  Reamsbottom (1971  and  1974)  of both Cogburn Creek Group and S e t t l e r S c h i s t Needle Formation.  S e t t l e r S c h i s t was  i n c l u d e d rocks  in h i s Cairn  named and d e s c r i b e d by  Lowes (1972) and dated as e a r l y Mesozoic The two  this  (Bartholomew  1979).  s t r a t i g r a p h i c u n i t s are separated by a s t e e p l y  d i p p i n g b e l t of imbricated t e c t o n i c s l i c e s of u l t r a m a f i c dioritic  and  rocks that has been i n t e r p r e t e d as a n o r t h e r n e x t e n s i o n  of the root zone of the Shuksan t h r u s t of the North (Lowes 1972).  Cascades  Some of the u n i t s i n t h i s zone resemble  Yellow  5A  Figure  1.2  Regional Geology of the North Cascade and South Coast  Mountains  Key f o r R e g i o n a l Geology Quaternary Miocene and 01igocene  23 22  21 Eocene E a r l y T e r t i a r y 20 -Late Cretaceous 19 Cretaceous  Mountains  Map  D r i f t and a l l u v i u m G r a n o d i o r i t e , quartz d i o r i t e ; C h i l l i w a c k B a t h o l i t h , Cogburn G r a n o d i o r i t e , Needle Peak P l u t o n , S i l v e r Peak P l u t o n , Conglomerate Diorite, granodiorite; Yale I n t r u s i o n s D i o r i t e ; Spuzzum Pluton and o u t l i e r s , Scuzzy Pluton  18  Gneiss; C u s t e r , Breakenridge Formation  17  Jackass Mountain  16  Brokenback  Hill  Group Formation  1 5 P e n i n s u l a Formation 1 4 Granodiorite; Late J u r a s s i c -Cretaceous Upper J u r a s s i c 13 A g a s s i z P r a i r i e Formation Mid J u r a s s i c  1 2 Kent Formation 1 1 B i l l h o o k Creek Formation 10  U-M-L Jurassic Tr i a s s i c - J r Pm-Tr(?)-Jr  Mysterious Creek Formation  9  Echo I s l a n d Formation  8 7 6 5 4 3 2 1  H a r r i s o n Lake Formation Ladner Group C u l t u s Formation Hozameen Group T r i a s s i c or o l d e r Settler Schist Cogburn Creek Group C h i l l i w a c k Group Yellow A s t e r Complex, B a i r d M e t a d i o r i t e  Tr or o l d e r Penn.-Permian Precambrian Early Paleozoic Age unknown A  u l t r a m a f i c rocks thrust  fault  *"V steep f a u l t , downthrown s i d e  5  30'  49°15'  122a00'  45'  30'  121°  15'  6  A s t e r Complex (Misch age  (Mattinson  1966), which i s p a r t i a l l y of Precambrian  1972).  D i o r i t i c and  g r a n o d i o r i t i c rocks of Cretaceous  i n t r u d e d the s c h i s t s and  gneisses.  age  have  These are syn- to post-  t e c t o n i c with respect to the r e g i o n a l metamorphism and deformation  but predate movement on the F r a s e r F a u l t zone.  Discordant midyoungest  to l a t e - T e r t i a r y g r a n o d i o r i t e bodies are  the  intrusives.  The youngest s t r a t i g r a p h i c u n i t s are Eocene conglomerates that have been deformed along the F r a s e r F a u l t zone (McTaggart and Thompson  1967), and P l e i s t o c e n e g l a c i a l d e p o s i t s .  McTaggart  (1970) c o n s i d e r e d that the b e l t of c r y s t a l l i n e  g n e i s s e s n o r t h of the I n t e r n a t i o n a l Border a x i a l zone of a Cretaceous  orogen.  was  the deeply  eroded  The metamorphosed and  deformed sediments of the North Cascade Mountains represented a shallower  level.  A l e n t i c u l a r body of gneiss (Custer  bounded by f a u l t s of the F r a s e r F a u l t zone was the core of t h i s orogen.  The  Gneiss)  i n t e r p r e t e d as  Custer Gneiss was  first  described  by McTaggart and Thompson (1967) as migmatitic gneiss with small areas of s c h i s t .  They c o r r e l a t e d i t with gneisses mapped i n the  North Cascade Mountains by Misch  (1966).  from E a r l y P a l e o z o i c to Cretaceous c o n s i d e r s that the Custer Gneiss may batholith.  Age  estimates  (McTaggart 1970).  range  McTaggart  be r e l a t e d to Spuzzum  Reamsbottom (1971) c o r r e l a t e d two  g r a n o d i o r i t i c g n e i s s i n the Mount Breakenridge  domes of area with Custer  Gneiss. Movement on the F r a s e r F a u l t zone was  once thought  to have  7  s t a r t e d i n the Mesozoic and continued u n t i l Eocene times 1970).  New  data now  i n d i c a t e s that most of the movement i s  T e r t i a r y , mostly Eocene  (Monger  1985).  The f a u l t zone i s s e a l e d  by the m i d - T e r t i a r y C h i l l i w a c k b a t h o l i t h related plutons. Washington  (Monger  (Appendix E) and  Major movement on the S t r a i g h t Creek f a u l t i n  ended by 33 Ma  ( M i l l e r and Vance  1981).  Correlation  of the S e t t l e r S c h i s t with the Chiwaukum S c h i s t of the Skagit Metamorphic postulate  S u i t e i n the Stevens Pass area l e d Misch (1977) to  150 to 200 km r i g h t  S t r a i g h t Creek f a u l t  l a t e r a l o f f s e t on the F r a s e r -  system (Figure  1.3).  Displacement of the  Hozameen and Bridge River Groups and Bridge River zone 1985)  (Monger  1978,  support that  Metamorphic  1985) and Tyaughton-Methow Basins (Kleinsphen i n f e r e n c e and may  to w i t h i n the Eocene.  bracket the f a u l t movement  Kleinspehn (1985) r e s t o r e s the Tyaughton-  Methow basin to a reasonable d e p o s i t i o n a l environment by reversing  110 km of d e x t r a l motion on the Fraser - S t r a i g h t  Creek F a u l t .  T h i s was preceded by 150 km of Cenomanian to  Turonian motion on the Yalakom  - Ross Lake F a u l t .  A more  d e t a i l e d d i s c u s s i o n of f a u l t movements i s i n Chapter 6, w i t h appropriate  figures.  8  Figure  1.3  G e o g r a p h i c a l l o c a t i o n of the S e t t l e r S c h i s t and Chiwaukum S c h i s t , m o d i f i e d from Evans and B e r t i (1985). I n d i v i d u a l p l u t o n s i n t r u d i n g the two s c h i s t u n i t s are named. This map i s used as a base for Figure 6.2.  9  2.  Geology of the Cogburn Creek area  Rocks i n the Cogburn Creek area are d i v i d e d i n t o f i v e main u n i t s , with a r e a l d i s t r i b u t i o n as shown i n F i g u r e 2.1.  The most  e x t e n s i v e u n i t s are two  distinct  l i t h o l o g i c a l l y and  structurally  s c h i s t s , the Cogburn Creek Group and the S e t t l e r S c h i s t , and Spuzzum b a t h o l i t h that i n t r u d e s them.  the  The main s t r u c t u r a l  f e a t u r e of the area i s a sinuous zone of almost  vertical  t e c t o n i c s l i c e s of m e t a d i o r i t e and u l t r a m a f i c rocks  (Plate  2.1)  that separates the Cogburn Creek Group and S e t t l e r S c h i s t . zone s t r i k e s east-west  i n the south p a r t of the area around Old  S e t t l e r Mountain, and turns to s t r i k e north-northwest northwestern  This  i n the  p a r t of the a r e a , with a d e f l e c t i o n around the  Spuzzum b a t h o l i t h , Hut Creek body. -  Lowes ( 1972)  zone with the Shuksan t h r u s t zone (Misch 1966)  correlated  this  on the b a s i s of  s t r u c t u r a l 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 i n the zone, and g e n e r a l c o n t i n u i t y of the b e l t along s t r i k e s i m i l a r rocks and  s t r u c t u r e s to the  with  south.  Both s c h i s t s have undergone at l e a s t three phases of deformation,  mostly  r e l a t e d to the Cretaceous  metamorphism and d i o r i t e  intrusion.  episode of  The dominant s t r u c t u r e s i n  the S e t t l e r S c h i s t were formed by the second r e c o g n i s a b l e deformation found graded  (Pigage bedding  1973,  Bartholomew 1979).  i n the S e t t l e r S c h i s t  area, no c o n v i n c i n g evidence of bedding Creek.  was  Although  Pigage  i n the Gordon Creek found around Cogburn  In the Cogburn Creek Group, metamorphosed c h e r t s e x h i b i t  what appears  to be o r i g i n a l bedding  transposition.  Schistosity  r a t h e r than metamorphic  i s s u b v e r t i c a l 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  6  Granodiorite  4-6  Agmatised d i o r i t e , mixture of S e t t l e r S c h i s t and Cogburn G r a n o d i o r i t e  5  D i o r i t e (Spuzzum)  5a  Foliated diorite  5b  Hornblende-hypersthene gabbro  4  Settler Schist  (Cogburn)  i n melange zone  Cogburn Creek Group 3a  -grey  pelite  3b  -greenschist  3c  -metachert  3d  -marble  2  Ultramafic  1  Baird Metadiorite  x  ^  rocks  Shuksan t h r u s t Cogburn F a u l t  debris  49° 35  49° 31'  121° 45'  121° 40'  121° 35'  11  study area, but has v a r i a b l e s t r i k e . Regional metamorphic grade changes across the area from g r e e n s c h i s t grade in the west  (on the shore of H a r r i s o n  Lake),  through g a r n e t - s t a u r o l i t e , k y a n i t e , f i b r o l i t e ± k y a n i t e , to sillimanite ± fibrolite  i n the n o r t h e a s t e r n c o r n e r .  The  i n c r e a s e i s c o n s i s t e n t with p r o x i m i t y to the l a r g e mass of d i o r i t e that comprises the Spuzzum b a t h o l i t h . k i l o m e t r e of the two  d i o r i t e bodies  Within a  i n the f i e l d area an  earlier  c o n t a c t metamorphism i s i n d i c a t e d in the S e t t l e r S c h i s t by r e l i c t c h i a s t o l i t i c andalusites. The  l a t e s t event  granodiorite plutons. stock  i s post-metamorphic i n t r u s i o n of small An example i s the Cogburn G r a n o d i o r i t e  (Bartholomew 1979)  which occurs east of the f o r k s of  Cogburn Creek. Age  r e l a t i o n s among the n o n - i n t r u s i v e rocks are not  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 s t r u c t u r a l evidence.  s c h i s t s are bounded by f a u l t s and  i n t r u s i v e c o n t a c t s , and  metamorphic grade and degree of deformation make i t u n l i k e l y that f o s s i l s would s u r v i v e . d e s c r i b e d below i n order of d e c r e a s i n g age, isotopic studies.  clear Both the  are high enough to The  rock u n i t s are  as deduced from  12  2.1  U n i t 1.  Baird Metadiorite  The name B a i r d M e t a d i o r i t e i s a p p l i e d u n i t of v a r i a b l y metamorphosed mafic  i n t h i s study to a  igneous r o c k s .  Occurrence  i s as l e n s e s and t a b u l a r masses i m b r i c a t e d with u l t r a m a f i c rocks, along the zone marking the northern e x t e n s i o n of the Shuksan Thrust which extends  (Lowes 1972). northwest  The  l a r g e s t body forms the r i d g e  from Old S e t t l e r Mountain t o Cogburn  Creek, and separates T a l c and S e t t l e r Creeks The mafic rocks are v a r i a b l e  ( F i g u r e 2.1).  i n composition  from gabbro to  q u a r t z d i o r i t e , and have been metamorphosed to a m p h i b o l i t e facies.  They are grey-green  i n c o l o u r , hard, and  form  massive  blocky outcrops and steep c l i f f s with sparse v e g e t a t i o n . T e x t u r a l l y the rocks show weak to moderate f o l i a t i o n due alignment  of mafic m i n e r a l s .  to  S t r a i n d u r i n g metamorphism and  deformation has been taken up along f i n e - g r a i n e d ,  chloritic  shear zones through the otherwise e q u i g r a n u l a r r o c k . C o m p o s i t i o n a l l y the mafic rocks c o n s i s t 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 ,  opaques, i n v a r y i n g p r o p o r t i o n s . The hornblende  and  R u t i l e i s a common a c c e s s o r y .  occurs both as p o r p h y r o b l a s t i c , probably  l a r g e c r y s t a l s and as small i n t e r g r a n u l a r prisms.  relict,  In the  samples s t u d i e d i n t h i n s e c t i o n the p l a g i o c l a s e has been almost e n t i r e l y converted to e p i d o t e , a l b i t e ,  s e r i c i t e and  calcite.  R e l i c t p l a g i o c l a s e compositions, measured by a-normal methods, were s i m i l a r t o those r e p o r t e d by Lowes (1972), and are i n the range An35 to An42. shear zones.  Minor  C h l o r i t e occurs mainly relict  pyroxene now  in fine-grained  overgrown by  hornblende  13  (sample MD1)  may  be p a r t of the o r i g i n a l mineral assemblage.  Dynamothermal metamorphism to andesine s u b f a c i e s d u r i n g the Cretaceous has  - epidote a m p h i b o l i t e  reconstituted  earlier  mineral assemblages i n the rocks, although there i s only minor i n t e r n a l deformation.  Lowes (1972) r e p o r t e d l i m i t e d  evidence  here f o r a p r e v i o u s a m p h i b o l i t e f a c i e s metamorphism observed Misch  by  (1966) i n the North Cascade Mountains. Lowes (1972) c o r r e l a t e d the rocks here c a l l e d B a i r d  M e t a d i o r i t e with the Yellow Aster Complex  (Misch 1966)  North Cascade Mountains, on the b a s i s of s t r u c t u r a l with respect to the Shuksan T h r u s t .  i n the  position  The Yellow A s t e r Complex  occurs as t e c t o n i c s l i c e s with sheared u l t r a m a f i c rocks i n melange below the s o l e of the Shuksan t h r u s t ; i n p l a c e s i t i s imbricated with the u n d e r l y i n g C h i l l i w a c k Group. c o n s i d e r e d the Yellow A s t e r Complex to be it  Misch  (1966)  "basement", and mapped  i n the type area as s c h i s t o s e and d i r e c t i o n l e s s metamorphic  h o r n b l e n d i t e s and a m p h i b o l i t e s , g n e i s s o s e and  directionless  metagabbros, m e t a d i o r i t e s , meta-quartz d i o r i t e s metatrondhjemites,  and migmatites  and  of v a r i e d composition.  He  speculated that the s c h i s t o s e h o r n b l e n d i t e s and a m p h i b o l i t e s are remnants of primary predominantly  oceanic c r u s t t h a t was  hornblende  quartz d i o r i t i c  Z i r c o n s dated by Mattinson Yellow A s t e r Complex.  l a t e r converted to  continental crust.  (1972) showed two age groups i n the  Pyroxene g n e i s s was  dated at 1400  to  1600  Ma and quartz d i o r i t e g n e i s s and pegmatite  g n e i s s a t 410  ± 85  Ma. Rock types i n the B a i r d M e t a d i o r i t e are s i m i l a r to those  comprising the Yellow Aster Complex. dating  (Chapter  Rb-Sr and z i r c o n U-Pb  5) on the B a i r d M e t a d i o r i t e suggest  an o l d age  s i m i l a r to that measured by Mattinson on Yellow A s t e r pyroxene gneiss.  Thus new i s o t o p i c evidence  made by Misch  supports the c o r r e l a t i o n  (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 U n i t 2.  U l t r a m a f i c Rocks  U l t r a m a f i c rocks i n the study area mainly elongate bodies of p e r i d o t i t e  occur as pods and  imbricated with the B a i r d  M e t a d i o r i t e along the root zone of the Shuksan t h r u s t .  The  s c a l e of i m b r i c a t i o n v a r i e s from metres t o k i l o m e t r e s .  Some  small bodies of p y r o x e n i t e and h o r n b l e n d i t e a r e a s s o c i a t e d with the Spuzzum b a t h o l i t h , and occur w i t h i n both the d i o r i t e and the surrounding s c h i s t s .  Outcrops  of u l t r a m a f i c rock are almost  devoid of v e g e t a t i o n and weather orange, so they a r e r e c o g n i s a b l e from a d i s t a n c e . rock  The l a r g e s t body of u l t r a m a f i c  (mapped by Lowes 1972, o u t s i d e the present study area) i s  faulted  i n t o the Cogburn Creek Group along the west s i d e of T a l c  Creek, g i v i n g the drainage  i t s name.  The p e r i d o t i t e composition to u n a l t e r e d d u n i t e .  i s variable,  from  serpentinite  The most common i s s e r p e n t i n i t e , but  metamorphic r e c o n s t i t u t i o n of the o r i g i n a l d u n i t e , h a r z b u r g i t e and p y r o x e n i t e i s q u i t e v a r i a b l e w i t h i n the r o c k s .  Some of the  s e r p e n t i n i t e 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 .  15  2.3 Unit 3. Pelitic along  Cogburn Creek Group s c h i s t , c h e r t and metavolcanic  the eastern  rocks cropping out  shore of H a r r i s o n Lake and west of the Shuksan  t h r u s t zone have been given the informal name Cogburn Creek Group i n t h i s study.  Because of t h e i r  similarity  in structural  p o s i t i o n beneath the Shuksan t h r u s t , Lowes (1972) had p r e v i o u s l y assigned  them t o the C h i l l i w a c k 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 l a c k of f o s s i l s I have chosen t o designate  them as the Cogburn Creek Group.  (1930) used the name S l o l l i c u m Group f o r metavolcanic  Crickmay rocks and  low grade p e l i t e s around S l o l l i c u m Peak t h a t may be part of the same u n i t .  Reamsbottom  (1971 and 1974) mapped rocks  s t r u c t u r a l p o s i t i o n i n the B i g S i l v e r R i v e r area  i n the same  t o the north of  Cogburn Creek as part of the C a i r n Needle Formation, along what i s h e r e i n c a l l e d S e t t l e r S c h i s t .  Monger  with  ( p e r s . comm. 1985)  has followed the u n i t north a t l e a s t as f a r as between the two g n e i s s domes i n the Mount Breakenridge  area.  The s t r a t i g r a p h i c t o p and bottom of the Cogburn Creek Group are not seen.  The u n i t i s t r u n c a t e d on the northeast  B a i r d M e t a d i o r i t e and u l t r a m a f i c rocks  against  i n t h e melange zone below  the Shuksan t h r u s t .  Since the rocks d i p t o the e a s t , t h i s i s  the s t r u c t u r a l top.  The southwest contact has not been s t u d i e d ;  Lowes (1972) d i d not recognise any s u b d i v i s i o n of h i s C h i l l i w a c k Group.  Monger (pers. comm. 1985) has found C h i l l i w a c k Group  north of the Fraser R i v e r , but c o n s i d e r s t h a t most of the rock along  the eastern s i d e of H a r r i s o n Lake does not belong  C h i l l i w a c k Group.  t o the  16  Three main rock types of the Cogburn Creek Group have been mapped.  They are, from west to e a s t : grey p e l i t e ,  chlorite-  a c t i n o l i t e g r e e n s c h i s t , metamorphosed ribbon c h e r t . been metamorphosed up to a t l e a s t garnet r e g i o n a l metamorphism.  A l l have  grade during  Cretaceous  Grade i n c r e a s e s eastward a c r o s s  strike.  Rock Types 1) Grey  Pelite  The  westernmost part of the Cogburn Creek Group a t Cogburn  Creek c o n s i s t s of l i g h t to dark grey p e l i t e .  These rocks are  very f i n e g r a i n e d , micaceous, and may c o n t a i n  porphyroblasts,  u s u a l l y embayed garnet  The c o l o u r of the  or s k e l e t a l  ilmenite.  rock depends on g r a p h i t e content, which may be up to 50% as i n a black shale with magnetite p o r p h y r o b l a s t s . Quartz and p l a g i o c l a s e a r e the main c o n s t i t u e n t s . P l a g i o c l a s e composition  v a r i e s from o l i g o c l a s e to andesine.  B i o t i t e or muscovite are common primary 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 . accessory m i n e r a l s  metamorphic m i n e r a l s ,  Minor a l t e r a t i o n and  include c a l c i t e , orthoclase, epidote,  i l m e n i t e and tourmaline. 2) C h l o r i t e - a c t i n o l i t e g r e e n s c h i s t Greenschist  occurs both as l a y e r s i n the ribbon c h e r t and  as a massive u n i t which c o n t a i n s bands of c h e r t . mineralogy  Present  i s q u a r t z , p l a g i o c l a s e (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 , e p i d o t e , sphene, and a c c e s s o r y m a f i c s .  C h l o r i t e r e p l a c e s hornblende i n most of  the t h i n s e c t i o n s s t u d i e d . unit.  Calcite  i s common i n p a r t s of the  G r a i n s i z e i s g e n e r a l l y f i n e , although  c o a r s e r , knobbly  17  g r e e n s c h i s t s were found. Mineralogy and  a s s o c i a t i o n with  the g r e e n s c h i s t s were o r i g i n a l l y  ribbon c h e r t s suggest that  submarine v o l c a n i c  rocks.  Lowes (1972) found evidence of p i l l o w s south of Cogburn Creek. Calcareous volcanic  rocks were probably  marls i n t e r l a y e r e d with  rocks.  3) Metamorphosed ribbon  chert  A rock c o n t a i n i n g bands of f i n e l y to c o a r s e l y quartz  separated  by  f i n e grained  tourmaline  l a y e r s , and  and  ribbon c h e r t .  z i r c o n are present  i n t o numerous minor f o l d s .  The  Chert  west of Old S e t t l e r Mountain i s i n t e r l a y e r e d and with a 20 m t h i c k band of marble. crystalline, The  and  The  been  Opaques and  i n the  p l a g i o c l a s e i s found with q u a r t z .  been c o n t o r t e d  crystalline  b i o t i t e - r i c h l a y e r s has  i n t e r p r e t e d as metamorphosed d i r t y accessory  the  biotite bands have  on the  ridge  tightly  folded  marble i s c o a r s e l y  i s dark grey with white patches.  u n i t of ribbon c h e r t c o n t a i n s  that become t h i n n e r and chlorite-actinolite  l a y e r s of  greenschist  l e s s numerous eastwards, away from the  greenschist.  The  a s s o c i a t i o n 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  U n i t 4. S e t t l e r The  Schist  S e t t l e r S c h i s t crops out east of the i m b r i c a t e zone  c o r r e l a t e d with the Shuksan T h r u s t . metasedimentary p e l i t i c quartzite. distinct  The  unit  s c h i s t s , with minor a m p h i b o l i t e  In the Gordon Creek area Pigage  rock types w i t h i n the p e l i t i c  schist, graphitic p e l i t i c micaceous q u a r t z i t e .  i s dominated by and  (1973) mapped four  schist: layered p e l t i c  schist, quartzofeldspathic schist,  A s i m i l a r s u b d i v i s i o n was  t h i s study, but although d i f f e r e n t  and  attempted i n  l i t h o l o g i e s c o u l d be  i d e n t i f i e d they d i d not form mappable u n i t s . Two  main episodes of metamorphism have r e c r y s t a l l i s e d  Settler Schist.  the  Contact metamorphism by the Spuzzum b a t h o l i t h  produced a n d a l u s i t e i n rocks of s u i t a b l e composition k i l o m e t r e of the c o n t a c t . metamorphism caused  within a  L a t e r r e g i o n a l (amphibolite  facies)  an i n c r e a s e i n grade up to s i l l i m a n i t e zone  in the northeast corner of the study a r e a .  The  from south to north across the S e t t l e r S c h i s t  grade i n c r e a s e s  i n the study  area,  with g a r n e t - s t a u r o l i t e grade rocks i n the south and east along the Shuksan t h r u s t , through to  sillimanite ± fibrolite  kyanite grade to f i b r o l i t e (Bartholomew 1979).  ± kyanite  This increase  c o r r e l a t e s with p r o x i m i t y to the main body of the Spuzzum b a t h o l i t h , although  i n t r u s i o n was  metamorphism (Bartholomew 1979).  before the peak of A general i n c r e a s e i n g r a i n  s i z e and the appearance of g n e i s s i c banding  and  thin  leucocratic  v e i n l e t s accompany the i n c r e a s e i n grade. Chemical  compositions  of s t a u r o l i t e - b e a r i n g 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 A l content,  low  19  alkali  and Ca content  (Hoschek 1967).  M i n e r a l assemblages i n  the S e t t l e r S c h i s t suggest  a stratigraphic  dominantly  shales with l e s s e r amounts of  Fe- and A l - r i c h  v o l c a n i c and c a l c a r e o u s rocks (Pigage  sequence of  1973).  Rock Types 1) The p e l i t i c  part of the S e t t l e r S c h i s t  i s fine-grained,  and c o l o u r v a r i e s 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  c l e a v a g e , while more micaceous p e l i t e has good cleavage and v a r i a b l e metamorphic c o m p o s i t i o n a l l a y e r i n g .  Some s l a t e on the  northern r i d g e and near d i o r i t e bodies has a spotted appearance due  to p o r p h y r o b l a s t s .  The dominant m i n e r a l assemblage i s  quartz ± p l a g i o c l a s e , b i o t i t e , g r a p h i t e , garnet, with minor and accessory muscovite, sillimanite,  andalusite, kyanite,  f i b r o l i t e , c h l o r i t e , magnetite,  t o u r m a l i n e , or z i r c o n  (detrital).  staurolite,  Garnet,  rutile,  epidote,  staurolite,  a n d a l u s i t e , k y a n i t e and s i l l i m a n i t e occur as p o r p h y r o b l a s t s i n rocks w i t h a p p r o p r i a t e bulk compositions  and metamorphic  A n d a l u s i t e i s pseudomorphed by quartz and muscovite or s i l l i m a n i t e .  Both k y a n i t e and s i l l i m a n i t e  can be found with garnet and s t a u r o l i t e .  Tourmaline  In rocks where i t i s present the b i o t i t e  distinctive  r e d d i s h brown.  in g r a i n s i z e and g r a p h i t e content.  i s a common is a  from the p e l i t e s  mainly  They a r e c o a r s e r than the  p e l i t e s , and c o n t a i n a lower percentage medium t o l i g h t b l u i s h grey.  with k y a n i t e  (and f i b r o l i t e )  accessory.  2) The q u a r t z o f e l d s p a t h i c rocks d i f f e r  grade.  Mineralogy  of g r a p h i t e .  Colour i s  i s s i m i l a r t o the  20  pelitic  s c h i s t s , but i n d i f f e r e n t p r o p o r t i o n s .  3) The micaceous q u a r t z i t e has a sandy appearance and c o l o u r from b i o t i t e .  Mineralogy  An40), b i o t i t e , muscovite, magnetite,  i s pink i n  i s quartz, plagioclase  (An25 to  with v a r y i n g amounts of g r a p h i t e ,  garnet, s t a u r o l i t e , a l u m i n o - s i l i c a t e s ,  tourmaline, epidote, r u t i l e ,  z i r c o n and a p a t i t e .  chlorite, Two  samples,  SS66 and SS125, a l s o c o n t a i n pale green amphibole along the margins of quartz v e i n s , i n d i c a t i n g c a l c - s i 1 i c a t e  assemblages.  In both these rocks the groundmass around  the euhedral  hornblendes  poikilitic  i s mylonitised.  Large  phenocrysts were found i n two  relict  samples,  SS134 and  plagioclase  SS143.  4) Amphibolites are a minor c o n s t i t u e n t i n the S e t t l e r and have been d i v i d e d i n t o two types as i n Pigage Bartholomew (1979).  blue-green hornblende,  The  o r i g i n a l rock was  They c o n t a i n p l a g i o c l a s e ,  q u a r t z , and b i o t i t e , with a c c e s s o r y  chlorite, calcite,  The second  (1973) and  Speckled a m p h i b o l i t e s were found  i n t e r l a y e r e d with other rock types.  magnetite,  Schist,  r u t i l e , c l i n o z o i s i t e and  apatite.  probably marl or c a l c a r e o u s mudstone.  type of amphibolite i s found i n small pods and  l e n s e s w i t h i n the s c h i s t . dark green hornblende,  These pods c o n t a i n l a r g e c r y s t a l s of  and commonly have a very mafic rim around  a l i g h t e r core ( P l a t e 2.2).  They are c o n s i d e r e d to have been  mafic  dykes that were boudinaged  igneous rocks, perhaps  metamorphism and Age  deformation.  r e l a t i o n s of the S e t t l e r S c h i s t w i t h i n the  s t r a t i g r a p h y have not been deciphered as the u n i t all  during  s i d e s by f a u l t s and  intrusive contacts.  regional  i s bounded on  E a r l i e r workers  21  have based c o r r e l a t i o n s on similarities.  The  l i t h o l o g i c a l and  S e t t l e r S c h i s t has  chemical  been c o r r e l a t e d  with the Chiwaukum S c h i s t of the  Skagit  Cascade Mountains (Lowes 1972).  L i t h o l o g i e s and  h i s t o r y are  remarkably s i m i l a r , and  together by  removing 150  the  to 200  S t r a i g h t Creek - Fraser  (Chapter 5)  Thus the  fault  the  two  i n the  metamorphic  u n i t s can  brought  of r i g h t l a t e r a l o f f s e t along system.  Geochronometry  Metamorphic S u i t e  in Washington.  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 The  Rb-Sr whole rock isochron  (Bartholomew, 1979, deposition  be  of the u n i t , which i s s i m i l a r to r e s u l t s of  (1972) f o r the Skagit  evidence.  North  i n d i c a t e s an o l d , p o s s i b l y Precambrian provenance  for at l e a s t part Mattinson  km  Suite  previously  of the  and  date of 210  t h i s study) i n d i c a t e s that  isotopic ± 27  the  the p r o t o l i t h of the S e t t l e r S c h i s t  Ma  time of was  probably e a r l y Mesozoic.  2.5  Premetamorphic i n t r u s i v e rocks Some f o l i a t e d rocks are  i n t r u s i v e rocks, the  i n t e r p r e t e d as pre-metamorphic  f o r example, a l a r g e f o l i a t e d f e l s i c  ridge north of Cogburn Creek.  felsite,  HL  111  is a light  which i s f l e c k e d with black b i o t i t e , and  p a r a l l e l to surrounding c h e r t . ( o l i g o c l a s e ) , quartz, with minor z i r c o n .  foliation.  grey  is foliated  muscovite, s i l l i m a n i t e , b i o t i t e ,  Igneous t e x t u r e s  Muscovite and  on  Mineralogy i s p l a g i o c l a s e  l a r g e myrmekitic p l a g i o c l a s e g r a i n s , recrystallised.  dyke  garnet,  are preserved i n patches of but most of the  rock i s  s i l l i m a n i t e are a l i g n e d along the-  22  G a r n e t - c l u s t e r Dykes Metamorphosed f e l s i c dykes and s i l l s are common i n the Settler Schist. knobbly  The most common type c o n t a i n s d i s t i n c t i v e  c l u s t e r s of pink garnets up to 0.5 cm a c r o s s with  depletion haloes. dykes.  Pigage  white  (1973) named them g a r n e t - c l u s t e r  The dated sample SS85 f a l l s  i n t h i s category.  The rock  i s l i g h t p i n k i s h grey, f i n e g r a i n e d q u a r t z - b i o t i t e s c h i s t garnet c l u s t e r s .  Mineralogy  b i o t i t e or muscovite,  i s quartz, plagioclase  with  (andesine),  garnet and opaques, with minor a p a t i t e ,  z i r c o n , and/or d i o p s i d e .  Relict  zoned p l a g i o c l a s e  p o r p h y r o b l a s t s i n d i c a t e an igneous o r i g i n .  D e p l e t i o n haloes  around the garnets c o n t a i n only quartz and p l a g i o c l a s e .  These  are not n e c e s s a r i l y p o s t - c r y s t a l l i n e r e a c t i o n t e x t u r e s but may be a r e s u l t of o r i g i n a l metamorphic growth of garnet. growing garnet  uses the mafic c o n s t i t u e n t s w i t h i n the  surrounding a r e a , l e a v i n g f e l s i c components Two cleavages a r e present  behind.  i n the dykes, as i n the  surrounding s c h i s t , and are marked by alignment f  2  foliation  The  of b i o t i t e .  The  i s d e f l e c t e d around both p l a g i o c l a s e p o r p h y r o b l a s t s  and garnet c l u s t e r s ,  i n d i c a t i n g a pre-deformation  timing f o r the  halo r e a c t i o n .  2.6  Foliated diorite  i n melange zone  SD 92 was c o 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 w i t h i n the t h r u s t S e t t l e r Creek.  f a u l t zone, on the western  The composition  s i d e of  of t h i s body i s b i o t i t e , q u a r t z ,  p l a g i o c l a s e , garnet, and e p i d o t e , with minor r u t i l e , and white  23  mica. can  Chlorite  i s a l a t e a l t e r a t i o n of b i o t i t e .  be separated i n t o two g e n e r a t i o n s based on t e x t u r e and  composition:  1) p o i k i l i t i c  igneous andesine, An30, and 2) w e l l -  twinned metamorphic andesine, An35. that of  Plagioclase  Geochronometry suggests  i t i s p a r t of the Spuzzum b a t h o l i t h ; however, the presence  metamorphic p l a g i o c l a s e and f o l i a t i o n means that the body  must be o l d e r than the Spuzzum b a t h o l i t h , or one of the e a r l i e s t phases.  2.7 U n i t 5.  Spuzzum B a t h o l i t h  I n t r u s i v e rocks belonging to the Spuzzum b a t h o l i t h form two d i s c r e t e 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.  Creek body i s U n i t 3d of Bartholomew  (1979).  The  Several  Settler  small  bodies of s i m i l a r composition have been mapped as part of the same i n t r u s i v e s u i t e .  Both l a r g e bodies i n t r u d e the S e t t l e r  S c h i s t and not the Cogburn Creek Group.  Crosscutting  r e l a t i o n s h i p s or otherwise with the t h r u s t been determined, as c r i t i c a l  f a u l t zone have not  areas are covered with  glacial  d e b r i s and/or t h i c k v e g e t a t i o n .  The f a u l t zone appears to have  been d i s t o r t e d d u r i n g i n t r u s i o n ,  implying that i t i s o l d e r  the  Spuzzum b a t h o l i t h . Because the d i o r i t e  i s more r e s i s t a n t to e r o s i o n than the  s u p r a c r u s t a l rocks, i t forms h i g h r i d g e s with steep c l i f f s deeply i n c i s e d v a l l e y s . the  than  into  The S e t t l e r Creek body forms part of  r i d g e north of Old S e t t l e r Mountain.  The Hut Creek body  24  spans the Cogburn Creek v a l l e y , up to 1000 m on e i t h e r Hut  side.  Creek b i s e c t s a tongue of d i o r i t e t h a t continues north i n t o  the area mapped by Reamsbottom (1971 and 1974). Most of the i n t r u s i v e rocks a r e h o r n b l e n d e - q u a r t z - d i o r i t e . Accessory b i o t i t e and garnet occur l o c a l l y e i t h e r together or separately.  Hornblende i s g e n e r a l l y brown, or has brown cores  with green metamorphic overgrowths ( P l a t e 2.3). mapped west of the Cogburn Creek f o r k s . hypersthene  - hornblende  Gabbro has been  The composition i s  or c l i n o p y r o x e n e . w i t h minor p l a g i o c l a s e  and m a f i c s , forming an e q u i g r a n u l a r rock that i s extremely resulting above.  i n the d i s t i n c t i v e l y  steep, blocky c l i f f s d e s c r i b e d  P l a t e 2.4 shows replacement  the gabbro.  planes a r e common i n the r e l i c t  1)  t e x t u r e s i n hornblende i n  F i n e grained opaque i n c l u s i o n s along  2.8 U n i t 6.  hornblende  cleavage  cores.  Younger I n t r u s i v e Bodies  Cogburn G r a n o d i o r i t e i s a medium g r a i n e d , l e u c o c r a t i c  granodiorite containing b i o t i t e , plagioclase An60), q u a r t z , with minor hornblende. p l u t o n c r o s s c u t both S e t t l e r S c h i s t Spuzzum b a t h o l i t h 2)  hard,  (Bartholomew  (zoned  The boundaries  of the  f o l i a t i o n and margins of the  1979).  SD 14 comes from a small area of agmatised  south fork of Cogburn Creek.  from An35 t o  The composition  diorite  i n the  suggests  a s s i m i l a t i o n of a g r a p h i t i c member of the S e t t l e r S c h i s t t h a t i t intruded.  The rock can be d i v i d e d  i n g r a i n s i z e and t e x t u r e . diorite,  i n t o two types by d i f f e r e n c e  The c o a r s e r p a r t i s medium grey  s p e c k l e d by b i o t i t e and g a r n e t s ; the f i n e r  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 S c h i s t . size transition change.  The  grain  i s sharp but the major mineralogy does not  It i s quartz, plagioclase  garnet, with minor magnetite,  (oligoclase),  biotite,  a p a t i t e and z i r c o n .  The  finer  part a l s o c o n t a i n s minor g r a p h i t e and green hornblende.  The  geochronometry i n d i c a t e s that t h i s rock i s younger than Cretaceous,  (see Chapter  5), and  i s one  of the younger  i n t r u s i v e s r a t h e r than Spuzzum b a t h o l i t h . 3)  A small body of g r a n o d i o r i t e i n t r u d e s the Cogburn Creek  Group southwest  of Old S e t t l e r Mountain.  The  rock i s l i g h t  grey, f i n e g r a i n e d b i o t i t e - h o r n b l e n d e g r a n o d i o r i t e . i s q u a r t z , p l a g i o c l a s e , b i o t i t e , and hornblende. t e x t u r e s are p r e s e r v e d , and there i s no 4)  Igneous  foliation.  B a s a l t i c dykes c r o s s c u t a l l u n i t s o l d e r than and  Spuzzum b a t h o l i t h .  hornblende,  including  They are f i n e g r a i n e d , grey to g r e e n i s h  grey, and undeformed.  2.9  Mineralogy  Mineralogy  i s quartz, p l a g i o c l a s e ,  and opaques, with minor l a t e c h l o r i t e  Breakenridge Formation  alteration.  Gneiss  Grey g n e i s s from the Breakenridge Formation mapped by Reamsbottom (1971,74) i n the Big S i l v e r River v a l l e y was f o r geochronometry, so a d e s c r i p t i o n and Hutchison  i s i n c l u d e d here.  sampled Roddick  (1967) and Reamsbottom c o n s i d e r e d the grey g n e i s s  c o r i n g the a n t i c l i n a l dome c e n t r e d on Mount Breakenridge the o l d e s t rock i n the r e g i o n . yields conflicting  results  Dating c a r r i e d out i n t h i s  (Chapter 5).  d e s c r i p t i o n i s from Reamsbottom  to be  (1974).  The  following  study  The  Breakenridge  Formation  i s composed mainly  of homogeneous grey g n e i s s and a m p h i b o l i t e . M i g m a t i t i c , banded t o i r r e g u l a r l y banded g n e i s s , pelitic  s c h i s t and skarn form heterogeneous zones  between the grey g n e i s s e s and a m p h i b o l i t e s . The  g r e y . g n e i s s e s are medium-grained with an  a l l o t r i o m o r p h i c granular t e x t u r e . b i o t i t e , quartz and p l a g i o c l a s e l o c a l l y with muscovite,  They c o n s i s t of  (AnlO to An34)  garnet or m i c r o c l i n e .  Myrmekite commonly develops m i c r o c l i n e and p l a g i o c l a s e .  between g r a i n s of  27  3.  Metamorphism  S c h i s t s i n the Cogburn Creek area have been subjected to contact metamorphism from the Spuzzum b a t h o l i t h followed by r e g i o n a l metamorphism d u r i n g the Cretaceous.  Lowes (1972)  recognised an e a r l i e r metamorphism i n the Precambrian  or  Paleozoic Baird Metadiorite.  3.1  Unit 1.  Baird Metadiorite  The present mineral assemblage of hornblende,  plagioclase,  q u a r t z , c l i n o z o i s i t e , c h l o r i t e and opaques r e p r e s e n t s metamorphism of mafic  intrusive  amphibolite s u b f a c i e s .  rocks to  Hornblende i s p a l e green  a c t i n o l i t i c , and occurs as l a r g e ragged appear to have been l e f t e s s e n t i a l l y surrounding may  andesine-epidote  porphyroblasts.  a u g i t e cores may  igneous o r i g i n . be of primary  Rare brown cores  In sample MD1  igneous o r i g i n .  rimmed by p a l e green hornblende.  These  undeformed while  f e l s i c m i n e r a l s were fragmented.  be of primary  and  rare r e l i c t They are  now  P l a g i o c l a s e compositions  cores of l a r g e p o r p h y r o b l a s t s i n MD1  reach An42, while rims  smaller g r a i n s have r e c r y s t a l l i s e d at around An35.  Epidote  in and and  a l b i t e have r e p l a c e d the p l a g i o c l a s e .  3.2  U n i t 2.  U l t r a m a f i c Rocks  Both the o r i g i n a l composition  of the u l t r a m a f i c rocks  t h e i r degree of metamorphic r e c o n s t i t u t i o n samples of d u n i t e were found,  is variable.  Several  f o r example HL30, c o n t a i n i n g  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  and  28  tremolite HL31  ( P l a t e 3.1).  HL33 c o n s i s t s almost e n t i r e l y of  and HL99 c o n t a i n t r e m o l i t e with minor l a t e  alteration.  The  other  c a l c i t e or magnesite, with chlorite.  opaques and  late chlorite lamellae.  strained after  l a t e r a l t e r a t i o n to t a l c ,  rock and continued  3.3  serpentine,  T h i s suggests that the rock  i t recrystallised.  f a u l t plane  serpentine  i n v e i n s , some of the c a l c i t e e x h i b i t s  t r i b u t a r y to S e t t l e r Creek visible  clinopyroxene,  In HL54, c o n s i s t i n g of c a l c i t e ,  deformed twin  serpentine  rocks a l l c o n t a i n v a r y i n g p r o p o r t i o n s of  o l i v i n e , chromite or magnetite, e n s t a t i t e or  and  talc;  Its location,  (Appendix D),  was  i n the western  i s very c l o s e to a  s e p a r a t i n g S e t t l e r S c h i s t from u l t r a m a f i c  Baird Metadiorite.  Thus there may  have been some  movement on t h i s f a u l t a f t e r metamorphism.  Unit 3.  Cogburn Creek Group  Lack of s u i t a b l e l i t h o l o g i e s i n the Cogburn Creek Group makes r e c o g n i t i o n of the metamorphic grade d i f f i c u l t . t e x t u r e s r e c o r d three phases of deformation and  f , with d e c r e a s i n g 3  respect to these  age.  The  and  folding, f,, f  timing of mineral  i s shown i n F i g u r e  3.1a. to the  Mica f o l i a t i o n developed during f , and 2  i n d i v i d u a l f l a k e s have r e c r y s t a l l i s e d w i t h i n t h i s f o l i a t i o n parallel  f . 3  2  growth with  1) In the grey p e l i t e , b i o t i t e ± muscovite belong e q u i l i b r i u m assemblage.  Mineral  In sample HL16  l a r g e brown p o r p h y r o b l a s t s  ( P l a t e 3.2)  biotite  to  i s found as  r e s u l t i n g from e a r l y contact  metamorphism by Spuzzum b a t h o l i t h .  These appear to have been  29  Phase 1  Phase 2  Phase3  Posttectonic  Chlorite Biotite Muscovite Garnet Tourjn aline Hornblende  Figure  3.1a  —  -  —  R e l a t i v e ages of metamorphic m i n e r a l growth with respect to deformation, Cogburn Creek Group.  Phase 1  Phase 2  Phase 3  Posttectonic  Chlorite Biotite Muscovite Andalusite Garnet Staurolite  —  Kyanite Fibrolite Sillimanite Tourmaline  Figure  3.1b  R e l a t i v e ages of metamorphic m i n e r a l growth with respect to deformation, S e t t l e r S c h i s t .  30  r o t a t e d - d u r i n g the formation of f by quartz i n p r e s s u r e shadows. p o r p h y r o b l a s t s are a l s o found  f o l i a t i o n , and are  2  Large s k e l e t a l in t h i s  surrounded  ilmenite  sample.  Textures i n garnet p o r p h y r o b l a s t s i n d i c a t e that they formed d u r i n g and a f t e r  f, and f  2  deformation.  They c o n t a i n i n c l u s i o n s  and show evidence of r o t a t i o n and synkinematic (e.g. HL15,  P l a t e 3.3).  i n c l u s i o n s that mark f k i n e m a t i c a l l y to f , . synkinematic  to f  Chlorite HL16,  2  t  A garnet  i n HL15  and a s o l i d  has a core with  rim that grew post-  Small euhedral garnets i n HL16  (Plate  i s p a r t of the main e q u i l i b r i u m assemblage i n  Tourmaline  during f . 2  are  3.2).  but i n HL103 i t i s a l a t e replacement  garnet.  growth  where present appears  of b i o t i t e  and  to have c r y s t a l l i s e d  A l u m i n o - s i l i c a t e s were not found, but that may  be a  f a c t o r of bulk composition r a t h e r than metamorphic c o n d i t i o n s . Lowes (1972) found s t a u r o l i t e p o r p h y r o b l a s t s i n p e l i t i c in the upper p a r t of Three M i l e Creek,  i n d i c a t i n g that  higher range of e p i d o t e a m p h i b o l i t e f a c i e s was  schist the  reached.  2) The main assemblage i n the g r e e n s c h i s t i s p l a g i o c l a s e , q u a r t z , hornblende, chlorite.  Two  e p i d o t e , c a l c i t e , magnetite,  g e n e r a t i o n s of p l a g i o c l a s e are p r e s e n t .  phenocrysts with graphic intergrowths are c a l c i c (An28), and  igneous  composition  (An8  segregations.  in o r i g i n .  The  Early  albite  to AnlO), and occurs a l i g n e d i n f e l s i c  Hornblende i s p a l e blue-green.  The  and  oligoclase  l a t e r g e n e r a t i o n has  (HL77) are found as r e l i c t s from the o r i g i n a l assemblage.  biotite,  Rare brown c o r e s igneous  g r a i n s are g e n e r a l l y s m a l l , euhedral, and  31  a l i g n e d with the f o l i a t i o n around i t , suggesting after  f . 2  In HL80 ( P l a t e 3.4) l a r g e hornblende g r a i n s 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  g r a i n s are randomised by r e c r y s t a l l i s a t i o n , c r y s t a l l i s a t i o n during and a f t e r calcite  crystallisation  fills  smaller  suggesting  f . In t h i s same sample 2  gashes that formed d u r i n g l a t e kink  folding.  HL142 ( P l a t e 3.5) c o n t a i n s l a r g e p o r p h y r o b l a s t s of dark browngreen  t o blue-green  hornblende with i n c l u s i o n s of e p i d o t e ,  p l a g i o c l a s e and opaques.  Although  these p o r p h y r o b l a s t s  deflect  the f o l i a t i o n , at l e a s t one c o n t a i n s i t and a t h i r d phase i n d i c a t i n g growth l a t e  i n the t h i r d phase.  at t h i s time, and are now rounded. sieve-textured porphyroblasts  Garnets  Epidote  a l s o formed  i s found both as  (HL77) and as f i n e g r a i n e d  replacement  of p l a g i o c l a s e (HL80).  replacement  of b i o t i t e , hornblende and garnet.  C h l o r i t e occurs as a l a t e  3) The ribbon c h e r t has been r e c r y s t a l l i s e d metamorphism to a c o a r s e - g r a i n e d aggregate plagioclase.  fold,  during  of q u a r t z and  B i o t i t e r i c h l a y e r s are f i n e r g r a i n e d and  represent more a r g i l l i c  l a y e r s i n the c h e r t .  A second  cleavage  i s marked by b i o t i t e f l a k e s o r i e n t e d at an angle a c r o s s the layers.  No other metamorphic index m i n e r a l s were r e c o g n i s e d .  3.4 U n i t 4, S e t t l e r  Schist  Metamorphism of the S e t t l e r S c h i s t was s t u d i e d i n d e t a i l by Pigage  (1973) i n the Gordon Creek area and by Bartholomew  in the Yale Creek a r e a .  The metamorphic c o n d i t i o n s i n the  Cogburn Creek area are s i m i l a r , producing  the same m i n e r a l  (1979)  32  assemblages. As  in the Cogburn Creek Group, three phases of f o l d i n g  recognised.  F i g u r e 3.1b  for S e t t l e r S c h i s t . inclusion t r a i l s Two The  episodes  first  The  g i v e s mineral earliest  in e a r l y garnets  growth age r e l a t i o n s h i p s  foliation  i s seen only i n  (Bartholomew 1979).  of metamorphism are recognised  in these  i s high temperature contact metamorphism due  Spuzzum b a t h o l i t h .  Both the Hut  produced t h i s e f f e c t . metamorphism has  up to a k i l o m e t r e  Creek body and  No  L a t e r upgrading due  from the  to r e g i o n a l  caused r e c r y s t a l l i s a t i o n of the a n d a l u s i t e i n t o kyanite or  a n d a l u s i t e remained i n most of the  s t u d i e d in t h i n s e c t i o n . SS43, SS80, SS96. before  the  S e t t l e r Creek body  aggregates of q u a r t z , muscovite, s t a u r o l i t e , and sillimanite.  to  rocks.  I t produced c h i a s t o l i t i c a n d a l u s i t e in  s c h i s t s of s u i t a b l e composition p l u t o n margins.  are  Some small r e l i c t  specimens  g r a i n s were found i n  Several c h i a s t o l i t e s were deformed during  recrystallisation  (SS110, P l a t e 3.6).  o r i e n t a t i o n of the minerals  The  2  lack of  in the pseudomorphs i n d i c a t e s that  r e c r y s t a l l i s a t i o n took p l a c e under s t a t i c c o n d i t i o n s . s t a u r o l i t e that has  f ,  In  SS110  grown across the boundary of a  p s e u d o c h i a s t o l i t e c o n t a i n s g r a p h i t i c i n c l u s i o n s of the e x t e r n a l f o l i a t i o n where i t i s o u t s i d e the p s e u d o c h i a s t o l i t e . the t i m i n g of pseudomorphing and and  before  f  3  s t a u r o l i t e growth as a f t e r  f  2  deformation.  C o e x i s t i n g b i o t i t e and muscovite are u b i q u i t o u s the s c h i s t .  This gives  They d e f i n e the  f  2  f o l i a t i o n , and  r e c r y s t a l l i s e d around small s c a l e f  3  kink  throughout  are  folds.  In SS135  33  ( P l a t e 3.7) f  2  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  fold. Garnet  to t,  t e x t u r e s are i n d i c a t i v e of growth from  to post f . 2  E a r l y garnets are now  q u a r t z pressure shadows or h a l o e s . with s i l l i m a n i t e are rounded and  rounded and may  Garnets  and  fibrolite  f ;  i t v a r i e s from synkinematic  3.9  shows p o s t - t e c t o n i c twinned s t a u r o l i t e  2  inclusions.  f  f  2  and  3  The  in a s s o c i a t i o n  by q u a r t z , muscovite  to f  3  to p o s t - t e c t o n i c . in p e l i t e .  after Plate  Many are  and c o n t a i n g r a p h i t e or  i n c l u s i o n t r a i n s mark f  2  foliation  than  is usually  staurolite.  Kyanite, where present,  i s i n p s e u d o c h i a s t o l i t e , having  r e p l a c e d a n d a l u s i t e during thermal and pressure upgrading metamorphism.  of  Most of the a l u m i n o - s i l i c a t e s i n rocks i n 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 . s i l l i m a n i t e p o r p h y r o b l a s t s reach n o r t h r i d g e of the area  10 to 20 cm  The  i n l e n g t h on  the  grade of r e g i o n a l  metamorphism i n c r e a s e s towards t h i s area. c r y s t a l s c o n t a i n patches  Crystalline  ( P l a t e 3.13), i n c l o s e p r o x i m i t y to the  Spuzzum b a t h o l i t h to the n o r t h .  of f i b r o l i t e and  b i o t i t e and opaques, and have ragged r e g u l a r halo of quartz and b i o t i t e Elsewhere,  and  f o l d s w i t h i n the mineral g r a i n s (SS53, P l a t e 3.11).  In many of the rocks c o n t a i n i n g both m i n e r a l s , garnet earlier  have  Staurolite crystallised  c o n c e n t r i c a l l y zoned (SS114, P l a t e 3.10) quartz  found  surrounded  (SS66, P l a t e 3.8).  synkinematic  In t h i n s e c t i o n these i n c l u s i o n s of q u a r t z ,  outlines within a  fairly  (SSl82a, P l a t e 3.14).  small c r y s t a l s produce a s p o t t e d or streaky  appearance i n 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 w i t h i n the f o l i a t i o n but randomly o r i e n t e d , and i s therefore post-kinematic.  I t appears to have formed a t the same  time as and s l i g h t l y before  porphyroblastic  sillimanite.  Tourmaline i s a minor but common c o n s t i t u e n t of the S e t t l e r Schist.  I t appears t o have formed during  grow p o s t - t e c t o n i c a l l y . zoning  with g r a p h i t e  the same rocks. distinctive  2  and continued t o  Some l a r g e r g r a i n s show c o n c e n t r i c  i n c l u s i o n s , s i m i l a r to s t a u r o l i t e g r a i n s i n  Tourmaline-bearing rocks a l s o c o n t a i n  r e d d i s h brown b i o t i t e .  On the b a s i s of the mineral zones of Barrovian Schist  f  a s s o c i a t i o n s four metamorphic  s e r i e s have been recognised  i n the study a r e a .  i n the S e t t l e r  They a r e as f o l l o w s , i n order of  i n c r e a s i n g grade: 1) G a r n e t - s t a u r o l i t e : b i o t i t e - garnet - quartz  - staurolite - (ilmenite - r u t i l e )  2) S t a u r o l i t e - k y a n i t e : b i o t i t e - garnet p l a g i o c l a s e - quartz  - fibrolite  ± kyanite  - muscovite - p l a g i o c l a s e -  ± staurolite - (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 quartz  - fibrolite  The represent  first  - muscovite - p l a g i o c l a s e -  - sillimanite ± staurolite  sillimanite  (fibrolite)  isograd i s c o n s i d e r e d t o  e q u i l i b r i u m (Bartholomew 1979); i t marks the f i r s t  appearance of f i b r o l i t e .  The coarse  the appearance of p o r p h y r o b l a s t i c from f i n e grained kyanite.  - kyanite - muscovite -  - 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 quartz  - muscovite - p l a g i o c l a s e  sillimanite  sillimanite  sillimanite  i s o g r a d marks  recrystallising  ( f i b r o l i t e ) and disappearance of  Bartholomew (1979) c o n s i d e r s  that i t r e p r e s e n t s  35  overstepping  of the kyanite to s i l l i m a n i t e r e a c t i o n .  i s found i n s i l l i m a n i t e - b e a r i n g rocks but  Staurolite  i s not n e c e s s a r i l y  a s s o c i a t e d with the f i b r o l i t e - b i o t i t e masses or  sillimanite  porphyroblasts. Although d e t a i l s of the r e l a t i o n s h i p s between metamorphic i s o g r a d s and  i n t r u s i v e bodies are not c l e a r , the grade  towards the n o r t h e a s t . recognised  On a l o c a l  increases  s c a l e , Bartholomew (1979)  a northwestern trend i n Yale Creek, towards h i s Unit  3e, F a g e r v i c k  body of the Spuzzum b a t h o l i t h .  T h i s was  part of the Spuzzum b a t h o l i t h i n h i s study area  the  last  to i n t r u d e ,  and  p r o v i d e d a d d i t i o n a l heat f o r the r e g i o n a l metamorphism to the northeast  of Cogburn Creek.  Metamorphic grade changes a b r u p t l y  a c r o s s Cogburn Creek, l e n d i n g support along  the south  eastern  to r e c o g n i t i o n of a  branch (Bartholomew 1979).  The  s c h i s t on  s i d e i s in s t a u r o l i t e - k y a n i t e zone, while  rock  fault the  directly  west i s i n g a r n e t - s t a u r o l i t e zone.  3.5  Premetamorphic i n t r u s i v e The  felsic  sill  HL111  c o n t a i n s s k e l e t a l garnet  that i n t r u d e s Cogburn Creek Group porphyroblasts  with b i o t i t e and muscovite. foliation.  The  rock was  rocks  the country  fibrolite  Large mica g r a i n s d e f i n e  c o l l e c t e d from w i t h i n 50 m of  c o n t a c t with Spuzzum b a t h o l i t h Hut i s most probably  and  Creek body.  the r e s u l t of l o c a l  The  intergrown f  2  the  sillimanite  i n c r e a s e i n temperature in  rock by heat from the d i o r i t e d u r i n g r e g i o n a l  metamorphi sm..  36  3.6  Discussion  Ultramafic  rocks  C o n d i t i o n s of metamorphism in the u l t r a m a f i c rocks appear c o n s i s t e n t with those of the surrounding the rocks were a l r e a d y juxtaposed  schist,  i n d i c a t i n g that  before the c u l m i n a t i o n of  r e g i o n a l metamorphism. The  mineral assemblages can be used to p l a c e broad  on metamorphic c o n d i t i o n s across the u l t r a m a f i c b e l t . (1973) notes  that c o e x i s t i n g serpentine and  temperature below 500°C with l e s s than the f l u i d phase (Greenwood 1967,  0.1  talc  limits Pigage  indicate a  mole f r a c t i o n C0  Johannes 1969).  2  in  The  a s s o c i a t i o n , seen here i n HL55, HL56, HL65, p r o v i d e s a lower temperature l i m i t of 300°C (Figure 3.2).  HL56 c o n t a i n s t a l c  magnesite, which i s i n the same, s t a b i l i t y  field.  c r y s t a l s of o l i v i n e  i n HL30 ( P l a t e 3.1)  and  Tabular  i n d i c a t e metamorphic  r e c o n s t i t u t i o n of s e r p e n t i n i t e back to o l i v i n e + t a l c , which r e q u i r e s a minimum temperature of 350°C.  Contact  Metamorphism  Bartholomew (1979) c o n s i d e r s that the a n d a l u s i t e formed under s t a b l e c o n d i t i o n s at a pressure  below the a l u m i n o - s i l i c a t e  t r i p l e point  (1969b)  metastable stability  (Figure 3.3).  crystallisation field  Hollister  of a n d a l u s i t e in the  kyanite  f o r s i m i l a r a n d a l u s i t e pseudomorphs in the  Kwoiek area, with subsequent r a p i d c o n v e r s i o n then  suggested  sillimanite.  to kyanite  However, t h i s would r e q u i r e a  steep  and  Figure  3.2  Schematic T~X(C0 ) diagram f o r the system MgO-Si0 H 0-CO at e l e v a t e d pressures and temperatures. Adapted from Johannes (1969). 2  2  2  2  38  2 400  '  1  1  1  1  600  800  T,%C 4 P r e s s u r e - T e m p e r a t u r e gradient /  s t a b l e f o r m a t i o n of a n d a l u s i t e  4 P r e s s u r e - T e m p e r a t u r e gradient I  assuming  m e t a s t a b l e f o r m a t i o n of a n d a l u s i t e kyanite  F i g u r e 3.3  assuming  field (Hollister  in  1969b)  Pressure-temperature c o n d i t i o n s i n the p e l i t i c assemblages, adapted from Pigage (1976). A Al Si0 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 S t a u r o l i t e + Muscovite + Q u a r t z = A l - s i l i c a t e + B i o t i t e + vapour (Hoschek 1969) C Minimum temperature e q u i l i b r i u m p o s i t i o n of curve C based on f i e l d evidence D F e - s t a u r o l i t e + Quartz=Almandine + S i l l i m a n i t e + 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). 2  5  39  temperature g r a d i e n t with l i t t l e change i n pressure with  i n t r u s i o n of d i o r i t e at depths of  1969b).  17 to 21  Subsequent s t u d i e s have shown that  interpretation pressure  is unlikely.  km  associated (Hollister  Hollister's  Bartholomew (1979) estimated  of r e g i o n a l metamorphism to be up to 3 kb above the  a l u m i n o - s i l i c a t e t r i p l e p o i n t , and  noted that  14 km of  burial  would be r e q u i r e d to produce the d i f f e r e n c e between contact r e g i o n a l metamorphic c o n d i t i o n s .  and  He c a l c u l a t e d that the Spuzzum  b a t h o l i t h must have been emplaced at depths l e s s than 14 km, that a depth of b u r i a l of 27 ± 2 km of 7.6  ± 0.5  to a  pressure  kb) must have been reached d u r i n g r e g i o n a l  metamorphism.  Since a n d a l u s i t e formed before  p e r i o d of deformation, by s t a b l e formation  Regional  (corresponding  and  the most  intense  the time and pressure d i f f e r e n c e implied  i s q u i t e reasonable  (Bartholomew 1979).  Metamorphism  P e l i t e s of the Cogburn Creek Group belong  to the  g r e e n s c h i s t f a c i e s of r e g i o n a l metamorphism, grading  from the  c h l o r i t e s u b f a c i e s i n the west on the shore of H a r r i s o n Lake ( o u t s i d e the map  area) to the b i o t i t e s u b f a c i e s .  c o n d i t i o n s range between 300°C and recognised AFM  s t a u r o l i t e grade rocks  450°C.  Lowes (1972)  i n upper Three M i l e Creek.  diagram r e p r e s e n t i n g the metamorphic mineral  found i n the Cogburn Creek Group p e l i t e Mineralogy  facies.  An  assemblage  i s in Figure  i n the g r e e n s c h i s t suggests that the  metamorphic grade i s upper g r e e n s c h i s t amphibolite  Temperature  3.4. present  f a c i e s to kyanite zone of  An ACF-AKF diagram of the  mineral  40  Key Bi=Biotite Cc*=Calcite ChfeChlorite Ep = E p i d o t e Gar-Garnet  F i g u r e 3.4  Hb=Hornblende llm=llmenite Mu=Muscovite PI=Plagioclase Rtl=Rutile  AFM p r o j e c t i o n of metamorphic m i n e r a l assemblages found i n p e l i t e s from the g a r n e t - s t a u r o l i t e zone, and ACF-AKF p r o j e c t i o n of assemblages i n g r e e n s c h i s t , from the Cogburn Creek Group.  41  Gar  Bi STAUROLITE-GARNET A  A  Fib  FIBROLITE  ZONE  ZONE  STAUROLITE-KYANITE A.  ZONE  Sil  SILLIMANITE Z O N E  Key Bi=Biotite FibsFibrollte Gar=Garnet llm=llmenite Ky=Kyanite F i g u r e 3.5  Mu=Muscovite PUPIagioclase OOuartz Rtl-Rutile Sil=Sillimanite  AFM p r o j e c t i o n of metamorphic m i n e r a l assemblages found i n p e l i t e from the g a r n e t - s t a u r o l i t e , s t a u r o l i t e - k y a n i t e , f i b r o l i t e , and s i l l i m a n i t e zones, from the S e t t l e r S c h i s t .  42  assemblage  found i n the rocks i s i n F i g u r e 3.4.  The temperature  of metamorphism must have been between 300 and 525°C, probably in the higher p a r t of the range.  Sampling was not d e t a i l e d  enough to enable r e c o g n i t i o n of d i s c o n t i n u i t i e s  i n metamorphic  grade. M i n e r a l assemblages  found i n each of the metamorphic  in the S e t t l e r S c h i s t around Cogburn diagrams i n F i g u r e 3.5.  Creek are p l o t t e d on AFM  Pigage (1973) d i d r e g r e s s i o n  on p e l i t i c m i n e r a l assemblages  zones  analysis  from the S e t t l e r S c h i s t to  provide i n f o r m a t i o n on p o s s i b l e r e a c t i o n s based on m i n e r a l compositions.  He found s e v e r a l r e a c t i o n s with too many c o -  e x i s t i n g phases, r e s u l t i n g reaction r e l a t i o n .  from d i s e q u i l i b r i u m or a u n i v a r i a n t  C o - e x i s t i n g k y a n i t e - s t a u r o l i t e - garnet -  b i o t i t e - muscovite - q u a r t z i s u n i v a r i a n t on the AFM projection.  T h i s assemblage  i s found i n a 3 km wide zone i n  Gordon Creek  (Pigage 1973), and i s r e c o g n i s e d i n Cogburn  Creek  (for example SS65, SS202). Bartholomew (1979) found that two r e a c t i o n s were i n v o l v e d in the replacement of a n d a l u s i t e .  Some took p l a c e v i a d i r e c t  polymorphic t r a n s f o r m a t i o n to k y a n i t e or s i l l i m a n i t e . Pseudomorphs c o n t a i n i n g s t a u r o l i t e  imply a replacement  reaction  of the type: a n d a l u s i t e + b i o t i t e + H 0 = s t a u r o l i t e + muscovite + q u a r t z . 2  Regression a n a l y s e s by Pigage (1973) suggest two s i l l i m a n i t e - forming r e a c t i o n s of the form: 1) s t a u r o l i t e + muscovite + q u a r t z + r u t i l e = s i l l i m a n i t e + b i o t i t e + i l m e n i t e +H 0 2  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 i n each r e a c t i o n and the m i n e r a l s represent 8 components, these r e a c t i o n s must have been continuous  r a t h e r than d i s c o n t i n u o u s (Bartholomew 1979). A l l  the phases c o - e x i s t  i n the s i l l i m a n i t e zone i n Cogburn Creek.  Bartholomew (1979) i n t e r p r e t s the f i b r o l i t e and s i l l i m a n i t e isograds as r e p r e s e n t i n g e q u i l i b r i u m and overstepped sillimanite transitions  kyanite-  respectively.  Estimates of metamorphic p r e s s u r e and temperature f o r pelitic  rocks i n the S e t t l e r S c h i s t were obtained by Pigage  (1973, 1976) and Bartholomew (1979) by means of mutual i n t e r s e c t i o n of s e v e r a l c a l c u l a t e d e q u i l i b r i a accuracy of the estimate i s based  ( F i g u r e 3.6). The  on the assumption  e q u i l i b r i u m , accuracy and p r e c i s i o n of microprobe  of chemical  analyses,  v a l i d i t y of s o l i d - s o l u t i o n models, and the q u a l i t y of thermodynamic data presence  (Pigage  1976).  of u b i q u i t o u s s t a u r o l i t e  Pigage notes that the i n the lowest grade rocks i n  the S e t t l e r S c h i s t d e f i n e s a minimum temperature f o r metamorphism of approximately  540°C.  He concludes that r e g i o n a l  metamorphism of the S e t t l e r S c h i s t took p l a c e under c o n d i t i o n s between 550 and 700°C, and 6 t o 8 kbar.  Bartholomew  estimated c o n d i t i o n s i n the v i c i n i t y of the f i b r o l i t e  (1979) (fine  g r a i n e d s i l l i m a n i t e ) i s o g r a d 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 l o g f ( 0 ) between -18.1 and -17.3. 2  He was not able to determine  upper l i m i t s on c o n d i t i o n s i n the p e l i t i c  rocks of the h i g h e s t ,  44  400  500  600  700  800  Temperature, ° C  F i g u r e 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 a t a(H O)=<0.81, p l u s 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 , a d a p t e d from Bartholomew (1979). I n t e r s e c t i o n s used t o c a l c u l a t e metamorphic p r e s s u r e and t e m p e r a t u r e conditions 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 s c a l e d o f f from P i g a g e and Greenwood (1982), a r e c e n t r e c a l c u l a t i o n of t h e e q u i l i b r i u m of I I I . 2  2  45  coarse s i l l i m a n i t e grade. Recent r e c a l c u l a t i o n of the e q u a t i o n : s t a u r o l i t e + quartz = almandine + A l S i 0 2  5  down the s t a b i l i t y  + H 0 (Pigage and Greenwood 1982), 2  field  narrows  f o r 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) e s t i m a t e of metamorphic  conditions  Metamorphic  (Figure 3.6).  c o n d i t i o n s i n the c o r r e l a t i v e Chiwaukum S c h i s t  (Lowes 1972) a r e s i m i l a r .  G e t s i n g e r (1978) found c h i a s o l i t i c  a n d a l u s i t e p r e d a t i n g kyanite and s i l l i m a n i t e ,  i n Chiwaukum  S c h i s t on Nason Ridge, Stevens Pass.  (1969,  Plummer  1980) had  found the same m i n e r a l assemblages and t e x t u r e s but m i s i n t e r p r e t e d the age r e l a t i o n s h i p s of the a l u m i n o - s i l i c a t e s (Evans and B e r t i clear  1985).  G e t s i n g e r (1978) was unable t o map  isograds i n the Chiwaukum S c h i s t , because of o v e r l a p i n  assemblages.  In the Stevens Pass area B e r t i  ( p e r s . comm. 1983)  c a l c u l a t e d a temperature of 550°C from g a r n e t - b i o t i t e p a i r s and a p r e s s u r e of 4 t o 6 kbar.  At Wenatchee Ridge, McLaughlin  (pers. comm. 1985) e s t i m a t e s 550°C and 6 t o 8 kbar using garnetp l a g i o c l a s e geobarometry,  suggesting a pressure gradient  a s s o c i a t e d with c r u s t a l t h i c k e n i n g a f t e r Stuart b a t h o l i t h  (Evans and B e r t i  i n t r u s i o n of the Mount  1985).  Isograds have been drawn f o r the Cogburn 3.7), based on m i n e r a l assemblages.  The complex  between zones suggests post-metamorphic r e l a t e d t o the d i o r i t e b o d i e s .  Creek area (Figure relationships  deformation, p o s s i b l y  In c o n t r a s t t o the r e l a t i o n s h i p s  found by Bartholomew (1979) around Gordon Creek, the zone c o n t a i n i n g 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  F i g u r e 3.7  Map of metamorphic mineral assemblages and i s o g r a d s for the Cogburn Creek a r e a . See. F i g u r e 2.1 f o r key to rock u n i t s . A b b r e v i a t i o n s of mineral names a r e : 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'  C Chlorit Q Garnet 1  G-St  49°30'  p  Garnet-  Staurolite S t - K y Staurolit -Kyanite "  Fib Fibrolit Sill Sillimanite \ Kyanite in > Fibrolite in * Sillimanite  121°45'  F i g u r e 3.8  121°30'  R e g i o n a l i s o g r a d s , m o d i f i e d from Bartholomew (1979), Lowes (1972), Pigage (1973), and Reamsbottom (1971, 1974). See F i g u r e 1.2 f o r key t o rock u n i t s .  48  d i r e c t l y r e l a t e d to contact metamorphism by the Cogburn Granodiorite.  The  throughout the  region i n d i c a t e s l a r g e volumes of magma at  shallow  presence of numerous small i n t r u s i v e  depths (Pigage  areas  F i g u r e 3.8.  For comparison  a r e g i o n a l s y n t h e s i s of isograds  Information  Bartholomew  fairly  1973), which c o u l d have s u p p l i e d some of  the heat f o r the r e g i o n a l metamorphism. surrounding  bodies  with  i s given i n  has been taken from Pigage  (1973),  (1979), Reamsbottom (1974) and Lowes (1972).  Grade  of r e g i o n a l metamorphism g e n e r a l l y i n c r e a s e s northeastward, toward the l a r g e northern the  p l u t o n of Spuzzum b a t h o l i t h .  However,  isograds are not p a r a l l e l to the pluton margins, and  p l a c e s are t r u n c a t e d by the  intrusives.  Evans and  in  Berti  (1985)  account f o r r e g i o n a l metamorphism of the Chiwaukum S c h i s t as r e l a x a t i o n of  isotherms  d u r i n g an episode  r e l a t e d to the Mount S t u a r t b a t h o l i t h ,  of c r u s t a l t h i c k e n i n g soon a f t e r  intrusion.  They propose that a f t e r dynamic c o n t a c t metamorphism heat from the b a t h o l i t h i n t o the surrounding about 600  °C.  flowed  rocks, l e v e l l i n g out  to  T h i s corresponds to a c o n t i n e n t a l - c r u s t  geothermal g r a d i e n t of 30  °C/km and  pressure  of 6 kbar.  Thus  they c o n s i d e r  t h a t the r e g i o n a l metamorphism of the Mount S t u a r t  b a t h o l i t h and  nearby contact-metamorphosed rocks was  and  retrograde,  that of the Chiwaukum S c h i s t d i s t a n t from the b a t h o l i t h was  prograde.  Since  c o r r e l a t i v e and i n t r u s i o n and  the Chiwaukum S c h i s t and probably  formed a continuous  b e l t at the time of  metamorphism, the same i n t e r p r e t a t i o n can  a p p l i e d to the area east of H a r r i s o n Lake. b a t h o l i t h and  the S e t t l e r S c h i s t are  Scuzzy D i o r i t e to the north  The  be  Spuzzum  form a much l a r g e r  49  body than the Mount Stuart b a t h o l i t h  (Figure 2.2), so that the  heat flow from i t would have been g r e a t e r . for  T h i s would account  the higher-grade metamorphic assemblages seen  S c h i s t than i n the Chiwaukum S c h i s t .  The metamorphism of the  Cogburn Creek Group rocks would be prograde. (1981) r e c o g n i s e d a 30° t i l t paleomagnetic  schist.  Beck e t a l .  down to the southeast  from  evidence from the Mount Stuart b a t h o l i t h .  were a p p l i e d to the Spuzzum b a t h o l i t h , northeastward  i n the S e t t l e r  i t c o u l d a l s o e x p l a i n the  increase of metamorphic grade  Recent  paleomagnetic  If t h i s  i n the surrounding  r e s u l t s of I r v i n g et a l .  (1985)  on the Spuzzum D i o r i t e c o u l d be i n t e r p r e t e d as i n d i c a t i n g 28° tilt  down t o the southeast, r a t h e r than l a t e r a l  (they c o n s i d e r that t h e i r data are i n s u f f i c i e n t alternative).  displacement to prove  either  50  4.  Structure  Megascopic s t r u c t u r e i n the Cogburn creek area in the steep i m b r i c a t e zone and d e f l e c t i o n s around bodies.  i s expressed intrusive  Three o u t c r o p - s c a l e phases of f o l d i n g have been  recognised, c o n s i s t e n t with o b s e r v a t i o n s of Lowes (1972) and Bartholomew (1979).  The  e a r l i e s t deformation  only from i n c l u s i o n t r a i l s  in p o r p h y r o b l a s t i c garnet and  o r i e n t a t i o n of b i o t i t e p o r p h y r o b l a s t s foliation assigned  Settler Schist i s  ( F i g u r e 4.1,  compositional  4.2).  isoclinal  l a y e r i n g , where  i s g e n e r a l l y p a r a l l e l or s u b p a r a l l e l to the f  foliation  area was  Compositional  re-  Strong mica  i s p a r a l l e l to the a x i a l planes of  2  f o l d s up to 1 m in amplitude. present,  (HL16).  i n both the Cogburn Creek Group and to f , and  (f,) i s recognised  2  Pigage (1973) considered that  l a y e r i n g i n S e t t l e r S c h i s t i n the Gordon Creek  o r i g i n a l graded bedding and was  younging d i r e c t i o n .  able to determine a  However in the area I have mapped i t may  a l s o be metamorphically  transposed  layering.  Outcrops showing  "bedding" were sparse around Cogburn Creek, and graded bedding or other  f a c i n g c r i t e r i a were not seen, making i t impossible  s p e c i f y a younging  direction.  F o l d s t y l e of minor f o l d s a s s o c i a t e d with f to t i g h t , rounded to chevron.  On  the map  o r i e n t a t i o n p a r a l l e l s the t r e n d of the map of p l u t o n s .  In the S e t t l e r S c h i s t f  northeast  ( F i g u r e 4.2),  southeast  corner of the map  southwest.  to  T h i s may  except  2  f o r one  2  is isoclinal  scale f o l i a t i o n u n i t s and  the margins  f o l i a t i o n dips s t e e p l y small area i n the  area, where i t d i p s s t e e p l y  represent a f o l d  hinge.  51  Figure  4.1  Equal area p r o j e c t i o n of poles' to c o m p o s i t i o n a l l a y e r i n g i n the S e t t l e r S c h i s t .  52  Figure  4.2  Equal area p r o j e c t i o n of p o l e s to f f o l i a t i o n f o r the S e t t l e r S c h i s t south of Cogburn Creek. 2  53  The  f  2  f o l i a t i o n has  north r i d g e  been r o t a t e d 40 to 50° westward on  ( F i g u r e 4.3).  I t may  of the Spuzzum b a t h o l i t h , Hut  the  have been r o t a t e d by i n t r u s i o n  Creek body.  In the Cogburn Creek Group rocks around Three M i l e Creek, bedding in c h e r t and  foliation  p a r a l l e l to the melange zone. northeast  ( F i g u r e 4.4).  foliation, has  in g r e e n s c h i s t and  pelite is  Dips are steep and  to  Whereas the g r e e n s c h i s t  l a y e r i n g in the c h e r t  the  shows strong  i s i n f e r r e d to be bedding that  s u r v i v e d because of r e s i s t a n c e of competent quartzose  layers  to t r a n s p o s i t i o n . The and  latest  f o l d i n g i s a r e s u l t of both f  intrusive a c t i v i t y .  F o l d s of t h i s generation  c h a r a c t e r i s t i c a l l y broad warps and margins a p e n e t r a t i v e mineral  and  (Bartholomew The  Close  cleavage-intersection lineations. to 40°  O r i e n t a t i o n s of f o l d axes to both f due  kinks.  2  to the  and  f  3  to d u c t i l i t y c o n t r a s t s causing  to p l u t o n  The f  2  f ,  producing  3  f  3  foliation  foliation.  f o l d s are  scattered,  local variation  1979).  melange zone c o n t a i n i n g  u l t r a m a f i c rocks and the study area  imbricated  t a b u l a r bodies of  B a i r d M e t a d i o r i t e curves s i n u o u s l y  from southeast  to northwest  appears to have been pushed a s i d e by l a t e r batholith.  are  f o l i a t i o n has developed with  i s t y p i c a l l y at an angle of 25°  probably  regional folding  3  (Figure 2.1).  the u l t r a m a f i c rocks  It  i n t r u s i o n of Spuzzum  I t c o u l d a l s o have bent around an a l r e a d y  r e s i s t a n t core of d i o r i t e .  across  emplaced  F i n e s t r i n g e r s of d i o r i t e i n t r u d i n g  i n the t h r u s t zone, on the r i d g e south of  Cogburn Creek appear to i n d i c a t e the former i n t e r p r e t a t i o n .  54  Figure  4.3  Equal area p r o j e c t i o n of poles to f f o l i a t i o n i n S e t t l e r S c h i s t north of Cogburn Creek. 2  F i g u r e 4.4  Equal area p r o j e c t i o n of p o l e s to f f o l i a t i o n i n Cogburn Creek Group, f o r the area north of 1 km south of Cogburn Creek. 2  56  In c o n t r a s t to the Shuksan t h r u s t f a r t h e r south, zone here was a c t i v e before metamorphism.  the imbricate  the c u l m i n a t i o n of r e g i o n a l  T h i s does not p r e c l u d e  c o r r e l a t i o n of the melange  zone with the Shuksan t h r u s t , as r e g i o n a l metamorphism of the Shuksan S u i t e was e a r l i e r than metamorphism i n the Cogburn Creek area  (Brown et a l . 1982). Metamorphic  batholith.  foliation  i s found i n p l a c e s i n the Spuzzum  Poles to f o l i a t i o n and mineral  i n t r u s i o n s have been p l o t t e d i n F i g u r e 4.5. randomly d i s t r i b u t e d . postdates  f  2  l i n e a t i o n s f o r the These appear to be  The main p e r i o d of d i o r i t e i n t r u s i o n  deformation  and predates  the c u l m i n a t i o n of  r e g i o n a l metamorphism. S e t t l e r S c h i s t i s s t r u c t u r a l l y above the i m b r i c a t e zone, which forms the northeast  boundary of the Cogburn Creek Group.  Lowes (1972) mapped i n d e t a i l  the s t r u c t u r e of the imbricate  zone around O l d S e t t l e r Mountain.  The upper c o n t a c t  of the zone  with the S e t t l e r S c h i s t i s a steep  (75°), n o r t h - d i p p i n g  surface.  The north face of Old S e t t l e r Mountain i s a d i p slope of B a i r d M e t a d i o r i t e p a r a l l e l to t h i s f a u l t , exposed by e r o s i o n of the narrow band of u l t r a m a f i c rocks between Settler Schist.  Foliation  that u n i t and the  i n the B a i r d M e t a d i o r i t e i s g e n e r a l l y  p a r a l l e l to t h i s s u r f a c e , which can be followed across the topography i n 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 c o n t a c t , where i t i s exposed west of Old S e t t l e r Mountain  ( P l a t e 4.1).  However,  j u s t south of the gorge i n Cogburn Creek the 2 km wide zone of m e t a d i o r i t e and u l t r a m a f i c rocks becomes no more than 100 m  57  wide, g i v i n g way  to Cogburn Creek Group c h e r t .  The  nature of  the c o n t a c t i s unknown, as steep t e r r a i n p r e c l u d e d mapping. Macroscopic  s t r u c t u r e i n the Cogburn Creek Group rocks i n  northern p a r t of Three M i l e Creek becomes complicated, mapping was  not continued  but  f a r enough to unravel i t . A s e r i e s of  minor n o r t h - e a s t e r l y o r i e n t e d f a u l t s may  have d i s r u p t e d the  s t r a t igraphy. In s p i t e of recent a l l u v i a l cover the Cogburn F a u l t recognised by Bartholomew (1979) i n the south branch Creek.  He  noted truncated g e o l o g i c c o n t a c t s  c o n t r a s t s i n rock types, metamorphic grade, the v a l l e y .  fault.  and  and  s t r u c t u r e across to the north with  and the occurrence  b r e c c i a on the r i d g e at i t s head, strengthen the  of Cogburn  ( F i g u r e 2.1)  Alignment of the t r i b u t a r y stream  the main v a l l e y of the south branch,  was  of  the evidence  fault for  58  N  Figure  4.5  Equal area p r o j e c t i o n of poles to f o l i a t i o n m i n e r a l l i n e a t i o n s i n Spuzzum b a t h o l i t h .  and  59  5.  Geochronometry  D a t i n g of rock u n i t s i n the study area was u s i n g Rb-Sr, K-Ar  and U-Pb  techniques.  carried  Analytical  out  techniques  are d e s c r i b e d i n Appendix A; a n a l y t i c a l data, i s o t o p e r a t i o s  and  c a l c u l a t e d dates are given i n Appendix B; sample d e s c r i p t i o n s and  l o c a t i o n s i n Appendix C, and l o c a t i o n s p l o t t e d i n F i g u r e D-  2. E i g h t y - s i x whole rocks and mineral separates from  40  samples of Spuzzum b a t h o l i t h , S e t t l e r S c h i s t , Cogburn Creek Group s c h i s t , Breakenridge and  s e v e r a l pre- and  Formation  gneiss, Baird Metadiorite  s y n - t e c t o n i c i n t r u s i v e bodies were analysed  f o r Rb-Sr and Sr i s o t o p i c composition. to c a l c u l a t e  The  data have been used  isochron dates f o r the d i f f e r e n t  i s o c h r o n p l o t s are shown i n F i g u r e s 5.2  rock u n i t s .  a to h.  Five  s e p a r a t e s from Spuzzum b a t h o l i t h were dated by K-Ar. plotted  i n F i g u r e 5.4a  and b.  Z i r c o n s f o r U-Pb  s e p a r a t e d from B a i r d M e t a d i o r i t e , Breakenridge Settler  S c h i s t , Spuzzum b a t h o l i t h  hornblende Data are  d a t i n g were Formation  gneiss,  (Hut Creek and S e t t l e r Creek  b o d i e s ) , and one pre-metamorphic i n t r u s i v e rock. diagram i s Figure 5.3.  The  The  concordia  A l l f i g u r e s are at the end of the  Chapter.  5.1  P r e v i o u s Geochronometry Although  no one has undertaken  an i n t e n s i v e d a t i n g study i n  the r e g i o n between H a r r i s o n Lake and the F r a s e r V a l l e y , a number of a n a l y s e s have been c a r r i e d out by p r e v i o u s workers.  Most of  these have been on i n t r u s i v e rocks, p a r t i c u l a r l y the Spuzzum  60  Table  5.1  T a b l e of d a t e s and age e s t i m a t e s  from p r e v i o u s  between H a r r i s o n  River  Reference  Rock  Baird  (Unit  Metadiorite  Lowes  Lake and F r a s e r  studies  D a t e Ma 1 ) ' and  Method  correlatives  (1972)  possibly  Precambrian  correlation Yellow  with  Aster  Complex Mattinson  (1972)  Yellow  Aster  pyroxene  gneiss  pegmatite quartz  gneiss  diorite  711  912  1452  ± 20  U-Pb  zircon  64  75  427  ? 75  U-Pb  zircon  368  375  411  ? 15  U-Pb  zircon  433  628  U-Pb  zircon  U-Pb  zircon  2  3  orthogneiss Swakane  gneiss  biotite  gneiss  pegmatite  Cogburn Creek Group Lowes  (Unit  gneiss  3) and  1419 ± 100  69  correlatives  (1972)  Permian-Pennsylvanian  Reamsbottom  (1971)  L. C r e t .  correlation Chilliwack  and/or o l d e r  Group  correlation Chilliwack  Monger  (1970)  Chilliwack  Group  Permian-Pennsylvanian  Group  191  Armstrong  unpub.  Chilliwack  Armstrong  unpub.  Bridge River  Settler Pigage  Schist  Bartholomew Mattinson  Group  fossils Rb-Sr WR"  n=14.  Group 104 ± 25 @ 0.70555  Rb-Sr WR  n=14  low  256 ± 35 @ 0.70394  Rb-Sr WR  n=28  grade s c h i s t s  4) and  correlatives  Settler Schist  (1979) S e t t l e r  (1972)  with  high grade s c h i s t s  (Unit  (1973)  ± 16 @ 0.70510  with  Skagit  Schist  early-mid  214 ±  Paleozoic  32  correlation Shuksan  Suite  Rb-Sr WR  n=7  gneiss  biotite  gneiss  gneissic pegmatite  qtz di gneiss  with  98  112  428 +  10  U-Pb  zircon  66  67  79 ±  10  U-Pb  zircon  90  U-Pb  zircon  57  U-Pb  zircon  61 A r m s t r o n g unpub.  Darrington  Phyllite  Shuksan b l u e s c h i s t and Brown e t a l . (1982)  132 ± 8 @ 0.70556  Rb-Sr WR  n=22  172 ± 18 @ 0.70387  Rb-Sr WR  n=l5  greenschist  Shuksan amphib.  148 ± 5  K-Ar  Hb  barroisite  164 ± 6  K-Ar  Hb  128 ± 5  Rb-Sr  Mu  . K-Ar  Mu  schist  blueschists  129 ± 5  Spuzzum b a t h o l i t h  (Unit  5) and  M c T a g g a r t and  tonalite  77 ± 4  K-Ar B i  Thompson  (1967)(6)tonalite  77 ± 4  K-Ar  Richards  (1971)  80 ± 4  K-Ar B i  82 ± 4  K-Ar  foliated  correlatives  Hb  (1) diorite  Wanless McLeod  fol.  diorite  84 ± 4  K-Ar B i  fol.  diorite  105 ± 3  K-Ar B i  (1973)(4) q u a r t z d i o r i t e (1975)  ( 5 )  76 ± 4  K-Ar B i , Hb  tonalite  80.7  ± 2.5  K-Ar B i  tonalite  86.6  ± 2.8  K-Ar  Hb  diorite  91.5 ± 2.8  K-Ar  Hb  diorite  91.2  ± 3.1  K-Ar  WR  hornblendite  96.4  ± 4  K-Ar  Hb  hornblendite  121.6  ± 4  K-Ar  Hb  fels.  106 ± 4  K-Ar  Hb  112 ± 4  K-Ar  Hb  hbite  hornblendite R i c h a r d s and McTaggart  diorite  (1976)  Bartholomew  Hb  91.0  ± 2.8  K-Ar  Hb  3b  77.3  ± 2.6  K-Ar  Hb  3e .  94.4  ± 3.2  K-Ar  Hb  K-Ar  hb  (1)  (1979) u n i t  (2) unit A r m s t r o n g unpub.  b i hb d i o r i t e  94.9 ± 7  (3) 101 bi  hb d i o r i t e  ± 3  91.3  ± 2.2  K-Ar B i @ 0.7038  Rb-Sr  WR-  Pl-Hb-Bi  62  A r m s t r o n g and Ryan unpub.  Younger  bi  (1979)  (2)  (Unit  112  122  331  U-Pb  zircon  and  A r m s t r o n g unpub.  6)  aplite quartz  Woodsworth  diorite  (3)  intrusives  Bartholomew  hb  diorite  34.4 ± 1.4  @ 0.7041  Rb-Sr  WR  30.7 ± 0.6  @ 0.7041  Rb-Sr Bi-WR  granodiorite  33.6  ±1.1  K-Ar B i  granite(GEM-1)  35 ±  0.7  Rb-Sr  WR  (3)  U n i t numbers a r e from t h i s s t u d y A l l d a t a r e c a l c u l a t e d w i t h IUGS c o n v e n t i o n d e c a y c o n s t a n t s ( S t e i g e r J a g e r 1 9 6 9 ) , e x c e p t U-Pb z i r c o n from M a t t i n s o n ( 1 9 7 2 ) . Z i r c o n dates a r e reported as Pb/ 'U, Pb/ U, Pb/ Pb * Initial S r / S r r a t i o s a r e r e p o r t e d w i t h Rb-Sr i s o c h r o n d a t e s n= number o f s a m p l e s a n a l y s e d f o r r e p o r t e d i s o c h r o n d a t e . WR=whole r o c k , H b = h o r n b l e n d e , 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 a r e r e f e r e n c e s f o r s y m b o l s on F i g u r e 5.1. 1  2  3  2 0 6  8 7  8 6  2 3  2 0 7  2 3 S  2 0 7  2 0 6  and  63  +  . . . r  * + + + + + + + +|+ + + + + + + + + \\++J ++ \ «+ + + + + + + + + + + +*70b+ + +\ V + + + +' \ + + + + + + + + + + + + + + + + + + \ \ + + + + H \ + + + + + + + + + + + + + + + + + +>+ + + + +  ' , +  '  -  -..  +  +  +  +  +  +  +  +  +  +  +  +  +  +  +  + ', +  Is.  x  \+  +  +  + Ha  + + + + + + + +..+ + + + + + + +,\+ + + -  \ + + + + Scuzzy Pluton + + + + +  \ IK  *  +  >+  +  +  +  +  +  +  +  +  +  +  +  +  +  +  +^+ + -  +*  i + ^ + H  + + + + + + + + + + + + + +  +  + + ^r-'.-.i + + • I'-'i- + +  1  + + + + + + + + + + + + + 4l ++++++++ n ,+ + Spuzzum Pluton + +-+ + -«  \  -+++++  ) + + + + + + + + + +  +'H  + + + + + + + + + + + H\ + '  A + + + + + + + +76 bh**™- + (\+ + + + + + + + + + + +>\ i\+++++++++++  105Z;  79Rr  />  \  N^/  + + + + + + + + + + + iS*  > + + + + + + + + + + \ lr\+ + + + + + + + + + + M y  V + + + + + + + ++ + + + 30.7R+ 33.6b 77 34.4r .77. 70.11 391 +  .  9  5  2  WW >1  i+i  h  794.4h  210i  7 7R '<v; v  30'  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) (3) References • (4) (5) (a\ Table 5.1  78.9h _^ 110z / + 96.4M  LlXV ^ 0  \ + -n  v  ++++  io6h 7l5. 7b +  *.\+ • 112 o 1 o KT 1.6h + i + + i+ J -  +  .i ^. . 6 h r  +  + + + i. ++ k Spuzzum Pluton +  + +91 +  A.  u  + + + +  + + + + + 1+ + + + + _+ <f + + j84.  ~r  r  r  Hope  B2h „ +*+ VA84b i  f r o m  + + + + + + +> + + + + + + + +\ -+ + + + + + +\_ v  *+ + + + + + + 0 >+  AS  49° 15'f 122°00'  F i g u r e 5.1  + + + + +(  /+ + + + +.+ +' 4- + + + + + + ' —-fe. + + + +_++>  Kilometres 45'  121° 30'  Map showing geochronometry of the H a r r i s o n Lake - Fraser River region See F i g u r e 1.2 f o r key t o map u n i t s . Symbols i n d i c a t e r e f e r e n c e s l i s t e d i n Table 5.1.  —  64  batholith. Mattinson  In the North Cascade Mountains, Washington, (1972) dated  by U-Pb  z i r c o n s from u n i t s that are  b e l i e v e d to be c o r r e l a t i v e s of rocks His r e s u l t s are t a b u l a t e d i n Table in  i n the present  5.1,  along with other  the H a r r i s o n Lake - F r a s e r River r e g i o n .  these p r e v i o u s l y dated  study  The  area. dating  l o c a t i o n s of  rocks have been p l o t t e d on a map  (Figure  5.1).  5.2  Unit  1•  Baird Metadiorite  T h i s u n i t has  been c o r r e l a t e d p r e v i o u s l y with the Yellow  A s t e r Complex of the North Cascades, Washington (Lowes 1972), because of i t s appearance, and  structural relationships within  the Shuksan t h r u s t i m b r i c a t e zone.  Mattinson  (1972) dated  z i r c o n s from pyroxene g n e i s s of the Yellow A s t e r Complex from the North Cascades as o r t h o g n e i s s as 430 The  1452  to 2000 Ma,  Rb-Sr and U-Pb  shows that  8 7  Rb/  8 6  s m a l l , a l l are < 0.02, The  resulting  d a t i n g r e s u l t s reported here have l a r g e  Sr  f o r the rocks.  r a t i o s of the samples are extremely the d i f f e r e n c e s h a r d l y  significant.  i s o c h r o n age  has a l a r g e e r r o r (3.4  ± 2.4  isotopic  have been homogenised toward a value of  0.7043 by Cretaceous metamorphism, from an i n i t i a l l y  approximately lower  that would have d e f i n e d a l i n e with even g r e a t e r s l o p e . i s the case,  Ga),  squares f i t of a t i g h t c l u s t e r of p o i n t s .  c l u s t e r of p o i n t s suggests that the strontium  r a t i o s may  Figure  and  because i t i s a l e a s t The  from the younger  Ma.  e r r o r s , but suggest a Precambrian age 5.2a  and  the c a l c u l a t e d i n i t i a l  value  If this  value of 0.7038 i s only  an  65  upper l i m i t  f o r the true i n i t i a l  ratio.  The  are c o n s i s t e n t with a p r i m i t i v e ocean c r u s t Mattinson  values  protolith.  (1972) noted that Rb-Sr analyses of the o l d e r  o r t h o g n e i s s i n the Yellow A s t e r Complex had meaningful  low rubidium  f a i l e d to y i e l d  ages because although Sr c o n c e n t r a t i o n s are normal,  the rocks are so d e p l e t e d i n Rb that Rb-Sr whole rock d a t i n g i s precluded  (J.C. Engels, 1968,  p e r s . comm. to M a t t i n s o n ) .  The  same comment holds f o r the Yellow A s t e r - t y p e rocks of the B a i r d M e t a d i o r i t e of t h i s  study.  A h i g h l y d i s c o r d a n t z i r c o n U-Pb suggests a pre-Mesozoic, f r a c t i o n was  analysed  date  (Table B-6)  perhaps Precambrian,  (Table C-2), due  The  2 0 7  Pb/  spiked runs are 704 to  2 0 6  concordia  2 0 7  Pb/  ± 180 Ma and  2 0 6  i t was  and  of poor  1653  ± 175 Ma  P b date  (Figure 5.3)  i s 1251  the averaged  2 0 7  respectively,  Pb/  ± 180 Ma.  2 0 6  Pb  the S e t t l e r S c h i s t .  The  due  ratios.  When p l o t t e d  on  value l i e s away from  c o n c o r d i a but c l o s e to the l i n e d e f i n e d by p o i n t s from of  one  P b dates c a l c u l a t e d f o r the n a t u r a l and  s i g n i f i c a n t d i f f e r e n c e s i n the measured  The averaged  Only  to low z i r c o n y i e l d ,  the a n a l y s i s c o u l d not be repeated although quality.  age.  also  analyses  s i n g l e a n a l y s i s f o r the m e t a d i o r i t e  r e s t r i c t s the c o n c l u s i o n s : e i t h e r the rock i s o l d and the  U-Pb  c l o c k has been reset by a m p h i b o l i t e - f a c i e s metamorphism d u r i n g the Mesozoic,  or o l d lead has been i n c o r p o r a t e d i n t o  zircons.  first  The  i n t e r p r e t a t i o n , which i s r e i n f o r c e d by  Rb/Sr a n a l y s e s , i s compatible interpretation  Mesozoic  with Mattinson's  the  (1972)  f o r rocks of the Yellow A s t e r Complex i n the  Cascade Mountains of Washington, namely i n t r u s i o n around 1400  to  66  1600  Ma  f o l l o w e d by metamorphism at 415 Ma  concordia  and  90 Ma.  His  f o r Yellow Aster Complex i s shown in F i g u r e 5.5  for  comparison with B a i r d M e t a d i o r i t e .  5.3  Unit 3.  Cogburn Creek Group  Rb-Sr data events, and  from t h i s group of rocks c l e a r l y d e f i n e  the e a r l i e r one  the l a t e r  i s the approximate time of d e p o s i t i o n  i s the Cretaceous metamorphism.  isochron date when b i o t i t e and i s 296  ± 58 Ma  (Table B-2,  The c a l c u l a t e d  c h e r t analyses  Figure  e i t h e r a l a t e P a l e o z o i c age rocks and  two  are not  5.2b), which c o u l d  f o r the  included represent  sequence of c h e r t , v o l c a n i c  p e l i t e s , or a r e s e t caused by Mesozoic metamorphism of  even o l d e r rocks.  The  first  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 d e f i n e the s t r a t i g r a p h i c age. Rb-Sr whole rock Ma  isochron date f o r g r e e n s c h i s t  (n=2), i s probably  not  g r e e n s c h i s t are 81  These dates represent metamorphic episode, S e t t l e r S c h i s t and On  and  8 7  Sr/  8 6  Sr  value  8 7  Sr/  8 6  Sr  initial  are  77 ± 1.6  Ma r e s p e c t i v e l y .  of the  Cretaceous  s i m i l a r to b i o t i t e dates from  Spuzzum b a t h o l i t h (see d i s c u s s i o n below). (Figure 5.2b)  the  (HL37a) whole rock has a higher  than the other  i s higher  ± 68  B i o t i t e dates from c h e r t  the waning stages  the Rb-Sr isochron diagram  representing chert  isochron  region.  i 5 Ma  and  438  s e n s i b l e g e o l o g i c a l l y , as no s i m i l a r  dates have been found i n the and  alone,  A  whole rocks.  date i s , however, s i m i l a r to other  Rb/ Sr 8 6  and  S i m i l a r l y , the  r a t i o , 0.7090, f o r the chert than those f o r other  8 7  point  (HL37a)-Bi  rock-biotite pairs.  metamorphic dates.  The  The  67  difference in  8 7  Sr/  8 6  Sr  may  be due  seawater at the time the c h e r t was No  Unit 4. The  deposited  z i r c o n s were dated from the c h e r t and  although accessory  5.4  to i n c o r p o r a t i o n of  z i r c o n i s present  (Jager  8 7  1979).  greenschist,  in sample HL37a.  Settler Schist  S e t t l e r S c h i s t i s bounded by major f a u l t s and  so s t r a t i g r a p h i c r e l a t i o n s with other b a s i s of s i m i l a r i t i e s  plutons,  u n i t s are u n c l e a r .  i n s t r u c t u r a l p o s i t i o n with the  an E a r l y - Mid  Paleozoic  age  to the S e t t l e r S c h i s t .  (1971) suggested a l a t e P a l e o z o i c  to Mesozoic age  s c h i s t s in the Mount Breakenridge area  plutons. in the area  for s i m i l a r  (see a l s o Pigage 1973).  pre-Jurassic Both these  rocks  in the  i s l i k e Chiwaukum S c h i s t with the Skagit  g r a n i t e c l a s t s have been found i n  i n the Bridge River area,  of e a r l y Mesozoic p l u t o n s .  The  age  Shuksan S u i t e b l u e s c h i s t s have been dated as Mesozoic  (Armstrong et a l . 1982), and Triassic  area  horizons  estimates have been i n v a l i d a t e d by subsequent s t u d i e s region.  assigned  north of t h i s study  because there are no  the  Reamsbottom  because g r a n i t o i d c l a s t s were found in conglomeratic s c h i s t s , and  On  Paleozoic  (?) Shuksan S u i t e i n the North Cascades, Pigage (1973)  w i t h i n the  Sr-rich  L i t h o l o g i c a l l y , the S e t t l e r S c h i s t  (Lowes 1972), which may  S u i t e and  be c o r r e l a t e d  i s d i f f e r e n t from the Shuksan S u i t e .  Shuksan S u i t e c o n t a i n s  metamorposed ocean f l o o r  i n d i c a t i n g the presence  b l u e s c h i s t and  (Dungan et a l .  greenschist 1981), and  that  are  offshore  sediments. Bartholomew (1979) obtained  an Rb-Sr isochron date from  68  Settler  S c h i s t in the Yale Creek area of 214  i n t e r p r e t e d t h i s as both the age metamorphic sediments and of the low  initial  8 7  Sr/  8 6  of the source  Sr  from seven rocks  B-1)  produce an  5.2c  and d ).  rock  He  f o r the  pre-  the time of t h e i r d e p o s i t i o n because r a t i o of 0.7043.  Eleven Rb-Sr whole rock analyses with data  ± 32 Ma.  from t h i s study  combined  from Bartholomew (1979) ( a l l in Table  isochron age  of 210  ± 27 Ma  (Table B-2;  This r e f l e c t s either T r i a s s i c  Figure  - Jurassic  d e p o s i t i o n or p a r t i a l to complete r e s e t t i n g by Mesozoic metamorphism of p r e - J u r a s s i c rocks.  If i t i s the age  of  d e p o s i t i o n , then i t does not n e c e s s a r i l y a l s o represent i n t e g r a t e d age The  mineral  phyllite  of the source  the  rocks, as w i l l be d i s c u s s e d  later.  isochron date from a b i o t i t e - b e a r i n g g r a p h i t i c  (SS82) i n d i c a t e s e i t h e r a s l i g h t l y l a t e r end  metamorphism, at 66 ± 1.6  Ma,  or a p a r t i a l  to the  reset by the  mid  T e r t i a r y Cogburn G r a n o d i o r i t e which crops out w i t h i n a k i l o m e t r e , on the east s i d e of Cogburn Creek. The  youngest isochron drawn i n F i g u r e 5.2c  quartz-biotite-garnet Cogburn G r a n o d i o r i t e . by Rb-Sr WR mineral to  schist  and  d i s from a  (SS128) from near the contact  Bartholomew (1979) dated  i s o c h r o n , Rb-Sr B i , and  K-Ar  the g r a n o d i o r i t e  Bi at 32 ± 4 Ma.  isochron date for the s c h i s t SS128 of 39 ± 4 Ma  r e s e t t i n g by t h i s young p l u t o n .  of 0.7066 for t h i s s c h i s t sample has  The  with  initial  8 7  Sr/  8 6  The  i s due  Sr  ratio  been p u l l e d 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 f e l s p a r components (Jager Z i r c o n separated  1979).  from two  samples of S e t t l e r S c h i s t i s  and  69  small and s u b h e d r a l , c e r t a i n l y d e t r i t a l .  They are unusual i n  that they c o n t a i n small opaque ( g r a p h i t e ?) i n c l u s i o n s . may  This  i n d i c a t e metamorphic overgrowths on the d e t r i t a l cores, with  i n c l u s i o n of m a t e r i a l from the sediments c o n t a i n i n g the z i r c o n s . The two samples 2 0 7  p  b  / 2 0 6  P  b  a  g  e  S  y i e l d e d h i g h l y d i s c o r d a n t U-Pb 1183 ± 12 Ma and  f  1279  ± 32 Ma  dates but  similar  (Table B-5).  P l o t t e d on c o n c o r d i a (Figure 5.3), the l i n e j o i n i n g the two p o i n t s has a lower i n t e r c e p t of 211 i n t e r c e p t of 2450 ± 230 Ma.  ± 30 Ma,  The young lower i n t e r c e p t i s  c o n s i s t e n t with the Rb-Sr isochron age upper  intercept  f o r the s c h i s t .  i n d i c a t e s a Precambrian  i n these sediments.  and an upper  The o l d  source f o r the z i r c o n s  The age o b t a i n e d i s c o n s i s t e n t with z i r c o n  ages o b t a i n e d from s c h i s t s and g n e i s s e s i n the North Cascade Mountains  of Washington  by Mattinson (1972), as w e l l as from the  pyroxene g n e i s s e s of the Yellow A s t e r Complex.  Z i r c o n s from the  s u p r a c r u s t a l Swakane Gneiss and Skagit Gneiss y i e l d e d d i s c o r d a n t ages with Ma  (Figure 5.5).  (Mattinson  1972)  2 0 7  Pb/  2 0 6  P b ages  i n the range  highly  1400  to  The c o n c o r d i a f o r z i r c o n s from the two i s shown i n F i g u r e 5.5,  r e s u l t s from the S e t t l e r S c h i s t .  1650  units  f o r comparison with  Mattinson c o n s i d e r e d those  from the Swakane Gneiss to approximate  s t r a t i g r a p h i c age and  those from the Skagit Gneiss to be d e t r i t a l and d e r i v e d from a Precambrian  source.  Thus the s t r a t i g r a p h i c age f o r the Skagit  Gneiss c o u l d be younger  than Precambrian.  c o n s i d e r e d that the pre-metamorphic Skagit Gneiss was  Misch (1966)  e q u i v a l e n t of much of the  immature greywacke.  The  isotopic  similarity  between the S e t t l e r S c h i s t and Skagit Gneiss i s not s u r p r i s i n g  70  since the S e t t l e r S c h i s t i s considered Chiwaukum S c h i s t  to be e q u i v a l e n t  (Lowes 1972), which i s part of the  Metamorphic S u i t e d e f i n e d by Misch  to  Skagit  (1966).  In summary, the z i r c o n geochronometry suggests that S e t t l e r S c h i s t i s at l e a s t in part d e r i v e d The  Rb/Sr date of 210  thus i t r e p r e s e n t s  does not  the  from an o l d source.  reflect  t h i s o l d source;  only the approximate time of d e p o s i t i o n  S e t t l e r S c h i s t and, the age  ± 27 Ma  the  because of the low  initial  ratio,  of almost contemporaneous source rocks.  l i t h o l o g i e s and  chemistry, Pigage (1973) and  s t a t e d that the  sediments forming the s c h i s t were  and  derived  age  as those sediments.  of  possibly  From t h e i r  Bartholomew  (1979)  eugeosynclinal  from a r i s i n g v o l c a n i c arc of approximately the same However, there a l s o must have been o l d  basement exposed nearby to supply f i n e - g r a i n e d sediments that now Settler Schist. Mesozoic, as  The  the o l d z i r c o n and  form the p e l i t i c  time of d e p o s i t i o n was  mature  p a r t of  the  most probably e a r l y  i n d i c a t e d by the Rb-Sr whole rock i s o c h r o n .  Waning  of the main Cretaceous metamorphic event i s shown by the  mineral  ( b i o t i t e ) ages obtained felsic  intrusive.  (Bartholomew 1979)  5.5  from both a p h y l l i t e and  The  last  a  syn-tectonic  i n t r u s i v e event at 32 ± 4 Ma  reset b i o t i t e  in the nearby s c h i s t .  Premetamorphic i n t r u s i v e rocks  1) Z i r c o n sample HL111, from a f e l s i c body (Figure D-2, 5), y i e l d e d U-Pb 20 7  P  b  /2 0 6  the s i l l  P  b  d  a  t  e  dates that are s l i g h t l y d i s c o r d a n t ; i  s 3 7 6  ±  8  3  M a #  the  Because of the p r o x i m i t y  to the Spuzzum b a t h o l i t h , Hut  Table  Creek body, and  of l a c k of  B-  71  s i m i l a r o l d dates from other i n t r u s i v e rocks i n the region the sill  i s probably Mesozoic.  b i o t i t e and muscovite,  Sillimanite  i s intergrown  i n d i c a t i n g that the s i l l  before the height of r e g i o n a l metamorphism. 2 0 7  Pb/  5.3). sill was  2 3 5  U  r a t i o s p l o t almost  T h i s Cretaceous emplacement, based  date  The  intruded.  The  contamination  old  on l o c a l geology.  U  and  (Figure  sill  intrusive  and  to have been deformed with the s c h i s t s i t 2 0 7  Pb/  2 0 6  P b date c o u l d be due  felsic  composition  to The Rb-Sr whole  f o r the s i l l  s i m i l a r to other d a t i n g that records the end of  and  2 3 8  I b e l i e v e the  by country rock l e a d or z i r c o n s .  The  Pb/  i s a reasonable estimate f o r time of  rock - mineral isochron date of 74 ± 10 Ma  metamorphism.  2 0 6  in place  on c o n c o r d i a at 110 Ma  emplaced e a r l y enough d u r i n g the Cretaceous  metamorphic event  was  with  is  Cretaceous  ( b i o t i t e , no hornblende)  the h i g h U and Pb contents compared with the Spuzzum  b a t h o l i t h , Hut Creek body (Table B-4)  suggests t h a t t h i s s i l l  is  not part of the main i n t r u s i v e phase.  2) M i n e r a l isochron dates from a f o l i a t e d , f e l s i c ,  garnet-  c l u s t e r dyke (SS85) c u t t i n g the S e t t l e r S c h i s t on the north ridge event. 20 Ma,  ( F i g u r e 5.2e), show the Cretaceous  igneous and metamorphic  E x c l u d i n g Bi from the c a l c u l a t i o n y i e l d s a date of 105 ± probably r e p r e s e n t i n g time of i n t r u s i o n d u r i n g the e a r l y  p a r t of the metamorphic c y c l e . p a r a l l e l to the s c h i s t b l o c k i n g temperature  The dyke became f o l i a t e d  f o l i a t i o n , and c o o l e d through  of 300°C (Jager  1979)  at 80 ± 6  the Ma.  biotite  72  5.6  Foliated diorite  i n f a u l t 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 s i d e of S e t t l e r Creek, i n t r u d i n g the metamorphosed u l t r a m a f i c rocks in the i m b r i c a t e zone of the Shuksan t h r u s t . intrusive  i s p a r a l l e l to the f a u l t zone.  isochron was  Foliation  i n the  A Rb-Sr mineral  obtained from t h i s rock i n the hope that i t might  h e l p date the t h r u s t i n g event. date of 77 ± 3 Ma  The  four p o i n t isochron gave a  (Figure 5.2f, Table 3-B).  Since t h i s  i s in  the range of metamorphic ages, the t h r u s t i n g e i t h e r took p l a c e during metamorphism or before i t so that a l l the rocks were equally affected.  T h i s small body was  emplaced a f t e r  t h r u s t i n g but before r e g i o n a l metamorphism, and the  the  isotopic  system has been r e s e t .  5.7  U n i t 5. Two  Spuzzum b a t h o l i t h  d i s c r e t e bodies of Spuzzum b a t h o l i t h i n the area were  mapped, and dated  ( F i g u r e s 1.2,  5.2).  The  small body i n t r u d i n g  only S e t t l e r S c h i s t north of Old S e t t l e r Mountain i s c a l l e d S e t t l e r Creek body and  i s unit 3d of Bartholomew (1979).  Hut Creek body spans Cogburn Creek and extends Creek  (Figure F i v e K-Ar  the two  bodies  The Hut  1.2). dates were obtained on hornblende (Table B-3).  c o n v e n t i o n a l date of 162 ± 7 Ma, dates between 100  separates  from  Hornblende from gabbro of the  Creek body (SD66) with very low %K  gabbro may  north up  the  ± 3 Ma and 77.5  Hut  (0.046 ± 0.002%) y i e l d e d a  and  the other samples gave  ± 3 Ma.  have l a r g e a n a l y t i c a l e r r o r s due  The dates from  the  to the very low K  73  content.  The  interpretation  i n c o r p o r a t i o n of i n i t i a l  i s f u r t h e r c o m p l i c a t e d by  r a d i o g e n i c argon.  content has a r e p r o d u c i b i l i t y of ± 4%  The measured K  (1a f o r 4 a n a l y s e s ) , but  such a low K means that the f r a c t i o n of the t o t a l radiogenic  i s a l s o low.  only 38% of t o t a l  U 0  For t h i s sample,  *°Ar that i s  the r a d i o g e n i c  A r , l e a d i n g to p o s s i b l e e r r o r  c a l c u l a t i o n of up to 5 %.  possible  a o  Ar is  i n the age  Apart from t h i s sample, ages o b t a i n e d  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 S e t t l e r Creek body, 100 ± 3 and  94.5  ± 4 Ma.  F i g u r e 5.5a  diagram of * ° A r / A r 36  shows the data p l o t t e d on an  v. " ° K / A r . 36  If no excess  initial  r a d i o g e n i c argon were present the isochron l i n e s would through an i n t e r c e p t of 295.5 on the argon a x i s , the  atmospheric " ° A r / A r 36  ratio.  isochron  pass  representing  Whether the p o i n t s a r e grouped  together or s p l i t a c c o r d i n g to d i o r i t e body, the i n t e r c e p t i s still  >295.5.  The slopes of the l i n e s through the p l o t t e d  i n t e r c e p t s represent dates of 70.1 78.9'±  2 Ma  ± 5 Ma  f o r the S e t t l e r Creek body.  f o r the North body and The a c t u a l amount of  excess argon can be found from F i g u r e 5.4b, isochron p l o t of  , 0  A r v. %K.  a concentration-  L i n e s on t h i s p l o t  through the o r i g i n ; a p o s i t i v e i n t e r c e p t on the the  amount of excess argon i n n l / g .  are  0.19  the  S e t t l e r Creek body.  ± 0.02  should pass 4 0  Ar  gives  For these rocks the v a l u e s  n l f o r the Hut Creek body and 0.14  isochrons represent 65.7  axis  ± 0.01  n l for  The slopes of the c o n c e n t r a t i o n ± 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 e s t i m a t e s f o r the d i o r i t e  74  bodies are those c a l c u l a t e d  from the i s o c h r o n s .  Of the  i n d i v i d u a l c o n v e n t i o n a l d a t e s , those f o r the S e t t l e r Creek body are c l o s e r to i n t r u s i v e ages than those from the Hut Creek body, as the hornblendes c o n t a i n l e s s excess argon.  S i n c e the d i o r i t e  was i n t r u d e d b e f o r e the peak of the h i g h p r e s s u r e and temperature r e g i o n a l metamorphism, a l l K-Ar dates must have been subject to some r e s e t t i n g .  Pigage  (1976) c a l c u l a t e d the  c o n d i t i o n s of metamorphism of the S e t t l e r S c h i s t kbar and 550 t o 700°C. hornblende i s 400°C.  to be 6 to 8  The K-Ar b l o c k i n g temperature f o r The d i o r i t e was subjected t o temperatures  high enough t o r e s e t the K-Ar c l o c k of a l l the c o n s t i t u e n t m i n e r a l s d u r i n g metamorphism.  In t h i n s e c t i o n , hornblendes i n  the d i o r i t e have brown c o r e s and green rims that have r e c r y s t a l l i s e d d u r i n g metamorphism. Rb-Sr d a t i n g of the Hut Creek body g i v e s 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 h i g h i n  r a d i o g e n i c s t r o n t i u m was not i n c l u d e d i n the WR-Pl-Hb i s o c h r o n . The b i o t i t e date, s i m i l a r t o a l l other b i o t i t e dates from the area, i n d i c a t e s that the d i o r i t e c o o l e d through the b i o t i t e b l o c k i n g temperature of 300°C at the same time as the s c h i s t s d u r i n g the waning metamorphism.  The WR-Pl-Hb i s o c h r o n  produces  a date o l d e r than, but not i n disagreement with, any date f o r diorites  i n the region r e p o r t e d by p r e v i o u s workers  (e.g. R i c h a r d s 1971, McLeod 1975).  K-Ar, Rb-Sr and U-Pb d a t i n g  along Highway 1 west of Hope has produced dates i n the range 70 to 110 Ma, and f u r t h e r n o r t h i n the range 70 t o 90 Ma.  The  75  o l d e r date o b t a i n e d i n t h i s study may be due to movement of radiogenic  s t r o n t i u m between mineral components during  ( c f . Wasserburg et a l . subjected  1964), e s p e c i a l l y as the d i o r i t e was  t o h i g h grade r e g i o n a l metamorphism along with the  schists i t intrudes, Zircon  or merely due t o random a n a l y t i c a l e r r o r .  from the Hut Creek body y i e l d e d a h i g h l y  date with l a r g e e r r o r s ; the overlapping calculated  2 0 7  Pb/  2 0 6  Pb  independently from unspiked and spiked  ± 360 Ma and 1235 ± 600 Ma. Ma.  cooling  discordant  dates runs were 778  The averaged value i s 1023 ± 500  T h i s date, which c o n t r a d i c t s  l o c a l geology, i n d i c a t e s the  presence of o l d l e a d or o l d z i r c o n .  There seems no l i k e l i h o o d  of p h y s i c a l contamination from the s c h i s t , as t h i s sample was c o l l e c t e d well within  the d i o r i t e body; however, the z i r c o n  y i e l d was extremely low, 210 z i r c o n s  from 35 kg of rock.  A n a l y t i c a l e r r o r s may be h i g h , because of the small and 2 0 7  U available  Pb/  2 3 5  U  f o r measurement.  When the  2 0 6  r a t i o s are p l o t t e d the point  mixing l i n e through t h i s p o i n t  2 3 8  U and  l i e s off concordia.  A  and those f o r S e t t l e r S c h i s t  i n t e r s e c t s c o n c o r d i a at 110 ± 5 Ma. i n t r u s i v e age,  Pb/  amount of Pb  This  i s a plausible  and i n d i c a t e s that the discordance might be due  to the presence of o l d z i r c o n picked  up from the country rock,  such as S e t t l e r S c h i s t , a t the time of i n t r u s i o n ( c f . Gebauer and  Grunenfelder  1979).  Rb-Sr and U-Pb i s o t o p i c a n a l y s i s of the S e t t l e r Creek body shows a pronounced e f f e c t of e i t h e r a s s i m i l a t i o n of s c h i s t i n the magma d u r i n g fluids.  i n t r u s i o n or m o b i l i s a t i o n  Since the dating  of Sr and Pb by  samples were c o l l e c t e d u n i n t e n t i o n a l l y  76  c l o s e t o margins of the body the former an  i s quite l i k e l y .  When  isochron i s c a l c u l a t e d f o r WR analyses of a l l samples except  SD97 a date of 274 ± 179 Ma i s o b t a i n e d .  T h i s date  i s obviously  too o l d , s i n c e i t i s g r e a t e r than the i s o c h r o n age obtained f o r the country rock, and most probably r e p r e s e n t s i n c o r p o r a t i o n of Sr from the s c h i s t s at the time of i n t r u s i o n . but s t i l l Pl-Hb.  A more r e a l i s t i c  o l d date of 167 ± 46 Ma ( F i g u r e 5.2h) i s obtained f o r  The data f o r sample SD97 were l e f t  c a l c u l a t i o n s above as they have lower other samples f o r s i m i l a r  8 7  8 7  Sr/  out of the isochron 8 6  R b / S r values. 8 6  a p p a r e n t l y i s o t o p i c a l l y heterogeneous,  S r values than the The d i o r i t e body i s  as SD97 was c o l l e c t e d  c l o s e t o SD96 and i s s i m i l a r 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 f o r sample SD97 i s 89 ± 7 Ma. The WR-Pl-Hb date i s 186 ± 49 Ma; t h i s because the hornblende All  the hornblende  i s probably too o l d  a n a l y s i s i s abnormally  high i n  8 7  Sr/  8 6  Sr.  samples analysed from the S e t t l e r Creek body  are h i g h i n r a d i o g e n i c strontium; s i n c e they were a l l c o l l e c t e d near the margin of the p l u t o n they may have been e n r i c h e d with r a d i o g e n i c strontium when they r e c r y s t a l l i s e d d u r i n g metamorphism. Zircons from the S e t t l e r Creek body y i e l d e d  2 0 7  Pb/  2 0 6  Pb  dates of 242 ± 194 Ma, 629 ± 88 Ma and 217 ± 138 Ma f o r the n a t u r a l and two spiked runs.  The spread  i s due to the p o i n t s  l y i n g on a chord very c l o s e to c o n c o r d i a (Figure 5.3). On the p l o t of  2 0 6  Pb/  2 3 8  U v.  2 0 7  Pb/  2 3 5  U the p o i n t s l i e near 95 ± 5 Ma  on the c o n c o r d i a , so t h i s i s taken as a minimum estimate of the age  of the S e t t l e r Creek body.  77  5.8  Agmatised q u a r t z  diorite  A d i o r i t e agmatite the south branch  (SD  14), c o l l e c t e d on the east s i d e of  of Cogburn Creek, gave a three p o i n t isochron  date of 42 ± 14 Ma  with a high i n i t i a l  8 7  Sr/  8 6  Sr  r a t i o of  0.7048, which i n d i c a t e s r e s e t t i n g by the 32 ± 4 Ma Granodiorite.  The WR-Pl date of 182  r a t i o of 0.7045) i s c l o s e to the WR Settler Schist.  ± 33 Ma  Cogburn  (initial  8 7  Sr/  8 6  Sr  date obtained f o r the  R e - e q u i l i b r a t i o n of r a d i o g e n i c s t r o n t i u m  between b i o t i t e and  some other mineral phase d u r i n g r e s e t t i n g  by  the Cogburn G r a n o d i o r i t e has p i v o t e d the isochron l i n e around the WR  p o i n t to g i v e a younger date.  The  data suggest  that the  body may  have been i n t r u d e d before the Spuzzum b a t h o l i t h , or  they may  be s p u r i o u s due  to  8 7  Sr  redistribution  during  resetting.  5. 9 Breakenridge  Formation  Roddick and Hutchison  (1967) and Reamsbottom (1971) thought  that t h i s u n i t c o n t a i n e d the o l d e s t rocks i n the area based on structural position deformation Two  i n the cores of the domes, and  the rocks had  the amount of  undergone.  samples of Breakenridge  Formation  g n e i s s were c o l l e c t e d  from the western dome of Reamsbottom (1971) and dated by Rb-Sr and U-Pb.  The  composite Rb-Sr m i n e r a l  79 ± 2 Ma,  so the metamorphism of the g r a n o d i o r i t e that  deformed i n t o g n e i s s Needle Formation  (Reamsbottom 1971)  s c h i s t s surrounding  isochron gave an age  of  was  and presumably the C a i r n  i t ended  with metamorphism of the s c h i s t s and d i o r i t e  simultaneously 10-15  km  south i n  78  the Cogburn Creek a r e a . Zircon 180 Ma  (  2 0 7  from CU2 Pb/  2 0 6  was  dated by U-Pb  P b ages).  r a t i o s are p l o t t e d  When the  (Figure 5.3)  c o n c o r d i a at 105 ± 5 Ma.  at 153 ± 87 Ma and 224 ±  2 0 6  Pb/  2 3 8  U and  2 0 7  Pb/  2 3 5  U  the p o i n t l i e s very c l o s e t o  T h i s i s a reasonable age f o r e i t h e r  i n t r u s i o n of the g r a n o d i o r i t e or i t s metamorphism to g n e i s s , and as with the Rb-Sr date i t i s s i m i l a r to dates i n the Cogburn Creek a r e a .  I f i t r e p r e s e n t s metamorphism then the  2 0 7  Pb/  2 0 6  Pb  date of 166 ± 115 Ma c o u l d represent i n t r u s i o n of the granodiorite. possibility,  There  i s no supporting evidence f o r t h i s  but i t i s f e a s i b l e as the date i s younger than the  Rb-Sr date f o r the S e t t l e r S c h i s t surrounding the dome.  The  z i r c o n s are c l e a r and euhedral and c o u l d have c r y s t a l l i s e d during e i t h e r  i n t r u s i o n or high-grade  r e g i o n a l metamorphism to  s t a u r o l i t e - k y a n i t e grade.  5.10 D i s c u s s i o n A summary of the t i m i n g of events based on geochronometry ( t h i s study) are given i n Table The o l d e s t u n i t  5.2.  i n the study area i s the B a i r d M e t a d i o r i t e ,  which i s e q u i v a l e n t to the Yellow Aster Complex of the North Cascade Mountains.  A c o n c l u s i v e date was  the Rb-Sr and z i r c o n U-Pb protolith.  The  (Table 5.2)  not o b t a i n e d but  suggest a  both  Precambrian  rocks have been metamorphosed twice to  amphibolite grade  (Lowes 1972), which probably scrambled  geochronological clocks.  Possible interpretations  i n t r u s i o n d u r i n g the Precambrian  the  include  f o l l o w e d by metamorphism  79  Table  5.2  Summary of e v e n t s new  Method  area b a s e d on  analyses  D a t e Ma'  Interpretation  Baird Metadiorite  (Unit  Rb-Sr  3.4  ± 2.4  U-Pb  269  400  2  i n the Cogburn Creek  1)  Ga  1251  Intrusion ± 180  followed  during Precambrian by a m p h i b o l i t e  metamorphism (Mattinson  Cogburn Creek Group Rb-Sr  296  Rb-Sr  81  Settler U-Pb  (Unit  Schist  ±  1.6  End of C r e t a c e o u s  intercept  Precambrian zircons  ±30  Rb-Sr  210  ± 27  Rb-Sr  66 ±  Rb-Sr  39 ± 4  Premetamorphic  intercept  80 ± 6,  U-Pb  111  source f o r  i n sediments of sediments  n  n  Cretaceous  intrusive  Rb-Sr  metamorphism  Deposition  Reset  105 ± 20  2 0 S  lower  1.6  Rb-Sr  deposition  4)  2450 ± 230 u p p e r  211  metamorphism  by C o g b u r n  Granodiorite  rocks Intrusion?  74 ±  Pb/  2 3 8  Ma  1972)  Approx. time of  (Unit  U-Pb  a t 415 and 90  3)  ± 58 ± 5, 76.6  facies  10 U  date  Cretaceous Reset  metamorphism  by Spuzzum  batholith?  80  Spuzzum b a t h o l i t h  (Unit  5)  a) Hut C r e e k body K-Ar  70.1 ± 5 i s o c h r o n  Minimum  i n t r u s i v e age,  metamorphism U-Pb  181 ± 7  Rb-Sr  86 ± 4  Z 0 S  Pb/  2 3 a  U  Meaningless  date  Minimum  i n t r u s i v e age,  metamorphism b) S e t t l e r K-Ar  C r e e k body  78.9 ± 2 i s o c h r o n  Minimum  intrusive  age,  metamorphism U-Pb  91.8 ± 1  Rb-Sr  90 ± 8, 89 ± 7  2 0 S  Pb/  2 3 8  U  Intrusion?  date  Minimum  i n t r u s i v e age,'  metamorphism  Breakenridge Formation Gneiss U-Pb  Rb-Sr  105 ± 1  2 0 S  Pb/  2 3 8  U  (Unit  date  7) Either  79 ± 1.6  intrusion  of  granodiorite  or i t s  metamorphism  to gneiss  Cooling after  metamorphism t o  gneiss  1 2  A l l a n a l y t i c a l d a t a and r e s u l t s a r e l i s t e d U-Pb z i r c o n d a t e s r e p o r t e d a s Pb/ U, 2 0 6  2 3 8  2 0 7  i n A p p e n d i x B. Pb/ U, Pb/ 2 3 5  2 0 7  2 0 S  Pb  81  i n t r u s i o n during the Precambrian f o l l o w e d by metamorphism and  90 Ma as proposed by Mattinson  at 415  (1972) f o r rocks of the  Yellow Aster Complex and Swakane Gneiss  (see Table 5.1  for h i s  dates). Cogburn Creek Group has been mapped p r e v i o u s l y as C h i l l i w a c k Group, based on s t 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 s e c t i o n of the C h i l l i w a c k Group i n the C h i l l i w a c k v a l l e y and are unfossiliferous.  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 r e q u i r e major r e - i n t e r p r e t a t i o n of the r e g i o n a l geology.  The whole rock  i s o c h r o n date from the Cogburn Creek Group of 296 ± 58 Ma at 0.7039 i n i t i a l  8 7  Sr/  8 6  Sr  r a t i o represents l a t e  Paleozoic  d e p o s i t i o n of c h e r t and p e l i t e s and e x t r u s i o n of v o l c a n i c B i o t i t e ages of 74 to 89 Ma of  rocks.  from a l l u n i t s represent the waning  the Cretaceous i n t r u s i v e and metamorphic episode.  The  C h i l l i w a c k Group has been dated by Rb-Sr whole rock i s o c h r o n at 191 ± 6 Ma at 0.7051 i n t i a l  8 7  Sr/  8 6  Sr  ratio.  T h i s 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 i n a l l d i v i s i o n s of the Group (Monger  1970). the  A b e t t e r c o r r e l a t i o n based on l i t h o l o g y would be with  Bridge River Group, which a l s o c o n s i s t s of c h e r t and  greenschist rocks.  in g r e a t e r abundance than p h y l l i t e and other  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 i n F i g u r e 5.6. and  clastic  Armstrong  others have dated the Bridge R i v e r Group (Rb-Sr isochron) at  256 ± 35 Ma at 0.7037 i n i t i a l  8 7  Sr/  B 6  Sr  ratio.  T h i s date i s  82  lower than expected  from the spread of p o i n t s , because the  p r o p o r t i o n a l e r r o r s assigned are h i g h . plotted  Data boundaries  in F i g u r e s 5.3b,  initial and  8 7  Sr/  8 6  S r - r a t i o samples  have been drawn using the data p o i n t s  d,  j , k, and  superimposed i n F i g u r e 5.6. although  to the low  The  the d i f f e r e n t  fields  f i e l d s do o v e r l a p c o n s i d e r a b l y ,  isochrons f o r a l l three u n i t s have somewhat d i f f e r e n t 8 7  Sr/  8 6  Sr  r a t i o s and  slopes.  The  Bridge R i v e r Group data are very  C h i l l i w a c k Group.  s i m i l a r and d i f f e r e n t  from  C a l c u l a t i o n s f o r the Cogburn Creek Group are  based on a r e l a t i v e l y b i a s e d the r e s u l t s . most probably  Cogburn Creek Group  small number of samples, which may Since the date  have  f o r the C h i l l i w a c k Group i s  metamorphic, the r e s u l t s presented  here do  not  r u l e out the c o r r e l a t i o n of t h i s u n i t with the Cogburn Creek Group.  Monger (1978) c o n s i d e r s that the Bridge R i v e r Group i s  e q u i v a l e n t to the Hozameen Group on the east s i d e of the F r a s e r Fault.  No data are a v a i l a b l e from the Hozameen rocks to confirm  this isotopically.  I propose t h a t the Cogburn Creek Group i s  c o r r e l a t i v e with the Bridge R i v e r Group, and  t h e r e f o r e a l s o the  Hozameen Group (See Chapter 6 ) . S e t t l e r S c h i s t c o n t a i n s at l e a s t some d e t r i t u s of Precambrian age, d e p o s i t i o n was isochron).  The  metamorphism. two  most probably s c h i s t has Contact  old.  around 210  The  ± 27 Ma  undergone c o n t a c t and  metamorphism was  time of (Rb-Sr  and Rb-Sr dates  r e l a t e d to i n t r u s i o n of  from s c h i s t s and d i o r i t e  indicate cooling after  WR  then r e g i o n a l  bodies of Spuzzum b a t h o l i t h at around 95 to 110 Ma.  K-Ar Ma  z i r c o n 2450 ± 230 Ma  Biotite  i n the range 80-60  r e g i o n a l metamorphism.  K-Ar  isochron  83  dates from hornblende 78.9  ± 2 Ma,  dates and  i n the two d i o r i t e bodies, 70.1  t 5 Ma  are minimum v a l u e s f o r time of i n t r u s i o n .  and  Rb-Sr  WR  z i r c o n upper i n t e r c e p t dates f o r Spuzzum b a t h o l i t h are  too o l d and not v a l i d , probably due  to a s s i m i l a t i o n of country  rock d u r i n g i n t r u s i o n . S e t t l e r S c h i s t has been c o r r e l a t e d with Chiwaukum S c h i s t of the Skagit Metamorphic Complex, based on s i m i l a r i t i e s i n l i t h o l o g y and metamorphic h i s t o r y . that  I have v i s i t e d  i n the Stevens  to the S e t t l e r S c h i s t .  The  Outcrops  of Chiwaukum S c h i s t  Pass area look very s i m i l a r  same unusual  rock types  metamorphic mineral assemblages are p r e s e n t . dykes and metatrondhjemites ultramafic  pods.  Foliated  felsic  are common i n both u n i t s , as are  Both the Mount Stuart  b a t h o l i t h and  Spuzzum b a t h o l i t h were i n t r u d e d before the end of metamorphism.  and  the  regional  The Chiwaukum S c h i s t and Shuksan S u i t e are not  r e l a t e d , and are separated by f a u l t s and metamorphic f a c i e s differences.  F i g u r e 2.2  shows the present g e o g r a p h i c a l  l o c a t i o n s of the S e t t l e r S c h i s t and the Chiwaukum S c h i s t . Chiwaukum S c h i s t has not yet been dated.  Babcock et a l . (1985)  have found that Rb-Sr i s o t o p i c s y s t e m a t i c s of the Skagit Gneiss and Cascade R i v e r S c h i s t c l e a r age,  show broad d i s p e r s i o n  but a very strong o v e r p r i n t  Tertiary re-equilibration.  Clear  of data with no  of Mesozoic  and  Early  i s o t o p i c c o r r e l a t i o n i s not  possible. Based on s t r u c t u r a l p o s i t i o n with respect to the Shuksan t h r u s t and comparison with the North Cascades i n Washington, a p o s s i b l e a l t e r n a t i v e c o r r e l a t i v e of the S e t t l e r S c h i s t  i s the  84  Darrington  P h y l l i t e of the Shuksan Metamorphic S u i t e .  Where I  have seen the Shuksan S u i t e i n 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 S c h i s t . T h e i r metamorphic h i s t o r y i s a l s o d i f f e r e n t .  Evidence p o i n t s t o  the p o s s i b l e c o r r e l a t i o n of the two u n i t s being u n j u s t i f i e d . Geochronometry on the Darrington Sr WR,  Figure  P h y l l i t e gives  132 ± 8 Ma (Rb-  5.2m, Armstrong unpub.), which i s f a r younger than  d e p o s i t i o n of the p r o t o l i t h of the S e t t l e r S c h i s t . b l u e s c h i s t and g r e e n s c h i s t  Dates from  i n the Shuksan S u i t e demonstrate a  Late J u r a s s i c to l a t e E a r l y Cretaceous metamorphic age (Figure 5.2m, Armstrong et a l .  1983), a l s o d i s s i m i l a r .  The d a t a - f i e l d s i n the c o r r e l a t i o n diagram ( F i g u r e 5.6) show an i s o t o p i c s i m i l a r i t y between the S e t t l e r S c h i s t and C h i l l i w a c k Group, although that i s probably q u i t e c o i n c i d e n t a l . Sampling of the C h i l l i w a c k Group may not have been r e p r e s e n t a t i v e , and c o u l d have i n c l u d e d Mesozoic p e l i t e melange  from a  (Armstrong pers. comm. 1985).  The Cogburn Creek Group and S e t t l e r S c h i s t were juxtaposed before  i n t r u s i o n of Spuzzum b a t h o l i t h .  intruded  i n t o the imbricate  gave a mineral age.  isochron  The imbricate  A s m a l l body of d i o r i t e  zone before  the end of metamorphism  date of 77 ± 3 Ma, a metamorphic  cooling  zone appears to have been f o l d e d and  p o s s i b l y pushed aside at the western margin of the Spuzzum b a t h o l i t h i n the Cogburn Creek a r e a .  Some l a t e s t r a i n i s  i n d i c a t e d by f o l i a t i o n of the small d i o r i t e Lowes (1972) c o r r e l a t e d the imbricate Fraser  body. zone n o r t h of the  River with the Shuksan t h r u s t of the N o r t h Cascades,  85  Washington.  Misch (1966) c o n s i d e r e d the Shuksan t h r u s t to be  post-metamorphic and mid-Cretaceous i n age.  In the Cogburn  Creek area the imbricate zone pre-dates the peak of r e g i o n a l metamorphism.  In the Mount Watson area Armstrong et a l .  dated metamorphism which i s e a r l i e r  in blueschist  (1980)  f a c i e s rocks as 120 to 130  than r e g i o n a l metamorphism  Ma,  i n Cogburn Creek.  If the i m b r i c a t e zone i n Cogburn Creek i s the Shuksan  thrust  then the j u x t a p o s i t i o n of the Cogburn Creek Group and S e t t l e r S c h i s t can be bracketed between b l u e s c h i s t metamorphism Shuksan S u i t e at 120 to 130 Ma and d i o r i t e 10 Ma.  of the  i n t r u s i o n at 100 ±  Current s t u d i e s i n the North Cascades, Washington  (Silverberg  1985, L e i g g i and Brown 1985) show that the Shuksan  t h r u s t changes i n s t y l e and amount of deformation along i t s length.  Movement may d i e out northwards, as Reamsbottom  (1974)  recognised no major s t r u c t u r a l break a c r o s s s c h i s t s i n the Mount Breakenridge a r e a . be at Cogburn Creek.  The northern l i m i t  of movement may  in fact  T h i s would e x p l a i n the sudden change i n  l i t h o l o g y a c r o s s the Cogburn Creek v a l l e y ,  from u l t r a m a f i c  to the south to Cogburn Creek Group c h e r t to the north  rocks  (Figure  2.1). A l l v a l i d dates from the Spuzzum b a t h o l i t h are Cretaceous (Table 5.2, Appendix B).  The d i o r i t e  intruded during  regional  metamorphism  1979); thus the z i r c o n U-Pb  dates of  110 ± 5 Ma  (Bartholomew  (Hut Creek body, lower i n t e r c e p t of chord through  QDHC and S e t t l , Sett2) and 91.8 ± 0.5 Ma 2 0 6  Pb/  2 3 8  and K-Ar  ( S e t t l e r Creek body,  U date) are minimum i n t r u s i v e ages (Table 5.2). i s o c h r o n dates of 66 Ma to 88 Ma r e f l e c t  cooling  Rb-Sr  86  through b l o c k i n g temperatures metamorphism. of  d u r i n g waning of r e g i o n a l  On a r e g i o n a l s c a l e , dates from the southern part  the Spuzzum b a t h o l i t h around the F r a s e r V a l l e y a r e o l d e r  than  those from i n and north of the study area (Table 5.1, F i g u r e 5.1).  A c o n c e n t r a t i o n - i s o c h r o n f o r a l l a v a i l a b l e Hb-Bi data  from Spuzzum b a t h o l i t h has been p l o t t e d dates range  dates younging  5.7). and  Slope  from 75 t o 97 Ma f o r i n d i v i d u a l sample p a i r s , with a  c a l c u l a t e d mean of 83 ± 5 Ma. of  i n F i g u r e 5.7.  eastward  He c o n s i d e r e d that  Bartholomew (1979) found a trend  towards the F r a s e r F a u l t  zone  (Figure  i t r e p r e s e n t e d greater u p l i f t , e r o s i o n  l a t e r c o o l i n g through the argon b l o c k i n g  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 i m p l i e s t i l t i n g suggested by paleomagnetic North Cascade Mountains southeast of around  i n the o p p o s i t e sense to that  measurements.  Evidence from the  indicates regional t i l t i n g  30° (Beck  down t o the  1982), and I r v i n g et a l .  (1985)  note that t h e i r data from the Spuzzum b a t h o l i t h c o u l d be i n t e r p r e t e d as 28° t i l t r i s e t o the northward  down to the southeast.  younging  T h i s would g i v e  t r e n d of dates from the Spuzzum  b a t h o l i t h that can be seen i n F i g u r e 5.1, and to the i n c r e a s e i n metamorphic grade  northwards.  Breakenridge Formation g r a n o d i o r i t i c g n e i s s gave a z i r c o n 2 0 6  Pb/  2 3 8  U date of 105 ± 1 Ma and Rb-Sr mineral i s o c h r o n date of  79 ± 1.6 Ma, s i m i l a r to dates from the Spuzzum b a t h o l i t h .  The  g r a n o d i o r i t e i s h i g h l y deformed, so these dates may represent either  i n t e n s e metamorphism of an o l d e r rock or synkinematic  intrusion.  87  The l a t e s t event was granodiorite.  i n t r u s i o n of small p l u t o n s of  One of these, the Cogburn G r a n o d i o r i t e ,  dated at 32 ± 4 Ma  (Bartholomew 1979) and l o c a l l y  i s o t o p i c a l l y r e s e t nearby s c h i s t s .  has  has been  88  0.7031  0.05 8 7  Figure  5.2a  -I  Rb/  8 6  L  0.1 Sr  Rb-Sr i s o c h r o n p l o t A l l a n a l y s e s are WR  for Baird Metadiorite (whole r o c k ) .  89  Rb/ Sr 8 6  F i g u r e 5.2b  Rb-Sr isochron p l o t f o r Cogburn Creek Group L i n e s represent 1) WR and P i data e x c l u d i n g c h e r t 2) c h e r t HL37a WR-Bi p a i r 3) metavolcanics and p h y l l i t e Inset shows p o i n t s near the i n t e r c e p t s , expanded scale, also i n i t i a l S r / S r ratios. 8 7  8 6  90  F i g u r e 5.2c  Rb-Sr isochron p l o t f o r S e t t l e r S c h i s t L i n e s represent 1) a l l data, not i n c l u d i n g B i and Mu 2) WR-Pl-Bi i s o c h r o n , p h y l l i t e SS82 3) WR-Pl-Bi isochron f o r quartz-biotite-garnet schist marginal to Cogburn G r a n o d i o r i t e (SS128).  91  0.709h  0.707^ co  (O 00  CO oo  0.705r  0.703 8 7  Figure  5 . 2d  Rb/ Sr 8 6  S e t t l e r S c h i s t , ..expanded  scale  F i g u r e 5.2e  Rb-Sr isochron p l o t f o r premetamorphic i n t r u s i v e rocks. L i n e s represent 1) WR-Pl-Bi, f e l s i c s i l l HL111 i n t r u d i n g Cogburn Creek Group. 2+3) foliated felsic g a r n e t - c l u s t e r dyke, SS85, intruding Settler Schist. 2) not i n c l u d i n g B i 3) i n c l u d i n g Bi  93  F O L I A T E D DIORITE in fault zone  0.715  0.710  1 £ 1  CO CO  CO  CO CO  0.705h  ^  + WR • PI A Bi T Mu  0.7040  0.700 6 8 7  F i g u r e 5.2f  Rb/  8 6  Sr  Rb-Sr i s o c h r o n p l o t f o r small body of f o l i a t e d g r a n o d i o r i t e i n imbricate zone (SD92).  • WR • PI • Hb  SPUZZUM B A T H O L I T H Hut C r e e k B o d y 0  10  0.2 0.4 8 7  F i g u r e 5.2g  15  A  B j  20  Rb/ Sr 8 6  Rb-Sr isochron p l o t f o r Spuzzum b a t h o l i t h , Hut Creek body L i n e s represent 1) a l l data not i n c l u d i n g Bi 2) a l l data i n c l u d i n g Bi Inset shows p o i n t s scale.  near i n t e r c e p t s  on expanded  95  0.730 CO  CO  co  CO  0.720  co  0.7044 0.7042 <0.7041 0.7040  >-0.7039  0.710 SPUZZUM B A T H O L I T H Settler Creek Body  0.7038  0.7031 0.1  F i g u r e 5.2h  0.2  15  10 8 7  Rb/  8 6  + WR o PI • Bi I I Hb  20  Sr  Rb-Sr isochron p l o t f o r Spuzzum b a t h o l i t h , S e t t l e r Creek body L i n e s represent 1) WR-Pl i s o c h r o n f o r a l l samples except SD97 2) WR-Pl-Hb isochron f o r a l l samples i n c l u d i n g SD97 3) i s o c h r o n f o r a l l data including Bi Inset shows p o i n t s near i n t e r c e p t s on expanded scale.  96  0.760K  0.740H  0 0.2 0.4  F i g u r e 5.2i  8 7  Rb/  8 6  Sr  Rb-Sr i s o c h r o n p l o t f o r a g m a t i t i c quartz d i o r i t e (SD14), marginal t o Cogburn G r a n o d i o r i t e . Inset shows p o i n t s near i n t e r c e p t s on expanded scale.  97  F i g u r e 5.2j  Rb-Sr isochron p l o t f o r Breakenridge Formation gneiss  .98  0.730 0.708  0.720 CO  0.706  CO 00  • 0.710 0.704  CHILLIWACK GROUP  expanded scale  0.700 8 7  F i g u r e 5.2k  Rb/ Sr 8 6  Rb-Sr isochron p l o t f o r C h i l l i w a c k Group sediments. Data from Armstrong and o t h e r s (unpub.). A l l a n a l y s e s a r e WR  F i g u r e 5.21  Rb-Sr isochron p l o t f o r Bridge R i v e r Group. Data from Armstrong and others (unpub.). L i n e s represent 1) a l l data 2) high grade s c h i s t s  100  F i g u r e 5.2m  Rb-Sr isochron p l o t f o r Shuksan S u i t e b l u e s c h i s t and g r e e n s c h i s t , and D a r r i n g t o n P h y l l i t e . Data from Armstrong and o t h e r s (unpub.).  101  0.4 2 0 7  F i g u r e 5.3  Pb/  2 3 5  U  U-Pb c o n c o r d i a diagram f o r z i r c o n d a t i n g . P l o t of Pb/ U v. Pb/ U for a l l zircon samples. L i n e 1) i s f o r S e t t l e r S c h i s t , 2) i s a l l samples combined. Sample numbers as l i s t e d i n Appendix C. 2 0 6  2 3 8  2 0 7  2 3 5  i  r  separates from Spuzzum b a t h o l i t h . L i n e s represent 1) Hut Creek body. Slope g i v e s 70.1 ± 5 Ma I n t e r c e p t 392 ± 25 2) S e t t l e r Creek body. Slope g i v e s 78.9 ± 2 Ma. I n t e r c e p t 388 ± 88 F i g u r e 5.4b  P l o t of %K v. Ar n l / g f o r hornblende separates from Spuzzum b a t h o l i t h . L i n e s represent 1) Hut Creek body. Slope g i v e s 65.7 ± 5 Ma Intercept 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 g i v e s 79.7 ± 2 Ma Intercept i n i t i a l A r 0.139 0.010 n l / g fl0  a o  4 0  103  F i g u r e 5.5  U-Pb c o n c o r d i a diagram f o r Yellow A s t e r Complex, Skagit Gneiss and Swakane G n e i s s , from Mattinson (1972). P l o t of Pb/ U v. Pb/ U for zircon samples. 2 0 6  2 3 8  2 0 7  2 3 5  1 04  0.710r  0.708r C o t t i e r  87  210±27Ma eo.7043  C^m*»  i_j settler Schist Chilliwack Group  Sr/ Sr| 86  l i  0.706 • SR  Vo^osi*  i~ . . . 296±58Ma i C o g b u r n C k Group •0.7039 Bridge River Group  ISOTOPIC  2  4 fg| 6  Ma  COMPARISONS  0.704k  87,  Figure  5.6  Rb/86Sr  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 correlative stratigraphic units.  from  possible  105"  •  .  .  i  i  i  i  %K F i g u r e 5.7  P l o t of %K v. A r n l / g f o r r e g i o n a l data from Spuzzum b a t h o l i t h , not i n c l u d i n g t h i s study. L i n e s represent 1) lower l i m i t , slope g i v e s 76.0 ± 5 Ma 2) c a l c u l a t e d mean, slope gives 83.1 ± 5 Ma 3) upper l i m i t , slope g i v e s 97.5 ± 5 Ma 4 0  Lower and upper l i m i t isochron dates represent HbBi p a i r s from s i n g l e samples; mean i s f o r a l l data. Conventional K-Ar dates are l i s t e d i n Table 5.1.  106  West  14  12  10  8  East  D I S T A N C E , km  F i g u r e 5.8  Graph of eastward younging t r e n d of K-Ar dates from Spuzzum b a t h o l i t h , from Bartholomew (1979). K-Ar date versus p e r p e n d i c u l a r d i s t a n c e from a N-S g r i d l i n e on NTS 92/H, approximately 121° 20'. Boxes i n d i c a t e age and d i s t a n c e e r r o r margins. S t i p p l e d boxes a r e averaged hornblendebiotite pairs.  1 07  6. The  Regional  Synthesis  region between the F r a s e r R i v e r , H a r r i s o n Lake, and  Canada-United S t a t e s Border i s g e o l o g i c a l l y complex. Berg  (1984) separate  the Intermontane superterrane  Quesnel, Cache Creek and superterrane terranes  S t i k i n e t e r r a n e s ) and  ( W r a n g e l l i a and  (Figure 6.1).  Monger  and  (comprising  the I n s u l a r  Alexander t e r r a n e s ) with f i v e  They are  the  small  (from Monger 1985):  1) Methow; T r i a s s i c ( ? ) b a s a l t o v e r l a i n by J u r a s s i c and Cretaceous c l a s t i c  sediments,  2) Bridge River-Hozameen; Permian to m i d - J u r a s s i c basalt, chert, c l a s t i c  and  u l t r a m a f i c rocks,  3) Cadwallader; T r i a s s i c b a s a l t , c l a s t i c J u r a s s i c and  Cretaceous c l a s t i c  4) Shuksan; J u r a s s i c ( ? ) b a s a l t and 5) Chilliwack-Nooksack;  disrupted  and carbonate  rocks,  rocks, clastic  rocks,  late Paleozoic volcanic, c l a s t i c  and  carbonate rocks, Mesozoic c l a s t i c and v o l c a n i c rocks.  He  has  r e s t o r e d these  t e r r a n e s to t h e i r p r e - T e r t i a r y  c o n f i g u r a t i o n by removing 80 S t r a i g h t Creek f a u l t these Ma  f a u l t s has  km of d e x t r a l motion on the  zone (Figure 6.2a).  been bracketed  (Monger 1985), and  Fraser-  Timing of movement on  between Cretaceous and around  p o s s i b l y a l l w i t h i n the Eocene.  Monger c o n s i d e r s that the Skagit Metamorphic S u i t e may derived largely  from rocks of the Methow, Bridge River  Cadwallader t e r r a n e s . but  i t i s not  35  T h i s may  be true f o r the Custer  be  and Gneiss  f o r the S e t t l e r S c h i s t and Chiwaukum S c h i s t , which  are l i t h o l o g i c a l l y  different  from the rocks i n those  terranes.  108  kilometres 0  5 0  1 0 0  ±  Present  day  configuration  Bridge  R iv e r - H o z a me e n  terrane  BR, H  c CG ss&d."  Methow-Tyaughton  • vw\  Cadwallader  •  Shuksan  •  h Z-I-_i  N  o  Q  k  s  a  Skagit  Figure  6.1  c  terrane  terrane  =  K  terrane  Suite  terrane  MT  C  Sh N  Sk  Present day c o n f i g u r a t i o n of t e c t o n o - s t r a t i g r a p h i c t e r r a n e s , modified from Monger and Berg (1984).  109  Sh igure 6.2  R e c o n s t r u c t i o n of Bridge R i v e r and Methow t e r r a n e s , 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 i n a s u i t a b l e time and  c o n f i g u r a t i o n f o r the d e r i v a t i o n .  Figures  6.3a  and  space  b show post-  Cretaceous, pre-Eocene, r e s t o r a t i o n of the S e t t l e r S c h i s t Chiwaukum S c h i s t based on a) S t r a i g h t Creek f a u l t s , from Monger (1985).  150  km  o f f s e t on  from Misch (1977), and  Figure  6.3a  the  Fraser-  b) 80  r e s t o r e s the two  km o f f s e t ,  u n i t s to a  reasonable c o n f i g u r a t i o n , but  Bridge River and Methow  are d i s r u p t e d  Figure  separated  (Figure 6.4a).  by about 80  km.  If the 80  there must have been at l e a s t 80 another, p a r a l l e l , S c h i s t and  fault  km  km  leaves  Methow b a s i n appear to be too great t e r r a n e map  (Monger and  to separate the S e t t l e r 150  fault  150  km  (1985) f o r the Tyaughtonwhen a p p l i e d to the most  Berg 1985).  She  and  km  km  a fairly  km  6.4b)  River fault  Movement on  l a r g e orthogonal  component as w e l l as d e x t r a l s l i p , a l l o w i n g  for greater  total  t h i s does not a f f e c t the r e l a t i v e p o s i t i o n s of  the S e t t l e r S c h i s t and 1985)  110  on the F r a s e r  on the Yalakom-Ross Lake f a u l t .  the Ross Lake f a u l t probably had  displacement, but  (Figure  f o r r e c o n s t r u c t i o n of the Bridge  Tyaughton-Methow basin are 80  around 100  proposed  of movement on the Yalakom-Ross Lake F a u l t .  More reasonable values Group and  km.  reversal  of o f f s e t on the F r a s e r - S t r a i g h t Creek f a u l t s preceded by  the s c h i s t s  of d e x t r a l movement on  Chiwaukum S c h i s t by a t o t a l of  displacements proposed by Kleinspehn  terranes  o f f s e t i s c o r r e c t , then  system i n order  There i s a dilemma here, as the  recent  6.3b  and  Chiwaukum S c h i s t .  surmises that the dilemma can  of h o r i z o n t a l p l a t e s i n v o l v e d  i n the  be  Monger (pers. comm.  solved by having a  right-lateral  stack  111  F i g u r e 6.3  R e c o n s t r u c t i o n of Settle.r Schist-Chiwaukum S c h i s t using a) Misch (1977); b) Monger (1985). Base map i s F i g u r e 1.3.  1 12  F i g u r e 6.4  D i s r u p t e d r e c o n s t r u c t i o n of. Bridge R i v e r and Methow t e r r a n e s using a) 150 km displacement, a f t e r Misch (1977); b) 110 km displacement, a f t e r Kleinspehn (1985).  1 13  displacement. the stack  By invoking  d i f f e r e n t i a l movement down through  i t i s p o s s i b l e to r e c o n c i l e 150 km displacement f o r  the S e t t l e r Schist-Chiwaukum S c h i s t with 80 km displacement f o r the Bridge River-Kozameen Groups. Monger (1985) notes that post-Cretaceous deformations have masked the boundaries of the small r e l a t i o n s h i p s are not c l e a r . small  terranes  records  He c o n s i d e r s  c l o s u r e of oceanic  between the superterranes. orthogonal,  terranes,  transcurrent,  so that  their  that e v o l u t i o n of the and/or m a r g i n a l  basins  The c l o s u r e may have been by or t r a n s p r e s s i v e movement.  I s o t o p i c and g e o l o g i c a l 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 f a u l t movements ( F i g u r e 6.2 and 4)  does not juxtapose them, but there a r e p o s s i b l e The  first  and simplest  explanations.  i s that the Cogburn Creek Group never was  continuous with the Bridge River Group, but i s a sequence of similar  sediments and v o l c a n i c rocks deposited  from the same source i n a nearby b a s i n .  a t the same time  The second i s that  d i s r u p t i o n of the Bridge River Group-Cogburn Creek Group Basin by  f a u l t i n g such as the Shuksan t h r u s t , combined w i t h i n t r u s i o n  of voluminous d i o r i t e p l u t o n s ,  has separated  the two u n i t s .  Shuksan t h r u s t has brought i n the S e t t l e r S c h i s t and s l i c e s of o l d e r u n i t s and u l t r a m a f i c  rocks  The  imbricate  from the southeast to  o v e r l i e the Cogburn Creek Group. Timing of movement on the Shuksan Thrust  i s b r a c k e t e d to  A l b i a n by i s o t o p i c d a t i n g of r e g i o n a l b l u e s c h i s t metamorphism i n the Shuksan Metamorphic S u i t e and i n t r u s i o n of Spuzzum  1 14  batholith.  Hence the Shuksan t h r u s t was a c t i v e before d e x t r a l  motion on the other f a u l t s .  Movement on the Shuksan t h r u s t  must  have d i e d out northwards, ending j u s t north of Cogburn Creek, as Reamsbottom  (1971, 1974) d i d not f i n d a major break between the  " C h i l l i w a c k " Group (Cogburn Creek Group) and C a i r n Needle Formation  (Settler Schist).  The geometry i n v o l v e d i n  r e c o n s t r u c t i o n of the Cogburn Creek Group-Bridge River Group as one continuous u n i t  is difficult,  Bridge River Groups now  because the Cogburn Creek and  form p a r a l l e l b e l t s  ( F i g u r e 6.1).  However, i t i s not impossible, because they are separated by a wide b e l t of l a t e Cretaceous and T e r t i a r y  i n t r u s i v e rocks that  must have pushed a s i d e the country rock.  Not enough evidence i s  a v a i l a b l e to choose between the a l t e r n a t e  relationships;  however, the f i r s t simpler.  (that the two u n i t s were never continuous) i s  Perhaps the Cogburn Creek Group was d e p o s i t e d i n an  intermediate p o s i t i o n between the Bridge R i v e r Group and the Cadwallader t e r r a n e , which i s a v o l c a n i c a r c . The s t r u c t u r a l p o s i t i o n of the S e t t l e r S c h i s t ,  overlying  the Shuksan t h r u s t , i s s i m i l a r to that of the D a r r i n g t o n Phyllite.  However, the D a r r i n g t o n P h y l l i t e has been dated as  128 + 6 Ma  (Armstrong unpub. d a t a ) , thus at face value i s  c o n s i d e r a b l y younger than the S e t t l e r S c h i s t . s t r u c t u r a l p o s i t i o n may  The s i m i l a r i t y in  be purely c o i n c i d e n t a l , as a r e s u l t of  complex movements on v a r i o u s f a u l t systems.  115  Imbricate zone, Shuksan Thrust, shown on saddle east of S e t t l e r Lake. Steeply d i p p i n g s l i c e s of u l t r a m a f i c rock between B a i r d M e t a d i o r i t e ( l e f t ) and S e t t l e r S c h i s t (grassy slope on r i g h t ) .  Mafic amphibolite pod i n S e t t l e r S c h i s t . Dark rim c o n t a i n i n g l a r g e hornblende c r y s t a l s , surrounding f e l s i c core.  116  P l a t e 2.3  Photomicrograph of sample SD101. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Hornblende with brown igneous c o r e s and green metamorphic rims.  P l a t e 2.4  Photomicrograph of sample SD66. Photo r e p r e s e n t s 10 mm i n s e c t i o n . A l t e r a t i o n of hornblende d u r i n g metamorphism, i n the hornblende-hypersthene gabbro.  117  P l a t e 3.1  Photomicrograph of sample HL30. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Chromite and o l i v i n e r e c r y s t a l l i s e d d u r i n g metamorphism, a l t e r e d to c a l c i t e and t r e m o l i t e . Crossed n i c o l s .  P l a t e 3.2  Photomicrograph of sample HL16. Photo r e p r e s e n t s 10 mm i n s e c t i o n . B i o t i t e p o r p h y r o b l a s t s i n quartz b i o t i t e schist. Crossed n i c o l s .  Plate  3.3  P h o t o m i c r o g r a p h o f 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 rim. Crossed n i c o l s .  Plate  3.4  P h o t o m i c r o g r a p h o f 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 hornblende c r y s t a l l i s e d a l o n g an f f o l d a x i s , f i n e h o r n b l e n d e r a n d o m l y r e oriented. 2  119  P l a t e 3.5  Photomicrograph of sample HL142. Photo r e p r e s e n t s 10 mm i n s e c t i o n . P o r p h y r o b l a s t i c green hornblende ( s i d e s of photo) c o n t a i n i n g f and f . Staurolite p o r p h y r o b l a s t s i n c e n t r e , epidote with b i o t i t e . 2  P l a t e 3.6  3  Photomicrograph of sample SS110. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Deformed p s e u d o c h i a s t o l i t e with random i n t e r n a l texture and s t a u r o l i t e c r o s s i n g the boundary. Note s i l l i m a n i t e i n lower r i g h t corner.  120  . ^, c  Plate 3.7  Photomicrograph of sample SS135. Photo r e p r e s e n t s 10 mm i n s e c t i o n . 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 t i g h t f f o l d , r e c r y s t a l l i s e d around the f kink f o l d . 2  3  Plate 3.8  Photomicrograph of sample SS66. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Rounded garnet with q u a r t z , muscovite, f i b r o l i t e h a l o , 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 .  o  o  s  121  P l a t e 3.10  Photomicrograph of sample SS114. Photo r e p r e s e n t s 10 mm i n s e c t i o n . Twinned s t a u r o l i t e , zoned with graphite.  122  P l a t e 3.12  S i l l i m a n i t e p o r p h y r o b l a s t s from r i d g e n o r t h of Cogburn Creek. L o c a t i o n 49° 35.3'N 121° 39'W.  123  Plate  3.13  Photomicrograph of s i l l i m a n i t e p o r p h y r o b l a s t . Photo represents 10 mm in s e c t i o n . F i b r o l i t e patches (fuzzy) and quartz i n c l u s i o n s , rounded garnet i n quartz r e a c t i o n rim.  124  P l a t e 4.1  F a u l t c o n t a c t below the i m b r i c a t e zone. Looking north, at 49° 38.5'N 121° 31'W. Cogburn Creek Group c h e r t ( l e f t ) below B a i r d M e t a d i o r i t e (forms peak).  1 25  REFERENCES Armstrong, R.L.  1980.  Geochronometry of the Shuksan Metamorphic  S u i t e , North Cascades, Washington. America, A b s t r a c t s with Programs,  G e o l o g i c a l S o c i e t y of 12, p. 94.  Armstrong, R.L., Harakal, J.E., Brown, E.H., Rady, P.M.  1983.  B e r n a r d i , M.L.  Late P a l e o z o i c high-pressure  and  metamorphic  rocks i n northwestern Washington and southwestern B r i t i s h Columbia: The Vedder Complex.  G e o l o g i c a l S o c i e t y of  America, B u l l e t i n , 94, p. 451-458. Babcock, R.S.,  Armstrong, R.L.  and Misch, P. 1985.  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M.Sc. t h e s i s ,  B r i t i s h Columbia, Vancouver,  Reamsbottom, S.B. 1974.  thesis,  The geology of the Mount Breakenridge  a r e a , H a r r i s o n Lake, B.C. of  Cascades.  University  161 p.  Geology and metamorphism of the Mount  Breakenridge area, H a r r i s o n Lake, B.C.  Unpub. PhD.  t h e s i s , U n i v e r s i t y of B r i t i s h Columbia, Vancouver, R i c h a r d s , T. 1971.  P l u t o n i c rocks between Hope, B.C., and the  49th P a r a l l e l .  Unpub. Ph.D. t h e s i s , U n i v e r s i t y of B r i t i s h  Columbia, Vancouver, Richardson, S.W.  1968.  178 p.  S t a u r o l i t e s t a b i l i t y i n a p a r t of the  system Fe-Al-Si-O-H. Richardson, S.W.,  155 p.  J o u r n a l of P e t r o l o g y , 9, p. 467-488.  G i l b e r t , M.C. and B e l l , P.M.  1969.  Experimental determination of k y a n i t e - a n d a l u s i t e and andalusite-sillimanite equilibria; t r i p l e point.  the aluminium s i l i c a t e  American J o u r n a l of S c i e n c e , 267, p. 259-  272. Roddick, J.A. and Hutchison, W.W.  1969.  Northwestern p a r t of  1 32  the Hope map-area, B.C. activities,  April  (92H W/2):  to October  1968;  in Report of G e o l o g i c a l Survey of  Canada Paper 69-1A, p. 29-38. S i l v e r b e r g , D.S.  1985.  The  Shuksan F a u l t i n the Whitechuck  Mountain - Mount Pugh area, North Cascades, Washington. G e o l o g i c a l S o c i e t y of America, A b s t r a c t s with Programs, 17, p. Stacey,  408.  S.J. and  Kramers, J.D.  terrestrial E a r t h and S t e i g e r , R.H.  1975.  l e a d isotope e v o l u t i o n by a two-stage model.  P l a n e t a r y Science L e t t e r s , 26, p. 207-221.  and Jager, E.  1977.  geochronology: Convention geo-  Approximation of  and cosmochronology.  Subcommission on  on the use of decay c o n s t a n t s i n E a r t h and P l a n e t a r y  Science  L e t t e r s , 36, p. 359-362. V i n i n g , M.R.  1977.  The  Unpub. M.Sc.  t h e s i s , U n i v e r s i t y of B r i t i s h  Vancouver, 147 Wanless, R.K., R.N.  Age  R.D.,  Lachance, G.R.  determinations  Ar i s o t o p i c ages, r e p o r t 11. Paper  B.C.  Columbia,  p.  Stevens,  1973.  Spuzzum Pluton northwest of Hope,  and  and  Delabio,  g e o l o g i c a l s t u d i e s : K-  G e o l o g i c a l Survey of Canada  73-2.  Wasserburg, G.J.,  Albee, A.L.  and Lanphere, M.A.  1964.  M i g r a t i o n of r a d i o g e n i c strontium d u r i n g metamorphism. J o u r n a l of G e o p h y s i c a l York, D.  1967.  The  L e t t e r s , 2, p. York, D.  1969.  best  Research, 69, p. 4395-4401.  isochron.  E a r t h and P l a n e t a r y  Science  479-  Least squares f i t t i n g  of a s t r a i g h t l i n e  with  1 33  correlated errors. p. 320-324.  E a r t h and  P l a n e t a r y Science L e t t e r s 5,  134  Appendix A.  I s o t o p i c D a t i n g ; A n a l y t i c a l Methods  I s o t o p i c d a t i n g using K-Ar, was  Rb-Sr and U-Pb  z i r c o n methods  c a r r i e d out at the U n i v e r s i t y of B r i t i s h Columbia, with  advice and a s s i s t a n c e from R.L.  Armstrong, K r i s t a S c o t t , Joe  Harakal, Randy P a r r i s h and Peter Van  der Heyden.  Analytical  techniques are as f o l l o w s : K-Ar Potassium  analyses are c a r r i e d out i n d u p l i c a t e by  a b s o r p t i o n , u s i n g a Techtron AA4  spectrophotometer.  measured by i s o t o p i c d i l u t i o n with a high p u r i t y an AEI MS-10  mass spectrometer.  standard d e v i a t i o n .  Reported  3 B  Argon i s  Ar  spike i n  e r r o r s are  The constants are l i s t e d  atomic  one  i n Table  B-3.  Rb-Sr Determination  of Rb and Sr c o n c e n t r a t i o n s i s by  replicate  a n a l y s i s of pressed powder p e l l e t s using X-Ray f l u o r e s c e n c e . Mass a b s o r p t i o n c o e f f i c i e n t s are obtained from Mo s c a t t e r i n g measurements, and U.S. standards are used r a t i o s i s 2 % (1 a)  Ka Compton  G e o l o g i c a l Survey  for c a l i b r a t i o n .  E r r o r on the  where Rb and Sr ppm  are >50,  8 7  rock Rb/ Sr 8 6  and  p r o p o r t i o n a l to the r e c i p r o c a l of the smaller value Sr i s o t o p i c composition  i s measured on unspiked  prepared u s i n g standard ion exchange techniques. spectrometer  i s a Micromass 54-R.  and automated u s i n g a Hewlett Experimental  8 7  8 6  Sr  The mass  Data a c q u i s i t i o n  data have been normalised to an  Sr/  samples  is digitised  Packard HP-85 computer.  0.1194 and a d j u s t e d so that the NBS gives an  otherwise.  8 7  Sr/  8 6  S r r a t i o of  standard SrC03 (SRM  987)  r a t i o of 0.71020 ± 2 and the Eimer and Amend  1 35  Sr a r a t i o of 0.70800 ± 2. ratio  i s 0.00010 (1 a ) .  are i n Table B-2.  The  the technique of York  The  p r e c i s i o n of a s i n g l e  Decay c o n s t a n t s for age  8 7  Sr/  8 6  Sr  calculations  r e g r e s s i o n s are c a l c u l a t e d a c c o r d i n g to (1967).  U-Pb Z i r c o n was  separated from  samples u s i n g wet separator.  f i n e l y crushed 20 to 40 kg rock  shaking t a b l e , heavy l i q u i d , and a magnetic  They were a c i d washed i n strong aqua r e g i a and hand  picked as r e q u i r e d .  Cleaned  dissolved in acid, s p l i t runs, then put through  z i r c o n separate was  f o r s p i k e d and  2ospj _ 3  2 3 5rj  S  pik . e  a Hewlett  Data a c q u i s i t i o n and  c a l i b r a t e d u s i n g the NBS  errors  s i l i c a g e l on a V.G.  Packard HP-85 computer.  composition:  of Krogh (1973).  6/4:  use a  981  17.75, 7/4:  Re  Isomass 54R mass  r e d u c t i o n are c a r r i e d out The mass spectrometer  standard.  The  15.57, 8/4:  system blank  37.00.  U-Pb  on  is has  date  ( l a ) 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 u n c e r t a i n t i e s through c a l c u l a t i o n program and to  We  Samples were analysed u s i n g s i n g l e  f i l a m e n t s loaded with and spectrometer.  isotope composition  ion exchange columns to separate the l e a d  and uranium, using the procedure mixed  weighed,  the t o t a l v a r i a n c e .  the e n t i r e  date  summing a l l the i n d i v i d u a l c o n t r i b u t i o n s  1  Appendix Table  B.  B-1.  Sample Number  Isotopic Dating: Rb-Sr a n a l y t i c a l  Analytical  data  and  dates.  data  Rock D e s c r i p t i o n  Sr ppm  Rb ppm  ' Rb Sr 7  86  86  l[ Sr  Sr  Baird Metadiorite 493  1 .4  0 .008  0 .7040  metamorphosed  1 35  0 .8  0 .016  0 .7045  and g a b b r o  179  0 .3  0 .004  0 .7043  WR  145  0 .6  0 .012  0 .7045  WR  175  0 .4  0 .006  0 .7040  MD1  WR  highly  foliated,  MD3  WR  white,  MD6  WR  diorite  MD8 MD9  green  and  0.7043  Cogburn C r e e k Group HL37a  WR  miceous  Bi  ribbon  recrystallised  18.1 16.7  chert  24 .6 252  3 .94  0 .71 35  43 .92  0 .7594 • 0 .7599  HL76  WR  knobbly  chloritic  green-  349  8 .4  0 .070  0 .7044 0 .7059  PI  schist,  coarse  metavolc.  55  Hb HL97  WR  268  micaceous  quartzite  HL103  WR  grey  phyllite  HL125  WR  fine  grained  PI  actinolite  biotite-  greenschist  Settl  WR  S e t t 2 WR SS82  WR PI  0 .025  0 .7037  3. 1  0 .161  0 .7041  141  58 .3  1 .20  0 .7082  386  35 .2  0 .264  0 .7048  209  41 .7  0 .578  0 .7073  , 483  1 .8  0 .011  0 .7068  12.5  Settler  2 .4  139  32 .37  0 .7420  Schist graphitic "  phyllite  224  71 . 3  0.923  0.7074  284  70. 7  0.720  0.7070  314  76. 3  0.703  0.7071  0.185  0.7066  73.5  4.7  36  1 37  61 . 2  •••• B i SS109  WR  SSI 14  WR  SS128  WR PI  WR  1 1 .26 ,  0.,7170  1 50  52, .8  1 .021 ,  0.,7072  164  81 .2 ,  1 .436  0,.7088  quartz-biotite-garnet  1 72  73,.7  1 .24  0,.7075  schist,  285  4,.2  0 .043  0 .7065  graphitic  phyllite  marginal  to  Cogburn G r a n o d i o r i t e SS130  238  graphitic  23. 6  phyllite/black  356  252 28 .9  30 .95 0 .235  0 .7234 0 .7065 0 .7062  slate SSI 32  WR  black  SS135  WR  graphitic  SS143  WR  coarse  slate phyllite  sandy b i s c h i s t  pelitic  schist  1 57  98 .8  1 .824  0 .7090  331  65 .4  0 .571  0 .7070  370  30 .4  0 .24  0 .7045  317  73 .5  0 .672  0 .7065  PRB  11 7  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  quartzofelspathic  640  5 .3  0 .024  0 .7038  PRB  90  pelitic  204  80 .7  1 . 1 45  0 .7079  PRB  80  it  213  80 .2  1 .089  0 .7068  schist  schist i?  0 .7066 PRB  samples  from  Bartholomew  (1979)  S i l l s in Schists HL111  WR  , . PI Bi SS85  ....  foliated  dyke  flecked  with  biotite,  contact  with  Spuzzum  WR  foliated  PI  cluster  Mu  Settler  •••• Bl  felsic  felsic dyke Schist  near D.  garnet-  intruding  356  39 .3  0 .319  0 .7040  352  25 . 1  0 .206  0 .7043  22 .2  331  43 .3  0 .7493  368  22 . 1  0 .174  0 .7041  367  2 .9  0 .023  0 .7036  250  83 .0  0 .96  0 .7051  5 .06  0 .7095  1 20  209  138  F o l i a t e d G r a n o d i o r i t e , s m a l l body i n i m b r i c a t e zone SD92 WR PI .... Mu  medium g r a i n e d f o l i a t e d  390  26.9  0.199  0.7042  granodiorite with b i o t i t e ,  615  5.0  0.024  0.7040  muscovite,  155  108  2.02  0.7064  210  6.43  0.7109  garnets  .... B i  94.4  Spuzzum D i o r i t e , Hut Creek body SD36 WR  quartz d i o r i t e  PI SD66 WR  hornblende-hypersthene  724  1 .4  0. 005  0.7037  1 1 58  0. 9  0. 002  0.7036  456  0. 4  0. 003  0.7036!  650  0. 8  0. 004  0.7038  11 1  0. 9  0. 024  0.7040  gabbro SD67 WR Hb 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  4. 3  0. 272  0.7042  45.6  Hb  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  fine grained  668  10. 5  0. 046  0.7037  hornblende  gabbro  0.7035 13. 7  0. 028  0.7037  104  4. 1  0. 1 15  0.7038  422  1 .8  0. 012  0.7036  385  5.6  0.042  0.7039  17.3  0.044  0.7039  1.2  0.054  0.7041  6.6  0.053  0.7039  1415 Hb SD119 WR  fine grained  hornblende  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  1 143  PI  65.3  Hb SD96 WR  quartz d i o r i t e  361  1 39  PI  922  ....  Hb  SD97  WR  quartz d i o r i t e  PI  biotite  71 . 7 with  354 496  Hb  30. 2  .... B i  14. 9  7.5  0.024  0.7038  2.5  0.101  0.70425  0. 132  0.7039  7.3  0.043  0.7038  2.7  0.256  0.70435  16.1  127  24.71  0.7347 0.7343  SD98 WR  quartz  diorite  PI  358  12.0  0.097  0.7041  1 148  29.4  0.074  0.7039 0.7037  Hb  53. 3  Aqmatised  quartz  SD14  WR  agmatised d i o r i t e ,  PI  contact  B1  Granodiorite  ....  Breakenridqe CU1  WR  near  w i t h Cogburn  leucocratic  granodioritic  muscovite  705  56.0  0.318  0.7053  0.009  0.7044E  2.11 250  45.6  0.7313  21 1  33. 1  0.452  0.7041  1 46  2.1  0.042  0.7038  3.30  0.7075  83 .7 50 .3  ... B i  95.3 191  10.98  0.7157  leucocratic  granodioritic  207  31.2  0.436  0.7042  gneiss with  biotite  139  5.0  0.105  0.7038  ... B i WR Bi  0.7042  Formation  ... Mu  ... PI  509  15,.9  g n e i s s w i t h b i o t i t e and  WR  0. 134  diorite  ... PI  CU2  2.5  = whole r o c k , PI = p l a g i o c l a s e , = b i o t i t e , Mu = m u s c o v i t e  20 .7  231  Hb = h o r n b l e n d e ,  32.4  0.7428  1 40  Table  B-2.  Rb-Sr  isochron  dates  Rock U n i t  n  Sr 8 6 8 7  Init.  S r  Baird Metadiorite Cogburn Creek  Date X10" 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  Group  1) a l l d a t a  less Bi,chert,  HL125 P I 2) c h e r t HL37a WR-Bi 3) m e t a v o l c . Settler  pair  HL125  Schist  1) a l l WR  data  (incl.  PRB)  0.70429 + 32  210 + 27  2) p h y l l i t e SS82  3  0.70643 + 8  66 + 1 .6  3) s c h i s t  3  0.70664 + 18  39  3  0.70388 + 21  74 + 10  2) F e l s i c dyke SS85 l e s s B i  3  0.70369 ±  16  3) F e l s i c d y k e , S S 8 5  4  0.70381  ±  15  4  0.70403 ± 9  77 + 3  14  0.70368 ± 4  127 + 41  15  0.70370 ± 4  86 + 4  Sills  SS128  4  in Schists  1) f e l s i c  Foliated  sill  HL111  g r a n o d i o r i t e , SD92  Spuzzum D i o r i t e , 1) a l l d a t a  Hut C r e e k  less Bi  Spuzzum D i o r i t e ,  Settler  105 + 20 80 + 6 <  body  2) a l l d a t a  a)  16  Creek b 274 + 179  3  0.70372 ±  8  0.70382 ± 7  167 + 46  3) WR-Pl-Hb, a l l d a t a  12  0.70368 ± 6  285 + 97  4) a l l d a t a  13  0.70388 ± 4  90 + 8  1) WR,  a l l less  SD97  2) P l - H b , a l l d a t a  17  b) 1) SD97 WR-Pl-Hb  3  0.70364 ±  12  2) SD97 a l l d a t a  4  0.70379 ±  10  186 + 49 89  + 7  141  Agmatite,  SD14  WR-P1 Breakenridge  Pm.  gneiss  3  0.70478 ± 34  42 ± 14  2  0.70448 ± 13  182 ± 33  7  0.70369 ± 6  79  ± 1.6  I s o c h r o n d a t e s c a l c u l a t e d u s i n g l e a s t s q u a r e s f i t t i n g methods o f York (1967). Programme w r i t t e n f o r HP-85 m i c r o c o m p u t e r by R.L. Armstrong. E r r o r i n S r / S r i s 0.0001; Rb/Sr r a t i o a s s i g n e d 2% e r r o r when Rb and S r c o n c e n t r a t i o n s o v e r 50 ppm, o t h e r w i s e p r o p o r t i o n a l e r r o r o f r a t i o d i v i d e d by l o w e s t ppm. The d a t e 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 u s i n g 2% e r r o r i n Rb/Sr b e c a u s e t h e York l e a s t s q u a r e s c a l c u l a t i o n would n o t c o n v e r g e w i t h proportional errors assigned. The s l o p e o b t a i n e d i s c l o s e t o 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 c o n s t a n t ( S t e i g e r and J a g e r 1977): X = 1.42 x 10- y r " E r r o r s are iff. 8 7  11  /  1  8 6  142  Table  B-3.  Sample Number SD66 Hb  K-Ar a n a l y t i c a l  Rock Type  %K  hb-hyp g a b b r o ,  Hut  Creek  Hut C r e e k  Creek  ratio  Creek  0.470  0.310  37.5  162 ± 7  0.462  2.468  1.472  1.49  80.6  80.9 ± 3  0.391  1.388  0.929  1.20  68.8  77.5 ± 3  0.206  0.754  0.742  0.822  60.6  100 ± 3  1.025  0.869  1.16  ( F i g u r e 5.5a)  s l o p e 70.1 ± 5 Ma initial Body  initial  isochrons  3 6  392 ± 25  "°Ar/ Ar 3 S  388 ± 88  ( F i g u r e 5.5b)  1) Hut C r e e k body  s l o p e 65.7 ± 5 Ma  n=3  initial Creek  «°Ar/ Ar  s l o p e 78.9 ± 2 Ma  n=2  Concentration  66.5  body  n=3  2) S e t t l e r  0.178  isochron dates  Creek  3  0.046  0.308  1) Hut C r e e k body  2) S e t t l e r  s  Date Ma + a.  body  quartz d i o r i t e , Settler  Ar(rad.) nl/g %~~tot , 0  body  hb g a b b r o , Settler  Ar Ar x10  a o 3 6  body  SD117 Hb hb-hyp g a b b r o ,  Isotope  K Ar X10  , 0  Diorite  body  SD103 Hb q u a r t z d i o r i t e ,  SD97 Hb  Spuzzum  36  Hut C r e e k  SD71 Hb  data,  Body  *°Ar  0.185 ± 0.016 n l / g  s l o p e 79.7 ± Ma  n=2  initial  , 0  A r 0.139 ± 0.010 n l / g  Hb=hornblende, H y p = h y p e r s t h e n e , rad.=radiogenic Decay c o n s t a n t s ( S t e i g e r a n d J a g e r 1 9 7 7 ) : X 0 = 4.96 x 1 0 " y r " 1 0  1  X e = 0.581 x 1 0 " y r " *°K/K = 0.01167 atom.% E r r o r s a r e 1a. 1 0  1  94.5 ± 4  143  Table  B-4  Sample  weights for  Sample Wt  No  spike mg  U-Pb  analyses  Wt d i s s o l v e d zircon mg  Pb b l a n k % tot.Pb  Obs, 2 0 6] TUTj  Baird  MD  HL111  184  0.20  13  60  22  0.10  4  135  Sett  1  13.3  1 .2  1 .4  950  Sett  2  12  1.5  2.0  330  Qtz  Di  HC  26.2  0.010  27  Qtz  Di  SC  21 . 1  9.2  6  40 1300 3560  CU2/1  55.2  9.0  2.5  400  CU2/2  28.5  2.5  4  160  Solution one  containing  o f w h i c h was  z i r c o n was  spiked.  split  into  2 aliquots,  1  Table B-5,  U-Pb a n a l y t i c a l data i n c l u d i n g i s o t o p e r a t i o s  Sample No  U ppm  Pb ppm  PB r PBc + r  T-nrjj  20 7 p 2 35u  B a i r d MD  356.8  29.3  0.52  0.04258  0.3691  0.06287  29.9  0.53  0.04253  0.5956  0.10157  228.2  0.83  0.01729  0.1290  0.05411  227.3  0.84  0.01741  0.1230  0.05123  53.4  0.95  0.04418  0.4820  0.07913  53.1  0.97  0.04436  0.4877  0.07973  27.5  0.88  0.04635  0.5262  0.08233  27.6  0.88  0.04637  0.5403  0.08450  24.5  0.40  0.02865  0.2573  0.06512  24.7  0.39  0.02816  0.3166  0.08154  945.6  13.3  0.99  0.01398  0. 1170  0.06071  944.5  13.4  1 .03  0.01481  0.1031  0.05048  14.0  0.96  0.01453  0. 1022  0.05103  3.9  0.94  0.01654  0. 1273  0.05583  4.0  0.89  0.01645  0.1114  0.04911  4.0  0.89  0.01649  0. 1148  0.05046  5.0  0.73  0.01641  0.1173  0.05187  4.9  0.73  0.01643  0.1119  0.04937  HL1 1 1  Sett 1  Sett 2  Qtz Di HC  Qtz Di SC  CU 2/1  CU 2/2  5189  1117  493.5  326.4  218.6  218.6  20 6  Pb  b  20 7 20 6  Pb Pb  c=common Pb, r=radiogenic Pb. I s o t o p i c abundance of common Pb based on 200 Ma Pb d e r i v e d from the growth curve of Stacey and Kramers (1975), except f o r B a i r d MD and S e t t l e r S c h i s t samples. 2000 Ma Pb was more a p p r o p r i a t e f o r B a i r d M e t a d i o r i t e , and 1000 Ma Pb f o r S e t t l e r S c h i s t . C a l c u l a t e d U-Pb ages are l i s t e d i n Table B-6. Rock and z i r c o n d e s c r i p t i o n s and sample l o c a t i o n s are given i n Appendix C.  1 46  Table B-6.  C a l c u l a t e d 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  20 6  B a i r d MD  269 ± 10  400 ± 34  1251 ± 180  Sett  279 ± 2  401 ± 3  1183 ± 12  Sett 2  292 ± 2  434 ± 7  1279 ± 32  HL111  111 ± 1.5  121 ± 13  315 ± 250  1  p  b  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 c o n s t a n t s ( S t e i g e r and Jager 1977): X = 1 . 551 25 x 1 0 " y r X = 9.8485 x 1 0 " y r ' X = 4.9475 x 1 0" y r " 2 3 8U/2 3 5TJ = 7. E r r o r s (1a) a r e d e r i v e d from spike c a l i b r a t i o n and f r a c t i o n a t i o n u n c e r t a i n t y and mass spectrometer within-run isotope r a t i o measurement u n c e r t a i n t i e s . 238  10  2 3 5  1  1 0  232  11  1 3  8 8  1  1  1 47  Appendix  C.  T a b l e C-1.  Sample Number Baird  Isotopic  Dating:  Rock d e s c r i p t i o n s  Rock  Sample  and sample  Description  descriptions  and l o c a t i o n s ,  locations  Location  Latitude N Longitude E  S e t t l e r Lake W s h o r e f l o a t on beach  49° 31.4' 121° 37.6'  49° 33.4' 121° 43.2'  Metadiorite  MD1,3, 6,8,9  h i g h l y f o l i a t e d , green and w h i t e , metamorphosed d i o r i t e and gabbro  Cogburn  Creek  HL37a  r e c r y s t . ribbon chert, micaceous p a r t i n g s  Cogburn C r e e k above gorge  HL76  c o a r s e knobbly c h l o r i t i c metavolcanic greenschist  Cogburn C r e e k r o a d , c u t 49° 32.85' b e f o r e 1st t r i b u t a r y 121° 44.9*  HL97  grey micaceous  1510', S s i d e s p u r betw. 49° 32.45' T a l c S. Cogburn Cks 121° 43.75'  HL103  grey, p h y l l i t e  750', S s i d e s p u r betw. 49° 32.65' T a l c & Cogburn Cks 121° 44.3'  HL125  f i n e grained b i o t i t e a c t i n o l i t e greenschist  On W s l o p e above upper fork i n 3-mile Creek  49° 34.6' 121° 44.5'  Settler  Schist  Sett  1  graphitic  phyllite  Roadend, S f o r k C r e e k , a t 2500'  49° 31.9' 121 ° 35.15*  Sett  2  graphitic  phyllite  S f o r k Cogburn C r e e k , bridge over N t r i b . , W s i d e of v a l l e y  49° 33.65' 121° 36.4*  SS82  graphitic  phyllite  2995' S s i d e 2nd t r i b W s i d e S f o r k Cogburn  49° 32.6' 121 ° 35.8'  SS109  graphitic  phyilite  4150' on r i d g e S e t t l e r Creek  E of  49° 32.85' 121° 38.9'  SS114  graphitic  phyllite  2300', N o f t r i b , W s i d e S e t t l e r Creek  49° 32.75' 121 ° 39.9'  SS128  q t z - b i - g a r s c h i s t , near margin o f Cogburn Granodiorite  In N f o r k Cogburn Ck, 0.5km up from j u n c t i o n  49° 34.9' 121° 37. 1 '  SSI 30  graphitic phyllite-black slate  H a l f w a y up s t r e a m from Cogburn L a k e , S f o r k Cogburn Creek  49° 31.4' 121° 35.72'  H a l f w a y up s t r e a m from Cogburn Lake, S f o r k  49° 31.4* 121° 36.0'  SS1 32  black  Group  slate  quartzite  road, cut  Cogburn  1 48 SS1 35  graphitic  SS143  coarse schist  Sills HL111  phyllite  sandy  biotite  in Schists 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  SS85  Foliated SD92  foliated felsic dyke i n S e t t l e r granodiorite,  gar c l u s t e r Schist  49° 31 . 5' 121° 35. 3'  5500', N end r i d g e N of O l d S e t t l e r  49° 32.,35' 121° 37.,6'  Roadcut a t 1000', N s i d e Cogburn C r e e k  49° 33.,5' 121 ° 42..55'  W s i d e S f o r k Cogburn Ck, N of stm, a t 3700' from Cogburn Lake  49° 31 ,.95' 121 35..7'  s m a l l body i n i m b r i c a t e  medium g r a i n e d g r a n o d i o r i t e w i t h b i , mu, g a r n e t s  Spuzzum D i o r i t e ,  2500' i n s t r e a m from Cogburn Lake  Hut Creek  0  zone  S i d e s t m W of Creek  Settler  49° 32,.2' 121 ° 40,.4'  body  SD36  fine grained leucocratic hb q u a r t z d i o r i t e  S s i d e Cogburn Ck, on W r d t o S e t t l e r Lake  49° 33 .45' 121 ° 4-1 .5'  SD66  f i n e grained hornblendeh y p e r s t h e n e gabbro  1750' above r o a d on N s i d e Cogburn C r e e k  49° 33 .95' 121 ° 39 .55'  SD67  f i n e grained hornblendeh y p e r s t h e n e gabbro  Rdcut a t 1800' Cogburn Creek  49° 34 .0' 121° 38 .8'  SD1 01  fine quartz  3600' on W r i d g e above Hut Ck, N of Cogburn Ck  SD103  coarse quartz hb+bi  SD1 10  fine quartz hb+bi  diorite diorite  diorite  N  side  4 9 ° 34 .45' 121 ° 42 .25'  49° 34 .5' On r i d g e c r e s t between 3-mi and Hut Cks, 4500' 121° 43 .2' In r i v e r b e d , Cogburn Ck  49° 34 .45' 121 37 .15'  f o r k s of  0  49° 34 .5' 1850' on N s i d e Cogburn Ck, a t f o r k s 121 ° 38 .25'  SD1 17  f i n e g r a i n e d hornblende gabbro  At of  SD1 19  f i n e grained hornblendeh y p e r s t h e n e gabbro  W o f SD117, a t 1850' •  49° 34 .6* 121 ° 38 .2'  2100' on N s l o p e Cogburn Creek  above  49° 33 .93' 121 ° 42 .4'  3800', W s l o p e s above S b r a n c h Cogburn C r e e k  49° 32 .15' 121 36 .2'  E side S e t t l e r Creek, r o a d c u t a t 2700'  49° 31 .85' 121 ° 39 .3' 49° 32 .5' 121° 39 .3'  QtzDiHC q u a r t z  diorite  Spuzzum D i o r i t e , SD71  coarse  Settler  C r e e k Body  gabbro  SD96  hb-quartz  SD97  fine leucocratic quartz d i o r i t e , hb+bi+ g a r  E side S e t t l e r Creek, r o a d c u t a t 2450'  SD98  coarse  E side S e t t l e r Creek, r o a d c u t a t 2500'  QtzDiSC  diorite  quartz'"diorite  Composite  sample o f SD71,96,98  0  .  49° 32 .5' 121 ° 39 .3'  149  Aqmatised q u a r t z SD14  diorite  fine foliated bi,garnets  Breakenridge  quartz  dior ite  2000' b e s i d e N b r i d g e W s i d e S f o r k Cogburn Creek  49° 33.45' 121° 36.5'  Formation  CU 1  leucocratic granodioritic g n e i s s w i t h b i and mu  Big S i l v e r River, r d c u t i n 1st g o r g e  49° 38' 121° 49.5'  CU  leucocratic gneiss with  Big cut  49° 38' 121° 49.5'  2  granodioritic biotite  Description  Baird  MD  Sett 1  of z i r c o n  Clear, colourless, euhedral  grains.  colourless.  <325 mesh. and  Fairly  Similar  325  to Sett  Clear  r o u n d e d , a n d many c o n t a i n  Most p r o b a b l y  detrital. mostly  Size kyanite  1, b u t a few a b o u t  l o n g and t h i n  and c l e a r .  mesh, <5% c o n t a m i n a t i o n .  Very  200 mesh  size.  contamination.  have opaque D i NB  in size.  Up t o 20% c o n t a m i n a t i o n ,  Zircons mostly  Qtz  contamination.  graphite.  5-10%  HL1 1 1  broken p i e c e s and complete 5%  opaque i n c l u s i o n s .  2  samples.  Z i r c o n s a r e a l l s m a l l and s i m i l a r and  Sett  Silver River, r d i n 1st g o r g e  grains  inclusions.  s m a l l sample,  fragments.  Some l a r g e r  S i z e 200-  210 h a n d p i c k e d  C l e a r and c o l o u r l e s s .  z i r c o n s and S i z e 200-325  mesh. Qtz  D i SC  Zircons clear, prisms,  colourless,  200-325 mesh.  amber c o l o u r e d m i n e r a l CU  2  Zircons pink, inclusions,  euhedral.  About  7% c o n t a m i n a t i o n , an  (rutile?).  200-325 mesh.  C l e a r , e u h e d r a l , no  <5% c o n t a m i n a t i o n .  Most a r e s t u b b y .  Most a r e l o n g  Some b r o k e n g r a i n s .  150  Appendix D  Figure  D-1  Map of Cogburn. Creek area showing l o c a t i o n s of samples s t u d i e d i n t h i n s e c t i o n . See F i g u r e 2.1 f o r key to rock u n i t s .  151  Figure  D-2  Map showing l o c a t i o n s of geochronometry samples, Cogburn Creek a r e a .  1 52  Appendix E. Table E-1.  A n a l y t i c a l data f o r C h i l l i w a c k  Rb-Sr a n a l y t i c a l data  Sample Number  Sr ppm  ChilliWR  batholith  main phase d i o r i t e  PI  Rb BS 87  Sr  Sr at B 7  S r  372  33. 8  0.263  0.7040  473  4. 7  0.029  0.7039  10. 3  1 .03  0.7048  29. 0 -  Rb ppm  14. 3  232  47.2  0.7262 0.7278  JV216 WR  Bi  sample from J . Vance  309  U n i v e r s i t y of Washington,  31 .5  Seattle  29. 2  57. 3  0.537  0.7042  51 .1  4.70  0.7058  130  12.8  0.7098 0.7097  WR = whole rock, P i = p l a g i o c l a s e , Hb = hornblende, Bi = b i o t i t e  153  1  CHILLIWACK  F i g u r e E-1.  1  •  7  1  r  BATHOLITH  Rb-Sr i s o c h r o n p l o t f o r C h i l l i w a c k b a t h o l i t h , North Cascades Mountains, Washington.  

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