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K[40] - Ar[40] isotopic age determination of the Nelson batholith, B.C. 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 THESIS SUBMITTED I N PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department o f GEOPHYSICS We accept t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1968 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make it freely available for reference and Study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by h.ils representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of Brit ish Columbia Vancouver 8, Canada Date A l ^ . f i i ABSTRACT The purposes of t h i s thesis are to examine ages of the Nelson bat 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 previously published i n the l i t e r a t u r e , and to test hornblende and pyroxene further f o r t h e i r applica- t i o n i n K 4 0 - A r 4 0 dating as r e l i a b l e minerals. K4-0 - A r 4 0 model ages obtained on b i o t i t e , hornblende and clinopyroxene from the present study indicate with certainty that the various phases of the Nelson bat h o l i t h were emplaced during a short span of time, centered around 156 m.y. corres- ponding to the Upper-Middle Jurassic boundary of the Kulp Ts time scale. At le a s t a period of hydrothermal a l t e r a t i o n has occurred i n t h i s area since that time. It i s evident that these phases cannot be distinguished on the basis of t h e i r K4-0 _ A r 4 0 model ages. I t i s also evident that hornblende gives re- l i a b l e model ages. Clinopyroxene contains s i g n i f i c a n t excess of radiogenic argon and should not be used f o r K 4 0 - Ar40 dating. B i o t i t e may occasionally contain excess of radiogenic argon, p a r t i c u l a r l y under high temperature and high argon pressure environmental conditions. A n a l y t i c a l techniques used and descriptions of the samples analyzed are given as appendixes. i i i 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 1.1 Introduction 1 1.2 The K 4 0 - A r 4 0 Method of Age Determina- t i o n 3 1 .2.1 Introduction 3 1 . 2 . 2 K 4 0 - A T 4 0 Model Age 3 1 . 2 . 2 . 1 Decay Constants 6 1 . 2 . 2 . 2 Excess of Radiogenic Argon 8 1 . 2 . 2 . 3 Argon loss hy D i f f u s i o n 11 1 . 2 . 3 Materials Used for K 4 0 - A r 4 0 Dating 17 CHAPTER I I . GENERAL GEOLOGY OF THE NELSON BATHOLITH ... 20 2.1 Introduction 20 211.1 History of Literature 20 2 . 1 . 2 Composite Nature 20 2.2 Different Phases of the Nelson Batholith. 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 i v PAGE 2.3 O r i g i n Due t o Metasomatism o f P a l e z o i c and E a r l y M e s o z o i c ..... 27 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 S t r u c t u r e ;... 28 2 . 5 P o s t - C r y s t a l l i z a t i o n H i s t o r y 30 CHAPTER I I I . PHYSICAL DETERMINATION 32 3.1 P r e v i o u s Work: 32 3.2 C u r r e n t P r o j e c t 34 3 . 2 . 1 I n t r o d u c t i o n 34 3 . 2 . 2 The K40 - Ar40 R e s u i t s , 37 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 3.3 D i s c u s s i o n 31 CONCLUSION 61 BIBLIOGRAPHY 63 APPENDIX I - EXPERIMENTAL PROCEDURE 70 APPENDIX I I - DESCRIPTIONS OF SPECIMENS FOR K 4 0 _ A r 4 0 DATING 84 V L I S T OF FIGURES PAGE FIGURE 1-1 Decay Scheme o f K 4 0 4 FIGURE 1 - 2 Schematic P l o t of L o g D Vs. 1 .... 14 a2 T FIGURE 1 - 3 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 D i s t a n c e from an I n t r u s i v e C o n t a c t 17 FIGURE 2-1 L o c a t i o n Map o f K 4 0 _ A r 4 0 Samples. 21 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 hy F i e l d Geology 26 FIGURE 3-1 U.B.C. Argon E x t r a c t i o n and A n a l y t i c a l System 36 FIGURE 3-2 P l o t o f L o g A r 4 0 Vs. K I s o c h r o n s .. 38 FIGURE 3 -3 P l o t o f Frequency Vs. L o g Apparent Ages 39 FIGURE 3-4 T y p i c a l A t m o s p h e r i c Argon 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 F r a c t i o n a t i o n C o r r e c t i o n 82 r v i LIST OF TABLES PAGE TABLE 3 . 1 K 4 0 - A r 4 0 ANALYTICAL DATA 4 0 TABLE 3 . 2 U.B.C. RESULTS ON STANDARD MINERALS 4 4 TABLE 3 . 3 INTERLABORATORY RESULTS 4 3 TABLE 3 . 4 AGES OF VARIOUS ROCK UNITS 4 9 TABLE 3 . 5 RELIABLE MODEL AGES 5 0 v i i ACKNOWLEDGEMENTS The writer wishes to express h i s sincere thanks to Dr. T.J. Ulrych who supervised the research and whose guid- ance and enthusiasm has contributed immeasurably to this study. The writer also wishes to thank Dr. W.F. Slawson f o r his supervision during the absence of Dr. Ulrych and fo r h i s help i n formulating the present thesis. P a r t i c u l a r thanks are due to Drs. A.J. S i n c l a i r and W.G. Libby, of the Department of Geology, for supplying the samples and f o r t h e i r many h e l p f u l suggestions, and to Mr. J.E. Harakal for h i s valuable assistance. A teaching assistantship offered to the writer by the Department of Physics was sincerely appreciated. Sincere appreciation i s due to Professor J.A. Jacobs who made i t possible f o r the writer to come to the University of B r i t i s h Columbia. The writer also wishes to express his deep gratitude t o the authorities of both governments of Vietnam and Canada f o r the r e a l i z a t i o n of t h i s thesis. 1 CHAPTER I K 4 0 - A r 4 0 METHOD OF AGE DETERMINATION' 1.1. Introduetion Isotope geophysics is one of the main fields of research i n the Departments of Geophysics and Geology of the University of British Columbia. In parallel with the extensive research i n the variation of lead isotopes and the isotopic composition of common strontium, the potassium-argon studies are currently under pro- gress. The potasslum-argon laboratory was constructed early i n 1963 and became operational late i n 1964. The dual objectives are to investigate further the techniques of the potassium- argon method and to help unravel the history of complex geo- logic areas. Today, more than 200 samples have been analyzed i n the laboratory. The ages obtained range from less than 10 million years to greater than 3,000 million years. The main interest of the present writer has been to apply these f a c i l i t i e s to solve the geologic history of a ̂ complex area. The Nelson Batholith, a massive grmite pluton between Kootenay and Slocan Lakes, i s in ao uthern British Columbia. It i s exposed over an area of approximately 900 square miles. 2 The major portion of the Nelson Batholith l i e s within the area covered by the' Geological Survey of Canada, map 1090A on a scale of four miles to the inch ( L i t t l e , I960), or by the Geological Survey of Canada, map 272A on a scale of one mile t© the inch (Cairnes, 1934). (a) The purposes of the present study are: To apply the f a c i l i t i e s provided i n the potassium- argon laboratory f o r dating b i o t i t e s , hornblendes, pyroxene and oogenetic minerals. To te s t hornblende and pyroxene further f o r t h e i r a p p l i c a t i o n i n potassium-argon age determination as r e l i a b l e minerals. To determine the potassium-argon ages of the Nelson Batholith with the aim of obtaining more information. on geo- l o g i c h i s t o r y . (b) The Nelson Batholith was selected for invest i g a t i o n because: Lead isotopes studies of the area have been i n v e s t i - gated recently by P. Reynolds (1967). In addition, - . S i n c l a i r (1964), Kanasewich (1962), Leech and Wanless (1962) investigated lead isotopes on the areas around Nelson, and: The area contains economically important ore deposits. 3 The stratigraphy and structure of the area i s rea- sonably well-known ( L i t t l e , I960). Current f i e l d works have been carried out hy members of Geology Department ( S i n c l a i r & Libby, 1967). Potassium-argon age determinations c a r r i e d out by the Geological Survey of Canada have been made only on b i o t i t e s . Moreover, the published data by Gabrielse and Reesor (1964) exhibit abroadrange between 49 to 171 m.y.. 1.2 The K 4 0 - A r 4 0 method of age determination. 1.2.1 Introduction Isotopic age determination by means of radioactive decays was f i r s t suggested by Rutherford i n 1904. He postulated that the r a t i o of uranium to helium i n a mineral should be time de- pendent. Subsequent investigations by Rutherford and others (Thompson, 1905$ Campbell and Wood,;i906), indicated that helium i s usually p a r t i a l l y l o s t by the mineral making the time of "geological clock" unreliable. Thompson (1905) and Campbell and Wood(1906), f i r s t proved the radioactive decay of potassium to calcium. I t was not un- t i l 1937 when Von Weizsactter discovered the branching decay of K40 to A r 4 0 . This provided a promising p o s s i b i l i t y of two "geological clocks" within a single mineral. * m.y.: m i l l i o n years 4 Potassium c o n s i s t s of three n a t u r a l l y o c c u r r i n g isotopes w i t h the mass numbers 3 9 , 40 and 41. Only potassium 40 i s r a d i o a c t i v e . The percent atomic abundances of the i s o t o p e s of potassium are: K 3 9 = 93.08 i 0.04; K 40 -. 0.01181 ± 0.0001; K41 a 6.91 ± 0.04; (N i e r , 1950; Goldich et a l v 1 9 6 l ; A.E.0. Nuclear Data Tables, 1959). The i s o t o p i c eompsosition of a i r argon was also determined by Nier (1950): Ar?6 s 0.337%; Ar38 - 0 . 0 6 3 % ; Ar40 = 99.600%. Potassium decays, by both beta emission and o r b i t a l e l e c t r o n capture, t o calcium 40 and argon 40 r e s p e c t i v e l y , ( F i g . 1.1). F i g . 1.1 : Decay scheme of K40 ( a f t e r A l d r i c h & W e t h e r i l l , 1938). The equations f o r the decay can be w r i t t e n as f o l l o w s : K40 i - e° —>• A r 4 0 -+• If + X-ray or Auger e l e c t r o n K40 » C a 4 0 + I t i s known that approximately 89 pereent of potassium 40 decays to c a l c i u m 40 hy beta emission but the K40 - C a 4 0 method has been of l i m i t e d use. This/sdue 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 40 forms 97 pereent of common calcium, t h e r e f o r e the small amount of radi o g e n i c calcium 40 i s very d i f f i c u l t t o d i s t i n g u i s h from the l a r g e r amount o f common calcium. Moreover, Polevaya et a l , , (I960), mentioned that the K 4 0 - C a 4 0 method can only be used f o r r o u tine age determination i f the f o l l o w i n g problems are s a t i s f a c t o r i l y solved: l ) f i n d a s u i t a b l e method of se p a r a t i o n f o r s m a l l amount of Ca from K;2)the Ca-44 i n n a t u r a l calcium must be p r e c i s e l y determined;3) a "•powerful" mass spectrometric technique f o r the determination of t r a c e Ca. This i s not the case i n the Ar^method,where the anount of common argon contained i n minerals and rooks i s v e r y low and the measurement of argon, which i s one of the r a r e gases, i s much e a s i e r . Broad time ranges are measurable w i t h the K40 - A r 4 0 method because potassium 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 the potassium-argon h a l f - l i f e i s very long- * •-• 1.2.2 K 40 - A r 4 0 model age The K40 - A r 4 0 method of i s o t o p i c age determination has been developed during the past f i f t e e n years t o a h i g h degree of s o p h i s t i c a t i o n . With 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 of very young age and low potassium content can now be dated 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 (Gale et aL, 1 9 6 6 ; C u r t i s , 1 9 6 & ) . 6 T h i s makes the K*® - A r 4 u method a w i d e l y used t o o l i n modern ge©chronology. However, the accuracy i n a n a l y t i c a l p r o v i d techniques supplying a p r e c i s e and accurate determination of the AT40/K40 r a t i o , i s only a necessary but not s u f f i c i e n t c o n d i t i o n . The K40 - Ar40 "ages 0 are meaningful i f and o n l y 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 con- s 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 excess radiogenic argon - 40, i . e . no radiogenic argon was i n c l u d e d i n the sample at the time i t was formed ;3)the sample must have been a c l o s e d system, i . e . no l o s s or g a i n of potassium or r a d i o g e n i c argon has occurred since i t was formed, other than by r a d i o a c t i v e decay of potassium 40. The consistency of r e s u l t s w i l l be a key point t o check whether or not these assumptions are f u l f i l l e d . 1.2.2.1 Decay constants As mentioned above, K4-0 ± 3 r a d i o a c t i v e and decays by K - e l e c t r o n capture t o Ar^O and by beta emission t© Ca40. The determination of these decay constants i s d i f f i c u l t . However, a knowledge of these i s a b s o l u t e l y necessary t o p r e d i c t the accumulation of radiogenic argon i n a potash 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 constants and the branching r a t i o , p a r t i c u l a r l y during the p e r i o d between 1950 and I960. Extensive reviewing of the a v a i l a b l e data were given by Smith (1964) and Houtermans (1966). Two methods have been used f o r t h i s purpose: the p h y s i c a l or counting method; and the g e o l o g i c a l method. 7 Physical determination This method i s based on d i r e c t counting measurements. As no suitable Remitting standards are available, the measurement of the emission of 1 .46 MeV rad i a t i o n i s quite d i f f i c u l t . However, i n making use of Co^O and N a 2 4 standards and by extrapolation technique Wetherill (19 57), a r r i v e d to determine the s p e c i f i c gamma a c t i v i t y . A value of 3 . 3 9 - 0.12 S/g sec. which corresponds to a K - electron capture decay constant Ae of O.585 x 10-10 y r - 1 has been obtained. Due to the maximum of the high beta energy of 1 .34 MeV, the determination of the beta decay constant i s l e s s d i f f i - c u l t than the previous case. A s p e c i f i c beta a c t i v i t y of 27.6 ^/g s e c , corresponding to a beta-decay constant A^ of 4 .72 x 1 0 - 1 0 y r - 1 has been obtained. Indt and Kluyver (1954), A l d r i c h and Wetherill (1958 )v' reviewed the a v a i l - able physical measurements and suggested values of O.585 x 1 0 - 1 0 y r - 1 for p a r t i a l electron capture decay constant "V" and 4 . 7 2 x 10-10 y r - l f 0 r , the p a r t i a l beta decay con- stant. I t should mention that a new determination of the s p e c i f i c beta a c t i v i t y has been made by Glendenin (1961) . By usingvliquid s c i n t i l l a t o r counting technique, Glendenin obtained a value of 28.2 ± 0 . 3 f/g sec. However, most laboratories have continued to adopt the values suggested by A l d r i c h and We t h e r i l l . 8 Geological determination By determining the production o f radiogenic A r 4 - 0 i n minerals of known age, the geological method may y i e l d s a t i s f a c t o r y values for the decay constants. The best values obtained by t h i s method areinquite i^a good agree- ment with physical counting determinations (Wetherill et a l . , 1956). The work of Wetherill et a l . . (1956) yielded a s p e c i f i c gamma a c t i v i t y of 3.24 ± 0.15 Vg sec. and a s p e c i f i c beta a c t i v i t y of 27.4 f / g s e c , corresponding to a beta decay constant of 4 . 7 0 x 10-10 y r . - l , and K - electron capture decay constant \ of 0 . 5 5 7 - 0.026 x 10-10 yr-1. E a r l i e r investigations using geological methods included the work of Russell^tl953), Wasserburg and Hayden (1954), S h i l l i b e e r et al. (1954). However, t h e i r determinations yielded a too low branching r a t i o . The reason f o r t h i s was found out l a t e r to be due to the i n - complete extraction of argon or the closed system condition was not s a t i s f i e d . 1.2.2.2 Excess of radiogenic argon some It i s known that invnatural environments; p a r t i c u l a r l y at great depth within the crust or where high argon pressure might develop, excess radiogenic argon i s present. Mineral phases which c r y s t a l l i z e i n these environments are more or le s s sub- ject to t h i s argon "contamination". The excess of radiogenic argon, occluded by the minerals at the time of c r y s t a l l i z a t i o n i s probably held i n the c r y s t a l i m p e r f e c t i o n s such as d i s l o c a t i o n s or s t r u c t u r a l h o l e s . The excess of radiogenic argon i n pyroxene has been observed by many i n v e s t i g a t o r s . Hart and Dodd (1962) found s u r p r i s i n g l y h i g h n a g e s n f o r pyroxenes from a pyroxene gneiss and an amphi- b o l i t e . They concluded t h a t these pyroxenes have i n c o r p o r a t e d excess 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 or during a subsequent r e c r y s t a l l i z a t i o n . ^cDougall and Green (1964) dating pyroxenes from the Norwegian e c l o g i t e s found a l s o the occurrence of excess argon i n these samples.A S i m i l a r r e - s u l t has been reported by A l l s o p p (1965) f o r pyroxene from the great dyke of Southern Rhodesia. However, McDougall (1963), and Stern et a l . (1965) reported 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. More complete degassing might be expected i n low pressure environments so t h a t these observations are not i n c o n f l i c t w i t h the occurr- ence of excess of radiogenic argon i n pyroxene, from deep-seated environments. Damon and Kulp (1958) observed the excess of 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 excess of radiogenic argon i n amphiboles i s p o s s i b l e . Funkhouser et a l . (1965) reported a l s o excess argon i n hornblende from young Hawaiian rocks. The excess of radi 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 micas, has been concluded by Rama et a l . (1965). However, Evernden and Richards (1962), Hart (i960, 1961) and G e r l i n g et a l (1965) d i d not prove any evidence of excess argon i n amphiboles. They concluded that amphiboles may give 10 r e l i a b l e K*0 - A r 4 0 "ages". In 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 4 0 - A r 4 0 d a t i n g , Hart (1964), A l d r i o h ot a l 4 i 9 W and G e r l i n g et a l (1965) showed t h a t amphiboles are 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 , through d i f f u s i o n than micas because the former have a higher r a d i o g e n i c argon d i f f u s i o n t h r e s h o l d temperature ( f i g . l - 3 ) . The most unfortunate i s the p o s s i b i l i t y of excess of radiogenic argon i n hornblende and mica, the two most commonly used fo i " K 4 0 - A r 4 0 d a t i n g (Hunt ,1962; Leech e t a l . , 1 9 6 3 ) . This r a i s e s the question whether, i n c e r t a i n cases, the higher argon r e t e n t i v i t y i n hornblende represents i n f a c t a compensa- t i n g e f f e c t between the excess and l o s s of argon. I n ge n e r a l , one favours the i d e a t h a t micas do not c o n t a i n excess of r a d i o - genic argon. However, g e o l o g i c a l and experimental evidences show t h a t i n some oases they do ( S t o c k w e l l , 1963; Pepin et a l . , 1 9 6 4 ) . Richards and Pi«geon (1963) reported t h a t some b i o t i t e samples of 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 twice the Rl^Sr^ages of the same phase. Excess radiogenic argon i n phlogopite and b i o t i t e from young Hawaiian rocks has been reported by Funkhouser et a l . (1965). p i e e r r o r i n K 4^ - A r 4 ^ d a t i n g , caused by the excess argon, w i l l be very s e r i o u s i f low potassium content minerals such as pyroxene, quartz, f l u o r i t e e t c . are 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 these minerals 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 or vacancy d e f e c t s . 11 P o s s i b l e occurrence of excess of r a d i o g e n i c argon i n datable minerals suggest t h a t the i n t e r p r e t a t i o n o f K 4 0 - A r 4 0 data 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 excess helium may be used as a good i n d i c a t i o n of*>excess ra d i o g e n i c argon (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 obtained when the s t a b i l i t y o f the A r 4 0 rati© i n minerals possessing d i f f e r e n t K40" potassium content i s assured. This 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 p a i r s , t o t e s t the r e l i a b i l i t y and the usefulness of the d a t i n g m a t e r i a l s . The use of other r a d i o m e t r i c methods, f o r example; the R b ^ Sr^ or l e a d - uranium methods w ould be another a l t e r n a t i v e . I n cases where oogenetic 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 assessed from i n t e r n a l c onsistency between the r e s u l t s and the known geology. 1.2.2.3 Argon l o s s by d i f f u s i o n Due t o i t s great importance i n the £.40 - A r 4 0 method, the problem of d i f f u s i o n of r a d i o g e n i c argon w i l l be discussed ±o a greater extent i n the f o l l o w i n g p a r t of t h i s chapter* One knows, th a t the argon l o s s is. dependent on the mineral and rock type. While micas are the most s u i t a b l e minerals 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 , Folinsbee e t a l . . (1936); Zartman, (1964). Dates obtained from whole rock samples i n which an appreciable amount of potassium i s i n f e l d s p a r s are t h e r e f o r e suspect. E r i c k s o n and Kulp (1961) a p p l i e d the whole 12 rock technique to the basic rocks from Palisades s i l l and showed that the argon r e t e n t i v i t y i s much better i n the f i n e r than i n the coarser grained f a c i e s . In general, the discordance i n K 4 0 - A r 4 0 i s o t o p i c "ages" can most often and most e a s i l y be explained by daughter product d i f f u s i o n . D i f f u s i o n i s defined by the American Geological I n s t i t u t e , "A process of spreading out of molecules, atoms or ions into 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 parts of a system". The problem of argon leakage by d i f f u s i o n i s of such impor- tance i n K 4 0 - A r 4 0 dating that numerous papers dealing with both experimental and t h e o r e t i c a l aspect of the subject have been published. Argon d i f f u s i o n from minerals has been studied under laboratory conditions by Gerling and Morozova (1958, 1 9 6 2 ) ; Evernden et a l . ( i 9 6 0 ) ; Hart ( i 9 6 0 ) ; Baadgaard et a l . (1961); Brandt (1962) and under geologic f i e l d conditions by Hurley et a l . (1962); Hart (1964) and Hanson and Gast ( 1967) . Recent re- views of argon d i f f u s i o n have been made by Fechtig and Kalbitzer ( 1 9 6 6 ) . Fundamental Laws of D i f f u s i o n The f i r s t and sedond laws postulated by Adolf Fick, are given as follows: 1. The d i f f u s i o n f l u x across a given plane i s propor- t i o n a l to the concentration gradient across that plane: 13 where J i s the d i f f u s i o n f l u x , D i s the d i f f u s i o n constant and V c i s the gradient concentration. 2. The divergence of the d i f f u s i o n f l u x i s equal t o the change of concentration w i t h time: ^£ - _ d;v. or - c/fv. (ovc) The d i f f u s i o n constant D i s temperature dependent- as shown by the Arrhenius r e l a t i o n : D = D Q e - E / R T where E i s the a c t i v a t i o n energy, D 0 i s the charac- t e r i s t i c constant, R i s the gas constant and T i s the absolute temperature. The experimental determination of the a c t i v a t i o n energy i s based on the k i n e t i c s study o f argon rel e a s e d from minerals at d i f f e r e n t temperatures. With an i n i t i a l homogeneous concentra- t i o n o f argon and zero c o n c e n t r a t i o n of argon at the s u r f a c e , the process of gas release;, from minerals may be given i n the f o l l o w i n g equations (Crank, 1956; J o s t , I960): p - 1 - £ r ( - k * ) where F i s the f r a c t i o n of t o t a l argon r e l e a s e d at any time t . This has been obtained from s o l u t i o n of F i c k f s law f o r a s p h e r i c a l g r a i n of r a d i u s a. Reichenberg (1953), by using a F o u r i e r i n t e g r a l transform, obtained an approximation f o r 0 <F <0.85, as f o l l o w s : F = A ( D t )i _ 3 J 3 t 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 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 o f t o t a l a r g o n r e l e a s e d a t a g i v e n temperature 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 r e c e i v e d from d i f f u s i o n measurements. I n g e n e r a l , argon i n t h e m i n e r a l o c c u p i e s s e v e r a l d i f f e r e n t 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 range between 2 5°C and 1,000°C. I t i s o f t e n o b s e r v e d t h a t t h e g e n e r a l r e s u l t i s s e v e r a l s t r a i g h t l i n e segments ( H a r t , I 960; G e r l i n g and Morozova, 1962;aGerling'.©.fea3,i$^3ii c o r r e s p o n d i n g t o 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 , as p r e s e n t e d s c h e m a t i c a l l y i n f i g u r e 1-2. ^ f o l l o w s : T F i g u r e 1-2: Schematic p l o t o f l o g D v e r s u s 1. a 2 T 15 I n 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 r e t e n t i o n o f r a d i o g e n i c a r g o n . The a c t i v a t i o n energy o f m i n e r a l s can a l s o s e r v e as c r i t e r i o n of t h e r e t e n t i o n o f r a d i o g e n i c argon i n m i n e r a l s . S a r d a r o v (19&3) c a r r i e d 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 the 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 c o n c l u d e d 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 t h e g l a u c o n i t e s up t o 400-oc and the escape o f argon i s due t o s t r u c t u r a l decomposi- t i o n o f t h e m i n e r a l a t t h i s t e m p e r a t u r e , W i t h an a c t i v a t i o n e nergy o f E - 60 K c a l / m o l e g l a u c o n i t e s h o u l d r e t a i n a r g o n com- p l e t e l y a t a l o w e r t e m p e r a t u r e . The works o f G e r l i n g and Morozova (1958, 1 9 6 2 ) , G e r l i n g e t a l . (1963) showed t h a t 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 are c h a r a c t e r i s t i c 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 t h a t the a r g o n i n micas was h e l d i n t h r e e t y p e s o f s i t e s : a few p e r - cent i n the f i r s t , 60 - 70 p e r c e n t i n t h e second and the remainder i n t h e t h i r d . However, th e work o f B r a n d t (1962) i n d i c a t e d o n l y one s i n g l e a c t i v a t i o n energy f o r m u s c o v i t e . G e r l i n g and Morozova (1962) o b s e r v e d a l s o t h a t the argon was h e l d i n m i c r o c l i n e - p e r t h i t e i n f i v e d i f f e r e n t s i t e s , c o r r e s p o n d i n g t o f i v 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 ; t h e f i r s t t h r e e phases a c c o u n t e d f o r 20 p e r c e n t o f the a r g o n , t h e f o u r t h c o n t a i n e d about 15 p e r c e n t o f t h e argon and t h e f i f t h c o n t a i n e d t h e remainder o f t h e argon, 65 p e r c e n t . ^ Amirkhanov e t a l . (1959) has made some argon d i f f u s i o n 16 measurements on v a r i o u s p o t a s h m i n e r a l s and obse 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 each o f two f e l d s p a r samples. F e c h t i g e t a l . (1961) i n v e s t i g a t e d the argon d i f f u s i o n f r o m a number o f m i n e r a l s and r o c k s . 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 om 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 not 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 . However, i n a 2 T the case o f a u g i t e , two s t r a i g h t l i n e segments were o b t a i n e d . I t i s f o u n d t h a t the r e s u l t s o f d i f f u s i o n measurements made by 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 but 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 have c o r r e l a t e d t h i s 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 o r l a t t i c e changes i n m i n e r a l s (Baadsgaard e t a l . , 1 961). I n any c a s e , most o f t h e above a u t h o r s p r o v e d e s s e n t i a l l y t h a t m i c a s and h o r n b l e n d e s are s u i t a b l e f o r K 4 0 - A r 4 " 0 d a t i n g . H u r l e y e t a l . , ( 1962), H a r t . (1964) and Hanson and Gast (1967) showed t h a t the apparent ages o b t a i n e d by K 4 0 - A r 4 0 method or Rb-'sr^method are a f u n c t i o n o f d i s t a n c e from a c o n t a c t zone as shown i n f i g u r e 1 - 3 . 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 over b i o t i t e may a l s o be seen from t h i s d i a g r a m . 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 accoun- t e d t o c h e m i c a l a l t e r a t i o n (Kulp and E n g e l § ) 1 9 6 3 ; Ma^visr e t a l . , •1-9&3 and Gqili'ch' e t a l . , 1966), and t h e r m a l e f f e c t due t o m e c h a n i c a l a b r a s i o n ( G e r l i n g e t a l . , 1 9 6 l ) . 17 1400 a fl o •r-i r-i -P fl CD U Age of o1- in t r u s i o n 10 3 ^ 1 0 0 0 10000 Figure 1-3 i Distance from contact (meters) • 1 (°C-1) T Va r i a t i o n of apparent ages of b i o t i t e , hornblende and feldspar as a function of distance from an intrusive contact (After Hart, 1964). 1.2.3 Materials Used for K40 _ Ar40 Dating Depending on the type of materials 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 formation i n others, may be events recorded by K 4 0 - A r 4 0 method. The laboratory geochronologist must therefore be con- stantly aware of the s u f f i c i e n t conditions for the K 4 0 - A r 4 0 model age, p a r t i c u l a r l y the second and t h i r d conditions. 18 In general, micas, sanidine, glauconite are the most widely used minerals f o r dating because of t h e i r large range of occurrence, high potassium content and high argon r e t e n t i v i t y . However, with improved a n a l y t i c a l techniques, geological materials other than micas can also be used. A l l igneous metamorphic and sedimentary rocks have been used as geological materials for dating. The K 4 0 - A r 4 0 "age" obtained on minerals of an igneous rock gives the time of emplacement, provided that argon has not been l o s t by d i f f u s i o n process and the rock has not been subjected to 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 yielded from minerals of a metamorphic rock w i l l indicate the time of the metamorphic event as previously accumulated argon i s completely l o s t by t h i s metamorphic event. An excellent description of dating of sediments by K 4 ° - A r 4 0 method has been made by Hurley (1966) and Hower et a l . (1963). The e s s e n t i a l things are, they must r e t a i n well radiogenic argon and must not contain fragments of d e t r i t a l minerals. The reason f o r t h i s i s based on the f a c t that these fragments may contain r e l i c t argon. Using f o s s i l bones, limestones and f l u o r i t e as dating material for K 4 0 - Ar 4 <^ method, an acceptable t o t a l f o s s i l bone age has been obtained and reported by L i p p o l t and Gentner 11963) . 19 I t should be noted that, from t h e i r three dated f l u o r i t e s , contained about 50 ppm of potassium; only one sample yielded an acceptable date. The other two gave high ages implying pro- bably the s u f f i c i e n t condition (2) was not f u l f i l l e d . There- fore, i n order to obtain r e l i a b l e r e s u l t s , careful selection of samples i s of primary importance. Weathered samples should be discarded, fresh materials are highly desirable for K 4 0 _ A r 4 0 dating. 2 0 CHAPTER I I GENERAL GEOLOGY OF THE NELSON BATHOLITH 2.1 Introduction ^ 2.1.1 History of Lit e r a t u r e After seventy years of mining a c t i v i t y the f i r s t geologic report of the West Kootenay D i s t r i c t , B r i t i s h Columbia was published by G.M. Dawson (1889). The stimulation of m e t a l l i c mineral deposits p a r t i c u l a r l y gold and s i l v e r , spurred others to look enquiringly at the Nelson region. F i e l d investigations and geological studies have therefore been car r i e d out by prospectors and members of the Geological Survey of Canada, (Cairnes, 1934; Mulligan, 1952 e t c . ) . Most of the previous works, as well as h i s own are summar- ized by H.W. L i t t l e , (i960). Recent geological investigation of the northern part of the Nelson batholith has been c a r r i e d out by members of the Geology Department ( S i n c l a i r and Libby, 1967). Much of the geological informations 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 of Batholiths" indicate a composite petrography i s c h a r a c t e r i s t i c of batholiths. The Nelson batholith can be 21 Figure 2 - l : Location Map of K-Ar"samples of Nelson Batholith and Related Rocks New Denver - Kaslo, 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 and L i b b y , 1967), V a l h a l l a r o c k s ( L i t t l e , I 960) and Lamprophyre dykes t h a t c u t t h e N e l s o n b a t h o l i t h . E a ch 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 terms of f i e l d o r l a b o r a t o r y c r i t e r i a . 2.2 D i f f e r e n t Phases o f t h e N e l s o n B a t h o l i t h 2.2.1 N e l s o n Rocks The main p a r t o f the a r e a o f the p r e s e n t s t u d y i s u n d e r l a i n by N e l s o n r o c k s . Compared w i t h many o t h e r b a t h o l i t h s , ( H a m i l t o n and Myers, 1967), the N e l s o n 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 t h a n f a l l i n g i n t h e broad " g r a n i t i c " c l a s s i f i c a t i o n o f the S i e r r a Nevada and o t h e r s . Toward t h e c o n t a c t b o r d e r s w i t h the c o u n t r y r o c k s the c o n t e n t o f ferromagnesiam m i n e r a l s tends t o i n c r e a s e . I n many s e c t i o n s the N e l s o n r o c k s may be d e s c r i b e d as be- 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 by 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 o f s o d i c o r t h o c l a s e . I n a few l o c a l i t i e s , t h e s e p h e n o c r y s t s may c o n t r i b u t e up t o 5© p e r c e n t of t h e r o c k and range i n s i z e van average o f about two i n c h e s . I n a r e a s ex- h i b i t i n g d e f o r m a t i o n , t h e s e a re r e p l a c e d by m i c r o c l i n e . The groundmass 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 by a n abundance of hornblende o v e r t h e b i o t i t e . P a r t i a l c h l o r i t i c 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 L E G E N D L Tertiary intrusions Nel on batholith and satellites Late Palaeozoic and Early to Middle Mesozoic * 1 \ Early Palaeozoic Precambrian District boundary.... i G.S. C 20 l_ S c a l e of M i l e s o Figure 2-2: Setting of- the Nelson Batholith and some s a t e l l i t e s (After L i t t l e , 1 9 6 p 0 24 The p o r p h y r i t i c member i s cutby.or sometimes grades i n t o y ^ the n o n - p o r p h y r i t i c member. The l a t t e r tends t o be g r a n o d i o r i t e as the a c i d i c phenocrysts are missing and the other m i n e r a l c o n s t i t u e n t s are the same as i n the p o r p h y r i t i c member. The np n - p o r p h y r i t i c member o f the Nelson b a t h o l i t h i s considered t o be s l i g h t l y younger than the p o r p h y r i t i c member. T h i s i s based on the f a c t t h a t the former c r o s s c u t s , i n p l a c e s , the l a t t e r . 2.2.2 Nelson S a t e l l i t e s Rocks The Nelson s a t e l l i t e s comprise apophyses and stocks of the Nelson b a t h o l i t h , i n c l u d i n g the Mount C a r l y l e stock. The Slocan s e r i e s south o f the Mount C a r l y l e stock forms a septum seve r a l hundred f e e t wide separating the stock from the northern edges of the b a t h o l i t h . ( S i n c l a i r and L i b b y , 1967). With some excep- t i o n s , the composition of the s a t e l l i t i c r o c k s , on the whole, resembles very much t h a t of the Nelson r o c k s . Both p o r p h y r i t i c and n o n - p o r p h y r i t i c members are present. Most of these s a t e l l i t e s have been observed t o intru d e the T r i a s s i c Slocan s e r i e s and an important number of them i n t e r s e c t other formations. M u l l i g a n (19 52), b e l i e v e d t h a t the stocks are of e a r l y phases of the Nelson i n t r u s i o n . However, Cairnes ( 1 9 3 4 ) , suggested t h a t the 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 of the Nelson b a t h o l i t h , . L i t t l e ( i 9 6 0 ) , 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 than the Nelson b a t h o l i t h . 25 2.2.3 V a l h a l l a Rocks Chemically and m i n e r a l o g i c a l l y , the 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 to the Nelson rocks. The d i f f e r e n c e s are predominately t e x t u r a l , f i n e r grained and a l l o t r i o m o r p h i c i n s t e a d of hypidiomorphic t e x t u r e as i n the case of the Nelson rocks . The contacts between the V a l h a l l a rocks and the Nelson rocks are commonly g r a d a t i o n a l but sharp contacts occur where the former i n t r u d e s the l a t t e r . The s i m i l a r i t y between these phases l e d Cairnes (1934), t o drop the name V a l h a l l a , but L i t t l e ( i 9 6 0 ) , preserves the d i s t i n c t i o n . The V a l h a l l a rocks outcrop toward the west, but a hat- shaped mass of 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 of Maurier, F e n n e l l and S i l v e r t o n creeks. The r a r i t y of V a l h a l l a rocks i n the eastern Slocan r i v e r and Slocan lake and "the Nelson rocks comprise a l a r g e p r o p o r t i o n of t h i s p a r t of the 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 the p o r p h y r i t i c m a t e r i a l which may have flowed out' from the auto- chtonous core,pushing the e a s t e r n p e r i p h e r a l rocks i n t o t h e i r present a r c - l i k e p o s i t i o n " , has been suggested by L i t t l e ( i 9 6 0 ) . As the c o n t a c t s between the Nelson rocks and V a l h a l l a rocks are most o f t e n i n d i s t i n c t , L i t t l e ( i 9 6 0 ) , assumed contemporaneity of these two phases. 2.2.4 Lamprophyre Dykes Dykes have been observed t o cut the Nelson p l u t o n i c rocks. rrvvesTl GATORS Cairn.es arid FOLlMSBEtJ Newr-narch, C ) 180 ? AAAA1 •WWHi 1 so (40 I 2 0 100 go .Ages (V) miUron y^ors Figure 2-J>- Previous age determinations by f i e l d geology, Nelson batholith 27 Their composition, on the whole, resembles very much that of the Nelson rocks. They are considered to represent a l a t e staga i n the i n t r u s i o n of the Nelson batholith, (Cairnes, 1934). 2.3 Origin Due to Metasomatism of Paleozoic and E a r l y Mesozoic From observations of i t s components, (the paragneiss, the Nelson s a t e l l i t e s and V a l h a l l a rocks) and i t s structure, the o r i g i n of the Nelson batholith may be deduced. The paragneiss exposed i n the Nelson b a t h o l i t h core and «l«swih©r© has been correlated with the.Hall formation of Middle (?) and TJppe-r Jurassic age. The paragneiss, and V a l h a l l a rocks form successive l a y e r s with gradational borders. Lying above the paragneiss and grading l a t e r a l l y into i t are the Nelson rocks are which, i n turn , voverlain, and- with gradational contact by Va l h a l l a non-porphyritic rocks. The gradational contacts between the V a l h a l l a and the Nelson are often observed. However i n some places the. dykes of the V a l h a l l a rocks cut the Nelson, normally without e x h i b i t i n g d i l a t i o n . I t i s concluded by L i t t l e (I960), that the core of the • < Nelson b a t h o l i t h exhibits evidence of a metasomatism o r i g i n , i.e., the rocks were formed by metasomatism of pre-existing rocks such as sedimentary roeks. L i t t l e showed also evidence to suggest that the region now occupied by the Nelson b a t h o l i t h hack been deeply buried. Moreover, the autochtonous core of the Nelson batholith i s t y p i c a l of Read*s deep-seated batholith, (Read; 1951). This has been used by L i t t l e t o ensure his hypothesis. 28 However, L i t t l e (i960), pointed out that the rocks about the periphery, tinlike those of the core, have been mobilized and injected into older rocks. 2.4 Age of Metasomatism and Intrusion by Stratigraphy and Structure The Nelson batholith seems to be the oldest major batho- l i t h i c mass formed during the Mesozoic orogeny i n the Arc. Cairnes (1934), reported that the emplacement of the Nelson bat h o l i t h followed intense f o l d i n g of the Sloe an s e r i e s . The orogenic disturbances, have caused the f r a c t u r i n g and hreccia- t i o n exhibited by the Nelson rocks. Based on/the chain.-relation between the orogenic distrubance and the mineralization which i s , i n turn, c l o s e l y r e l a t e d i n o r i g i n to the Nelson rocks, the age of the Nelson b a t h o l i t h may be deduoed i f the date of the orogenic disturbance i s known. This orogenic disturbance was believed to occur at about the same time i n the Sel k i r k as i n the Rocky- Mountains. Cairnes (1934)-, assigned consequently, the invasion of the Nelson batholith to l a t e Cretaceous. E a r l i e r investigators^based on the s i m i l a r i t y of the Nelson rocks and the Coast rocks, suggested the same age for both Coast Range and Nelson rocks. The Coast Range intrusives have been observed to cut Jurassic rocks, (Middle and Lower Jurassic f o s s i l s were observed) and i n places, the ba t h o l i t h i e rocks i n - trude sediments of lower Cretaceous age. From those evidences, i t was suggested that the Coast plutonic rocks represent a long 29 period of i n t r u s i o n , from Upper Jurassic to Cretaceous. Based on the f o s s i l s of l a t e Upper Jurassic age observed from the basal Kootenay bed and on the assumption that the Kootenay was c h i e f l y derived from the geanticline b a t h o l i t h , Newmar.oh (1953), suggested the Upper Jurassic €.3 the lower l i m i t time of em- placement of the Nelson b a t h o l i t h . However, the existence of the f o s s i l s of Lower Cretaceous age are also observed, because of these inconclusive evidence;, available, Newmarch stated that the Upper Jurassic or Lower Cretaceous as the lower l i m i t time of emplacement of the Nelson batholith. In observing the Nelson rocks to contain a heavy mineral suite 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 that the Nelson rock was unroofed just a f t e r the deposition of the basal Blairmore conglomerate. The age of t h i s conglomerate has been assigned to Lower Creta- ceous, (Albian) by B e l l (1956). L i t t l e ( i 9 6 0) concluded that "the lower l i m i t of the age of the Nelson plutonic rocks, with some confidence, i s post Middle Juras s i c " , and, "the upper l i m i t of the age of the Nelson rocks can be set, with rather l e s s confidence, as pre-Upper Cretaceous". The lower l i m i t was based on the fa c t that the; Nelson rocks cut sedimentary and volcanic rocks overlying the H a l l formation whose yongest f o s s i l s are of e a r l y Middle Jurassic age. 30 The upper l i m i t was based on the fact that the plant f o s s i l s from the Sophie Mountain formation are probably of Upper Cre- taceous age and the Sophie Mountain conglomerate contains g r a n i t i c pebbles derived from Nelson. On the other hand, the Sophie Mountain formation rests unconformably upon the Rossland forma- t i o n and i s cut by the Nelson, (figure 2-2). In any case, the surest l i m i t s of the emplacement 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 can be set as following: the Nelson rocks cut the T r i a s s i c or Lower Jurassic Slocan s e r i e s , t h i s indicates that they are younger than the Slocan series and of course they must be older than the mineralization period. 2.5 P o s t - C r y s t a l l i z a t i o n History The Nelson ba t h o l i t h i s considered to be a massive g r a n i t i c pluton, however areas of strong f o l i a t i o n s may be observed within the pluton. 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 part of the Nelson bathol i t h , along the Slocan Lake. ( L i t t l e , I960). The eastern part of the ba t h o l i t h has been subjected to s i g n i f i c a n t post- c r y s t a l l i n e deformation which affected the enclosing sedimentary and metamorphic rocks. The period of deformation was marked by the f i s s u r i n g and shearing whose d i r e c t i o n i s controlled by well developed j o i n t f r a c t u r e s . As mineral deposits occur, i n general, i n these f r a c t u r e s , i t i s believed that the mineralization took place after most of these fractures occurred. In e f f e c t , the age of the Nelson b a t h o l i t h determined by geological i n v e s t i g a t i o n indicatesasomewhat complete i n t r u s i o n h i s t o r y . The geological relationships are^clearcut as there not are d i f f e r e n t opinions concerning the age of the batholith, as described above, (figure 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 to be s l i g h t l y younger or contemporaneous with the main batholith, ( L i t t l e , I960). Even, so, geological i n v e s t i - gations could not determine an "absolute age" of the formation. The s i t u a t i o n i s resolvable only within the l a s t decade with the method of physical determination. \ 32 CHAPTER I I I PHYSICAL DETERMINATION 3.1 Previous Works As the "age" of the Nelson Batholith could not be solved d e f i n i t e l y by geological means, some workers have attempted to solve t h i s complex h i s t o r y b a t h o l i t h by means of physical deter- mination. By using lead alpha a c t i v i t y r a t i o s on accessory minerals zircons, monazite and xenotime, Larson et a l . (1934), arrived t-e determineathe age of rocks from the Southern C a l i f o r n i a , S i e r r a Ova- Nevada and Idaho batholiths. Five age determinations onvldaho batholith average 103 m.y.. Beveridge and Folinsbee (1956), i n using the radioactive method have evaluated the age of zircon from the g n e i s s i c phase of the Nelson batho 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 batholiths, Beveridge and Folinsbee suggested that the emplacement of the Nelson ba t h o l i t h was i n l a t e lower Cretaceous time. A K 4 0 - A r 4 0 age determination on b i o t i t e s separated from granodiorite near the c i t y of Nelson, yieldedavalue of 86 m.y. as reported by L i t t l e (I960). However, L i t t l e stated that "thi s figure, which of course appears to be too small, may be incorrect because 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 long after emplacement of the batholith."-. Following t h i s , the Geological Survey of Canada conducted a series of age determinations by K40- Ar4(method on b i o t i t e . The 33 samples have been collected and interpreted hy Reesor and re- ported by Lowdon et a l . (1961, 1963), Leech et a l . (1963) and Gabrielse and Reesor (1964). A total of 10 age determinations have been resulted as follows: GSC 60-21 (49 m.y.), GSG 60-22 (55 m.y.), GSC 6 l - l ? (131 m.y.), GSC 62-26 (165 m.y.), GSC 62-27 (159 m.y.), (JSC 62-28 (171 m.y.), GSC 62-29 (163 m.y.), GSC 62-30 (171 m.y.), GSC 62-31 (105 m.y.), GSC 62-32 (63 m.y.), The widespread in ages led Reesor to propose that the emplacement of Nelson batholith has evolved through a very long time, I t developed in three main stages X cfis+inguishec/ by aftervthree groups of ages: X (a) The older group (from 159 to 171 m.y.) represents the emplacement and consolidation of hornblende, biotite and granodiorite before 171 m.y.. (b) The intermediate group (from 86 to 131 m.y.) indicates a mobilization and reintrusion, due to an intense structural episode occuring about the earliest Cretaceous, of a relatively incompetent ^ mass. (c) Finally, the younger igroup (from 49 to 63 m.y.) repre- sents further emplacement of leucocratic quartz monzonite. Reesor then concluded that i f this hypothesis i s accepted, there i s thus no conflict between the age data and stratigraphic - structural data. However, the considerable spreading in ages even-, V v aft, within a very short distance between l o c a l i t i e s , which generally 34 assumed to be of nearly the same age, appears to be unsatisfac- t o r i l y j u s t i f i e d . Although, Reesor has c a r e f u l l y formulated h i s hypothesis within three possible major f a c t o r s that could af f e c t the age measurement. These factors are: the complex succession of st r u c t u r a l events within the mesozoic cycle of deformation, the associated complex evolution of rocks throughout the development of the mobile be l t during the frtesozoic and f i n a l l y the complex gradual cooling of emplaced mass during u p l i f t and uproofing. One can therefore see that Reesor 1 s views of the Nelson bat h o l i t h are somewhat di f f e r e n t or i t could be said d i r e c t l y that the required a new d e f i n i t i o n for the b a t h o l i t h to warrant h i s hypothesis. 3.2 Current Project 3.2.1 Introduction To t e s t the v a l i d i t y of the "Reesor hypothesis" of the evolu- t i o n of the Nelson bath o l i t h , i t i s considered necessary to ex- tend further s t r u c t u r a l , petrologic studies as well as to make additional isotopic age determinations. Geological study of the northern part of the Nelson batholith has been car r i e d out by S i n c l a i r and Libby of the Department of Geology* who have k i n d l y provided a t o t a l of eleven specimens for the present study. Four plutonic rock units are considered i n t h i s study: i*' U n i v e r s i t y of Br i t ish - Columbia 3 5 1. The northern part of the Nelson b a t h o l i t h . 2. Mont Carlyle stock ( S i n c l a i r and Libby, 1967). 3. L i t t l e ' s " V a l h a l l a plutonic rocks" centered on the headwaters of the Fennel Creek. 4. Lamprophyre dykes that cut the Nelson ba t h o l i t h . A l l samples used f o r K40 . A r 4 0 isotopic age determination were chosen a f t e r c a r e f u l examination of t h i n sections of speci- mens. Sample locations shown i n figure 2.1 are l i s t e d i n the Appendix I I with the petrographic descriptions. Experimental procedures used are l i s t e d i n Appendix I. Three major steps involved i n the K40 . A r 4 0 isotopic age determination of the present study are: 1. The separation of the minerals from rocks. 2. The determination of potassium content i n minerals by flame photometry (Dean, I960; Cooper, 1963; Cooper et a l . , 1966 and Dirom, 1965). 3» The determination of argon content i n minerals by mass spectrometry s t a t i c isotope d i l u t i o n method (Reynolds, 1956; Farrar et a l . , 1964) using a highly enriched A r ^ tracer. The argon a n a l y t i c a l system used i s shown i n figure 3-1• Figure 3-1: UBC Argon Extraction and A n a l y t i c a l System E L E C T R O M E T E R S A M P L E IN ° Ho C R U C I B L E x . . z . . . z ; : ^ L z z z : : : z z z . . . z : : z : n:n^j±f\n:r^ L E A K V A L V E TO P U M P S C 1Z3ZLL_Z2 M E T A L V A L V E S T i IN S i 0 2 T U B E IZZZZZ '0 PUMPS! 37 3 . 2 . 2 The K*0 _ A r 4 G Results " N The r e s u l t s obtained by K^O _ A r 4 0 age determinations on b i o t i t e , hornblende and pyroxene from the various 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 given i n fugure 2 - 1 . The data i s conven- i e n t l y presented i n a graphical form i n figure 3 - 2 , a plo t of log A r 4 ^ versus log K. The 1%' m.y. isochron, a straight l i h e containing a l l the points of the same age, i s shown. I t i s accompanied by two other isochrons of 100 m.y. and 200 m.y. respectively. A l l plotted points located on the upper side of the Straight l i n e are of older ages and those on the lower side are of younger ages. The sign of excess radiogenic argon or argon l o s s of the samples can therefore be checked with t h i s kind of graphical presentation. The data obtained by Reesor, (Grabrielse and Reesor, 1964) are also plotted i n the same figure 3^2. A histogram of K 4^ - Ar4*-* dates i s shown i n figure 3 - 3 , where frequency versus l o g ages are presented. 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 the estimated a n a l y t i c a l e r r or. Eleven age determinations reported by Gabrielse and Reesor (1964) are concurrently presented. An error of ± 8% (Lowdon et a l . 1963) has been attached to each.date before p l o t t i n g . The advantage of t h i s kind of presentation has been discussed by Ross ( 1 9 6 6 ) . Table 3 . 2 gives U.B.C. res u l t s obtained on interlaboratory 38 FIGURE 3 . 3 : A p l o t of frequency versus l o g apparent ages TABLE 3.1 K 4 0 - A r 4 0 A n a l y t i c a l Data - Nelson Batholith *e = 0.385 x 10-10yr-l; A f - 4.72 x 10-lQyr- 1; K 4 0/K - 0.01181% Sample Number Mineral K (%) A r 4 0 A r 4 0 A r 4 0 Rock Type Analyzed Total Ar40 xl0- 1 0moles/g K4U Apparentage im.y.) R - l l Granodiorite B i o t i t e 5.83 5.76 5.81 5.74 Av=3.78 ± 0 . 0 4 0.86 12.690 0.007268 120+5 Q,uartzidiorite B i o t i t e 7.52 7.55 7.51 7.5? Av«7.53± 0.02 0.90 20.163 0.008870 1462:5 R-13 Granodiorite B i o t i t e ( C h l o r i t i c ) 6.43 6.33 6.31 M l Av«6.36± 0.05 0.86 15.087 0.007860 130*5 L-236 Fine grained hornfels B i o t i t e ( C h l o r i t i c ) 6.63 6.62 6.68 6.66 Av=6.65*0.05 0.72 16 .431 0.008179 135±5 : radiogenic argon 4-0 TABLE 3 . 1 a K 4 0 _ 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 Sample Number Rook Type M i n e r a l Analyzed Ar40 Ar 40' Ar 40' Total Ar4U ( x l 0 - 1 0 m o l e s / g j E4TT Apparantage (m.y.) A -66 G r a n o d i o r i t e B i o t i t e 6.66 6.63 6.58 6.61 Av= 6.621Q.03 0.87 18.194 0.00909? 149*6 C-66 Hornblende Quartz Monzonite B i o t i t e 5.88 5.85 5 . 8 3 5.93 Av= 3.88±o .05 0.81 16.327 0.009193 131*6 D-66 B i o t i t e Lamprophyre Dyke 7.12 7.16 7.19 7.18 Av= 7 . 1 6 ± 0 . 0 3 0 . 8 0 22.410 O.OIO363 169±6 K -66 P o r p h y r i t i c Hornblende Quartz Monzonite Hornblende Av» 1 .213 1.208 1.209 1.209 1.210+.005 0 . 5 9 3.387 0.009816 I 6 l£ 6 TABLE 3.1b K40 _ ̂ .AQ A n a l y t i c a l Data - Nel son Batholith Sample Number Mineral K (%) A r 4 0 ' A r 4 0 A r 4 0 Apparentage Rock Type Analyzed Total Ar4U x l 0 - 1 0 m o l e s/g K W ~ (m.y.) C-66 Hornblende 1.104 O.56 3.342 O . O I O O 3 2 164+6 Hornblende 1.106 Quartz Monzonite 1.099 1.104 Av= 1.103^0.005 SILV-66 Hornblende Quartz Monzonite Hornblende 0.886 0.888 0.884 0.886 Av= 0.886±0.003 0.69 2.442 0.009123 150+5 4=- 257-66 Nelson Batholith Quartz Diordte Porphyry B i o t i t e 6.85 6.86 6.87 6 . 8 9 Av= 6 . 8 7 * 0 . 0 3 0 . 9 0 1 9 . 0 0 5 0.009159 150±5 R-12 Clinopyroxene Quartz D i o r i t e 0 . 0 . 0 . 0 . Av- 0.^585^0.002 0.71 0.42 2.731 0.016207 258+12 2.902 0.017191 273+16 43 standard minerals,1 including U.B.C. standard b i o t i t e GD-12. A comparison of analyses ca r r i e d oat at U.B.C. isotope geophysics laboratory with others made i n various laboratories on the same mineral samples (Muscovite standard r Pt- 207; b i o t i t e standards GE-3203 and GE-2060) i s shown i n Table 3 . 3 . Table 3»4 gives the "ages" of the various rock units on the Nelson b a t h o l i t h . 3 . 2 . 3 A n a l y t i c a l Precision and Accuracy The interference e f f e c t i s one of the most controversial problems i n the accuracy and pre c i s i o n of the potassium de- termination by flame photometry. The interference e f f e c t s of elements involved, i n the solutions used for K40 - A r 4 - 0 dating have been investigated by many authors, (Dean, I960; Cooper, 1963; and Cooper et a l . , 1966). Various types of buffering techniques have been proposed. The main purpose of the buffering action i s to make both the standard and the sample solutions close to the same phys i c a l , chemical and ioni c condition within the flame. This involves the addition of the same amount of an i n t e r f e r i n g element to the standard solutions as . i s present i n the sample solutions. Cooper (1963) indicated that the interference of other ions depends considerably on the charac- t e r i s t i c s of the flame photometer type used (for example, burner, f u e l ) . Dirom (1965) reported that with the instrument and the buffering technique used f o r the present study exeept H 2 - S O 4 , otiier Hian f f 2 S 0 4 the interference e f f e c t s of the—©the-r elements,Yare n e g l i g i b l e . 1Analyzed by J . Harakal 44 TABLE 3.2 RESULTS OE U.B.G. LABORATORY ON STANDARD MINERALS Sample Mineral Ar^O' Total Ar40 Ar40 (xlO"^moles/g) GD-12 B i o t i t e x - 5.56 <r s 10.03 (Zaralyses i n quadrup- l i c a t e ) O.83 0.77 0.83 0.71 x = <r a 2.081 2.076 2.098 2.124 2.095 to.022 GE-2060 B i o t i t e x « 6.83 cr = ±0.06 (14 analyses i n quadrup- l i c a t e ) 0.88 0.54 0.61 0.84 2.459 2.430 2.498 2.302 sc.- 2.472 O-a ±0.034 B-3203 B i o t i t e 0.72 17.121 P-207 Muscovite x = 8.58 <r 5 tO.05 (13 analyses i n duplicate or quadruplicate) 0.88 1.240 O.85 1.252 0.75 1.274 0.68 1.244 O.83 1.225 0.90 1.261 0.59 1.239 0.82 1.244 0.86 1.220 x ~ 1.244 6"= 10.017 Arithmetic mean Standard deviation or mean deviation 4 5 TABLE 3 . 3 COMPARISON OF ANALYSES CARRIED OUT IN U.B.C. WITH OTHERS MADE IN VARIOUS LABORATORIES ON STANDARD MINERALS. Sample Mineral Data Source (Date) Ar 40- UBC Difference (xL0-9moles/g) i n Ar40* Analyses ( % ) GE-2060 B i o t i t e Columbia Yale (1963) x = 6.87 <r • ±0.05 x = 2.500 cr = + 0 . O i l 1.12 UBG (1965-1967) 5 = 6.83 <r = ±0.06 x - 2.472 cr = 1 0 . 0 3 4 B-3203 B i o t i t e Summary (Hurley et a l , 1962) 7.58 17.30 to .06 UBC (1965) x = 17.121 1.03 LanphereancJ x = 7.42 x s 17.33 1-21 Dalrymple <r = i0.02 <r= *0.06 P-207 Muscovite Summary (Lanphere and Dalrymple 1967) x 8.58 tO.12 5 = 1.260 <3"= ±0.024 UBC (1965-1967) x CP- 8.58 t o . 0 5 x = 1.244 <r = ± 0 . 0 1 7 I . 2 7 = Arithmetic mean cr = Standard deviation or mean deviation 46 J Moreover, tine interference e f f e c t of H2SO4 was only s i g n i f i c a n t for low potassium content amphibole and pyroxene solutions. For t h i s reason, the standard solutions used f o r b i o t i t e analysis did not require any addition of H2SO4 for buffering purpose. In the present study, for those samples which resistedcomplete d i s s o l u t i o n , further additional heating or n i t r i c acid was used to replace per c h l o r i c acid. This was t o avoid the possible interference e f f e c t s of perch l o r i c acid which may cause some depression of potassium emission. The potassium analysis was c a r r i e d out at l e a s t i n quad- ruplicat e f o r each sample. The obtained average values of potassium content were then used i n the ca l c u l a t i o n of the apparent age. The r e s u l t s obtained on t h i r t y - f i v e analyses i n duplicate or quadruplicate of the L.G.O. b i o t i t e standard GE-20 60, U.S.G.S. muscovite standard P-207 and Nelson project b i o t i t e samples indicate a pr e c i s i o n of about 0 .7 percent (Tables 3.1 and 3 . 2 ) . From Table 3 . 3 , the absolute agreement obtained on the potassium determination of the standard P - 2 0 7 , between the U.B.C. r e s u l t s (8.58 ± 0 . 0 3 ) and the averaged r e s u l t s of twelve d i f f e r e n t laboratories (8.38 ± 0 . 1 2 ) i s noteworthy. From the standard GE-2060, a difference of about 0 . 6 percent resulted. The potassium analysis i s therefore believed to be accurate with- i n 0 . 6 percent or better. For hornblende and pyroxene, the re s u l t s obtained from four samples, i n at lea s t quadruplicate, on yNelson project show a 47 p r e c i s i o n of about 0.4 percent (Table 3.1). Unfortunately no hornblende or pyroxene standard i s available at present. However, with c a r e f u l work, on the preparation of samples, sample and standard solutions, i t i s believed that the accuracy i s better than two percent. One of the most serious problems i n the argon analysis by mass spectrometry s t a t i c method i s the contamination of the previous sample on the sample t o be processed. This causes a ".; change i n the i s o t o p i c r a t i o s measured with time. However, with the mass spectrometer MS-10 used i n the present study, no "memory e f f e c t " has been detected. The standard deviation of the aver- age is o t o p i c r a t i o s was always l e s s than 0.2 percent. Farrar et a l . , (1964) ascribed t h i s to the low accelerating voltage, about 100 v o l t s , used i n the mass spectrometer MS-10. The re- . one solving power of the MS-10 i s approximately 50, t h i s permits vto resolve p e r f e c t l y a l l mass peaks of argon isotopes36, 38 and 40. A sample run for atmospheric argon, given i n figure 3-4, shows the resolution i n the mass 36 - 40 range. Based on the high p r e c i s i o n and accuracy obtained previously i n the laboratory, (White et a l . , 1967), only ca single run was carried out for each sample, (except the clinopyroxene sample R-12 which was run i n duplicate). Seventeen analysis on b i o t i t e standards indicate a p r e c i s i o n of approximately one percent (Table 3 .2). I t may be seen that the U.B.C. argon analysis results are, systematically lower than those reported by other laboratories. The difference i s approximately one percent (Table 3 « 3 . ) » It may be deduced from Table 3.2 that the results obtained i n the present study are i n t e r n a l l y consistent within two percent A r 5 x ZOmV. 48 8- 40 Ar* 10 V 7 4- 38 Ar x 30mV. L J Figure 3 - 4 : Typical atmos- pheric argon run shows the resolu- t i o n i n the mass 36-40 range. TABLE 3.4 Ages of the Various Rock Units on the Nelson Batholith Rock Unit Sample Number Rock Type Mineral Dated Model Age (m.y.) Remark on Sample Analyzed Nelson batholith R - l l : Granodiorite B i o t i t e 120 C h l o r i t i z e d R-13 Granodiorite B i o t i t e 130 C h l o f i t i z e d R-12 Quartz d i o r i t e B i o t i t e 146 Ere sh Clinopyroxene 265 Fresh C-66 Quartz monzonite B i o t i t e 151 Very fresh Hornblende 164 Very fresh SILV-66 Quartz monzonite Hornblende 150 B i o t i t e completely c h l o r i t i z e d but hornblende i s fresh i n appearance. L - 2 3 6 Hornfels B i o t i t e 135 About 15% of B i o t i t e has ragged edges. S O Nelson s a t e l l i t e s L -237 Quartz d i o r i t e B i o t i t e 150 Altered (including Mont Carlyle stock) K-66 Quartz monzonite Hornblende 161 Very fresh V a l h a l l a Plutonic Rocks A-66 Granodiorite B i o t i t e 149 S l i g h t l y altered Post-Nelson batholith D-66 Lamprophyre dyke B i o t i t e 169 Fresh 50 of the apparent age. However, due to the discrepancy met when comparing the r e s u l t s obtained by various laboratories, larger error l i m i t s are adopted. The apparent ages are assigned error l i m i t s ranging from three to six percent. The lower p r e c i s i o n was assigned to the analysis which indicated l e s s than 50 percent radiogenic A r 4 0 as suggested by Lipson (1958). TABLE 3 . 5 RELIABLE MODEL AGES SELECTED FOR DATA INTERPRETATION Mineral dated Sample number Model age (m.y.) Standard deviation (m.y.) R -12 146 5 B i o t i t e A -66 C -66 U9 151 6 6 D -6 6 169 6 SILV -66 150 5 Hornblende C -66 164 6 K -66 161 6 51 3«3 Discussion The K40 A r 4 0 isotopic age determination of the Nelson batho l i t h , i n the present study, yielded values which agree reasonably well with the r e s u l t obtained from geological evidences. On the whole, the values range from 120 m.y. to 169 m.y., with the exception of the clinopyroxene sample R-12 which yielded an anomalous age of 265 m.y.. A large group of ages f a l l around 150 m.y. Without further considerations, the values indicate that the various phases of the Nelson b a t h o l i t h were emplaced during a considerable i n t e r v a l of time, from Middle Jurassic to Lower Cretaceous. The age re- s u l t s f i t therefore the sequences of emplacement of the gran- i t i c rocks as established by contact relationships observed i n the f i e l d . Cairnes ( 1 9 3 4 ) ^ i n h i s memoir stated that, "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 followed by the invasion of stocks and minor i n t r u s i v e s and concluded with the mafic dykes". However, a more detailed i n t e r p r e t a t i o n of the data needs • H i e further refinement. From^welveage determinations obtained, of which three have been obtained on hornblendes, one on c l i n o - pyroxene and the remainder on b i o t i t e s , Age determinations were ca r r i e d out f o r two oogenetic minerals p a i r s . Five age determinations have been considered unreliable, based on the fact that the prerequisite s u f f i c i e n t conditions posed prev- iouslywtre not f u l f i l l e d . 52 F i r s t of a l l , the clinopyroxene sample R-12 yielded an age of 265 m.y. which i s unquestionably anomalous, re- and yeoLogic i-etofions garding the r e s u l t s obtained on the other samples*. The re s u l t s were checked a n a l y t i c a l l y f o r both potassium and argon determinations. On the flame photometry determina- t i o n , an average of 0 .5585 percent with a standard deviation from the mean of l e s s than 0 .5 percent has been obtained from eight d i f f e r e n t aliquots. I t s potassium content i s re- l a t i v e l y low. However, based on the p r e c i s i o n and accuracy obtained from previous analyses on very low potassium bearing materials (White et a l . , 1967), i t i s believed that the potassium determination i s accurate withlte<2 peroent, i f not better. The argon analyses were ca r r i e d out i n duplicate. The experimental data are therefore considered to be r e l i a b l e . The accuracy and p r e c i s i o n are believed to be within the error l i m i t s assigned. On the other hand, the highly discordant apparent ages obtained from the pair of oogenetic minerals sample R-12, 146 m.y. from b i o t i t e against 265 m.y. from the clinopyroxene sample, suggest to the writer that c l i n o - pyroxene contains cJ excess radiogenic argon. The use of the clinopyroxene f o r K 4 0 - A r 4 0 dating should therefore be avoided. Hart (196l), i n v e s t i g a t i n g pyroxene f o r K40 _ A r 4 0 dating stated that "pyroxenes are unlike l y hosts for excess argon as they do not have s t r u c t u r a l vacancies or holes as amphi- boles". However, further works of Hart and Dodd (1962) and Lovering and Richards (1964), proved the presence of excess radiogenic argon i n pyroxene. This "age" may be disregarded on the basis of the prerequisite conditions for a K40 - Ar40 33 model age are not f u l f i l l e d . Therefore, highly anomalous clinopyroxene "age" must not he included i n the data i n t e r - pretation. The other four samples, L - 2 3 6 , R - l l , R-13 and L - 2 3 7 , do not s a t i s f y the t h i r d s u f f i c i e n t condition of the K 4 0 - A r 4 0 model age. In p a r t i c u l a r , the sample L -257 i s so heavily c h l o r i t i z e d and altered that even the yielded value of 150 m.~y. which i s f a i r l y concordant with the "age" of the Nelson plutonic rocks, T-h-i-s date i s s t i l l considered un- r e l i a b l e . Moreover, the sample was c o l l e c t e d from an i n - trusion i n rocks of the Slocan series, north of the Nelson bath o l i t h . I t was found that the sample, petr©graphically, i s very d i s t i n c t from the Nelson plutonic rocks. The date i s believed t o indicate only a l a t e stage of hydrothermal a l t e r a t i o n . Samples R - l l , R-13 and L - 2 3 6 yielded 120 m.y., 130 m.y. and 135 m.y. respectively. The low ages may be ex- plained by the argon l o s s through d i f f u s i o n . The deleterious e f f e c t s of a l t e r a t i o n on the K 4 0 - A r 4 0 ages of b i o t i t e s and hornblendes have been discussed by Goldich & ;,Gast ,~XL966). They showed that a l t e r a t i o n has reducedb-approximately 25 per- cent the K 4 0 - A r 4 0 age of b i o t i t e s from Morton gneiss of southwestern Minnesota. Kulp and Engels (1963), Ma^in- et a l . fl -9^34, also discussed the possible e f f e c t s of a l t e r a - t i o n on K 4 0 - A r 4 0 ages of b i o t i t e s . These e f f e c t s are 8 7 8 7 generally more d r a s t i c s ! on Rb - Sr ages than on K 4 0 Q A r 4 0 ages mainly because of the large exponential e f f e c t i n the A 4 0 r a t i o age r e l a t i o n . In any case, those samples did not E4TJ- 54 f u l f i l l the prerequisite conditions f o r the K 4 0 - A r 4 0 model age. They may be disregarded i n the i n t e r p r e t a t i o n of the emplacement h i s t o r y of the Nelson ba t h o l i t h . F i n a l l y , a remark should be made on samples D-66 and SILV -66. The sample D-66 yielded an age of 169 m.y. which appears high i n l i g h t of geologic evidence. From f i e l d geology, i t was observed that the lamprophyre dyke d e f i n i - t e l y cut the Nelson rocks (A.J. S i n c l a i r , personal communi- cation). This anomolous b i o t i t e age could again be explained by the fact that the prerequisite conditions might not be completely f u l f i l l e d . An excess of radiogenic argon i n the sample, i s not improbable. Richards and Pidgeon (1963), re- ported that some b i o t i t e samples of Broken H i l l of New South Wales, A u s t r a l i a , gave ages which were nearly twice the Rb-Sr age of the same phases. A'similar excess of radiogenic argon i n micas have also been reported by many others (Stockwell, 1963; Pepin et a l . , 1964). I t i s known that the measured radiogenic Ar40 content of mineral depends not only on the argon r e t e n t i v i t y and the temperature h i s t o r y of the _phase, but also on p a r t i a l argon pressure of the geological environment where the mineral was formed. I t seems therefore possible, that the high K 4 0 - A r 4 0 model ages appear to occur i n rocks formed under high pressure and temperature conditions. This implies that r e l a t i v e l y high argon pressure could possibly develop and consequently 55 excess radiogenic argon could be trappedifcito the existing or newly formed minerals. Nevertheless, the idea that argon content of mica may be affected by external argon pressure has been shown by Karpinskaya et a l . (1961). They proved experimentally that a considerable excess of argon can be forced i n t o muscovite under high argon p a r t i a l pressures and at r e l a t i v e l y high temperature. This has been supported by Hart (1961). Thus the s l i g h t l y high model age of 169 m.y. yielded by the b i o t i t e sample D-66, may indicate l o c a l l y high argon p a r t i a l pressures operating at some time i n the h i s t o r y of b i o t i t e . This supports further the hypothesis of deep-seated o r i g i n of the Nelson batholith proposed by L i t t l e (I960). However, i n the case of the present study the excess argon i s quite i n s i g n i f i c a n t . The error involved i s s t i l l within experimental l i m i t s , and the date obtained i s there- fore considered to be r e l i a b l e . Sample SILV-66 has been coll e c t e d from the eastern edge of a major fracture zone forming the western contact of the Nelson batholith. Ex- tensive c h l o r i t i z a t i o n i s recognized along t h i s zone (A.J. S i n c l a i r , personal communication). The rock has undergone considerable c a t a c l a s i s and b i o t i t e has been completely re- placed by pseudomorphs of c h l o r i t e . However, hornblende i s undeformed and appears fresh. The apparent age of 150 m.y. yielded by hornblende sample SILV-66 should therefore be i n - cluded i n data interpretation. In any case, the date seems 56 to be very i n t e r e s t i n g ; the high 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 . I t s usefulness i n the £40 - Ar40 dating i s unquestionably s i g n i f i c a n t . The high r e l i a b i l i t y of the hornblende dates has been affirmed by Hart (1964) and Gerling et a l . (1965). After the above-mentioned " r e s t r i c t i o n s " , the remaining data are four fresh b i o t i t e model ages, which y i e l d a mean value of 154±10 m.y. and three hornblende model ages which y i e l d a mean value of 158+4* m.y. (Table 3 . 5 ) . These val ues are believed to be r e l i a b l e within l i m i t s of experimental error. It may be seen that the pa i r of oogenetic b i o t i t e - hornblende from the specimen C-66 gives f a i r l y concordant isotopic ages; 151 m.y. f o r b i o t i t e against 164 m.y. for hornblende. The low b i o t i t e age may be explained by the fact that loss of 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 that i n hornblende. In other words, hornblende has had higher r e t e n t i v i t y of radiogenic argon than b i o t i t e (Hart, 1964); Gerling et a l . , (1965); Love ring & Richards, (1964). The low apparent age from b i o t i t e sample C-66 may be caused by heat produced along the contact zone, between V a l h a l l a and Nelson plutonic rocks, which would result i n argon l o s s from the re- l a t i v e l y older Nelson rocks. As the K4° - Ar40 model ages obtained from rocks and minerals may vary with the distance from an intrusive contact (Aldr-teh -etj—al., 1̂ 60-; Hart, 1964; Hanson and Gast, 1 9 6 7 ) , b i o t i t e may lose up to 5 ° percent of 57 i t s argon within one mile whereas within a very small distance even as near as 1,000 feet from the contact, hornblende may r e t a i n well a l l i t s argon (figure 1 -2 ) . Some p o s s i b i l i t y of error may occur i n these r e s u l t s which were^based on two- dimension studies instead of three. Further geophysical inves- t i g a t i o n s should be ca r r i e d out f o r a more accurate r e s u l t . However, quantitatively, i t may be concluded that the horn- blende model age of 164 m.y. i s more r e l i a b l e and HtGr-9 closer, to the true age than i t s oogenetic b i o t i t e model age of 151 m.y.. From the r e s u l t s obtained i n t h i s investigation, one may see^that i t i s impossible to d i s t i n g u i s h the various phases of the Nelson batholith by means of K40 _ Ar^O dating. The v a r i a - tions i n the model ages obtained were within the a n a l y t i c a l l i m i t s of e r r o r . This reaffirms the importance of f i e l d geology i n the i n v e s t i g a t i o n of phase relationships of an area. Many laboratory geochronologists have e-ften the tendency of over-" estimating the role of £40 _ Ar40 dating i n ge©chronology i n - vestigations. However, on the basis of the present study and the work of K. Northcote of the Geology Department, on the Guichon batho l i t h , i t i s c l e a r l y r e a l i z e d that without f i e l d geology controls, misinterpretation may a r i s e . In the case of the Nelson and Guichon batholiths, 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 various phases. The time at which minerals begin to r e t a i n the radiogenic argon i s nearly the same for each phase. Conversely, f i e l d geologists attempt to think of geological processes expanding at time i n t e r - v a l s of tens of m i l l i o n s of years, whete-aB^ih r e a l i t y , the periods * Univers i ty of B r i t i s h - C o l u m b i a . 59 V ' w h i c h From geological f i e l d study the sequence of eventsroccurred i n the Nelson batholith. may be summarized i n rough geochrono- l o g i c order as follows: The Nelson rocks, the Nelson s a t e l l i t e s , the V a l h a l l a rocks and the Lamprophyre dyke. I f the age of the Nelson batholith i s defined as the time r n i c a e e o u s at which the vminerals of the batholith s t a r t to r e t a i n radio- genic argon, then the model age of 1.54110 m.y. i s the represent- ing f i g u r e . However i t i s believed that the model age of 1J?8±7 m.y. obtained on hornblende i s closer to the true age of the Nelson b a t h o l i t h . The higher 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 recognized by many workers i n the f i e l d of geochronology (Hart, 1964; Gerling et a l . , 1965). I f the model age for the lamprophyre dyke i s accepted, then on the basis of geological evidence, 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 therefore older than 170 m.y,. Structural r e l a t i o n s require that the sample D -66 be younger than the other samples. The model age obtained of l 6 9 i 6 m.y. i s s l i g h t l y higher than one would expect on s t r u c t u r a l evidence. Possible excess of radiogenic argon i n t h i s sample has been discussed above. X-ray d i f f r a c t i o n was also used to check whether or not the sample has been contaminated by other minerals such as clino-pyroxene. The result from X-ray d i f f r a c t i o n affirmed further the "purity" of the sample. In any case, the "excess age" i s so small, within the l i m i t s of experimental error, that 60 the date does not cause any s i g n i f i c a n t c o n f l i c t to the model age obtained on hornblende samples. From the above discussion, i t i s preferable t o accept a true "age" f o r the Nelson batholith within the l i m i t s of con- t r o l of data on b i o t i t e , hornblende and geological (lampro- phyre dyke') ; model ages. Therefore the average K40 _ A r 4 Q model age of 156+9 m.y. may be considered to be ta:: close approximat'oh<to the true age of the batholith. 61 CONCLUSION The K40 _ Ar^O is o t o p i c age determinations on b i o t i t e , hornblende and pyroxene of the Nelson ba t h o l i t h i n the present study indicate that the various phases of the batholith (at least i n the northern part), were emplaced during a short i n - te r v a l of time, not more than 18 m.y., centered around 156 m.y., corresponding to the Upper-Middle Jurassic boundary of the Kulp's time scale. At some l a t e r time, le s s than 120 m.y.. ago, the batholith was subjected to at l e a s t one period of hydrothermal a l t e r a t i o n , r e s u l t i n g i n l o c a l c h l o r i t i z a t i o n and therefore variable loss of radiogenic argon from b i o t i t e s , thus accounting f o r the scatter of model ages between 120 - 146 m.y.. It i s also concluded that the various phases of the Nelson batholith cannot be distinguished on the basis of the K 4 0 ^ A r 4 0 i s o t o p i c dating; the variations i n "ages" of these phases were within experimental error l i m i t s , of the an a l y t i c a l tech- niques used. On the basis of the res u l t s obtained on two pai r s of oogenetic samples, biotite-hornblende and biotite-clinopyroxene, i t i s evident that: Clinopyroxene contains s i g n i f i c a n t excess of radiogenic argon and therefore i t should not be used f o r K40 _ Ar^O dating. Hornblendes give r e l i a b l e K40 _ A r 4 0 model ages. Their high r e t e n t i v i t y of radiogenic argon i s worthy for praise, B i o t i t e s may contain excess e f radiogenic argon, p a r t i c u l a r l y under high temperature and high argon pressure environmental conditions. I t i s therefore evident that 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, for i d e n t i f y i n g hitherto unsuspected complexities. However, i t i s necessary to consider the K 4 0 - A r 4 ^ dates along with a l l available petrographic and f i e l d geology evid- ences to be able to gifre a clearcut conclusion. •the Further investigations on vNelson batholith, e s p e c i a l l y i n the southern part, should involve further detailed mapping i n conjunction with isotopic age determinations, p a r t i c u l a r l y by R b ^ - S r ^ method. This might unravel the metamorphic , st r u c t u r a l and int r u s i v e h i s t o r y of the whole area. 63 BIBLIOGRAPHY Al d r i c h , L.T. and G.W. Weth e r i l l . Geochronology hy radio- active decay. Ann. Rev. Nuclear S c i . , 8., 273 (1938) Allsopp, H.L. Rb - Sr and K - Ar age measurements on the Great Dyke of Southern Rhodesia, J . Geophys. 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Some d i f f u s i o n measurements r e l a t i n g the K - Ar dating method. Eighth Ann. Prog. Report US Atomic Energy Comm., M.I.T., Cambridge 87 (I960) 66 Hart, S.R. The use of hornblendes and pyroxene for K - Ar dating. J . Geophys. Res., 2995 (1961) Hart, S.R. The petrology and isotopic mineral age relations of a contact zone i n the Front Range. Colorado, J . Geoc.-, 1 2 , 493 (1964) Hart, S.R. A te s t f o r excess radiogenic argon i n micas. J". Geophys., Res., 21, .1769 (1966) Hart, S.R. and R.T. Dodd. Excess radiogenic argon i n nyroxenes, J. Geophys. Res., 6£, 2998 ^1962) Houtermans, F.G. History of the K - Ar method of geochronology i n Potassium-argon dating (Schaeffer, O.A. and J . Zahringer, Eds.) Springer-Verlag, New York (1966) Hower, J., Hurley, P.M., Pinson, W.H. and H.W. Fa i r b a i r n . The dependence of K - Ar age on the mineralogy of various p a r t i c l e size ranees i n a shale. Geochim. et Cosmochim. Acta, 22, 403 (1963) Hunt, G. Time of P u r c e l l eruption i n southeastern B r i t i s h Columbia and southwestern Alberta. Alberta Soc. P e t r o l . Geol. J . , 10, 438 (1962) Hurley, P.M.,- Hughes, H., Pinson, W.H. and H.W. F a i r b a i r n . Radiogenic argon and strontium d i f f u s i o n parameters i n b i o t i t e at low temperatures obtained from Alpine f a u l t u p l i f t i n New Zealand. Geochim. et Cosmochim. Acta, 26, 67 (1962) Hurley, P.M. E - Ar dating of sediments, i n Potassium-argon dating (Schaeffer, O.A. and J". Zahringer, Eds.), Springer- Verlag, New York (1966) Jost, W. 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. Academic press, New York, ( i 9 6 0 ) Kanasewich,E.R. Quantitative interpretations of anomalous lead isotope abundances. Ph.D Thesis, Univ. of B.C. (1962) Karpinskaya, T.B., Ostrovskii, I.A. and L.I. Shanin. A r t i f i c i a l introduction of argon in t o mica under conditions of high pressure and temperature. Izv. Akad. Nauk. USSR Izv., Ser., Geol., &, 99 (1961) Kulp, J.L. Geologic time scale. Science, 133. 1105 (1961) 67 Kulp, J.L. and J . Engels. Discordances i n K - Ar and Rb - Sr isot o p i c ages: Discussion i n Radioactive dating. Inter- national Atomic Energy Agency, Vienna, 219 (1963) Lamphere, M.A. and G.B. Dalrymple. P -207: An interlaboratory standard muscovite for argon and potassium analyses. J . Geophys., Res., JO.* 34-97 U 9 6 3 ) Lamphere, M.;Ju. and G.B. Dalrymple. K - Ar and Rb - Sr measure- ments on P - 2 0 7 ; the USGS interlaboratory standard mus- covite. Geochim. et Cosmochim. Acta, 1091 (1967) Larson, E.S., Gottfried, D., Ja f f e , H. and C L . Waring. Age of the southern C a l i f o r n i a , S i e r r a Nevada and Idaho batholiths. B u l l . Geol. Soc. Am., 6£, 1277 (1954) Leech, G.B. and R.K. Wanless. Lead-isotope and Potassium-argon studies i n the East Kootenay d i s t r i c t , B r i t i s h Columbia. Geol. Soc. Amer., Buddington Volume, 241 (1962) Leech, G.B., Lowdon, J.A., Stockwell, CH. and R.K. Wanless. Age determinations and Geological Studies. Geol. Surv. Can., Paper 63-17 (1963) L i p p o l t , H.J. and W. Gentner. K - Ar dating of limestones and f l u o r i t e s , i n Radioactive dating; International Atomic Energy Agency. Vienna, Proc. Sev., 239 (1963) Lipson, J . J . Pot as s i urn-argon dating of sedimentary rocks. B u l l . Geol. Soc. Am. 6 2 , 137 (1953) L i t t l e , H.W. Nelson map-area, west-half, B r i t i s h Columbia. Geol. Surv. Can., Memoir 308 ( i 9 6 0 ) Lovering, J.F. and J.R. Richards. Potassiurn-argon age study of possible lower-crust and upper-mantle inclusions i n deep- seated intrusions. J . Geophys., Res., 6_9, Lowdon, J.A. Age determinations by the Geological Survey of Canada, Report 2 - Isotopic ages; Geol. Surv. Can., paper 61-17 (1961) Lowdon, J.A., Wanless, R.K., Stockwell, CH. and G.B. Leech. Age determinations and geological studies, Report 4 , Geol. Surv. Can., paper 63-17 (1963) Mulligan, R. Bonnington map-area, B r i t i s h Columbia. Geol. Surv. Can., paper 52-13, 13 (1952) Mc^ougall, I. Potassium-argon age measurements on dolerites from Antartica and South A f r i c a . J. Geophys. Res., 6 8 , 1535 (1963) 68 McDougall, I. and D.H. Green. Excess radiogenic argon i n pyroxenes and isotopic ages on minerals from Norweigian eiilogites. Norsk, Geol. Tidsskr, 44, I83 ̂ 1964) Newmarch, G.B. Geology of the Crowsnest Coal Basin, B.C. Dept. Mines, B u l l . 33 (1953) Nier, A.O.A. Redetermination of the r e l a t i v e abundances of the isotopes of carbon, nitrogen, oxygen, argon and potassium. Phys. Rev. 21, 789 (1950) Pepin, R.O., Reynolds, J.H. and F. Turner. Shock emplaced argon i n a stony meteorite. 2. A comparison with natural argon i n i t s d i f f u s i o n . J. Geophys. Res., 69» 1406 (1964) Polevaya, N.I., Kazakov, G.A. and G.A. Murina. Glauconite as an indicator of geologic time. Geokhimiya, 1, 3 ( i 9 6 0 ) Rama, Si.N.I., Hart, 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., 10, 509 (1965) Read, H.H. Metamorphism and granatization; Geol. Soc. S. A f r i c a , Annexure to v o l . 54 ( l 9 5 l ) Reichenberg, D. Properties of Ion-Exchange r e s i n i n r e l a t i o n to t h e i r structure. I I I . Kin 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 analysis. Rev. S c i . Inst., 27, 928 (1956) Reynolds, P.H. A lead isotope study of ores and adjacent rocks. Ph.D Thesis, Univ. of B.C. (1967) Richards, 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. Geol. S o c , Au s t r a l i a , 10 243 (1963) Ross, J.V. A note on histogram analysis of isotopic age data. Can. J. Earth S c i . , ^ 293 (1966) Russell, R.D., S h i l l i b e e r , H.A., Earguhar, R.M. and A.K. M o a s u f . The branching r a t i o of potassium 40. Phys. Rev., 91, 1223 (1953) ~ Sardarov, S.S. Retention of radiogenic argon i n glauconites. Geokhimiya, 1 0 , 905 (1963) 69 S h i l l i b e e r , H.A., Russell, 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 lead isotope study of mineral deposits i n the Kootenay Arc..-.,Ph.D Thesis, Univ. of B.C. (1964) S i n c l a i r , A.J. and W.C. Lihhy. D i s t r i b u t i o n of major minerals i n a stock of Nelson plutonic rocks, south central B r i t i s h Columbia (abstract), Can. Miner 308 (1967) Smith, A.G. Potassium-argon decay constants and age tables. The phanerozoic time scale, sympos. Geol. Soc. (London), Bartholomen Press, England, 129 (1964) Stern, T.W., Newell, M.F., K i s t l e r , R.W. and D.R. Shawe. Z i r - con uranium-lead and thorium-lead ages and mineral po- tassium-argon ages of La Sal Mountains rocks, Utah. J . Geophys. Res., J o , 1503 (1965) Stockwell,. CH. Third report on s t r u c t u r a l provinces, orogen- ies and t i m e - c l a s s i f i c a t i o n of rocks of the Canadian Precambrian s h i e l d . Geol. Surv. Can., paper 63-17, 125 (1963) Thompson, J . J . On the emission of negative corpuscles by the a l k a l i metals. P h i l . Mag., 10, 584 (1905) Wassenburg, G.J. and R.J. Hayden. The branching r a t i o of K40. Phys. Rev., 9_3, 645 (1954) Von Weizsacker, C.F. Uber die ^ 6'glichkeit eines dualen Beta- Z e r f a l l s von Kalium, Phys.Z.38, 623 (1937) Wetherill, G.W. Radioactivity of potassium and geologic time. Science, 126, 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., T i l t o n , G.R. and R.J. Hayden. Decay of constants of K40 as determined by the radiogenic argon content of potassium minerals. Phys. Rev., 10J5, 987 (1956) White, W.H., Erickson, G.P., Northcote, K.E., Dirom, G.E. and J.E. Harakal. Isotopic dating of the Guichon batholith, B.C. Can. J. Earth Sc., 4, 677 (1967) Zartman, R.E. A geochrologic study of the Lone @rove pluton from the Llano U p l i f t , Texas. J. Petrology,' 359 (1964) 70 APPENDIX I EXPERIMENTAL PROCEDURE A - I - l Mineral Separation i The samples were extracted to detain concentrate py- . roxene, hornblende and b i o t i t e . The equipment;r wasD c a r e f u l l y cleaned and decontaminated from the previous samples with a s p l i t of the sample to be processed. Rocks were crushed, gribuiid and sieved into size f r a c - t ions ranging from 35 mesh t o 100 mesh (in the case of hornblende, i t was often necessary to reach a f i n e r s i z e ) . Using i n various 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 separation techniques. The procedures may be summarized i n the following flow, sheet i n figure A - I - l . To obtain pure minerals, great care was taken. As a f i n a l step of clean-up of the sample, i t was always necessary to use "hand picking" to remove any impurity, the surest way to obtain a clean sample of not l e s s than 99% pure hornblende or b i o t i t e . 71 F i g . A - I - l • Flowsheet showing various steps of mineral separation. Hand size rock I jaw crusher I gyratory-crusher 1 * cone crusher I screen 14 mesh I Pulverizer - 14 mesh Screen 1 i I I I 33 mesh -33 + 50 mesh - 5 0 +70 mesh - 7 0 +100 mesh - 1 0 0 mesh I Wash i n H 2 Q column 1 Dry (65° - 100°0) I Ding e l e c t r o s t a t i c separator A: BIOTITE CONCENTRATE B: REJECT A: B i o t i t e Concentrate . I sink ( b i o t i t e concentrated) i P l a s t i c sheet tetrabromoethane B i o t i t e concentrated reject I f l o a t (reject) handpicking under binocular microscope f o r complete cleaning I b i o t i t e )> 99% pure 72 Fig.. A - I - l (Cont'd.. . ) B; Reject Remove strongly magnetic p a r t i c l e with hand magnet Frantz isodynamic separator ( v e r t i c a l position) r e j e c t , quartz, feldspar hornblende concentrate tetrabromoethane f l o a t (reject, quartz, feldspar) sink (hornblende concentrate) Diiodomethane f l o a t (hornblende concentrate) sink (epidote) 1 Frantz i n c l i n e d feed (20° forward, 15° side, «r 0.425 amp.) rej e c t hornblende concentrate I p l a s t i c sheet (remove b i o t i t e ) handpicking under binocular microscope for further cleaning hornblende 99% p u r i t y 73 In the separation of the minerals from rocks, the only- d i f f i c u l t y met was the removal of a l l epidote grains (the common impurity) from amphihole concentrates. To achieve an optimum separation i t was found necessary to use a high-power binocular microscope, which was also employed to monitor each step of separation, as an a i d for hand picking of impurities. However, the presence of a small amount of epidote i n the amphibole concen- tr a t e s introduces i n s i g n i f i c a n t error i n potassium con- tent as the content of potassium i n epidote i s negligible (Dirom, 1965). The p u r i f i e d pyroxene, hornblende and b i o t i t e concen- trates were washed several times i n acetone or methyl alcohol to remove the heavy l i q u i d before being used for analysis, A-1-2 The Determination of Potassium Content i n Mineral by Same Photometry The potassium content i n mineral was determined by flame photometric techniques involving two main steps: 1. Acid digestion of mineral sample by chemical pro- cedures. 2. Determination of potassium content by flame photometer. 74 Chemical Procedures 1. About four aliquots of approximately 0.25 gm of b i o t i t e sample were extracted by micros- p l i t t i n g ( i n the case of muscovite, aliquots of about 0.18 gm should be reasonably adequate). 2. Weigh samples in t o t e f l o n dishes. 3 . Add 1 .5 ml concentrated H2SO4 and 25 ml 49% HF. 4. Place on hot plate and evaporate down to about 5 ml at a temperature of approximately 250°C (using asbestos mats on the hot plate to protect t e f l o n dishes). 5. Add 10 ml HF and evaporate down again to about 5ml» 6. Add 2 ml concentrated HNO^, the temperature was then increased u n t i l the f i r s t v&ite fumes of H2SO4 appear. 7. The samples were allowed to cool, about 50 ml of pure water were added and evaporate down again. 8. Add 50 ml of pure H2O, warm and transfer solution to a 250 ml beaker. 9. Place on hot plate and bring to a b o i l , stop and.cool. 10. To each a l i q u o t , add 50 ml of stock solution contain- ing 5 ,000 ppm Na and 2 ,000 ppm l i . 75 1 1 . Transfer each aliquot t o a 500 ml volumetric f l a s k and dilute with pure water to a f i n a l volume of 500 ml. 1 2 . Store i n clean, dry and tight locking cap polyethylene bottles f o r flame photometric analysis. In the case of hornblende or pyroxene, the above-described procedure was used. Due to the low potassium contents i n hornblende and pyroxene, samples of 0 . 7 5 gram were decomposed i n a t o t a l of 3 nil of concentrated HgSO^ 35 ml and 49% HF and 2 ml concentrated HNO3. However, i f the samples r e s i s t complete d i s s o l u t i o n , further additional heating and i f necessary, an addition of approximately 1 ml of concentrated HNO^ was found to y i e l d s a t i s f a c t o r y digestion. The use of other reagents such as perchloric a c i d was therefore avoided. The solution was transferred t o a 250 ml volumetric f l a s k to which was added 25 ml of the stock solution contain- ing 5 , 0 0 0 ppm Na and 2 , 0 0 0 ppm l i . The solution was then made to volume with pure water and stored i n poly- ethylene bott l e s for flame photometric analysis. Flame Photometric Measurement The sample solutions were run on a Baird Atomic model KY-1 flame photometer, bracketed by standard solutions ranging from 0 ppm to 50 ppm i n approximately 4 ppm increments. These standard solutions contain the same 76 concentration of l i , Na and H2SO4 as the sample solutions. However, H2SO4 was not necessarily added to thte standard solutions for b i o t i t e solutions because the interference e f f e c t of H^04r. t was negligible i n t h i s case (Dirom, 1965). The r e s u l t s were then recorded on a 2-second response, 10 m i l l i v o l t zero centre WESTON s t r i p chart recorded connected i n series with*: photometer output. A f t e r standardizing the flame photometer as recommended by the maker's instructions> four runs for each analysis were made i n alternative d i r e c t i o n . The r e s u l t s of the four runs were then averaged. The concentration i n t e r v a l between the higher and lower standard solutions was^reasonably kept small so that the r e l a t i omship between concentration of potassium i n the standard and sample solutions may be considered l i n e a r . The concentration of potassium i n the sample solut i o n was then calculated by using the following equation. x = y - y l (x2 - x l ) + x l 72 - y i where x - concentration of potassium i n the sample soluti o n x l a concentration of potassium i n the lower standard solution x2 = concentration of potassium i n the upper standard sol u t i o n 77 y s d i a l reading f o r the sample solution y l = d i a l reading f o r the lower standard solution y2 » d i a l reading for the higher standard solution The Determination of Argon Content i n Minerals The radiogenic argon content i n minerals was determined by mass spectrometric isotope d i l u t i o n techniques. The argon a n a l y t i c a l system used f o r the present study consists of 3 main sections, as shown i n figure 3*1 • - the fusion and f i r s t p u r i f i c a t i o n section - the second p u r i f i c a t i o n section - the mass spectrometer section - the argon 38 spike metering section - 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 high vacuum metal valves, any of these sections can be i s o l a t e d or connected properly. Except f o r the mass spectrometer section where an i r o n pump was used, the other four sections were evacuated by mercury d i f f u s i o n pumps, backed by high vacuum rotary pumps. Five p i r a n i gauges were also added into the system for the measurement of the pressure of the d i f f e r e n t sections. The Radiogenic Argon Extraction and P u r i f i c a t i o n s A sample charge of from 0 . 5 to 0.8 gram b i o t i t e ( i n the case of hornblende and pyroxene, the charges used were from 2 t o 6 grams), contained i n an alundum en- 78 closed molybdenum crucible, was placed i n the fusion jar (water jacketed pyrex j a r ) , which, by a copper gasketed flange coupling, was connected to the argon l i n e . I t was outgassed by overnight baking at a temperature of approximately 200°C, which was done only i n the case of the hornblende and pyroxene sample. In the case of the b i o t i t e sample, overnight taking at room tempera- ture was found s u f f i c i e n t l y adequate. The sample was heated by means of a 6 KW radio frequency generator and induction c o i l f i t around the fusion j a r . Gold trap and U trap containing activated charcoal were used and kept at tempera- ture of nitrogen l i q u i d . Hydrogen and carbon monoxide are oxidized to become water and carbon dioxide which together with the other impurities were removed by the cold finger trap which prevents the water from reaching the charcoal trap; nitrogen was removed by allowing the gas to react with the hot titanium sponge. When the temperature of the sample attained approximately 1,000°C an Argon 38 spike was admitted into the fusion and f i r s t p u r i f i c a t i o n section by means of two high-vacuum metal valves. The temperature of the sample was then raised gradually to i t s f i n a l value of approximately 1,500°C, and at t h i s temperature the fusion of the sample may be considered as complete. 79 However, i t was maintained at t h i s temperature f o r almost 15 minutes to ensure the complete removal of argon from the sample. A l l the argon was then absorbed by activated charcoal at low temperature, (the temperature of the nitrogen l i q u i d ) . To ensure that 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 titanium sponges and pumping system i n the fusion and f i r s t - p u r i f i - cation and second p u r i f i c a t i o n sections. The system was so designed that any number of p u r i f i c a t i o n cycles could be conveniently r e a l i z e d by allowing 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 single cleanup cycle i s found to be adequate, e s p e c i a l l y i n the case of the b i o t i t e sample. After the p u r i f i c a t i o n steps have been completed, argon was absorbed on charcoal trap at nitrogen l i q u i d temperature and the remaining gas was pumped away. Mass Spectrometric Measurement Before each analysis, leak testing was car r i e d out to check the build-up of the mass 40 s i g n a l . This was done simply by opening each section i n turn t o 80 the mass spectrometer. A measure of the r e l a t i v e abundance of argon 40 and argon 36 contained i n the sample enabled one to make a correction f o r any contamination of the sample by atmospheric argon. The p u r i f i e d gas from the second p u r i f i c a t i o n section was then admitted to the mass spectrometer section of the system to measure i t s is o t o p i c com- po s i t i o n . The mass spectrometer used for the present study i s a eli-nieal model MS10, made by Associated E l e c t r i c Industries Ltd., using a two inch radius of curvature with 180° d e f l e c t i o n and a permanent magnet producing a magnetic f i e l d of approximately I83O gauss. Some s l i g h t modifications have been made, for ex- ample a more sensitive v i b r a t i n g reed electrometer amplifier with a 10H ohm input r e s i s t o r to replace the maker's electrometer. The mechanical scanner was also replaced by an e l e c t r o s t a t i c scanner which causes the appropriate variat i o n s i n accelerating voltage. Therefore any mass peak may be reached immediately, the recording time becomes shorter and the s i g n a l decay becomes consequently n e g l i g i b l e . Other detailed descriptions may be found i n the maker's in s t r u c t i o n s catalogue. A Honeywell - 81 Brown potentiometer-type recorder, of \ second res- ponse, 10 m i l l i v o l t f a l l scale, 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 to the ma spectrometer tube which was then i s o l a t e d by an u l t r a high vacuum metal valve and analyzed. As the mass spectrometer i s maintained continuously at a pressure of l e s s than 10-8 mm Hg, by means of an l i o n pump, the vacuum inside the mass spectro- 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 for the s t a t i c a l analysis to be carried on. Before each run, the background was checked; i t was observed that the argon 3 6 and argon 38 background was reduced to minimum so that they were nearly undetectable. Argon 40 background was also reduced to minimum so that i t was only detectable on the most sensitive range. Only mass 28 signal which i s supposed to be due to nitrogen and carbon monoxide was s i g n i f i c a n t . Mass 44 signal which i s supposed to be due to carbon dioxide was also reduced to minimum. It was found that an amount of between 5 and 10 v o l t s of argon 40 signal admitted to the mass spectrometer i s e f f e c t i v e l y adequate -for the analysis to be ca r r i e d out. Figure A-I -2 : Fractionation correction vs % admitted t o the spectrometer tune. 8? Isotopic r a t i o s Ar40/Ar38 and Ar at lea s t 10 times f o r each sample, were measured and averaged. Before being used i n the age c a l c u l a t i o n these r a t i o s had to be subjected to two corrections: discrimination correction and f r a c t i o n a t i o n correction. Discrimination i n the mass spectrometer has been evaluated by analyzing atmospheric argon. According to Graham's Law, is o t o p i c f r a c t i o n a t i o n i s caused by the slower passage of the heavier isotope into the leak valve r e l a t i v e to the l i g h t e r isotope. In order to have the f r a c t i o n a t i o n at the i n l e t leak valve evaluated, a check of the t o t a l quantity of argon sample admitted to the mass spectrometer was always made. With the designed system used i n the present study, t h i s requires no d i f f i c u l t i e s . A portion of gas sample of approximately i n volume of the second p u r i f i c a t i o n section, could be measured accurately with mass spectrometer. A graph showing fr a c t i o n a t i o n correction against the percentage of quantity of gas admitted to the mass spectrometer for analysis i s given i n figure A-I-2. I 84 APPENDIX I I NELSON PLUTONIC ROCKS DESCRIPTIONS OF SPECIMENS FOR K-VDATING (PREPARED BY DR. A . J . SINCLAIR) Specimen A - 6 6 : The rock i s a massive, medium-grained, leucocratic granod- i o r i t e of the V a l h a l l a plutonic suite that appears fresh i n hand specimen. A th i n section mode i s : plagioclase (50%), quartz (24%), K-feldspar-perthitic microcline (15%), b i o t i t e (5%), horn- blende (1%), sphene (1%), opaque minerals (1%), museovite and s e r i c i t e (1%), and trace amounts of epidote, c h l o r i t e , apatite, carbonate and zirco n . The rock has undergone very s l i g h t defor- mation as indicated by shadowy extincti o n of quartz and gently curved cleavage traces of some b i o t i t e grains. 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. B i o t i t e i s fresh i n appearance. A small amount of c h l o r i t e i s present as mono- mi n e r a l l i c grains. L o c a l l y , plagioclase has been altered to epidote, c a l c i t e and s e r i c i t e , although most of the epidote i n the rock occurs as f a i r l y large, anhedral grains associated with hornblende. Specimen C - 6 6 : The specimen of Nelson plutonic rock i s a medium - to coarse- grained, massive hornblende quartz monzonite containing 10% mafic minerals (mainly hornblende) and 90% f e l s i c constituents (mainly feldspar) with a few percent subhedral, K-feldspar phenocrysts up to 15 mm. i n length. A mode determined from t h i n section i s : plagioclase-Ango (47%), K-fsldspar-microcline (30%), quartz (15%), hornblende (5%), b i o t i t e (3%), and trace amounts of myrmekite, 85 apatite, sphene and opaque minerals. There i s no evidence of superimposed a l t e r a t i o n e f f e c t s . Specimen D - 6 6 : The rock i s a fine-grained, grey dyke that cuts Nelson plutonic rock. Rock-forming minerals could not be i d e n t i f i e d i n hand speci- men although a minor amount of chalcopyrite was observed. A mode determined from t h i n section i s as follows: plagioelase-An^i (56%), clinopyroxene (50%), b i o t i t e (12%), o l i v i n e and iddingsite (2%) and trace amounts of apatite and opaque minerals. Plagioelase occurs mainly 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. Olivine and idd i n g s i t e occur as r e l i c t anhedral cores texture. The specimen shows no eff e c t s of a l t e r a t i o n apart from what can be ascribed to deuteric processes. Specimen K - 6 6 : The specimen i s a medium-grained, massive, porphyritic horn- blende quartz monzonite from Mount Carlyle stock containing abun- dant pale pink phenocrysts of K-feldspar up to 2 . 5 cm. i n length. A medium-grained matrix consists predominantly of plagioelase, quartz and about 10% hornblende as narrow prisms about 2 mm. long. A mode estimated from t h i n section i s : K -feldspar-perthitic micro- c l i n e (45%), plagioclase - A n 2 0 (37%), quartz (10%), hornblende (5%), epidote-including a l l a n i t e (1%), sphene (1%), b i o t i t e (0 .5%) and trace amounts of myrmekite, c h l o r i t e , carbonate, apatite, zircon and opaque minerals. A l t e r a t i o n e f f e c t s are minor. Minerals are fresh i n appearance, although very small amounts of ch l o r i t e are present, apparently pseudamorphous after b i o t i t e . 86 Specimen S i l v - 6 6 : The rock from the northwest corner of the Nelson Batholith i s a c h l o r i t i z e d hornblende quartz monzonite, medium-grained, containing about 30% mafic constituents. Rare phenoerysts e x i s t up to 1 cm. i n maximum dimension. Trace amounts of pyrite can be seen i n the hand specimen. The specimen l o c a l i t y i s on the eastern edge of a major fracture zone that forms the western contact of the northern part of the Nelson Batholith. Extensive c h l o r i t i z a - t i o n i s recognized along t h i s zone and south of the sample l o c a l i t y quartz-microcline pegmatites have been introduced l o c a l l y within the zone. The rock has undergone ^considerable c a t a c l a s i s — a b o u t 20 pereent of the specimen i s very fine-grained c a t a c l a s t i c material that has been r e c r y s t a l l i z e d . A mode estimated from t h i n section i s as follows: p l a g i o c l a s e - A n ^ (30%), K-feldspar-micro- c l i n e (30%), hornblende (20%), quartz (15%), c h l o r i t e (3%), sphene (1%) and trace amounts of epidote, apatite and opaque minerals. A l t e r a t i o n i s extensive i n that b i o t i t e has been completely re- placed by pseudomorphs of c h l o r i t e . No other e f f e c t s of chemical al t e r a t i o n s are apparent. Hornblende i s undeformed and fresh i n appearance. Specimen R - l l : The specimen i s a medium-grained, melanocratic granodiorite with subparallel alignment of mafic minerals and tabular feldspar c r y s t a l s . I t contains rare phenocrysta of K-feldspar up to 1.5 cm long. A mode estimated from t h i n section i s : plagioclase (50%), K-feldspar (15%), hornblende (15%), b i o t i t e (10%), quartz (10%), 87 and trace amounts of epidote, ;sphene, apatite, myrmekite, c h l o r i t e and opaque minerals. A l t e r a t i o n e f f e c t s are s l i g h t with only trace amounts of c h l o r i t e occurring as interleaves with some b i o t i t e c r y s t a l s . Specimen R-12: The rock i s a medium - to coarse-grained melanocratic quartz d i o r i t e with vague f o l i a t i o n of mafic minerals ( b i o t i t e and horn- blende). The specimen i s fresh i n appearance. A r e l a t i v e l y un- common feature compared with most Nelson plutonic rocks i s the absence of K-feldspar phenocrysts. A mode determined from t h i n section i s : plagioelase (60%), clinopyroxene (12%), b i o t i t e (10%), quartz (10%), K-feldspar (5%), apatite (2%), hornblende (1%), and trace amounts of sphene. The rock i s generally fresh 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 plagioelase i s distributed sporadically but i s not extensive. Specimen R-l3 : The rock, a medium-grained granodiorite from the Nelson Batholith, has rare K-feldspar phenocrysts up to 2 cm. long. A f a i n t f o l i a t i o n i s evident due to alignment of mafic minerals and tabular K-feldspar phenocrysts. Mafic minerals ( b i o t i t e and horn- blende) account f o r about 20% of rock volume. The specimen has an ove r a l l grey caste and appears f r e s h . A mode estimated from t h i n section examination i s : plagioclase-An25 (40%), quartz (25%), b i o t i t e (15%), K-feldspar (12%), hornblende (5%), epidote (2%) and trace amounts of c h l o r i t e , clinopyroxene, myrmekite, sphene, apatite, s e r i e i t e and opaque minerals. The only evidence of a l t e r a t i o n i s li m i t e d formation of s e r i e i t e and epidote i n cores of some plagioelase grains, and some interleaves of c h l o r i t e i n b i o t i t e c r y s t a l s . 88 Specimen L - 2 3 6 : The specimen i s a fine-grained, grey hornfels taken from an in c l u s i o n within Nelson plutonic rocks. Original character of the rock i s uncertain although i t may represent metamorphosed and metasomatized rock of the Slocan Series. A vague suhparallel alignment of h i o t i t e c r y s t a l s can he seen i n t h i n section. Exam- ina t i o n of a t h i n section revealed the following mode: plagioclase (64%), b i o t i t e (20%), quartz (10%), K-feldspar-microcline (2%), epidote (2%), apatite (2%), and trace amounts of sphene, s e r i c i t e and opaque minerals. The specimen i s f a i r l y fresh i n appearance although a s i g n i f i c a n t proportion (about 15%) of the b i o t i t e has ranged edges and appears almost opaque along some cleavage planes suggesting that i t i s i n the i n i t i a l 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 dyke-like i n t r u s i o n i n rocks of-the Slocan Series and i s considered a " s a t e l l i t e " of the Nelson batholith to the south. It consists of about 85% medium - to coarse-grained phenocrysts i n a fine grained matrix. Plagioclase i s the only i d e n t i f i a b l e f e l s i c mineral i n hand specimen. Scattered anhedral blebs of pyrite can be seen and the rock effervesces on treatment with d i l u t e EG1 i n d i c a t i n g the presence of c a l c i t e . About 5% mafic minerals can be seen, only 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 . A mode determined from t h i n section i n v e s t i g a t i o n i s plagioclase - A n 2 g (67%), quartz (20%), b i o t i t e (3%), c a l c i t e (3%), c h l o r i t e (2%), muscovite and s e r i c i t e (2%), apatite (1%), K-feldspar-microcline (1%) and trace amounts of sphene, horn- 89 blende, epidote and opaque minerals. The rock has been extensively- altered with c h l o r i t e having formed p a r t i a l and complete pseudomorphs aft e r b i o t i t e , and plagioelase having been altered extensively to c a l c i t e , epidote and s e r i e i t e .

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