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The metamorphism and alteration of the basement rocks in the Carswell circular structure, Saskatchewan Herring, Bernard Geoffrey 1976

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THE METAMORPHISM AND ALTERATION OF THE BASEMENT ROCKS IN THE CARSWELL CIRCULAR STRUCTURE, SASKATCHEWAN. BY BERNARD GEOFFREY HERRING B.Sc. Queen's U n i v e r s i t y o f B e l f a s t , 1959 A" THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FCR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f GEOLOGY We a c c e p t 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 F e b r u a r y , 19 7 6 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r ee t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rpo se s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Co l umb i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date J_J_j_J__ i i • ABSTRACT This thesis reports on a petrographic study of the meta-morphosed Precambrian basement rocks exposed i n the Carswell C i r c u l a r Structure i n the Athabasca sandstone of northern Saskatchewan. A descriptive c l a s s i f i c a t i o n of the rocks present i s set up, and comprehensive i l l u s t r a t i o n s are provided for the benefit of subsequent workers i n the area. The basement rocks consist of gneisses and pegmatites, and have been metamorphosed to the granulite f a c i e s , with subsequent p a r t i a l retrogression to upper amphibolite f a c i e s . This retrogression evidently resulted from increased a v a i l a -b i l i t y of water, and may be related to the emplacement of pegmatites. A study of Mg-Fe p a r t i t i o n i n g between coexisting proxenes indicates that prograde metamorphism occurred at a temperature of about 8 00°C, consistent with the granulite f a c i e s . The basement rocks are highly altered by low-temperature hydration. Geochronologic studies suggest that t h i s a l t e r a t i o n occurred at about one b i l l i o n years before present, and i s therefore d i s t i n c t from prograde metamorphism (two b i l l i o n years) and from the C i r c u l a r Structure "event" (48 0 m i l l i o n years). The a l t e r a t i o n i s considered to have been related to uranium mineralisation, and a possible model for the emplace-ment of mineralisation i s discussed. CONTENTS LIST OF ILLUSTRATIONS LIST OF TABLES ACKNOWLEDGEMENTS CHAPTER I - I n t r o d u c t i o n G e n e r a l Statement L o c a t i o n and a c c e s s i b i l i t y - P r e v i o u s g e o l o g i c a l work G e o l o g i c a l s e t t i n g The C i r c u l a r S t r u c t u r e Uranium m i n e r a l i s a t i o n T e c t o n i c framework Scope and o b j e c t i v e s of stu d y CHAPTER I I - P e t r o g r a p h y o f t h e basement r o c k s I n t r o d u c t i o n P e l i t i c g n e i s s e s G e n e r a l d i s c u s s i o n G a r n e t - c o r d i e r i t e g n e i s s e s G a r n e t - s i l l i m a n i t e g n e i s s e s Q u a r t z o f e l d s p a t h i c g n e i s s s u i t e G e n e r a l d i s c u s s i o n Q u a r t z o f e l d s p a t h i c g n e i s s sensu s t r i c t o G r a n i t e g n e i s s e s M a f i c g n e i s s s u i t e G e n e r a l d i s c u s s i o n G r a n u l i t e s A m p h i b o l i t e s Pegmatoid r o c k s C a t a c l a s t i c r o c k s CHAPTER I I I - C o n d i t i o n s o f metamorphism I n t r o d u c t i o n i v Page Geographic d i s t r i b u t i o n o f assemblages 75 The s t a b i l i t y o f c o r d i e r i t e 7 7 Metamorphism o f t h e m a f i c g n e i s s e s 84 The o r t h o p y r o x e n e i s o g r a d 84 The i r o n - t i t a n i u m o x i d e s 8 5 The pyroxene geothermometer 8 8 S i g n i f i c a n c e o f pegmatoids 95 Metamorphic e v o l u t i o n 9 8 Summary o f metamorphic c o n d i t i o n s 9 8 R e t r o g r a d e metamorphism 9 9 R e t r o g r e s s i o n and m i g m a t i s a t i o n 1.01 CHAPTER IV - Post-metamorphic a l t e r a t i o n 103 G e n e r a l 103 S p e c i f i c a l t e r a t i o n r e a c t i o n s 105 I n t e r p r e t a t i o n 110 CHAPTER V - G e o l o g i c a l h i s t o r y and c o r r e l a t i o n w i t h o t h e r a r e a s 114 Geochronology o f t h e C a r s w e l l a r e a 114 G e o l o g i c h i s t o r y 116 C o r r e l a t i o n w i t h o t h e r a r e a s 119 CHAPTER VI - I m p l i c a t i o n s f o r m i n e r a l i s a t i o n 123 Source o f uranium 12 3 N a t u r e o f m i n e r a l i s a t i o n 124 A model f o r m i n e r a l i s a t i o n 126 BIBLIOGRAPHY 131 APPENDIX - E l e c t r o n m i c r o p r o b e a n a l y s e s 134 Po t a s s i u m - a r g o n age-dates 137 V F i g u r e i F i g u r e 2 F i g u r e 3 F i g u r e 4" F i g u r e 5 F i g u r e 6 F i g u r e 7 F i g u r e 8 F i g u r e 9 F i g u r e 10 F i g u r e 11 F i g u r e 12 F i g u r e 13 F i g u r e 14 F i g u r e 15 F i g u r e 16 F i g u r e 17 F i g u r e 18 F i g u r e 19 F i g u r e 20 LIST OF ILLUSTRATIONS L o c a t i o n Map and G e n e r a l i s e d Geology o f N o r t h e r n Saskatchewan Map o f t h e C a r s w e l l C i r c u l a r S t r u c t u r e showing l o c a t i o n s o f d i a m o n d - d r i l l h o l e s D e f o r m a t i o n l a m e l l a e i n q u a r t z C l u f f B r e c c i a Sheared q u a r t z i t e c l a s t from t h e b a s a l A t h a b a s c a F o r m a t i o n G a r n e t - c o r d i e r i t e g n e i s s C o a r s e l y p o r p h y r o b l a s t i c g a r n e t - c o r d i e r i t e g n e i s s T h i n - s e c t i o n o f g a r n e t - c o r d i e r i t e g n e i s s P o i k i l o b l a s t i c h a b i t o f c o r d i e r i t e P o i k i l o b l a s t i c h a b i t o f g a r n e t and c o r d i e r i t e P i n i t e a l t e r a t i o n o f c o r d i e r i t e C o r d i e r i t e pseudomorph w i t h s i l l i m a n i t e i n c l u s i o n s G a r n e t - s i l l i m a n i t e g n e i s s T h i n - s e c t i o n o f g a r n e t - s i l l i m a n i t e g n e i s s S i l l i m a n i t e and q u a r t z i n c l u d e d i n a g a r n e t p o r p h y r o b l a s t T h i n s e c t i o n of g a r n e t - s i l l i m a n i t e g n e i s s showing c r a c k i n g o f m i n e r a l g r a i n s S i l l i m a n i t e g r a i n s w i t h m u s c o v i t e r e a c t i o n S i l l i m a n i t e pseudomorph and c r a c k e d q u a r t z g r a i n s F i n e - g r a i n e d q u a r t z o f e l d s p a t h i c g n e i s s T h i n - s e c t i o n o f f i n e - g r a i n e d q u a r t z o f e l d s -p a t h i c g n e i s s Page 2 8 9 11 18 F i g u r e 21 F o l d e d h o r n b l e n d e - b e a r i n g g r a n i t o i d °-nei ss 21 2k 26 27 28 31 32 33 35 36 37 39 41 43 v i Page F i g u r e 22 C h l o r i t e r e p l a c i n g g a r n e t i n q u a r t z o - 44 f e l d s p a t h i c g n e i s s e s F i g u r e 23 G r a n i t o i d g n e i s s w i t h s t r o n g l i n e a t i o n 45 F i g u r e 24 G r a n i t o i d q u a r t z o f e l d s p a t h i c g n e i s s 46 F i g u r e 25 T h i n s e c t i o n o f g r a n i t o i d g n e i s s 47 F i g u r e 2 6 Two-pyroxene g r a n u l i t e 51 F i g u r e 27 T h i n s e c t i o n o f two-pyroxene g r a n u l i t e 52 F i g u r e 28 G r a n u l i t e w i t h l a t e h o r n b l e n d e 54 F i g u r e 29 C o a r s e - g r a i n e d a m p h i b o l i t e 56 F i g u r e 3 0 T h i n s e c t i o n o f a m p h i b o l i t e w i t h f r e s h 57 . h o r n b l e n d e and a l t e r e d f e l d s p a r F i g u r e 31 C o a r s e - g r a i n e d pegmatoid 60 F i g u r e 3 2 M a r g i n o f pegmatoid body 61 F i g u r e 33 C o n t a c t o f p e l i t i c g n e i s s and pegmatoid 62 showing d e f o r m a t i o n f a b r i c F i g u r e 34 Pegmatoid w i t h abundant g a r n e t s 63 F i g u r e 3 5 T h i n s e c t i o n o f pegmatoid d e m o n s t r a t i n g 64 complex p a r a g e n e s i s F i g u r e 3 6 M a r g i n o f pegmatoid body showing i n v a s i o n 6 6 o f h o s t - r o c k F i g u r e 37 P o r p h y r o c l a s t i c u l t r a m y l o n i t e 69 F i g u r e 38 M y l o n i t i s e d q u a r t z o f e l d s p a t h i c g n e i s s 70 F i g u r e 3 8a AFM d i a g r a m 73 F i g u r e 3 9 Phase d i a g r a m showing th e breakdown r e a c t i o n s 7 9 o f c o r d i e r i t e i n hydrous c o n d i t i o n s F i g u r e 40 Phase d i a g r a m showing t h e breakdown o f 8 3 magnesium c o r d i e r i t e i n anhydrous c o n d i t i o n s F i g u r e 41 P l o t o f c o m p o s i t i o n s o f c o e x i s t i n g m a g n e t i t e . 86 and i l m e n i t e s o l i d s o l u t i o n s F i g u r e 42 I l m e n i t e g r a i n i n g r a n u l i t e 87 v i i Page Figure 43 Composition diagram for pyroene-pairs 93 Figure 44 Phase diagram for hydrous aluminum s i l i c a t e s 111 Figure U5 Correlation of geological events i n the 120 Carswell and Beaverlodge areas VI11 T a b l e 1 T a b l e 2 T a b l e 3 Table 4 T a b l e 5 T a b l e 6 T a b l e 7 T a b l e 8 LIST OF TABLES Page M i c r o p r o b e a n a l y s e s o f g a r n e t from 2 9 p e l i t i c g n e i s s M i c r o p r o b e a n a l y s i s o f g a r n e t from p e g m a t i t e 66 M i n e r a l c o m p a t i b i l i t i e s f o r a l l r o c k t y p e s 72 Magnesium-iron r a t i o s o f g a r n e t s i n p e l i t i c 80 g n e i s s e s M i c r o p r o b e a n a l y s e s o f c o e x i s t i n g m a g n e t i t e 8 9 and i l m e n i t e M i c r o p r o b e a n a l y s e s o f c o e x i s t i n g o r t h o - 9 2 p y r o x e n e s and c l i n o p y r o x e n e s M i c r o p r o b e a n a l y s e s o f c o r d i e r i t e a l t e r a t i o n 106 p r o d u c t s M i c r o p r o b e a n a l y s i s o f m u s c o v i t e r e p l a c i n g 108 c o r d i e r i t e ACKNOWLEDGEMENTS The a u t h o r i s d e e p l y i n d e b t e d t o Amok L t d . f o r t h e i r generous f i n a n c i a l s u p p o r t f o r t h i s work and f o r p r o v i d i n g f a c i l i t i e s f o r f i e l d work i n c l u d i n g t r a n s p o r t a t i o n and t h e a v a i l a b i l i t y o f diamond d r i l l c o r e . Mr. G. G e o r g a k o p o u l i s I s t o be thanked f o r h i s h e l p and a d v i c e i n o p e r a t i n g t h e e l e c t r o n m i c r o p r o b e , and Mr. J . H a r a k a l f o r c a r r y i n g out t h e p o t a s s i u m - a r g o n d a t i n g . Thanks a r e due t o Dr. H. Wynne-Edwards f o r h i s a d v i c e and h i s r e a d i n g o f t h e manu-s c r i p t , as w e l l as t o . s e v e r a l o f t h e f a c u l t y and s t u d e n t s a t th e U n i v e r s i t y o f B r i t i s h Columbia f o r t h e i r many s t i m u l a t i n g d i s c u s s i o n s . I . INTRODUCTION 1 General Statement This study was undertaken in order tc determine the characte and the geologic hi s t o r y of the metamorphic rocks exposed i n the Carswell C i r c u l a r Structure. The Carswell Structure has attracte great inte r e s t i n recent years, as a r e s u l t of the discovery of Uranium i n 1969 by Mokta Ltd. Over one hundred diamond-core holes have been d r i l l e d on a reconnaissance basis In the Ci r c u l a r Structure, together with a large number of diamond-core and p e r c u s s i o n - d r i l l holes i n the mineralized zones (see figure 2). The present work i s based on examination of d r i l l - c o r e made available to the author by Mokta Ltd. and co l l e c t e d i n the summers of 1973 and 197*1 • Petrologic examination of about 100 thin-sections, supplemented by electron microprobe analyses, was conducted at the University of B r i t i s h Columbia. The author also had the opportunity to v i s i t the Beaverlodge mining d i s t r i c t , for the purpose of f a m i l i a r i z a t i o n with regional geology and to f a c i l i t a t e comparisons with the geology of the Carswell Area. Location and a c c e s s i b i l i t y The Carswell C i r c u l a r Structure i s located i n northwestern Saskatchewan, south of Lake Athabasca, about 130 km. south of Uranium City, and 30 km. east of the Alberta-Saskatchewan border 60° 110° 109° 108° 107° 105° 102! 5 9 ° 58°-^Tazin-Lake to bos B e a v e r l o d g e A r e a . C o m p l e x G e o l o g y . Gran i tes , g n e i s s e s a n d A m p h i b o l i tes B E A V E R L O D G E C o 0 ' U R A N I U M P R O D U C I N j S r ^ r - . D I S T R I C T ooooo ooo fo o o o o o o o 'OO O O O O O Oj O O O O o o o iO o o o o o o o o o o o lo o o o + + + + + 1K6 ; ^ Fort oewyar o::r\...;'Vr-> Fond ft-Lac,, • 5 9 ° ' v.- ••••• q v - O y ^ O o 0 0: 0= roo\om«e0^cS.. ^ ly"o.-..---Ci-.°-^/:o0-'a ^ ° ;:./ c^omplex z o n e ^ W ^ V ' »v'..vr0cffS - ™ • o 1 J>/ U r a n i u m P ' o p e r r } ^ v + 5 8 ° /• • Rabbit Lake Uran ium Proper ty t - 5 7 ' 3 ^ Methyl Lake[J I 0 10 20 I LES Sca le l " = 4 0 M i F i g . I : L o c a t i o n m a p a n d gene ra l i zed geology of Nor thern Saska tchewan Metasedimentary rocks o o o o ooo o o o ol Amphibol ites M i gmatites Granitic Rocks P r e c a m b r i a n Crys ta l l i ne B a s e m e n t V0 c> lo Athabasca Formation ( P r o t e r o z o i c ) ' o ) 3 (see figure 1 ) . Access i s by scheduled a i r service from Prince Albert (600 km.) or Uranium City to the C l u f f Lake mining camp which serves as a base for geological a c t i v i t i e s i n the area. ^ ^ y j ; 0 y s 0"H.9J;°_Sl5,al__V7ork The s t r u c t u r a l complexity of the Carswell area, as opposed to the surrounding h o r i z o n t a l l y bedded Athabasca Sandstone, was recognised by Blake (1956) > but Fabrig (1961) was the f i r s t to report the outcropping of metamorphosed basement rocks i n the area. Fahrig's work revealed the c i r c u l a r nature of the Carswell structure, and Currie (1967) l a t e r mapped the area i n some d e t a i l . Beck (3 970) gives an excellent summary of the regional geology of northern Saskatchewan, and considers theories of the origins of uranium mineralization. Geological Setting The northern part of Saskatchewan i s underlain by a complex series of pre-Cambrian gneisses of the C h u r c h i l l Province, with a regional s t r i k e i n a northeasterly d i r e c t i o n (see figure 1 ) . South of Lake Athabasca, the basement complex i s overlain by a body of f l a t - l y i n g unmetamorphosed Proterozoic sandstone (Fahrig, 1961) named the Athabasca Sandstone, and covering an area 400 km. from east to west, and 200 km. from north to south. Over most of this area, the sandstone i s about 200 m. thick and consists of generally quartzose d e l t a i c sediments. Two major f o l d - b e l t s can be traced along s t r i k e beneath the Athabasca sandstone, the Fond-du-Lac belt and the Wollaston Lake 4-Figure 2 . T H E C A R S W E L L C I R C U L A R S T R U C T U R E Locat ion of diamond drill holes Scale I 150, 000 Carswell Formation ( Dolomite) Athobascc Sandstone Metamorphic Basement Diamond dril l core examined by the author. ( A d -° ditional control within the mineralized zones not shown) Q Location of core specimen illustrated in text Outcrop sample illustrated in text u \J belt (see figure 1 ) , and the continuity of these belts i s confirmed by geophysical data (Wallis, 1970). The Carswell Structure may l i e i n a t h i r d such b e l t , as garnet-cordierite gneisses i d e n t i c a l to those at C l u f f Lake have been reported southwest of the Athabasca Sandstone body, but these gneisses have not been studied i n d e t a i l . S t r uctural complexity in the Beaverlodge area precludes confirmation of the extension of such a f o l d - b e l t to the northeast. The Carswell Structure i s the only known i n l i e r of basement rocks within the Athabasca sandstone. L i t t l e d e t ailed p e t r o l o g i c a l work has been published on the basement rocks of northwestern Saskatchewan s with the exception of l o c a l i z e d work i n areas of economic importance such as the Beaverlodge mining d i s t r i c t (e.g. Tremblay 1972, C h r i s t i e 1952), although regional mapping has been carried out by the Geological Survey of Canada (e.g. Alcock, 1936). The regional d i s t r i b u t i o n of metamorphic grade i s not w e l l -known, owing larg e l y to the sc a r c i t y of diagnostic mineral assemblages i n p e l i t i c or other rocks, but i t appears that there i s a range from low-grade c h l o r i t e schists (which are commonly retrograde) to rocks of the granulite f a c i e s . In general, the most widespread rock-types are granite-gneisses and quartzo-feldspathic gneisses which are probably metasedimentary i n o r i g i n . The C i r c u l a r Structure The Carswell Structure Is the only well-documented major structure i n the Athabasca Sandstone. Kornik(1969) suggests on the basis of geophysical evidence that another structure may be present at P a s f i e l d Lake, i n the eastern part cf the sandstone body, but his evidence i s inconclusive. Outside the Carswell 6 Structure, the Athabasca formation i s e s s e n t i a l l y undisturbed and at least 1000 m. thick, no d r i l l - h o l e as yet having penetrated i t beyond t h i s depth i n the v i c i n i t y of the structure-The Carswell Structure consists of three concentric zones (see figure 2). The outermost, and most d i s t i n c t i v e topograph-i c a l l y , consists of a set of almost i s o c l i n a l f o l d s , the axes of which form a horizontal c i r c l e 20 miles i n diameter, with the a x i a l planes dipping r a d i a l l y away from the centre of the structure at about 60 degrees (Currie, 1 9 6 7 ) . The rocks involved i n these folds are s t r o m a t o l i t i c dolomites, which are not seen elsewhere within the Athabasca sandstone area; they are considered to be Proterozoic strata younger than the Athabasca formation. Apparent-l y , the dolomites have been preserved at t h i s - l o c a l i t y due to down-folding, but were otherwise removed by erosion. The next zone, a belt about three miles wide, i s a region of extreme complexity, apparently consisting of a mass of randomly disposed allochthonous blocks of sandstone and basement rocks, the blocks commonly having dimensions of several hundred metres. Lack of outcrop masks the true nature of t h i s zone. It i s not uncommon in d r i l l i n g to encounter blocks i n which the sandstone-basement unconformity i s present, but inverted. The blocks generally appear to be fault-bounded. Diamond d r i l l i n g shows that t h i s "melange" of blocks i s underlain, usually at a depth of a few hundred feet, by autochthonous basement rocks. The inner zone i s a c i r c u l a r area, twelve miles i n diameter, i n which only basement rocks outcrop. The c i r c u l a r margin of t h i s zone i s modified by a series of r a d i a l l y oriented d i s l o c a -tions, which are considered to be f a u l t s . A s t r u c t u r a l synthesis carried out by Bassaget and Camps (1973) reveals no major imprint 7 o f the c i r c u l a r s t r u c t u r e on the t e c t o n i c f a b r i c o f t h e basement r o c k s , a l l s t r u c t u r a l elements b e i n g c o m p a t i b l e w i t h a " r e g i o n a l " model. The C a r s w e l l C i r c u l a r S t r u c t u r e has a s s o c i a t e d w i t h i t many o f the c l a s s i c f e a t u r e s a t t r i b u t e d t o " c r y p t o - e x p l o s i o n " s t r u c t u r e s , ( C u r r i e , 1967). I t s c i r c u l a r , c r a t e r - i i k e morphology i s , o f c o u r s e , t h e most s t r i k i n g c f t h e s e c h a r a c t e r i s t i c s . S h a t t e r - c o n e s , w h i c h a r e c u r v e d f r a c t u r e s d e c o r a t e d w i t h s l i c k e n s i d e - l i k e s t r i a t i o n s , a r e found t h r o u g h o u t t h e a r e a , but as t h e i r r e c o g n i t i o n depends on t h e i r b e i n g p r e s e n t i n a weathered o u t c r o p ( i . e . t h e y a r e not v i s i b l e i n d r i l l - c o r e ) t h e i r t r u e d i s t r i b u t i o n cannot be a c c u r a t e l y a s s e s s e d . However, t h e y do appear t o be c o n c e n t r a t e d a t t h e margins o f the c e n t r a l zone. Quartz g r a i n s i n many r o c k - t y p e s show p l a n a r d e f o r m a t i o n l a m e l l a e ( f i g u r e 3 ) . P a g e l (1974) measured t h e o r i e n t a t i o n o f p o l e s o f t h e s e l a m e l l a e ( w h i c h a r e commonly a s s o c i a t e d w i t h a l i g n m e n t s o f f l u i d i n c l u s i o n s ) r e l a t i v e t o t h e c - a x i s o f q u a r t z g r a i n s and found a d i s t i n c t c o n c e n t r a t i o n a t about 23 d e g r e e s . T h i s d i r e c t i o n c o r r e s p o n d s t o t h e 1013 c r y s t a l l o g r a p h i c p l a n e , and l a m e l l a e w i t h t h i s o r i e n t a t i o n a r e s a i d t o be d i s t i n c t i v e o f shock-metamorphism (see F r e n c h and S h o r t , 19 6 8 ) . Commonly seen i n d r i l l - c o r e , and more r a r e l y i n o u t c r o p , a r e the " C l u f f B r e c c i a s " ( C u r r i e 1967). These a r e p o l y m i c t b r e c c i a s c o n t a i n i n g a n g u l a r fragments r e p r e s e n t i n g e v e r y r o c k t y p e p r e s e n t i n t h e a r e a , s e t i n a m a t r i x o f a p h a n i t i c m a t e r i a l ( f i g u r e 4 ) . As sandstone fragments appear i n the s e b r e c c i a s , t h e y can w i t h c o n f i d e n c e be a s s i g n e d a. p o s t - s a n d s t o n e d e p o s i t i o n age, and i t would be l o g i c a l t o r e l a t e them t o t h e f o r m a t i o n o f t h e c i r c u l a r F i g u r e 3 C r o s s e d n i c o l s . T h i n s e c t i o n BH-74-159, from b o r e - h o l e Car - 7 7 , s o u t h o f C a r s w e l l Lake. M a g n i f i c a t i o n 110X. Quartz ( r i g h t , n e a r e x t i n c t i o n ) showing w e l l - d e v e l o p e d d e f o r m a t i o n l a m e l l a e . Abundant t r a i n s of minute f l u i d i n c l u s i o n s . L i g h t c o l o u r i s p e r t h i t e . Figure 4 From'bore-hole C a r - 5 3 , at eastern rim of C i r c u l a r Structure. C l u f f Breccia. Poiymict breccia containing c l a s t s of pegmatoid, quartzofeldspathic gneiss etc. i n aphanitic groundmass. Structures resembling flow-banding are v i s i b l e i n the groundmass. 10 s t r u c t u r e . They a r e emplaceo as d y k e - l i k e b o d i e s w i t h t h i c k n e s s e s from a few c e n t i m e t r e s t o s e v e r a l m e t r e s . Q u a r t z g r a i n s i n t h e b r e c c i a fragments show w e l l - d e v e l o p e d d e f o r m a t i o n l a m e l l a e . C l u f f B r e c c i a s are p r e s e n t i n a l l p a r t s of the C i r c u l a r S t r u c t u r e . In the b a s a l conglomerate of the A t h a b a s c a Sandstone, th e q u a r t z i t e c l a s t s a r e h i g h l y sheared ( f i g u r e 5) a l t h o u g h the h o s t r o c k i s e s s e n t i a l l y undeformed. The shear p l a n e s have been recemented. T h i s phenomenon may be r e l a t e d t o shock-metamorphism, but no s i m i l a r o c c u r r e n c e s i n shocked ter-ranes a r e known t o t h e a u t h o r . Uranium M i n e r a l i z a t i o n Uranium m i n e r a l i z a t i o n i n t h e C a r s w e l l Area o c c u r s i n two d i s t i n c t e n v i r o n m e n t s : 1. I n carbonaceous f i n e - g r a i n e d sediments near th e base o f t h e A t h a b a s c a f o r m a t i o n . 2. A l o n g p l a n a r shear zones i n t h e metamorphosed basement r o c k s . The most s i g n i f i c a n t m i n e r a l i z e d b o d i e s d i s c o v e r e d t o d a t e a r e i n t h e r e g i o n i m m e d i a t e l y e a s t o f C l u f f Lake (see f i g u r e 2) and i n t h e Claude Lake zone, a l t h o u g h o t h e r s m a l l showings have been r e p o r t e d , f o r t h e most p a r t s i t u a t e d near t h e edge of the basement i n l i e r . The m i n e r a l i z a t i o n o c c u r r i n g i n the basement r o c k s c o n s i s t s p r e d o m i n a n t l y o f p i t c h b l e n d e and c o f f i n i t e , and i s a s s o c i a t e d w i t h minor copper s u l p h i d e m i n e r a l s . M i n e r a l i z a t i o n i s c o n c e n t r a t e d a l o n g p l a n a r zones w h i c h may be i n t e r p r e t e d as f a u l t s o r m y l o n i t i c zones. These zones a r e c h a r a c t e r i z e d by i n t e n s e a l t e r a t i o n t o 11 F i g u r e 5 Sheared, q u a r t z l t e c l a s t f r o m b a s a l c o n g l o m e r a t e o f t h e A t h a b a s c a f o r m a t i o n . 12 s e r i c i t e , k a o l i n i t e and c h l o r i t e and t h e o r i g i n a l r o c k t y p e i s as a r u l e u n r e c o g n i s a b l e . P i t c h b l e n d e o c c u r s as c o n c r e t i o n s i n a q u a r t s gangue, whereas c o f f i n i t e n o r m a l l y impregnates t h e a r g i l l a c e o u s a l t e r a t i o n p r o d u c t s . The i m p o r t a n t m i n e r a l i z a t i o n o c c u r r i n g i n p e l i t i c h o r i z o n s o f t h e A t h a b a s c a f o r m a t i o n i s c h a r a c t e r i s e d by u n u s u a l p a r a g e n e s e s . U r a n i n i t e and p i t c h b l e n d e c o e x i s t ( t h e s e m i n e r a l s a r e u s u a l l y c o n s i d e r e d t o r e p r e s e n t q u i t e d i f f e r e n t e n v i r o n m e n t s ) , and t h e s e m i n e r a l s a r e a s s o c i a t e d w i t h p l a t i n i f e r o u s g o l d , s e l e n i d e s , c o f f i n i t e and u r a n i f e r o u s h y d r o c a r b o n s such as t h u c h o l i t e . The parageneses of t h e uranium m i n e r a l i z a t i o n suggest a h y d r o t h e r m a l r a t h e r t h a n a supergene o r i g i n f o r t h e m i n e r a l i z e d b o d i e s . The u l t i m a t e s o u r c e o f m i n e r a l i z a t i o n u n d o u b t e d l y l a y i n t h e p e g m a t i t e s w h i c h are w i d e l y d e v e l o p e d i n the basement r o c k s , and w h i c h commonly c o n t a i n non-commercial a c c u m u l a t i o n s o f uranium and t h o r i u m . T e c t o n i c Framework The s m a l l amount and poor q u a l i t y o f o u t c r o p i n t h e a r e a p r e c l u d e s a d e t a i l e d t e c t o n i c s y n t h e s i s . However, by m e t i c u l o u s e x a m i n a t i o n o f a v a i l a b l e o u t c r o p s and by s t e r e o g r a p h i c a n a l y s i s o f a l l s t r u c t u r a l i n f o r m a t i o n which c o u l d be o b t a i n e d over a l a r g e p o r t i o n o f t h e a r e a , Mokta g e o l o g i s t s were a b l e t o e s t a b l i s h t h e f o l l o w i n g f a c t s (Bassaget and Camps, 1 9 7 3 ) -1. There i s one dominant f o l i a t i o n , u s u a l l y p a r a l l e l t o c o m p o s i t i o n a l l a y e r i n g i n t h e g n e i s s e s . A l a t e r f o l i a -t i o n i s seen i n p l a c e s , but I t i s not w e l l - d e v e l o p e d . 2. L i n e a t i o n s , expressed mainly by the e l o n g a t i o n of mineral g r a i n s , are developed throughout the area. Each area, of outcrop i s c h a r a c t e r i z e d by a p a r t i c u l a r o r i e n t a t i o n of l i n e a t i o n s , and Bassaget considers that t h i s i s i n d i c a t i v e of f o l d i n g on a s c a l e too large to be rendered v i s i b l e by the a v a i l a b l e outcrop. 3. Three f o l d i n g episodes are.proposed: P l - represented by the dominant f o l i a t i o n . P2 - minor f o l d s , plunging 20 degrees to the NW. P3 - plunging 60 degrees to. the NNE. P3 i s the youngest phase and i s revealed only by stereographic a n a l y s i s ; i t i s not v i s i b l e on the s c a l e of an outcrop. This f o l d i n g phase would appear to c o i n c i d e ' w i t h the r e g i o n a l s t r u c t u r a l " g r a i n " (see index map, f i g u r e 1). Scope and Objectives of Study The primary o b j e c t i v e of t h i s study i s to determine the character of the basement r o c k s , and t h e i r p r e - C i r c u l a r S t r u c t u r e h i s t o r y . The work of Bassaget and Camps c i t e d above i s probably a l l that can u s e f u l l y be done towards a study of t e c t o n i c f a b r i c s as such work i s hampered by the small amount ( l e s s than one percent) of outcrop, the m a j o r i t y of the t e r r a i n being covered .by swamps and sand deposits of g l a c i a l o r i g i n . What outcrops are present tend to be h i g h l y weathered, and the m a j o r i t y of outcrops are of the more massive q u a r t z o f e l d s p a t h i c rocks. The w e l l f o l i a t e d p e l i t i c gneisses are r a r e l y seen at the surface. The l a c k of outcrops of rocks whose bulk compositions give r i s e t o g r a d e - s e n s i t i v e m i n e r a l assemblages d i c t a t e s t h a t any p e t r o l o g i c s t u d y must r e l y on r o c k specimens r e c o v e r e d by diamond d r i l l i n g . T h i s i n t u r n l a r g e l y p r e c l u d e s t h e p o s s i b i l i t y o f c o r r e l a t i o n o f parageneses w i t h m a c r o s c o p i c s t r u c t u r e s . I n i t i a l l y , i t was i n t e n d e d t o examine a l l r o c k - t y p e s and t o a p p l y v a r i o u s m i n e r a l o g i c a l "geothermometers ?' i n o r d e r t o d e t e r m i n e t h e c o n d i t i o n s o f p r o g rade metamorphism from a thermo-dynamic s t a n d p o i n t . However, many of t h e assemblages c r i t i c a l f o r t h i s work have e i t h e r been e n t i r e l y r e p l a c e d by l a t e , low t e m p e r a t u r e a l t e r a t i o n p r o d u c t s , as i n the case o f c o r d i e r i t e , or have r e - e q u i l i b r a t e d , as w i t h t h e i r o n - t i t a n i u m o x i d e s . The p e r v a s i v e a l t e r a t i o n o f c e r t a i n m i n e r a l phases i n the a r e a i s o f i n t e r e s t i n i t s e l f , as i t i s an i m p o r t a n t i n d i c a t o r o f t h e post-metamorphic h i s t o r y o f t h e a r e a . The r e s u l t s o f the s t u d y w i l l be p r e s e n t e d i n f i v e main p a r t s 1. A d e s c r i p t i v e s e c t i o n o u t l i n i n g t h e main r o c k - t y p e s p r e s e n t . 2. A d i s c u s s i o n o f the metamorphic f a c i e s r e p r e s e n t e d , and the thermodynamic i n f o r m a t i o n which can be e x t r a c t e d by c o n s i d e r -a t i o n o f t h e c o m p o s i t i o n s o f v a r i o u s phases. 3. A d i s c u s s i o n o f a l t e r a t i o n and i t s p l a c e i n the g e o l o g i c a l h i s t o r y o f the a r e a . 4. The c h r o n o l o g y o f events w h i c h have o c c u r r e d i n the C a r s w e l l A r e a , and c o m p a r i s i o n w i t h the g e o l o g i c h i s t o r y of s u r r o u n d i n g a r e a s . 5- A d i s c u s s i o n o f t h e i m p l i c a t i o n s o f t h i s work r e g a r d i n g uranium m i n e r a l i z a t i o n . I I . PETROGRAPHY-OP THE BASEMENT ROCKS I n t r o d u c t i o n The o b j e c t o f t h i s c h a p t e r i s t o p r e s e n t a c l a s s i f i c a t i o n o f t h e m e t a m o r p h i c r o c k s , t o comment upon t h e p o s s i b l e r e l a t i o n s h i p s b e t w e e n t h e v a r i o u s r o c k t y p e s , and t o p r o v i d e a framework f o r t h e d i s c u s s i o n o f g e o l o g i c a l h i s t o r y . I n summary t h e m a j o r g r o u p s o f r o c k s p r e s e n t a r e as f o l l o w s : 1. P e l i t i c g n e i s s e s a. G a r n e t - c o r d i e r i t e g n e i s s e s . b, G a r n e t - s i l l i m a n i t e g n e i s s e s 2. Q u a r t z o f e l d s p a t h i c g n e i s s e s a. F i n e - g r a i n e d q u a r t z - p l a g i o c l a s e - b i o t i t e g n e i s s e s b. P o r p h y r o b l a s t i c g r a n i t o i d g n e i s s e s 3. M a f i c g n e i s s e s a. P y r o x e n e - b e a r i n g g r a n u l i t e s b. A m p h i b o l i t e s 4. P e g m a t o i d r o c k s 5. C a t a c l a s t i c a l l y d e f o r m e d e q u i v a l e n t s o f e a c h o f t h e above. . As most o f t h e p e t r o g r a p h i c work i s b a s e d on d r i l l - c o r e s a m p l e s , i t i s n o t p o s s i b l e t o a n a l y s e t e c t o n i c f a b r i c s and r e l a t e them t o m i n e r a l p a r a g e n e s e s . N e v e r t h e l e s s , a t l e a s t two d e f o r m a t i o n p h a s e s may be p o s t u l a t e d on t h e b a s i s o f p e t r o l o g i c work, and a s i m p l i f i e d d e f o r m a t i o n h i s t o r y may be s u g g e s t e d : 1. . P r e - o r s y n - m e t a m o r p h i c d e f o r m a t i o n p h a s e , p r o d u c i n g t h e d o m i n a n t f o l i a t i o n . T h i s f o l i a t i o n , as f a r as c a n be e s t a b -l i s h e d , i s p a r a l l e l t o t h e l i t h o l o g i c c o n t a c t s between t h e p e l i t i c and q u a r t z o f e l d s p a t h i c g n e i s s e s . I t i s r e p r e s e n t e d 16 by t h e s t r o n g c o m p o s i t i o n a l b a n d i n g i n t h e p e l i t i c g n e i s s e s , w h i c h i s p r o b a b l y e n h a n c e d by m e t a m o r p h i c d i f f e r e n t i a t i o n , and by t h e p e r v a s i v e p r e f e r r e d o r i e n t a t i o n o f b i o t i t e and o t h e r m i n e r a l s i n t h e q u a r t z o f e l d s p a t h i c and m a f i c g n e i s s e s . 2. Metamorphism, f o l l o w e d by r e t r o g r a d e metamorphism' and t h e emplacement o f g r a n i t i c m a t e r i a l . 3. A p h a s e o f m i n o r f o l d i n g , i m p a r t i n g a m i n o r f o l i a t i o n t o t h e p e g m a t o i d r o c k s , w i t h d e v e l o p m e n t o f a l i n e a t i o n and o f augen s t r u c t u r e s i n t h e g r a n i t o i d g n e i s s e s . T h i s d e f o r m a t i o n p h a s e may have a l s o p r o d u c e d t h e l o c a l i s e d d e v e l o p m e n t o f m y l o n i t e z o n e s . 4. One o r more p h a s e s o f l a t e a l t e r a t i o n , t h e s t a t u s o f w h i c h w i l l be d i s c u s s e d i n a l a t e r c h a p t e r . 5. The s u p e r i m p o s i t i o n o f s h o c k metamorphism and f a u l t i n g a s s o c i a t e d w i t h t h e f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e . P e l i t i c G n e i s s e s 17 G e n e r a l d i s c u s s i o n . The t e r m p e l i t i c g n e i s s i s h e r e u s e d t o d e n o t e s t r o n g l y banded r o c k s o f h i g h l y a l u m i n o u s c o m p o s i t i o n , w h i c h a r e c o n s i d e r e d t o be m e t a s e d i m e n t a r y . The m i n e r a l o g y o f t h e s e g n e i s s e s i s s i m p l e and t h e y c a n be c o n v e n i e n t l y d i v i d e d i n t o two g r o u p s : g a r n e t - c o r - d i e r i t e g n e i s s e s and g a r n e t - s i l i i m a n i t e g n e i s s e s . B o t h c o n t a i n q u a r t z , p o t a s s i u m f e l d s p a r , g a r n e t and b i o t i t e and e i t h e r c o r d i e r i t e o r s i l l i m a n i t e a s m a j o r p h a s e s , and b o t h a r e commonly I n t e n s e l y a l t e r e d . The p e l i t i c g n e i s s e s show l i t t l e r e s i s t a n c e t o w e a t h e r i n g , p r o b a b l y • o w i n g t o t h e i r p e r v a s i v e a l t e r a t i o n , and as a r e s u l t t h e y a r e p o o r l y e x p o s e d . N e v e r t h e l e s s , t h e y c o n s t i t u t e a v o l u -m e t r i c a l l y i m p o r t a n t basement r o c k - t y p e and a r e commonly e n c o u n t e r e d i n d r i l l - c o r e and i n t r e n c h e s . F i g u r e 2 shows t h e d i s t r i b u t i o n o f t h e two p e l i t i c g n e i s s t y p e s . O n l y t h e s i l l i -m a n i t e - b e a r i n g t y p e o c c u r s i n t h e s o u t h e a s t p a r t o f t h e s t r u c t u r e , and o n l y c o r d i e r i t e i n t h e n o r t h w e s t , b u t t h e r e i s a b r o a d t r a n s i t i o n zone where b o t h t y p e s a r e r a n d o m l y d i s t r i b u t e d . The s i g n i f i c a n c e o f t h i s d i s t r i b u t i o n p a t t e r n w i l l be d i s c u s s e d i n c h a p t e r I I I . G a r n e t C o r d i e r i t e G n e i s s e s . Rocks o f t h i s g r o u p a r e t y p i c a l l y medium.to c o a r s e g r a i n e d and p o r p h y r o b l a s t i c . They a r e t y p i c a l l y f o l i a t e d a l t h o u g h a few s p e c i m e n s c o n t a i n l a r g e e q u i d i m e n s i o n a l g a r n e t o r c o r d i e r i t e p o r p h y r o b l a s t s and show no o b v i o u s t e c t o n i t e f a b r i c . I n hand s p e c i m e n ( f i g u r e 6) . t h e more s t r o n g l y f o l i a t e d r o c k s show c o m p o s i t i o n a l b a n d i n g on a s c a l e o f a b o u t one c e n t i -m e t e r , a r i s i n g f r o m an a l t e r n a t i o n between .grey g r a n o b l a s t i c 18 Figure 6 Specimen BH-74-159 from bore-hole Car -77, south of Carswell Lake. Polished surface. Typical garnet-cordierite gneiss. Rough segregation of quartzo-feldspathic bands ( l i g h t e r colour) and c o r d i e r i t e r i c h bands (dark). Small pink equidimensional garnets occur throughout, but are concentrated i n c o r d i e r i t e r i c h bands. Cor d i e r i t e porphyroblasts (actually e n t i r e l y altered) are flattened p a r a l l e l to f o l i a t i o n . Lightest coloured bands are concordant veins of g r a n i t i c material. q u a r t z o f e l d s p a t h i c bands and bands o f streakedr-out b l a c k or b l u e c o r d i e r i t e p o r p h y r o b l a s t s . The m a f i c bands a l s o c o n t a i n s m a l l a n h e d r a l g a r n e t p o r p h y r o b l a s t s w h i c h , a l t h o u g h e l o n g a t e d , a re more e q u i d i m e n s i o n a l t h a n t h e c o r d i e r i t e . D i s s e m i n a t e d p y r i t e i s c o n c e n t r a t e d i n t h e m a f i c bands. A l t h o u g h c o r d i e r i t e i s the d i a g n o s t i c m i n e r a l o f t h i s group o f g n e i s s e s , i t i s always c o m p l e t e l y pseudomorphed by v a r i o u s a l t e r a t i o n p r o d u c t s . I t s o r i g i n a l p r e s e n c e can be i n f e r r e d by the d i s t i n c t i v e b l u e - g r e e n c o l o u r o f i t s a l t e r a t i o n p r o d u c t s i n hand specimen, and by t h e c h a r a c t e r i s t i c " p i n i t e " r e p l a c e m e n t • p a t t e r n and p o i k i l o b l a s t i c t e x t u r e i n t h i n s e c t i o n . W i t h i n t h e group a- c o m p o s i t i o n a l s e r i e s e x i s t s , a c c o r d i n g t o the abundance o f q u a r t z o f e l d s p a t h i c m a t e r i a l i n the r o c k . The c o m p o s i t i o n a l v a r i a t i o n i s ' r e f l e c t e d i n t e x t u r e s . The more m a f i c t y p e s are crowded w i t h g a r n e t and c o r d i e r i t e p o r p h y r o b l a s t s two c e n t i m e t r e s o r more i n d i a m e t e r ( e . g . f i g u r e 7), whereas the more q u a r t z o f e l d s p a t h i c r o c k s a r e r e l a t i v e l y f i n e - g r a i n e d and e q u i g r a n u l a r w i t h poor f o l i a t i o n and s m a l l s c a t t e r e d p o r p h y r -o b l a s t s . I n t h i n - s e c t i o n the f e l s i c bands a r e seen t o be composed l a r g e l y o f q u a r t z and p e r t h i t e . Q u a r t z i s u s u a l l y f i n e - g r a i n e d w i t h s u t u r e d g r a i n b o u n d a r i e s " ( f i g u r e 8) and shows s t r o n g u n d u l a t o r y e x t i n c t i o n . I t commonly e x h i b i t s m u l t i p l e s e t s o f d e f o r m a t i o n l a m e l l a e ( f i g u r e 3) and i s c h a r a c t e r i s t i c a l l y packed w i t h minute f l u i d i n c l u s i o n s (see P a g e l , 197*0. P e r t h i t e forms l a r g e g r a i n s w i t h r a r e , p o o r l y d e v e l o p e d c r y s t a l f a c e s , and i s c o n s i d e r e d t o be p o r p h y r o b l a s t i c . P l a g i o c l a s e about An3Q i n c o m p o s i t i o n i s p r e s e n t i n s m a l l amounts. I n a few-cases the p l a g i o c l a s e i s a l b i t e , such o c c u r r e n c e s seeming t o be r e l a t e d F i g u r e 7 Specimen BH-74-169 from b o r e - h o l e Clu-503, i n Claude Lake m i n e r a l -i z e d zone. C o a r s e l y p o r p h y r o b l a s t i c , almost u n f o l i a t e d g a r n e t c o r d i e r i t e r o c k . P i n k g a r n e t and b l u e a l t e r e d c o r d i e r i t e p o r p h y r o b l a s t s a r e packed w i t h rounded q u a r t z i n c l u s i o n . Quartz and f e l d s p a r o c c u r i n amounts l e s s t h a n 30$. Figure 8a P l a n e - p o l a r i z e d l i g h t . i ^ ^ ^ ^ ^ ^ Figure 8b Crossed n i c o l s . Thin s e c t i o n , specimen J?B-73-l8. Outcrop i n NW part of str u c M a g n i f i c a t i o n 27-5X. For c a p t i o n see f o l l o w i n g page. 22 C a p t i o n f o r F i g u r e 8 G a r n e t c o r d i e r i t e g n e i s s . Upper r i g h t and u p p e r l e f t ( i n e x t i n c -t i o n ) p e r t h i t e p o r p h y r o b l a s t w i t h r o u n d e d q u a r t z i n c l u s i o n . F i n e -g r a i n e d q u a r t z j w i t h u n d u l a t o r y e x t i n c t i o n and s u t u r e d g r a i n b o u n d a r i e s . C o r d i e r i t e p s eudomdrphs: n o t e y e l l o w c o l o u r i n p l a n e -p o l a r i s e d l i g h t . - G r a i n a t b o t t o m l e f t i s m a i n l y one m u s c o v i t e c r y s t a l : n o t e c l e a v a g e and c r o s s - c u t t i n g " p i n i t e " c r a c k s . 23 t o an I n t i m a t e a s s o c i a t i o n w i t h v e i n s o f g r a n i t i c m a t e r i a l . I n t h e m a f i c bands and i n t h e u n f o l i a t e d r o c k s , t h e t e x t u r e o f the g a r n e t and c o r d i e r i t e g r a i n s may be p o r p h y r o b l a s t i c ( f i g u r e 7) or p o i k i l o b l a s t i c ( f i g u r e s 9 and 1 0 ) . P o r p h y r o b l a s t s ( i n c l u d i n g p e r t h i t e ) a r e commonly f l a t t e n e d p a r a l l e l t o t h e f o l i a t i o n and c o n t a i n rounded i n c l u s i o n s o f q u a r t z . B i o t i t e i s p r e s e n t as s m a l l d i s s e m i n a t e d . g r a i n s i n amounts up t o t e n p e r c e n t . I n t h e banded r o c k s i t tends t o be o r i e n t e d p a r a l l e l t o t h e f o l i a t i o n . The m a j o r i t y o f b i o t i t e g r a i n s have a ragged appearance, s u g g e s t i n g t h a t i t i s u n s t a b l e , but s m a l l e r ' g r a i n s p r e s e n t as i n c l u s i o n s i n q u a r t z , g a r n e t o r c o r d i e r i t e a r e l e s s 'corroded. The u b i q u i t o u s pseudomorphous re p l a c e m e n t o f c o r d i e r i t e t a k e s s e v e r a l forms. Common t o a l l i s the c h a r a c t e r i s t i c p i n i t e p a t t e r n , i n wh i c h minute s e r i c i t e c r y s t a l s n u c l e a t e a l o n g c r a c k s , growing i n t o the body o f t h e g r a i n . I n most c a s e s , t h e c o r e s o f t h e c o r d i e r i t e g r a i n s a r e a l t e r e d t o e x t r e m e l y f i n e - g r a i n e d m a t e r i a l , - the r e l i e f o f wh i c h may v a r y c o n s i d e r a b l y from p l a c e t o p l a c e i n a t h i n - s e c t i o n (see f i g u r e 1 1 ) . I n one specimen, c o r d i e r i t e pseudomorphs c o n t a i n a f e l t e d mass o f minute a c i c u l a r c r y s t a l s of s i l l i m a n i t e ( f i g u r e 1 2 ) . These may r e p r e s e n t an i n t e r m e d i a t e s t a g e i n t h e a l t e r a t i o n o f the m i n e r a l t o k a o l i n i t e ••-(see c h a p t e r 4 ) . M i c r o p r o b e a n a l y s e s were c a r r i e d out on t h e g a r n e t s o f two samples, and t h e r e s u l t s a re shown i n t a b l e 1. The g a r n e t s a r e c l o s e t o almandine' i n c o m p o s i t i o n , and i n specimen PC - 7 3 - 2 some z o n i n g was d e t e c t e d . The r i m i s e n r i c h e d i n i r o n and t h e c o r e i n magnesium. Manganese was found t o be e n r i c h e d a t t h e r i m , wh i c h i s t h e r e v e r s e o f t h e p a t t e r n most o f t e n o b s e r v e d . 24 I F i g u r e 9a F i g u r e 9b C r o s s e d n i c o l s . T h i n s e c t i o n P C - 7 3 - 2 . Outcrop i n NW p a r t o f s t r u c t u r e . M a g n i f -i c a t i o n 2 7 . 5 X . F o r c a p t i o n see f o l l o w i n g page. C a p t i o n f o r F i g u r e 9 Shows p o i k i l o b l a s t i c h a b i t o f c o r d i e r i t e ( l o w r e l i e f ) . Replaceme o f c o r d i e r i t e by low b i r e f r i n g e n t m a t e r i a l and by s e r i c i t e . " P i n i t e " a l t e r a t i o n p a t t e r n . N o t e a l i g n m e n t o f b i o t i t e g r a i n s . 26 F i g u r e 1 0 P l a n e - p o l a r i s e d l i g h t . Specimen ,TPB-73-it6b. Outcrop i n NW p a r t o f s t r u c t u r e . M a g n i f i c a -t i o n 27.5X. P o i k i l o b l a s t i c h a b i t o f g a r n e t ( h i g h r e l i e f ) and c o r d i e r i t e ( y e l l o w ) i n p o o r l y f o l i a t e d g a r n e t - c o r d i e r i t e f e l s . F i g u r e 11 Cr o s s e d n i c o l s . T h i n s e c t i o n JPB-73-18. Outcrop i n NW p a r t o f s t r u c t u r e . Magni-f i c a t i o n 1100X. " P i n i t e " a l t e r a t i o n o f c o r d i e r i t e . At t h i s m a g n i f i c a t i o n t h e g r a n u l a r n a t u r e o f t h e l o w - b i r e f r i n g e n t m a t e r i a l becomes a p p a r e n t . B l a d e d m u s c o v i t e c r y s t a l s n u c l e a t e a l o n g c r a c k s and at g r a i n b o u n d a r i e s , and grow i n t o t h e g r a i n body. 1 F i g u r e 12 C r o s s e d n i c o l s . T h i n s e c t i o n , specimen PC - 7 3 - 2 , E l i z a b e t h a r e a . M a g n i f i c a t i o n 2 7 .5X. Large c o r d i e r i t e pseudomorph, r e p l a c e d by f i n e - g r a i n e d s e r i c i t e ( l i g h t c o l o u r ) i n o u t e r zone, and l o w - b i r e f r i n g e n c e m a t e r i a l a t c o r e . Note minute i n c l u s i o n s o f s i l l i m a n i t e ( r e d and b l u e i n t e r f e r e n c e c o l o u r s ) . OXIDE RIM WEIGHT % FORMULA CENTRE WEIGHT % FORMULA S i 0 2 . 37.45 • 3.01.0 37.76 2.998 A1 20 3 20.72 1. 964 21.06 1.972 MgO 2.42 0.290 2.98 0.471 FeO 37.79 2.540 36.42 2,419 CaO 0.96 0.083 1.07 0.091 MnO 1.78 0.122 0.9-6 0,064 TOTAL 101.13 101.24 Table 1 Microprobe analyses of garnet from garnet-cordierite gneiss specimen PC-73-2. Note zoning i n iron, magnesium and . manganese. Formulae calculated on the basis of 12 oxygens. . . •  ' 30 G a r n e t - S i l l i m a n i t e G n e i s s e s . The s i l l i m a n i t e b e a r i n g p e l i t i c g n e i s s e s a r e c h a r a c t e r i s e d , by s t r o n g d i f f e r e n t i a t i o n i n t o a l m o s t m o n o m i n e r a l i c 'zones, sometimes on a s c a l e l a r g e r . t h a n a t h i n - -s e c t i o n . . T h i s d i f f e r e n t i a t i o n r e s u l t s i n a s t r o n g c o m p o s i t i o n a l b a n d i n g , w i t h an a l t e r n a t i o n o f . s i l l i r a a n i t e - r i c h and q u a r t z - r i c h bands. The s i l l i m a n i t e - r i c h bands o f t e n have a d i s t i n c t i v e a p p e a r -a n c e , b e i n g composed o f a s e r i e s , o f pod-shaped bulges,, each "pod" h a v i n g a l a r g e g a r n e t p o r p h y r o b l a s t a t i t s c o r e ( f i g u r e 1 3 ) . The s i l l i m a n i t e o c c u r s i n a c l o s e l y packed mass o f p r i s m a t i c c r y s t a l s w i t h a v e r y s t r o n g p r e f e r r e d o r i e n t a t i o n s u b - p a r a l l e l t o the f o l i a t i o n ( f i g u r e 1 4 ) . I n each !'pod : i t h e o r i e n t a t i o n o f t h e s i l l i m a n i t e c u r v e s around t h e g a r n e t , g i v i n g t h e i m p r e s s i o n t h a t e i t h e r t h e g a r n e t has grown w i t h i n t h e o r i g i n a l l y p l a n a r s i l l i m a n -i t e band i n f l a t i n g i t i n t o a s e r i e s o f nodes, or t h a t t h e g a r n e t s have caused d i f f e r e n t i a l f l a t t e n i n g . S i l l i m a n i t e , . o r s i l l i m a n i t e pseudomorphs, a r e commonly e n c l o s e d or p a r t i a l l y ; overgrown w i t h i n t h e g a r n e t g r a i n s ( f i g u r e 15), i n d i c a t i n g l a t e f o r m a t i o n o f the g a r n e t s . G a r n e t s c h a r a c t e r i s t i c a l l y c o n t a i n abundant s m a l l q u a r t z i n c l u s i o n s . B i o t i t e i s p r e s e n t i n s m a l l amounts and i s g e n e r a l l y ragged i n appearance. L e s s c o r r o d e d g r a i n s a r e commonly p r e s e r v e d w i t h i n g a r n e t p r o p h y r o b l a s t s . . The q u a r t z - r i c h bands a r e composed of f a i r l y l a r g e g r a i n s w i t h b o u n d a r i e s l e s s s u t u r e d i n appearance t h a n t h o s e seen i n t h e c o r d i e r i t e - b e a r i n g g n e i s s e s . S m a l l q u a n t i t i e s o f p e r t h i t e o r p l a g i o c l a s e (Anr$^), b o t h p a r t i a l l y a l t e r e d t o s e r i c i t e , . o c c u r i n the q u a r t z r i c h bands. D i s s e m i n a t e d g r a p h i t e and p o i k i l o -b l a s t i c - g r a i n s o f p y r i t e a r e commonly p r e s e n t . . 31 F i g u r e 13 P o l i s h e d c o r e s l a b BH-74-5. B o r e - h o l e C a r - 8 0 , n e a r c e n t r e o f s t r u c t u r e . Garnet s i l l i m a n i t e g n e i s s . White bands are q u a r t z w i t h minor f e l d s p a r . Dark bands a r e o f s t r o n g l y a l i g n e d s i l l i m a n i t e , w i t h g a r n e t s f o r m i n g "nodes". The g r een r i m o f t h e d a r k m a t e r i a l i s a " r e a c t i o n r i m " o f s i l l i m a n i t e pseudomorphs. F i g u r e 14 C r o s s e d n i c o l s . T h i n s e c t i o n o f specimen BH-74-141 from b o r e - h o l e Car-40 i n Hubert Lake a r e a , s o u t h e a s t p a r t o f C i r c u l a r S t r u c t u r e . M a g n i f i c a t i o n 44X. G a r n e t - s i l l i m a n i t e g n e i s s . S e c t i o n t h r o u g h a s i l l i m a n i t e - r i c h band, from g a r n e t a t c o r e ( l e f t ) t h r o u g h m a s s i v e s i l l i m a n i t e ( y e l l o w t o b l u e c o l o u r s ) many g r a i n s h a v i n g t h i n m u s c o v i t e s h e a t h s , pseudo-morphed s i l l i m a n i t e ( l o w - b i r e f r i n g e n c e ) t o q u a r t z w i t h g r a i n s o f opaque h e m a t i t e . I F i g u r e 15 C r o s s e d n i c o l s . T h i n s e c t i o n Car-40-1. Hubert Lake a r e a . M a g n i f i c a t i o n 176X. I n c l u s i o n o f s i l l i m a n i t e and q u a r t z w i t h i n g a r n e t ( d a r k ) . Q u a r t z and s i l l i m a n i t e a r e i n e q u i l i b r i u m , a s i t u a t i o n n e v e r o b s e r v e d except i n such i n c l u s i o n s . 34 S i l l i m a n i t e i s commonly r e p l a c e d pseudomorphously by a low-b i r e f r i n g e n c e m i n e r a l aggregate ( f i g u r e 16). X-ray powder photo-graphs show t h i s to c o n s i s t wholly or i n p a r t of very, f i n e - g r a i n e d k a o l i n i t e . The replacement appears to be due to a r e a c t i o n between s i l l i m a n i t e and q u a r t z , . as the s i l l i m a n i t e . . pods are commonly surrounded by a sheath of pseudomorphously r e p l a c e d m a t e r i a l ( f i g u r e s 13 and 14). Indeed, s i l l i m a n i t e and q u a r t z are never found i n c o n t a c t , except when both m i n e r a l s occur as an i n c l u s i o n i n garnet ( f i g u r e 15). S i l l i m a n i t e g r a i n s , or t h e i r pseudomorphs, are commonly surrounded by a t h i n envelope of muscovite ( f i g u r e 17). The m a t e r i a l r e p l a c i n g s i l l i m a n i t e i s m i n e r a l o g i c a l l y v e r y s i m i l a r to t h a t which r e p l a c e s c o r d i e r i t e , and consequently the s t r o n g l y a l t e r e d c o r d i e r i t e and s i l l i m a n i t e -b e a r i n g g n e i s s e s may be d i f f i c u l t to d i s t i n g u i s h i n hand specimen. The g a r n e t - s i l l i m a n i t e g n e i s s e s are cut by a f i n e network of c r a c k s ( f i g u r e s 16 and 18). The c r a c k s pervade the whole ro c k , but are most e a s i l y r e c o g n i s e d i n the q u a r t z o s e bands. I n d i v i d u a l g r a i n s are t h o r o u g h l y d i s s e c t e d and the c r a c k s are f i l l e d w i t h f i n e - g r a i n e d , low b i r e f r i n g e n c e m a t e r i a l . The q u a r t z g r a i n s are otherwise q u i t e c l e a r and u n s t r a i n e d , and i t seems l i k e l y t h a t the c r a c k i n g a cted as a mechanism f o r r e l i e v i n g s t r a i n w i t h i n the c r y s t a l s . The o r i g i n of these c r a c k s i s u n c l e a r , but i s p o s s i b l y r e l a t e d to expansion r e s u l t i n g from the h y d r a t i o n of s i l l i m a n i t e . F i g u r e 18 shows an i s o l a t e d pseudo-morph surrounded by r a d i a t i n g c r a c k s , which may support t h i s i n t e r p r e t a t i o n . The phenomenon i s not c o n s i d e r e d to be r e l a t e d to. shock-metamorphism, as i t i s r e s t r i c t e d to the g a r n e t - s i l l i -manite g n e i s s e s , and i t s d i s t r i b u t i o n i s not c o n t r o l l e d by the geometry of the C i r c u l a r S t r u c t u r e . F i g u r e 16 C r o s s e d n i c o l s . T h i n s e c t i o n BH-7 il-72 from b o r e - h o l e C a r - 3 4 , e a s t s i d e o f C i r c u l a r S t r u c t u r e . M a g n i f i c a t i o n 27.5X. G a r n e t - s i l l i m a n i t e g n e i s s . Q u a r t z , w h i t e ; s i l l i m a n i t e , y e l l o w . S i l l i m a n i t e g r a i n s r e p l a c e d s e l e c t i v e l y by f i n e - g r a i n e d low-b i r e f r i n g e n c e m a t e r i a l , and p e r v a s i v e c r a c k s f i l l e d w i t h s i m i l a r m a t e r i a l . F i g u r e 17 C r o s s e d n i c o l s . T h i n s e c t i o n BH-72+-39a. B o r e - h o l e Car-54, e a s t s i d e o f s t r u c t u r e . M a g n i f i c a t i o n G a r n e t - s i l l i m a n i t e g n e i s s . S i l l i m a n i t e g r a i n s i n q u a r t z w i t h t h i n m u s c o v i t e r e a c t i o n r i m s . 37 F i g u r e 18 Crossed n i c o l s . T h i n s e c t i o n Car-40-1, from Hubert Lake a r e a . M a g n i f i c a t i o n 44X. G a r n e t - s i l l i m a n i t e g n e i s s . A l l yellow m a t e r i a l i s q u a r t z . R e c t a n g u l a r g r a i n at c e n t r e i s a s i l l i m a n i t e pseudomorph. S h a t t e r e d appearance of q u a r t z g r a i n s i s p e c u l i a r t o the s i l l i m a n i t e - r i c h p e l i t i c g n e i s s e s , and may t h e r e f o r e be due t o expansion d u r i n g h y d r a t i o n of the s i l l i m a n i t e . Quart z o f e l d s p a t h i c Gneiss Suite. 38 General D i s c u s s i o n . The q u a r t z o f e l d s p a t h i c g n e i s s e s f a l l i n t o two groups, both of which are widely, d i s t r i b u t e d , b e i n g encountered i n . v i r t u a l l y .every bore-hole p e n e t r a t i n g the basement • r o c k s . The f i r s t group are monotonous, grey, f i n e - g r a i n e d r o c k s c o n s i s t i n g mainly of q u a r t z , p l a g i o c l a s e . f e l d s p a r and b i o t i t e , and are h e r e a f t e r . d e s i g n a t e d " q u a r t z o f e l d s p a t h i c g n e i s s sensu., s t r i c t o " . The second group, " g r a n i t e g n e i s s e s " , have a more g r a n i t o i d aspect w i t h s t r o n g f o l i a t i o n and an abundance of p o r p h y r o b l a s t i c a l k a l i f e l d s p a r s . Both groups c o n t a i n v a r i a b l e amounts >of ferromagnesian m i n e r a l s ; these are most commonly b i o t i t e and garnet, but i n more mafic examples hornblende i s l o c a l l y p r e s e n t . T h e - q u a r t z o f e l d s p a t h i c g n e i s s e s may b e . c o n s i d e r e d to form a continuous c o m p o s i t i o n a l g r a d a t i o n i n t o a m p h i b o l i t e s . The f i n e - g r a i n e d , w e l l f o l i a t e d nature of the q u a r t z o f e l d s -p a t h i c g n e i s s e s , and t h e i r c o m position, suggest that, they were formed by the metamorphism of psammitic sediments. The g r a n i t o i d g n e i s s e s are best i n t e r p r e t e d as q u a r t z o f e l d s p a t h i c g n e i s s e s which have been m o d i f i e d by the p o r p h y r o b l a s t i c growth of a l k a l i f e l d -spars and by l i t - p a r - l i t i n t r o d u c t i o n of g r a n i t i c m a t e r i a l . Q u a r t z o f e l d s p a t h i c Gneiss Sensu S t r i c t o . T h i s rock i s f i n e -g r a i n e d ( l e s s than 0.5 mm.) t y p i c a l l y l i g h t g r e y , and l a c k s any m a c r o s c o p i c a l l y v i s i b l e c o m p o s i t i o n a l banding. I t s f o l i a t i o n i s due t o p r e f e r r e d o r i e n t a t i o n of d i s s e m i n a t e d b i o t i t e g r a i n s . I t i s composed o f about equal p r o p o r t i o n s of q u a r t z and p l a g i o c l a s e with u s u a l l y l e s s than ten percent b i o t i t e ( f i g u r e 19). F i g u r e 19 Specimen Clu-295-1 from C l u f f Lake m i n e r a l i s e d zone. F i n e - g r a i n e d q u a r t z o f e l d s p a t h i c g n e i s s . P o o r l y f o l i a t e d grano-b l a s t i c q u a r t z , f e l d s p a r and b i o t i t e w i t h u n u s u a l l y abundant pink garnet p o r p h y r o b l a s t s . Edge of minor pegmatoid v e i n i s v i s i b l e at upper r i g h t . 40 . P l a g i o c l a s e i s u s u a l l y a b o u t AnijQ i n c o m p o s i t i o n , , b u t I n a . few c a s e s i t i s p u r e a l b i t e . P o t a s s i u m f e l d s p a r i s . a b s e n t , , e x c e p t where t h e r o c k i s s p a t i a l l y a s s o c i a t e d w i t h g r a n i t i c m a t e r i a l . S m a l l p o i k i l o b l a s t i c g a r n e t s a r e common. T h e y a r e t y p i c a l l y c o n c e n t r a t e d i n p l a n e s p a r a l l e l t o t h e f o l i a t i o n , s u g g e s t i n g t h a t t h e y r e p r e s e n t i r o n - r i c h bands i n t h e o r i g i n a l s e d i m e n t a r y r o c k . H o r n b l e n d e i s d e v e l o p e d l o c a l l y ; i t does n o t c o e x i s t w i t h g a r n e t . A p a t i t e and z i r c o n a r e common a c c e s s o r i e s . The t e x t u r e o f t h e r o c k i s f i n e - g r a i n e d g r a n o b l a s t i c . ( f i g u r e 20). Q u a r t z commonly shows s u t u r e d q u a r t z i t i c g r a i n , b o u n d a r i e s . Most b i o t i t e g r a i n s a r e c o r r o d e d i n a p p e a r a n c e , and b i o t i t e i s c o n s i s t e n t l y a p a r a g e n e t i c a l l y e a r l y m i n e r a l , b e i n g o v e r g r o w n by q u a r t z and f e l d s p a r . The p o i k i l o b l a s t i c h a b i t o f g a r n e t s u g g e s t s t h a t i t was r e l a t i v e l y l a t e i n t h e p a r a g e n e s i s . The q u a r t z o f e l d -s p a t h i c g n e i s s e s a r e v e r y s i m i l a r i n t e x t u r e t o t h e f i n e r - g r a i n e d c o r d i e r i t e - b e a r i n g p e l i t i c g n e i s s e s . The q u a r t z o f e l d s p a t h i c g n e i s s e s a r e f o r t h e most p a r t q u i t e f r e s h , b u t i n a few s p e c i m e n s a l l m i n e r a l s a r e a l t e r e d c o m p l e t e l y t o f i n e - g r a i n e d s e r i c i t e and c h l o r i t e . B i o t i t e , i n s p i t e o f i t s c o r r o d e d a p p e a r a n c e , i s o n l y r a r e l y a l t e r e d t o c h l o r i t e and g a r n e t a l s o i s u s u a l l y f r e s h . T h e s e g n e i s s e s a r e commonly s e e n i n o u t -c r o p , b u t i n d r i l l - c o r e t h e y a r e g e n e r a l l y r e s t r i c t e d t o beds a few t e n s o f c e n t i m e t r e s t h i c k , a p p a r e n t l y b e i n g p r e f e r e n t i a l l y e x p o s e d as a r e s u l t o f t h e i r r e s i s t a n c e t o w e a t h e r i n g . G r a n i t e G n e i s s e s . The g r a n i t e g n e i s s e s a r e i n c l u d e d with, t h e q u a r t z o f e l d s p a t h i c g n e i s s e s b e c a u s e t h e r e e x i s t s a t e x t u r a l and c o m p o s i t i o n a l s e r i e s o f i n c r e a s i n g l y g r a n i t o i d c h a r a c t e r , b e g i n n i n g 41 F i g u r e 20 C r o s s e d n i c o l s . T h i n s e c t i o n , specimen BH-74-66, from b o r e - h o l e C l u - 4 2 5 , i n C l u f f Lake m i n e r a l i s e d zone. M a g n i f i c a t i o n 27-5X. F i n e - g r a i n e d q u a r t z o f e l d s p a t h i c g n e i s s showing q u a r t z i t i c g r a n o -b l a s t i c t e x t u r e . S m a l l , r a gged b i o t i t e c r y s t a l s show s l i g h t degree o f p r e f e r r e d o r i e n t a t i o n . w i t h t h e growth o f i s o l a t e d p e r t h i t e p o r p h y r o b l a s t s w i t h i n f i n e -g r a i n e d g r e y g n e i s s o r a m p h i b o l i t e s arid p a s s i n g i n t o f o l i a t e d r o c k s o f t r u e g r a n i t e composition... The s e r i e s i s c h a r a c t e r i s e d by an. I n c r e a s e i n g r a i n s i z e , and by p r o g r e s s i v e enhancement of f o l i a t i o n -c u l m i n a t i n g i n a l i t - p a r - l i t a l t e r n a t i o n o f gneisso.se and g r a n i t o i d o r pegmatoid m a t e r i a l as seen i n f i g u r e 21. T h i s i l l u s t r a t i o n a l s o d e m o n s t r a t e s t h a t t h e f o l i a t i o n o f t h e g r a n i t e g n e i s s i s m o d i f i e d by assymmetric f o l d s o f a few c e n t i m e t r e s w a v e l e n g t h . Where p o r p h y r o b l a s t i c growth o f a l k a l i f e l d s p a r o c c u r s w i t h i n i n i t i a l l y u n a l t e r e d q u a r t z o f e l d s p a t h i c g n e i s s , g a r n e t and b i o t i t e , w h i c h i n t h e u n m o d i f i e d p a r t s o f t h e g n e i s s may be q u i t e f r e s h , a r e t y p i c a l l y a l t e r e d t o c h l o r i t e near t h e m a r g i n s o f t h e g r a n i t o i d m a t e r i a l ( f i g u r e 2 2 ) . F i g u r e 23 demonstrates an i n t e r m e d i a t e s t a g e o f r e p l a c e m e n t , i n w h i c h abundant p o r p h y r o b l a s t s o f p i n k p e r t h i t e a r e s e t i n a m a t r i x o f t y p i c a l q u a r t z o f e l d s p a t h i c gneiss.. The p o r p h y r o b l a s t s a r e deformed i n t o augen-shaped r o d s w h i c h i m p a r t a s t r o n g l i n e a t i o n t o t h e r o c k , i n d i c a t i n g a p o s t - g r a n i t i s a t i o n d e f o r m a t i o n phase. The g n e i s s e s most t h o r o u g h l y permeated w i t h g r a n i t i c m a t e r i a l a r e r e p r e s e n t e d i n f i g u r e 24, i n w h i c h t h o r o u g h l y r e c r y s t a l l i s e d q u a r t z o f e l d s p a t h i c g n e i s s m a t e r i a l c o n c e n t r a t e d i n t w o - m i l l i m e t r e b i o t i t e - r i c h bands i s s u b o r d i n a t e t o a r e l a t i v e l y c o a r s e - g r a i n e d i n t e r g r o w t h o f q u a r t z , p e r t h i t e and a l b i t e . P o t a s s i u m f e l d s p a r i n t h e g r a n i t o i d m a t e r i a l commonly shows w e l l - d e v e l o p e d p e r t h i t e t e x t u r e s , and t h e b o r d e r s o f t h e p o r p h y r o -b l a s t s may be m o d i f i e d by m y r m e k i t i c i n t e r g r o w t h s o f q u a r t z and a l b i t e ( f i g u r e 2 5 ) . The myrmekite t e x t u r e i s commonly p r e s e r v e d even where the f e l d s p a r i s c o m p l e t e l y a l t e r e d . The p i n k c o l o u r 43 F i g u r e 21 C a r - 5 2 - 4 f r o m e a s t e r n m a r g i n o f t h e C i r c u l a r S t r u c t u r e . Upper s p e c i m e n s t a i n e d f o r p o t a s s i u m f e l d s p a r . G r a n i t e g n e i s s , s h o w i n g d e v e l o p m e n t o f p o t a s s i u m f e l d s p a r p o r p h y r o b l a s t s i n f i n e r -g r a i n e d ( h o r n b l e n d e - b e a r i n g ) p a r t , and c o n f o r m a b l e v e i n o f pegma-t o i d m a t e r i a l . N o t e p o s t - g r a n i t i s a t i o n a s y m m e t r i c a l f o l d i n g . I F i g u r e 22 Crossed, n i c o l s . T h i n s e c t i o n , s p e c i m e n Clu - 377 -1 f r o m C l u f f m i n e r a l i s e d z o n e . M a g n i f i c a t i o n 27-5X. G a r n e t i n q u a r t z o f e l d s p a t h i c g n e i s s , l a r g e l y r e p l a c e d by c h l o r i t e . 45 I F i g u r e 23 Specimen BH-1 from E l i z a b e t h Area i n NW p a r t o f C i r c u l a r S t r u c t u r e . Potassium f e l d s p a r p o r p h y r o b l a s t s i n f i n e - g r a i n e d q u a r t z o f e l d -s p a t h i c g n e i s s . L i n e a t i o n a r i s e s from r o d - l i k e e l o n g a t i o n of p o r p h y r o b l a s t s , and rods appear as augen i n c r o s s - s e c t i o n . F i g u r e 24 Specimen Car - 5 9-1, n o r t h e r n p a r t of C i r c u l a r S t r u c t u r e , 1 . 5 km, s o u t h o f C a r s w e l l L a k e . S t a i n e d and u n s t a i n e d s l a b s . Q u a r t z o f e l d s p a t h i c g n e i s s w i t h abundant g r a n i t o i d m a t e r i a l . P o t a s s i u m f e l d s p a r , y e l l o w ; a l b i t e , p i n k . Dark bands a r e c h l o r i t i s e d remnants o f o r i g i n a l g n e i s s . Q u a r t z i n g r a n i t o i d p a r t s e l o n g a t e d p a r a l l e l t o f o l i a t i o n . F i g u r e 25 C r o s s e d n i c o l s . T h i n s e c t i o n , specimen Car-59-1 as i n f i g u r e 24. M a g n i f i c a t i o n 27.5X. S e c t i o n t h r o u g h f i n e - g r a i n e d g n e i s s remant on l e f t and g r a n i t o i d m a t e r i a l on r i g h t . I n t h e f i n e - g r a i n e d p a r t , n o t e g a r n e t ( d a r k ) r a gged b i o t i t e and c h l o r i t e . I n g r a n i t o i d p a r t , n o t e m i c r o c l i n e p o r p h y r o b l a s t s b o r d e r e d w i t h m y r m e k i t i c p a t c h e s . 4 8 o f t h e p e r t h i t e i s due t o t h e p r e s e n c e o f f i n e l y d i s s e m i n a t e d h e m a t i t e . P l a g i o c l a s e i n t h e g r a n i t e - g n e i s s e s i s i n a l l c a s e s a l b i t e . Some a m p h i b o l i t e s h a v e b e e n s u b j e c t e d t o e n r i c h m e n t i n g r a n i t i c m a t e r i a l i n t h e same manner as h a v e t h e q u a r t z o f e l d s p a t h i c . g n e i s s e s -F i g u r e 21 shows a r o c k r i c h i n h o r n b l e n d e i n v a d e d i n a l i t - p a r - l i t manner by q u a r t z . a n d p e r t h i t e , w h i c h i n p l a c e s d e v e l o p i n t o p l a n a r s e g r e g a t i o n s o f a p e g m a t o i d n a t u r e . M a f i c Gneiss S u i t e 49 General D i s c u s s i o n . The term mafic g n e i s s i n c l u d e s a l l rocks which are composed e s s e n t i a l l y of ferromagnesian m i n e r a l s (pyrox-enes and hornblende) and p l a g i o c l a s e f e l d s p a r , and i n which q u a r t z i s absent.or p r e s e n t only i n s m a l l q u a n t i t i e s . W i t h i n t h i s group t h e r e i s a wide v a r i a t i o n i n p a r a g e n e s i s , and i t i s probable t h a t the v a r i o u s members have somewhat d i f f e r e n t o r i g i n s and metamorphic h i s t o r i e s . Two major s u b d i v i s i o n s are r e c o g n i s e d w i t h i n the group: a m p h i b o l i t e s , i n which the predominant mafic phase i s hornblende, and g r a n u l i t e s ( c o n s i d e r e d to be t r u e i n d i c a t o r s o f the g r a n u l i t e f a c i e s ) , i n which two pyroxenes are p r e s e n t . S e v e r a l i n t e r m e d i a t e types are r e p r e s e n t e d , i n which horn-blende may c o e x i s t w i t h orthopyroxene, c l i n o p y r o x e n e or both. The mafic g n e i s s e s do not form l a r g e b o d i e s , and they are not v o l u m e t r i c a l l y a major r o c k type. Amphibolites are found i n a l l p a r t s of the basement i n l i e r . , but the two-pyroxene g r a n u l i t e s have on l y been c o l l e c t e d i n two are a s : at the extreme e a s t e r n margin of the i n l i e r , and at a depth of 500 metres i n the zone east of C l u f f Lake (bore-hole Clu-425)- Amphibolites outcrop at a few l o c a l i t i e s , but g r a n u l i t e s are never exposed at the s u r f a c e . In a d d i t i o n to the rocks d e s c r i b e d i n t h i s s e c t i o n , s e v e r a l dark, f i n e - g r a i n e d r o c k s probably r e p r e s e n t mafic g n e i s s e s so a l t e r e d t h a t t h e i r o r i g i n a l mineralogy i s completely obscured. Prom t h e i r c h e m i c a l compositions, the g r a n u l i t e s and most of the a m p h i b o l i t e s may be c o n s i d e r e d t o r e p r e s e n t metamorphosed b a s a l t i c igneous r o c k s , a l t h o u g h some w e l l - f o l i a t e d q u a r t z - b e a r i n g a m p h i b o l i t e s are probably metasedimentary and grade i n t o q u a r t z o -f e l d s p a t h i c g n e i s s e s . None of the- mafic r o c k s r e t a i n any obvious 50 i g n e o u s t e x t u r e s , and g e o t h e r m o m e t r i c . s t u d i e s o f t h e g r a n u l i t e s (see. c h a p t e r I I I ) i n d i c a t e e q u i l i b r a t i o n under- m e t a m o r p h i c c o n d i -t i o n s . G r a n u l i t e s . The g r a n u l i t e s . a r e f i n e - g r a i n e d (0,1mm,-1mm.),. g r a n o b l a s t i c , d a r k g r e y o r g r e e n , . a n d p o o r l y f o l i a t e d . A n t i p e r t h -i t i c p l a g i o c l a s e (AnL(5_5o) g e n e r a l l y c o m p r i s e s a b o u t 50 p e r c e n t o f t h e r o c k , t h e r e m a i n d e r c o n s i s t i n g o f f e r r o m a g n e s l a n m i n e r a l s and • opaque m i n e r a l s . Brown b i o t i t e i s i n v a r i a b l y p r e s e n t i n s m a l l q u a n t i t i e s , a nd a l t h o u g h t h i s m i n e r a l i s g e n e r a l l y somewhat c o r r o d e d , w i t h a n e a r l y p a r a g e n e t i c p o s i t i o n i n d i c a t e d by p a r t i a l o v e r g r o w t h by p y r o x e n e s , i t s p r e f e r r e d o r i e n t a t i o n i s l a r g e l y r e s p o n s i b l e f o r t h e m a c r o s c o p i c f o l i a t i o n o f t h e r o c k . P l a g i o c l a s e i s q u i t e d a r k i n c o l o u r , due t o i t s f a i r l y c a l c i c c o m p o s i t i o n . E t c h i n g o f t h e f e l d s p a r w i t h h y d r o f l u o r i c a c i d e n h a n c e s t h e r o c k t e x t u r e , ( f i g u r e 2 6 ) , and shows t h a t a l l m i n e r a l s have some d e g r e e o f d i m e n s i o n a l p r e f e r r e d o r i e n t a t i o n . P y r o x e n e s o c c u r i n e q u i d i m e n s i o n a l x e n o b l a s t i c g r a i n s ( f i g u r e 2 7 ) . P i n k p l e o c h r o i c h y p e r s t h e n e and g r e e n i s h s a l i t i c c l i n o p y r o x -ene a r e t y p i c a l l y p r e s e n t i n a p p r o x i m a t e l y e q u a l amounts. A n a l y s e s o f t h e p y r o x e n e s a r e p r e s e n t e d i n c h a p t e r I I I . H o r n b l e n d e i s p r e s e n t i n some g r a n u l i t e s , a nd i t s p a r a g e n e t i c p o s i t i o n i s u n c l e a r . I n some s p e c i m e n s i t o c c u r s a s s m a l l i n c l u -s i o n s w i t h i n p y r o x e n e g r a i n s , whereas i n o t h e r s i t a p p e a r s t o be l a t e , r e p l a c i n g o r f o r m i n g o v e r g r o w t h s ' on p y r o x e n e s ( f i g u r e 2 8 ) . S u c h t e x t u r a l r e l a t i o n s h i p s , however, a r e r a r e l y u n e q u i v o c a l , and t h e y may be c o n t r a d i c t o r y w i t h i n one t h i n s e c t i o n . H o r n b l e n d e i n t h e g r a n u l i t e s i s t y p i c a l l y brown.. F i g u r e 26 S p e c i m e n BH-74-24 f r o m b o r e - h o l e C a r - 5 2 , e a s t s i d e o f s t r u c t u r e . Two p y r o x e n e g r a n u l i t e c u t by p e g m a t o i d v e i n . K - f e l d s p a r s t a i n e d y e l l o w , p l a g i o c l a s e w h i t e , q u a r t z m i l k y g r e y . Note c o n c o r d a n c e o f v e i n w i t h f o l i a t i o n , p y r o x e n e s i n c o r p o r a t e d i n t o v e i n , and s m a l l K - f e l d s p a r p o r p h y r o b l a s t s w i t h i n g r a n u l i t e . 52 I F i g u r e 27B C r o s s e d n i c o l s . T h i n s e c t i o n s p e c i m e n Car-23-10 f r o m e a s t s i d e o f s t r u c t M a g n i f i c a t i o n 27.5X. F o r c a p t i o n s e e f o l l o w i n g p a g e . 53 Caption f o r Figure 27 Two pyroxene gr a n u l i t e . Note granoblastic texture, preferred o r i e n t a t i o n of b i o t i t e (at p o s i t i o n of maximum absorption i n upper p i c t u r e ) . Hypersthene i n large grains, top centre, S a l i t e i n greenish grains along lower- margin. F i g u r e 28 C r o s s e d n i c o l s . T h i n s e c t i o n Car-34-3, e a s t s i d e o f s t r u c t u r e . M a g n i f i c a t i o n 27. 5X. G r a n u l i t e . L a r g e l a t e h o r n b l e n d e c r y s t a l s f o r m i n g p a r t i a l o v e r g r o w t h s on h y p e r s t h e n e ( g r e y , h i g h r e l i e f ) a n d b i o t i t e . 55 I l m e n i t e i s commonly p r e s e n t , l o c a l l y i n amounts up t o f i v e p e r c e n t , w i t h minor m a g n e t i t e and. p y r i t e . A p a t i t e i s a common a c c e s s o r y m i n e r a l . F i g u r e 26 shows a c o n f o r m a b l e pegmatoid band c u t t i n g a g r a n u l i t e body. Some m a f i c m a t e r i a l has been i n c o r p o r a t e d , but pyroxene g r a i n s w i t h i n t h e l e u c o c r a t i c band do not appear t o have s u f f e r e d e x t e n s i v e a l t e r a t i o n . M i n o r p o r p h y r o b l a s t s o f p e r t h i t i c p o t a s s i u m f e l d s p a r a r e d e v e l o p e d w i t h i n t h e g r a n u l i t e I n a f a s h i o n a n a l o g o u s t o t h a t i n t h e q u a r t z o f e l d s p a t h i c g n e i s s e s . A m p h i b o l i t e s . The a m p h i b o l i t e s a r e d a r k g r e y t o g r e e n , and a r e g e n e r a l l y somewhat c o a r s e r - g r a i n e d (up t o two m i l l i m e t r e s ) and b e t t e r f o l i a t e d t h a n t h e g r a n u l i t e s . H ornblende i s t y p i c a l l y r e c o g n i s a b l e i n hand specimen. P l a g i o c l a s e (Anij5_5o) i s p r e s e n t i n amounts up t o 50 p e r c e n t , t h e r e m a i n d e r of t h e r o c k e i t h e r c o n s i s t i n g e n t i r e l y o f b l u e g r e e n h o r n b l e n d e o r o f h o r n b l e n d e and a g r e e n i s h c l i n o p y r o x e n e . Where c l i n o p y r o x e n e i s p r e s e n t , i t i s commonly s e g r e g a t e d i n t o h a l f - c e n t i m e t r e zones, i m p a r t i n g a marked c o m p o s i t i o n a l b a n d i n g t o t h e r o c k ( f i g u r e 2 9 ) , Q u a r t z i s absent o r p r e s e n t o n l y i n s m a l l amounts. I l m e n i t e i s a common a c c e s s o r y m i n e r a l . Hornblende i s t y p i c a l l y f r e s h , f o r m i n g l a r g e , w e l l - d e v e l o p e d p o i k i l o b l a s t s . E u h e d r a l f a c e s a r e not uncommon. Hornblende may be f r e s h even when p l a g i o c l a s e and c l i n o p y r o x e n e i n t h e same specimen a r e h i g h l y a l t e r e d ( f i g u r e 3 0 ) . Hornblende i s c o n s i d e r e d t o have formed l a t e i n t h e p a r a g e n e s i s . F i g u r e 29 Specimen JPB-56. Northwest of Claude Lake. Coarse-grained a m p h i b o l i t e . B l a c k , hornblende; dark gree c l i n o p y r o x e n e ' l i g h t c o l o u r , p l a g i o c l a s e f e l d s p a r . S t r o n c o m p o s i t i o n a l banding. 5 7 F i g u r e 30 Specimen J P B - 7 3 - 5 , northwest of Claude Lake. M a g n i f i c a t i o n 2 7 .5X. Large p o i k i l o b l a s t o f hornblende e n c l o s i n g p l a g i o c l a s e f e l d s p a r which i s h i g h l y a l t e r e d to s e r i c i t e . C l i n o p y r o x e n e g r a i n s (top l e f t ) are somewhat a l t e r e d . P e g m a t o i d R o c k s 58 The t e r m " p e g m a t o i d " i s a p p l i e d t o t h e s u i t e o f c o a r s e - g r a i n e d l e u c o c r a t i c r o c k s w h i c h have a n a p p a r e n t i n t r u s i v e o r v e i n - l i k e r e l a t i o n s h i p w i t h t h e basement g n e i s s e s . T h i s . t e r m i s . c h o s e n t o e m p h a s i s e t h a t , a l t h o u g h t h e y a p p e a r t o be p e g m a t i t i c i n o r i g i n , t h e c h a r a c t e r i s t i c t e x t u r e s o f t r u e p e g m a t i t e s a r e n o t w e l l - d e v e l -o p e d . P e g m a t o i d b o d i e s a r e s e e n on a l l s c a l e s , , f r o m c e n t i m e t r e t h i c k c o n c o r d a n t s h e e t s i n q u a r t z o f e l d s p a t h i c g n e i s s e s o r a m p h i -b o l i t e s , t o b o d i e s o f v e r y c o a r s e - g r a i n e d r o c k w i t h d i m e n s i o n s o f t e n s o f m e t r e s . The m a r g i n s o f t h e b o d i e s a r e f o r t h e most p a r t p a r a l l e l o r s u b - p a r a l l e l t o t h e f o l i a t i o n o f t h e h o s t r o c k , b u t a few m i n o r p e g m a t o i d o r a p l i t i c v e i n s a r e t r a n s g r e s s i v e , c u t t i n g b o t h t h e f o l i a t i o n and t h e o t h e r p e g m a t o i d r o c k s o b l i q u e l y . The p e g m a t o i d s a r e r e l a t i v e l y r e s i s t a n t t o w e a t h e r i n g and f o r m t h e l a r g e s t o u t c r o p s i n t h e a r e a . T h e y a r e a b u n d a n t t h r o u g h -out t h e a r e a , a n d no s y s t e m a t i c g e o g r a p h i c a l v a r i a t i o n c a n be d e t e c t e d . Q u a r t z , p e r t h i t e and a l b i t e a r e v i r t u a l l y t h e o n l y m i n e r a l s p r e s e n t i n t h e p e g m a t o i d r o c k s , e x c e p t f o r p h a s e s w h i c h a p p e a r t o have b e e n a s s i m i l a t e d f r o m t h e c o u n t r y r o c k s . The amount o f q u a r t z p r e s e n t i s b e t w e e n f i v e and 50 p e r c e n t , and a l t h o u g h most s p e c i m e n s c o n t a i n b o t h p e r t h i t e and a l b i t e , p e r t h i t e o r l e s s f r e q u e n t l y a l b i t e may c o n s t i t u t e a l m o s t t h e e n t i r e r o c k . The two f e l d s p a r s c a n be d i s t i n g u i s h e d w i t h some c e r t a i n t y i n hand s p e c i m e n by t h e p i n k c o l o u r o f t h e p o t a s s i u m f e l d s p a r , b u t s t a i n i n g t e c h n i q u e s 59 r e n d e r t h e t e x t u r e s more, r e a d i l y v i s i b l e . The l a r g e s t p e g m a t o i d b o d i e s a r e composed o f q u a r t z and p e r t h i t e . T h e i r t e x t u r e i s an i n t e r g r o w t h o f v e r y . l a r g e (up t o 5 cm.) c r y s t a l s ( f i g u r e 31), commonly s h o w i n g f o l i a t i o n r e s u l t i n g f r o m a s l i g h t e l o n g a t i o n o f q u a r t z g r a i n s ( f i g u r e 32). T h i s f a b r i c r e s u l t s f r o m a f o l d i n g p h a s e l a t e r than' t h e m a i n f o l i a t i o n o f t h e g n e i s s o s e r o c k s , as shown by t h e p a r a l l e l i s m o f t h e p e g m a t i t e f o l i a t i o n t o t h e a x i a l p l a n e s o f m i n o r f o l d s i n p e l i t i c g n e i s s e s ( f i g u r e 33). A l l g r a i n s a r e a n h e d r a l , and t h e f e l d s p a r s t y p i c a l l y h a v e embayed o u t l i n e s s u g g e s t i v e o f c o r r o s i o n . The f i n e r - g r a i n e d p e g m a t o i d r o c k s c h a r a c t e r i s t i c a l l y show a more g r a p h i c t e x t u r e t h a n t h e c o a r s e - g r a i n e d t y p e s , and g e n e r a l l y c o n t a i n l a r g e r p r o p o r t i o n s o f a l b i t e ( f i g u r e '3*0', Tn t h e r o c k s c o n t a i n i n g two f e l d s p a r s , no p a r a g e n e t i c s e q u e n c e c a n be e s t a b l i s h e d . A l b i t e i s p r e s e n t as l a t e e u h e d r a l . c r y s t a l s ( f i g u r e 35) and as . c o r r o d e d g r a i n s o v e r g r o w n b y . p e r t h i t e . Many o f t h e p e g m a t o i d s c o n t a i n up t o t e n p e r c e n t o f f e r r o m a g -n e s i a n m i n e r a l s , n o t a b l y g a r n e t , w h i c h a r e c o n s i d e r e d t o be x e n o -c r y s t s i n v i e w o f t h e i r a p p a r e n t s t r o n g d i s e q u i l i b r i u m w i t h t h e h o s t r o c k . G a r n e t s a r e s m a l l ( l e s s t h a n 5 mm.) and r o u n d e d , and a r e w i t h o u t e x c e p t i o n c o r r o d e d and rimmed w i t h c h l o r i t e . Rounded b l e b s o f f i n e - g r a i n e d c h l o r i t e , a s i n f i g u r e 31 , a r e i n t e r p r e t e d as c o m p l e t e l y r e p l a c e d g a r n e t s . The p e g m a t o i d b o d i e s a r e commonly most e n r i c h e d i n g a r n e t s a t t h e i r m a r g i n s , where e v i d e n c e o f a s s i m i l a t i o n o f g a r n e t - b e a r i n g r o c k s may be f o u n d ( f i g u r e 36). M i c r o p r o b e a n a l y s i s o f a c h l o r i t e - r i m m e d g a r n e t f r o m a p e g m a t o i d body ( t a b l e 2) shows i t t o be c o m p a r a b l e i n c o m p o s i t i o n t o g a r n e t s i n t h e p e l i t i c g n e i s s e s . S m a l l b u n d l e s and i s o l a t e d c r y s t a l s o f s i l l i m a n i t e , t y p i c a l l y 60 F i g u r e 31 Specimen BH-73-25, west o f c e n t r e o f C i r c u l a r S t r u c t u r e . C o a r s e - g r a i n e d pegmatoid r o c k . I n t e r g r o w t h o f c o a r s e - g r a i n e d q u a r t z and p o t a s s i u m f e l d s p a r . Dark p a t c h e s a r e f i n e - g r a i n e d c h l o r i t e , p r o b a b l y g a r n e t pseudomorphs. Note d e f o r m a t i o n f a b r i c a r i s i n g from e l o n g a t i o n o f q u a r t z g r a i n s . F i g u r e 32 Specimen Clu-91-2 from C l u f f Lake m i n e r a l i s e d zone. Potassium f e l d s p a r s t a i n e d y e l l o w . Margin of pegmatoid body. Abundant potassium f e l d s p a r p o r p h y r o -b l a s t s i n a d j o i n i n g q u a r t z o f e l d s p a t h i c g n e i s s . Pegmatoid i s e n t i r e l y composed of quartz and potassium f e l d s p a r , and has marked f o l i a t i o n due t o elongated quartz g r a i n s . 62 F i g u r e 33 Sp e c i m e n Car-50-2a, e a s t m a r g i n o f C i r c u l a r S t r u c t u r e . C o n t a c t o f p e g m a t o i d and g a r n e t - s i l l i m a n i t e p e l i t i c g n e i s s . T h e r e i s some a s s i m i l a t i o n o f t h e p e l i t i c m a t e r i a l by t h e p e g m a t o i d . F o l d i n g i n t h e g n e i s s i s r e p r e s e n t e d by an a x i a l -p l a n e f o l i a t i o n i n t h e p e g m a t o i d , d e m o n s t r a t i n g p o s t - e m p l a c e m e n t d e f o r m a t i o n . 63 F i g u r e 34 S p e c i m e n Car - 5 4-1, f r o m e a s t s i d e o f t h e C i r c u l a r S t r u c t u r e . P o o r l y d e v e l o p e d g r a p h i c i n t e r g r o w t h o f q u a r t z , p o t a s s i u m f e l d s p a r ( s t a i n e d y e l l o w ) and a l b i t e ( s t a i n e d p i n k ) . Abundant c o r r o d e d g a r n e t s , e x t e n s i v e l y r e p l a c e d by c h l o r i t e . F i g u r e 35 C r o s s e d n i c o l s . T h i n s e c t i o n specimen Car-73-4, n o r t h w e s t p a r t o f C i r c u l a r S t r u c t u r e . M a g n i f i c a t i o n 55X. Pegmatoid. L e f t s i d e : l a r g e s e r i c i t i s e d p e r t h i t e c r y s t a l . C e n t r e : e u h e d r a l a l b i t e w i t h c o r r o d e d p e r t h i t e c o r e . R i g h t s i d e : i n t e r g r o w t h o f q u a r t z and p e r t h i t e . 65 F i g u r e 36 Specimen BH - 7 4 - 6 6 from b o r e - h o l e C l u - 4 2 5 , C l u f f Lake m i n e r a l i s e d zone. Lower s l a b s t a i n e d f o r p o t a s s i u m f e l d s p a r . M a r g i n o f pegmatoid body a g a i n s t q u a r t z o f e l d s p a t h i c g n e i s s ( s e e f i g u r e 2 0 ) . A s s i m i l a t i o n and i n v a s i o n o f g n e i s s by pegmatoid m a t e r i a l a r e a p p a r e n t . S m a l l p i n k g a r n e t s , w i t h c h l o r i t e r e a c t i o n r i m s not v i s i b l e i n p h o t o g r a p h , p r o b a b l y o r i g i n a t e d i n g n e i s s (see t e x t ) . C o n t a c t i s s u b - p a r a l l e l t o f o l i a t i o n i n t h e g n e i s s . OXIDE WEIGHT PERCENT FORMULA SI02 38.81 2.971 A 1 2 0 3 22.77 2.054 MgO 5-89 0.672 FeO 34.09 2.182 CaO 0.64 0,053 MnO 1.03 0.067 TOTAL '. 103.24 T a b l e 2 M i c r o p r o b e a n a l y s i s o f c h l o r i t e - r i m m e d g a r n e t from p e g m a t i t e . Formula c a l c u l a t e d on t h e b a s i s o f 12 oxygens. Compare t h i s a n a l y s i s w i t h t h a t o f g a r n e t from p e l i t i c g n e i s s , T a b l e 1 . 67 p a r t i a l l y r e p l a c e d by m u s c o v i t e , a r e l o c a l l y p r e s e n t I n t h e pegma-t o i d s . B i o t i t e and o t h e r m a f i c p h a s e s a r e n o t s e e n b u t t h e s e may be r e p r e s e n t e d by some o f t h e c h l o r i t e p s e u d o m o r p h s . -68 C a t a c l a s t i c R o c k s A l l o f t h e r o c k t y p e s d e s c r i b e d above a r e a f f e c t e d by l o c a l - .-. i s e d z o n e s o f i n t e n s e d e f o r m a t i o n . F i g u r e s 37 and 38 show examples o f g r a n i t e g n e i s s e s and q u a r t z o f e l d s p a t h i c . g n e i s s e s w h i c h have b e e n h i g h l y s h e a r e d and have d e v e l o p e d m y l o n i t i c and p o r p h y r o -c l a s t i c t e x t u r e s . A l t h o u g h t h e a v a i l a b l e b o r e - h o l e c o n t r o l d o e s n o t a l l o w a d e t a i l e d s t r u c t u r a l a n a l y s i s o f t h e s e c a t a c l a s t i c f e a t u r e s , t h e y a p p a r e n t l y r e p r e s e n t p l a n a r s h e a r - z o n e s w h i c h , when a l t e r e d by p r o c e s s e s s u c h as w e a t h e r i n g , a r e i m p o r t a n t i n t h e l o c a l i s a t i o n o f u r a n i u m m i n e r a l i z a t i o n . S u c h z o n e s a r e a n a l o g o u s t o t h e l i n e a r m y l o n i t e z o n e s w h i c h c o n t a i n much o f t h e m i n e r a l i -z a t i o n i n t h e B e a v e r l o d g e M i n i n g D i s t r i c t ( s e e F a h r i g , 1967). As t h e c a t a c l a s t i c f e a t u r e s c u t a l l basement u n i t s , i n c l u d i n g t h e i n t r u s i v e p e g m a t o i d s , t h e y must be p o s t - m e t a m o r p h i c . As a l a t e d e f o r m a t i o n p h a s e i s known t o have p r o d u c e d p e r v a s i v e f o l i a -t i o n and f o l d i n g i n t h e p e g m a t o i d s ( s e e f i g u r e 21), i t may be t h a t t h e m y l o n i t e z o n e s r e p r e s e n t t h e s h e a r e d l i m b s o f s u c h l a t e f o l d s . The f o l d i n g d e s c r i b e d h e r e may be a s m a l l - s c a l e e x p r e s s i o n o f t h e l a t e f o l d - p h a s e (p-3) d e s c r i b e d by B a s s a g e t and Camps. 69 F i g u r e 37 S p e c i m e n C a r - 7 0 - 5 , f r o m n o r t h e a s t p a r t o f s t r u c t u r e . P o r p h y r o c l a s t i c u l t r a m y l o n i t e . Groundmass r e d u c e d t o s u c h f i n e - g r a i n e d m a t e r i a l t h a t o r i g i n a l r o c k t y p e i s no l o n g e r i d e n t i f i a b l e . F i g u r e 38 Specimen Car-77-1, from n o r t h w e s t s i d e o f s t r u c t u r e . M y l o n i t i s e d q u a r t z o f e l d s p a t h i c g n e i s s . M a f i c m i n e r a l s e x t r e m e l y s t r e a k e d - o u t . P o r p h y r o c l a s t i c zones may r e p r e s e n t p egmatoid bands w h i c h were more r e s i s t a n t t o c a t a c l a s t i c d e f o r m a t i o n . 71 I I I . CONDITIONS OF METAMORPHISM I n t r o d u c t i o n T h i s c h a p t e r i s c o n c e r n e d w i t h r e c o n s t r u c t i n g t h e h i s t o r y o f p r o g r a d e metamorphism as d i s t i n c t f r o m . l a t e r e v e n t s s u c h as t h e f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e . The c r i t e r i a w h i c h w i l l be d i s c u s s e d h e r e a r e : 1. The m e t a m o r p h i c f a c i e s r e p r e s e n t e d j as r e f l e c t e d i n t h e m i n e r a l a s s e m b l a g e s i n g n e i s s e s o f v a r i o u s b u l k c o m p o s i -t i o n s . 2. The r e s u l t s o f a t t e m p t s t o c a l c u l a t e t h e a b s o l u t e p h y s i c a l c o n d i t i o n s o f metamorphism u s i n g t h e r m o d y n a m i c t e c h n i q u e s . 3- M e t a m o r p h i c h i s t o r y as shown by p a r a g e n e s e s o f m e t a m o r p h i c m i n e r a l s . T a b l e 3 i s a m i n e r a l c o m p a t i b i l i t y d i a g r a m f o r a l l r o c k t y p e s , s h o w i n g t h e m a j o r p h a s e s w h i c h a r e c h a r a c t e r i s t i c a l l y s e e n i n a p p a r e n t e q u i l i b r i u m c o n t a c t i n t h i n s e c t i o n , t o g e t h e r w i t h t h o s e w h i c h , by t h e p r e s e n c e o f p s e u d o m o r p h s , a r e i n f e r r e d t o have b e e n i n s u c h c o n t a c t a t t h e t i m e o f p r o g r a d e metamorphism. The m i n e r a l a s s e m b l a g e s i n a l l r o c k - t y p e s s u g g e s t e q u i l i b r a t i o n u n d e r t h e p h y s i c a l c o n d i t i o n s o f t h e u p p e r a m p h i b o l i t e o r g r a n -u l i t e f a c i e s . The p e l i t i c and m a f i c g n e i s s e s a r e t h e r o c k s whose b u l k c o m p o s i t i o n s l e a d t o t h e d e v e l o p m e n t o f t h e most g r a d e - s e n s i t i v e m i n e r a l a s s e m b l a g e s . The number o f a s s e m b l a g e s p r e s e n t i n t h e s e t y p e s i s s m a l l , and t h e i m p o r t a n t t y p e s a r e as f o l l o w s : P e l i t i c g n e i s s e s 72 1 CO O a 03 o H • O • > H O > > • tr1 . a • w O H w o i-3 3-tr1 H H H H Cd O O O tsi w H3 tr1 T) CO o > W M K tr1 H S3 > > H l"3 O O CO H3 w W X M M 'M NE ORTHOPYROXENE X X X 0 0 0 X X CLINOPYROXENE X X X 0 0 0 X X HORNBLENDE X X X 0 0 0 X BIOTITE X X X X X X GARNET X X X X X CORDIERITE X X X 0 SILLIMANITE X X X QUARTZ X X PLAGIOCLASE X T a b l e 3 M i n e r a l c o m p a t i b i l i t y d i a g r a m f o r a l l r o c k t y p e s . X i n d i c a t e s m i n e r a l s t y p i c a l l y i n e q u i l i b r i u m c o n t a c t . x i n d i c a t e s m i n e r a l s i n f e r r e d t o have been i n e q u i l i b r i u m by t h e p r e s e n c e o f pseudomorphs. 0 i n d i c a t e s an assemblage not f o u n d . Figure 3 8a AFM diagram showing mineral c o m p a t i b i l i t i e s i n p e l i t i c rocks. g a r n e t + c o r d i e r i t e + q u a r t z + b i o t i t e + p l a g i o c l a s e + k - f e l d s p a r i g r a p h i t e i p y r i t e . g a r n e t + s i l l i m a n i t e +' quart-z + b i o t i t e + p l a g i o - . c l a s e + k ~ f e l d s p a r t g r a p h i t e t p y r i t e M a f i c g n e i s s e s h y p e r s t h e n e + s a l i t e + b i o t i t e ± h o r n b l e n d e + p l a g i o c l a s e t k - f e l d s p a r q u a r t z + a p a t i t e +• i l m e n i t e h y p e r s t h e n e + b i o t i t e + h o r n b l e n d e + p l a g i o c l a s e ± k - f e l d s p a r ± q u a r t z + a p a t i t e + i l m e n i t e . h o r n b l e n d e ± c l i n o p y r o x e n e + p l a g i o c l a s e + q u a r t z S e v e r a l c o m p a t i b i l i t i e s w h i c h do n o t o c c u r a r e c o n s i d e r e d i m p o r t a n t . I n t h e r o c k s o f m a f i c c o m p o s i t i o n , n e i t h e r g a r n e t n o r c o r d i e r i t e i s d e v e l o p e d . I n t h e p e l i t i c g n e i s s e s , p y r o x e n e s a r e n o t s e e n , and t h e a s s e m b l a g e c o r d i e r i t e + s i l l i m a n i t e does n o t o c c u r . I n t h e r o c k s o f m a f i c c o m p o s i t i o n , n e i t h e r g a r n e t n o r s i l l i m a n i t e n o r c o r d i e r i t e i s d e v e l o p e d . M u s c o v i t e i s a b s e n t a s a p r o g r a d e p h a s e . The n o n - o c c u r r e n c e o f c o r d i e r i t e + s i l l i m a n i t e i s p r o b l e m a t i c . An AFM d i a g r a m ( F i g . 3 8 ^ shows t h a t c o r d i e r i t e s h o u l d be s t a b l e i n . any r o c k c o n t a i n i n g s u f f i c i e n t a l u m i n a and s i l i c a t o p r o d u c e s i l l i m a n i t e . Geographic D i s t r i b u t i o n of Assemblages 75 As i n d i c a t e d e a r l i e r , sample c o n t r o l i s inadequate for: a d e t a i l e d a n a l y s i s of the d i s t r i b u t i o n of m i n e r a l assemblages. N e v e r t h e l e s s , two g e n e r a l i s a t i o n s may be made ( f i g u r e 2 ) . In the p e l i t i c g n e i s s e s , a broad zone may be r e c o g n i s e d i n the northwest p a r t o f the area where on l y the c o r d i e r i t e - b e a r i n g type i s encountered, and In the southeast t h e r e Is a.zone i n which o n l y s i l l i m a n i t e i s found. The l i n e s on the map are intend.*-ed t o i n d i c a t e o n l y g e n e r a l t r e n d s , but i t may be noted t h a t they l i e i n a d i r e c t i o n approximating t h a t of r e g i o n a l s t r i k e (compare f i g u r e 1 ) . In the broad band between the two l i m i t i n g l i n e s , both assemblages are encountered, with an a p p a r e n t l y random d i s t r i b u -t i o n . Nowhere are roc k s c o n t a i n i n g the two assemblages encount-ered i n j u x t a p o s i t i o n ( i . e . i n the same bore-hole) so t h e i r r e l a t i o n s h i p remains u n c l e a r . The apparent complexity of d i s t r i -b u t i o n i n t h i s t r a n s i t i o n a l area i s exaggerated-by the h i g h e r d e n s i t y o f c o n t r o l i n the m i n e r a l i s e d zones. F i g u r e 2 a l s o shows the apparent l i m i t o f development of h y p e r s t h e n e - b e a r i n g mafic g n e i s s e s . T h i s l i n e i s based on o n l y f o u r known occurrences o f such r o c k s , but they are d i s t r i b u t e d a c r o s s the area o u t l i n e d , and c a r e f u l s e arch has i n d i c a t e d no s i m i l a r rocks elsewhere. Taken i n d i v i d u a l l y , the two assemblage boundaries d e s c r i b e d above might be c o n s i d e r e d t o be a r t i f a c t s of the sparse c o n t r o l , . but t h e i r c o i n c i d e n c e , t o g e t h e r with t h e i r p a r a l l e l i s m t o the r e g i o n a l g r a i n suggests t h a t they may r e p r e s e n t a r e a l metamorphic f a c i e s boundary, w i t h c o r d i e r i t e u n s t a b l e i n p e l i t i c rocks to the and hypersthene unstable i n mafic rocks to the 7 7 The S t a b i l i t y o f C o r d i e r i t e . I f t h e i n c o m p a t i b i l i t y o f c o r d i e r i t e and s i l l i m a n i t e i n t h e p e l i t i c g n e i s s e s t r u l y r e p r e s e n t s a m e t a m o r p h i c r e a c t i o n , , some e s t i m a t e s may be made o f t h e p h y s i c a l c o n d i t i o n s w h i c h p r e v a i l e d a t t h e r e a c t i o n b o u n d a r y . No b u l k - r o c k a n a l y s e s a r e a v a i l a b l e , a nd as a r e s u l t o f t h e l a r g e g r a i n s i z e and t h e s t r o n g m e t a m o r p h i c d i f f e r e n t i a t i o n i n t h e p e l i t i c g n e i s s e s , m o d a l a n a l y s e s w o u l d be u n r e l i a b l e , so i t i s not. c l e a r w h e t h e r t h e two g r o u p s o f g n e i s s e s r e p r e s e n t i d e n t i c a l b u l k c h e m i s t r i e s . N e v e r t h e l e s s , a l t h o u g h a few s i l l i m a n l t e - r l c h r o c k s a r e o b v i o u s l y more a l u m i n o u s t h a n any o f t h e c o r d i e r i t e g n e i s s e s , t h e a u t h o r i s s a t i s f i e d t h a t t h e r e i s a l a r g e o v e r l a p b e t w e e n t h e c o m p o s i t i o n s o f t h e two g r o u p s . I f t h e d i s t i n c t i o n b etween t h e g r o u p s were p u r e l y c o m p o s i t i o n a l , t h e n r o c k s o f i n t e r m e d i a t e c o m p o s i t i o n w o u l d be e x p e c t e d t o c o n t a i n b o t h s i l l i m a n i t e and c o r d i e r i t e . T h i s a s s e m b l a g e d o e s n o t o c c u r , The r e a c t i o n w h i c h most e a s i l y a c c o u n t s f o r t h e o b s e r v e d s i t u a t i o n i s t h e c o r d i e r i t e breakdown; 3 c o r d i e r i t e = 2 g a r n e t + 4 s i l l i m a n i t e + 5 q u a r t z T h i s r e a c t i o n was s t u d i e d e x p e r i m e n t a l l y by Hensen and G r e e n (1971, 1972, 1973), A p a r t f r o m g a r n e t and c o r d i e r i t e , t h e o n l y f e r r o -m a g n e s i a n p h a s e p r e s e n t i n t h e p e l i t i c r o c k s i s b i o t i t e . As t h i s i s p r e s e n t i n r e l a t i v e l y s m a l l amounts i n t h e c o r d i e r i t e -f r e e r o c k s , i t i s u n l i k e l y t h a t i t i s a m a j o r p r o d u c t o f c o r d -i e r i t e b r e akdown. The breakdown r e a c t i o n o f c o r d i e r i t e as w r i t t e n i s d i v a r i a n t ( Hensen and G r e e n , 1973), t h a t i s , t h e f o u r r e a c t a n t s c o e x i s t 78 o v e r . a - s i g n i f i c a n t r e g i o n o f P-T s p a c e , Two c o n c l u s i o n s may be drawn f r o m t h e l a c k o f c o e x i s t e n c e o f c o r d i e r i t e . a n d . s i l l i m a n i t e : 1. t h a t t h e d i v a r i a n t band i s e i t h e r d e v e l o p e d o v e r a v e r y r e s t r i c t e d g e o g r a p h i c a l a r e a , o r . i s a b s e n t as a r e s u l t o f f a u l t i n g o r a s i m i l a r p r o c e s s 2. t h a t t h e b u l k c o m p o s i t i o n o f t h e p e l i t i c r o c k s i s n o t s u f f i c i e n t l y r i c h i n a l u m i n a f o r s i l l i m a n i t e t o be p r e s e n t i n c o r d i e r i t e - b e a r i n g rocks,,. A phases-diagram s h o w i n g t h e v a r i o u s b r e a k d o w n r e a c t i o n s o f c o r d i e r i t e as d e t e r m i n e d by Hensen and G r e e n , i s shown i n f i g u r e I t w i l l be s e e n t h a t t h e r e a c t i o n c o r d i e r i t e = g a r n e t + s i l l i m a n i t e + q u a r t z i s r e p r e s e n t e d by a d i v a r i a n t f i e l d . . The c o m p o s i t i o n s o f g a r n e t i n e q u i l i b r i u m w i t h c o r d i e r i t e and c o r d i e r -i t e i n e q u i l i b r i u m w i t h g a r n e t a t v a r i o u s P~T c o n d i t i o n s a r e shown by c o n t o u r l i n e s . A t t h e commencement o f t h e p r e s e n t work, i t was hoped t o m e a s u r e c o m p o s i t i o n s o f g a r n e t ^ c o r d i e r i t e p a i r s i n o r d e r t o r e l a t e them t o a m odel s u c h as t h a t o f H e n s e n and G r e e n , However, t h i s i n t e n t i o n was f r u s t r a t e d by t h e u b i q u i t o u s c o m p l e t e a l t e r a t i o n o f c o r d i e r i t e . A l l t h a t c a n be s a i d i s t h a t t h e Mg/Mg + P e 2 + r a t i o s o f g a r n e t show a g e n e r a l . i n c r e a s e f r o m west t o e a s t , w i t h t h e most m a g n e s i u m - r i c h g a r n e t s b e i n g f o u n d i n t h e s i l l i m a n -i t e - b e a r i n g r o c k s ( t a b l e d ) . T h i s t r e n d i s c o m p a t i b l e w i t h a m o d e l o f i n c r e a s i n g l y m a g n e s i a n g a r n e t s b e i n g i n e q u i l i b r i u m w i t h c o r d i e r i t e w i t h i n c r e a s i n g p r e s s u r e . However, t h e t r e n d i s e q u a l l y l i k e l y t o be due t o t h e f o r t u i t o u s s e l e c t i o n o f r o c k s w i t h i n c r e a s i n g l y M g r - r i c h b u l k c o m p o s i t i o n s , as t h e s i l l i m a n i t e -f r e e r o c k s do n o t f a l l w i t h i n t h e d i v a r i a n t f i e l d o f t h e r e a c t i o n 79 I I — J t I 7 0 0 9 0 0 1100 Temperature, degrees C, Figure 39 Phase diagram showing the breakdown reactions of c o r d i e r i t e under hydrous conditions. Contours show the compositions of garnet and c o r d i e r i t e i n equilibrium f o r the divariant r e a c t i o n : c o r d i e r i t e = garnet + s i l l i m a n i t e + quartz. After Hensen and Green, 1973. Some high-temperature reactions are omitted. 80 SPECIMEN LOCATION Mg/Mg+Fe 2+ PCr.73-2 Gar-cord NW part of s t r u c t u r e 102 BRV74~l69 Gar^cord Claude Lake 278 Car-50-1 G a r - s i l l SE p a r t of s t r u c t u r e ,334 T a b l e ^ Magnesium-iron r a t i o s determined from microprobe a n a l y s e s of t h r e e garnets, i n p e l i t i c g n e i s s e s demonstrating p o s s i b l e t r e n d of magnesium enrichment from west t o east a c r o s s the a r e a . 81 c o n s i d e r e d . To c o n f i r m t h e t r e n d i t would.be n e c e s s a r y t o c o n s i d e r g a r n e t - c o r d i e r i t e d i s t r i b u t i o n . c o e f f i c i e n t s , but t h i s i s p r o h i b -i t e d by t h e a l t e r a t i o n o f c o r d i e r i t e . The above d i s c u s s i o n w o u l d a p p e a r t o i n d i c a t e t h a t t h e c o r d i e r i t e - s i l l i m a n i t e r e a c t i o n , i f i t i s r e a l , . r e p r e s e n t s an e s s e n t i a l l y i s o b a r i c s u r f a c e . However, t h i s i n v o l v e s t h e a s s u m p t i o n made by Hensen and G r e e n t h a t t h e s t a b i l i t y o f c o r d -i e r i t e i s I n d e p e n d e n t o f w a t e r f u g a c i t y . A p p l i c a t i o n o f t h i s m o d e l w o u l d r e q u i r e t h a t t h e c o r d i e r i t e - f r e e p e l i t i c g n e i s s e s l i e above t h e d i v a r i a n t f i e l d , and r e p r e s e n t p r e s s u r e s i n e x c e s s o f 10 k i l o b a r s . A l s o , t h e p o s i t i o n o f t h e k y a n i t e - s i l l i m a n i t e u n i v a r i a n t b o u n d a r y r e q u i r e s t e m p e r a t u r e s a b o v e 9 0 0 d e g r e e s c e n t i g r a d e . T h e s e c o n d i t i o n s a r e b e y o n d t h e r e g i m e o f t h e g r a n u l i t e f a c i e s as d e f i n e d by T o u r e t ( 1 9 7 1 ) , and w o u l d i n d i c a t e e x c e e d i n g l y deep b u r i a l and s u b s e q u e n t u p l i f t o f a s e d i m e n t a r y r o c k s e q u e n c e . R e c e n t e x p e r i m e n t s s u c h as t h o s e o f Newton ( 1 9 7 1 ) , i n d i c a t e t h a t t h e p a r t i a l p r e s s u r e o f H2O has a p r o f o u n d e f f e c t on t h e s t a b i l i t y r e l a t i o n s o f c o r d i e r i t e , p r o b a b l y a s a r e s u l t o f t h e a b i l i t y , o f t h a t m i n e r a l t o c o n t a i n s t r u c t u r a l w a t e r . The m a j o r i t y o f s y n t h e t i c e x p e r i m e n t s on c o r d i e r i t e s t a b i l i t y , s u c h as t h o s e o f Hensen and G r e e n ( o p . c i t . ) have b e e n c o n d u c t e d i n t h e p r e s e n c e o f a h y d r o u s p h a s e , w i t h PH2O ?z P t o t a l - A n h y d r o u s o r low PH2O e x p e r i m e n t s c a n n o t . b e p e r f o r m e d w i t h e a s e i n t h e P-T f i e l d o f t h e g r a n u l i t e f a c i e s , a s a t t e m p e r a t u r e s b e l o w 1 0 0 0 ° C , r e a c t i o n r a t e s become e x t r e m e l y s l o w . The h i g h - t e m p e r a t u r e e q u i l i b r i a M g - s a p p h i r i n e + e n s t a t i t e + q u a r t z = a n h y d r o u s M g - c o r d i e r -82 i t e and e n s t a t i t e + s i l l i m a n i t e = M g - s a p p h i r l n e + q u a r t z , i n t e r s e c t a t an i n v a r i a n t p o i n t w i t h t h e l o w e r - t e m p e r a t u r e r e a c t i o n . e n s t a t i t e + s i l l i m a n i t e + q u a r t z = a n h y d r o u s M g - c o r d i e r -i t e . By c o m b i n i n g e x p e r i m e n t a l d a t a on t h e f i r s t two r e a c t i o n s u n d e r h i g h - t e m p e r a t u r e a n h y d r o u s c o n d i t i o n s , Newton has c a l c u l a t e d an a n h y d r o u s P-T c u r v e f o r t h e t h i r d , w h i c h c a n be e x t r a p o l a t e d b a c k t o c o n d i t i o n s r e a s o n a b l e f o r t h e g r a n u l i t e f a c i e s , Newton's p h a s e d i a g r a m f o r t h e r e a c t i o n s c o n s i d e r e d i s shown i n f i g u r e , and shows the. d i f f e r e n c e i n s t a b i l i t y o f c o r d i e r i t e i n t h e h y d r o u s and a n h y d r o u s s y s t e m s . Newton e t a l . (197**) p r o p o s e t h a t c o r d -i e r i t e may be s t a b l e a t p r e s s u r e s 4-5 k i l o b a r s h i g h e r i n t h e h y d r o u s s y s t e m t h a n i n t h e a n h y d r o u s s y s t e m . The h i g h p r e s s u r e breakdown o f c o r d i e r i t e t o o r t h o p y r o x e n e t s i l l i m a n i t e + q u a r t z o n l y o c c u r s i n v e r y M g - r i c h b u l k - c o m p o s i t i o n s ( H e n s en and G r e e n , 1971)• I n t h e c o m p o s i t i o n a l r a n g e o f p e l i t i c r o c k s - t h i s i s r e p l a c e d by t h e g a r n e t + s i l l i m a n i t e + q u a r t z a s s e m b l a g e , I t i s p r o p o s e d h e r e t h a t t h e breakdown p r e s s u r e o f c o r d i e r i t e i n r o c k s o f p e l i t i c c o m p o s i t i o n w o u l d be l o w e r e d i n an a n h y d r o u s e n v i r o n m e n t i n a manner a n a l o g o u s t o t h a t d e d u c e d by Newton f o r t h e Mg-end-member s y s t e m . The above h y p o t h e s i s c o u l d e x p l a i n t h e p o s t u l a t e d c o r d i e r i t e -o u t r e a c t i o n . i n t h e C a r s w e l l r o c k s e n t i r e l y i n t e r m s o f a d e c r e a s e o f p a r t i a l p r e s s u r e o f w a t e r . T h i s w o u l d i n t u r n r e l a t e t h e breakdown o f c o r d i e r i t e t o t h e commencement o f t h e g r a n u l i t e f a c i e s as d e f i n e d by T o u r e t . 83 CO cc! o rH •H -id t CD CO ra 0) 20 1 0 E N S T A T I T E S I L L I M A N I T E Q U A R T Z . ( H Y D R O U S C O R D I E R I T E ) SAPPKIRIKE ( E N S T A T I T E ) Q U A R T Z A N H Y D R O U S C O R D I E R I T E 1 1 JL 600 800 1000 1200 1400 T e m p e r a t u r e - d e g r e e s C. F i g u r e 40 Breakdown o f c o r d i e r i t e i n a n h y d r o u s c o n d i t i o n s . C u r v e s above 800°C were d e t e r m i n e d e x p e r i m e n t a l l y , and t h e l o w - t e m p e r a t u r e c u r v e c a l c u l a t e d f r o m them. A f t e r Newton, 1972. 84 Metamorphism o f . t h e M a f i c G n e i s s e s The O r t h o p y r o x e n e I s o g r a d . The a p p e a r a n c e o f h y p e r s t h e n e i n r o c k s o f m a f i c c o m p o s i t i o n i s w i d e l y a c c e p t e d as i n d i c a t i n g t h e commencement o f g r a n u l i t e f a c i e s metamorphism ( e , g . T o u r e t , 1971; B i n n s , 1964; De Waard, 1965). I n t h e C a r s w e l l a r e a , h y p e r s t h e n e -b e a r i n g r o c k s a r e r e s t r i c t e d t o t h e s o u t h e a s t e r n p a r t o f t h e C i r c u l a r S t r u c t u r e ( f i g u r e 2 ) , and I t i s c o n s i d e r e d t h a t t h e l i m i t o f d e v e l o p m e n t o f h y p e r s t h e n e shown-on t h e map may i n d i c a t e t h e a p p r o x i m a t e p o s i t i o n o f a n " o r t h o p y r o x e n e i s o g r a d " . The work o f T o u r e t (op. c i t . ) and t h e e x p e r i m e n t s o f B i n n s (1968) i n d i c a t e t h a t t h e r e a c t i o n s e p a r a t i n g t h e a m p h i b o l i t e f a c i e s f r o m t h e g r a n u l i t e f a c i e s i s one o f d e h y d r a t i o n , T o u r e t p o i n t s o u t t h a t e x p e r i m e n t a l s t u d i e s on t h e s t a b i l i t y o f o r t h o -p y r o x e n e - b e a r i n g r o c k s i n c o n d i t i o n s o f PH2O = ^ t o t a l r e q u i r e . c o n d i t i o n s o f h i g h p r e s s u r e and t e m p e r a t u r e w h i c h a r e q u i t e u n r e a l i s t i c i n t h e g e o l o g i c a l e n v i r o n m e n t s o f most g r a n u l i t e t e r r a n e s ; t h e s t a b i l i t y f i e l d o f o r t h o p y r o x e n e i n h y d r o u s c o n d i -t i o n s as p r o p o s e d by W i n k l e r (1967) i s by no means c o m p a t i b l e w i t h t h e s t a b i l i t i e s o f a s s e m b l a g e s i n p e l i t i c r o c k s o b s e r v e d i n a s s o c i a t i o n w i t h p y r o x e n e - b e a r i n g g r a n u l i t e s , I n t h e c o u r s e o f t h e p r e s e n t work, two a t t e m p t s have b e e n made t o e s t i m a t e d i r e c t l y t h e t e m p e r a t u r e s a t w h i c h t h e g r a n u l i t e s o f t h e C a r s w e l l a r e a e q u i l i b r a t e d . The f i r s t , a t r e a t m e n t o f t h e p a r t i t i o n i n g o f i r o n and t i t a n i u m between c o e x i s t i n g i l m e n i t e and m a g n e t i t e , , was u n s u c c e s s f u l , as t h e o x i d e p h a s e s have a p p a r e n t -l y r e - e q u i l i b r a t e d a t low t e m p e r a t u r e , However, a study, o f t h e p a r t i t i o n i n g o f i r o n and magnesium between c o e x i s t i n g o r t h o p y r o x - . 8 5 ene and clinopyroxene using the method described by Wood and Banno (1973) was successful, and indicates e q u i l i b r a t i o n tempera-tures of the order of 800°C, exactly i n the range postulated by Touret and others for.the granulite f a c i e s . The d e t a i l s of the two geothermometric methods are set out below. The Iron-Titanium Oxides. The work of Buddington and Lindsley (1964) was an experimental i n v e s t i g a t i o n of the compos-i t i o n s of ilmenite-hematite and magnetite-ulvospinel s o l i d - s o l u t i o n s coexisting at various temperatures and oxygen f u g a c i t i e s . The primary r e s u l t of t h e i r work was the c a l c u l a t i o n of two surfaces i n T-X-frj2 space, one for each of the s o l i d - s o l u t i o n s considered (see figure 41). In the diagram, each composition i s projected as a curve on the T-frj2 surface and. the i n t e r s e c t i o n of these curves for two coexisting s o l i d solutions should indicate the unique temperature and oxygen fugacity of e q u i l i b r a t i o n , The hypersthene-bearing granulites of the Carswell area are characterised by large grains of ilmenite associated with sma.ll patches of magnetite (figure 4 2 ) . The texture i s suggestive of exsolution from a previous homogeneous phase, possibly a high-temperature sp i n e l of the o r i g i n a l mafic igneous rock,. It was o r i g i n a l l y expected that the two phases would have equilibrated at. the temperature of granulite f a c i e s metamorphism. To test t h i s hypothesis, microprobe analyses of the phases were made. The accuracy of probe analyses of oxide minerals ( p a r t i c u l a r -l y those of high Pe3 + content) i s d i f f i c u l t to estimate, as the amount of excess oxygen i n the mineral structure cannot be 86 - Percent ulvospinot in magnetite - Percent hematite in i lmenite 600 700 800 900 1000 Temperature - degrees C. Figure 1+1 Compositions of coexisting Magnetite - ulvospinel and ilmenite - hematite s o l i d solutions for various temperatures and oxygen f u g a c i t i e s . After Buddington and Lindsley, 1964. 87 F i g u r e 4 2 R e f l e c t e d l i g h t , p a r t i a l l y c r o s s e d n i c o l s . S p e c i m e n C a r - 2 3 - 8 , e a s t s i d e o f s t r u c t u r e , M a g n i f i c a t i o n 90X. I l m e n i t e g r a i n i n h y p e r s t h e n e - b e a r i n g g r a n u l i t e . D a r k g r e y w i t h l i g h t l a m e l l a e and l i g h t g r e y a t l o w e r r i g h t a r e i l m e n i t e . Medium g r e y s m a l l i n c l u s i o n s i n i l m e n i t e a r e m a g n e t i t e . A l l o t h e r m a t e r i a l i s s i l i c a t e . 88 3 + d e t e r m i n e d d i r e c t l y . E s t i m a t e s o f t h e p e r c e n t a g e o f Fe may be made by t h e t e c h n i q u e o u t l i n e d i n the a p p e n d i x , o r by c o n s i d e r i n g s t o i c h i o m e t r y . The f o r m e r method i s u s e d h e r e . The m o l e c u l a r p e r c e n t a g e s o f h e m a t i t e i n i l m e n i t e and u l v o s p i n e l i n m a g n e t i t e , measured i n s p e c i m e n Car-2 3-8 a r e shown i n t a b l e 5, and t h e c o r r e s p o n d i n g c u r v e s a r e h i g h - l i g h t e d on t h e B u d d i n g t o n - L i n d s l e y p l o t ( f i g u r e i+i). The c u r v e s f o r t h e s e c o m p o s i t i o n s do n o t i n t e r s e c t i n a u n i q u e T - f g p o i n t , and c o n -s e q u e n t l y an e q u i l i b r i u m , temperature, cannot be d e t e r m i n e d by t h i s method. E x t r a p o l a t i o n o f t h e c u r v e s f o r o b s e r v e d phase c o m p o s i t i o n s i n d i c a t e s t h a t i f t h e phases e q u i l i b r a t e d a t a l l , i t must have been under low-T, low f g c o n d i t i o n s . Under t h e s e c o n d i t i o n s , p h a s e - r e l a t i o n s h i p s become more complex; f o r i n s t a n c e a s o l v u s e x i s t s i n t h e m a g n e t i t e - u l v o s p i n e l s o l i d - s o l u t i o n ( V i n c e n t , 1957). I t must be c o n c l u d e d t h a t t h e i r o n t i t a n i u m o x i d e s have r e - e q u i l -i b r a t e d a t low t e m p e r a t u r e s subsequent t o t h e p e r i o d o f p r o g r a d e metamorphism. F i g u r e 42 shows t h a t s e v e r a l s e t s o f l a m e l l a e o f h i g h - b i r e -f l e c t a n c e m a t e r i a l a r e d e v e l o p e d i n t h e i l m e n i t e . These may r e p r e s e n l a l o w - t e m p e r a t u r e e x s o l u t i o n f e a t u r e , but m i c r o p r o b e t r a v e r s e s were u n a b l e t o d e t e c t any c o m p o s i t i o n a l d i f f e r e n c e between t h e l a m e l l a e and t h e h o s t g r a i n . The Pyroxene Geothermometer. The p a r t i t i o n i n g o f i r o n and magnesium between c o e x i s t i n g o r t h o p y r o x e n e and c l i n o p y r o x e n e i s s t r o n g l y t e m p e r a t u r e - d e p e n d e n t . D a v i s and Boyd (1966) e s t a b l i s h e d t h a t e q u i l i b r a t i o n t e m p e r a t u r e s on t h e e n s t a t i t e - d i o p s i d e s o l v u s ILMENITE  Wt% F o r m u l a ( 3 0 2 " ) S I 0 2 0,09 .0022 A 1 2 0 3 ^ . FeO 47,72 1.0167 F e 2 0 3 nd nd MgO 0. 04 . 0015 CaO _ T I 0 2 50 . 72 • 9715 Mn0 2 1,52 .0328 C r 2 0 3 0.05 . 0011 100.14 Mole % h e m a t i t e = 1% ( c a l c u l a t e d from e x c e s s Fe i n f o r m u l a ) MAGNETITE  Wtg F o r m u l a ( 4 0 2 " ) 0,3 .0113 26.89 .8469 68,07 1 ,929 0.09 ,0048 0.09 .0035 2.1 .0595 0 .02 .0007 2.44 .0726 (100) Mole % u l v o s p i n e l = 9% ( c o n s i d e r i n g a l l T i as Fe 2 T i O i | ) T a b l e 5 do not vary s i g n i f i c a n t l y with pressure, even up to 30 ki l o b a r s . Boyd (1969) used p a r t i t i o n i n g in low-FeO, low-Al 20 3 pyroxenes as a geothermometer u t i l i s i n g the approximate l i n e a r r e l a t i o n s h i p a c p x 1 Mg Si 0g , , - = constant X In — — • m „OpX Mg 2Si 20 6 where a^ i s the a c t i v i t y of component p i n phase q. Experimental work on i r o n - r i c h two-pyroxene systems (e.g. Green, 1973) shows that the above r e l a t i o n s h i p becomes non-ideal when s i g n i f i c a n t amounts of iron are present, but according to Boyd (1970) the presence of AI2O3 has no s i g n i f i c a n t e f f e c t . Wood and Banno (1973), devised a simple two-site s o l u t i o n model to allow estimation of Mg 2Si 20g a c t i v i t i e s i n i r o n - r i c h pyroxenes, 2+ 2 + making the assumption that Fe and Mg are d i s t r i b u t e d randomly between Ml and M2 l a t t i c e s i t e s , except that the large cations 2+ H" 21 (Ca , Na , Mn ) occupy M2 s i t e s , and the smaller cations ( A l 3 + , C r 3 + , Ti^" 1", F e 3 + ) occupy Ml s i t e s . Making the further assumption that both pyroxene phases behave as i d e a l solutions, the a c t i v i t y of Mg 2Si 20g i n either pyroxene i s given by a M g o s i o 0 , = — ^ „ m.2+ 2" X2 U6 " ~2+ 2+ 2+ 5+ 5 1 Ca" + Mg^+ + F e 2 + + Mn 2 + + N a 2 + m Mg 2 +  F e 3 + + F e 3 + + A l 3 + + T i 3 + + C r 3 + + Mg 3 + 2+ By adding terms i n the form of A F e 2+ 2+ Fe* +Mg* Ml n to equation (1) and f i t t i n g the data, Wood and Banno derived expression: r e s u l t i n g e q u a t i o n t h e f o l l o w i n g s e m i -91 t o e x p e r i m e n t a l e m p i r i c a l rp = - 1 0 2 0 2 c p x aMg , S l p O f i _ o -Spi - 7-65 X J P X + 3.88 ( X ° f ) 2 - 4.6 a M g 2 S i 2 0 6 where T i s the e q u i l i b r a t i o n temperature of the pyroxene p a i r i n degrees K. Wood and Banno tested t h i s model with a l l a v a i l a b l e experi-mental data, and found that calculated temperatures consistently f e l l within 50°C of observed temperatures. As the range of temperatures tested i s from 780°C to 1500°C, t h i s geothermometer can be applied with confidence to pyroxene p a i r s i n rocks of the granulite f a c i e s . Analyses of coexisting orthopyroxenes and clinopyroxenes from the Carswell Structure are shown i n table 6 , and on the tri a n g u l a r diagram figure 43 . No s i g n i f i c a n t compositional zoning was detected i n the pyroxenes and as hypersthene and s a l i t e are commonly observed i n contact, i t i s considered that they represent an equilibrium assemblage. The e q u i l i b r a t i o n temperatures calculated for the three specimens considered are 838°C, 821°C and 737°C. These temper-atures are based on median values of microprobe analyses which vary over a range of about + 1% of the amount present for major oxides. As a r e s u l t of the + 50°C correspondence of calculated and observed values noted by Wood and Banno, and the high sensi-92 Car 23-10 Car 52-4 Clu 425 OPX CPX OPX CPX OPX CPX S i 0 2 5 0 . 9 51.6 48,9 48.69 5 0 , 7 5 0 . 1 6 A1 20 3 0.94 1 .81 1.2 3 . 0 5 0 ,57 1.62 PeO 32. 32 14.64 36.43 1 8 . 1 8 22.06 8 . 9 6 MgO 16.21 11.47 12 .38 9.11 24.86 14.46 CaO 0 . 8 7 21.04 1.4 19 .97 0 ,55 23.03 Na 20 _ 0.26" 0. 02 0.27 0.4 K 20 „ „ - 0.02 T i 0 2 0 . 1 6 0.26 0 . 4 5 0 .51 0,04 0.26 Mn02 0.5 0.27 _ 0,68 0.2 Total 101 .9 101.35 100.78 9 9 , 7 8 99.46 99.11 Calculated equilibrium temperature 838°C 821°C 737°C Table 6 Microprobe analyses of coexisting pyroxenes from three gr a n u l i t e s . E q u i l i b r a t i o n temperatures calculated by the method of Wood and Banno, 1973 (see t e x t ) . MgSi0 3 50 (Mn.Fe)SlO Figure 43 Compositions of pyroxene-pairs from mafic gneisses. t i v i t y o f t h e method t o a n a l y t i c a l e r r o r , t h e c a l c u l a t e d t e m p e r -a t u r e s c a n a t b e s t be r e g a r d e d a s ' a g e n e r a l i n d i c a t i o n o f e q u i l i b r a t i o n t e m p e r a t u r e s . However, t h e t h r e e p y r o x e n e - p a i r t e m p e r a t u r e s c a l c u l a t e d h e r e , a l t h o u g h e a c h p a i r i s o f q u i t e d i f f e r e n t c o m p o s i t i o n ( f i g u r e 4 3 ) , a l l f a l l w i t h i n a o n e - h u n d r e d d e g r e e r a n g e , and i t i s t h e r e f o r e r e a s o n a b l e t o p o s t u l a t e an e q u i l i b r a t i o n t e m p e r a t u r e f o r t h e g r a n u l i t e s o f a b o u t 800°C. i S i g n i f i c a n c e o f Pegmatoids 95 The w i d e s p r e a d o c c u r r e n c e o f pegmatoid r o c k s i n t i m a t e l y a s s o c i a t e d w i t h t h e basement g n e i s s e s , and t h e r e l a t e d growth of K - f e l d s p a r p o r p h y r o b l a s t s w i t h i n the g n e i s s e s , i n d i c a t e t h a t , a t some s t a g e i n t h e i r development, t h e g n e i s s e s o f t h e C a r s w e l l A r e a were at or near e q u i l i b r i u m w i t h a m e l t o r o t h e r f l u i d phase c a p a b l e o f p r e c i p i t a t i n g g r a n i t i c m a t e r i a l . S e v e r a l f e a t u r e s o f t h e pegmatoids and t h e i r h o s t - r o c k s i n d i c a t e t h a t t h i s m o b i l e phase was a t l e a s t i n p a r t an aqueous s o l u t i o n ( i . e . a p e g m a t i t i c phase) r a t h e r t h a n a magma: 1. The s u b - g r a p h i c t e x t u r e s a re more c h a r a c t e r i s t i c o f pegma-t i t e s t h a n o f i g n e o u s r o c k s . 2, The t y p i c a l c o a r s e g r a i n s i z e , a c c o r d i n g t o Jahns and B u r n -ham (1969)j i s more c h a r a c t e r i s t i c o f c r y s t a l l i s a t i o n from a h i g h l y m o b i l e aqueous phase t h a n from a v i s c o u s g r a n i t i c magma, 3- The embayed,, c o r r o d e d n a t u r e o f many c r y s t a l s and t h e "observed complex p a r a g e n e s i s , suggest t h a t f o r m a t i o n and d e s t r u c t i o n o f c r y s t a l phases o c c u r r e d s i m u l t a n e o u s l y w i t h i n a s i n g l e body. Such r e l a t i o n s h i p s a r e c i t e d by Jahns and Burnham ( o p . c i t . ) as e v i d e n c e f o r d e p o s i t i o n f rom an aqueous phase. 4 . The e x t e n s i v e , a p p a r e n t l y metasomatic g r o w t h o f K - f e l d s p a r p o r p h y r o b l a s t s w i t h i n t h e host g n e i s s e s i m p l i e s m i g r a t i o n t h r o u g h s o l i d r o c k o f a phase a b l e t o d e p o s i t K - f e l d s p a r . The p r e s e n c e o f a l b i t e i n host r o c k s a s s o c i a t e d w i t h pegma-t o i d s may s i m i l a r l y i m p l y metasomatic a l b i t i s a t i o n o f p l a g i o -c l a s e by a p e g m a t i t e - r e l a t e d f l u i d . 5.. The p e g m a t o i d b o d i e s a r e i n v a r i a b l y c o n c o r d a n t w i t h t h e g n e i s s o s i t y o f t h e h o s t r o c k s . I n many c a s e s , m a r g i n a l r e l a t i o n s h i p s show a t r a n s i t i o n f r o m p e g m a t o i d i n t o g r a n i t e -g n e i s s , w i t h f o l i a t i o n p r e s e r v e d i n t h e t r a n s i t i o n a l z o n e . T h e s e r e l a t i o n s h i p s a r e not', s u g g e s t i v e o f emplacement by magmatic i n j e c t i o n b u t o f m i g r a t i o n o f a h i g h l y m o b i l e p h a s e , s u c h m i g r a t i o n b e i n g c o n t r o l l e d by a n i s o t r o p i c p e r m e a b i l i t y r e s u l t i n g f r o m t h e f o l i a t i o n o f t h e g n e i s s e s . Most i n t e r p r e t a t i o n s o f p e g m a t i t e g e n e s i s ( e . g . J a h n s a n d Burnham op. c i t . ) c o n s i d e r t h a t a p e g m a t i t e p h a s e i s a l a t e s t a g e o f t h e e v o l u t i o n o f a magma i n w h i c h an i n i t i a l l y s m a l l q u a n t i t y o f d i s s o l v e d w a t e r i s i n c r e a s i n g l y c o n c e n t r a t e d by t h e c r y s t a l -l i s a t i o n o f s o l i d p h a s e s u n t i l a n aqueous f l u i d a p p e a r s as a s e p a r a t e p h a s e . I n t h e C a r s w e l l a r e a t h e r e i s no d i r e c t e v i d e n c e o f t h e p r e s e n c e o f a p l u t o n i c a s s o c i a t i o n w h i c h c o u l d e x p l a i n t h e g e n e r a t i o n o f a p e g m a t i t e p h a s e by t h i s m o d e l . The c o m p o s i t i o n s o f t h e m a j o r i t y o f p e g m a t o i d s i n t h e C a r s w e l l A r e a l i e n e a r t h e K - f e l d s p a r - Q u a r t z j o i n o f t h e t e r n a r y P l a g i o c l a s e - K - f e l d s p a r - Q u a r t z s y s t e m . The e x p e r i m e n t a l l y d e t e r m i n e d "minimum m e l t " c o m p o s i t i o n o f t h i s s y s t e m ( T u t t l e & Bowen, 1958 and l a t e r w o r k e r s ) f a l l s n e a r t h e c e n t r e o f t h e t e r n a r y d i a g r a m . The g r a n i t o i d r o c k s o f t h e C a r s w e l l a r e a a r e t h e r e f o r e u n l i k e l y t o have o r i g i n a t e d a s a p a r t i a l m e l t o f a p r e - e x i s t i n g r o c k . An e n r i c h m e n t i n p o t a s s i u m i n t h e basement r o c k s , i s s u g g e s t e d by t h e a b u n d a n t p e g m a t o i d s and t h e m e t a s o m a t i c g r o w t h o f K - f e l d s p a r p o r p h y r o b l a s t s . T h i s i m p l i e s t h e m i g r a t i o n o f a potassium-bearing phase, which from the above d i s c u s s i o n must have been a p e g m a t i t i c phase r a t h e r than a magma. The most obvious source f o r such a potassium-rich phase would be a r e m o b i l i s a t i o n of deep-seated g r a n i t i c basement rocks during the orogenic episode which deformed and metamorphosed the Carswell r o c k s . Such a r e m o b i l i s a t i o n . o f early g r a n i t e s i s proposed by Beck (1970), In the Carswell Area pegmatiteSj s i l l i m a n i t e i s commonly present as small needles which, although they are now rimmed by t h i n sheaths of muscovite, were almost c e r t a i n l y o r i g i n a l l y i n e q u i l i b r i u m w i t h K - f e l d s p a r , The r e a c t i o n muscovite + quartz = K-feldspar + s i l l i m a n i t e + H^ O places d e f i n i t e c o n s t r a i n t s on the c o m p a t i b i l i t y of K-feldspar and s i l l i m a n i t e (Winkler, 1967) and f o r these minerals to be i n e q u i l i b r i u m i n the presence of water and at moderate pressure r e q u i r e s temperatures i n excess of 700°C, This i n d i c a t e s a high temperature of formation f o r the pegmatoids. However, the muscovite sheaths, and the c h l o r i t i c a l t e r a t i o n of a s s i m i l a t e d garnets show that p a r t i a l low-temperature r e - e q u i l i b r a t i o n has occurred. 98 Metamorphic E v o l u t i o n ; Summary o f Metamorphic C o n d i t i o n s . The p r e c e d i n g s e c t i o n s a l l o w t h e f o l l o w i n g g e n e r a l i s a t i o n s t o be made, 1. A p o s s i b l e " i s o g r a d " c u t s t h e basement r o c k s o f t h e C a r s w e l l A r e a . T h i s i s c h a r a c t e r i s e d by t h e appearance o f o r t h o p y -roxene i n r o c k s o f m a f i c c o m p o s i t i o n , and t h e d i s a p p e a r a n c e o f c o r d i e r i t e i n t h e p e l i t i c r o c k s . 2. A v a i l a b l e e x p e r i m e n t a l e v i d e n c e and f i e l d e v i d e n c e f rom o t h e r g r a n u l i t e t e r r a n e s I d e n t i f y t h i s " I s o g r a d " as t h e boundary between t h e upper a m p h i b o l i t e and g r a n u l i t e f a c i e s . 3- B o t h t h e " c o r d i e r i t e - o u t " and " h y p e r s t h e n e - i n " r e a c t i o n s may be e x p l a i n e d i n terms o f a r e d u c t i o n o f p a r t i a l p r e s s u r e o f w a t e r , and t h e two f a c i e s c o u l d be r e p r e s e n t a t i v e o f t h e same P t o t a l " " ^ c o n d i t i o n s . 4. The h i g h - p r e s s u r e breakdown o f c o r d i e r i t e i s a d i v a r i a n t r e a c t i o n . The f a c t t h a t no d i v a r i a n t zone ( i , e . a zone where b o t h r e a c t a n t s and p r o d u c t s c o e x i s t s t a b l y ) i s e v i d e n t i n t h e f i e l d c o u l d be e x p l a i n e d by an a b r u p t change i n Pj^O' On t h e o t h e r hand, t h e f a u l t i n g - o u t o f such a zone would appear r a t h e r u n l i k e l y i n v i e w o f t h e b r o a d a r e a i n wh i c h b o t h c o r d i e r i t e - b e a r i n g and c o r d i e r i t e - f r e e r o c k s a r e foun d . 5. A major phase o f m o b i l i s a t i o n has o c c u r r e d , t h e t r a n s p o r t a -t i o n medium p r o b a b l y b e i n g a p e g m a t i t i c f l u i d phase. T h i s r e q u i r e s an abundant a v a i l a b i l i t y o f w a t e r . The l a t e p a r a -g e n e t i c p o s i t i o n o f K - f e l d s p a r p o r p h y r o b l a s t s and the l i t -p a r - l i t emplacement o f t h e p e g m a t i t e s w i t h a s s i m i l a t i o n o f phases from t h e h o s t g n e i s s e s , i n d i c a t e t h a t t h e pegmatoids formed at a t i m e subsequent t o t h e h i g h e s t grade of metamor-phism. However, t h e r e i s . no e v i d e n c e o f polymetamorphism, and so i t i s - l i k e l y t h a t t h e g r a n u l i t e / a m p h i b ' o l i t e •metamorphism and t h e p e g m a t i t e emplacement be l o n g e d t o t h e same metamorphic e p i s o d e . R e t r o g r a d e Metamorphism. I f t h e i n t e r p r e t a t i o n i s a c c e p t e d o f a f a c i e s boundary s e p a r a t i n g r o c k s formed under hydrous and anhydrous c o n d i t i o n s , t h e r e a r e s e v e r a l l i n e s o f e v i d e n c e w h i c h s u g g e s t t h a t t h e e n t i r e a r e a was i n i t i a l l y s u b j e c t e d t o g r a n u l i t e f a c i e s c o n d i t i o n s , and t h a t t h e a m p h i b o l i t e s and c o r d i e r i t e -b e a r i n g r o c k s a r e a r e s u l t o f r e t r o g r e s s i v e h y d r a t i o n . The arguments i n s u p p o r t o f t h i s h y p o t h e s i s a r e as f o l l o w s ? 1, P a r a g e n e t i c sequences. I n t h e p e l i t i c g n e i s s e s a p p r o x i m a t e p a r a g e n e t i c sequences can be e s t a b l i s h e d f o r b o t h t h e c o r d i e r i t e - b e a r i n g and c o r d i e r i t e r - f r e e r o c k s . I n b o t h t y p e s , b i o t i t e , by v i r t u e o f i t s i n c l u s i o n i n a l l o t h e r m i n e r a l s was a p p a r e n t l y t h e e a r l i e s t m i n e r a l t o c r y s t a l l i s e , Q u a r t z and p l a g i o c l a s e occupy an i n t e r m e d i a t e p o s i t i o n , and g a r n e t c h a r a c t e r i s t i c -a l l y forms l a t e p o r p h y r o b l a s t s . The i m p o r t a n t d i f f e r e n c e between t h e two t y p e s o f p e l i t i c g n e i s s i s t h e r e l a t i v e l y e a r l y f o r m a t i o n o f s i l l i m a n i t e ( i t i s commonly p a r t i a l l y o r c o m p l e t e l y e n c l o s e d by g a r n e t c r y s t a l s ) , and the l a t e , p o r p h y r o b l a s t i c p o s i t i o n o f c o r d i e r i t e . I f t h e s e r o c k s a r e i n d e e d r e l a t e d by t h e r e a c t i o n c o r d i e r i t e = g a r n e t + s i l l i m a n i t e + q u a r t z , t h e n c o r d i e r i t e i s t h e p r o d u c t , and s i l l i m a n i t e t h e r e a c t a n t . 100 I n t h e m a f i c g n e i s s e s , p a r a g e n e t i c sequences are l e s s c l e a r . S m a l l amounts o f e a r l y b i o t i t e , w h i c h g i v e the r o c k i t s f o l i a t i o n , a r e t y p i c a l l y overgrown by orthopyroxene.. A l l o t h e r hydrous m i n e r a l s ( i . e . h o r n b l e n d e ) a r e a p p a r e n t l y l a t e r t h a n t h e p yroxenes of t h e g r a n u l i t e s , a l t h o u g h i n d i s -p u t a b l e o v e r g r o w t h s or r e p l a c e m e n t t e x t u r e s a r e r a r e . I n t h e a m p h i b o l i t e s n o r t h w e s t o f t h e "pyroxene i s o g r a d " , h o r n b l e n d e forms l a r g e f r e s h p o i k i l o b l a s t i c g r a i n s , and must c e r t a i n l y be c o n s i d e r e d as l a t e i n t h e p a r a g e n e s i s . 2. Hornblende c o l o u r . J a k e s ( 1968) c i t e s h o r n b l e n d e c o l o u r and c o m p o s i t i o n as c r i t e r i a f o r degree o f metamorphic r e t r o g r e s s i o n . I n p a r t i c -u l a r , h o r n b l e n d e s p r o d u c e d by h y d r a t i o n o f pyroxenes i n g r a n u l i t e f a c i e s r o c k s i n t h e Bohemian M a s s i f grade from brown t o g r e e n w i t h i n c r e a s i n g " r e t r o g r e s s i o n " . P r e c i s e l y t h i s p a t t e r n o c c u r s i n t h e C a r s w e l l r o c k s , w i t h brown h o r n -b l e n d e d e v e l o p e d i n t h e o r t h o p y r o x e n e - b e a r i n g r o c k s , and b l u e - g r e e n h o r n b l e n d e i n t h e a m p h i b o l i t e s n o r t h w e s t o f the " o r t h o p y r o x e n e i s o g r a d " , 3. Garnet z o n i n g . I f c o r d i e r i t e forms by t h e p a r t i a l breakdown of g a r n e t ( w i t h a d d i t i o n o f s i l i c a and a l u m i n a ) t h e newly formed g a r n e t would be d e p l e t e d i n pyrope c o n t e n t , as magnesium i s p a r t i t i o n e d p r e f e r e n t i a l l y i n t o c o r d i e r i t e (Hensen, 1 9 7 1 ) . T h i s c o u l d e x p l a i n t h e o b s e r v e d p a t t e r n o f z o n i n g i n t h e g a r n e t s o f t h e c o r d i e r i t e - b e a r i n g p e l i t i c g n e i s s e s ( t a b l e 1 ) i n w h i c h magnesium i s more c o n c e n t r a t e d i n t h e c o r e s t h a n i n t h e r i m s . The z o n i n g , t h e n , i s c o n s i s t e n t w i t h a model 101 I n w h i c h t h e g a r n e t - c o r d l e r i t e g n e i s s e s f o r m by r e t r o g r a d e m etamorphism o f t h e g a r n e t - s i l l i m a n i t e g n e i s s e s . The o b s e r v e d e n r i c h m e n t o f manganese a t t h e r i m s o f g a r n e t s i s a l s o s u g g e s t e d by Hensen and G r e e n (1973) t o be i n d i c a t i v e o f r e t r o g r a d e metamorphism.. 4 . F l u i d i n c l u s i o n s . The f l u i d i n c l u s i o n s i n t h e r o c k s o f t h e C a r s w e l l S t r u c t u r e a r e t h e s u b j e c t o f a s t u d y by P a g e l (1974 and i n p r e p a r a t i o n ) . T o u r e t (1971b) d e m o n s t r a t e d t h a t i n t h e t w o -p y r o x e n e g r a n u l i t e s o f s o u t h e r n Norway, t h e f l u i d i n c l u s i o n s a r e f i l l e d e x c l u s i v e l y w i t h C 0 2 3 and T o u r e t p r o p o s e s t h a t t h e g r a n u l i t e s a r e f o r m e d i n c o n d i t i o n s o f low PH2O A S A r e s u l t o f a C 0 2 - r i c h f l u i d p h a s e . P a g e l has f o u n d t h a t t h e h y p e r s t h e n e - b e a r i n g r o c k s o f t h e C a r s w e l l A r e a c o n t a i n o n l y C 0 2 - f i l l e d f l u i d i n c l u s i o n s , s h o w i n g an e x a c t a n a l o g y w i t h T o u r e t ' s g r a n u l i t e s . P a g e l a l s o d e m o n s t r a t e s t h a t a n e v o l u t i o n a r y s e q u e n c e c a n be r e c o g n i s e d i n t h e C a r s w e l l f l u i d i n c l u s i o n s . The i n c l u s i o n s a r e t y p i c a l l y s e e n i n p l a n a r c o n c e n t r a t i o n s , and where t h e s e p l a n e s i n t e r s e c t , t h e r e l a t i v e a g e s o f i n c l u s i o n s i n e a c h p l a n e c a n be d e t e r m i n e d . V/here b o t h C 0 2 - f i l l e d and H 2 0 -f i l l e d i n c l u s i o n s e x i s t i n one r o c k , t h e C 0 2 ~ f i l l e d i n c l u -s i o n s a r e o l d e s t . T h i s i s i n a c c o r d w i t h t h e m o d e l o f g r a n u l i t e - f a c i e s m etamorphism w i t h l a t e r r e t r o g r a d e h y d r a t i o n . R e t r o g r e s s i o n and M i g m a t i s a t i o n . From t h e above d i s c u s s i o n s i t f o l l o w s t h a t t h e d e v e l o p m e n t o f t h e basement r o c k s c a n be e x p l a i n e d by m e t a m o r p h i s m t o t h e g r a n u l i t e f a c i e s f o l l o w e d by 102 t h e g e n e r a t i o n o f a m o b i l e p e g m a t i t e p h a s e and s t r o n g r e t r o g r a d e m e tamorphism. B o t h t h e m o b i l i s a t i o n and r e t r o g r e s s i o n c a n be e x p l a i n e d by t h e i n t r o d u c t i o n o f w a t e r i n t o r o c k s u n d e r h i g h -p r e s s u r e , h i g h - t e m p e r a t u r e c o n d i t i o n s . As t h e r e I s no t e x t u r a l e v i d e n c e f o r more t h a n one m a j o r p h a s e o f metamorphism, i t i s p r o p o s e d t h a t t h e m i g m a t i s a t i o n and t h e r e t r o g r e s s i o n r e s u l t e d f r o m t h e same h y d r a t i o n e p i s o d e . 103 IV. POST-METAMORPHIC ALTERATION G e n e r a l The basement r o c k s o f t h e C a r s w e l l a r e a have been s u b j e c t e d t o r e g i o n a l metamorphism and d e f o r m a t i o n w h i c h , as w i l l be shown i n a l a t e r s e c t i o n , o c c u r r e d d u r i n g t h e H u d s o n i a n O r o g e n y . The m e t a m o r p h i c h i s t o r y o f t h e C a r s w e l l basement r o c k s i s s u m m a r i s e d as f o l l o w s : 1) M a i n d e f o r m a t i o n p h a s e and g r a n u l i t e f a c i e s metamorphism. 2) R e t r o g r e s s i o n t o t h e a m p h i b o l i t e f a c i e s , a c c o m p a n i e d by emplacement o f p e g m a t i t i c m a t e r i a l . 3) L a t e f o l d i n g and m y l o n i t i s a t i o n . The m i n e r a l o g y p r o d u c e d by t h i s s t r o n g r e g i o n a l metamorphism has b e e n h i g h l y m o d i f i e d by p o s t - m e t a m o r p h i c a l t e r a t i o n . E v e r y r o c k s p e c i m e n shows some d e g r e e o f a l t e r a t i o n , and i n s e v e r a l c a s e s r e p l a c e m e n t i s so i n t e n s e t h a t t h e o r i g i n a l r o c k t y p e i s u n r e c o g n i s a b l e . The a l t e r a t i o n d i s c u s s e d i n t h i s c h a p t e r i s d i s t i n c t f r o m t h e r e t r o g r a d e m e tamorphism p r o p o s e d i n C h a p t e r I I I , as t h e r e t r o g r e s s i o n i s c h a r a c t e r i s e d by m i n e r a l s s t a b l e u n d e r t h e p r e s s u r e s and t e m p e r a t u r e s o f r e g i o n a l metamorphism, w h e r e a s t h e a l t e r a t i o n shows c o m p a r a t i v e l y l o w - t e m p e r a t u r e m i n e r a l o g y . I n a d d i t i o n , i t w i l l be shown i n a l a t e r c h a p t e r t h a t t h e r e t r o g r e s s i o n and t h e a l t e r a t i o n a r e s e p a r a t e d i n t i m e by as much as one b i l l i o n y e a r s . P o s t - m e t a m o r p h i c a l t e r a t i o n may be a t t r i b u t e d t o one o r more o f t h e f o l l o w i n g a g e n c i e s : 1) W e a t h e r i n g o f t h e p r e - A t h a b a s c a s a n d s t o n e e r o s i o n s u r f a c e , 104 2) "Hydrothermal" a c t i v i t y 3) A l t e r a t i o n associated with the formation of the C i r c u l a r structure. This chapter w i l l be concerned with the nature of the a l t e r a t i o n and the physical conditions under which i t occurred. The most strongly altered zones i n the basement rocks occur i n two environments: 1) The zone several metres thick immediately underlying the basal contact of the Athabasca sandstone 2) Localised planar zones representing f a u l t s or mylonite zones. In these al t e r e d zones, v i r t u a l l y , every mineral i s replaced by fine-grained clay material, although the o r i g i n a l f o l i a t i o n of the gneisses i s commonly recognisable as a colour-banding. The d i s t r i b u t i o n of t h i s most intense a l t e r a t i o n suggests that i t resulted from weathering during the pre-Athabasca sandstone erosion period. The fact that some mylonite zones are completely alt e r e d and others are r e l a t i v e l y fresh (see f i g . 3 8 ) could be explained i f the former outcropped on the paleo-surface and were deeply weathered, but the l a t t e r remained unconnected with the atmosphere. The nature of these highly altered zones i s the subject of a study by D. Joseph ( i n preparation). Away from the most intensely altered zones, a l t e r a t i o n i s expressed by the se l e c t i v e replacement of various rock-forming minerals. The mineralogy of these a l t e r a t i o n products w i l l be described i n the following section. 105 S p e c i f i c A l t e r a t i o n R e a c t i o n s The p e l i t i c g n e i s s e s a r e c h a r a c t e r i s e d by t h e s e l e c t i v e a l t e r a t i o n o f c e r t a i n m i n e r a l s . I n t h e c o r d i e r i t e g n e i s s e s , a l t h o u g h o t h e r m i n e r a l s a r e n o r m a l l y f r e s h , c o r d i e r i t e i s w i t h o u t e x c e p t i o n e n t i r e l y a l t e r e d t o a f i n e - g r a i n e d a g g r e g a t e w i t h low b i r e f r i n g e n c e . At a m a g n i f i c a t i o n o f 1100X ( f i g u r e H ) , t h e m i c r o c r y s t a l -l i n e n a t u r e o f t h e c o r d i e r i t e a l t e r a t i o n i s j u s t d i s c e r n a b l e . The a l t e r e d m a t e r i a l was s e p a r a t e d from specimen B H - 7 4 - I 6 9 (a c o a r s e l y p o r p h y r o b l a s t i c g a r n e t - c o r d i e r i t e g n e i s s from th e C l aude Lake m i n e r a l i s e d zone, F i g u r e 7 ) by h e a v y - l i q u i d t e c h -n i q u e s . I t has a d e n s i t y o f about 2 . 5 - A n a l y s i s by X-ray d i f f r a c t i o n , u s i n g copper r a d i a t i o n , showed t h e a g g r e g a t e t o c o n s i s t l a r g e l y o f k a o l i n i t e . M i n o r c h l o r i t e peaks were a l s o o b s e r v e d . The b r e a d t h (up t o two degrees 29) o f t h e peaks o b t a i n e d a t t e s t s t o t h e e x t r e m e l y f i n e - g r a i n e d n a t u r e o f t h e a g g r e g a t e s . A s i n g l e fragment (about 40 m e s h - s i z e ) o f t h e s e p a r a t e d m a t e r i a l was mounted i n a D e b y e - S c h a i r e r X-ray camera and p r o d u c e d a r e c o g n i s a b l e k a o l i n i t e p o w d e r - p a t t e r n , a g a i n d e m o n s t r a t i n g t h a t t h e a l t e r a t i o n c o n s i s t s o f an a g g r e g a t e o f m inute c r y s t a l s w i t h o u t marked p r e f e r r e d o r i e n t a t i o n . E l e c t r o n m i c r o p r o b e a n a l y s i s o f t h e a g g r e g a t e i s c o n s i s t e n t w i t h a m i x t u r e o f k a o l i n i t e and c h l o r i t e , e x c e p t f o r t h e h i g h v a r i a b l e p e r c e n t a g e of p o t a s s i u m ( t a b l e ^ ). The p o t a s s i u m c o n t e n t i s o v e r 3% i n t h e v i c i n i t y o f p i n i t i s e d c r a c k s , and so may be a t t r i b u t e d t o t h e p r e s e n c e o f f i n e - g r a i n e d s e r i c i t e . I n samples c o l l e c t e d from th e " E l i z a b e t h " A r e a i n the 106 A B OXIDE ' WEIGHT % WEIGHT % S I 0 2 43.12 41 . 4 3 A 1 2 0 3 40 , 6 3 1 , 0 6 PeO 6 , 0 6 . 4 4 MgO v • 3 . 1 2 6 , 0 CaO 0 .17 0 , 2 Na 20 0 . 2 9 0 . 0 9 K 20 0 . 2 3 3 , 2 7 T i 0 2 0 0 , 0 2 Mn0 2 . 0 . 0 9 0 , 0 2 H 20 6 . 3 9 11.48 TOTAL ( 1 0 0 ) (100) T a b l e 7 M i c r o p r o b e a n a l y s e s o f c o r d i e r i t e . a l t e r a t i o n p r o d u c t s , specimen BH -74-169 from b o r e - h o l e Clu - - 5 0 3 , Water a n a l y s i s c a l c u l a t e d by m o d i f i e d Bence-Albee method Csee a p p e n d i x ) . A n a l y s i s A i s i n body o f g r a i n and i s c o m p a t i b l e w i t h a m i x t u r e o f k a o l i n i t e and c h l o r i t e , as i n d i c a t e d by X- r a y d i f f r a c t i o n . A n a l y s i s B i s c l o s e t o a. " p i n i t i s e d " c r a c k and shows h i g h e r p o t a s s i u m c o n t e n t due t o p r e s e n c e o f s e r i c i t e . 1 0 7 n o r t h w e s t p a r t o f t h e basement i n l i e r , , some c o r d i e r i t e g r a i n s a r e e n t i r e l y r e p l a c e d by m u s c o v i t e , a l t h o u g h o t h e r g r a i n s i n t h e same t h i n s e c t i o n have t h e more u s u a l p o l y m i n e r a l i c p a t t e r n . The g r a i n i l l u s t r a t e d i n f i g u r e I s l a r g e l y r e p l a c e d by a s i n g l e m u s c o v i t e c r y s t a l , a l t h o u g h t h e p r e s e n c e o f p i n i t i s e d c r a c k s w o u l d seem t o s u g g e s t t h a t t h e l a r g e c r y s t a l i s a r e p l a c e m e n t o f an o r i g i n a l s e r i c i t e - k a o l i n i t e - q u a r t z p a t t e r n p s e u d o m o r p h i n g c o r d i e r i t e . M i c r o p r o b e a n a l y s i s o f t h i s m u s c o v i t e ( t a b l e 8 ) shows i t t o be h i g h l y p h e n g i t l c I n c o m p o s i t i o n . The p o t a s s i u m c o n t e n t shown i n t h i s a n a l y s i s i s v e r y l o w , and. t h e i n f e r r e d w a t e r c o n t e n t i s h i g h . R e p e a t e d a n a l y s e s o n d i f f e r e n t g r a i n s gave f a i r d u p l i c a t i o n o f r e s u l t s . I t i s s u g g e s t e d t h a t t h e m i n e r a l i s i n f a c t h y d r o m u s c o v i t e , w h i c h w o u l d i n d i c a t e t h a t r e p l a c e m e n t o c c u r r e d a t a r e l a t i v e l y low t e m p e r a t u r e ( D e e r , Howie and Zussman, 1966). U n f o r t u n a t e l y , i n s u f f i c i e n t m a t e r i a l c o u l d be e x t r a c t e d t o v e r i f y t h i s h y p o t h e s i s by X - r a y t e c h n i q u e s . I n t h e g a r n e t - s i l l i m a n i t e g n e i s s e s , a r e a c t i v e r e l a t i o n s h i p i s c o n s i s t e n t l y a p p a r e n t between s i l l i m a n i t e a n d q u a r t z ( s e e C h a p t e r I I ) . W h e r e v e r t h e s e m i n e r a l s o c c u r i n j u x t a p o s i t i o n , a " r e a c t i o n r i m " o f f i n e - g r a i n e d b l u e - g r e e n m a t e r i a l i s f o r m e d p s e u d o m o r p h o u s l y r e p l a c i n g s i l l i m a n i t e . T h i s m a t e r i a l i s i n d i s t i n g u i s h a b l e i n c h e m i s t r y o r by X - r a y d i f f r a c t i o n f r o m t h e k a o l i n i t e - r i c h m a t e r i a l r e p l a c i n g c o r d i e r i t e a s d e s c r i b e d a b o v e . I n some r o c k t y p e s ( e . g . t h e p e g m a t o i d s ) s i l l i m a n i t e - K - f e l d s p a r c o n t a c t s have a t h i n r e a c t i o n - r i m o f m u s c o v i t e . I n t h e m a f i c g n e i s s e s , some r e p l a c e m e n t o f p y r o x e n e s a n d h o r n b l e n d e i s common. C h l o r i t e i s t h e most common a l t e r a t i o n p r o d u c t , b u t a g r e e n m i c a c e o u s m i n e r a l i s a l s o w i d e l y d e v e l o p e d , 108 OXIDE •' WEIGHT PERCENT FORMULA S I 0 2 47-54 5.963 A 1 2 0 3 32.53 4.811 MgO 4.02 0,751 FeO 5.38 0.564 CaO 0.10 0,013 K 20 2.39 0.382 Na 20 0.05 0.011 MnO .0.04 0,004 T i 0 2 0.02 0.002 H 20 7-95 6.649 TOTAL (100) T a b l e 8 M i c r o p r o b e a n a l y s i s of m u s c o v i t e r e p l a c i n g c o r d i e r i t e i n specimen J P B - 7 3 - 1 8 . Water c o n t e n t c a l c u l a t e d by m o d i f i e d Bence-Albee method, (see a p p e n d i x ) , P o t a s s i u m c o n t e n t i s low and i n f e r r e d water c o n t e n t h i g h , s u g g e s t i n g s u b s t i t u t i o n . 1 0 9 p a r t i c u l a r l y associated with the pyroxenes. This mineral occurs 0 i n grains about 0.1 mm i n s i z e , and has the o p t i c a l properties of a mica, being b i a x i a l negative with a 2V angle of about 15°. This material i s probably a clay mineral. Feldspars, p a r t i c u l a r l y microcline, are commonly replaced by fine-grained s e r i c i t e or k a o l i n i t e . In some pegmatoids, t h i s replacement i s complete. Replacement of minerals by hematite occurs l o c a l l y . The i r o n s t a i n i n g of K-feldspar i s ubiquitous, but probably does not represent l a t e a l t e r a t i o n . 110 I n t e r p r e t a t i o n The basement r o c k s have o b v i o u s l y s u f f e r e d a t l e a s t one e p i s o d e o f i n t e n s e a l t e r a t i o n , t h e p r o d u c t s o f w h i c h a r e " c l a y " m i n e r a l s ( k a o l i n i t e , l o w - t e m p e r a t u r e m i c a s , a n d c h l o r i t e ) s u g g e s t i n g f o r m a t i o n a t a r e l a t i v e l y low t e m p e r a t u r e . P a g e l , 1974, has m e a s u r e d h o m o g e n i s a t i o n t e m p e r a t u r e s o f f l u i d i n c l u s i o n s i n t h e a l t e r e d r o c k s , and s u g g e s t s t e m p e r a t u r e s o f f o r m a t i o n o f 150°C t o 200°C. The d e v e l o p m e n t o f r e a c t i o n r i m s o f k a o l i n i t e b etween s i l l i m a n i t e and q u a r t z i s o f i n t e r e s t , a s i t s u g g e s t s t h e r e a c t i o n : s i l l i m a n i t e + q u a r t z + 2H2O = k a o l i n i t e (2) K a o l i n i t e + q u a r t z h as b e e n shown e x p e r i m e n t a l l y t o be u n s t a b l e above 350°C (Thomson, 1 9 7 0 ) , as t h e s e m i n e r a l s r e a c t t o f o r m p y r o p h y l l i t e + H2O. The s t a b i l i t y f i e l d s o f k a o l i n i t e and s i l l i m a n i t e do n o t i n t e r s e c t f o r any P^o-^ c o n d i t i o n s ( s e e f i g u r e 4 4 , - a f t e r Zen and Thomson, 1 9 7 4 ) , so t h a t e q u a t i o n (2) must r e p r e s e n t a m e t a s t a b l e e q u i l i b r i u m r e a c t i o n . No i n t e r m e d -i a t e s o l i d p h a s e s u c h as p y r o p h y l l i t e i s p r e s e r v e d , e v e n t h o u g h t h e r e a c t i o n i s shown by t h e p r e s e n c e o f b o t h r e a c t a n t s and p r o d u c t t o be i n c o m p l e t e . The f o r e g o i n g d i s c u s s i o n s u g g e s t s t h a t t h e a l t e r a t i o n r e p r e s e n t s l o w - t e m p e r a t u r e metamorphism i n t h e p r e s e n c e o f w a t e r . A t e m p e r a t u r e o f f o r m a t i o n o f l e s s t h a n 350°C i s r e q u i r e d by t h e s t a b i l i t y o f k a o l i n i t e + q u a r t z , and P a g e l ' s work on f l u i d i n c l u s i o n s s u g g e s t s 150°C t o 200°C. The n a t u r e o f t h e f l u i d c a u s i n g a l t e r a t i o n i s unknown, b u t f l u i d i n c l u s i o n s c o n t a i n I l l P hase d i a g r a m f o r h y d r o u s aluminum s i l i c a t e s . A f t e r Zen and Thompson, 1974.. 112 w a t e r o f low s a l i n i t y . The a l t e r a t i o n o f c o r d i e r i t e p r o d u c e s : k a o l i n i t e + c h l o r i t e a s s e m b l a g e s . s i m i l a r t o t h o s e o r i g i n a t i n g f r o m s i l l i m a n i t e , and-s i m i l a r c o n d i t i o n s o f a l t e r a t i o n may be p o s t u l a t e d . M u s c o v i t e i s commonly p r e s e n t i n t h e f o r m o f p i n i t e , and a n a l y s e s ( t a b l e 8) s u g g e s t t h a t t h i s m i c a f o r m e d I n a l o w - t e m p e r a t u r e e n v i r o n - , ment. The c h l o r i t e a l t e r a t i o n p r o d u c t s o f m a f i c m i n e r a l s a s s i m i l -a t e d i n t o p e g m a t o i d s and a s s o c i a t e d w i t h g r a n i t i s a t i o n a r e p r o b l e m a t i c . T h e i r a s s o c i a t i o n s s u g g e s t i n s t a b i l i t y o f t h e m a f i c m i n e r a l s i n t h e p r e s e n c e o f a p e g m a t i t i c f l u i d , p h a s e , b u t o t h e r e v i d e n c e ( c h a p t e r I I I ) s u g g e s t s t h a t t h e p e g m a t o i d s were f o r m e d a t t e m p e r a t u r e s o u t s i d e t h e s t a b i l i t y f i e l d o f c h l o r i t e . T he c h l o r i t i s a t i o n t h u s p r o b a b l y o c c u r r e d i n t h e same e p i s o d e a s t h e k a o l i n i s a t i o n . Two e x p l a n a t i o n s may be a d v a n c e d f o r t h e p e r v a s i v e a l t e r a -t i o n o b s e r v e d . The f i r s t , w e a t h e r i n g a t t h e p r e - A t h a b a s c a 1 p a l e o s u r f a c e , i s s u g g e s t e d m a i n l y by t h e i n t e n s e r e g o l i t h - l i k e a l t e r a t i o n c l o s e t o t h e u n c o n f o r m i t y , b u t t h e t e m p e r a t u r e o f f o r m a t i o n i n d i c a t e d by m i n e r a l s t a b i l i t i e s and f l u i d i n c l u s i o n s d o e s n o t s u p p o r t t h i s h y p o t h e s i s . The s e c o n d e x p l a n a t i o n , l o w - t e m p e r a t u r e h y d r o t h e r m a l a l t e r -a t i o n , i s s u g g e s t e d by t h e u b i q u i t o u s n a t u r e o f t h e a l t e r a t i o n a n d by t h e s p e c i f i c r e a c t i o n s d i s c u s s e d a b o v e . I t i s t h e o p i n i o n o f t h e a u t h o r t h a t t h e t r u e e x p l a n a t i o n o f t h e o b s e r v e d a l t e r a t i o n l i e s i n a c o m b i n a t i o n o f t h e s e two p r o c e s s e s , and t h a t t h e h i s t o r y o f a l t e r a t i o n i s a s f o l l o w s : 1) Deep w e a t h e r i n g o f t h e basement s u r f a c e c a u s i n g a l t e r -113 a t i o n t o c l a y m i n e r a l s . T h i s c o u l d l e a d , to- t h e d e v e l o p m e n t o f some p e r m e a b i l i t y due t o l e a c h i n g and p o s s i b l y a l s o t o d i f f e r e n -t i a l e x p a n s i o n o f m i n e r a l g r a i n s as a r e s u l t , o f h y d r a t i o n . 2) D e p o s i t i o n o f t h e A t h a b a s c a s a n d s t o n e , , and s u b s e q u e n t " h y d r o t h e r n i a l " a l t e r a t i o n . The w e a t h e r i n g a l t e r a t i o n i n t h e r e g o l i t h zone and i n t h e m y l o n i t e z o n e s may have c r e a t e d c h a n n e l s f o r t h e m i g r a t i o n o f s o l u t i o n s and a c t e d as c e n t r e s f o r h y d r o -t h e r m a l a l t e r a t i o n . The new g e o c h r o n o l o g i c d a t a p r e s e n t e d i n C h a p t e r V s u p p o r t t h i s e x p l a n a t i o n . An a d d i t i o n a l p o s s i b i l i t y i s t h a t t h e a l t e r a t i o n i s r e l a t e d i n some way t o t h e f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e , f o r example t h e f o r m a t i o n o f u n s t a b l e g l a s s e s f r o m s i l l i m a n i t e by s h o c k - m e t a m o r p h i s m , w i t h s u b s e q u e n t l o w - t e m p e r a t u r e r e - e q u i l i b r a -t i o n . However, t h e a p p a r e n t age o f a l t e r a t i o n i s q u i t e d i f f e r e n t f r o m t h a t o f t h e C i r c u l a r s t r u c t u r e . 114 V. GEOLOGICAL HISTORY AND CORRELATION WITH OTHER AREAS . Geochronology o f t h e C a r s w e l l A r e a I n t h e c o u r s e o f t h e p r e s e n t s t u d y , two new p o t a s s i u m - a r g o n d a t e s were c a l c u l a t e d , b o t h on d i f f e r e n t m i n e r a l s from t h e same r o c k - s p e c i m e n ( g a r n e t - c o r d i e r i t e g n e i s s from t h e Claude Lake zone, sample BH-74-169 i l l u s t r a t e d i n f i g u r e 7 ) The r o c k was c r u s h e d t o -40, +80 m e s h - s i z e , and heavy l i q u i d s were used t o s e p a r a t e t h e v a r i o u s d e n s i t y f r a c t i o n s . F a i r l y p u r e s e p a r a t i o n s o f b i o t i t e and o f a l t e r e d c o r d i e r i t e were p r o d u c e d . These f r a c t i o n s were a n a l y s e d f o r p o t a s s i u m by at o m i c a b s o r p t i o n , and r a d i o g e n i c a r g o n d e t e r m i n a t i o n s were made by m a s s - s p e c t r o m e t r y * . The apparent age computed f o r t h e b i o t i t e was 1,973 m i l l i o n y e a r s . I n t h i s r o c k t y p e , as i n most r o c k t y p e s o f t h e C a r s w e l l A r e a , b i o t i t e i s d e v e l o p e d e a r l y i n t h e p a r a g e n e s i s . However, i t would appear u n l i k e l y t h a t t h e c a l c u l a t e d age c o u l d r e p r e s e n t a " s u r v i v a l age" from an e a r l y phase o f metamorphism, as t h e extreme c o n d i t i o n s e n v i s a g e d f o r metamorphism o f t h e g r a n u l i t e f a c i e s s h o u l d be adequate t o r e s e t K-Ar d a t e s i n any m i n e r a l ( W e t h e r i l l , 1965)- I t i s c o n c l u d e d t h a t t h i s d a t e i s r e p r e s e n t a -t i v e o f t h e main phase o f metamorphism o f t h e basement r o c k s , and i t s s i m i l a r i t y w i t h ages o f p r o g r a d e metamorphism i n s u r r o u n d -i n g a r e a s (see below) s u g g e s t s e i t h e r t h a t r e t r o g r e s s i o n as d e s c r i b e d i n C h a p t e r I I I was penecontemporaneous w i t h peak meta-morphism, o r t h a t r e t r o g r e s s i o n d i d not r e s e t t h e r a d i o m e t r i c c l o c k . ' *• Mr. J . H a r a k a l , U n i v e r s i t y o f B r i t i s h C o l u m b i a , see a p p e n d i x . 115 The " a l t e r e d c o r d i e r i t e " f r a c t i o n i s a s o f t g r e e n i s h m a t e r i a l o f low d e n s i t y , and c o n s i s t s o f k a o l i n i t e , c h l o r i t e and s e r i c i t e (see c h a p t e r I V ) . The a l t e r a t i o n , i s c o n s i d e r e d t o be h y d r o t h e r m a l i n o r i g i n . M i c r o p r o b e a n a l y s e s i n d i c a t e t h a t t h e p o t a s s i u m i n t h i s f i n e - g r a i n e d a g g r e g a t e i s c o n c e n t r a t e d i n s e r i c i t e a l o n g p i n i t i s e d c r a c k s , but as two b u l k p o t a s s i u m a n a l y s e s o f t h e m a t e r i a l gave c o n s i s t e n t r e s u l t s , t h e a g g r e g a t e may be c o n s i d e r e d e f f e c t i v e l y homogeneous w i t h r e s p e c t t o p o t a s -sium. The apparent age o f t h e a l t e r a t i o n ( i . e . the. f o r m a t i o n o f t h e s e r i c i t e ) i s 988 m i l l i o n y e a r s . Two w h o l e - r o c k K-Ar d a t e s on t h e C l u f f B r e c c i a s (Wanless et a l . , 1967) gave d a t e s o f 467 + 28 m i l l i o n y e a r s and 486 + 55 m i l l i o n y e a r s . These d a t e s are c o n s i d e r e d t o r e p r e s e n t t h e t i m e o f f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e . I t i s s i g n i f i c a n t t h a t n e i t h e r o f t h e basement, ages d i s c u s s e d above were r e s e t a t t h i s t i m e , as t h e p r e s e n c e o f shock-metamorphic f e a t u r e s i n d i c a t e t h a t h i g h - e n e r g y p r o c e s s e s were a s s o c i a t e d w i t h t h e s t r u c t u r e f o r m a t i o n . A p p a r e n t l y t h e C i r c u l a r S t r u c t u r e event was a t r a n -s i e n t phenomenon, and d i d not open t h e p o t a s s i u m - b e a r i n g m i n e r a l s t o a r g o n . P r e l i m i n a r y u r a n i u m - l e a d d a t e s on a p i t c h b l e n d e - u r a n i n i t e b o u l d e r gave i m p e r f e c t c o n c o r d a n c e , but i n d i c a t e an age o f t h e o r d e r o f 1,100 m.y. (Mokta r e p o r t ) . I f a + 10% c o n f i d e n c e l e v e l i s p l a c e d on t h i s d a t e and t h o s e d i s c u s s e d above, i t may be s u g g e s t e d t h a t t h e h y d r o t h e r m a l a l t e r a t i o n and u r a n i u m m i n e r a l -i s a t i o n were contemporaneous, e s p e c i a l l y i n v i e w o f t h e s p a t i a l a s s o c i a t i o n o f t h e s e phenomena. 116 G e o l o g i c H i s t o r y The f o l l o w i n g i s a c o m p i l a t i o n o f t h e major e v e n t s w h i c h have a f f e c t e d t h e r o c k s o f t h e C a r s w e l l A r e a . Tt i s based p a r t l y on p e t r o l o g i c o b s e r v a t i o n s , p a r t l y on g e o c h r o n o l o g i c e v i d e n c e , and p a r t l y on a n a l o g i e s w i t h a d j o i n i n g a r e a s . 1. A r c h e a n basement. No r o c k s i n t h e C a r s w e l l A r e a can be i d e n t i f i e d w i t h a g r a n i t i c basement complex e a r l i e r t h a n t h e s u p r a -c r u s t a l g n e i s s e s , but t h e m e t a s e d i m e n t a r y n a t u r e o f t h e g n e i s s e s i n d i c a t e s t h a t t h e y must have been d e r i v e d from p r e - e x i s t i n g r o c k s . " S u r v i v a l " ages (Beck, 1970) o f m i n e r a l s i n r o c k s t e n t a t i v e l y c o r r e l a t e d w i t h t h e C a r s w e l l A r e a g n e i s s e s i n d i c a t e an age f o r t h i s basement complex i n e x c e s s o f 2500 m.y. .2. G n e i s s e s . The t w o - b i l l i o n - y e a r age o f metamorphism and t h e s u p r a c r u s t a l n a t u r e o f t h e C a r s w e l l g n e i s s e s s u g g e s t s t h a t t h e y a r e c o r r e l a t i v e w i t h s i m i l a r r o c k s o f t h e B e a v e r l o d g e A r e a (Tremblay, 1 9 72). The t i m e span o f t h e metamorphism a t C a r s w e l l i s unknown, as o n l y one r e l e v a n t age-date i s a v a i l a b l e , but two m ajor phases can be r e c o g n i s e d : metamorphism t o g r a n u l i t e f a c i e s and l a t e r r e t r o g r e s s i o n ( p r o b a b l y due t o a h y d r a t i o n e p i s o d e ) t o t h e upper a m p h i b o l i t e f a c i e s . The p o t a s s i u m metasoma-t i s m and t h e emplacement of pegmatoids p r o b a b l y accom-p a n i e d r e t r o g r e s s i o n . I n i t i a l u r a n i u m m i n e r a l i s a t i o n i n t h e t h o r i u m - b e a r i n g p e g m a t i t e s p r o b a b l y o c c u r r e d a t t h i s 117 t i m e . The p o s t u l a t e d age o f t h e metamorphism i d e n t i f i e s i t w i t h t h e H u d s o n i a n O r o g e n y . T h i s o r o g e n y was r e s p o n s i b l e f o r t h e d e v e l o p m e n t o f t h e d o m i n a n t f o l i a t i o n , and t h e m a j o r f o l d - b e l t s o f N o r t h e r n S a s k a t c h e w a n ( f i g u r e 1 ) . The H u d s o n i a n O r o g e n y l a s t e d f r o m 2 b . y . t o 1.75 b . y . ago ( B e c k , 1970). 3. Basement t e c t o n i s m . The m e t a m o r p h i c and g r a n i t o i d r o c k s a r e a f f e c t e d by l a t e f o l d i n g , i n d i c a t i n g t h a t t e c t o n i s m p e r s i s t e d somewhat l o n g e r t h a n metamorphism. A l t h o u g h i t c a n n o t be p r o v e n , t h i s f o l d i n g may be t e n t a t i v e l y c o r r e l a t e d w i t h t h e f o r m a t i o n o f f a u l t s and m y l o n i t e z o n e s i n t h e basement r o c k s . 4. D e p o s i t i o n o f C o v e r R o c k s . D i r e c t e v i d e n c e f o r t h e age o f t h e A t h a b a s c a S a n d s t o n e i s l a c k i n g . However, b a s a l t s o f t h e B e a v e r -l o d g e A r e a M a r t i n F o r m a t i o n , w h i c h i s c o n s i d e r e d t o be o l d e r t h a n t h e A t h a b a s c a F o r m a t i o n ( B e c k , 1970) have b e e n d a t e d a t 1630 m.y., and d i a b a s e d y k e s c u t t i n g a t l e a s t t h e l o w e r p a r t o f t h e A t h a b a s c a S a n d s t o n e a t C r e e L a k e ( f i g u r e 1) a r e d a t e d a t 1230 m.y. The d e p o s i t i o n o f t h e A t h a b a s c a S a n d s t o n e t h e r e f o r e o c c u r r e d b e t w e e n t h e s e d a t e s . A c o n s t r a i n t on t h e maximum age o f t h e . s a n d s t o n e i s i m p o s e d by t h e f a c t t h a t d e e p e r o s i o n t o e x p o s e t h e g r a n u l i t e f a c i e s r o c k s must have o c c u r r e d b e t w e e n t h e end o f t h e H u d s o n i a n O r o g e n y and t h e b e g i n -n i n g o f s a n d s t o n e d e p o s i t i o n . 118: Some o f t h e ob s e r v e d a l t e r a t i o n o f t h e basement r o c k s no doubt o c c u r r e d i n t h e h i a t u s p r o c e e d i n g t h e d e p o s i t i o n o f t h e A t h a b a s c a f o r m a t i o n , but t h i s was p r o b a b l y r e s t r i c t e d t o a t h i n r e g o l i t h zone. H y d r o t h e r m a l A l t e r a t i o n and I n i t i a l Uranium Mineral!™ s a t i o n . These e v e n t s most l i k e l y o c c u r r e d penecontemporaneou l y , as i n d i c a t e d by t h e i r s i m i l a r d a t e s o f about one b i l l i o n y e a r s , and may be r e l a t e d t o a " t h e r m a l e v e n t " a t 1110 m.y. ( d e s c r i b e d by K o e p p e l , 1967) w h i c h caused r e m o b i l i s a t i o n o f some p i t c h b l e n d e d e p o s i t s a t B e a v e r -l o d g e . K n i p p i n g ( u n p u b l i s h e d ) g i v e s a d a t e o f 929-1127 m.y. f o r a major p e r i o d o f p i t c h b l e n d e m i n e r a l i s a t i o n i n t h e R a b b i t Lake A r e a . F o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e . T h i s event i s d a t e d a t 470-480 m.y. Numerous shock-metamorphic f e a t u r e s a r e a s s o c i a t e d w i t h t h e s t r u c t u r e , but whether t h e e x p l a n a t i o n o f m e t e o r i t e impact o f " c r y p t o - e x p l o s i o n " i s a c c e p t e d i s s t i l l a m a t t e r o f p e r s o n a l p r e f e r e n c e . 1 1 9 C o r r e l a t i o n W i t h O t h e r A r e a s As t h e g e o l o g y o f t h e B e a v e r l o d g e M i n i n g D i s t r i c t i s w e l l known, n o t a b l y f r o m t h e work o f C h r i s t i e ( 1 9 5 3 ) s K o e p p e l ( 1967) and T r e m b l a y ( 1 9 7 2 ) , i t i s i n s t r u c t i v e t o draw c o m p a r i s o n s b e t w e e n t h i s a r e a and t h e C a r s w e l l A r e a . F i g u r e shows t h e m a j o r e v e n t s i n d i c a t e d by t h e g e o l o g i c r e c o r d i n the. C a r s w e l l A r e a , and t h e i r p o s s i b l e r e l a t i o n s h i p s w i t h e v e n t s a t B e a v e r -l o d g e as p o s t u l a t e d by T r e m b l a y and by B e c k ( 1 9 7 0 ) . I n b o t h a r e a s , t h e o l d e s t r o c k s e x p o s e d a r e m e t a s e d i m e n t s d e p o s i t e d b e t w e e n a b o u t 2 . 5 b i l l i o n and 2 b i l l i o n , y e a r s a g o . However, t h e n a t u r e o f t h e r o c k s i n t h e two a r e a s i s q u i t e d i f f e r e n t , m a f i c g n e i s s e s a n d q u a r t z o f e l d s p a t h i c g n e i s s e s b e i n g d o m i n a n t a t B e a v e r l o d g e , and p e l i t i c g n e i s s e s a t C a r s w e l l . R a d i o m e t r i c d a t e s i n b o t h a r e a s were r e s e t by r e g i o n a l metamor-p h i s m a s s o c i a t e d w i t h t h e H u d s o n i a n O r o g e n y . The name " T a z i n G r o u p " a p p l i e d t o t h e m e t a m o r p h i c r o c k s a t B e a v e r l o d g e by v a r i o u s a u t h o r s , r e p r e s e n t s r o c k s d e p o s i t e d o v e r t h e 500 m i l l i o n - y e a r p e r i o d p r e c e e d i n g t h e H u d s o n i a n O r o g e n y . W i t h t h i s d e f i n i t i o n , t h e C a r s w e l l r o c k s may be i n c l u d e d i n t h e T a z i n G r o u p . The g n e i s s e s o f t h e B e a v e r l o d g e A r e a a r e c o n s i d e r e d by T r e m b l a y t o have b e e n metamorphosed t o t h e a l m a n d i n e - a m p h i b o l i t e f a c i e s . R ocks o f t h e g r a n u l i t e f a c i e s h a v e n o t b e e n r e p o r t e d , and t h e peak metamorphism i n t h i s a r e a a p p e a r s t o h a v e o c c u r r e d a t a somewhat l o w e r t e m p e r a t u r e , and w i t h g r e a t e r a v a i l a b i l i t y o f w a t e r , t h a n t h a t a t C a r s w e l l . The B e a v e r l o d g e r o c k s have b e e n s t r o n g l y , and v a r i a b l y , r e t r o g r a d e d and r o c k s o f g r a d e as low as t h e g r e e n s c h i s t f a c i e s a r e common. T h i s r e t r o g r e s s i o n .HYDROTHERMAL ACTtVITY AND MINERALISATION TECTONISM GRAN ITlSAT ION METAMORPHISM SEDIMENTATION HYDROTHERMAL ACTIVITY AND MINERALISATION TECTONISM GRAN ITI SAT I ON LATE PITCHBLENDE MOBILISATION PITCHBLENDE REJUVENATION PITCHBLENDE SYNOENETIC _ URANINITE ® LATE FOLDING 8 MOVEMENTS MYLONIT1SATION EARLY FOLDING "Y" TYPE "o" TYPE AMPHIBOLITE FACIES RETROGRESSION ATHABASCA MARTIN TAZIN GROUP HYDROTHERMAL ALTERATION PITCHBLENDE SYNGENETIC MINERALISATION CIRCULAR STRUCTURE FOLDING & MYLON1TISATION EARLY FOLDING METAMORPHISM SEDIMENTATION PEGMATOIDS GRANULITE FACIES RETROGRESSION (CREE LAKE DIABASE) TAZIN GROUP PHANEROZOIC HELIK1AN HUDSON I AN APHEBIAN PROTEROZOIC — : 1 ARCHEAN ~J r -3 *\ r u a CD rH > £> a e CD CD PQ M LH c CTJ rH O rH U rH •P CD c o ra o U cti CD o hO rd CD x: o - P •H O H c o •H -p o ra ra •P •H C • CD CD ^ > •P CD OJ M o o rH •H ^ crj O C -P •H •H C CD O ra w rH CD CD O rH U CD o a faD u •H -O u O C o a '<-> ci o S cti o H t— C M 03 0> O rH c L O o Cti S t •H CD O - P SH CD ct) cd PQ CD rH SH CD CD •*> bOCM bO C— •H o O CTN o rH rH 121 r e s u l t s a t l e a s t i n p a r t f r o m r e c r y s t a l l i s a t i o n a s s o c i a t e d w i t h t h e "dynamic metamorphism" w h i c h f o r m e d t h e m y l o n i t e z o n e s o f t h e a r e a , s u g g e s t i n g t h a t t h e m y l o n i t i s a t i o n o c c u r r e d - l a t e r t h a n t h e p r o g r a d e metamorphism. T r e m b l a y a l s o n o t e s p r e - m y l o n i t e r e t r o g r e s s i o n w h i c h i s s p a t i a l l y a s s o c i a t e d w i t h H u d s o n i a n g r a n i t e b o d i e s . T h i s p h a s e may c o r r e s p o n d w i t h t h e p e r i o d o f r e t r o g r e s s i o n and. g r a n i t i s a -t i o n p o s t u l a t e d f o r t h e C a r s w e l l A r e a . B e c k , i n h i s c o m p i l a t i o n o f t h e g e o l o g y o f n o r t h e r n S a s k -a t c h e w a n , p r o p o s e s t h a t two p e r i o d s o f w i d e s p r e a d g r a n i t e emplacement have o c c u r r e d . T h e s e a r e d e s i g n a t e d as " O - t y p e " g r a n i t e s (2200 + 100 m i l l i o n y e a r s ) and " Y - t y p e " (1820 + 100 m i l l i o n y e a r s ) . The g r a n i t i s a t i o n o f r o c k s i n t h e C a r s w e l l A r e a may be a s s i g n e d t o t h e "Y" p e r i o d , b u t no o l d e r g r a n i t e s c a n be r e c o g n i s e d i n t h i s a r e a . C o r r e l a t i o n o f t h e unmetamorphosed M a r t i n F o r m a t i o n a t B e a v e r l o d g e w i t h t h e A t h a b a s c a s a n d s t o n e i s p r o b l e m a t i c . The M a r t i n f o r m a t i o n i s d a t e d f a i r l y r e l i a b l y a t 1630 m i l l i o n y e a r s by a b a s a l t f l o w w i t h i n t h e s e d i m e n t s , b u t no s u c h d i r e c t e v i d e n c e i s a v a i l a b l e f o r t h e A t h a b a s c a . B e c k c o n s i d e r s t h e M a r t i n f o r m a -t i o n t o be t h e o l d e r , b u t t h a t t h e two f o r m a t i o n s may be c o n t e m -p o r a n e o u s i n p a r t . T h e r e i s e v i d e n c e f o r r e g i o n a l h y d r o t h e r m a l a c t i v i t y a t a b o u t one b i l l i o n y e a r s b e f o r e p r e s e n t . A p p a r e n t l y s t r o n g l o w -t e m p e r a t u r e a l t e r a t i o n and u r a n i u m m i n e r a l i s a t i o n o c c u r r e d a t a b o u t t h i s t i m e i n b o t h t h e C a r s w e l l and R a b b i t L a k e A r e a s , and K o e p p e l (1967) r e p o r t s a 1100 m i l l i o n y e a r r e j u v e n a t i o n o f d e p o s i t s a t B e a v e r l o d g e . I n a l l a r e a s , u r a n i u m r e m o b i l i s a t i o n has o c c u r r e d 122 t h r o u g h o u t th e g e o l o g i c r e c o r d , b u t e x i s t i n g d a t a do not a l l o w any a d d i t i o n a l r e g i o n a l c o r r e l a t i o n s . The f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e appears t o have been an i s o l a t e d i n c i d e n t . . No s i m i l a r s t r u c t u r e s , e xcept p o s s i b l y t h e P a s f i e l d Lake S t r u c t u r e ( K o r n i k , 1 9 6 9 ) , a r e known i n N o r t h e r n Saskatchewan, and t h e date o f f o r m a t i o n o f t h e C a r s w e l l S t r u c t u r e , does not l i n k i t with, any r e g i o n a l ; h y d r o -t h e r m a l c r t e c t o n i c e v e n t . 123 V I . IMPLICATIONS FOR MINERALISATION  Source o f Uranium The p r i m a r y c o n c e n t r a t i o n o f uranium i n t h e c r y s t a l l i n e basement r o c k s o f n o r t h e r n Saskatchewan I s c o n s i d e r e d by Beck (1970) t o have been i n t h e abundant g r a n i t i c b o d i e s d e v e l o p e d i n t h e r e g i o n . A c c o r d i n g t o t h e model o f McKe l v e y , E v e r h a r t . 4+ and G a r r e l s (1955), t h e U i o n i s s t a b l e i n t h e r e d u c i n g 4+ c o n d i t i o n s o f magmas. The U i o n i s p r e v e n t e d from s u b s t i t u t i n g i n t h e l a t t i c e s o f common r o c k - f o r m i n g m i n e r a l s by i t s l a r g e o 2+ o s i z e (0.97A). The Ca c a t i o n (0.99A) i s t h e o n l y common c a t i o n o f comparable s i z e , and t h i s s u b s t i t u t i o n i s p r e v e n t e d by t h e 4+ charge d i s c r e p a n c y . C o n s e q u e n t l y , U i o n s t e n d t o become IH-c o n c e n t r a t e d i n l a t e magmatic f l u i d s . The s i m i l a r Th i o n becomes c o n c e n t r a t e d i n t h e same way. Uranium and t h o r i u m , t h e n , t e n d t o be c o n c e n t r a t e d i n p e g m a t i t e s , and uranium may be s u b s e q u e n t l y r e d i s t r i b u t e d by h y d r o t h e r m a l o r supergene 4+ 4+ s o l u t i o n s . U and Th i o n s a re v e r y i n s o l u b l e i n w a t e r , but g^. 2-t-uranium i s r e a d i l y o x i d i s e d t o t h e s o l u b l e (U Og) u r a n y l i o n , and i s e a s i l y r e m o b i l i s e d , whereas t h o r i u m cannot f o r m s o l u b l e complexes. The p r e s e n c e o f t h o r i u m I n a p e g m a t i t e may i n d i c a t e t h a t uranium and t h o r i u m were b o t h o r i g i n a l l y c o n c e n t r a t e d i n t h e r o c k , but t h a t an e p i s o d e o f o x i d a t i o n has p e r m i t t e d p r e f e r e n -t i a l l e a c h i n g o f uranium (McKelvey et a l . , op. c i t . ) . The t h o r i u m - r i c h p e g m a t i t e s o f t h e C a r s w e l l a r e a may t h u s have r e p r e s e n t e d t h e s i t e o f p r i m a r y c o n c e n t r a t i o n o f uranium. 124 N a t u r e o f t h e M i n e r a l i s a t i o n As d e s c r i b e d i n c h a p t e r I , t h e i m p o r t a n t zones o f uranium m i n e r a l i s a t i o n i n t h e C a r s w e l l a r e a o c c u r i n two g e o l o g i c a l e n v i r o n m e n t s , 1. i n t h e a l t e r e d " r e g o l i t h " zone b e n e a t h t h e A t h a b a s c a Sandstone and i n t h e i m m e d i a t e l y a d j a c e n t s e d i m e n t s , . 2. i n a l t e r e d basement f r a c t u r e - z o n e s . S e v e r a l l i n e s o f g e o c h r o n o l o g i c evidence> as d i s c u s s e d i n c h a p t e r V, i n d i c a t e t h a t t h i s m i n e r a l i s a t i o n was r e l a t e d t o an e p i s o d e o f a l t e r a t i o n a t about one b i l l i o n years.. T h i s d a t e i s s i g n i f i c a n t , as i t i m p l i e s " h y d r o t h e r m a l " a c t i v i t y b e n e a t h t h e A t h a b a s c a Sandstone a t a t i m e when no t e c t o n i c o r p l u t o n i c a c t i v i t y i n t h e r e g i o n has been r e c o g n i s e d . The f o l l o w i n g e v e n t s a l l o c c u r r e d c l o s e t o one b i l l i o n y e a r s b e f o r e p r e s e n t : 1) A phase o f uranium m i n e r a l i s a t i o n i n t h e C a r s w e l l A r e a . 2) A l t e r a t i o n i n t h e basement r o c k s , o r r e c r y s t a l l i s a -t i o n o f a l t e r a t i o n p r o d u c t s a s s o c i a t e d w i t h t h e p a l e o -s u r f a c e w e a t h e r i n g . T h i s a l t e r a t i o n may have caused r e c r y s t a l l i s a t i o n o f q u a r t z and d e p o s i t i o n o f secondary c l a y m i n e r a l s i n t h e b a s a l p a r t o f t h e A t h a b a s c a forma-t i o n . 3) S i m i l a r a l t e r a t i o n and m i n e r a l i s a t i o n a t R a b b i t L a k e . . . 4) R e j u v e n a t i o n o f t h e uranium d e p o s i t s a t B e a v e r l o d g e . G e o c h r o n o l o g i c e v i d e n c e s u g g e s t s t h a t t h e a l t e r a t i o n and t h e m i n e r a l i s a t i o n were contemporaneous and, by i n f e r e n c e , t h a t 125 t h e y were r e l a t e d . Prom t h e d i s c u s s i o n i n c h a p t e r IV, i t appears t h a t t h e a l t e r a t i o n o c c u r r e d i n an aqueous environment a t a t e m p e r a t u r e o f about 200°C. The t i m i n g and t h e c o n d i t i o n s o f f o r m a t i o n o f t h e a l t e r a t i o n i n d i c a t e t h a t i t i s not a r e s u l t o f r e g i o n a l metamorphism, nor o f w e a t h e r i n g a t t h e p r e - A t h a b a s c a p a l e o s u r f a c e , but a superimposed p o s t - A t h a b a s c a Sandstone e v e n t . A s t u d y o f t h e A t h a b a s c a Sandstone as seen i n a deep d r i l l -c o r e a t Rumpel Lake ne a r t h e c e n t r e o f t h e A t h a b a s c a Sandstone body ( G u l f M i n e r a l s u n p u b l i s h e d r e p o r t ) r e v e a l s t h e p r e s e n c e o f a u t h i g e n i c c l a y m i n e r a l s o f s i m i l a r m i n e r a l o g y t o t h e a l t e r a t i o n p r o d u c t s o b s e r v e d i n the C a r s w e l l basement r o c k s . P a g e l ( p e r s o n -a l communication) has r e c o g n i s e d q u a r t z o v e r g r o w t h s on s a n d - g r a i n s i n t h e A t h a b a s c a Sandstone i n b o t h t h e C a r s w e l l a r e a and i n t h e Rumpel Lake c o r e . F l u i d i n c l u s i o n s i n such q u a r t z near t h e base o f t h e sandstone i n d i c a t e f o r m a t i o n under a p r e s s u r e o f 1.5 k i l o b a r s a t a t e m p e r a t u r e . o f about 220°C. The n a t u r e o f t h e a l t e r a t i o n , and t h e f l u i d i n c l u s i o n d a t a suggest t h a t a l t e r a t i o n o c c u r r e d under c o n d i t i o n s o f b u r i a l metamorphism. The 1.5 k i l o b a r p r e s s u r e p o s t u l a t e d by P a g e l a t Rumpel Lake would r e q u i r e b u r i a l o f up t o f i v e t h o u s a n d m e t r e s , t h r e e t i m e s t h e known pre s e n t t h i c k n e s s o f t h e s a n d s t o n e ( K o r n i k , 1969). The r e g i o n may a t t h a t time have had a t h i c k e r c o v e r o f s e d i m e n t s . 126 A Model f o r M i n e r a l i s a t i o n I n t h e l i g h t o f t h e p e t r o l o g i c a l and g e o c h r o n o l o g i c a l r e s u l t s p r e s e n t e d i n t h i s t h e s i s , any e x p l a n a t i o n f o r t h e " s e c o n -d a r y " m i n e r a l i s a t i o n i n t h e C a r s w e l l a r e a must s a t i s f y t h e f o l -l o w i n g c o n d i t i o n s : 1. M i n e r a l i s a t i o n i s l o c a l i s e d n e a r t h e u n c o n f o r m i t y a t t h e base o f t h e A t h a b a s c a S a n d s t o n e , and I s p r e s e n t i n b o t h t h e metamorphic and t h e s e d i m e n t a r y r o c k s . 2. M i n e r a l i s a t i o n was independent o f r e g i o n a l meta-morphism, as m i n e r a l i s a t i o n o c c u r s i n unmetamorphosed r o c k s . 3. M i n e r a l i s a t i o n o c c u r r e d a f t e r d e p o s i t i o n o f t h e A t h a b a s c a Sandstone and b e f o r e t h e f o r m a t i o n o f t h e C i r c u l a r S t r u c t u r e a t a time when no p l u t o n i c a c t i v i t y and no t e c t o n i s m ( e x c e p t , p o s s i b l y , some minor e p e i r o g e n i c w a r p i n g ) a r e known. 4. M i n e r a l i s a t i o n a p p a r e n t l y o c c u r r e d i n t h e p r e s e n c e o f an aqueous f l u i d , a t a t e m p e r a t u r e o f about 200°C. As i n d i c a t e d above, s e c o n d a r y m i n e r a l i s a t i o n , was p r o b a b l y due t o l e a c h i n g o f u r a n i f e r o u s p e g m a t i t e s by an aqueous f l u i d and subsequent d e p o s i t i o n i n . a s u i t a b l e t r a p . Such m i g r a t i o n o f an aqueous f l u i d c o u l d o n l y o c c u r i n a r o c k w i t h s i g n i f i c a n t p e r m e a b i l i t y , and i s t h e r e f o r e u n l i k e l y i n u n a l t e r e d h i g h - g r a d e g n e i s s e s and p e g m a t i t e s . The p r e s e n c e o f m i n e r a l i s a t i o n and " h y d r o t h e r m a l " a l t e r a t i o n show t h a t such a m i g r a t i o n has i n d e e d o c c u r r e d , and i t i s s u g g e s t e d t h a t t h e n e c e s s a r y p e r m e a b i l i t y was p r o d u c e d i n i t i a l l y by deep w e a t h e r i n g a t t h e p a l e o - s u r f a c e . However, t h e p o s t u l a t e d t i m i n g and P-T e n vironment o f t h e 127 a l t e r a t i o n and m i n e r a l i s a t i o n r e q u i r e t h a t t h e o b s e r v e d m i n e r a l . a s s e m b l a g e s were f o r m e d b e n e a t h t h e s e d i m e n t a r y c o v e r r o c k s . I t i s p r o p o s e d , t h e n , t h a t t h e " r e g o l i t h " zone a c t e d as a s u b -A t h a b a s c a S a n d s t o n e a q u i f e r a t t h e t i m e o f m i n e r a l i s a t i o n . P o r o s i t y and p e r m e a b i l i t y i n t h i s zone were p r o b a b l y due t o m i c r o f r a c t u r i n g p r o d u c e d by volume c h a n g e s i n h y d r a t i o n r e a c t i o n s s u c h as t h e a l t e r a t i o n o f s i l l i m a n i t e t o k a o l i n i t e . F r a c t u r e z o n e s a n d a l t e r e d m y l o n i t e z o n e s i n t h e basement r o c k s may h a v e a c t e d a s m a j o r c o n d u i t s . The d e p t h o f t h e zone o f a l t e r a t i o n b e n e a t h t h e u n c o n f o r m i t y i s unknown, b u t t h e most i n t e n s e l y a l t e r e d r o c k s a r e o b s e r v e d i n c l o s e p r o x i m i t y t o t h e u n c o n f o r m i t y , i n a zone some t e n s o f m e t r e s t h i c k . A l l t h e basement r o c k s o f t h e C a r s w e l l S t r u c t u r e a r e a l t e r e d t o some d e g r e e , b u t t h e amount o f u p l i f t , o f t h e c e n t r a l " p l u g " , a n d t h e r e f o r e i t s d e p t h b e l o w t h e u n c o n f o r m i t y , a r e unknown. The l o c a l i s a t i o n o f m i n e r a l i s a t i o n n e a r t h e p l a n e o f u n c o n f o r m i t y r e q u i r e s t h a t movement o f t h e o r e - b e a r i n g f l u i d was p r i m a r i l y i n a h o r i z o n t a l d i r e c t i o n , t h a t i s , p a r a l l e l t o t h e u n c o n f o r m i t y . M i n e r a l i s a t i o n f r o m a d e e p - s e a t e d s o u r c e w o u l d r e q u i r e a "Deus ex m a c h i n a " , and v e r t i c a l m i g r a t i o n t h r o u g h t h e s a n d s t o n e away f r o m t h e a l t e r e d zone s h o u l d p r o d u c e m i n e r a l i s a -t i o n w i t h i n t h e s a n d s t o n e body. However, t h e l a c k o f o b s e r v e d m i n e r a l i s a t i o n a t h i g h s t r a t i g r a p h i c l e v e l s i n t h e s a n d s t o n e may be due t o t h e l a c k o f s u i t a b l e t r a p s , and t h e s e c o n d p o s s i b i l i t y c a n n o t be r u l e d o u t . I f u r a n i u m - r i c h w a t e r i s assumed t o h a ve m i g r a t e d t h r o u g h p o r o u s r o c k s c l o s e t o t h e u n c o n f o r m i t y , m i n e r a l i s a t i o n w o u l d 128 o c c u r i n any s t r u c t u r a l o r c h e m i c a l t r a p i n i t s p a t h . Two t r a p t y p e s a p p a r e n t l y e x i s t i n t h e C a r s w e l l a r e a : 1. A l t e r e d zones a s s o c i a t e d w i t h f r a c t u r e s o r m y l o n i t e zones. A p p a r e n t l y uranium m i n e r a l s were p r e c i p i t a t e d as i m p r e g n a t i o n s i n a r g i l l a c e o u s m a t e r i a l . 2. C o n c e n t r a t i o n s o f carbonaceous m a t e r i a l i n b a s a l zones o f t h e A t h a b a s c a f o r m a t i o n . The r e d u c i n g e f f e c t 2} + o f c a r b o n causes p r e c i p i t a t i o n o f I n s o l u b l e U compounds. The A t h a b a s c a Sandstone may be r e g a r d e d as an e s s e n t i a l l y homogeneous a q u i f e r , and t h e permeable r e g o l i t h zone p o s t u l a t e d above may have been an e f f e c t i v e e x t e n s i o n o f t h i s a q u i f e r . The w e l l - e s t a b l i s h e d p r i n c i p l e s o f ground-water c i r c u l a t i o n may t h e r e f o r e be I n v o k e d t o e x p l a i n a h o r i z o n t a l m i g r a t i o n o f w a t e r a l o n g t h e base o f t h e s a n d s t o n e body. By f a r t h e most i m p o r t a n t agency p r o d u c i n g ground w a t e r c i r c u l a t i o n i s g r a v i t y . A c c o r d i n g t o Domenico ( 1 9 7 7 ) , q u o t i n g T o t h ( 1 9 6 3 ) , a s m a l l r e g i o n a l t o p o g r a p h i c g r a d i e n t at t h e s u r f a c e o f an u n c o n f i n e d homogeneous a q u i f e r i s s u f f i c i e n t t o produce a r e g i o n a l c i r c u l a t i o n system. The net e f f e c t o f such a system i s t h a t groundwater t e n d s t o f l o w t h r o u g h t h e a q u i f e r from a t o p o g r a p h i c a l l y h i g h a r e a t o a low a r e a . Such f l o w would be a t t e n u a t e d w i t h d e p t h , but c o u l d r e s u l t i n e f f e c t i v e l y h o r i z o n t a l t r a n s p o r t o f w a t e r a l o n g t h e base o f an a q u i f e r . As t h e A t h a b a s c a s a n d s t o n e i s almost f l a t -l y i n g , i t i s l i k e l y t h a t i t s s u r f a c e never d e v e l o p e d a s t r o n g r e l i e f , and so any. r e g i o n a l t i l t i n g o r e p e i r o g e n i c u p l i f t c o u l d have produced d e e p l y c i r c u l a t i n g r e g i o n a l g r o u n d w a t e r f l o w . I f g roundwater f l o w was i n f a c t an i m p o r t a n t mechanism f o r uranium m i g r a t i o n , t h e n t h e i d e n t i f i c a t i o n o f c i r c u l a t i o n systems 129 a t t h e t i m e o f m i n e r a l i s a t i o n would be o f g r e a t i m p o r t a n c e , i n p r o s p e c t i n g f o r u r a n i u m . . The d o m i n a n t t r a n s p o r t a t i o n d i r e c t i o n o f A t h a b a s c a f o r m a t i o n s e d i m e n t s was t o t h e n o r t h w e s t ( F a h r i g , . 1961). Knowledge o f t h e p a l e o s l o p e , and a d e t a i l e d a n a l y s i s c f g r o u n d w a t e r movement i n and b e n e a t h t h e A t h a b a s c a S a n d s t o n e c o u l d be o f m a j o r s i g n i f i c a n c e i n u n d e r s t a n d i n g t h e p a t t e r n s o f m i n e r a l i s a t i o n , and h e n c e i n f u t u r e e x p l o r a t i o n programs,, Any m o d e l must t a k e i n t o a c c o u n t t h e e v i d e n c e t h a t u r a n i u m m i n e r a l i s a t i o n i s l o c a l i s e d a t t h e b a s e o f t h e s a n d s t o n e b o d y , an d was emplaced by a l o w - t e m p e r a t u r e h y d r o t h e r m a l p r o c e s s a t a t i m e o f no p l u t o n i c o r t e c t o n i c a c t i v i t y . 130 BIBLIOGRAPHY . A l c o c k , F . J . , 1 9 3 6 . G e o l o g y o f t h e n o r t h s h o r e o f L a k e A t h a b a s c A l b e r t a and S a s k a t c h e w a n : G e o l . S u r v . Can., Memoir 1 9 6 . B a s s a g e t , J . P . and Camps, P., 1973- R a p p o r t de f i n de m i s s i o n : U n p u b l i s h e d Mokta r e p o r t . B l a k e , D.A.W., 1 9 5 6 . G e o l o g i c a l n o t e s o n t h e r e g i o n s o u t h o f L a k e A t h a b a s c a : G e o l . S u r v . Can. P a p e r 5 5 - 3 3 . B e c k , L . S . , 1 9 7 0 . G e n e s i s o f u r a n i u m i n t h e A t h a b a s c a r e g i o n a n d i t s s i g n i f i c a n c e i n e x p l o r a t i o n : Can. I n s t . M i n . T r a n s v . L X X I I I , p . 5 9 - 6 9 . B e n c e , A . E . and A l b e e , A . L . , 1 9 6 8 . E m p i r i c a l c o r r e c t i o n f a c t o r s f o r t h e e l e c t r o n m i c r o a n a l y s i s o f s i l i c a t e s and o x i d e s : J o u r , o f G e o l . , v . 7 6 , p . 3 8 2 - 4 0 3 . B i n n s , R.A., 1 9 6 4 . Zones o f p r o g r e s s i v e r e g i o n a l m e tamorphism i n t h e W i l l y a m a c o m p l e x , B r o k e n H i l l d i s t r i c t , New S o u t h W a l e s : J o u r . G e o l . S o c . 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Les i n c l u s i o n s f l u i d e s dans l e s r o c h e s de l a s t r u c t u r e C a r s w e l l . R e s u l t a t s p r e l i m i n a i r e s : U n p u b l . r e p o r t . T o t h , J . , 1963. A t h e o r e t i c a l a n a l y s i s o f ground-water f l o w i n s m a l l d r a i n a g e b a s i n s : J o u r . Geophys. Res., v. 68, no. 16, pp. 4795-4812. T o u r e t , J . , 1971. Le f a c i e s g r a n u l i t e en Norvege M e r i d i o n a l e , 133 I . L e s a s s o c i a t i o n s m i n e r a l o g i q u e s : L i t h o s 4, p. 239-249. T o u r e t , J . , 1971b. Le f a c i e s g r a n u l i t e en No r v e g e M e r i d i o n a l e , I I . L e s i n c l u s i o n s f l u i d e s : L i t h o s 4, p.. 423-426. T r e m b l a y , L.P., 1972. G e o l o g y o f t h e B e a v e r l o d g e M i n i n g A r e a , S a s k a t c h e w a n : G e o l . S u r v . Can. Memoir 3 6 7 . T u t t l e , O.P. and Bowen, N.L., 1 9 5 8 . O r i g i n o f g r a n i t e i n t h e l i g h t o f e x p e r i m e n t a l s t u d i e s i n t h e s y s t e m NaAlSi308-KAlSi303 -Si0 2-H 20: G e o l . S o c ' Amer., Memoir 74. W a l l i s , 1 9 7 0 . A g e o l o g i c a l i n t e r p r e t a t i o n o f g r a v i t y and m a g n e t i c d a t a , n o r t h w e s t S a s k a t c h e w a n : Can. J o u r . E a r t h S c i . , 7, p. 8 5 8 - 8 6 8 . W a n l e s s , R.K., S t e v e n s , R.D., L a c h a n c e , G.R. and Edmonds, C M . , 1 9 6 8 . Age d e t e r m i n a t i o n s and g e o l o g i c a l s t u d i e s : G e o l . S u r v . Can., P a p e r 67-2, P a r t A, p . 84. W e t h e r i l l , G.W., 1 9 6 5 . G e o c h r o n o l o g y o f N o r t h A m e r i c a : N a t i o n a l Academy o f S c i e n c e s - N a t i o n a l R e s e a r c h C o u n c i l , P u b l i c a t i o n ' 1 2 7 6 . W i n k l e r , H.G.P., 1 9 6 7 . P e t r o g e n e s i s o f t h e m e t a m o r p h i c r o c k s : S p r i n g e r - V e r l a g , New Y o r k , I n c . Wood, B . J . and Banno, S., 1973« G a r n e t - o r t h o p y r o x e n e and o r t h o -p y r o x e n e - c l i n o p y r o x e n e r e l a t i o n s h i p s i n s i m p l e and complex s y s t e m s : C o n t r i b u t i o n s t o m i n e r a l o g y and p e t r o l o g y , 42, p. 109-124. Zen, E . , and Thompson, A.B., 1 9 7 4 . Low g r a d e r e g i o n a l metamor-p h i s m : M i n e r a l e q u i l i b r i u m r e l a t i o n s : i n A n n u a l r e v i e w o f E a r t h and P l a n e t . S c i . ( e d . F r e d D o n a t h ) , v . 2, New Y o r k : A n n u a l R e v i e w s I n c . 134 APPENDIX E l e c t r o n m i c r o p r c b e a n a l y s e s . The c h e m i c a l a n a l y s e s p r e s e n t e d i n t h i s t h e s i s were made u s i n g t h e A.R.L. "SEMQ" a u t o m a t i c p r o b e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . T h i s i n s t r u m e n t , has t h e c a p a b i l i t y o f m e a s u r i n g q u a n t i t a t i v e l y t h e i n t e n s i t i e s o f c h a r a c t e r i s t i c X - r a y r a d i a t i o n p r o d u c e d when an e l e c t r o n beam f o c u s e d t o s u b - m i c r o n s i z e s t r i k e s t h e p o l i s h e d s u r f a c e o f a m i n e r a l s p e c i m e n . A PDP 11 computer i n c o r p o r a t e d i n t h e equipment c o n t r o l s t h e s p e c t r o m e t e r s and r e d u c e s d a t a t o p r o d u c e c h e m i c a l a n a l y s e s . T h r e e X - r a y s p e c t r o m e t e r s a r e p r o v i d e d } e a c h e q u i p p e d w i t h a c r y s t a l c a p a b l e o f r e s o l v i n g l i n e s i n d i f f e r e n t p a r t s o f t h e X - r a y s p e c t r u m . The c r y s t a l s e mployed i n t h i s s t u d y were as f o l l o w s : o S p e c t r o m e t e r C r y s t a l Range(A) . E l e m e n t s 1 L i P 1 - 3 . 5 . F e , Ca, Mn, T i , C r 2 ADP 1 . 8 5 - 1 0 . 2 3 RbAP 4 . 6 - 2 5 . 3 Na, Mg, A l , S i D a t a r e d u c t i o n was a c c o m p l i s h e d by means o f a computer p r o g r a m i n BASIC l a n g u a g e , u s i n g t h e method o f Bence and A l b e e , 1968. . B r i e f l y , t h i s method a p p l i e s e m p i r i c a l l y d e r i v e d c o r r e c t i o n s t o t h e i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n o f e a c h e l e m e n t i n o r d e r t o compensate f o r a b s o r p t i o n o r f l u o r e s c e n c e p r o d u c e d by e a c h o t h e r e l e m e n t i n t h e sample. T h e s e c o r r e c t i o n s a r e a p p l i e d t o an- i n i t i a l e s t i m a t e (E^) o f t h e p r o p o r t i o n o f e a c h e l e m e n t i n t h e sample g i v e n by t h e r e l a t i o n : E = S X i ^ e f l s l t y o f X - r a y s f r o m sample , 1 i n t e n s i t y o f X - r a y s f r o m s t a n d a r d 135. where S i s t h e c o n c e n t r a t i o n o f t h e same e l e m e n t i n t h e s t a n d a r d . A p p l i c a t i o n o f t h e c o r r e c t i o n f a c t o r s p r o d u c e s a new s e t o f e s t i m a t e s (Ep) and t h e p r o c e s s i s r e i t e r a t e d u n t i l c o n v e r g e n c e • i s a c h i e v e d . I n p r a c t i c e , as o x y g e n does n o t p r o d u c e a d e t e c t a b l e e m i s s i o n l i n e , t h e a n a l y s e s a r e computed i n t e r m s o f s i m p l e o x i d e s r a t h e r t h a n e l e m e n t s . I r o n i s n o r m a l l y assumed t o be p r e s e n t as FeO. I f . e x c e s s o x y g e n i s p r e s e n t , e i t h e r i n H^O o r Fe-^Q^ ( a s s u m i n g o t h e r p o l y v a l e n t e l e m e n t s a r e p r e s e n t i n n e g l i g i b l e a m o u n t s ) , i t w i l l be r e f l e c t e d i n an a n a l y s i s t o t a l l i n g l e s s t h a n 100 p e r c e n t . S u b t r a c t i n g t h e t o t a l o b t a i n e d f r o m 100 t h e r e f o r e a l l o w s an e s t i m a t e o f t h e amount o f e x c e s s o x y g e n p r e s e n t , and t h i s c a n t h e n be r e c a l c u l a t e d as o r Fe203 ( b u t n o t b o t h ) . A d d i t i o n a l c o r r e c t i o n s c a n be made f o r a b s o r p t i o n o r f l u o r e s c e n c e due t o o x y g e n . A n h y d r o u s , u n o x i d i s e d s p e c i m e n s g e n e r a l l y g i v e a n a l y s e s t o t a l l i n g between 98.5$ and 101.5$. R e p e a t e d a n a l y s e s and a n a l y s e s o f s t a n d a r d s i n d i c a t e t h a t a c c u r a c y o f b e t t e r t h a n + 2$ o f t h e amount p r e s e n t o f e a c h o x i d e c a n be e x p e c t e d . I n unknown sa m p l e s where o x y g e n c o r r e c t i o n s must be a p p l i e d , no e s t i m a t e o f a c c u r a c y c a n be g i v e n , a s t h e t e s t o f t o t a l s a p p r o x i m a t i n g 100$ c a n n o t be u s e d . I n g e n e r a l , H^O c o r r e c t i o n s a p p e a r t o be l e s s s e n s i t i v e t o a n a l y t i c a l e r r o r t h a n t h e t y p i c a l l y s m a l l e r F e 2 0 3 c o r r e c t i o n s . Any e r r o r i n a n a l y s i s w i l l o b v i o u s l y d i r e c t l y a f f e c t t h e e x c e s s o x y g e n e s t i m a t e . Note t h a t a l l o x y g e n - c o r r e c t e d a n a l y s e s t o t a l 100$. A BASIC p r o g r a m was w r i t t e n by t h e a u t h o r f o r t h e computa-3+ t i o n o f a n a l y s e s , w a t e r and F e c o r r e c t i o n s and s t r u c t u r a l 13 6 f o r m u l a e . 137 P o t a s s i u r n - A r g o n A g e - d a t e s Sample No. M a t e r i a l A n a l y s e d P o t a s s i u m (%k) A r * 4 0 / T o t a l Ar^O A r * 4 0 ( 1 0 - 5 c c STP/g) A p p a r e n t age BH-74-169 B i o t i t e X = 6.14 t 0.19 (2 a n a l y s e s ) 0.994 8.107 x 10' 1.9 t 0.07 b . y . Sample No. M a t e r i a l a n a l y s e d P o t a s s i u m (%k) A r * 4 0 / T o t a i A r 4 0 A r * 4 0 / k 4 0 A p p a r e n t age C o n s t a n t s BH .74-169 A l t e r e d c o r d i e r i t e c o n t a i n i n g s e r i c i t e X = 1.94 t 0.01 (2 a n a l y s e s ) 0.967 6.889 x 1 0 " 2 915 t 3 5 m.y. X £ = 0.585 x 1 0 " 1 0 y r _ 1 Xa = 4.72 x 1 0 ~ 1 0 y r " 1 P K 4 0 / K = 1.19 x l O - 4 A r * 4 0 r e f e r s t o r a d i o g e n i c Ar 1"^ 

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