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K[40] - Ar[40] isotopic age determination of the Nelson batholith, B.C. Nguyeh, Kim-Khanh 1968

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K40 _ A r  4 0  ISOTOPIC AGE DETERMINATION  OF THE NELSON BATHOLITH,  B.C.  by  NGUYEN KIM-KHANH B.A.Sc. , U n i v e r s i t e L a v a l ,  1965  A T H E S I S SUBMITTED I N PARTIAL FULFILLMENT THE REQUIREMENTS FOR THE DEGREE OF MASTER OF A P P L I E D SCIENCE i n the Department of GEOPHYSICS  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g to the required standard  THE U N I V E R S I T Y OF B R I T I S H COLUMBIA April,  1968  OF  In presenting this thesis  in p a r t i a l fulfilment of the requirements  for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make it freely available Study. thesis  I further agree that permission for extensive  copying of this  for scholarly purposes may be granted by the Head of my  Department or by h.ils representatives. or publication of this thesis  Department The University of B r i t i s h Columbia Vancouver 8, Canada A l ^ .  f  It is understood that  copying  for financial gain shall not be allowed  without my written permission.  Date  for reference and  ii ABSTRACT The purposes o f t h i s t h e s i s are t o examine ages of the Nelson b a t h o l i t h and i t s s a t e l l i t e s because of the wide range of b i o t i t e model ages p r e v i o u s l y p u b l i s h e d i n the l i t e r a t u r e , and t o t e s t hornblende and pyroxene f u r t h e r f o r t h e i r tion i n K K4  0  4 0  - Ar  - Ar  4 0  4 0  applica-  d a t i n g as r e l i a b l e m i n e r a l s .  model ages obtained on b i o t i t e , hornblende  and  c l i n o p y r o x e n e from the p r e s e n t study i n d i c a t e w i t h c e r t a i n t y t h a t the v a r i o u s phases of the Nelson b a t h o l i t h were  emplaced  d u r i n g a s h o r t span o f time, c e n t e r e d around 156 m.y.  corres-  ponding t o the Upper-Middle J u r a s s i c boundary o f the K u l p s T  time s c a l e .  At l e a s t  a p e r i o d o f hydrothermal a l t e r a t i o n  o c c u r r e d i n t h i s a r e a s i n c e t h a t time.  I t i s evident  has  that  these phases cannot be d i s t i n g u i s h e d on the b a s i s of t h e i r K4-0 _ Ar  4 0  model ages.  I t i s a l s o e v i d e n t t h a t hornblende g i v e s r e -  l i a b l e model ages.  Clinopyroxene c o n t a i n s s i g n i f i c a n t  of r a d i o g e n i c argon and should not be used f o r K B i o t i t e may particularly  4  0  excess  - Ar40 d a t i n g .  o c c a s i o n a l l y c o n t a i n excess of r a d i o g e n i c argon, under h i g h temperature and h i g h argon p r e s s u r e  environmental c o n d i t i o n s . Analytical  t e c h n i q u e s used and d e s c r i p t i o n s of the  analyzed are g i v e n as appendixes.  samples  iii TABLE OF CONTENTS PAGE ABSTRACT ?  i i  TABLE OF CONTENTS  i i i  LIST OF FIGURES  v  LIST OF TABLES  vd  ACKNOWLEDGEMENTS  v i i  CHAPTER I . &40 _ A r  4 0  METHOD OF AGE DETERMINATION  1.1  Introduction  1.2  The K tion  4 0  - Ar  1 1  4 0  Method  o f Age Determina-  3  1.2.1  Introduction  3  1.2.2  K  3  4 0  - AT  4 0  Model Age  1.2.2.1  Decay Constants  6  1.2.2.2  E x c e s s o f Radiogenic Argon  8  1.2.2.3  Argon l o s s hy D i f f u s i o n  1.2.3  M a t e r i a l s Used f o r K  4 0  11 - Ar  4 0  Dating  CHAPTER I I . GENERAL GEOLOGY OF THE NELSON BATHOLITH ... 2.1  17 20  Introduction  20  211.1  History  20  2.1.2  Composite Nature  20  D i f f e r e n t Phases o f t h e Nelson B a t h o l i t h .  22  2.2.1  Nelson Rocks  22  2.2.2  Nelson S a t e l l i t e s & Rocks  24  2.2.3  V a l h a l l a Rocks  25  2.2.4  Lamprophyre Dykes  25  2.2  of L i t e r a t u r e  iv PAGE 2.3 2.4 2.5  O r i g i n Due t o M e t a s o m a t i s m o f P a l e z o i c and E a r l y M e s o z o i c  .....  Age o f M e t a s o m a t i s m a n d I n t r u s i o n b y S t r a t i g r a p h y and S t r u c t u r e ;...  27 28 30  Post-Crystallization History PHYSICAL DETERMINATION  32  3.1  P r e v i o u s Work:  32  3.2  Current Project  34  3.2.1  Introduction  34  3.2.2  The K40 - A r 4 0 R  3.2.3  A n a l y t i c a l P r e c i s i o n and A c c u r a c y  43  Discussion  31  CHAPTER I I I .  3.3  e  s  u  its  37  ,  CONCLUSION  61  BIBLIOGRAPHY  63  APPENDIX I - EXPERIMENTAL PROCEDURE  70  APPENDIX I I - DESCRIPTIONS OF SPECIMENS FOR K 0 _ A r DATING 4  4 0  84  V  L I S T OF FIGURES PAGE FIGURE  1-1  D e c a y Scheme o f K  FIGURE  1-2  Schematic P l o t of L o g  FIGURE  1-3  4  4 0  D V s . 1 ....  a  14  T  2  V a r i a t i o n o f Apparent Ages as a F u n c t i o n o f Distance from an Intrusive Contact  17 21  FIGURE  2-1  L o c a t i o n Map o f K 0 _ A r  FIGURE  2-2  S e t t i n g o f the N e l s o n B a t h o l i t h and Some S a t e l l i t e s  23  FIGURE  2-3  P r e v i o u s Age D e t e r m i n a t i o n s h y F i e l d Geology  26  FIGURE  3-1  4  4 0  Samples.  U.B.C. A r g o n E x t r a c t i o n a n d 36  A n a l y t i c a l System FIGURE  3-2  Plot of Log A r  FIGURE  3-3  Plot  4  0  V s . K I s o c h r o n s ..  o f Frequency Vs. L o g Apparent 39  Ages FIGURE  3-4  38  Typical  A t m o s p h e r i c A r g o n Run  48  FIGURE  A - I - l F l o w - s h e e t o f M i n e r a l S e p e r a t i o n ..  71  FIGURE  A-I-2 Fractionation  82  Correction r  vi LIST OF TABLES PAGE TABLE  3.1 K  TABLE  3 . 2 U.B.C. RESULTS ON STANDARD MINERALS  44  TABLE  3 . 3 INTERLABORATORY RESULTS  43  TABLE  3 . 4 AGES OF VARIOUS ROCK UNITS  49  TABLE  3 . 5 RELIABLE MODEL AGES  50  4 0  - Ar  4 0  ANALYTICAL DATA  40  vii ACKNOWLEDGEMENTS The w r i t e r wishes t o express h i s s i n c e r e thanks t o Dr. T . J . U l r y c h who s u p e r v i s e d t h e r e s e a r c h and whose g u i d ance and enthusiasm has c o n t r i b u t e d immeasurably  to this  study. The w r i t e r a l s o wishes to thank Dr. W.F. Slawson f o r h i s s u p e r v i s i o n d u r i n g the absence o f Dr. U l r y c h and f o r h i s h e l p i n f o r m u l a t i n g the present  thesis.  P a r t i c u l a r thanks are due t o D r s . A . J . S i n c l a i r and W.G. Libby,  of t h e Department  o f Geology, f o r s u p p l y i n g t h e samples  and f o r t h e i r many h e l p f u l suggestions,  and t o Mr. J.E. H a r a k a l  for h i s valuable assistance. A t e a c h i n g a s s i s t a n t s h i p o f f e r e d t o t h e w r i t e r by the Department  o f P h y s i c s was s i n c e r e l y a p p r e c i a t e d .  S i n c e r e a p p r e c i a t i o n i s due t o P r o f e s s o r J.A. Jacobs who made i t p o s s i b l e f o r t h e w r i t e r t o come t o the U n i v e r s i t y o f B r i t i s h Columbia.  The w r i t e r a l s o wishes t o express h i s deep  g r a t i t u d e t o the a u t h o r i t i e s of both governments and Canada f o r the r e a l i z a t i o n o f t h i s  thesis.  o f Vietnam  1 CHAPTER I K 1.1.  4 0  - Ar  4 0  METHOD OF AGE DETERMINATION'  Introduetion Isotope geophysics i s one of the main f i e l d s of research  i n the Departments of Geophysics  and Geology of the University  of B r i t i s h Columbia. In p a r a l l e l with the extensive research i n the v a r i a t i o n of lead isotopes and the isotopic composition of common strontium, the potassium-argon studies are currently under progress. The potasslum-argon laboratory was constructed early i n 1963  and became operational l a t e i n 1964.  The dual objectives  are to investigate further the techniques of the potassiumargon method and t o help unravel the history of complex geol o g i c areas. Today, more than 200 samples have been analyzed i n the laboratory.  The ages obtained range from l e s s than 10 m i l l i o n  years to greater than 3,000 m i l l i o n years. The main i n t e r e s t of the present writer has been t o apply these f a c i l i t i e s to solve the geologic h i s t o r y of a ^complex area. The Nelson Batholith, a massive g r m i t e pluton between Kootenay and Slocan Lakes, i s i n ao uthern B r i t i s h Columbia. It i s exposed over an area of approximately 900 square miles.  2  The major p o r t i o n o f the Nelson B a t h o l i t h l i e s w i t h i n the a r e a covered by the' G e o l o g i c a l Survey o f Canada, map 1090A on a s c a l e o f f o u r m i l e s t o the inch  ( L i t t l e , I960), or by the  G e o l o g i c a l Survey o f Canada, map 272A on a s c a l e o f one m i l e t© the i n c h (a)  (Cairnes,  1934).  The purposes o f the present study a r e : To apply t h e f a c i l i t i e s p r o v i d e d i n t h e potassium-  argon l a b o r a t o r y f o r d a t i n g b i o t i t e s , hornblendes,  pyroxene  and oogenetic m i n e r a l s . To t e s t hornblende  and pyroxene  a p p l i c a t i o n i n potassium-argon  further f o r their  age d e t e r m i n a t i o n as r e l i a b l e  minerals. To determine the potassium-argon  ages of the Nelson  B a t h o l i t h with the aim of o b t a i n i n g more i n f o r m a t i o n . on geologic  history. (b)  The Nelson B a t h o l i t h was s e l e c t e d f o r i n v e s t i g a t i o n  because: Lead i s o t o p e s s t u d i e s o f the a r e a have been i n v e s t i gated r e c e n t l y by P. Reynolds In  (1967).  addition, - . S i n c l a i r  (1964), Kanasewich  (1962),  Leech and Wanless (1962) i n v e s t i g a t e d l e a d i s o t o p e s on t h e areas around Nelson, and: The a r e a c o n t a i n s e c o n o m i c a l l y important ore d e p o s i t s .  3  The  s t r a t i g r a p h y and s t r u c t u r e o f t h e a r e a i s r e a -  sonably well-known ( L i t t l e , Current f i e l d  I960).  works have been c a r r i e d out hy members o f  Geology Department  ( S i n c l a i r & L i b b y , 1967).  Potassium-argon age determinations  c a r r i e d out by the  G e o l o g i c a l Survey o f Canada have been made o n l y on b i o t i t e s . Moreover, t h e p u b l i s h e d d a t a by G a b r i e l s e and Reesor  (1964) exhibit  a b r o a d r a n g e between 49 t o 171 m.y.. 1.2  The K  1.2.1  4 0  - Ar  4 0  method o f age d e t e r m i n a t i o n .  Introduction I s o t o p i c age d e t e r m i n a t i o n by means o f r a d i o a c t i v e decays  was f i r s t  suggested  by R u t h e r f o r d i n 1904.  He p o s t u l a t e d t h a t  the r a t i o of uranium t o h e l i u m i n a m i n e r a l should be time dependent.  Subsequent i n v e s t i g a t i o n s by R u t h e r f o r d and others  (Thompson, 1905$ Campbell and Wood,;i906), i n d i c a t e d t h a t helium i s u s u a l l y p a r t i a l l y l o s t by the m i n e r a l making the time of "geological clock" unreliable. Thompson (1905) and Campbell and Wood(1906), f i r s t the r a d i o a c t i v e decay o f potassium t o c a l c i u m .  proved  I t was n o t un-  t i l 1937 when Von Weizsactter d i s c o v e r e d the branching decay o f K40 t o A r  4 0  .  T h i s p r o v i d e d a promising p o s s i b i l i t y o f two  "geological clocks" within a single mineral.  *  m.y.: m i l l i o n  years  4  P o t a s s i u m c o n s i s t s of t h r e e n a t u r a l l y o c c u r r i n g w i t h the mass numbers 3 9 , 40 and 41. radioactive.  K41 a 6.91 ± 0.04;  ( N i e r , 1950;  N u c l e a r D a t a T a b l e s , 1959).  Ar38 -  Only p o t a s s i u m 40 i s  The p e r c e n t atomic abundances of t h e i s o t o p e s o f  p o t a s s i u m a r e : K 3 9 = 93.08 i 0.04;  argon was  K0 4  -. 0.01181 ± 0.0001;  G o l d i c h e t a l 1 9 6 l ; A.E.0. v  The i s o t o p i c e o m p s o s i t i o n of a i r  a l s o determined by N i e r (1950):  0 . 0 6 3 % ; Ar40 =  Ar?6  s  0.337%;  99.600%.  P o t a s s i u m decays, by b o t h b e t a e m i s s i o n and e l e c t r o n c a p t u r e , t o c a l c i u m 40 and argon 40 (Fig.  isotopes  orbital  respectively,  1.1).  F i g . 1.1  :  Decay scheme o f K40  (after Aldrich & Wetherill,  1938).  The e q u a t i o n s f o r t h e decay can be w r i t t e n as f o l l o w s : K40 i K40  e° —>• »  Ar C a  -+• If  4 0  40  +  +  X-ray or Auger  electron  I t i s known t h a t a p p r o x i m a t e l y  89 pereent  o f p o t a s s i u m 40 decays  t o c a l c i u m 40 hy b e t a e m i s s i o n b u t t h e K40 - C a o f l i m i t e d use.  This/sdue  4 0  method has been  to the f a c t t h a t calcium i s a very  common c o n s t i t u e n t i n g e o l o g i c a l m a t e r i a l s and cjaloium forms 97 p e r e e n t  40  of common c a l c i u m , t h e r e f o r e t h e s m a l l amount o f  radiogenic calcium  40 i s v e r y d i f f i c u l t t o d i s t i n g u i s h from  t h e l a r g e r amount o f common c a l c i u m .  Moreover, P o l e v a y a e t a l , ,  ( I 9 6 0 ) , mentioned t h a t t h e K  method c a n o n l y be used  for  4 0  - Ca  4 0  r o u t i n e age d e t e r m i n a t i o n i f t h e f o l l o w i n g problems a r e  s a t i s f a c t o r i l y s o l v e d : l ) f i n d a s u i t a b l e method o f s e p a r a t i o n for  s m a l l amount o f Ca f r o m K ; 2 ) t h e Ca-44 i n n a t u r a l c a l c i u m  must be p r e c i s e l y determined;3) a "•powerful" mass s p e c t r o m e t r i c technique  f o r t h e d e t e r m i n a t i o n of t r a c e Ca.  T h i s i s not t h e case i n t h e  Ar^method,where t h e anount  o f common a r g o n c o n t a i n e d i n m i n e r a l s and r o o k s i s v e r y l o w and t h e measurement o f a r g o n , w h i c h i s one o f t h e r a r e g a s e s , i s much e a s i e r . Ar  4 0  Broad time ranges a r e measurable w i t h t h e K40 -  method because p o t a s s i u m  i s a common element i n g e o l o g i c a l  m a t e r i a l s and t h e p o t a s s i u m - a r g o n h a l f - l i f e i s v e r y long1.2.2  K 0 - Ar 4  4 0  The K40 - A r developed  *  •-•  model age 4 0  method o f i s o t o p i c age d e t e r m i n a t i o n has been  d u r i n g t h e p a s t f i f t e e n y e a r s t o a h i g h degree o f  sophistication.  W i t h improved a n a l y t i c a l t e c h n i q u e s , g e o l o g i c a l  m a t e r i a l s o f v e r y young age and l o w p o t a s s i u m  c o n t e n t c a n now  be d a t e d w i t h p r e c i s e and r e l i a b l e r e s u l t s ( G a l e e t a L , 1 9 6 6 ; Curtis, 196&).  6  T h i s makes t h e K*® - A r modern ge©chronology.  method a w i d e l y used t o o l i n  4 u  However, the a c c u r a c y i n a n a l y t i c a l  provi d  t e c h n i q u e s s u p p l y i n g a p r e c i s e and a c c u r a t e d e t e r m i n a t i o n o f the AT40/K40 r a t i o , i s o n l y a n e c e s s a r y but not condition.  The K40 - Ar40 " a g e s  sufficient  are m e a n i n g f u l i f and o n l y  0  i f , the f o l l o w i n g c o n d i t i o n s are s a t i s f i e d : l ) t h e decay cons t a n t s must be a c c u r a t e l y known;2)the sample must not c o n t a i n e x c e s s r a d i o g e n i c argon - 40, i . e . no r a d i o g e n i c argon i n c l u d e d i n t h e sample a t t h e time i t was  formed;3)the  was sample  must have been a c l o s e d system, i . e . no l o s s or g a i n o f p o t a s s i u m or r a d i o g e n i c argon has o c c u r r e d s i n c e i t was o t h e r t h a n by r a d i o a c t i v e decay of p o t a s s i u m  formed,  40.  The c o n s i s t e n c y o f r e s u l t s w i l l be a key p o i n t t o check whether o r n o t t h e s e assumptions 1.2.2.1  are f u l f i l l e d .  Decay c o n s t a n t s  As mentioned above, K4-0 ± e l e c t r o n c a p t u r e t o Ar^O  3  r a d i o a c t i v e and decays by K -  and by b e t a e m i s s i o n t© Ca40.  d e t e r m i n a t i o n of t h e s e decay c o n s t a n t s i s d i f f i c u l t .  The However,  a knowledge o f t h e s e i s a b s o l u t e l y n e c e s s a r y t o p r e d i c t the a c c u m u l a t i o n of r a d i o g e n i c argon i n a p o t a s h m i n e r a l .  Many  workers have t r i e d w i t h a l l a v a i l a b l e means t o determine  these  decay c o n s t a n t s and t h e b r a n c h i n g r a t i o , p a r t i c u l a r l y d u r i n g the p e r i o d between 1950 and I 9 6 0 .  E x t e n s i v e r e v i e w i n g o f the  a v a i l a b l e d a t a were g i v e n by Smith  (1964) and Houtermans (1966).  Two methods have been used f o r t h i s purpose: c o u n t i n g method; and the g e o l o g i c a l method.  the p h y s i c a l or  7  Physical  determination  T h i s method i s based on d i r e c t c o u n t i n g measurements. As no s u i t a b l e R e m i t t i n g  standards are a v a i l a b l e , t h e  measurement o f the e m i s s i o n o f 1 . 4 6 MeV quite  difficult.  radiation i s  However, i n making use of Co^O and N a  standards and by e x t r a p o l a t i o n  technique W e t h e r i l l  a r r i v e d t o determine t h e s p e c i f i c  gamma a c t i v i t y .  of 3 . 3 9 - 0 . 1 2 S/g sec. which corresponds capture decay constant  Ae  2 4  (19 5 7 ) , A value  to a K - electron  o f O.585 x 1 0 - 1 0 y r - 1 has been  obtained. Due  t o the maximum o f the h i g h b e t a energy of 1 . 3 4 MeV,  the d e t e r m i n a t i o n o f t h e b e t a decay c o n s t a n t i s l e s s c u l t than the p r e v i o u s case. 27.6  ^/g s e c ,  of 4 . 7 2 x 1 0 (1954),  A specific  yr-  1  has been o b t a i n e d .  A l d r i c h and W e t h e r i l l  constant A^  I n d t and K l u y v e r  (1958 )v' reviewed t h e a v a i l -  a b l e p h y s i c a l measurements and suggested 10-  b e t a a c t i v i t y of  corresponding t o a beta-decay 1 0  diffi-  v a l u e s o f O.585 x  y r - 1 f o r p a r t i a l e l e c t r o n capture decay constant " V "  1 0  and 4 . 7 2 x 1 0 - 1 0  y  r  - l  f  0  r  , the p a r t i a l b e t a decay  con-  stant. I t should mention t h a t a new d e t e r m i n a t i o n o f t h e s p e c i f i c b e t a a c t i v i t y h a s been made by Glendenin By u s i n g v l i q u i d s c i n t i l l a t o r c o u n t i n g t e c h n i q u e , obtained a value o f 28.2 ± 0 . 3 f/g  sec.  Glendenin  However, most  l a b o r a t o r i e s have continued t o adopt t h e v a l u e s by A l d r i c h and W e t h e r i l l .  (1961).  suggested  8 Geological  determination  By determining the p r o d u c t i o n o f r a d i o g e n i c A r m i n e r a l s of known age, the g e o l o g i c a l method may satisfactory  v a l u e s f o r the decay c o n s t a n t s .  The  v a l u e s obtained by t h i s method areinquite i^a good  in  4 - 0  yield best agree-  ment w i t h p h y s i c a l c o u n t i n g determinations ( W e t h e r i l l e t al.,  1956).  The  work o f W e t h e r i l l e t a l . . (1956) y i e l d e d  a s p e c i f i c gamma a c t i v i t y of 3.24 s p e c i f i c b e t a a c t i v i t y o f 27.4 to  a b e t a decay constant  ± 0.15  f/g sec, of 4 . 7 0  K - e l e c t r o n capture decay constant 10-10  yr-1.  Vg  a  corresponding  x 10-10 \  s e c . and  yr.-l,  and  of 0 . 5 5 7 - 0.026 x  E a r l i e r i n v e s t i g a t i o n s using g e o l o g i c a l  methods i n c l u d e d the work o f R u s s e l l ^ t l 9 5 3 ) , Wasserburg and Hayden (1954), S h i l l i b e e r e t a l . (1954).  However, t h e i r  d e t e r m i n a t i o n s y i e l d e d a too low branching r a t i o . reason f o r t h i s was  found out l a t e r t o be due  The  to the i n -  complete e x t r a c t i o n of argon or the c l o s e d system c o n d i t i o n was 1.2.2.2  not s a t i s f i e d . Excess  o f r a d i o g e n i c argon some  It  i s known t h a t i n v n a t u r a l environments; p a r t i c u l a r l y  at  great depth w i t h i n the c r u s t or where h i g h argon p r e s s u r e might develop, excess r a d i o g e n i c argon i s p r e s e n t .  M i n e r a l phases  which c r y s t a l l i z e i n these environments are more or l e s s j e c t t o t h i s argon The  sub-  "contamination".  excess of r a d i o g e n i c argon,  occluded by the m i n e r a l s  at the time o f c r y s t a l l i z a t i o n i s p r o b a b l y h e l d i n the  crystal  i m p e r f e c t i o n s such a s d i s l o c a t i o n s o r s t r u c t u r a l h o l e s .  The  e x c e s s o f r a d i o g e n i c argon i n pyroxene has been o b s e r v e d by many i n v e s t i g a t o r s . high ages n  bolite.  n  H a r t and Dodd (1962) found s u r p r i s i n g l y  f o r pyroxenes from a pyroxene g n e i s s and an amphi-  They c o n c l u d e d t h a t t h e s e pyroxenes have i n c o r p o r a t e d  e x c e s s argon e i t h e r d u r i n g t h e i r i n i t i a l c r y s t a l l i z a t i o n o r d u r i n g a subsequent r e c r y s t a l l i z a t i o n .  ^ c D o u g a l l and Green  (1964) dating pyroxenes from the Norwegian e c l o g i t e s found a l s o the o c c u r r e n c e o f e x c e s s argon i n t h e s e s a m p l e s . A S i m i l a r r e s u l t h a s been r e p o r t e d by A l l s o p p (1965) f o r pyroxene from t h e g r e a t dyke o f S o u t h e r n R h o d e s i a .  However, M c D o u g a l l (1963), and  S t e r n e t a l . (1965) r e p o r t e d t h a t pyroxene from s h a l l o w l y i n truded b a s i c igneous rooks y i e l d e d s a t i s f a c t o r y dates. complete d e g a s s i n g might be e x p e c t e d i n l o w p r e s s u r e  More  environments  so t h a t t h e s e o b s e r v a t i o n s a r e n o t i n c o n f l i c t w i t h t h e o c c u r r ence o f e x c e s s o f r a d i o g e n i c argon i n pyroxene, from deep-seated environments.  Damon and K u l p (1958) observed t h e e x c e s s o f  argon i n b e r y l and c o r d i e r i t e .  They suggested f u r t h e r t h a t an  e x c e s s o f r a d i o g e n i c argon i n amphiboles i s p o s s i b l e .  Funkhouser  e t a l . (1965) r e p o r t e d a l s o e x c e s s argon i n hornblende from young H a w a i i a n r o c k s .  The e x c e s s of r a d i o g e n i c argon i n f l u i d  i n c l u s i o n s occurring g e n e r a l l y i n most m i n e r a l s , except p o s s i b l y m i c a s , h a s been c o n c l u d e d by Rama e t a l . (1965). However, E v e r n d e n and R i c h a r d s  (1962), H a r t ( i 9 6 0 , 1961)  and G e r l i n g e t a l (1965) d i d n o t prove any e v i d e n c e o f e x c e s s argon i n a m p h i b o l e s .  They c o n c l u d e d t h a t amphiboles may g i v e  10  r e l i a b l e K*0 - A r  "ages".  4 0  I n p r a i s i n g amphibole as a r e l i a b l e m i n e r a l f o r K Ar et  4 0  d a t i n g , H a r t (1964), A l d r i o h o t a l 4 i 9 W  4 0  -  and G e r l i n g  a l (1965) showed t h a t amphiboles a r e g e n e r a l l y more r e -  s i s t a n t t o argon l o s s , t h r o u g h d i f f u s i o n t h a n m i c a s because the  former have a h i g h e r r a d i o g e n i c a r g o n d i f f u s i o n t h r e s h o l d  temperature  (fig.l-3).  The most u n f o r t u n a t e i s t h e p o s s i b i l i t y o f e x c e s s o f r a d i o g e n i c a r g o n i n h o r n b l e n d e and m i c a , t h e two most commonly used f o i " K  4 0  - Ar  d a t i n g (Hunt,1962; Leech e t a l . ,  4 0  1963).  T h i s r a i s e s t h e q u e s t i o n whether, i n c e r t a i n c a s e s , t h e h i g h e r argon r e t e n t i v i t y i n h o r n b l e n d e r e p r e s e n t s i n f a c t a compensat i n g e f f e c t between t h e e x c e s s and l o s s o f argon.  I n general,  one f a v o u r s t h e i d e a t h a t m i c a s do not c o n t a i n e x c e s s of r a d i o g e n i c argon.  However, g e o l o g i c a l and e x p e r i m e n t a l e v i d e n c e s  show t h a t i n some oases t h e y do ( S t o c k w e l l , 1 9 6 3 ; P e p i n e t a l . , 1964).  R i c h a r d s and Pi«geon (1963) r e p o r t e d t h a t some b i o t i t e  samples o f Broken H i l l d i s t r i c t , A u s t r a l i a , gave ages which were n e a r l y t w i c e t h e R l ^ S r ^ a g e s o f t h e same phase.  Excess radiogenic  argon i n p h l o g o p i t e and b i o t i t e from young H a w a i i a n r o c k s h a s r e p o r t e d by Funkhouser e t a l . pie  been  (1965).  e r r o r i n K ^ - A r ^ d a t i n g , caused by t h e e x c e s s a r g o n , 4  4  w i l l be v e r y s e r i o u s i f l o w p o t a s s i u m c o n t e n t m i n e r a l s such a s p y r o x e n e , q u a r t z , f l u o r i t e e t c . a r e used f o r d a t i n g . The s i t u a t i o n i s s t i l l worse i f t h e s e m i n e r a l s c o n t a i n i n a d d i t i o n , f l u i d i n c l u s i o n s o r vacancy d e f e c t s .  11 P o s s i b l e occurrence  of e x c e s s of r a d i o g e n i c argon i n  d a t a b l e m i n e r a l s suggest t h a t t h e i n t e r p r e t a t i o n o f K Ar  4 0  4 0  -  d a t a must be done w i t h as much c a u t i o n as p o s s i b l e .  U s u a l l y , the presence  of e x c e s s h e l i u m may  be used as a good  i n d i c a t i o n of*>excess r a d i o g e n i c a r g o n (Damon and Green, 1963). The most r e l i a b l e r e s u l t s are b e l i e v e d t o be o b t a i n e d when t h e s t a b i l i t y o f the A r  4 0  rati© i n m i n e r a l s p o s s e s s i n g d i f f e r e n t  K40"  potassium  content i s assured.  T h i s suggests the use o f oogene-  t i c m i n e r a l s , f o r example; b i o t i t e - hornblende  pairs, to test  the r e l i a b i l i t y and t h e u s e f u l n e s s of t h e d a t i n g m a t e r i a l s . The  use o f o t h e r r a d i o m e t r i c methods, f o r example; the R b ^  o r l e a d - uranium methods w o u l d be a n o t h e r  Sr^  alternative.  I n c a s e s where o o g e n e t i c m i n e r a l s are not a v a i l a b l e , the r e l i a b i l i t y must be a s s e s s e d from i n t e r n a l c o n s i s t e n c y between the r e s u l t s and the known geology. 1.2.2.3 Due  Argon l o s s by d i f f u s i o n t o i t s g r e a t importance  i n the £.40 - A r  4 0  method, the  problem o f d i f f u s i o n o f r a d i o g e n i c a r g o n w i l l be d i s c u s s e d  ±o  a g r e a t e r e x t e n t i n t h e f o l l o w i n g p a r t of t h i s c h a p t e r * One  knows, t h a t t h e argon l o s s i s . dependent on the m i n e r a l  and r o c k t y p e .  While m i c a s are the most s u i t a b l e m i n e r a l s f o r  d a t i n g , f e l d s p a r s commonly show argon l o s s , F o l i n s b e e e t a l . . (1936); Zartman, (1964).  D a t e s o b t a i n e d from whole r o c k samples  i n w h i c h an a p p r e c i a b l e amount o f p o t a s s i u m i s i n f e l d s p a r s are therefore suspect.  E r i c k s o n and K u l p  (1961) a p p l i e d the whole  12 rock technique  t o the b a s i c r o c k s from Palisades s i l l  t h a t the argon r e t e n t i v i t y i s much b e t t e r i n the the c o a r s e r g r a i n e d  and showed  f i n e r than  facies.  In g e n e r a l , the d i s c o r d a n c e  in K  4 0  - Ar  4 0  i s o t o p i c "ages"  can most o f t e n and most e a s i l y be e x p l a i n e d by daughter diffusion. Institute,  in  product  D i f f u s i o n i s d e f i n e d by the American G e o l o g i c a l "A process  of spreading  out of molecules,  atoms or  i o n s i n t o a vacuum, a f l u i d or a porous medium, i n a d i r e c t i o n tending to equalize concentrations  i n a l l p a r t s of a system".  The problem of argon leakage by d i f f u s i o n i s of such importance  in K  4 0  - Ar  4 0  both experimental been p u b l i s h e d .  d a t i n g t h a t numerous papers d e a l i n g with  and t h e o r e t i c a l aspect o f the s u b j e c t have Argon d i f f u s i o n from m i n e r a l s has been s t u d i e d  under l a b o r a t o r y c o n d i t i o n s by G e r l i n g and Morozova (1958, 1 9 6 2 ) ; Evernden e t a l . ( i 9 6 0 ) ; Hart Brandt al.  (1962) and  ( 1 9 6 2 ) ; Hart  ( i 9 6 0 ) ; Baadgaard e t a l . ( 1 9 6 1 ) ;  under g e o l o g i c f i e l d  c o n d i t i o n s by H u r l e y et  (1964) and Hanson and Gast ( 1 9 6 7 ) .  Recent r e -  views of argon d i f f u s i o n have been made by F e c h t i g and K a l b i t z e r (1966).  Fundamental Laws of D i f f u s i o n The  f i r s t and  sedond laws p o s t u l a t e d by A d o l f F i c k , are  g i v e n as f o l l o w s : 1.  The  d i f f u s i o n f l u x across a g i v e n plane  t i o n a l t o the  i s propor-  c o n c e n t r a t i o n g r a d i e n t across t h a t  plane:  13 where J i s t h e d i f f u s i o n f l u x , D i s t h e d i f f u s i o n c o n s t a n t and V c 2.  The d i v e r g e n c e  i s the gradient concentration.  of the d i f f u s i o n f l u x i s equal t o  the change o f c o n c e n t r a t i o n w i t h t i m e : ^£  -  _  d;v. o r  (ovc)  c/fv.  -  The d i f f u s i o n c o n s t a n t D i s temperature  dependent- as  shown by t h e A r r h e n i u s r e l a t i o n :  D = D eQ  E / R T  where E i s t h e a c t i v a t i o n energy, D  i s the charac-  0  t e r i s t i c c o n s t a n t , R i s t h e gas c o n s t a n t and T i s t h e absolute  temperature.  The e x p e r i m e n t a l d e t e r m i n a t i o n o f t h e a c t i v a t i o n energy i s based on t h e k i n e t i c s s t u d y o f argon r e l e a s e d from m i n e r a l s a t d i f f e r e n t temperatures.  With an i n i t i a l homogeneous c o n c e n t r a -  t i o n o f argon and z e r o c o n c e n t r a t i o n o f argon a t t h e s u r f a c e , the p r o c e s s o f gas release;, from m i n e r a l s may be g i v e n i n t h e following equations  p - 1 -  (Crank, 1956; J o s t ,  £r  I960):  (-k*  )  where F i s t h e f r a c t i o n of t o t a l argon r e l e a s e d a t any t i m e t . T h i s has been o b t a i n e d from s o l u t i o n o f F i c k s l a w f o r a f  spherical  g r a i n o f r a d i u s a.  Reichenberg  (1953), by u s i n g a  F o u r i e r i n t e g r a l t r a n s f o r m , o b t a i n e d an a p p r o x i m a t i o n f o r 0 <F <0.85, as f o l l o w s :  F = A  (Dt  )i _  3  J3t  14 I n t h e case o f an i n f i n i t e  s l a b ; the f r a c t i o n F i s g i v e n as  follows: where h i s t h e h a l f - t h i c k n e s s o f t h e s l a b . The  f r a c t i o n of t o t a l argon released at a given  f o r a g i v e n t i m e i n t e r v a l i s commonly t h e d a t a  temperature  received  from  d i f f u s i o n measurements. In g e n e r a l , argon i n the m i n e r a l occupies  several  different  s t r u c t u r a l p o s i t i o n s w i t h a d i f f e r e n t a c t i v a t i o n energy a s s o c i a t e d w i t h each p o s i t i o n .  T h i s a r i s e s as t h e f u n c t i o n l o g D  v e r s u s JL i s g e n e r a l l y n o n - l i n e a r f o r t h e e n t i r e t e m p e r a t u r e T r a n g e b e t w e e n 2 5°C a n d 1,000°C. I t i s often observed t h a t the general  result i s several straight line  segments ( H a r t ,  I960;  G e r l i n g a n d M o r o z o v a , 1962;aGerling'.©.fea3,i$^3 i c o r r e s p o n d i n g i  different  a c t i v a t i o n energies, as presented  f i g u r e 1-2.  Figure  1-2:  schematically i n  ^  Schematic p l o t o f l o g D versus a 2  to  1. T  15  In  g e n e r a l , good p r e s e r v a t i o n o f t h e m i n e r a l i t s e l f i s  c o n s i d e r e d as a s u f f i c i e n t  c o n d i t i o n o f complete  radiogenic argon.  The  s e r v e as c r i t e r i o n  of the r e t e n t i o n of r a d i o g e n i c argon i n  minerals.  Sardarov  a c t i v a t i o n energy  retention  (19&3) c a r r i e d  o f m i n e r a l s can  also  out an e x p e r i m e n t a l i n v e s -  t i g a t i o n on t h e r e t e n t i v i t y o f r a d i o g e n i c a r g o n .  He  concluded  t h a t argon i s q u a n t i t a t i v e l y p r e s e r v e d i n the g l a u c o n i t e s to  400-oc and  the escape  o f a r g o n i s due  to structural  t i o n o f the m i n e r a l a t t h i s temperature,With an energy  of E  - 60 K c a l / m o l e g l a u c o n i t e  p l e t e l y at a lower The et are  characteristic  the  a r g o n i n m i c a s was  one  activation  (1958, 1962),  o f a r g o n i n m u s c o v i t e , and t h e y f o u n d  60 -  held i n three types of s i t e s :  However, the work o f Brandt  i n five different  sites,  ent a c t i v a t i o n e n e r g i e s ; the f i r s t  a few p e r -  t h e a r g o n and t h e f i f t h  remainder  (1962) i n d i c a t e d  only  G e r l i n g and M o r o z o v a  held i n microcline-  corresponding t o five t h r e e phases  differ-  accounted f o r  20 p e r c e n t o f t h e a r g o n , t h e f o u r t h c o n t a i n e d a b o u t  65 p e r c e n t .  that  70 p e r c e n t i n t h e s e c o n d a n d t h e  (1962) o b s e r v e d a l s o t h a t t h e a r g o n was  of  Gerling  activation energies  s i n g l e a c t i v a t i o n energy f o r muscovite.  perthite  decomposi-  temperature.  a l . (1963) showed t h a t t h r e e d i f f e r e n t  i n the t h i r d .  up  s h o u l d r e t a i n a r g o n com-  w o r k s o f G e r l i n g and M o r o z o v a  cent i n the f i r s t ,  of  contained the remainder  15  percent  of the  argon,  ^  A m i r k h a n o v e t a l . (1959) h a s made some a r g o n  diffusion  16  m e a s u r e m e n t s on v a r i o u s p o t a s h m i n e r a l s and o b s e r v e d t h r e e d i f f e r e n t a c t i v a t i o n e n e r g i e s i n e a c h o f two f e l d s p a r F e c h t i g e t a l . (1961) i n v e s t i g a t e d t h e a r g o n f r o m a number  o f m i n e r a l s and r o c k s .  samples.  diffusion  They f o u n d t h a t t h e argon  d i f f u s i o n f r o m p h o n o l i t e , a t t e m p e r a t u r e s up t o 600°C d i d n o t show a s t r a i g h t l i n e  on t h e l o g D v e r s u s 1 p l o t . a T  However, i n  2  t h e c a s e o f a u g i t e , two s t r a i g h t l i n e It  s e g m e n t s were o b t a i n e d .  i s found t h a t the r e s u l t s of d i f f u s i o n  measurements  made b y v a r i o u s i n v e s t i g a t o r s on t h e same m i n e r a l s b u t d i f f e r e n t samples v a r y s i g n i f i c a n t l y .  Many a u t h o r s h a v e c o r r e l a t e d  this  v a r i a t i o n t o s t r u c t u r a l d i s l o c a t i o n s or l a t t i c e changes i n minerals  (Baadsgaard e t a l . ,  1961).  I n a n y c a s e , most o f t h e a b o v e a u t h o r s p r o v e d t h a t m i c a s and h o r n b l e n d e s a r e s u i t a b l e f o r K Hurley et a l . ,  (1962),  Hart.  (1964)  4 0  essentially  - Ar " 4  0  and H a n s o n and G a s t  showed t h a t t h e a p p a r e n t a g e s o b t a i n e d b y K  40  - Ar  4 0  dating. (1967)  method o r  Rb-'sr^method a r e a f u n c t i o n o f d i s t a n c e f r o m a c o n t a c t z o n e a s shown i n f i g u r e 1 - 3 . o v e r b i o t i t e may  The h i g h e r r e t e n t i v i t y o f h o r n b l e n d e  a l s o be s e e n f r o m t h i s  diagram.  Other p o s s i b l e l o s s o f r a d i o g e n i c argon has been accounted to chemical a l t e r a t i o n •1-9&3 and G q i l i ' c h ' e t a l . ,  ( K u l p and E n g e l § 1 9 6 3 ;  1966),  )  Ma^visr e t a l . ,  and t h e r m a l e f f e c t due t o  mechanical abrasion (Gerling et a l . ,  196l).  17  1400  a  fl  o  •r-i r-i  -P fl CD U  Age of o intrusion 1  3^  10  1000  10000  D i s t a n c e from contact  (meters)  •  1 (°C-1) T V a r i a t i o n of apparent ages of b i o t i t e , hornblende and f e l d s p a r as a f u n c t i o n of d i s t a n c e from an i n t r u s i v e contact ( A f t e r H a r t , 1964).  F i g u r e 1-3 i  1.2.3  M a t e r i a l s Used f o r K40  _ Ar40 Dating  Depending on the type of m a t e r i a l s analyzed, c r y s t a l l i z a t i o n metamorphic events i n some cases and l o c a l i n t r u s i o n or o r i g i n of rock f o r m a t i o n i n o t h e r s , may K  4 0  - Ar The  4 0  be events recorded by  method.  l a b o r a t o r y g e o c h r o n o l o g i s t must t h e r e f o r e be  s t a n t l y aware of the s u f f i c i e n t  c o n d i t i o n s f o r the K  model age, p a r t i c u l a r l y the second and t h i r d  4 0  con- Ar  conditions.  4 0  18  In g e n e r a l , micas, sanidine, g l a u c o n i t e are the most w i d e l y used m i n e r a l s f o r  d a t i n g because of t h e i r l a r g e  range of occurrence, h i g h potassium retentivity.  content and h i g h argon  However, w i t h improved a n a l y t i c a l  techniques,  g e o l o g i c a l m a t e r i a l s other t h a n micas can a l s o be used. igneous metamorphic and  sedimentary  r o c k s have been used  All as  geological materials for dating. The K  - Ar  4 0  4 0  "age"  o b t a i n e d on m i n e r a l s of an  igneous  rock g i v e s the time of emplacement, p r o v i d e d t h a t argon  has  not been l o s t by d i f f u s i o n p r o c e s s and the r o c k has not been s u b j e c t e d t o a l a t e r metamorphism ( s u f f i c i e n t  conditions).  S i m i l a r l y , the age y i e l d e d from m i n e r a l s of a metamorphic rock w i l l i n d i c a t e the time of the metamorphic event as p r e v i o u s l y accumulated  argon i s completely l o s t by t h i s metamorphic event.  An e x c e l l e n t d e s c r i p t i o n o f d a t i n g o f sediments by K ° 4  Ar  4 0  method has been made by H u r l e y  (1963).  The  -  (1966) and Hower e t a l .  e s s e n t i a l t h i n g s a r e , they must r e t a i n w e l l  r a d i o g e n i c argon and must not c o n t a i n fragments of d e t r i t a l minerals.  The reason f o r t h i s i s based  fragments may  contain r e l i c t  on the f a c t t h a t these  argon.  U s i n g f o s s i l bones, l i m e s t o n e s and f l u o r i t e as d a t i n g material for K  40  - A r ^ method, an acceptable t o t a l 4<  fossil  bone age has been obtained and r e p o r t e d by L i p p o l t and Gentner  11963).  19  It  should be noted t h a t , from t h e i r t h r e e dated  contained about 50 ppm a c c e p t a b l e date.  The  b a b l y the s u f f i c i e n t  o f potassium; other two  o n l y one  sample y i e l d e d an  gave h i g h ages i m p l y i n g pro-  c o n d i t i o n (2) was  not f u l f i l l e d .  f o r e , i n order t o o b t a i n r e l i a b l e r e s u l t s , o f samples i s o f primary  fluorites,  importance.  careful  There-  selection  Weathered samples should  be d i s c a r d e d , f r e s h m a t e r i a l s are h i g h l y d e s i r a b l e f o r K Ar  4 0  dating.  4 0  _  20  CHAPTER I I GENERAL GEOLOGY OF THE NELSON BATHOLITH 2.1  Introduction  2.1.1  ^  History of Literature A f t e r seventy y e a r s o f m i n i n g a c t i v i t y the f i r s t  report  geologic  of the West Kootenay D i s t r i c t , B r i t i s h Columbia was  published The  by G.M. Dawson (1889). stimulation of m e t a l l i c mineral deposits p a r t i c u l a r l y  g o l d and s i l v e r , spurred Nelson r e g i o n .  others t o l o o k e n q u i r i n g l y at the  F i e l d i n v e s t i g a t i o n s and g e o l o g i c a l  studies  have t h e r e f o r e been c a r r i e d out by p r o s p e c t o r s and members o f the G e o l o g i c a l  Survey o f Canada, (Cairnes, 1934; M u l l i g a n , 1952  etc.). Most o f the p r e v i o u s works, as w e l l as h i s own a r e summari z e d by H.W.  Little,  (i960).  Recent g e o l o g i c a l i n v e s t i g a t i o n  of the n o r t h e r n p a r t o f the Nelson b a t h o l i t h has been c a r r i e d out by members o f t h e Geology Department 1967).  ( S i n c l a i r and L i b b y ,  Much o f t h e g e o l o g i c a l i n f o r m a t i o n s  i n the present  study has been based mainly on the above-mentioned authors' work;. , p a r t i c u l a r l y on L i t t l e ' s memoir. 2.1.2  Composite Nature  Hamilton and Myers (19670, i n t h e i r paper e n t i t l e d "The Nature o f B a t h o l i t h s " i n d i c a t e a composite petrography i s characteristic of batholiths.  The Nelson b a t h o l i t h can be  21  Figure 2 - l  :  L o c a t i o n Map o f K-Ar"samples of Nelson B a t h o l i t h and R e l a t e d Rocks New Denver - K a s l o , B.C. ,Base Map, H.W.  L i t t l e , Map 1090A, Nelson Quad («)  22 r e a s o n a b l y d e s c r i b e d i n terms o f f o u r phases o f i n t r u s i o n . These a r e , N e l s o n r o c k s , N e l s o n s a t e l l i t e s i n c l u d i n g t h e Mount G a r l y l e s t o c k , ( S i n c l a i r a n d L i b b y , 1 9 6 7 ) , rocks  ( L i t t l e , I960) and Lamprophyre d y k e s t h a t c u t t h e N e l s o n  batholith. terms 2.2  E a c h o f t h e s e may be f u r t h e r s u b d i v i d e d e i t h e r i n  of f i e l d  or laboratory  criteria.  D i f f e r e n t Phases o f t h e Nelson  2.2.1  main p a r t of the area o f the present study i s u n d e r l a i n  by Nelson r o c k s . and M y e r s ,  Compared w i t h many o t h e r b a t h o l i t h s ,  (Hamilton  1967), the Nelson b a t h o l i t h i s i n t h e main g r a n i t e  r a t h e r than f a l l i n g i n t h e broad t h e S i e r r a Nevada and o t h e r s .  to  Batholith  Nelson Rocks The  the  Valhalla  "granitic"  c l a s s i f i c a t i o n of  Toward t h e c o n t a c t b o r d e r s w i t h  country r o c k s the content o f ferromagnesiam  m i n e r a l s tends  increase. I n many s e c t i o n s t h e N e l s o n r o c k s may be d e s c r i b e d a s b e -  l o n g i n g t o e i t h e r a p r o p h y r i t i c o r a n o n - p o r p h y r i t i c member. The  p o r p h y r i t i c member i s c h a r a c t e r i z e d b y c o n s i d e r a b l y l a r g e  c r y s t a l s of sodic orthoclase. p h e n o c r y s t s may c o n t r i b u t e range  i n s i z e van  average  I n a few l o c a l i t i e s ,  these  up t o 5© p e r c e n t o f t h e r o c k a n d  o f about two i n c h e s .  I n areas ex-  h i b i t i n g deformation, these are r e p l a c e d by m i c r o c l i n e .  The  g r o u n d m a s s o f t h e p o r p h y r i t i c member i s c h a r a c t e r i z e d b y a n abundance o f h o r n b l e n d e  over t h e b i o t i t e .  Partial  chloritic  a l t e r a t i o n i s common i n t h e s e f e r r o m a g n e s i a n m i n e r a l s .  23  LEGEND L  Late Palaeozoic and Early Tertiary intrusions Nelson batholith and tosatellites Middle Mesozoic *1 \  Early Palaeozoic District boundary.... i  Precambrian  S c a l e of M i l e s  G.S.  Figure  20  C  o  l_  2-2:  S e t t i n g of- the Nelson B a t h o l i t h and some s a t e l l i t e s (After L i t t l e ,  196p0  24 The p o r p h y r i t i c member i s cutby.or sometimes grades i n t o t h e n o n - p o r p h y r i t i c member.  y  ^  The l a t t e r t e n d s t o be g r a n o d i o r i t e  as t h e a c i d i c p h e n o c r y s t s a r e m i s s i n g and t h e o t h e r m i n e r a l c o n s t i t u e n t s are t h e same as i n t h e p o r p h y r i t i c member.  The  n p n - p o r p h y r i t i c member o f t h e N e l s o n b a t h o l i t h i s c o n s i d e r e d to  be s l i g h t l y younger t h a n t h e p o r p h y r i t i c member.  This i s  based on t h e f a c t t h a t t h e former c r o s s c u t s , i n p l a c e s , t h e latter. 2.2.2  N e l s o n S a t e l l i t e s Rocks  The N e l s o n s a t e l l i t e s comprise apophyses and s t o c k s o f t h e N e l s o n b a t h o l i t h , i n c l u d i n g t h e Mount C a r l y l e s t o c k .  The S l o c a n  s e r i e s south o f t h e Mount C a r l y l e s t o c k forms a septum s e v e r a l hundred f e e t wide s e p a r a t i n g t h e s t o c k f r o m t h e n o r t h e r n edges of t h e b a t h o l i t h .  ( S i n c l a i r and L i b b y , 1967).  W i t h some excep-  t i o n s , t h e c o m p o s i t i o n o f t h e s a t e l l i t i c r o c k s , on the whole, resembles v e r y much t h a t o f t h e N e l s o n r o c k s . and n o n - p o r p h y r i t i c members a r e p r e s e n t .  Both p o r p h y r i t i c  Most o f t h e s e  satellites  have been o b s e r v e d t o i n t r u d e t h e T r i a s s i c S l o c a n s e r i e s and an i m p o r t a n t number o f them i n t e r s e c t o t h e r f o r m a t i o n s . M u l l i g a n (19 52),  b e l i e v e d t h a t t h e s t o c k s a r e o f e a r l y phases o f t h e  Nelson i n t r u s i o n .  However, C a i r n e s ( 1 9 3 4 ) , suggested t h a t t h e  s a t e l l i t e s represent a l a t e r stage of the i n t r u s i o n o f t h e Nelson batholith,. L i t t l e  (i960),  concluded that these s a t e l l i t e s are  i f not contemporaneous, s l i g h t l y younger t h a n t h e N e l s o n b a t h o l i t h .  25 2.2.3  V a l h a l l a Rocks C h e m i c a l l y and m i n e r a l o g i c a l l y , t h e V a l h a l l a r o c k s are  e s s e n t i a l l y e q u i v a l e n t t o the Nelson r o c k s . are p r e d o m i n a t e l y t e x t u r a l , f i n e r g r a i n e d and  The d i f f e r e n c e s allotriomorphic  i n s t e a d of h y p i d i o m o r p h i c t e x t u r e as i n the case o f t h e rocks.  The  Nelson  c o n t a c t s between the V a l h a l l a r o c k s and t h e N e l s o n  r o c k s are commonly g r a d a t i o n a l but sharp c o n t a c t s o c c u r where the former i n t r u d e s t h e l a t t e r .  The  s i m i l a r i t y between t h e s e  phases l e d C a i r n e s (1934), t o drop t h e name V a l h a l l a , but  Little  (i960), preserves the d i s t i n c t i o n . The V a l h a l l a r o c k s o u t c r o p toward t h e w e s t , b u t a h a t shaped mass o f V a l h a l l a i s exposed over f i v e square m i l e s about the headwaters o f M a u r i e r , F e n n e l l and S i l v e r t o n c r e e k s . r a r i t y o f V a l h a l l a r o c k s i n t h e e a s t e r n S l o c a n r i v e r and  The Slocan  l a k e and "the N e l s o n r o c k s comprise a l a r g e p r o p o r t i o n o f t h i s p a r t o f t h e b a t h o l i t h , p o s s i b l y owing t o m o b i l i z a t i o n of t h e p o r p h y r i t i c m a t e r i a l w h i c h may have f l o w e d out' from t h e a u t o chtonous c o r e , p u s h i n g the e a s t e r n p e r i p h e r a l r o c k s i n t o t h e i r p r e s e n t a r c - l i k e p o s i t i o n " , has been s u g g e s t e d by L i t t l e  (i960).  As the c o n t a c t s between t h e N e l s o n r o c k s and V a l h a l l a r o c k s are most o f t e n i n d i s t i n c t , L i t t l e of t h e s e two 2.2.4  (i960),  assumed  contemporaneity  phases.  Lamprophyre Dykes Dykes have been observed t o c u t t h e N e l s o n p l u t o n i c r o c k s .  rrvvesTl GATORS  Cairn.es  ?  arid FOLlMSBEtJ  AAAA1  Newr-narch,  C  •WWHi  )  180 Figure 2-J>-  1 so  (40  I20  100  .Ages  Previous age determinations by f i e l d geology,  (V)  go  miUron  Nelson  y^ors batholith  27 T h e i r composition, on the whole, resembles v e r y much t h a t o f the Nelson r o c k s .  They are c o n s i d e r e d t o r e p r e s e n t a l a t e  staga i n t h e i n t r u s i o n of t h e Nelson b a t h o l i t h , 2.3  ( C a i r n e s , 1934).  O r i g i n Due t o Metasomatism o f P a l e o z o i c and E a r l y  Mesozoic  From o b s e r v a t i o n s of i t s components, (the p a r a g n e i s s , the Nelson s a t e l l i t e s and V a l h a l l a r o c k s ) and i t s s t r u c t u r e , the o r i g i n o f t h e Nelson b a t h o l i t h may be deduced. The  p a r a g n e i s s exposed i n t h e Nelson b a t h o l i t h core and  «l«swih©r© has been c o r r e l a t e d w i t h t h e . H a l l f o r m a t i o n o f Middle (?) and TJppe-r J u r a s s i c age.  The p a r a g n e i s s , and V a l h a l l a r o c k s  form s u c c e s s i v e l a y e r s w i t h g r a d a t i o n a l borders.  L y i n g above  the p a r a g n e i s s and g r a d i n g l a t e r a l l y i n t o i t are t h e Nelson r o c k s are which, i n t u r n , v o v e r l a i n , and- w i t h g r a d a t i o n a l contact by V a l h a l l a non-porphyritic rocks.  The g r a d a t i o n a l c o n t a c t s between t h e  V a l h a l l a and the Nelson are o f t e n observed.  However i n some  p l a c e s the. dykes o f the V a l h a l l a r o c k s c u t the Nelson, n o r m a l l y without e x h i b i t i n g It  dilation.  i s concluded by L i t t l e  Nelson b a t h o l i t h e x h i b i t s  (I960), t h a t the core o f t h e  evidence  of a metasomatism o r i g i n , i.e.,  the r o c k s were formed by metasomatism o f p r e - e x i s t i n g r o c k s as sedimentary roeks.  Little  •<  such  showed a l s o evidence t o suggest  t h a t the r e g i o n now occupied by t h e Nelson b a t h o l i t h hack been deeply b u r i e d .  Moreover, the autochtonous  b a t h o l i t h i s t y p i c a l o f Read*s deep-seated 1951).  T h i s has been used by L i t t l e  core o f the Nelson batholith,  (Read;  t o ensure h i s h y p o t h e s i s .  28  However, L i t t l e the p e r i p h e r y ,  ( i 9 6 0 ) , p o i n t e d out t h a t t h e rocks about  t i n l i k e those  of the core, have been m o b i l i z e d  and i n j e c t e d i n t o o l d e r r o c k s . 2.4  Age  o f Metasomatism and I n t r u s i o n by S t r a t i g r a p h y and  Structure The  Nelson b a t h o l i t h seems t o be the o l d e s t major batho-  l i t h i c mass formed d u r i n g the Mesozoic orogeny i n the Cairnes  (1934), r e p o r t e d t h a t the emplacement of the  Arc. Nelson  b a t h o l i t h f o l l o w e d i n t e n s e f o l d i n g of t h e Sloe an s e r i e s .  The  orogenic d i s t u r b a n c e s , have caused the f r a c t u r i n g and h r e c c i a t i o n e x h i b i t e d by the Nelson r o c k s .  Based on/the c h a i n . - r e l a t i o n  between the orogenic d i s t r u b a n c e and the m i n e r a l i z a t i o n which i s , i n t u r n , c l o s e l y r e l a t e d i n o r i g i n t o the Nelson r o c k s , the of the Nelson b a t h o l i t h may disturbance  i s known.  be deduoed i f the date of the  T h i s orogenic d i s t u r b a n c e was  age  orogenic  believed to  occur at about the same time i n the S e l k i r k as i n the RockyMountains.  Cairnes  of the Nelson  (1934)-, assigned consequently,  batholith to late  the i n v a s i o n  Cretaceous.  E a r l i e r i n v e s t i g a t o r s ^ b a s e d on the s i m i l a r i t y of the rocks and the Coast Coast  r o c k s , suggested  Range and Nelson  been observed  rocks.  The  suggested  (Middle and Lower J u r a s s i c  i n p l a c e s , the b a t h o l i t h i e rocks i n -  trude sediments of lower Cretaceous i t was  f o r both  Coast Range i n t r u s i v e s have  t o cut J u r a s s i c rocks,  f o s s i l s were observed) and  the same age  Nelson  age.  From those  evidences,  that the Coast p l u t o n i c rocks represent a l o n g  29  p e r i o d of i n t r u s i o n , from Upper J u r a s s i c t o Cretaceous.  Based  on the f o s s i l s of l a t e Upper J u r a s s i c age observed from the b a s a l Kootenay bed and on t h e assumption  that the Kootenay  was  c h i e f l y d e r i v e d from the g e a n t i c l i n e b a t h o l i t h , Newmar.oh (1953), suggested t h e Upper J u r a s s i c €.3 the lower l i m i t time o f placement  o f the Nelson b a t h o l i t h .  em-  However, the e x i s t e n c e of  the f o s s i l s o f Lower Cretaceous age are a l s o observed,  because  of these i n c o n c l u s i v e evidence;, a v a i l a b l e , Newmarch s t a t e d t h a t the Upper J u r a s s i c or Lower Cretaceous as t h e lower l i m i t  time  of emplacement of the Nelson b a t h o l i t h . In  o b s e r v i n g the Nelson r o c k s t o c o n t a i n a heavy m i n e r a l  s u i t e which  i s i d e n t i c a l with that of^the Blairmore formation,  i-gseeus -pebble-s -from -M^Doiigall-Segur- ee-ngloaej'-afce, Be ve ridge and Folinsbee  (1956), proposed t h a t the Nelson rock was  j u s t a f t e r the d e p o s i t i o n o f the b a s a l B l a i r m o r e The  age o f t h i s conglomerate  ceous,  ( A l b i a n ) by B e l l  L i t t l e (i960) of  unroofed  conglomerate.  has been a s s i g n e d t o Lower C r e t a -  (1956).  concluded t h a t "the lower l i m i t o f t h e age  the Nelson p l u t o n i c r o c k s , w i t h some c o n f i d e n c e , i s p o s t  Middle J u r a s s i c " , and,  "the upper l i m i t of the age of the Nelson  rocks can be s e t , w i t h r a t h e r l e s s c o n f i d e n c e , as Cretaceous".  The lower l i m i t was  based  pre-Upper  on the f a c t t h a t the;  Nelson r o c k s c u t sedimentary and v o l c a n i c r o c k s o v e r l y i n g the H a l l f o r m a t i o n whose yongest f o s s i l s are o f e a r l y Middle age.  Jurassic  30  The  upper l i m i t was based on the f a c t t h a t the p l a n t  from the Sophie Mountain  fossils  f o r m a t i o n are p r o b a b l y of Upper Cre-  taceous age and t h e Sophie Mountain pebbles d e r i v e d from N e l s o n .  conglomerate  contains g r a n i t i c  On the other hand, the Sophie  Mountain f o r m a t i o n r e s t s unconformably  upon the Rossland  t i o n and i s cut by the Nelson,  2-2).  (figure  forma-  I n any case, the s u r e s t l i m i t s of the emplacement o f the Nelson b a t h o l i t h and i t s s a t e l l i t e s can be set as f o l l o w i n g : the Nelson r o c k s cut the T r i a s s i c or Lower J u r a s s i c S l o c a n s e r i e s , t h i s i n d i c a t e s t h a t t h e y are younger t h a n the S l o c a n s e r i e s and o f course t h e y must be o l d e r than the m i n e r a l i z a t i o n p e r i o d . 2.5  Post-Crystallization History The Nelson b a t h o l i t h i s c o n s i d e r e d t o be a massive  p l u t o n , however areas o f s t r o n g f o l i a t i o n s may the p l u t o n .  granitic  be observed w i t h i n  A deep break which has been a centre of deformation  and l a t e r i n t r u s i o n , occurs i n the western p a r t o f the Nelson b a t h o l i t h , a l o n g the S l o c a n Lake.  ( L i t t l e , I960).  The e a s t e r n  p a r t of t h e b a t h o l i t h has been s u b j e c t e d t o s i g n i f i c a n t p o s t c r y s t a l l i n e deformation which a f f e c t e d the e n c l o s i n g and metamorphic r o c k s .  sedimentary  The p e r i o d o f d e f o r m a t i o n was marked by  the f i s s u r i n g and s h e a r i n g whose d i r e c t i o n i s c o n t r o l l e d by w e l l developed j o i n t f r a c t u r e s .  As m i n e r a l d e p o s i t s occur, i n g e n e r a l ,  i n these f r a c t u r e s , i t i s b e l i e v e d t h a t the m i n e r a l i z a t i o n took p l a c e a f t e r most o f these f r a c t u r e s o c c u r r e d . In e f f e c t , the age of the Nelson b a t h o l i t h determined g e o l o g i c a l i n v e s t i g a t i o n indicatesasomewhat  complete  by  intrusion  history.  The g e o l o g i c a l r e l a t i o n s h i p s a r e ^ c l e a r c u t as there not  are d i f f e r e n t o p i n i o n s concerning the age of the b a t h o l i t h , as d e s c r i b e d above,  ( f i g u r e 2-3).  The only d e f i n i t e  result  i s that the igneous event of the Lamprophyre dykes which drre considered t o be s l i g h t l y younger or contemporaneous with the main b a t h o l i t h ,  ( L i t t l e , I960).  Even, so, g e o l o g i c a l i n v e s t i -  g a t i o n s c o u l d not determine an "absolute age" of the f o r m a t i o n . The s i t u a t i o n i s r e s o l v a b l e o n l y w i t h i n the l a s t decade w i t h the method o f p h y s i c a l d e t e r m i n a t i o n .  \  32 CHAPTER I I I PHYSICAL DETERMINATION 3.1  P r e v i o u s Works As the "age" of the Nelson B a t h o l i t h could not be s o l v e d  d e f i n i t e l y by g e o l o g i c a l means, some workers have attempted t o s o l v e t h i s complex h i s t o r y b a t h o l i t h by means of p h y s i c a l d e t e r mination. By u s i n g l e a d a l p h a a c t i v i t y r a t i o s on a c c e s s o r y m i n e r a l s z i r c o n s , monazite determineathe  and xenotime,  L a r s o n e t a l . (1934), a r r i v e d t-e  age of r o c k s from the Southern C a l i f o r n i a ,  Sierra  Ova-  Nevada and Idaho b a t h o l i t h s . b a t h o l i t h average 103 m.y..  F i v e age d e t e r m i n a t i o n s onvldaho Beveridge and F o l i n s b e e (1956), i n  u s i n g the r a d i o a c t i v e method have e v a l u a t e d the age of z i r c o n from the g n e i s s i c phase of the Nelson b a t h o l i t h , near Whatshan Lake, B.C..  A date of 10 5 m.y.  resulted.  Based on the good  agreement between t h i s r e s u l t and those from southern b a t h o l i t h s , Beveridge and F o l i n s b e e suggested that the emplacement of the Nelson b a t h o l i t h was i n l a t e lower Cretaceous time. A K  4 0  - Ar  4 0  age d e t e r m i n a t i o n on b i o t i t e s separated from  g r a n o d i o r i t e near the c i t y o f Nelson, y i e l d e d a v a l u e of 86 as r e p o r t e d by L i t t l e  (I960).  However, L i t t l e  s t a t e d that  f i g u r e , which o f course appears t o be t o o s m a l l , may because  m.y. "this  be i n c o r r e c t  of r e c r y s t a l l i z a t i o n of the b i o t i t e by metamorphism l o n g  a f t e r emplacement o f the batholith."-. F o l l o w i n g t h i s , the G e o l o g i c a l Survey o f Canada conducted a s e r i e s o f age d e t e r m i n a t i o n s by K 40  Ar method on b i o t i t e . 4(  The  33  samples have been c o l l e c t e d and interpreted hy Reesor and reported by Lowdon et a l .  (1961, 1963), Leech et a l .  (1963) and  Gabrielse and Reesor (1964). A t o t a l of 10 age determinations have been resulted as follows:  GSC 60-21 (49 m.y.), GSG 60-22 (55 m.y.), GSC  6l-l?  (131  m.y.), GSC 62-26 (165 m.y.), GSC 62-27 (159 m.y.), (JSC 62-28  (171  m.y.), GSC 6 2 - 2 9 (163 m.y.), GSC 62-30  (105 m.y.), GSC  62-32  (63 m.y.),  (171  m.y.), GSC  62-31  The widespread i n ages l e d  Reesor t o propose that the emplacement of Nelson batholith has evolved through a very long time, I t developed i n three main stages  X  cfis+inguishec/ by  aftervthree groups of ages: (a)  The older group (from 159 t o 171 m.y.)  X represents the  emplacement and consolidation of hornblende, b i o t i t e and granodiorite before 171 (b)  m.y.. The intermediate group (from 86 to 131 m.y.)  indicates  a mobilization and reintrusion, due to an intense structural episode occuring about the e a r l i e s t Cretaceous, of a r e l a t i v e l y incompetent  ^  mass. (c)  F i n a l l y , the younger igroup (from 49 t o 63 m.y.)  repre-  sents further emplacement of leucocratic quartz monzonite. Reesor then concluded that i f t h i s hypothesis i s accepted, there i s thus no c o n f l i c t between the age data and stratigraphic s t r u c t u r a l data.  However, the considerable spreading i n ages even-, V aft,  within a very short distance between l o c a l i t i e s , which generally  v  34 assumed t o be o f n e a r l y the same age, appears t o be u n s a t i s f a c torily  justified.  Although,  Reesor has c a r e f u l l y formulated h i s h y p o t h e s i s  w i t h i n t h r e e p o s s i b l e major f a c t o r s t h a t could a f f e c t t h e age measurement.  These f a c t o r s are:  the complex s u c c e s s i o n o f  s t r u c t u r a l events w i t h i n the mesozoic c y c l e of deformation, the a s s o c i a t e d complex e v o l u t i o n o f rocks throughout of the mobile  the development  b e l t d u r i n g the frtesozoic and f i n a l l y the complex  g r a d u a l c o o l i n g of emplaced mass d u r i n g u p l i f t and u p r o o f i n g . One can t h e r e f o r e see t h a t Reesor s views o f the Nelson 1  b a t h o l i t h are somewhat d i f f e r e n t or i t c o u l d be s a i d t h a t the  directly  r e q u i r e d a new d e f i n i t i o n f o r the b a t h o l i t h t o warrant  h i s hypothesis. 3.2 3.2.1  Current P r o j e c t Introduction To t e s t t h e v a l i d i t y o f t h e "Reesor h y p o t h e s i s " o f the e v o l u -  t i o n o f the Nelson b a t h o l i t h , i t i s c o n s i d e r e d necessary t o extend f u r t h e r s t r u c t u r a l , p e t r o l o g i c s t u d i e s as w e l l as t o make a d d i t i o n a l i s o t o p i c age d e t e r m i n a t i o n s .  G e o l o g i c a l study o f the  n o r t h e r n p a r t o f the Nelson b a t h o l i t h has been c a r r i e d out by S i n c l a i r and L i b b y o f the Department o f Geology* who have k i n d l y p r o v i d e d a t o t a l o f e l e v e n specimens f o r the present  study.  Four p l u t o n i c rock u n i t s are c o n s i d e r e d i n t h i s  study:  i*' U n i v e r s i t y  of B r i t i s h - Columbia  35  1.  The  n o r t h e r n p a r t of the Nelson  batholith.  2.  Mont C a r l y l e stock ( S i n c l a i r and L i b b y , 1967).  3.  L i t t l e ' s " V a l h a l l a p l u t o n i c r o c k s " centered on the  headwaters of the Fennel Creek. 4.  Lamprophyre dykes t h a t cut the Nelson  All  samples used f o r K40  . Ar  4 0  were chosen a f t e r c a r e f u l examination mens.  batholith.  i s o t o p i c age  o f t h i n s e c t i o n s of s p e c i -  Sample l o c a t i o n s shown i n f i g u r e 2.1  are l i s t e d i n t h e  Appendix I I w i t h the p e t r o g r a p h i c d e s c r i p t i o n s . procedures  determination  Experimental  used are l i s t e d i n Appendix I .  Three major steps i n v o l v e d i n the K40  . Ar  4 0  isotopic  age  d e t e r m i n a t i o n of the p r e s e n t study a r e : 1.  The  s e p a r a t i o n of the m i n e r a l s from r o c k s .  2.  The  d e t e r m i n a t i o n of potassium  content i n m i n e r a l s  flame photometry (Dean, I960; Cooper, 1963;  Cooper e t a l . ,  by 1966  and Dirom, 1965). 3»  The d e t e r m i n a t i o n o f argon content i n m i n e r a l s by mass  spectrometry  s t a t i c i s o t o p e d i l u t i o n method (Reynolds,  F a r r a r et a l . , 1964)  using a h i g h l y enriched A r ^  argon a n a l y t i c a l system used i s shown i n f i g u r e  1956;  tracer. 3-1•  The  Figure 3-1: UBC Argon E x t r a c t i o n and A n a l y t i c a l System  ELECTROMETER  SAMPLE °  Ho  CRUCIBLE  T i IN S i 0 x..z...z;:^Lzzz:::zzz...z::z: LEAK TO P U M P  n:n^j±f\n:r^ VALVE  1Z3ZLL_Z2  METAL  IZZZZZ  VALVES  SC  '0 PUMPS!  IN  2  TUBE  37 3.2.2  The K*0 The  _ Ar  Results  4 G  "  r e s u l t s o b t a i n e d by K^O  b i o t i t e , hornblende  _ Ar  4 0  N  age d e t e r m i n a t i o n s  on  and pyroxene from the v a r i o u s phases of the  Nelson b a t h o l i t h are presented i n Table 3 . 1 .  Sample numbers  correspond to those g i v e n i n fugure 2 - 1 .  data i s conven-  The  i e n t l y p r e s e n t e d i n a g r a p h i c a l form i n f i g u r e 3 - 2 , The 1%'  l o g A r ^ v e r s u s l o g K. 4  m.y.  a p l o t of  isochron, a straight  c o n t a i n i n g a l l the p o i n t s of the same age, i s shown. accompanied by two o t h e r i s o c h r o n s of 100 m.y. respectively.  lihe  It i s  and 200  m.y.  A l l p l o t t e d p o i n t s l o c a t e d on the upper s i d e of  the S t r a i g h t l i n e are of o l d e r ages and those on t h e lower s i d e are of younger ages.  The  s i g n o f excess r a d i o g e n i c argon  or  argon l o s s of the samples can t h e r e f o r e be checked w i t h t h i s k i n d of g r a p h i c a l p r e s e n t a t i o n .  The d a t a o b t a i n e d by Reesor,  ( G r a b r i e l s e and Reesor, 1964)  are a l s o p l o t t e d i n the same f i g u r e  3^2. A histogram of K ^ 4  - Ar *-* dates i s shown i n f i g u r e 4  where frequency v e r s u s l o g ages are p r e s e n t e d .  3-3,  The p l o t t i n g i n -  t e r v a l f o r each date i s equal to t h e estimated a n a l y t i c a l  error.  E l e v e n age determinations r e p o r t e d by G a b r i e l s e and Reesor (1964) are c o n c u r r e n t l y presented. et a l . 1 9 6 3 ) The  An e r r o r of  has been attached t o each.date  ±  8%  (Lowdon  before p l o t t i n g .  advantage of t h i s k i n d of p r e s e n t a t i o n has been d i s c u s s e d  by Ross  (1966).  Table 3 . 2  g i v e s U.B.C. r e s u l t s obtained on i n t e r l a b o r a t o r y  38  FIGURE 3 . 3  :  A plot  of frequency versus l o g apparent  ages  TABLE 3.1 K  4 0  - Ar  *e = 0.385 x 1 0 - 1 0 y r - l ;  A n a l y t i c a l Data - Nelson B a t h o l i t h  4 0  A  f - 4.72 x 1 0 - l Q y r - ;  4 0  Mineral Analyzed  R-ll Granodiorite  Biotite  5.83 5.76 5.81 5.74 Av=3.78 ± 0 . 0 4  0.86  12.690  0.007268  120+5  Biotite  7.52 7.55 7.51 7.5? Av«7.53± 0.02  0.90  20.163  0.008870  1462:5  0.86  15.087  0.007860  130*5  0.72  16.431  0.008179  135±5  R-13 Granodiorite  Biotite (Chloritic)  6.43 6.33 6.31  Ar T o t a l Ar40  K / K - 0.01181%  Sample Number Rock Type  Q,uartzidiorite  K (%)  1  4 0  Ar xl0- moles/g 4 0  10  Ar K4U  4 0  Apparentage im.y.)  Ml  Av«6.36± 0.05 L-236 Fine g r a i n e d hornfels  Biotite (Chloritic)  : radiogenic argon 4-0  6.63 6.62 6.68 6.66 Av=6.65*0.05  TABLE 3 . 1 a K40 _ A T  4 0  A n a l y t i c a l Data - Nelson B a t h o l i t h  A r 40'  Mineral Analyzed  A-66  Biotite  6.66 6.63 6.58 6.61 Av= 6.621Q.03  0.87  18.194  0.00909?  149*6  Biotite  5.88 5.85 5.83 5.93 Av= 3 . 8 8 ± o . 0 5  0.81  16.327  0.009193  131*6  Biotite  7.12 7.16 7.19 7.18 Av= 7 . 1 6 ± 0 . 0 3  0.80  22.410  O.OIO363  169±6  0.59  3.387  0.009816  I6l£6  Granodiorite  C-66  Hornblende Q u a r t z Monzonite  D-66 Lamprophyre  K-66  Dyke  Porphyritic Hornblende Q u a r t z Monzonite  Ar40  A r 40'  Sample Number Rook Type  T o t a l Ar4U  Hornblende  Av»  1.213 1.208 1.209 1.209 1.210+.005  (xl0-10 oles/gj m  E4TT  Apparantage (m.y.)  TABLE 3 . 1 b K40 _ ^.AQ A n a l y t i c a l Data - Nel son B a t h o l i t h  Sample Number Rock Type  Mineral Analyzed  C-66 Hornblende Hornblende Quartz Monzonite  K (%)  Ar ' T o t a l Ar4U  1.104 1.106 1.099 1.104  O.56  3.342  O.OIOO32  164+6  0.886 0.888 0.884 0.886  0.69  2.442  0.009123  150+5  6.85 6.86 6.87 6.89 6.87*0.03  0.90  19.005  0.009159  150±5  0.71  2.731  0.016207  258+12  0.42  2.902  0.017191  273+16  4 0  Ar xl0-10moles/g 4 0  Ar KW~ 4 0  Apparentage (m.y.)  Av= 1.103^0.005 Hornblende SILV-66 Hornblende Quartz Monzonite  Av= 0.886±0.003 257-66 Biotite Nelson B a t h o l i t h Quartz D i o r d t e Porphyry Av= R-12 Quartz  Clinopyroxene Diorite  0. 0. 0. 0. Av- 0 . ^ 5 8 5 ^ 0 . 0 0 2  4=-  43  standard m i n e r a l s , 1 i n c l u d i n g U.B.C. standard b i o t i t e GD-12. A comparison  o f a n a l y s e s c a r r i e d oat a t U.B.C. i s o t o p e  geophysics l a b o r a t o r y w i t h o t h e r s made i n v a r i o u s l a b o r a t o r i e s on the same m i n e r a l samples (Muscovite standard standards GE-3203 and GE-2060)  r  Pt- 2 0 7 ;  biotite  i s shown i n Table 3 . 3 .  Table 3»4 g i v e s the "ages"  of the v a r i o u s rock u n i t s on the  Nelson b a t h o l i t h . 3.2.3 The  A n a l y t i c a l P r e c i s i o n and Accuracy  interference effect  i s one o f the most  controversial  problems i n t h e accuracy and p r e c i s i o n of the potassium det e r m i n a t i o n by flame photometry.  The i n t e r f e r e n c e e f f e c t s of  elements i n v o l v e d , i n t h e s o l u t i o n s used f o r K40 - A r have been i n v e s t i g a t e d by many a u t h o r s , 1963;  and Cooper e t a l . , 1966).  t e c h n i q u e s have been proposed.  the flame.  dating  (Dean, I960; Cooper,  Various types of b u f f e r i n g The main purpose of the b u f f e r i n g  a c t i o n i s t o make both the standard and t h e sample c l o s e t o t h e same p h y s i c a l ,  4 - 0  solutions  chemical and i o n i c c o n d i t i o n w i t h i n  T h i s i n v o l v e s t h e a d d i t i o n o f the same amount o f an  i n t e r f e r i n g element t o t h e standard s o l u t i o n s as . i s p r e s e n t i n the sample s o l u t i o n s .  Cooper (1963)  indicated that the  i n t e r f e r e n c e o f other i o n s depends c o n s i d e r a b l y on the charact e r i s t i c s of t h e flame photometer type fuel).  Dirom (1965)  b u f f e r i n g technique  used  ( f o r example, burner,  r e p o r t e d t h a t w i t h the instrument and t h e used f o r t h e p r e s e n t study exeept H 2 - S O 4 , otiier  Hian  the i n t e r f e r e n c e e f f e c t s o f the—©the-r elements,Yare  1  Analyzed  by J . H a r a k a l  ff S04 2  negligible.  44 TABLE 3.2 RESULTS OE U.B.G. LABORATORY ON STANDARD MINERALS  Ar^O' T o t a l Ar40  Sample Mineral  GD-12 Biotite  GE-2060 Biotite  x - 5.56 <r s 10.03 (Zaralyses i n quadruplicate ) x « cr = (14 in  6.83 ±0.06 analyses quadruplicate )  Ar40 (xlO"^moles/g)  O.83 0.77 0.83 0.71  0.88 0.54 0.61 0.84  2.081 2.076 2.098 2.124 2.095  x = <r a  to.022  sc.-  2.459 2.430 2.498 2.302 2.472  O-a ±0.034  B-3203 Biotite  P-207 Muscovite  x = 8.58 <r 5 tO.05 (13 a n a l y s e s i n d u p l i c a t e or quadruplicate)  0.72  17.121  0.88 O.85 0.75 0.68 O.83 0.90 0.59 0.82 0.86  1.240 1.252 1.274 1.244 1.225 1.261 1.239 1.244 1.220 1.244  x ~  6"= 10.017  A r i t h m e t i c mean Standard d e v i a t i o n or mean d e v i a t i o n  45  TABLE 3 . 3 COMPARISON OF ANALYSES CARRIED OUT I N U.B.C. WITH OTHERS MADE I N VARIOUS LABORATORIES ON STANDARD MINERALS.  Sample Mineral  Columbia GE-2060 Biotite  x = 6.87 <r • ± 0 . 0 5  x = 2.500 cr = + 0 . O i l  5 = 6.83 <r = ±0.06  x - 2.472 cr = 1 0 . 0 3 4  7.58  Summary (Hurley e t a l , 1962) LanphereancJ Dalrymple  Summary (Lanphere and Dalrymple 1967) UBC (1965-1967)  = cr =  1.12  17.30  1.03  to.06 x = 7.42 <r = i0.02  x 17.33 <r= *0.06 s  1-21  x = 17.121  UBC (1965)  P-207 Muscovite  UBC D i f f e r e n c e i n Ar40* Analyses ( % )  Yale (1963) UBG (1965-1967)  B-3203 Biotite  Ar 40(xL0-9moles/g)  Data Source (Date)  8.58 tO.12  5 = 1.260  x  8.58  x = 1.244  CP-  to.05  <r = ± 0 . 0 1 7  x  <3"= ±0.024  A r i t h m e t i c mean Standard d e v i a t i o n o r mean d e v i a t i o n  I.27  46  J  Moreover, tine i n t e r f e r e n c e e f f e c t of H2SO4 was o n l y s i g n i f i c a n t f o r low potassium  content amphibole and pyroxene  solutions.  For t h i s r e a s o n , the standard s o l u t i o n s used f o r b i o t i t e a n a l y s i s d i d not r e q u i r e any a d d i t i o n o f H2SO4 f o r b u f f e r i n g purpose. In t h e p r e s e n t  study, f o r those samples which resistedcomplete  d i s s o l u t i o n , f u r t h e r a d d i t i o n a l h e a t i n g or n i t r i c to replace p e r c h l o r i c acid.  a c i d was used  T h i s was t o a v o i d the p o s s i b l e  i n t e r f e r e n c e e f f e c t s o f p e r c h l o r i c a c i d which may cause some d e p r e s s i o n o f potassium The potassium  emission.  a n a l y s i s was c a r r i e d out a t l e a s t  r u p l i c a t e f o r each sample. potassium apparent  i n quad-  The obtained average v a l u e s o f  content were then used i n the c a l c u l a t i o n o f t h e age.  The r e s u l t s o b t a i n e d on t h i r t y - f i v e a n a l y s e s i n  d u p l i c a t e or q u a d r u p l i c a t e o f the L.G.O. b i o t i t e standard GE-20 6 0 , standard P - 2 0 7 and Nelson p r o j e c t b i o t i t e  U.S.G.S. muscovite  samples i n d i c a t e a p r e c i s i o n o f about 0 . 7 percent  (Tables 3 . 1  and 3 . 2 ) . From Table 3 . 3 , the a b s o l u t e agreement obtained on the potassium U.B.C. r e s u l t s  d e t e r m i n a t i o n o f t h e s t a n d a r d P - 2 0 7 , between the ( 8 . 5 8 ± 0 . 0 3 ) and t h e averaged  different laboratories standard G E - 2 0 6 0 , The  potassium  i n 0 . 6 percent  (8.38 ± 0 . 1 2 )  r e s u l t s o f twelve  i s noteworthy.  From t h e  a d i f f e r e n c e of about 0 . 6 percent  resulted.  a n a l y s i s i s t h e r e f o r e b e l i e v e d t o be accurate w i t h or b e t t e r .  For hornblende samples, i n at l e a s t  and pyroxene, t h e r e s u l t s o b t a i n e d from f o u r q u a d r u p l i c a t e , o n N e l s o n p r o j e c t show a y  47 p r e c i s i o n of about 0.4 no hornblende  percent  (Table 3 . 1 ) .  Unfortunately  or pyroxene standard i s a v a i l a b l e at p r e s e n t .  However, w i t h c a r e f u l work, on the p r e p a r a t i o n o f samples, sample and  standard s o l u t i o n s , i t i s b e l i e v e d t h a t the  i s b e t t e r than two One  accuracy  percent.  o f the most s e r i o u s problems i n the argon a n a l y s i s by  mass spectrometry  s t a t i c method i s the contamination of the  p r e v i o u s sample on the sample t o be processed.  T h i s causes a ".;  change i n the i s o t o p i c r a t i o s measured with time.  However, w i t h  the mass spectrometer MS-10  used i n the present study, no "memory  e f f e c t " has been d e t e c t e d .  The  age  i s o t o p i c r a t i o s was  standard d e v i a t i o n of the  always l e s s than 0.2  percent.  aver-  Farrar  e t a l . , (1964) a s c r i b e d t h i s t o the low a c c e l e r a t i n g v o l t a g e , about 100 v o l t s , used i n the mass spectrometer MS-10.  The r e . one  s o l v i n g power of the MS-10  i s approximately  50, t h i s  permits to v  r e s o l v e p e r f e c t l y a l l mass peaks of argon i s o t o p e s 3 6 , 38 and A sample run f o r atmospheric  argon, g i v e n i n f i g u r e 3-4,  the r e s o l u t i o n i n the mass 36 - 40 range.  40.  shows  Based on the h i g h  p r e c i s i o n and accuracy obtained p r e v i o u s l y i n the l a b o r a t o r y , (White et a l . , 1967), only ca  s i n g l e run was  c a r r i e d out f o r  each sample, (except the clinopyroxene sample R-12 in duplicate).  Seventeen a n a l y s i s on b i o t i t e  a p r e c i s i o n o f approximately  one percent  which was  standards  (Table 3 . 2 ) .  run  indicate I t may  be  seen t h a t the U.B.C. argon a n a l y s i s r e s u l t s are, s y s t e m a t i c a l l y lower than those r e p o r t e d by other l a b o r a t o r i e s . i s approximately one percent I t may  The  difference  (Table 3 « 3 . ) »  be deduced from Table 3.2  t h a t the r e s u l t s obtained  i n the p r e s e n t study are i n t e r n a l l y c o n s i s t e n t w i t h i n two  percent  Ar x 5  ZOmV.  48  8-  Figure 3 - 4  40  7  4-  A r * 10 V  38 A r x 30mV.  L J  :  T y p i c a l atmosp h e r i c argon run shows the r e s o l u t i o n i n the mass 36-40 range.  TABLE 3.4 Ages of the V a r i o u s Rock U n i t s on the Nelson B a t h o l i t h  Rock U n i t  Sample Number  Mineral Dated  Rock Type  Model Age (m.y.) 120 130  Remark on Sample Analyzed Chloritized Chlofitized Ere sh Fresh Very f r e s h Very f r e s h B i o t i t e completely c h l o r i t i z e d but hornblende i s f r e s h i n appearance. About 15% of B i o t i t e has SO ragged edges.  C-66  Quartz monzonite  SILV-66  Quartz monzonite  Biotite Biotite Biotite Clinopyroxene Biotite Hornblende Hornblende  L-236  Hornfels  Biotite  135  L-237  Quartz d i o r i t e  Biotite  150  Altered  ( i n c l u d i n g Mont C a r l y l e stock)  K-66  Quartz monzonite  Hornblende  161  Very f r e s h  Valhalla Plutonic Rocks  A-66  Granodiorite  Biotite  149  Slightly altered  Post-Nelson batholith  D-66  Lamprophyre  Biotite  169  Fresh  Nelson b a t h o l i t h  Nelson  satellites  R-ll: R-13 R-12  Granodiorite Granodiorite Quartz d i o r i t e  dyke  146 265 151  164 150  50 of the apparent  age. However, due t o the d i s c r e p a n c y met when  comparing the r e s u l t s obtained by v a r i o u s l a b o r a t o r i e s , e r r o r l i m i t s are adopted.  The apparent  larger  ages are assigned e r r o r  l i m i t s ranging from three t o s i x p e r c e n t .  The lower  precision  was assigned t o t h e a n a l y s i s which i n d i c a t e d l e s s than 50 percent radiogenic A r  4 0  as suggested  by L i p s o n  (1958).  TABLE 3 . 5 RELIABLE MODEL AGES SELECTED FOR DATA INTERPRETATION  Mineral dated  Biotite  Hornblende  Sample number  Model age (m.y.)  Standard deviation (m.y.)  R-12  146  5  A-66  U9  6  C-66  151  6  D-6 6  169  6  SILV-66  150  5  C-66  164  6  K-66  161  6  51  3«3  Discussion The K40  Ar  4 0  i s o t o p i c age d e t e r m i n a t i o n of the  Nelson  b a t h o l i t h , i n the present study, y i e l d e d v a l u e s which agree r e a s o n a b l y w e l l w i t h the r e s u l t obtained from g e o l o g i c a l evidences. On the whole, the v a l u e s range from 120 m.y. m.y.,  169  w i t h the e x c e p t i o n of the c l i n o p y r o x e n e sample  which y i e l d e d an anomalous age of  to  ages f a l l  around 150 m.y.  of 265 m.y..  R-12  A l a r g e group  Without f u r t h e r c o n s i d e r a t i o n s ,  the v a l u e s i n d i c a t e t h a t the v a r i o u s phases of the  Nelson  b a t h o l i t h were emplaced d u r i n g a c o n s i d e r a b l e i n t e r v a l time, from M i d d l e J u r a s s i c t o Lower Cretaceous.  The  of  age r e -  s u l t s f i t t h e r e f o r e the sequences of emplacement o f the grani t i c r o c k s as e s t a b l i s h e d by c o n t a c t r e l a t i o n s h i p s i n the f i e l d .  observed  C a i r n e s ( 1 9 3 4 ) ^ i n h i s memoir s t a t e d t h a t ,  "the sequence commenced with the b a t h o l i t h i c i n t r u s i o n which was f o l l o w e d by the i n v a s i o n o f s t o c k s and minor i n t r u s i v e s  and  concluded w i t h the m a f i c dykes". However, a more d e t a i l e d i n t e r p r e t a t i o n o f the d a t a needs •Hie  f u r t h e r refinement.  From^welveage determinations o b t a i n e d , o f  which t h r e e have been o b t a i n e d on hornblendes, pyroxene and the remainder  on b i o t i t e s ,  Age  one  on  clino-  determinations  were c a r r i e d out f o r two oogenetic m i n e r a l s p a i r s .  F i v e age  d e t e r m i n a t i o n s have been c o n s i d e r e d u n r e l i a b l e , based on the f a c t t h a t the p r e r e q u i s i t e i o u s l y w t r e not  fulfilled.  sufficient  c o n d i t i o n s posed prev-  52  First an age  o f a l l , the clinopyroxene  o f 265 m.y.  sample R-12  yielded  which i s unquestionably anomalous, r e and  yeoLogic i-etofions  g a r d i n g the r e s u l t s o b t a i n e d on the other samples*.  The  r e s u l t s were checked a n a l y t i c a l l y f o r both potassium argon d e t e r m i n a t i o n s .  On the flame photometry  and  determina-  t i o n , an average of 0 . 5 5 8 5 percent w i t h a standard d e v i a t i o n from the mean of l e s s than 0 . 5 from e i g h t d i f f e r e n t a l i q u o t s . l a t i v e l y low.  percent has been o b t a i n e d I t s potassium content i s r e -  However, based on the p r e c i s i o n and  accuracy  obtained from p r e v i o u s a n a l y s e s on very low potassium  bearing  materials  (White e t a l . , 1967), i t i s b e l i e v e d t h a t the  potassium  d e t e r m i n a t i o n i s accurate withlte<2 p e r o e n t , i f not  better.  The  argon analyses were c a r r i e d out i n d u p l i c a t e .  The experimental The  d a t a are t h e r e f o r e c o n s i d e r e d t o be  accuracy and p r e c i s i o n are b e l i e v e d t o be w i t h i n the  l i m i t s assigned. apparent  clinopyroxene  error  On the o t h e r hand, the h i g h l y d i s c o r d a n t  ages obtained from the p a i r o f oogenetic  sample R-12,  146 m.y.  clinopyroxene f o r K  4 0  minerals  from b i o t i t e a g a i n s t 265 m.y.  sample, suggest  pyroxene c o n t a i n s cJ  Hart  reliable.  to the w r i t e r t h a t  excess r a d i o g e n i c argon. - Ar  4 0  from  clino-  The  use of the  d a t i n g should t h e r e f o r e be  ( 1 9 6 l ) , i n v e s t i g a t i n g pyroxene f o r K40  _ Ar  the  4 0  avoided.  dating  s t a t e d t h a t "pyroxenes are u n l i k e l y h o s t s f o r excess  argon  as t h e y do not have s t r u c t u r a l v a c a n c i e s or h o l e s as amphiboles".  However, f u r t h e r works of Hart and Dodd (1962) and  L o v e r i n g and R i c h a r d s  (1964), proved t h e presence  r a d i o g e n i c argon i n pyroxene.  T h i s "age" may  of  excess  be d i s r e g a r d e d  on the b a s i s of the p r e r e q u i s i t e c o n d i t i o n s f o r a K40  - Ar40  33  model age are not f u l f i l l e d .  T h e r e f o r e , h i g h l y anomalous  c l i n o p y r o x e n e "age" must not he i n c l u d e d i n the d a t a i n t e r pretation. The o t h e r f o u r samples, L - 2 3 6 , R - l l , R-13 do not s a t i s f y the t h i r d Ar  s u f f i c i e n t c o n d i t i o n of t h e K  4 0  -  I n p a r t i c u l a r , the sample L - 2 5 7 i s so  model age.  4 0  and L - 2 3 7 ,  h e a v i l y c h l o r i t i z e d and a l t e r e d t h a t even the y i e l d e d v a l u e of  150 m.~y.  which i s f a i r l y  Nelson p l u t o n i c r o c k s , reliable.  Moreover,  concordant with the "age" of the  T-h-i-s date i s s t i l l  the sample was  c o n s i d e r e d un-  c o l l e c t e d from an i n -  t r u s i o n i n r o c k s of t h e S l o c a n s e r i e s , n o r t h of t h e Nelson batholith.  I t was  found t h a t t h e sample,  petr©graphically,  i s v e r y d i s t i n c t from the Nelson p l u t o n i c r o c k s . i s believed t o indicate only a l a t e alteration. 130 m.y.  Samples R - l l , R-13  and 135 m.y.  stage o f hydrothermal  and L - 2 3 6 y i e l d e d 120  respectively.  The low ages may  p l a i n e d by the argon l o s s through d i f f u s i o n . e f f e c t s of a l t e r a t i o n on the K  4 0  The date  - Ar  4 0  The  m.y., be  ex-  deleterious  ages o f b i o t i t e s  and  hornblendes have been d i s c u s s e d by G o l d i c h & ;,Gast ,~XL966). They showed t h a t a l t e r a t i o n has reducedb-approximately 25 p e r cent the K  4 0  - Ar  4 0  age o f b i o t i t e s from Morton g n e i s s of  southwestern Minnesota. al.  Kulp and E n g e l s ( 1 9 6 3 ) , Ma^in- et  fl-9^34, a l s o d i s c u s s e d the p o s s i b l e e f f e c t s o f a l t e r a -  t i o n on K 0 4  - A r 0 ages of b i o t i t e s . 4  These  e f f e c t s are  8 7 8 7 g e n e r a l l y more d r a s t i c s ! on Rb - Sr ages than on K 0 4  ages m a i n l y because  Q Ar  4 0  o f the l a r g e e x p o n e n t i a l e f f e c t i n the  A 0 4  r a t i o age r e l a t i o n .  E4TJ-  In any case, those samples  d i d not  54  f u l f i l l the p r e r e q u i s i t e c o n d i t i o n s f o r the K model age.  They may  4 0  -  Ar  4 0  be d i s r e g a r d e d i n the i n t e r p r e t a t i o n  of t h e emplacement h i s t o r y o f the Nelson b a t h o l i t h . F i n a l l y , a remark should be made on samples D-66 SILV-66.  The  sample D-66  y i e l d e d an age  o f 169 m.y.  appears h i g h i n l i g h t o f g e o l o g i c evidence. geology, i t was  cation).  (A.J. S i n c l a i r , p e r s o n a l  T h i s anomolous b i o t i t e age  which  From f i e l d  observed t h a t the lamprophyre dyke  t e l y cut the Nelson rocks  and  could a g a i n be  definicommuniexplained  by the f a c t t h a t the p r e r e q u i s i t e c o n d i t i o n s might not completely  fulfilled.  be  An excess o f r a d i o g e n i c argon i n the  sample, i s not improbable. p o r t e d t h a t some b i o t i t e  Richards  and Pidgeon (1963), r e -  samples of Broken H i l l o f New  South  Wales, A u s t r a l i a , gave ages which were n e a r l y twice the Rb-Sr age  of the same phases. A'similar excess of r a d i o g e n i c  i n micas have a l s o been r e p o r t e d by many o t h e r s 1963;  argon  (Stockwell,  P e p i n et a l . , 1964). It  i s known t h a t the measured r a d i o g e n i c Ar40 content  m i n e r a l depends not o n l y on the argon r e t e n t i v i t y and  of  the  temperature h i s t o r y o f the _phase, but a l s o on p a r t i a l argon pressure  of the g e o l o g i c a l environment where the m i n e r a l  formed.  I t seems t h e r e f o r e p o s s i b l e , t h a t the h i g h K  model ages appear t o occur i n rocks formed under h i g h and temperature c o n d i t i o n s . h i g h argon p r e s s u r e  This implies that  could p o s s i b l y develop and  4 0  was -  Ar  4 0  pressure  relatively consequently  55  excess r a d i o g e n i c argon could be trappedifcito the e x i s t i n g or newly formed m i n e r a l s . content o f mica may  N e v e r t h e l e s s , the i d e a t h a t  argon  be a f f e c t e d by e x t e r n a l argon pressure  has been shown by K a r p i n s k a y a e t a l . (1961).  They proved  e x p e r i m e n t a l l y t h a t a c o n s i d e r a b l e excess o f argon can be f o r c e d i n t o muscovite under h i g h argon p a r t i a l p r e s s u r e s and a t r e l a t i v e l y h i g h temperature. by Hart m.y.  (1961).  T h i s has been supported  Thus the s l i g h t l y h i g h model age o f 169  y i e l d e d by the b i o t i t e  sample D-66,  may  indicate  locally  h i g h argon p a r t i a l p r e s s u r e s o p e r a t i n g a t some time i n the history of b i o t i t e .  T h i s supports f u r t h e r the h y p o t h e s i s of  deep-seated o r i g i n of the Nelson b a t h o l i t h proposed by L i t t l e (I960). However, i n the case o f the p r e s e n t study the excess argon i s q u i t e i n s i g n i f i c a n t . w i t h i n experimental l i m i t s ,  The e r r o r i n v o l v e d i s s t i l l  and the date obtained i s t h e r e -  f o r e c o n s i d e r e d t o be r e l i a b l e .  Sample SILV-66 has been  c o l l e c t e d from the e a s t e r n edge of a major f r a c t u r e zone forming the western c o n t a c t of the Nelson b a t h o l i t h .  Ex-  t e n s i v e c h l o r i t i z a t i o n i s r e c o g n i z e d along t h i s zone (A.J. S i n c l a i r , p e r s o n a l communication).  The rock has undergone  c o n s i d e r a b l e c a t a c l a s i s and b i o t i t e has been completely r e p l a c e d by pseudomorphs of c h l o r i t e . undeformed and appears f r e s h . y i e l d e d by hornblende  The  However, hornblende i s apparent age of 150  sample SILV-66  cluded i n d a t a i n t e r p r e t a t i o n .  m.y.  should t h e r e f o r e be i n -  In any case, the date seems  56  to  be v e r y i n t e r e s t i n g ; the h i g h argon r e t e n t i v i t y of  hornblende i s worthy f o r p r a i s i n g . the  I t s usefulness i n  £40 - Ar40 d a t i n g i s unquestionably s i g n i f i c a n t .  high r e l i a b i l i t y  The  o f the hornblende dates has been a f f i r m e d  by Hart (1964) and G e r l i n g e t a l . (1965). After the above-mentioned  " r e s t r i c t i o n s " , the remaining  data are f o u r f r e s h b i o t i t e model ages, which y i e l d a mean value o f 154±10 m.y. and t h r e e hornblende model ages which y i e l d a mean value o f 158+4* m.y. are  (Table 3 . 5 ) .  These val ues  b e l i e v e d t o be r e l i a b l e w i t h i n l i m i t s o f experimental  error.  I t may be seen t h a t the p a i r o f oogenetic b i o t i t e -  hornblende from the specimen C - 6 6 g i v e s f a i r l y  concordant  i s o t o p i c ages; 151 m.y. f o r b i o t i t e a g a i n s t 164 m.y. f o r hornblende.  The low b i o t i t e age may be e x p l a i n e d by t h e f a c t  t h a t l o s s o f argon i n b i o t i t e i s more s i g n i f i c a n t than t h a t i n hornblende.  I n other words, hornblende has had h i g h e r  r e t e n t i v i t y o f r a d i o g e n i c argon t h a n b i o t i t e  (Hart, 1964);  G e r l i n g e t a l . , (1965); Love r i n g & R i c h a r d s , ( 1 9 6 4 ) .  The low  apparent age from b i o t i t e sample C-66 may be caused by heat produced a l o n g the c o n t a c t zone, between V a l h a l l a and Nelson p l u t o n i c r o c k s , which would l a t i v e l y o l d e r Nelson r o c k s .  r e s u l t i n argon l o s s from t h e r e As the K4° - Ar40 model ages  o b t a i n e d from r o c k s and m i n e r a l s may v a r y w i t h t h e d i s t a n c e from an i n t r u s i v e contact  (Aldr-teh -etj—al., 1^60-; H a r t , 1 9 6 4 ;  Hanson and Gast, 1 9 6 7 ) , b i o t i t e may l o s e up t o 5 ° p e r c e n t of  57  i t s argon w i t h i n one m i l e whereas w i t h i n a v e r y s m a l l d i s t a n c e even as near as 1,000  f e e t from the  c o n t a c t , hornblende  r e t a i n w e l l a l l i t s argon ( f i g u r e 1 - 2 ) . e r r o r may  occur i n these  Some p o s s i b i l i t y  r e s u l t s which were^based on  dimension studies i n s t e a d of t h r e e .  may of  two-  Further geophysical inves-  t i g a t i o n s should be c a r r i e d out f o r a more accurate r e s u l t . However, q u a n t i t a t i v e l y , i t may blende model age to  of 164 m.y.  be concluded  t h a t the  i s more r e l i a b l e and  horn-  HtGr-9  the true age t h a n i t s oogenetic b i o t i t e model age  closer,  of 151  From the r e s u l t s obtained i n t h i s i n v e s t i g a t i o n , one  m.y.. may  see^that i t i s i m p o s s i b l e to d i s t i n g u i s h the v a r i o u s phases of the Nelson  b a t h o l i t h by means of K40  _ Ar^O  dating.  t i o n s i n the model ages obtained were w i t h i n the l i m i t s of e r r o r .  The  varia-  analytical  T h i s r e a f f i r m s the importance of f i e l d  i n the i n v e s t i g a t i o n of phase r e l a t i o n s h i p s of an area.  geology Many  l a b o r a t o r y g e o c h r o n o l o g i s t s have e-ften the tendency of over-" e s t i m a t i n g the r o l e o f £40 _ Ar40 d a t i n g i n ge©chronology i n vestigations.  However, on the b a s i s of the present  the work o f K.  Northcote  study  of the Geology Department, on the  Guichon b a t h o l i t h , i t i s c l e a r l y r e a l i z e d t h a t without geology  c o n t r o l s , m i s i n t e r p r e t a t i o n may  the Nelson  and  arise.  field  I n the case  of  and Guichon b a t h o l i t h s , the £40 _ Ar40 r e s u l t s i n -  dicate very l i t t l e  about the. d i s t i n c t i o n of v a r i o u s phases.  The  time at which m i n e r a l s b e g i n t o r e t a i n the r a d i o g e n i c argon i s n e a r l y the same f o r each phase.  Conversely,  f i e l d geologists  attempt t o t h i n k o f g e o l o g i c a l processes expanding a t time  inter-  v a l s o f t e n s of m i l l i o n s of y e a r s , whete-aB^ih r e a l i t y , the p e r i o d s *  U n i v e r s i t y of  British - Columbia .  59  V '  From g e o l o g i c a l f i e l d study the sequence i n the Nelson b a t h o l i t h . may l o g i c order as f o l l o w s :  which  of e v e n t s r o c c u r r e d  be summarized i n rough  geochrono-  The Nelson r o c k s , the Nelson  the V a l h a l l a r o c k s and t h e Lamprophyre  satellites,  dyke.  I f the age of the Nelson b a t h o l i t h i s d e f i n e d as the time r n i c a e e o u s  at which t h e m i n e r a l s of the b a t h o l i t h s t a r t to r e t a i n r a d i o v  g e n i c argon, then the model age of 1.54110 m.y. ing figure. 1J?8±7 m.y.  i s the r e p r e s e n t -  However i t i s b e l i e v e d t h a t the model age of obtained on hornblende i s c l o s e r t o the t r u e age of  the Nelson b a t h o l i t h .  The h i g h e r r e t e n t i v i t y of argon of horn-  blende over b i o t i t e has been r e c o g n i z e d by many workers i n the f i e l d of geochronology  (Hart, 1964;  G e r l i n g et a l . ,  I f the model age f o r t h e lamprophyre  1965).  dyke i s accepted, then  on the b a s i s of g e o l o g i c a l e v i d e n c e , an assumption that a l l the other samples have l o s t argon must be made; the age of the Nelson b a t h o l i t h i s t h e r e f o r e o l d e r than 170 m.y,. r e l a t i o n s r e q u i r e t h a t the sample D-66 samples.  Structural  be younger than the other  The model age o b t a i n e d of l 6 9 i 6 m.y.  i s slightly  h i g h e r than one would expect on s t r u c t u r a l evidence.  Possible  excess of r a d i o g e n i c argon i n t h i s sample has been d i s c u s s e d above.  X-ray d i f f r a c t i o n was  a l s o used t o check whether or not  the sample has been contaminated by other m i n e r a l s such as clino-pyroxene.  The  r e s u l t from X-ray d i f f r a c t i o n a f f i r m e d  f u r t h e r the " p u r i t y " of the sample.  I n any case, the "excess  age" i s so s m a l l , w i t h i n the l i m i t s of experimental e r r o r , that  60  the  date does not cause any s i g n i f i c a n t  age obtained on hornblende  c o n f l i c t t o the model  samples.  From the above d i s c u s s i o n , i t i s p r e f e r a b l e t o accept a t r u e "age" f o r the Nelson b a t h o l i t h w i t h i n the l i m i t s o f cont r o l o f data on b i o t i t e , hornblende and g e o l o g i c a l  (lampro-  phyre dyke') ; model ages. T h e r e f o r e the average K40 _ A r may  4 Q  model age of 156+9  m.y.  be considered t o be ta:: c l o s e approximat'oh<to the t r u e age  of the b a t h o l i t h .  61 CONCLUSION The K40 _ Ar^O i s o t o p i c age d e t e r m i n a t i o n s on b i o t i t e , hornblende and pyroxene  o f t h e Nelson b a t h o l i t h i n t h e p r e s e n t  study i n d i c a t e t h a t the v a r i o u s phases o f t h e b a t h o l i t h (at l e a s t i n t h e n o r t h e r n p a r t ) , were emplaced d u r i n g a s h o r t i n t e r v a l o f time, not more than 18 m.y., centered around 156 m.y.,  c o r r e s p o n d i n g t o the Upper-Middle  the Kulp's time s c a l e .  J u r a s s i c boundary o f  At some l a t e r time, l e s s than 120 m.y..  ago, the b a t h o l i t h was s u b j e c t e d t o a t l e a s t one p e r i o d o f hydrothermal  alteration, resulting i n local  chloritization  and t h e r e f o r e v a r i a b l e l o s s o f r a d i o g e n i c argon from b i o t i t e s , thus a c c o u n t i n g f o r the s c a t t e r of model ages between 120 146  m.y.. I t i s a l s o concluded that the v a r i o u s phases of the Nelson  b a t h o l i t h cannot be d i s t i n g u i s h e d on the b a s i s o f the K Ar  4 0  i s o t o p i c d a t i n g ; the v a r i a t i o n s i n "ages" o f these  ^  4 0  phases  were w i t h i n experimental e r r o r l i m i t s , o f t h e a n a l y t i c a l t e c h niques  used.  On the b a s i s o f the r e s u l t s o b t a i n e d on two p a i r s o f oogenetic samples, it  i s evident that:  b i o t i t e - h o r n b l e n d e and b i o t i t e - c l i n o p y r o x e n e , Clinopyroxene c o n t a i n s s i g n i f i c a n t  excess  o f r a d i o g e n i c argon and t h e r e f o r e i t should not be used f o r K  40  _ Ar^O d a t i n g . Hornblendes  g i v e r e l i a b l e K40 _ A r  4 0  model ages.  Their  h i g h r e t e n t i v i t y o f r a d i o g e n i c argon i s worthy f o r p r a i s e , B i o t i t e s may c o n t a i n excess e f r a d i o g e n i c argon, under h i g h temperature conditions.  particularly  and h i g h argon pressure environmental  I t i s t h e r e f o r e e v i d e n t t h a t K40 - Ar 40 dates are  62  p a r t i c u l a r l y s u i t a b l e , e s p e c i a l l y i n an i n i t i a l  reconnaissance,  f o r i d e n t i f y i n g h i t h e r t o unsuspected c o m p l e x i t i e s . However, i t i s necessary t o c o n s i d e r the K  4 0  along with a l l a v a i l a b l e p e t r o g r a p h i c and f i e l d  - Ar ^ 4  dates  geology e v i d -  ences t o be able t o gifre a c l e a r c u t c o n c l u s i o n . •the  Further i n v e s t i g a t i o n s on Nelson b a t h o l i t h , v  especially  i n the southern p a r t , should i n v o l v e f u r t h e r d e t a i l e d mapping i n c o n j u n c t i o n w i t h i s o t o p i c age by R b ^ - S r ^ method.  determinations,  particularly  T h i s might unravel the metamorphic ,  s t r u c t u r a l and i n t r u s i v e h i s t o r y of the whole a r e a .  63  BIBLIOGRAPHY A l d r i c h , L.T. and G.W. W e t h e r i l l . Geochronology hy r a d i o a c t i v e decay. Ann. Rev. Nuclear S c i . , 8., 273 (1938) A l l s o p p , H.L. Rb - Sr and K - A r age measurements on the Great Dyke o f Southern Rhodesia, J . Geophys. Res., .20., 977  (1965)  Amirkhanov, K.I., Brand, S.B. and E.B. B a r t n i t s k i i . 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D i f f u s i o n i n s o l i d s , l i q u i d s and gases. p r e s s , New York, ( i 9 6 0 )  Academic  Kanasewich,E.R. Q u a n t i t a t i v e i n t e r p r e t a t i o n s o f anomalous l e a d i s o t o p e abundances. Ph.D T h e s i s , U n i v . o f B.C. (1962) K a r p i n s k a y a , T.B., O s t r o v s k i i , I.A. and L . I . Shanin. Artificial i n t r o d u c t i o n of argon i n t o mica under c o n d i t i o n s o f h i g h p r e s s u r e and temperature. I z v . Akad. Nauk. USSR I z v . , Ser., Geol., &, 99 (1961) Kulp, J.L.  G e o l o g i c time s c a l e .  S c i e n c e , 133. 1105  (1961)  67 K u l p , J.L. and J . E n g e l s . D i s c o r d a n c e s i n K - Ar and Rb - Sr i s o t o p i c ages: D i s c u s s i o n i n R a d i o a c t i v e d a t i n g . Intern a t i o n a l Atomic Energy Agency, Vienna, 219 (1963) Lamphere, M.A. and G.B. Dalrymple. P - 2 0 7 : An i n t e r l a b o r a t o r y standard muscovite f o r argon and potassium a n a l y s e s . J . Geophys., Res., JO.* 34-97  U963)  Lamphere, M.;Ju. and G.B. Dalrymple. K - Ar and Rb - Sr measurements on P - 2 0 7 ; the USGS i n t e r l a b o r a t o r y standard muscovite. Geochim. e t Cosmochim. A c t a , 1091 (1967) L a r s o n , E.S., G o t t f r i e d , D., J a f f e , H. and C L . Waring. Age of the s o u t h e r n C a l i f o r n i a , S i e r r a Nevada and Idaho b a t h o l i t h s . B u l l . G e o l . Soc. Am., 6 £ , 1277 (1954) Leech, G.B. and R.K. Wanless. L e a d - i s o t o p e and Potassium-argon s t u d i e s i n the E a s t Kootenay d i s t r i c t , B r i t i s h Columbia. G e o l . Soc. Amer., Buddington Volume, 241 (1962) Leech, G.B., Lowdon, J.A., S t o c k w e l l , C H . and R.K. Wanless. Age d e t e r m i n a t i o n s and G e o l o g i c a l S t u d i e s . G e o l . Surv. Can., Paper 6 3 - 1 7 (1963) L i p p o l t , H.J. and W. Gentner. K - Ar d a t i n g o f l i m e s t o n e s and f l u o r i t e s , i n R a d i o a c t i v e d a t i n g ; I n t e r n a t i o n a l Atomic Energy Agency. V i e n n a , P r o c . Sev., 239 (1963) L i p s o n , J . J . Pot as s i urn-argon d a t i n g of sedimentary r o c k s . B u l l . G e o l . Soc. Am. 6 2 , 137 (1953) L i t t l e , H.W. Nelson map-area, w e s t - h a l f , B r i t i s h G e o l . Surv. Can., Memoir 308 ( i 9 6 0 )  Columbia.  L o v e r i n g , J . F . and J.R. R i c h a r d s . Potassiurn-argon age study of p o s s i b l e l o w e r - c r u s t and upper-mantle i n c l u s i o n s i n deepseated i n t r u s i o n s . J . Geophys., Res., 6_9, Lowdon, J.A. Age d e t e r m i n a t i o n s by the G e o l o g i c a l Survey o f Canada, Report 2 - I s o t o p i c ages; G e o l . Surv. Can., paper 61-17  (1961)  Lowdon, J.A., Wanless, R.K., S t o c k w e l l , C H . and G.B. Leech. Age d e t e r m i n a t i o n s and g e o l o g i c a l s t u d i e s , Report 4 , G e o l . Surv. Can., paper 63-17 (1963) M u l l i g a n , R. Can.,  Bonnington map-area, B r i t i s h  paper 5 2 - 1 3 ,  13  Columbia.  G e o l . Surv.  (1952)  M c ^ o u g a l l , I . Potassium-argon age measurements on d o l e r i t e s from A n t a r t i c a and South A f r i c a . J . Geophys. Res., 6 8 , 1535  (1963)  68  McDougall, I . and D.H. Green. E x c e s s r a d i o g e n i c argon i n pyroxenes and i s o t o p i c ages on m i n e r a l s from Norweigian eiilogites. Norsk, G e o l . T i d s s k r , 4 4 , I 8 3 ^1964) Newmarch, G.B. Geology o f the Crowsnest Dept. Mines, B u l l . 33 (1953)  C o a l B a s i n , B.C.  N i e r , A.O.A. Redetermination of t h e r e l a t i v e abundances o f the i s o t o p e s of carbon, n i t r o g e n , oxygen, argon and potassium. Phys. Rev. 2 1 , 789 (1950) P e p i n , R.O., Reynolds, J.H. and F. Turner. Shock emplaced argon i n a stony m e t e o r i t e . 2 . A comparison w i t h n a t u r a l argon i n i t s d i f f u s i o n . J . Geophys. Res., 69» 1406  (1964)  P o l e v a y a , N.I., Kazakov, G.A. and G.A. Murina. G l a u c o n i t e as an i n d i c a t o r of g e o l o g i c time. Geokhimiya, 1, 3 (i960)  Rama, Si.N.I., H a r t , S.R. and E . Roedder. Excess radiogenic argon i n f l u i d i n c l u s i o n s . J . Geophys. Res., 1 0 , 509 (1965)  Read, H.H. Metamorphism and g r a n a t i z a t i o n ; G e o l . Soc. S. A f r i c a , Annexure t o v o l . 54 ( l 9 5 l ) Reichenberg, D. P r o p e r t i e s o f Ion-Exchange r e s i n i n r e l a t i o n to t h e i r s t r u c t u r e . I I I . K i n e t i c s of Exchange. Am. Chem. S o c , 21, 589 (1953) Reynolds, J.H. High s e n s i t i v i t y mass spectrometer f o r noble gas a n a l y s i s . Rev. S c i . I n s t . , 2 7 , 928 (1956) Reynolds, P.H. A l e a d i s o t o p e study o f ores and adjacent r o c k s . Ph.D T h e s i s , Univ. o f B.C. (1967) R i c h a r d s , J.R. and l . T . Pidgeon. Some age measurements on micas from Broken H i l l , A u s t r a l i a . J . G e o l . S o c , A u s t r a l i a , 10 243  (1963)  Ross, J.V. A note on histogram a n a l y s i s o f i s o t o p i c age d a t a . Can. J . E a r t h S c i . , ^ 293 (1966) R u s s e l l , R.D., S h i l l i b e e r , H.A., Earguhar, R.M. and A.K. M f. The b r a n c h i n g r a t i o o f potassium 40. Phys. Rev., 91, 1223 o  (1953)  a  s  ~  Sardarov, S.S. R e t e n t i o n o f r a d i o g e n i c argon i n g l a u c o n i t e s . Geokhimiya, 1 0 , 905 (1963)  u  69  S h i l l i b e e r , H.A., R u s s e l l , R.D., FarQuhar, R.M. and S.A.W. Jones. Radiogenic argon measurements. Phys. Rev. 94. 1793  (1954)  S i n c l a i r , A . J . A l e a d i s o t o p e study of m i n e r a l d e p o s i t s i n the Kootenay Arc..-.,Ph.D T h e s i s , Univ. o f B.C. (1964) S i n c l a i r , A . J . and W.C. L i h h y . D i s t r i b u t i o n o f major m i n e r a l s i n a s t o c k o f Nelson p l u t o n i c r o c k s , south c e n t r a l B r i t i s h Columbia ( a b s t r a c t ) , Can. Miner 308 (1967) Smith, A.G. Potassium-argon decay c o n s t a n t s and age t a b l e s . The phanerozoic time s c a l e , sympos. G e o l . Soc. (London), Bartholomen P r e s s , England, 129 (1964) S t e r n , T.W., Newell, M.F., K i s t l e r , R.W. and D.R. Shawe. Zircon uranium-lead and t h o r i u m - l e a d ages and m i n e r a l potassium-argon ages of L a S a l Mountains r o c k s , Utah. J. Geophys. Res., J o , 1503 (1965) Stockwell,. C H . T h i r d r e p o r t on s t r u c t u r a l p r o v i n c e s , orogeni e s and t i m e - c l a s s i f i c a t i o n of r o c k s o f the Canadian Precambrian s h i e l d . G e o l . Surv. Can., paper 63-17, 125 (1963)  Thompson, J . J . On the e m i s s i o n o f n e g a t i v e c o r p u s c l e s by the a l k a l i m e t a l s . P h i l . Mag., 10, 584 (1905) Wassenburg, G.J. and R.J. Hayden. Phys. Rev., 9_3, 645 (1954)  The  branching r a t i o o f  K40.  Von Weizsacker, C.F. Uber d i e ^ 6 ' g l i c h k e i t e i n e s dualen BetaZ e r f a l l s von Kalium, Phys.Z.38, 623 (1937) W e t h e r i l l , G.W. R a d i o a c t i v i t y o f potassium and g e o l o g i c time. S c i e n c e , 1 2 6 , 545 (1957) W e t h e r i l l , G.W., Wasserburg, G.J., A l d r i c h , L.T., i l t o n , G.R. and R.J. Hayden. Decay of c o n s t a n t s o f K40 as determined by the r a d i o g e n i c argon content o f potassium m i n e r a l s . Phys. Rev., 10J5, 987 (1956) T  White, W.H., E r i c k s o n , G.P., Northcote, K.E., Dirom, G.E. and J.E. H a r a k a l . I s o t o p i c d a t i n g of the Guichon b a t h o l i t h , B.C. Can. J . E a r t h Sc., 4, 677 (1967) Zartman, R.E. A g e o c h r o l o g i c study of the Lone @rove p l u t o n from the L l a n o U p l i f t , Texas. J . Petrology,' 359 (1964)  70 APPENDIX I EXPERIMENTAL  A-I-l  PROCEDURE  Mineral Separation i  The  samples were e x t r a c t e d t o d e t a i n concentrate py-  roxene, hornblende The  equipment;  r  .  and b i o t i t e .  wasD c a r e f u l l y cleaned and  decontaminated  from t h e p r e v i o u s samples w i t h a s p l i t o f t h e sample t o be processed. Rocks were crushed, gribuiid and s i e v e d i n t o s i z e  frac-  t i o n s r a n g i n g from 35 mesh t o 100 mesh ( i n the case o f hornblende, finer  i t was o f t e n n e c e s s a r y t o reach a  size).  U s i n g i n v a r i o u s combinations  hydraulic c l a s s i f i c a t i o n ,  heavy l i q u i d , e l e c t r o s t a t i c and magnetic s e p a r a t i o n techniques. The procedures may be summarized i n the f o l l o w i n g f l o w , sheet i n f i g u r e  A-I-l.  To o b t a i n pure m i n e r a l s , g r e a t care was taken. step o f clean-up  As a f i n a l  o f the sample, i t was always necessary  t o use "hand p i c k i n g " to remove any i m p u r i t y , the s u r e s t way t o o b t a i n a clean sample o f not l e s s than 99% pure hornblende  or b i o t i t e .  71  Fig. A-I-l  •  Flowsheet  showing v a r i o u s steps of m i n e r a l s e p a r a t i o n . Hand s i z e  I I  rock  jaw c r u s h e r gyratory-crusher  *  1  cone crusher  I  screen - 14 mesh  14 mesh  I  Pulverizer  i  Screen  I  33 mesh  1 I  -33 + 50 mesh  - 5 0 +70 mesh  I  - 7 0 + 1 0 0 mesh  - 1 0 0 mesh  I  Wash i n H Q column 2  1  Dry (65° - 100°0)  I A:  Ding e l e c t r o s t a t i c BIOTITE CONCENTRATE A:  .  Biotite  tetrabromoethane  I  (reject)  i  sheet  Biotite  concentrated  reject  h a n d p i c k i n g under b i n o c u l a r microscope f o r complete c l e a n i n g  I  biotite  )> 99% pure  I float  concentrated)  Plastic  B: REJECT  Concentrate  sink (biotite  separator  72 Fig.. A - I - l  (Cont'd.. . ) B; R e j e c t  Remove s t r o n g l y magnetic p a r t i c l e with hand magnet  F r a n t z isodynamic  separator  (vertical position) r e j e c t , quartz, f e l d s p a r  hornblende  concentrate  tetrabromoethane float  ( r e j e c t , quartz, feldspar)  sink (hornblende  concentrate)  Diiodomethane  float  (hornblende  1  concentrate)  s i n k (epidote)  F r a n t z i n c l i n e d f e e d (20° forward, 15° s i d e ,  reject  hornblende  «r 0.425 amp.)  concentrate  I  p l a s t i c sheet  (remove b i o t i t e )  h a n d p i c k i n g under b i n o c u l a r microscope for further cleaning hornblende  99% p u r i t y  73  In t h e s e p a r a t i o n o f the m i n e r a l s from r o c k s , t h e onlyd i f f i c u l t y met was t h e removal o f a l l e p i d o t e g r a i n s (the common i m p u r i t y ) from amphihole c o n c e n t r a t e s . To achieve an optimum s e p a r a t i o n i t was found necessary t o use a high-power b i n o c u l a r microscope, which was a l s o employed t o monitor each step o f s e p a r a t i o n , as an a i d f o r hand p i c k i n g o f i m p u r i t i e s .  However, t h e presence  o f a small amount o f epidote i n t h e amphibole  concen-  t r a t e s i n t r o d u c e s i n s i g n i f i c a n t e r r o r i n potassium cont e n t as t h e content o f potassium i n epidote i s n e g l i g i b l e (Dirom, 1965). The p u r i f i e d pyroxene, hornblende and b i o t i t e  concen-  t r a t e s were washed s e v e r a l times i n acetone o r methyl a l c o h o l t o remove t h e heavy l i q u i d before b e i n g used for A-1-2  analysis,  The D e t e r m i n a t i o n o f Potassium Content i n M i n e r a l by Same  Photometry  The potassium content i n m i n e r a l was determined by flame photometric techniques i n v o l v i n g two main s t e p s : 1.  A c i d d i g e s t i o n of m i n e r a l sample by chemical procedures.  2.  D e t e r m i n a t i o n o f potassium content by flame  photometer.  74  Chemical Procedures 1.  About f o u r a l i q u o t s o f approximately 0.25 gm of b i o t i t e sample were e x t r a c t e d by m i c r o s plitting of  ( i n t h e case of muscovite,  aliquots  about 0 . 1 8 gm should be r e a s o n a b l y adequate).  2.  Weigh samples i n t o t e f l o n d i s h e s .  3.  Add 1 . 5 ml c o n c e n t r a t e d  4.  P l a c e on hot p l a t e and evaporate down t o about 5 ml a t a temperature  H2SO4  and 25 ml  49% HF.  o f a p p r o x i m a t e l y 250°C  ( u s i n g asbestos mats on the h o t p l a t e t o p r o t e c t teflon dishes). 5.  Add 10 ml HF and evaporate down a g a i n t o about 5ml»  6.  Add 2 ml c o n c e n t r a t e d HNO^, t h e temperature was then i n c r e a s e d u n t i l t h e f i r s t  H2SO4 7.  v&ite fumes of  appear.  The samples  were allowed t o c o o l , about 50 ml o f  pure water were added and evaporate down a g a i n . 8.  Add 50 ml o f pure H2O, warm and t r a n s f e r to  9. 10.  solution  a 250 ml beaker.  P l a c e on h o t p l a t e and b r i n g t o a b o i l ,  stop a n d . c o o l .  To each a l i q u o t , add 50 ml of stock s o l u t i o n c o n t a i n ing  5 , 0 0 0 ppm Na and 2 , 0 0 0 ppm l i .  75  11.  T r a n s f e r each a l i q u o t t o a 500 ml v o l u m e t r i c f l a s k and d i l u t e w i t h pure water t o a f i n a l volume o f 500 ml.  12.  Store i n c l e a n , d r y and t i g h t l o c k i n g cap p o l y e t h y l e n e b o t t l e s f o r flame photometric  I n the case of hornblende procedure was used. i n hornblende  analysis.  or pyroxene,  the above-described  Due t o the low potassium  and pyroxene,  contents  samples of 0 . 7 5 gram were  decomposed i n a t o t a l o f 3 nil of concentrated HgSO^ 35 ml and 49% HF and 2 ml concentrated HNO3. i f t h e samples r e s i s t complete d i s s o l u t i o n ,  However, further  a d d i t i o n a l h e a t i n g and i f necessary, an a d d i t i o n o f approximately 1 ml o f c o n c e n t r a t e d HNO^ was found t o yield satisfactory digestion.  The use o f other reagents  such as p e r c h l o r i c a c i d was t h e r e f o r e avoided.  The  s o l u t i o n was t r a n s f e r r e d t o a 250 ml v o l u m e t r i c f l a s k t o which was added 25 ml o f t h e stock s o l u t i o n c o n t a i n i n g 5 , 0 0 0 ppm Na and 2 , 0 0 0 ppm l i .  The s o l u t i o n was  then made t o volume w i t h pure water and s t o r e d i n p o l y ethylene b o t t l e s f o r flame photometric  analysis.  Flame Photometric Measurement The  sample s o l u t i o n s were r u n on a B a i r d Atomic model  KY-1 flame photometer, b r a c k e t e d by standard s o l u t i o n s ranging from 0 ppm t o 50 ppm i n approximately 4 ppm increments.  These standard s o l u t i o n s c o n t a i n t h e same  76  c o n c e n t r a t i o n of l i , solutions.  Na and H2SO4 as the sample  However, H2SO4 was not n e c e s s a r i l y  added t o thte standard s o l u t i o n s f o r b i o t i t e s o l u t i o n s because the i n t e r f e r e n c e e f f e c t o f H^04r. The  t  was n e g l i g i b l e i n t h i s case  (Dirom, 1965).  r e s u l t s were then r e c o r d e d on a 2-second  response,  10 m i l l i v o l t zero centre WESTON s t r i p  c h a r t recorded connected i n s e r i e s with*: photometer output.  A f t e r s t a n d a r d i z i n g t h e flame photometer  as recommended by the maker's i n s t r u c t i o n s > four runs f o r each a n a l y s i s were made i n a l t e r n a t i v e d i r e c t i o n . The r e s u l t s o f the four runs were then averaged. The  c o n c e n t r a t i o n i n t e r v a l between t h e higher and  lower standard s o l u t i o n s was^reasonably kept  small  so t h a t t h e r e l a t i omship between c o n c e n t r a t i o n o f potassium  i n t h e standard and sample s o l u t i o n s may  be c o n s i d e r e d l i n e a r .  The c o n c e n t r a t i o n o f potassium  i n the sample s o l u t i o n was then c a l c u l a t e d by using the f o l l o w i n g e q u a t i o n . x =  y - yl  (x2 - x l ) + x l  72 - y i where x - c o n c e n t r a t i o n o f potassium solution  i n the sample  x l a c o n c e n t r a t i o n of potassium standard s o l u t i o n  i n the lower  x2 = c o n c e n t r a t i o n o f potassium standard s o l u t i o n  i n the upper  77 y s d i a l r e a d i n g f o r the sample  solution  y l = d i a l r e a d i n g f o r the lower standard solution y2 » d i a l reading f o r t h e h i g h e r standard solution The D e t e r m i n a t i o n o f Argon Content i n M i n e r a l s The r a d i o g e n i c argon content i n m i n e r a l s was determined by mass s p e c t r o m e t r i c isotope d i l u t i o n t e c h n i q u e s . The argon a n a l y t i c a l system used f o r the present study c o n s i s t s of 3 main s e c t i o n s , as shown i n f i g u r e 3*1 • - the f u s i o n and f i r s t p u r i f i c a t i o n - the second p u r i f i c a t i o n  section  section  - the mass spectrometer s e c t i o n - the argon 38 spike metering s e c t i o n - the a i r c a l i b r a t i o n  section.  With a system of u l t r a h i g h vacuum metal v a l v e s , any o f these s e c t i o n s can be i s o l a t e d or connected p r o p e r l y . Except f o r the mass spectrometer s e c t i o n where an i r o n pump was  used, the o t h e r f o u r s e c t i o n s were evacuated  by mercury d i f f u s i o n pumps, backed by h i g h vacuum r o t a r y pumps.  F i v e p i r a n i gauges were a l s o added i n t o  the system f o r the measurement o f the p r e s s u r e o f the different  sections.  The Radiogenic Argon E x t r a c t i o n and P u r i f i c a t i o n s A sample charge o f from 0 . 5 t o 0 . 8 gram b i o t i t e ( i n the case o f hornblende and pyroxene, the charges used were from 2 t o 6 grams), c o n t a i n e d i n an alundum en-  78  c l o s e d molybdenum c r u c i b l e , was p l a c e d i n the f u s i o n j a r (water j a c k e t e d pyrex j a r ) ,  which,  by a copper gasketed f l a n g e c o u p l i n g , was connected t o the argon l i n e .  I t was  o v e r n i g h t baking at a temperature 200°C, which was hornblende biotite  by  of approximately  done o n l y i n the case of the  and pyroxene  sample.  I n the case of the  sample, overnight t a k i n g a t room tempera-  t u r e was The  outgassed  found s u f f i c i e n t l y  sample was  adequate.  heated by means o f a 6 KW  radio  frequency generator and i n d u c t i o n c o i l f i t around the f u s i o n j a r .  Gold trap and U t r a p c o n t a i n i n g  a c t i v a t e d c h a r c o a l were used and kept at temperat u r e of n i t r o g e n l i q u i d .  Hydrogen and  carbon  monoxide are o x i d i z e d t o become water and  carbon  d i o x i d e which t o g e t h e r w i t h the o t h e r i m p u r i t i e s were removed by t h e c o l d f i n g e r t r a p which p r e v e n t s the water from r e a c h i n g the c h a r c o a l t r a p ; n i t r o g e n was  removed by a l l o w i n g the gas t o r e a c t with the  hot t i t a n i u m sponge.  When the temperature  of the  sample a t t a i n e d approximately 1,000°C an Argon 38 spike was  admitted i n t o the f u s i o n and  first  p u r i f i c a t i o n s e c t i o n by means of two high-vacuum metal v a l v e s .  The temperature  of the sample  was  then r a i s e d g r a d u a l l y t o i t s f i n a l value o f approximately 1,500°C, and at t h i s temperature f u s i o n of the sample may  be c o n s i d e r e d as  the  complete.  79 However, i t was maintained at t h i s temperature f o r almost 15 minutes t o ensure the complete removal of argon from the sample.  A l l the argon was  then  absorbed by a c t i v a t e d c h a r c o a l at low temperature, (the  temperature of the n i t r o g e n l i q u i d ) .  To ensure t h a t the p u r i f i c a t i o n was  complete, the  sample was r e p u r i f i e d , p a r t i c u l a r l y i n the case of amphibole, by means of separate t i t a n i u m sponges and pumping system i n the f u s i o n and  first-purifi-  c a t i o n and second p u r i f i c a t i o n s e c t i o n s . was  The  system  so designed t h a t any number of p u r i f i c a t i o n  c y c l e s could be c o n v e n i e n t l y r e a l i z e d by a l l o w i n g the  gas back and f o r t h between the p u r i f i c a t i o n  sections.  However, a s i n g l e  cleanup c y c l e i s  found t o be adequate, e s p e c i a l l y i n the case o f the biotite  sample.  A f t e r the p u r i f i c a t i o n steps have been argon was  completed,  absorbed on charcoal t r a p at n i t r o g e n  l i q u i d temperature and the remaining gas  was  pumped away. Mass S p e c t r o m e t r i c Measurement Before each a n a l y s i s , l e a k t e s t i n g was c a r r i e d out to was  check t h e b u i l d - u p o f the mass 40 s i g n a l .  This  done s i m p l y by opening each s e c t i o n i n t u r n t o  80  the mass spectrometer.  A measure of the r e l a t i v e  abundance o f argon 40 and argon 36 contained i n the sample enabled one t o make a c o r r e c t i o n f o r any contamination of the sample by atmospheric The p u r i f i e d gas from the second s e c t i o n was  argon.  purification  then admitted t o the mass spectrometer  s e c t i o n of the system t o measure i t s i s o t o p i c composition. The mass spectrometer used f o r the p r e s e n t study i s a e l i - n i e a l model MS10,  made by A s s o c i a t e d  E l e c t r i c I n d u s t r i e s L t d . , u s i n g a two i n c h r a d i u s of c u r v a t u r e with 180° d e f l e c t i o n and a permanent magnet producing a magnetic I83O  f i e l d of approximately  gauss.  Some s l i g h t m o d i f i c a t i o n s have been made, f o r example a more s e n s i t i v e v i b r a t i n g reed e l e c t r o m e t e r amplifier with a 1 0 H  ohm  the maker's e l e c t r o m e t e r . was  input r e s i s t o r t o replace The mechanical  scanner  a l s o r e p l a c e d by an e l e c t r o s t a t i c scanner which  causes the a p p r o p r i a t e v a r i a t i o n s i n a c c e l e r a t i n g voltage.  T h e r e f o r e any mass peak may  be reached  immediately, the r e c o r d i n g time becomes s h o r t e r and the s i g n a l decay becomes consequently n e g l i g i b l e . Other d e t a i l e d d e s c r i p t i o n s may maker's i n s t r u c t i o n s c a t a l o g u e .  be found i n the A Honeywell  -  81  Brown potentiometer-type recorder, o f \ second ponse, 10 m i l l i v o l t  fall  scale,  res-  recordedthe r e s u l t s .  Mass spectrometry s t a t i c method was used.  A small  f r a c t i o n of the t o t a l gas sample was admitted t o the ma spectrometer tube which was then i s o l a t e d by an u l t r a h i g h vacuum metal v a l v e and analyzed. As the mass spectrometer i s maintained c o n t i n u o u s l y at a p r e s s u r e o f l e s s than 10-8 mm Hg, by means of an l i o n pump,  the vacuum i n s i d e the mass s p e c t r o -  meter tube was proved s i g n i f i c a n t l y s u f f i c i e n t f o r the s t a t i c a l a n a l y s i s t o be c a r r i e d on. Before each r u n , the background  was checked; i t was  observed t h a t the argon 3 6 and argon 38 background was reduced t o minimum so t h a t they were n e a r l y undetectable. t o minimum  Argon 40 background  was a l s o reduced  so t h a t i t was o n l y d e t e c t a b l e on t h e  most s e n s i t i v e range.  Only mass 28 s i g n a l which  i s supposed t o be due t o n i t r o g e n and carbon monoxide was s i g n i f i c a n t .  Mass 44 s i g n a l which i s supposed  t o be due t o carbon d i o x i d e was a l s o reduced t o minimum. I t was found t h a t an amount of between 5 and 10 v o l t s of argon 40 s i g n a l admitted t o the mass spectrometer i s e f f e c t i v e l y adequate -for the a n a l y s i s t o be c a r r i e d out.  Figure A-I-2  :  F r a c t i o n a t i o n c o r r e c t i o n vs % admitted t o the spectrometer tune.  8? I s o t o p i c r a t i o s Ar40/Ar38  and Ar  at l e a s t  10 times f o r each sample, were measured and averaged. Before b e i n g used i n t h e age c a l c u l a t i o n these r a t i o s had t o be s u b j e c t e d t o two c o r r e c t i o n s :  discrimination  c o r r e c t i o n and f r a c t i o n a t i o n c o r r e c t i o n . D i s c r i m i n a t i o n i n the mass spectrometer has been e v a l u a t e d by a n a l y z i n g atmospheric A c c o r d i n g t o Graham's Law, i s o t o p i c i s caused by t h e slower passage  argon. fractionation  of the h e a v i e r isotope  i n t o the l e a k v a l v e r e l a t i v e t o the l i g h t e r In  isotope.  order t o have the f r a c t i o n a t i o n at the i n l e t l e a k  v a l v e e v a l u a t e d , a check of the t o t a l  quantity of  argon sample admitted t o the mass spectrometer was always made.  With the designed system used i n the  present study, t h i s r e q u i r e s no d i f f i c u l t i e s . p o r t i o n of gas sample of approximately  A  i n volume  of the second p u r i f i c a t i o n s e c t i o n , could be measured a c c u r a t e l y w i t h mass spectrometer. A graph showing f r a c t i o n a t i o n c o r r e c t i o n a g a i n s t the percentage o f q u a n t i t y o f gas admitted t o the mass spectrometer f o r a n a l y s i s i s g i v e n i n f i g u r e  I  A-I-2.  84 APPENDIX I I NELSON PLUTONIC ROCKS DESCRIPTIONS OF SPECIMENS FOR (PREPARED BY  A . J . SINCLAIR)  A-66:  Specimen The iorite  DR.  K-VDATING  rock i s a massive, medium-grained, l e u c o c r a t i c granod-  of the V a l h a l l a p l u t o n i c  specimen.  s u i t e t h a t appears  A t h i n s e c t i o n mode i s :  plagioclase  f r e s h i n hand  (50%), q u a r t z  (24%), K - f e l d s p a r - p e r t h i t i c m i c r o c l i n e (15%), b i o t i t e blende  (1%),  sphene (1%),  (1%),  sericite carbonate  opaque m i n e r a l s (1%),  museovite  and t r a c e amounts of e p i d o t e , c h l o r i t e ,  and z i r c o n .  The  r o c k has  (5%),  horn-  and  apatite,  undergone v e r y s l i g h t d e f o r -  mation as i n d i c a t e d by shadowy e x t i n c t i o n of q u a r t z and g e n t l y curved cleavage t r a c e s of some b i o t i t e g r a i n s .  Some a l t e r a t i o n  e f f e c t s are apparent but these are not pronounced. f r e s h i n appearance. minerallic grains.  Biotite i s  A s m a l l amount o f c h l o r i t e i s present as monoL o c a l l y , p l a g i o c l a s e has been a l t e r e d t o e p i d o t e ,  c a l c i t e and s e r i c i t e , although most o f the e p i d o t e i n the rock occurs as f a i r l y l a r g e , anhedral g r a i n s a s s o c i a t e d w i t h C-66:  Specimen The  hornblende.  specimen of Nelson p l u t o n i c rock i s a medium - t o coarse-  g r a i n e d , massive hornblende minerals  (mainly hornblende)  quartz monzonite c o n t a i n i n g 10% mafic and  90% f e l s i c c o n s t i t u e n t s (mainly  f e l d s p a r ) w i t h a few percent s u b h e d r a l , K - f e l d s p a r phenocrysts t o 15 mm.  i n length.  plagioclase-Ango hornblende  A mode determined  from t h i n s e c t i o n i s :  (47%), K - f s l d s p a r - m i c r o c l i n e ( 3 0 % ) , quartz  (5%), b i o t i t e  up  (15%),  ( 3 % ) , and t r a c e amounts of myrmekite,  85  a p a t i t e , sphene and opaque m i n e r a l s . superimposed a l t e r a t i o n  There i s no evidence o f  effects.  Specimen D - 6 6 : The rock.  r o c k i s a f i n e - g r a i n e d , g r e y dyke t h a t c u t s Nelson p l u t o n i c  Rock-forming m i n e r a l s c o u l d not be i d e n t i f i e d i n hand  men although a minor amount o f c h a l c o p y r i t e was observed. determined  from t h i n s e c t i o n i s as f o l l o w s :  clinopyroxene  (50%), b i o t i t e  speci-  A mode  p l a g i o e l a s e - A n ^ i (56%),  (12%), o l i v i n e and i d d i n g s i t e (2%)  and t r a c e amounts o f a p a t i t e and opaque m i n e r a l s .  Plagioelase  occurs m a i n l y as lath-shaped c r y s t a l s showing vague p i l o t a x i t i c texture.  O l i v i n e and i d d i n g s i t e occur as r e l i c t  anhedral  cores  texture.  The specimen shows no e f f e c t s of a l t e r a t i o n apart  from  what can be a s c r i b e d t o d e u t e r i c p r o c e s s e s . Specimen K - 6 6 : The blende  specimen i s a medium-grained, massive, p o r p h y r i t i c  quartz monzonite from Mount C a r l y l e s t o c k c o n t a i n i n g abun-  dant p a l e p i n k phenocrysts  o f K - f e l d s p a r up to 2 . 5 cm. i n l e n g t h .  A medium-grained m a t r i x c o n s i s t s predominantly  of plagioelase,  quartz and about 10% hornblende as narrow prisms A mode estimated from t h i n s e c t i o n i s : cline  horn-  (45%), p l a g i o c l a s e - A n 2 0  epidote-including a l l a n i t e  K-feldspar-perthitic  long. micro-  (37%), quartz (10%), hornblende (5%),  (1%), sphene (1%), b i o t i t e  t r a c e amounts of myrmekite, c h l o r i t e , carbonate, and opaque m i n e r a l s .  about 2 mm.  ( 0 . 5 % ) and  apatite,  A l t e r a t i o n e f f e c t s are minor.  zircon  M i n e r a l s are  f r e s h i n appearance, although v e r y s m a l l amounts o f c h l o r i t e are p r e s e n t , a p p a r e n t l y pseudamorphous a f t e r  biotite.  86 Specimen S i l v - 6 6 : The  r o c k from t h e northwest corner  o f the Nelson B a t h o l i t h  i s a c h l o r i t i z e d hornblende quartz monzonite, medium-grained, c o n t a i n i n g about 30% mafic c o n s t i t u e n t s . up t o 1 cm. i n maximum dimension. seen i n the hand specimen.  Rare phenoerysts e x i s t  Trace amounts of p y r i t e can be  The specimen l o c a l i t y i s on t h e e a s t e r n  edge of a major f r a c t u r e zone t h a t forms t h e western contact o f the n o r t h e r n p a r t o f t h e Nelson B a t h o l i t h . t i o n i s recognized quartz-microcline the zone.  Extensive  along t h i s zone and south o f the sample  The rock has undergone ^considerable  m a t e r i a l t h a t has been r e c r y s t a l l i z e d . s e c t i o n i s as f o l l o w s :  (1%)  locality  pegmatites have been i n t r o d u c e d l o c a l l y w i t h i n  20 pereent of t h e specimen i s v e r y f i n e - g r a i n e d  cline  chloritiza-  (30%), hornblende  quartz  cataclastic  A mode estimated  plagioclase-An^ (20%),  cataclasis—about  (30%),  K-feldspar-micro-  (15%), c h l o r i t e  and t r a c e amounts o f e p i d o t e , a p a t i t e and opaque  A l t e r a t i o n i s extensive  from t h i n  (3%),  sphene  minerals.  i n t h a t b i o t i t e has been completely r e -  p l a c e d by pseudomorphs of c h l o r i t e . a l t e r a t i o n s are apparent.  No other e f f e c t s of chemical  Hornblende i s undeformed and f r e s h i n  appearance. Specimen R - l l : The with  s u b p a r a l l e l alignment o f m a f i c m i n e r a l s  crystals. long.  specimen i s a medium-grained, melanocratic  I t contains  A mode estimated  K-feldspar  granodiorite  and t a b u l a r f e l d s p a r  rare phenocrysta o f K - f e l d s p a r  up t o 1 . 5 cm  from t h i n s e c t i o n i s : p l a g i o c l a s e  (15%), hornblende  (15%), b i o t i t e  (10%), quartz  (50%), (10%),  87 and t r a c e amounts of e p i d o t e , sphene, a p a t i t e , myrmekite, c h l o r i t e ;  and opaque m i n e r a l s .  A l t e r a t i o n e f f e c t s are s l i g h t w i t h o n l y  t r a c e amounts o f c h l o r i t e o c c u r r i n g as i n t e r l e a v e s w i t h some biotite  crystals.  Specimen R-12: The rock i s a medium - t o c o a r s e - g r a i n e d m e l a n o c r a t i c quartz d i o r i t e w i t h vague f o l i a t i o n o f mafic m i n e r a l s ( b i o t i t e and hornblende).  The specimen i s f r e s h i n appearance.  A r e l a t i v e l y un-  common f e a t u r e compared w i t h most Nelson p l u t o n i c rocks i s t h e absence of K - f e l d s p a r phenocrysts.  A mode determined  s e c t i o n i s : p l a g i o e l a s e (60%), clinopyroxene quartz  (10%), K - f e l d s p a r ( 5 % ) , a p a t i t e  t r a c e amounts o f sphene.  from t h i n  (12%), b i o t i t e  ( 2 % ) , hornblende  (10%),  ( 1 % ) , and  The rock i s g e n e r a l l y f r e s h i n appearance.  A s l i g h t d u s t - l i k e , dark a l t e r a t i o n of p l a g i o e l a s e i s d i s t r i b u t e d s p o r a d i c a l l y but i s not e x t e n s i v e . Specimen R-l3 : The  rock, a medium-grained g r a n o d i o r i t e from the Nelson  B a t h o l i t h , has r a r e K - f e l d s p a r phenocrysts f a i n t f o l i a t i o n i s e v i d e n t due t o alignment tabular K-feldspar phenocrysts.  up to 2 cm. l o n g .  of m a f i c m i n e r a l s and  M a f i c m i n e r a l s ( b i o t i t e and horn-  blende) account f o r about 20% o f rock volume. o v e r a l l grey c a s t e and appears s e c t i o n examination  A  fresh.  A mode e s t i m a t e d from  i s : plagioclase-An25  (15%), K - f e l d s p a r (12%), hornblende  The specimen has an thin  (40%), quartz (25%), b i o t i t e  ( 5 % ) , epidote  (2%) and t r a c e  amounts o f c h l o r i t e , c l i n o p y r o x e n e , myrmekite, sphene, a p a t i t e , s e r i e i t e and opaque m i n e r a l s .  The o n l y evidence of a l t e r a t i o n i s  l i m i t e d f o r m a t i o n o f s e r i e i t e and epidote i n cores of some p l a g i o e l a s e g r a i n s , and some i n t e r l e a v e s o f c h l o r i t e i n b i o t i t e  crystals.  88  Specimen L - 2 3 6 : The  specimen i s a f i n e - g r a i n e d , g r e y h o r n f e l s taken  i n c l u s i o n w i t h i n Nelson p l u t o n i c r o c k s .  from an  O r i g i n a l c h a r a c t e r of  the r o c k i s u n c e r t a i n although i t may r e p r e s e n t metamorphosed and metasomatized rock o f t h e S l o c a n S e r i e s . alignment  of h i o t i t e  A vague  suhparallel  c r y s t a l s can he seen i n t h i n s e c t i o n .  Exam-  i n a t i o n o f a t h i n s e c t i o n r e v e a l e d the f o l l o w i n g mode: p l a g i o c l a s e (64%), b i o t i t e  ( 2 0 % ) , quartz (10%), K - f e l d s p a r - m i c r o c l i n e ( 2 % ) ,  (2%), apatite  epidote  and opaque m i n e r a l s .  ( 2 % ) , and t r a c e amounts o f sphene, s e r i c i t e The specimen i s f a i r l y f r e s h i n appearance  although a s i g n i f i c a n t p r o p o r t i o n (about 15%) o f t h e b i o t i t e has ranged  edges and appears  almost opaque a l o n g some cleavage  s u g g e s t i n g t h a t i t i s i n the i n i t i a l  planes  stages of a l t e r a t i o n .  Specimen L - 2 5 7 : The  specimen i s a quartz d i o r i t e porphyry from a d y k e - l i k e  i n t r u s i o n i n rocks o f - t h e S l o c a n S e r i e s and i s c o n s i d e r e d a " s a t e l l i t e " o f the Nelson b a t h o l i t h t o the south. about 85% medium - t o c o a r s e - g r a i n e d phenocrysts matrix.  Plagioclase i s the only i d e n t i f i a b l e  hand specimen.  I t consists of i n a f i n e grained  f e l s i c mineral i n  S c a t t e r e d anhedral b l e b s o f p y r i t e can be seen and  the r o c k e f f e r v e s c e s on treatment w i t h d i l u t e EG1 i n d i c a t i n g t h e presence  of c a l c i t e .  About 5% m a f i c m i n e r a l s can be seen, o n l y  r a r e l y i d e n t i f i a b l e as b i o t i t e . investigation i s plagioclase-An2g calcite  (3%), chlorite  A mode determined (67%),  ( 2 % ) , muscovite  from t h i n  section  quartz ( 2 0 % ) , b i o t i t e ( 3 % ) ,  and s e r i c i t e  (2%), apatite  (1%), K - f e l d s p a r - m i c r o c l i n e (1%) and t r a c e amounts o f sphene, horn-  89  blende, e p i d o t e and opaque m i n e r a l s . a l t e r e d with c h l o r i t e after biotite,  The rock has been extensively-  h a v i n g formed p a r t i a l and complete pseudomorphs  and p l a g i o e l a s e h a v i n g been a l t e r e d e x t e n s i v e l y t o  c a l c i t e , e p i d o t e and s e r i e i t e .  

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