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Pasco Creek breccia, Horseshoe Bay, B.C. Von Rosen, G. E.A. 1966

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PASCO CREEK BRECCIA Horseshoe Bay, B. C. by G. E. A. von Rosen B.So., U n i v e r s i t y of B r i t i s h Columbia, 1962 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF ' THE REQUIREMENTS FOR THE DEGREE OF . Master of Science i n the Department of GEOLOGY -We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1966 In p r e s e n t i n g t h i s t h e s i s i n 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 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 , I agree 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 a g r e e 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 p u r p o s e 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 The U n i v e r s i t y o f B r i t i s h C olumbia Vancouver 8, Canada ABSTRACT An 800 f o o t - s q u a r e outcrop of a g n e i s s - b r e c c i a body a t Pasco c r e e k , 3 m i l e s n o r t h a l o n g the highway from Horseshoe Bay, B.C. was mapped, and i t s r e l a t i o n s h i p to the g n e i s s c o u n t r y r o c k s t u d i e d , A survey of the l i t e r a t u r e of b r e c c i a s i n c l u d e d i n the p r e s e n t t h e s i s was used as a b a s i s , t o g e t h e r w i t h the f i e l d d a t a , f o r a method of b r e c c i a f o r m a t i o n proposed as a r e s u l t of t h i s work. The body was found to be p i p e - l i k e i n shape, f o l l o w i n g d i r e c t i o n s of s t r u c t u r a l weakness i n the r o c k . The s i z e of b r e c c i a fragments and v a r y i n g amounts o f m a t r i x , as w e l l as the b o r d e r phase d i o r i t i c r o c k s were thought to have r e s u l t e d from s e v e r a l p r o c e s s e s a c t i v e i n the f o r m a t i o n of the b r e c c i a , Amorig these e x p l o s i v e a c t i o n of gases ahead of an i n t r u s i v e body, t o -g e t h e r w i t h f l u i d i z a t i o n of a m i x t u r e of these gases and s h a t t e r e d c o u n t r y r o c k are thought to be of prime importance i n the f o r m a t i o n of the b r e c c i a . i i . iv. TABLE OF CONTENTS I. INTRODUCTION 1 • Location and. Access 2 Physiography and Topography 2 Aknowledgements 3 II. GENERAL GEOLOGY OF THE AREA . III. GNEISS-BRECCIA PIPE 5 Fragments 6 1. Schistose fragments 6 2. Gneissic fragments 7 3. Gneiss blocks 7 4 . Granitic pebbles & Pegm. inter-clast material . 8 Matrix 10 Structures 11 1. Orientation of clasts 11 2. Deformation of clasts 11 3. Shear zones 12 4 . Gouge-filled faults 12 5. Joints 12 6. Foliation in border phases 12 7* Secondary foliation 13 Relation of Pipe to Country Rocks 13 1. Gneiss 13 2. Hornblendite 14 V TABLE OP CONTENTS Rel a t i o n of Pipe to Country Rocks (cont'd) 13 3. Border phases of b r e c c i a 14 a. P o r p h y r i t i c d i o r i t e 15 b. F o l i a t e d granite 16 c. Dikes 19 Summary of C h a r a c t e r i s t i c s of t h i s Breccia , 20 IV. EXISTING THEORIES OF BRECCIA FORMATION •„ ' 23 V. INTERPRETATION OF BRECCIA FORMATION ,28 Pre-breccia Geology 28 O r i g i n of Pasco Gneiss-Breccia Pipe 29 A p p l i c a t i o n of Data to Theory 31 Conclusion ; 35 v i . LIST OF PLATES AND FIGURES Figures 1 Index Map: Vancouver North, British Columbia .opp. p. 2 2a Outcrop Distribution Flap, Pasco Creek, Horseshoe Bay, British Columbia appendix 2b Map of Breccia Body, Pasco Creek, Horseshoe Bay, B.C. appendix 3 Diagrammatic Cross-sections, Pasco Creek appendix Plates l a . Mottled plagioclase in typical groundmass opp. p. 7 b. Contact of amphibolite clast with groundmass . opp. p. 7 c. Pyrite-epidote associations opp. p. 7 Ila. Pegmatitic space-fillings and rimmed clasts opp. p. 8 b. Pegmatitic plagioclase against matrix opp. p. 8 c. Recrystallized intruded matrix opp. p. 8 I l i a . Breccia near contact with diorite opp. p. 10 b. Breccia at contact with diorite opp. p. 10 TVa. Granitic pebbles in breccia opp. p. 11 b. Sub-horizontal orientation of fragments opp. p. 11 Va. Gneiss fragment with ellipsoidal shape opp. p. 12 ' b. Deformed elongate breccia clast; opp. p. 12 v i i . Va. Gneiss country rock .opp. p. 13 b. Augen gneiss i n country rock ©PP. P« 13 V i l a . Contact of p o r p h y r i t i c d i o r i t e with b r e c c i a ....... opp. p. 15 b. Contact of d i o r i t e dike with b r e c c i a opp. p. 15 V i l l a . P l a g i o c l a s e phenocryst opp. p. 16 b. A l t e r e d p l a g i o c l a s e phenocryst and groundmass ..... opp. p. 16 IXa. Pink, f o l i a t e d border phase rock opp. p. 17 b. S e r i o i t i z e d , f o l i a t e d border phase rock opp. p. 17 Xa. Zoned p l a g i o c l a s e phenocryst- crossed n i c o l s opp. p. 18 b. Zoned p l a g i o c l a s e phenocryst- p l a i n p o l a r i z e d l i g h t opp. p. 18 XIa. South contact of b r e c c i a on road opp. p. 20 b. Relations near north contact of b r e c c i a pipe opp. p. 20 INTRODUCTION B r e c c i a o c c u r r e n c e s have l o n g r e c e i v e d s p o r a d i c a t t e n t i o n by g e o l o g i s t s . T h i s i n t e r e s t has i n c r e a s e d s i n c e the r e a l i z a -t i o n of the a s s o c i a t i o n between ore d e p o s i t s and b r e c c i a p i p e s . Thus t h e r e have been many attemp t s a t d e t a i l e d d e s c r i p t i o n and c l a s s i f i c a t i o n of a l l types of b r e c c i a . B r e c c i a s , l i k e o t h e r o r e - l o c a l i z i n g s t r u c t u r e s , are not always m i n e r a l i z e d . A c c o r d i n g to Walker (1928) o n l y one p e r c e n t are m i n e r a l i z e d , and even t h e n , o n l y s m a l l p a r t s of these are o r e - b e a r i n g . They g e n e r a l l y have no g e n e t i c r e l a t i o n s h i p w i t h ore d e p o s i t s , but are merely phenomena of t e c t o n i c and v o l c a n i c a c t i v i t y . The term " B r e c c i a p i p e ' , a c c o r d i n g t o B r y n e r ( l 9 6 l ) , r e f e r s t o * . . . a c r u d e l y c y l i n d r i c a l , h i g h l y i n c l i n e d o r v e r t i c a l struC' t u r a l u n i t composed w h o l l y o r p a r t l y of a n g u l a r , or rounded, ro c k fragments w i t h o r w i t h o u t m a t r i x . " (p. 490) The purpose of t h i s t h e s i s i s t o d e s c r i b e a b r e c c i a m a s s i n c o u n t r y r o c k of g n e i s s , and to c l a s s i f y a»d a s c r i b e a mode o f f o r m a t i o n f o r the same. A r e v i e w of the l i t e r a t u r e on b r e c c i a s i n form o f a summary s h a l l be i n c l u d e d i n t h i s work v 2 Location and Access The l o c a t i o n o f t h e b r e c c i a i s s h o w n i n F i g u r e 1. I t c r o p s o u t on t h e s t e e p w e s t e r n a s p e c t o f t h e C o a s t M o u n t a i n s , w h i c h h e r e r i s e s r a p i d l y f r o m t h e s h o r e o f Howe S o u n d t o 3,992 f e e t , t h e s u m m i t o f B l a c k M o u n t a i n . Two e x c e l l e n t e x p o s u r e s o f t h e b r e c c i a a r e f o u n d i n t h e c u t s o f t h e V a n c o u v e r - S q u a m i s h h i g h w a y and t h e P a c i f i c G r e a t E a s t e r n R a i l w a y , ( f i g , 2) Much b l a s t e d r o c k c o v e r s t h e s l o p e b e t w e e n t h e h i g h w a y a n d t h e r a i l r o a d , o b s c u r i n g some o f t h e o u t c r o p . T h r e e m i l e s s o u t h a l o n g t h e h i g h w a y i s H o r s e s h o e B a y , . B . C . located some 20 m i l e s n o r t h o f V a n c o u v e r . The a r e a c a n easily be r e a c h e d b y b o a t f r o m H o r s e s h o e B a y , a n e c e s s i t y f o r s h o r e -. l i n e m a p p i n g . A p o w e r l i n e r i g h t - o f - w a y o f t h e B . C . H y d r o P o w e r A u t h o r i t y c u t s a c r o s s t h e map a r e a , u p h i l l o f t h e h i g h w a y , l e a v -i n g a t a n g l e o f b r a m b l e s a n d f e l l e d l o g s . Physiography and Tonography The a r e a i s t y p i c a l o f t h e r u g g e d C o a s t M o u n t a i n s S t e e p s l o p e s r i s i n g f r o m t h e s h o r e s o f f j o r d s , i n many p l a c e s t o 5,000 f e e t a l t i t u d e , make t r a v e l on t h e f o r e s t e d s l o p e s h a z a r d o u s , e s -p e c i a l l y b e c a u s e o f t h e s u d d e n p r e c i p i c e s , 1 0 ' s t o 100 ' s o f f e e t h i g h w e l l h i d d e n b y c o n i f e r o u s f o r e s t s a n d u n d e r g r o w t h . O u t c r o p s a r e s c a r c e , s t e e p a n d u s u a l l y c o v e r e d b y moss a n d l i c h e n s . G l a c i a l a n d f l u v i a l g r a v e l c o v e r s many p a r t s o f t h e a r e a , , a n d b e d s o f h i g h l y c o m p e t e n t ' f r e s h e t ' s t r e a m s i n c i s e s l o p e s and widen i n t o s m a l l g r a v e l beaches, t h a t dot the b l u f f y s hore. Acknowledgements I am i n d e b t e d to Dr, J.V. Ross who made me aware of t h i s t h e s i s t o p i c , and hel p e d o r g a n i z e the m a n u s c r i p t . F u r t h e r g r a t i t u d e i s due to Dr.. R.M, Thompson who hel p e d w i t h th© X-ray i d e n t i f i c a t i o n of some m i n e r a l s , and to Dr. K.C. McTaggart wh© a i d e d w i t h h e l p f u l . " d i s c u s s i o n s GENERAL GEOLOGY OF THE AREA Howe Sound, w i t h i t s s e v e r a l i s l a n d s , and the Vancouver A N o r t h map a r e a were mapped by Armstrong (1954) and l a t e r by Roddick (1965). P l u t o n i c r o c k of the Coast Mountain i n t r u s i v e s u n d e r l i e most of the a r e a . P r e p l u t o n i c and p o s t - p l u t o n i c meta-morphic r o c k s comprise the remainder of the a r e a , t o g e t h e r w i t h some sedimentary r o c k s of low metamorphic grade and v o l c a n i c s . P l u t o n i c r o c k s are m a i n l y d i o r i t i c ; they are thought to be c o r r e l a t a b l e on a h o r n b l e n d e - b i o t i t e p ercentage b a s i s r a t h e r , than a q u a r t z - f e l d s p a r r a t i o . (Armstrong, 1954) T h i s p o i n t of view i s supported by Roddick (1965) who m a i n t a i n s t h a t the p l u t o n i r o c k s of t h i s r e g i o n can b e s t be c l a s s i f i e d a c c o r d i n g t o the ma f i c s r a t h e r than q u a r t z - f e l d s p a r i n the c o n v e n t i o n a l manner. Such a c l a s s i f i c a t i o n i s based upon the o b s e r v a t i o n t h a t t h e r e i s a d e f i n i t e p r e v a i l i n g g e n e t i c o r d e r i n the m i n e r a l s , as f o l l o w s : p l a g i o c l a s e - h o r n b l e n d e - b i o t i t e - q u a r t z - p o t a s h f e l d s p a r . He a l s o c o n c l u d e s t h a t through the a c t i o n of s y n p l u t o n i c f a u l t s r o o f pendants are brought to p r e s e n t l e v e l s , whereby p l u t o n i c rock i s brought to c o n t a c t w i t h p r e - e x i s t i n g metamorphic rock i n which connate water has been r e t a i n e d . The r e s u l t t h a t p l u t o n i c r o c k s are c r e a t e d from metamorphic r o c k s i s i n f e r r e d i n the f o l l o w i n g sequence: hornblende d i o r i t e - b i o t i t e g r a n i t e - a l a s k i t e -p e g m a t i t e , d u r i n g r e t r o g r a d e metamorphism. Roddick (1965'p. 29) f i n d s t h a t the c o u n t r y r o c k s of the Pasco b r e c c i a b e l o n g to a s l i c e of metamorphic rock (Twin I s l a n d s Group) which he c a l l s the Horseshoe Bay Pendant. 5. Armstrong (1954) r e p o r t s t h a t the metamorphic r o c k s are m a i n l y , o r e n t i r e l y Mesozoic i n age. The Bowen I s l a n d group c o n s i s t i n g o f s l i g h t l y metamorphosed, f o l d e d b a s a l t i c and ande-s i t i c l a v a s , w i t h i n t e r b e d d e d t u f f s and sed i m e n t a r y r o c k s , i s T r i a s s i c ? and/or e a r l i e r . The Gambier group, T r i a s s i c ? and/or l a t e r , c o n s i s t s m a i n l y o f p y r o c l a s t i c ^ r o c k s and l a v a s , w i t h minor i n t e r b e d d e d s e d i m e n t a r y m a t e r i a l . On Mt. Brunswick a 6,000 f o o t s e c t i o n c o n t a i n i n g a t l e a s t 2,000 f e e t o f s l a t e , a r g i l l i t e , q u a r t z i t e , and a r k o s e , b e l o n g s to the l a t t e r , P r e - g r a n i t i c r o c k s , to which g n e i s s i c . c o u n t r y r o c k s of the t h e s i s a r e a are thought to b e l o n g , had been 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 the Bowen I s l a n d group m a i n l y on l i t h o l o g i c grounds. Roddick (1965) a s s e r t s t h a t the r o c k s around the t h e s i s a r e a b e l o n g to the P r e - J u r a s s i c Twin I s l a n d s group p o s s i b l y the same as the Bowen I s l a n d group of s i m i l a r age. He s t a t e s t h a t the Gambier group i s Upper J u r a s s i c i n age. S e v e r a l v a r i e t i e s of young d i k e s o c c u r i n the a r e a , of which he b e l i e v e s many to be s y n p l u t o n i c . L i t t l e can be s a i d about the s t r u c t u r e of the a r e a to s u p p l a n t what was mentioned above. Armstrong (1954) mentions t h a t t h e r e "... i s the n e a r - v e r t i c a l average d i p of the p l a n e of u n c o n f o r m i t y s e p a r a t i n g the Gambier group from the o l d e r r o c k s , i n d i c a t i n g t h a t the l a t t e r , i n c l u d i n g the o l d e r p l u t o n i c r o c k s , w e r e ; a l l s e v e r e l y deformed." (Armstrong, 1954, p. l ) TIT. GNEISS-BRECCIA PIPE -The b r e c c i a p i p e , as i t i s c u t by topography, i s about 800 6. feet^square, plunges steeply, and has cross cutting relations with the gneissic country rock. Fragments in the dark breccia, mainly of amphibolitic composition similar to mafic bands i n the gneiss, are surrounded by scant matrix, mostly of nearly microscopic grain si z e , between closely packed fragments. The breccia face looks uniformly dark from a distance of several feet because of the predominance of- mafic fragments. This uniformity changes to variety on closer inspection when smaller compositional and textural changes, as well as the existence of minor structures, become evident. Fragments - Fragment size varies from microscopic, to 30 feet thick; and 35 feet long; a common size i s about 1 to 2 inches wide and 2 to ? inches long. . Small fragments are usually bounded by planar sides p a r a l l e l to s c h i s t o s i t y , but fragments have been noted where the f o l i a t i o n i s oblique to the length. Only one f o l i a t i o n i s evident i n the fragments. Texture and composition of fragments vary from gran i t i c and schistose, to f e l s i c and mafic, respectively, although mafic schists are moat common. The g r a n i t i c and f e l s i c fragments are rare enough to arouse attention especially for their rounded shapes. The term g r a n i t i c pebbles has been applied to these. 1. Schistose fragments--Most common are amphibolite fragments. They are normally dark, tabular in shape, 1 to 4 inches long, variously textured from spotted porphyroblastio, to fine«g?ained P l a t e I f a c i n g page'; 7 xlO c..Association of p y r i t e with epidote near v e i n of f r e s h q u a r t z . schistose. In thin section s c h i s t o s i t y i s defined by orientation of fresh grains of green hornblende, between s l i g h t l y cloudy plagio-clase (oligoclase-andesine), and quartz in a granoblastic texture, (plate lb) Fragment boundaries are sharp and there i s no v a r i a t i o n i n texture or composition near the margins. Where s l i v e r s have separated from schist-fragments brown b i o t i t e i s found to replace 'green hornblende. Epidote, s e r i c i t e , c h l o r i t e , pyrite and mag-netite occur i n minor amounts. 2. Gneissic fragments—Somewhat larger fragments exhibit gneissose banding comprising amphibollte similar to the ones just described, and f o l i a t e d f e l s i c bands composed of plagioclase, with some quartz i n variable proportions. These larger sizes are not common, and one such fragment (Plate V) i s well rounded. It has several gneissic bands obliquely to the axis of i t s sub-e l l i p s o i d a l form. 3. Gneiss blocks—There are larger inclusions of gneiss in the breccia. Two have been observed. One occurs at the north contact along the ra i l r o a d and the other i s within the section along the highway. The l a t t e r i s about 25 feet wide, has a "fault contact with the breccia on the south side and a porphyritic septum on the north contact. ( f i g . 3) Its attitude d i f f e r s from the normal country rock trend, i n d i c a t i n g r e l a t i v e rotation of the inclusion within the breccia. Another block i s about the same size as the f i r s t and i s Plate I I f a c i n g page 8 a. Pegmatitic s p a c e - f i l l i n g and rimmed c l a s t s . xlO 8. wedged apart from the "breccia wallrock by tightly-packed breccia and some porphyry. Rotation relative to the country rock again is indicated by the change in strike, as well as the change in dip. . 4. Granitic pebbles and pegmatitic inter-clast material— Pebbles of foliated granitic material and white plagioclase inter-clast f i l l i n g s are infrequent but contrast noticeably with the dark, angular fragments of the rest of the breccia, (plates Ila, IVa&b) Granitic pebbles with e l l i p t i c a l cross-sections appear to by cylin-drical in shape. Their sides are parallel to their foliations which generally impart to the rock structural weakness in at least two direc-' . .. tions. Various shapes and sizes, of this rock type occur in several places in the breccia. In thin section i t is seen to contain quartz, plagio-clase, and brown and green biotite, sericite, chlorite, and epidote with pyrite. .Most of the plagioclase is cloudy with sericite and kaolin. Quartz is clear. The fabric is medium-grained, or finer. The texture in part is granophyric, especially in plagioclase crystals, but for the most part i t tends to be granoblastic. Cracks in crystals, as well as inter-granular boundaries are f i l l e d with sodic plagioclase and quartz. Undu-latory extinction is common, and the shadowy and indistinct contacts between many grains are due to joined rims of sodic plagioclase. These textures are interpreted as indications of•recrystallization of a foliated, but i n i t i a l l y granitic textured rock. The hea led c r a c k s , overgrown boundar ie s , and granophyric. tex ture are thought to he s i gns of e i t h e r magmatic processes.-'or p o s s i b l y replacement of these areas by more sod ic p l a g i o c l a s e , a c t i v e d u r i n g the l a t e r .magmatic stages when c r y s t a l l i z a t i o n had a l ready ensued. On weathered su r f a ce white space f i l l i n g s c o n t r a s t sha rp l y with the norma l ly da rk - g reen b r e c c i a . ( p l a te I i ) From a short d i s t a n c e the o v e r a l l aspect of the white areas i s that they are fragments wi th a v a r i e t y of shapes between rounded and angu la r . In d e t a i l they seem r a t h e r to be space f i l l i n g s and rims around other 'fragments. The con tac t s with ba s i c fragments are g r a d a t i o n a l i n some ca se s , sharp i n o t h e r s . There are s c h i s t o s e fragments f o l i a t e d wi th l a y e r s of . s i a l i c m a t e r i a l i n d i r e c t connect ion, wi th the white s p a c e - f i l l i n g s . In such fragments s c h i s t q s i t y tends to be masked by coa r se r f o l i a t i o n , and p l a i n white areas grade i n t o grey ones c o n t a i n i n g wisps of o r i e n t e d ma f i c s . In t h i n s e c t i o n • these white s p a c e - f i l l i n g s are no tab ly j pegmat.i t i c , composed o f . p l a g i o c l a s e (An^g) or q u a r t z , se ldomly i n te rg rown, but always more c o a r s e l y c r y s t a l l i z e d than the remainder of the rock . ( p l a te II b) C rys ta l ' 'marg ins are l i g h t l y , overgrown and shadowed. The white areas i n c l u d e c loudy g rano-b l a s t i c i n te rg rowths of p l a g i o c l a s e which are f i n e - g r a i n e d and d i s p l a y a f i s h - n e t t ex tu re under p l a i n l i g h t . Such.a tex ture i s caused by the c o n t r a s t between c louded g ra in s of a l t e r e d p l a g i o c l a s e and sod ic p l a g i o c l a s e rims around these g r a i n s . An an i n t e r p r e t a t i o n of these tex tu re s i t i s thought that the white areas represent m a t e r i a l i n the process of s o l i d i f y i n g P l a t e I I I f a c i n g page 10 Br e c c i a near contact with d i o r i t e . Note p o r p h y r i t i c matrix and loose packing. (Numbers indicate origin of thinsections) b. B r e c c i a near contact with d i o r i t e . Note p o r p h y r i t i c matrix, f l o a t i n g fragments, and s t a r t of t i g h t e r packing. (Numbers indicate origin of thinsections) 10. f r o m l i q u i d f o r m ; some w h i t e a r e a s a p p e a r to he f r a g m e n t s o f the s o l i d form w h i l e o t h e r s have shapes d e p e n d e n t on the o u t l i n e s o f f r a g m e n t s s u r r o u n d i n g them, thus i n d i c a t i n g a t l e a s t p l a s t i c i t y , i f not l i q u i d i t y of the q u a r t z - f e l d s p a r m e l t . M a t r i x - The m a t r i x of the b r e c c i a a t the c o n t a c t w i t h d i o r i t e d i f f e r s from, the i n t e r c l a s t m a t e r i a l w i t h i n the b r e c c i a . At the c o n t a c t , f r a g m e n t s a r e w i d e l y s p a c e d and s u r r o u n d e d by p o r p h y r i t i c d i o r i t e , whereas w i t h i n the b r e c c i a p i p e they, a r e t i g h t l y packed and cemented t o g e t h e r by t h i n seams o f s i a l i c m a t e r i a l . A t the p e r i p h e r y , a p s e u d o - a g m a t i t'e of p o r p h y r y , s u r r o u n d i n g b r e c c i a f r a g m e n t s , r e s t r i c t e d , to the c o n t a c t . ( p l a t e I I I ) In t h i n s e c t i o n t e x t u r e s of t h e p o r p h y r i t i c d i o r i t e m a t r i x b e l o n g i n g t o t h e b o r d e r p h a s e s of the b r e c c i a , w i l l l a t e r be de-s c r i b e d . The m a t r i x of the t i g h t l y - p a c k e d b r e c c i a w i t h i n the p i p e i s p r e s e n t o n l y as t h i n seams as most o f the .fragments a r e i n d i r e c t contact, w i t h each o t h e r . These a earns c o n s i s t of s m a l l a l t e r e d l i t h i c f r a g m e n t s , t h a t have p a r t l y l o s t t h e i r m a f i c m i n e r a l o r i e n t a t i o n ' , and q u a r t z - f e l d s p a r a r e a s . G e n e r a l l y - p l a g i o -c l a s e . q u a r t z , b i o t i t e , c h l o r i t e , e p i d o t e , p y r i t e , and a p a t i t e a r e P r e s e n t i n v a r i e d p r o p o r t i o n s . P I a g i o c l a s e i s o f t e n r i d d l e d w i t h i n t e r g r c w t h s of e p i d o t e and p y r i t e . Q u a r t z i s c l e a r . Some gra.no-p h y r i c i n t e r g r p w t h i s p r e s e n t . Green c h l o r i t e w i t h anomalous b i r e f r i n g e n c e i s a s s o c i a t e d w i t h brown b i o t i t e . T e x t u r e v a r i e s o v e r s h o r t d i s t a n c e s b e c a u s e the l i t h i c f r a g m e n t s t h e m s e l v e s d i f f e r s l i g h t l y i n f a b r i c . On the whole g r a n o b l a s t i c t e x t u r e w i t h g r a n i t i c t e x t u r e d p a r t s i s common. G r a i n b o u n d a r i e s a r e Plate IV a. G r a n i t i c pebbles i n b r e c c i a with some o r i e n t a t i o n of fragments. b. D e f i n i t e sub-horizontal o r i e n t a t i o n of a l l types of fragments. 11. overgrown. . ' The i n t e r p r e t a t i o n g iven f o r these t e x t u r a l r e l a t i o n s i s .that the matr ix of the t i g h t l y packed b r e c c i a c o n s i s t s mainly of f i n e g ra ined l i t h i c fragments. ' The presence of f r e s h , as we l l as disrnembere d f r a gm e n t s , s u gge's t s so r« e process of c o in m i nu t j. o n . Re cry s t a l 1 i'/.a t i on of both smal l whole ~ and dismembered fragments' i s i n d i c a t e d by t h e i r g r a n o b l a s t i c texture ' , g ranophyr ic i n t e r -growth in some p l a c e s , and overgrowths on g ra in boundar ies in n e a r l y a l l i n s t a n c e s , ' S t ruo tu re s - o t rup tu re s encountered d u r i n g mapping are the f o l l o w i n g : o r i e n t a t i o n of c l a s t s . w i th i n the. b r e c c i a , bending of fragments with shea r i n g ac ros s .'small f o l d s , l a r g e r s t r u c t u r e s such as shear zones, gouge - r i ch f au l to , '. j o i n t s , f o l i a t i o n of g r a n i t i c r ock s , gn e i ss o s i ty - o f ten accompanied by j o i n t i n g , and secondary f o l i a t i o n in the gneiss. 1. O r i e n t a t i o n of c l a s t s . - - D e f i n i t e a l ignment of the l ong axes of a l l types o f fragments i n a. s u b - h o r i zon ta l d i r e c t i o n occurs over a 150 f oo t s t r e t c h of the highway s e c t i o n . ( p l a te IV a, b) " e a r v e r t i c a l a l ignment of l ong axes w i th i n the' p l a n e . o f s e c t i o n was vaguely v i s i b l e at two l o c a l i z e d p o i n t s , but these may be f o r t u i t o u s l y s i m i l a r o r i e n t a t i o n s , whereas the r .ub-horizon.tal a l ignment in tha t s e c t i o n i s very l i k e l y the exp re s s i on ' o f some' c o n t r o l l i n g f a c t o r in the fo rmat ion of the b r e c c i a . 2.. P l a s t i c d e f o r m a t i o n . — M o s t fragments in the brecc ia , are p 1 anar, ' 'many are bent, • some of them are not. only ben t bu t have sheared as shown i n p l a t e Vb. Bent fragments are u b i q u i t o u s in Plate V facing page 12 t h e b r e c c i a . F l e x u r e i s e s p e c i a l l y n o ' t i c e a h l e n e a r the l i n e o f i m p i n g e m e n t o f two f r a g m e n t s . 3. S h e a r zones.-—One 35 f o o t - w i d e - s h e a r zone o c c u r s a t t h e s o u t h c o n t a c t of the b r e c c i a i n the r o a d s e c t i o n . The r o c k i n the zone i s - d a r k g r e e n , w i t h brown a r e a s , and i s f i n e g r a i n e d . The c e n t r a l p o r . t i o n o f t h e s h e a r zone i s i n c i s e d by. P a s c o c r e e k , w h i c h c u t s t h r o u g h a 1 i g h t g r e e n , f r i a b l e v e i n o f v e r y f i n e -g r a i n e d e p i d o t e s h o t t h r o u g h w i t h a n a s t o m o s i n g v e i n l e t s o f f u s e d q u a r t z . T h i s r a t h e r i m p e r m e a b l e , m y l o n i t i c zone r e p r e s e n t s the b o u n d a r y b e t w e e n b r e c c i a and c o u n t r y r o c k i n the r o a d s e c t i o n , 11 c o u I d not. he f o l i o w o d a l o n g s t r i k e i n e i 1;her d i r e c t i o n . The a t t i t u d e o f t h e f r a c t u r e a t the c e n t r a l p a r t of the z o n e , whi ch would he the l a t e s t l o c u s o f movement, i s 06*0°/85° SS. 4. Gouge f i l l e d f a u l t s a r e m a i n l y f o u n d i n the b r e c c i a , and t o a l e s s e r e x t e n t :i.n. t h e c o u n t r y r o c k s . S c a r p s a r e v a g u e l y s l i c k e n s i d e d , ' a n d i n - some c a s e s c o n s t i t u t e the f a c e o f t h e s e c t i o n o f trie r a i l r o a d ' c u t , o b s c u r i n g much o f t h e d e t a i l o f t h e r o c k . J u n c t u r e o f f a u l t s o f t e n r e s u l t s i n c r a c k l e and f r a g m e n t e d b r e c c i a . 5. J o i n t s .--A1 though, many j o i n t s . a r e f o u n d i n . ho t h c.oun t r y r o c k and b r e c c i a , o n l y t h o s e p a r a l l e l t o ' g n e i s s o s i t y ( l 3 5°/70°NE) and one s e t w h i c h a- w e a t h e r e d d i k e p a r a l l e l s (l50 o/70°N W; have b e e n m e a s u r e d . f\. F o l i a t i o n i n b o r d e r p h a s e s , - - G r a n i t i c r o c k s a t the s o u t h b o r d e r o f t h e b r e c c i a a r e f o l i a t e d . . Prom t h r e e o u t c r o p s , m e a s u r e -ments o f f o l i a t i o n were t a k e n and a r e shown i n f i g u r e 2b. The p o s s i b i l i t y e x i s t s t h a t i f f u r t h e r o u t c r o p s - were t o he f o u n d , t h e y Plate VI f a c i n g page 13 b. •Typical band with augen from gneiss. would suggest that the f o l i a t i o n hears some r e l a t i o n s h i p to the p ipe o u t l i n e r a t h e r than the trend of the country rock.. 7. Secondary f Q 1 i a t i o n . - - P l a t e VI shows d e f i n i t e secondary f o l i a t i o n in g n e i s s i c country rock . (175°/?) Th.e f o l i a t i o n d ip could not he measured. Only few hands i n the gne i ss show t h i s s t r u c t u r e and the s c a r c i t y of good outcrop reduces the chance of f i n d i n g more examples of secondary f o l i a t i o n . Relat ion, of P:iye to Country Rocks - Va r i ous types of country rock surround/ ' the b r e c c i a p i p e ; they can be d e s c r i b e d as f o l l o w s 1. G n e i s s . - - T h e gne i s s , i n d e t a i l , i s minute ly f o l i a t e d , and on o l a r g e r s ca l e consp i cuous l y banded, and j o i n t e d p a r a l l e l , to the major f o l i a t i o n . A secondary f o l i a t i o n i n the c o a r s e r maf ic l a y e r s i s ev ident i n some smoothly weathered ou tc rops . '•• , ( p l a t e VI a) Minute f o l i a t i o n s f r e q u e n t l y d i sappear i n medium gra ined migrnatized zones with i n d i s t i n c t boundar ie s . In these zones the maf ic m inera l s occur i n c l o t s and f o l l o w s w i r l s which have no r e l a t i o n to the genera l trend of the rock. I n c i p i e n t m igmat i za t ion of t h i s type crops out i n some a rea . I n f i n i t e v a r i a t i o n s i n t e x t u r a l and compos i t i ona l changes w i t h i n very shor t d i s t a n c e s are seen i n many of the ou tc rops . The r o c k - f o r m i n g minera l s are mainly p l a g i o c l a s e ( o l i o g o c l a s e -andes i ne ) , green hornb lende, q u a r t s , and b i o t i t e , and the present assemblage i n c l u d e s smal l q u a n t i t i e s of a l t e r a t i o n minera l s such as p y r i t e , e p i d o t e , c h l o r i t e , b i o t i t e , a p a t i t e , s e r i c i t e , and c a l c i t e , main ly i n v e i n l e t s . Textures vary from f i n e gra ined s c h i s t o s e , through a range of g ra in s i z e s , to p o r p h y r o b l a s t i c with xen.oblast ic augen of 14. f e l d s p a r between a l i g n e d m a f i c s . These e l o n g a t e mafic m i n e r a l s . are o f t e n bent around the l a r g e r c r y s t a l s . ( p l a t e VI b) S p o t t e d s c h i s t s commonly c o n t a i n p a r a l l e l e l o n g a t e c l o t s of hornblende c r y s t a l s a s s o c i a t e d w i t h p l a g i o c l a s e augen s e t i n a f i n e g r a i n e d s c h i s t o s e m a t r i x . 2. H o r n b l e n d i t e . — D a r k green h o r n b l e n d i t e crops out on a p o i n t of the s h o r e l i n e near a c a v e - l i k e i n d e n t a t i o n of one of the s m a l l bays. The r o c k has no d i r e c t i v e t e x t u r e o t h e r than a vague p a r a l l e l e l o n g a t i o n of- the p i t s which r e p r e s e n t weathered out c l o t s of m a i n l y brown b i o t i t e surrounded by green h o r n b l e n d e . Ridges around the p i t s are due to more r e s i s t a n t p l a g i o c l a s e and h o r n b l e n d e - r i c h a r e a s . S m a l l areas of s e r i c i t i z e d p l a g i o c l a s e and brown b i o t i t e cut the r o c k . The brown b i o t i t e i s s i m i l a r to t h a t i n c i p i e n t l y r e p l a c i n g some of the h o r n b l e n d e . A s e l v a g e of f r e s h p l a g i o c l a s e , s i m i l a r to t h a t i n the rock., b o r d e r s the v e i n s . C o n t a c t r e l a t i o n s of t h i s h o r n b l e n d i t e w i t h the g n e i s s are not known. The l a c k of b a n d i n g and the o t h e r w i s e homogeneous c c h a r a c t e r of the r o c k , t o g e t h e r w i t h the h i g h c o n t e n t of w e l l -c r y s t a l l i z e d hornblende s u r r o u n d i n g i n t e r g r a n u l a r patches of p l a g i o c l a s e c r y s t a l s , i n d i c a t e an igneous o r i g i n , r a t h e r than metamorphic. 3. B o r d e r phases of the b r e c c i a . — R o c k s , here c l a s s e d under b o r d e r phases of the b r e c c i a , appear to be i g n e o u s . i n o r i g i n and o c c u r near the p e r i p h e r y of the b r e c c i a , but they d i f f e r i n c o l o u r , t e x t u r e , and c o m p o s i t i o n . C o m p o s i t i o n v a r i e s from v e r y dark green d i o r i t e , through l i g h t e r c o l o u r e d igneous r o c k s to Plate VII facing page 15 15. l e u c o c r a t i c p i n k g r a n i t e s . a. P o r p h y r i t i c d i o r i t e . Both a t the b r e c c i a c o n t a c t s and w i t h i n the b r e c c i a mass are o c c u r r e n c e s of p o r p h y r i t i c d i o r i t e . Those w e l l w i t h i n the b r e c c i a are l e s s p l e n t i f u l and s m a l l e r than those a t the p e r i p h e r y . A l t h o u g h ;the m a f i c m i n e r a l c o n t e n t , as w e l l as the degree of p h e n o c r y s t development and the c o n t a c t r e l a t i o n s d i f f e r from one o c c u r r e n c e of d i o r i t e to the n e x t , t e x t u r a l and c o m p o s i t i o n s i m i l a r i t y s u g g ests common o r i g i n f o r a l l of them. The b e s t exposure of p o r p h y r i t i c d i o r i t e , o c c u r r i n g a t the n o r t h c o n t a c t of the b r e c c i a , d i s p l a y s r e l a t i o n s t y p i c a l of t h i s r o c k t y p e . F i g u r e .3 shows the c o n t a c t s of d i o r i t e w i t h g n e i s s i c c o u n t r y r o c k s to the n o r t h and b r e c c i a to the s o u t h . D i o r i t e s l i g h t l y c r o s s c u t s , the g n e i s s i c f o l i a t i o n , y e t seen from the d i s t a n c e of few tens of f e e t the c o n t a c t i s s e n s i b l y p a r a l l e l to the g n e i s s o s i t y . The d i o r i t e - b r e c c i a c o n t a c t i s sharp when seen from t e n f e e t , but somewhat g r a d a t i o n a l i n d e t a i l . P l a t e V I I d e p i c t s two v a r i a t i o n s i n b r e c c i a d i o r i t e c o n t a c t s , one at the p e r i p h e r y and one a t the d i k e w i t h i n . t h e b r e c c i a i t s e l f . The c o n t a c t a t the d i k e w i t h i n the b r e c c i a i s s h a r p ; the d i k e or apo p h y s i s b e i n g younger than the b r e c c i a . The p e r i p h e r a l c o n t a c t , on the o t h e r hand, grades over a w i d t h of 2 to 3 i n c h e s from t i g h t l y packed b r e c c i a on one s i d e to d i o r i t e d e v o i d of b r e c c i a on the o t h e r . P l a t e I I I shows t h a t the p o r p h y r i t i c m a t r i x of the g r a d a t i o n a l c o n t a c t , d e s c r i b e d above, o c c u r s both as m a t e r i a l w i t h few s c a t t e r e d Plate VIII fac ing page 16 x 10 b. Myrmekite in zoned plagioclase phenocryst. Typcia l groundmass. Note zoned crysta l at arrow, (cf. plate X fac ing p. 17) 16. f r a g m e n t a l i n c l u s i o n s , and as s m a l l pockets between fragments i n more t i g h t l y packed b r e c c i a . F u r t h e r m o r e , changes i n mafic m i n e r a l c o n t e n t and ph e n o c r y s t s i z e w i t h i n s h o r t d i s t a n c e s , are e v i d e n t i n the o u t c r o p and are shown i n the photographs. The f r e q u e n c y of the l a r g e p l a g i o c l a s e p h e n o e r y s t s decreases away from the d i o r i t e -b r e c c i a c o n t a c t . I t sh o u l d be p o i n t e d out t h a t t h e r e are o n l y few o c c u r r e n c e s of d i o r i t e w i t h i n the b r e c c i a body, i n c o n t r a s t to f r e q u e n t o c c u r r e n c e near the p e r i p h e r y . Dark green d i o r i t i c r o c k , found a t the n o r t h b r e c c i a margin i n the highway s e c t i o n ( e s p e c i a l l y on the west s i d e of the r o a d ) , e x h i b i t s c o n t a c t r e l a t i o n s which grade from g r e e n i s h d i o r i t e , through f o l i a t e d d i o r i t e , t o dark g n e i s s i c c o u n t r y r o c k . Because t h i s g r a d a t i o n o c c u r s over a few i n c h e s the c o n t a c t i s s e n s i b l y sharp. Of i n t e r e s t , however, i s the f a c t t h a t green h o r n b l e n d i c d i o r i t e o c c u r s i n c l o s e p r o x i m i t y to p o r p h y r i t i c l i g h t c o l o u r e d d i o r i t e ' , a l o n g the c o n t a c t w i t h the same g n e i s s i c c o u n t r y r o c k . b. F o l i a t e d g r a n i t e . P i n k and o t h e r l i g h t c o l o u r e d g r a n i t i c r o c k s near the south margin of the b r e c c i a are w e l l f o l i a t e d w i t h r e s p e c t to b i o t i t e and f e l d s p a r s . C o n t a c t s are not exposed but by t h e i r p r o x i m i t y to the b r e c c i a margin these f o l i a t e s are here grouped under b o r d e r phase r o c k s . I n t h i n s e c t i o n c o m b i n a t i o n t w i n n i n g (Ross, 1957), grano-p h y r i c t e x t u r e , and s e r r a t e d edges on q u a r t z g r a i n s would i n -d i c a t e igneous o r i g i n f o r t h i s r o c k t y p e . A l t e r a t i o n , z o n i n g , and r e c r y s t a l l i z a t i o n of p l a g i o c l a s e can be seen i n some s e c t i o n s . The p i n k f o l i a t e d g r a n i t i c r o c k s c o n s i s t of medium g r a i n e d i Plate IX facing page 17 17. o r t h o c l a s e , q u a r t z , and s o d i c p l a g i o c l a s e mixed i n v a r i o u s p r o p o r t i o n s w i t h c h l o r i t e , e p i d o t e and p y r i t e . A l t e r a t i o n of the f e l d s p a r s i s s t r o n g , masking t w i n n i n g and i n t e r n a l s t r u c t u r e of the c y r s t a l s . F o l i a t i o n of the f o c k ( p l a t e IX a) i s produced by the s e g r e g a t i o n of f e l d s p a r s i n e l o n g a t e zones s u b - p a r a l l e l w i t h s i m i l a r zones of q u a r t z . These are d i v i d e d by l o n g smears of c h l o r i t e and p y r i t e . C r y s t a l b o u n d a r i e s are i r r e g u l a r , s u t u r e d and shadowy as seen w i t h p o l a r i z e d l i g h t . Shadowy m a t e r i a l a t the c o n t a c t s of p h e n o c r y s t s w i t h the m a t r i x surround s m a l l patches of o p t i c a l l y d i s c o n n e c t e d fragments of m a t e r i a l s i m i l a r to the l a r g e r c r y s t a l s . O r t h o c l a s e r e p l a c e s c l o u d y f e l d s p a r , and q u a r t z o c c u r s a l o n g some a l b i t e c l e a v a g e p l a n e s . Quartz and o r t h o c l a s e have u n d u l a t o r y e x t i n c t i o n . Other f o l i a t e d b o r d e r phase r o c k s are s i m i l a r . I n t e r p r e t a t i o n of the t e x t u r e s , such as c o m b i n a t i o n t w i n n i n g , suggests an igneous o r i g i n f o r the f o l i a t e d g r a n i t e , w i t h con-c o m i t a n t s t r e s s a l i g n m e n t d u r i n g c r y s t a l l i z a t i o n of the m a t e r i a l . R e c r y s t a l l i z a t i o n a t some l a t e r t i m e , i n d i c a t e d by the h e a l e d b r e c c i a t e d c r y s t a l b o u n d a r i e s , may have o c c u r r e d a f t e r q u a r t z and o r t h o c l a s e were fragmented a l o n g t h e i r c r y s t a l b o u n d a r i e s . C h l o r i t e a l t e r a t i o n of b i o t i t e o c c u r r e d d u r i n g r e c r y s t a l l i z a t i o n . P o r p h y r i t i c t e x t u r e and l a c k of f o l i a t i o n d i s t i n g u i s h d i o r i t i c r o c k s near the n o r t h margin of the b r e c c i a . T e x t u r e and c o m p o s i t i o n of t h i s r o c k v a r y e s p e c i a l l y near the b r e c c i a c o n t a c t ( p l a t e I I I ) as d e s c r i b e d b e f o r e on page 14. In t h i n s e c t i o n the ro c k i s seen to c o n s i s t of q u a r t z , o l i g o -fac ing page 18 Andesine grain a l tered from the center. Crossed n i co l s . (note that the very center of the c r y s t a l i s more c a l c i c than the zone surrounding i t , - t h i s point i s seen i n Xb where the center i s of higher r e l i e f than the area surrounding i t ) Andesine grain a l tered from the center. Incl ined p la in polar ized l i g h t . 18. c l a s e , w i t h b i o t i t e , s e r i c i t e , c h l o r i t e , a p a t i t e , e p i d o t e and p y r i t e i n a g r a n o b l a s t i c m a t r i x s u r r o u n d i n g zoned p l a g i o c l a s e p h e n o e r y s t s . Zoning of s o d i c p l a g i o c l a s e i s complex, but i t i s t y p i c a l l y of the normal type w i t h a v a r i a t i o n from more c a l c i c a t the c e n t e r through two or t h r e e c o m p o s i t i o n a l zones, to more s o d i c a t the p e r i p h e r y . The p e r i p h e r a l zone, u s u a l l y v e r y s o d i c , i s i n many cases e n l a r g e d to j o i n w i t h p l a g i o c l a s e g r a i n s i n the: m a t r i x . I n some g r a i n s ( p l a t e X ) , however, c o m p o s i t i o n changes from c a l c i c ( a n d e s i n e ) a t the ve r y c e n t e r through s o d i c ( o l i g o -c l a s e ) around the c e n t e r , to more c a l c i c near the p e r i p h e r y , w i t h a s o d i c r i m w i t h the same c o m p o s i t i o n as the m a t r i x . The s o d i c r i m s n o r m a l l y are f r e e of a l t e r a t i o n , w h i l e the c e n t r a l p a r t i s v e r y c l o u d y . Some phenoerysts are r i d d l e d w i t h b l e b s of q u a r t z , r e s u l t i n g i n a g r a n o p h y r i c t e x t u r e . Some c r y s t a l s or c r y s t a l fragments i n the m a t r i x are j o i n e d to,form l a r g e r ones. Twinned p h e n o c r y s t fragments may have overgrowths of untwinned m a t e r i a l , e n l a r g i n g them to e u h e d r a l p l a g i o c l a s e pheno-e r y s t s . I n g e n e r a l c r y s t a l s i n the m a t r i x are o f t e n enveloped by the o u t e r zones of p h e n o e r y s t s and j o i n e d t o g e t h e r to form l a r g e r ones. P l a t e I I c shows a g r a i n i n the top r i g h t hand c o r n e r which i s a c o m b i n a t i o n of two s m a l l e r g r a i n s ; a r e a c t i o n r i m s urrounds b o t h . P l a t e V I I I a,b d e p i c t s two more p o s s i b l e cases of t h i s phenomenon. I n t e r p r e t a t i o n of the z o n i n g i n these p l a g i o c l a s e c r y s t a l s , depending on whether i t o c c u r r e d d u r i n g o r a f t e r c r y s t a l l i z a t i o n , c o u l d be t w o - f o l d . D u r i n g c r y s t a l l i z a t i o n a change i n environment 19. of the melt would r e s u l t i n c o r r o s i o n and subsequent rimming of c r y s t a l s ; i t i s b e l i e v e d t h a t t h i s manner of z o n i n g accounts f o r a g r e a t p r o p o r t i o n of such t e x t u r e s i n these b o r d e r phase r o c k s . The second manner, t h a t of p o s t - c r y s t a l l i z a t i o n r e p l a c e m e n t ; O f p a r t s of c r y s t a l s by more s o d i c p l a g i o c l a s e , i s p o s s i b l e e s p e c i a l l y i n a case as shown i n p l a t e X a & b. The andesine g r a i n i n q u e s t i o n r e p r e s e n t s a v e r y s m a l l p r o p o r t i o n of the r o c k , t h i s i s e v i d e n t from p l a t e I I I b when i t i s r e a l i z e d t h a t o n l y one such c r y s t a l was o b s e r v e d , out of f i v e s e c t i o n s . The c e n t r a l p a r t of the c r y s t a l i s c a l c i c , whereas the zone next to the c e n t e r i s s o d i c h a v i n g a c o m p o s i t i o n s i m i l a r to the ground mass. No p a t h -ways f o r s o l u t i o n exchange are e v i d e n t i n the s e c t i o n , but such s t r u c t u r e s may pass through o t h e r p a r t s of the c r y s t a l . S o d i c a l t e r a t i o n of t h i s type i s thought to be r e l a t e d to the rimming of p l a g i o c l a s e p h e n o c r y s t s a t t e n d a n t w i t h f o r m a t i o n of the grano-b l a s t i c m a t r i x . Fragments and shadows around h e a l e d fragments of c r y s t a l s are b e l i e v e d to i n d i c a t e a p r o c e s s whereby an i g n e o u s , somewhat m e c h a n i c a l l y g r a n u l a t e d m a t r i x e n c l o s i n g p h e n o c r y s t s was r e c r y s t a l l i z e d r e s u l t i n g i n the p r e s e n t t e x t u r e and o v e r -growth on c r y s t a l s . a. D i k e s . One dark brown, h i g h l y weathered a n d e s i t e d i k e c u t s b r e c c i a i n both the r a i l r o a d and the highway s e c t i o n s and seems to f o l l o w a j o i n t s e t a t 050°/70°N. C a v i t y f i l l i n g s , r e s e m b l i n g amygdules, o c c u r near the n o r t h s i d e of t h i s 4-6 f o o t wide d i k e . I n p o l i s h e d hand specimen i t ' i s seen to have a f e l t y t e x t u r e of t h i n p l a g i o c l a s e l a t h s , except near c l u s t e r s of d i k t y -t a x i t i c c a v i t i e s f i l l e d w i t h w h i t e m a t e r i a l . Areas around these Plate XI f a c i n g page 20 a.. South contact at road of br e c c i a and gneiss. Note the 'pink' dike i n the shear zone, and the f a u l t to the r i g h t of i t . (dotted line approximates boundary of breccia; dashed line outlines pink dike; wavy symbol indicates gouge-rich fault zone and Pasco creek) b.. Contact near north end of road section of p o r p h y r i t i c d i o r i t e over b r e c c i a . Note small d i o r i t e 'dike'. 20. c a v i t i e s are s o l i d l y cemented by a secondary m i n e r a l . I n t h i n s e c t i o n the r o c k i s composed of f r e s h a n d e s i n e , o r t h o p y r o x e n e , c h l o r i t e , and p y r i t e , and the d i k t y t a x i t i c c a v i -t i e s are f i l l e d by q u a r t z , c a l c i t e , c h l o r i t e , c h a b a z i t e , and tho m s o n i t e . A dark green, f i n e - g r a i n e d , f r e s h a n d e s i t e d i k e (not shown on the map) c u t s the b r e c c i a i n both highway and r a i l r o a d s e c t i o n s . The c o n t a c t i s v e r y sharp and the d i k e i s f l i n t - l i k e due to c h i l l -i n g a t the margin. P y r i t e , c h l o r i t e , and e p i d o t e occur i n c r a c k s . A d i o r i t e d i k e c u t s the b r e c c i a near the n o r t h highway con-t a c t . I t i s an ap o p h y s i s of the p o r p h y r i t i c d i o r i t e mass of the , b r e c c i a b o r d e r phases. ( p l a t e X i ) Sma l l w h i t e d i k e s c ut the g n e i s s i c c o u n t r y r o c k s i n s e v e r a l l o c a t i o n s . T h e i r a t t i t u d e s are m a i n l y s u b - p a r a l l e l to the g n e i s s o s i t y . One such d i k e was found to be e x t r e m e l y f i n e g r a i n e d , somewhat p o r p h y r i t i c w i t h medium-grained c r y s t a l s of q u a r t z and f e l d s p a r , and a m a t r i x of p l a g i o c l a s e (about An. c) and q u a r t z c r y s t a l s w i t h l o n g i t u d i n a l o r i e n t a t i o n s e t i n a p l i t i c groundmass. The p h e n o c r y s t s are rimmed by s l i g h t l y c l e a n e r m a t e r i a l which a l s o e n v elops g r a i n s of the c l o u d y groundmass. '; U n d u l a t o r y e x t i n c t i o n i s common. C l o t s of c h l o r i t e and s e r i c i t e c o u l d have been d e r i v e d from the w a l l r o c k s . F r a c t u r e s are f i l l e d w i t h s e r i c i t e and p o s s i b l y l i m o n i t e . ' Summary of C h a r a c t e r i s t i c s of t h i s B r e c c i a - The f o l l o w i n g . p o i n t s c h a r a c t e r i z e the b r e c c i a and must be e x p l a i n e d by a t h e o r y of the mode of f o r m a t i o n of the b r e c c i a a t Pasco c r e e k . 21. 1. P r o x i m i t y of the b r e c c i a to the c o n t a c t of a mass of d i o r i t e i n t r u s i v e i n t o c o u n t r y r o c k . 2. G e n e r a l shape of the b r e c c i a body w i t h n e a r l y square p l a n and s t e e p l y d i p p i n g s i d e s . . 3. S u b p a r a l l e l i s m of the b r e c c i a o u t l i n e on two s i d e s w i t h the g n e i s s o s i t y of the c o u n t r y r o c k . 4. E x i s t e n c e of fragments of c o u n t r y r o c k w i t h o n l y s m a l l q u a n t i t i e s of f o r e i g n m a t t e r . 5. U n i f o r m s i z e , t a b u l a r shape, and e l o n g a t i o n p a r a l l e l to s c h i s t o s i t y , of the fragments. 6. R e l a t i v e r o t a t i o n of l a r g e and s m a l l fragments w i t h r e -s p e c t to c o u n t r y r o c k s . 7. H o r i z o n t a l a l i g n m e n t of t a b u l a r fragments i n some s m a l l a r e a s . 8. Loose p a c k i n g of the b r e c c i a fragments near the c o n t a c t of the b r e c c i a w i t h p o r p h y r i t i c d i o r i t e , i n c o n t r a s t t o the t i g h t l y -packed fragments w i t h i n the b r e c c i a . 9. Presence of s c a n t r e c r y s t a l l i z e d m a t r i x w i t h i n the t i g h t l y -packed p o r t i o n of the b r e c c i a , and p l e n t i f u l p o r p h y r i t i c m a t r i x near the l o o s e l y - p a c k e d b o r d e r of the b r e c c i a . 10. M a t r i x c o n s i s t i n g of f i n e l i t h i c f ragments. 11. S m a l l w h i t e p e g m a t i t i c areas w i t h i n the b r e c c i a . 12. G r a n i t i c pebbles i n the b r e c c i a . 13. B e n d i n g and d e f o r m a t i o n of fragments. 14. Occurrence of p o r p h y r i t i c d i o r i t e as a s k i n between c o u n t r y r o c k and b r e c c i a ^ p a r t l y e n g u l f i n g l o o s e fragments a t the b r e c c i a c o n t a c t . 22. 15. Small bodies of. f o l i a t e d granitic rock at the periphery of the breccia. .16. Presence of d i o r i t e within the breccia. 17. Signs of metamorphism in both fragments and matrix. i . • • . • • 23. IV. EXISTING THEORIES OF BRECCIA FORMATION Much has been w r i t t e n about b r e c c i a and accounts i n the l i t -e r a t u r e have, up to a few y e a r s ago, Often been based on b r e c c i a -ore body r e l a t i o n s h i p s . However, a l t h o u g h i n d i v i d u a l d e s c r i p t i o n s are c l e a r , the mode of f o r m a t i o n , and the r e l a t i o n s p a r t i c u l a r to the mapped l o c a l i t y u n d e r s t o o d , the .general a s p e c t of the problem of c o r r e l a t i n g and c l a s s i f y i n g b r e c c i a t y p e s of w i d e l y d i f f e r e n t c h a r a c t e r i s t i c s and o c c u r r e n c e s has o n l y been d e a l t w i t h i n the l a s t few y e a r s . An attempt a t " C l a s s i f i c a t i o n of V o l c a n i c B r e c c i a s " was made by F i s h e r ( i 9 6 0 ) , wherein he groups v o l c a n i c b r e c c i a s i n t o t h r e e major c a t e g o r i e s baaed ...on the p r o c e s s of f r a g m e n t a t i o n . He a l s o uses the term v o l c a n i c b r e c c i a "... as a g e n e r a l term a p p l y i n g to a l l c o a r s e - g r a i n e d r o c k s composed of a n g u l a r v o l c a n i c f r a g m e n t s . " ( i 9 6 0 , p. 973) Wright and Bowes (1963) c o u n t e r the proposed c l a s s i f i c a t i o n , p o i n t i n g out s i x i n a d e q u a c i e s and i n c o n s i s t e n c i e s , and c o n s i d e r i t " . . . n e i t h e r c o n s i s t e n t nor workable and submit the f o l l o w i n g d i s c u s s i o n of n o n - p y r o c l a s t i c v o l c a n i c b r e c c i a s , t h e i r nomenclature, and c l a s s i f i c a t i o n . (1963, p. 79). They f i n d t h a t f o u r c r i t e r i a have commonly been used i n the c l a s s i f i c a t i o n of v o l c a n i c b r e c c i a s , namely, cause, environment, l o c a t i o n , and r o c k - t y p e . These c r i t e r i a s h o u l d be used i n com-b i n a t i o n . A c c o r d i n g to them, of th e s e f o u r c r i t e r i a , cause of f r a g m e n t a t i o n can o n l y be s t a t e d d e f i n i t e l y i n the case where there-hag been l i t t l e 24. t r a n s p o r t of b r e c c i a t e d m a t e r i a l ; when a mass of b r e c c i a t e d m a t e r i a l has been c a r r i e d somewhere e l s e , i t i s d i f f i c u l t to e x p l a i n the i n i t i a l b r e c c i a t i o n , and thus c l a s s i f i c a t i o n a c c o r d i n g to the type of"environment i s more s i g n i f i c a n t . L o c a t i o n of the b r e c c i a has been used o n l y i n c e r t a i n i n s t a n c e s , i t s use c o u l d be a v o i d e d . A c l a s s i f i c a t i o n based on rock type i s l e a s t open to c o n t r o -v e r s y f o r i t s g e n e r a l n a t u r e , s i n c e the same type of r o c k can be b r e c c i a t e d i n many d i f f e r e n t ways. B r y n e r ( l 9 6 l ) d i v i d e s . m i n e r a l i z e d r o c k fragment columns i n t o two main groups, p r e - h y d r o t h e r m a l s t r u c t u r e s and c o h y d r o t h e r m a l s t r u c t u r e s ; t h i s s t a n d p o i n t i s . b a s e d on. m i n e r a l o g y and. economic geology. "From th e s t a n d p o i n t of o r i g i n , the b r o a d e s t c l a s s i f i c a t i o n would d i s t i n - g u i s h b r e c c i a s on the b a s i s of the main t r a n s p o r t of f r a g m e n t s . ..Such movement of fragments a r e . e i t h e r (a) i n t r u s i v e " , o r (b) c o l l a p s e , and c o u l d be (c) o s c i l l a t o r y , a c o m b i n a t i o n of (a) and ( b ) . He has the f o l l o w i n g e x p l a n a t i o n s . . I n t r u d e d f r a g m e n t s , ^ w r i t e s B r y n e r ( l 9 6 l ) , " g e n e r a l l y are i m p e l l e d by magma, or ...el se by gaseous or aqueous f l u i d s t h a t a r i s e from the magma or are m o b i l i z e d by magmatic h e a t . " (p. 505). C o l l a p s e can be e f f e c t e d by numerous means. S l a c k e n i n g of i n t r u s i v e a c t i v i t y , t h r ough w i t h d r a w a l of magma f o r example, would reduce the' s t a b i l i t y of the r o o f r o c k s by l o w e r i n g the p r e s s u r e from below. M e l t i n g of rock by gases compressed by an a d v a n c i n g magma column or s t r e t c h i n g of the c r u s t caused by dom-i n g may cause c o l l a p s e . F u r t h e r causes can be s o l u t i o n of l i m e -stone o r gypsum by water, s h r i n k a g e because of c o r r o s i o n by gase-25. ous or hydrothermal f l u i d s , or shrinkage due to c o o l i n g of int r u d e d rock, or c r y s t a l l i z a t i o n . C r a c k l e b r e c c i a s would, be- the . r e s u l t of e i t h e r i n t r u s i o n , c o l -llapse,, or o s c i l l a t i o n , i f any of these methods were i n c i p i e n t . Johnston and Lowell ( l 9 6 l ) , having s t u d i e d the l i t e r a t u r e on the problem of the o r i g i n and c l a s s i f i c a t i o n of these bodies of f r a c t u r e d rock,, concluded that "The o r i g i n of most 'of the p i p e -l i k e d e p o s i t s has been explained by the f o l l o w i n g t h e o r i e s or by m o d i f i c a t i o n and/or combination of them..." (p. 931 )•» and they l i s t the f o l l o w i n g : ( l ) e x p l o s i o n ; ( 2 ) t e c t o n i c ; ( 3 ) igneous i n t r u s i o n ; (4) f l u i d i n t r u s i o n ; ( 5 ) s o l u t i o n and replacement; ( 6 ) m i n e r a l i z a t i o n s t o p i n g ; and ( 7 ) shrinkage. Reynolds (1954) has int r o d u c e d f l u i d i z a t i o n i n t o the l i s t of g e o l o g i c a l processes. There are s i m i l a r i t i e s between ( l ) ex p l o s i o n mentioned above, (8) f l u i d i z a t i o n , and (4) f l u i d i n t r u -s i o n . (1) e x p l o s i o n due to blow-outs of gas a s s o c i a t e d with v o l c a n i c a c t i v i t y produce diatremes which are openings i n many cases across s t r a t i f i c a t i o n . These holes are subsequently f i l l e d with broken v o l c a n i c m a t e r i a l or country rock. ( 2 ) elongate b r e c c i a t e d bodies of rock may form at the i n t e r -s e c t i o n of f a u l t s and shear zones, or at bends i n f a u l t s , -these are t e c t o n i c b r e c c i a s . ( 3 ) p h y s i c a l f o r c e of i n t r u s i o n r e s u l t s i n formation of b r e c c i a columns or d i k e s . (4) f l u i d i n t r u s i o n of fragments broken off the s i d e s of the conduit somewhere below by magmatic gases would cause them to be 26. " . . . f o r c e d upward i n t o r o c k s above by a r e l a t i v e l y t h i c k mud." (Farmin, 1934, p. 370). (5) c o r r o s i o n of s h a t t e r e d r o c k by a s c e n d i n g s o l u t i o n s f a c i l i -.._ts,tes. .rounding of fragments and i n c r e a s e of open space, which sub-s e q u e n t l y may be r e f i l l e d through :the p r e c i p i t a t i o n of m i n e r a l s . (6) Locke's (1926) m i n e r a l i z a t i o n s t o p i n g t h e o r y i s s i m i l a r to the b l o c k - c a v i n g method of m i n i n g , -whereby "...removal of roc k a l o n g t r u c k c h a n n e l s by r i s i n g s o l u t i o n s d u r i n g an e a r l y stage of t h e i r a c t i v i t y , c o l l a p s e and b r e c c i a t i o n of the roc k thus l e f t u n s u p p o r t e d , and d e p o s i t i o n of ore: and gangue m i n e r a l s i n the b r e c c i a t e d mass." (p. 43l)» was. the p r o c e s s . (7) c o n t r a c t i o n on c r y s t a l l i z a t i o n ^ w i t h an e s t i m a t e d 10 p e r c e n t volume s h r i n k a g e i s r e s p o n s i b l e f o r much f r a c t u r i n g and b r e c c i a t i o n i n c e r t a i n - s t o c k s and c h o n o l i t h s , a c c o r d i n g to H u l i n (1948). VAn : i n c r e a s e i n volume w i t h a t t e n d a n t open spaces would r e s u l t . (8) f l u i d i z a t i o n i s a term t h a t o r i g i n a t e d - i n i n d u s t r y f o r a p r o c e s s by which g r a i n - s i z e d p a r t i c l e s c o u l d be s e t i n t o t u r b u -l e n t motion by gas s t r e a m i n g through a bed of them c a u s i n g them to f l o a t i n and w i t h the gas, thus p e r m i t t i n g t h e i r t r a n s p o r t by the movement of the gas. As a g e o l o g i c p r o c e s s i t i s v i s u a l i z e d as a c t i v a t i n g and moving b r e c c i a f r a g m e n t s , and may w e l l f u r n i s h the c h u r n i n g and c o r r o s i v e a c t i o n i n d i c a t e d by f o r example pebble d i k e s . To i n i t i a t e the p r o c e s s , upward s t r e a m i n g gaseous f l u i d s pass t h r o u g h . p r e - f r a c t u r e d r o c k u n t i l the m i x t u r e of r o c k fragments ;and-, gas: a c h i e v e a s t a t e of f l u i d i t y , the a c t i v a t e d fragments are then m o b i l e . Reynolds (1954) s t a t e s t h a t "...The a s s o c i a t i o n of t u r b u l e n t f l o w s t r u c t u r e w i t h abraded and w e l l - r o u n d e d rock f r a g -27. ments which have not a p p a r e n t l y - b e e n t r a n s p o r t e d away from t h e i r source r o c k s , t o g e t h e r w i t h l a c k of g r a d i n g of the fragments c o n c e r n e d , and the p o s s i b l e presence of d r u s e s . . . " (p. 579) are c r i t e r i a f o r gas t r a n s p o r t . F l u i d i z a t i o n of a b r e c c i a u ndoubtedly c o u l d a l s o produce s e l e c t i v e t r a n s p o r t a t i o n , a c c o r d i n g to fragment s i z e , a l l o w i n g l a r g e r fragments t o descend and s m a l l e r ones to r i s e w i t h i n the stream, r e l a t i v e to the o t h e r f ragments. B r y n e r ( l 9 6 l ) a s s e r t s t h a t the l a c k of f u s e d m a t r i x , which i s n o r m a l l y found i n i n t r u s i o n b r e c c i a s , i n pebble d i k e s " . . . i n -d i c a t e s an aqueous or gaseous medium o f t r a n s p o r t a t i o n f o r the p e b b l e s , and the rounding-.of t h e . p e b b l e s show t h a t a t t r i t i o n and p o s s i b l y c h e m i c a l c o r r o s i o n took p l a c e . " (p. 499). In c o n n e c t i o n w i t h gases, W i l l i a m s (193&) e x p l a i n s t h a t p r e s s u r e from an ad v a n c i n g magma column, e x p l o s i v e v o l c a n i s m b e i n g the d r i v i n g f o r c e , may form d i a t r e m e s i n the f o l l o w i n g manner, "... R i s i n g gases ' l u b r i c a t e ' the r o o f r o c k s ; p u l s a t i n g magma b r e c c i a t e s the c o v e r ; r e p e a t e d e x p l o s i o n s comminute the cap. r o c k s so t h a t f r o t h i n g magma i s i n t i m a t e l y mingled w i t h them and, near s u r f a c e , steam e r u p t i o n s caused by h e a t i n g of ground water f a c i l i t a t e the d r i l l i n g p r o c e s s w h i l e s l u m p i n g of s l a b s from the c o n d u i t w a l l s e n l a r g e s the c o n d u i t . " (p. 316). D e s p i t e the f r e q u e n t r e f e r e n c e i n the l i t e r a t u r e to v o l c a n i c o r i g i n and e x p l o s i v e a c t i v i t y , M c K i h s t r y (1955) f i n d s t h a t "... the r o l e of v o l c a n i c e x p l o s i o n i n c r e a t i n g o r e b e a r i n g p i p e s seems much l e s s i m p o r t a n t - t h a n an u n c r i t i c a l r e a d i n g of the l i t e r a t u r e 28. might suggest., (p. 214). .r Sedimentary b r e c c i a s must be added to t h i s d i s c u s s i o n f o r the sake of completeness. T h i s rock type can.be d e r i v e d from t a l u s and o t h e r s u b - a r e a l o r sub-aqueous c l a s t i c a c c u m u l a t i o n s . These, on b e i n g covered by more m a t e r i a l , and i n c o r p o r a t e d i n the s e d i m e n t a r y s e c t i o n , which i n t u r n w i l l e v e n t u a l l y be meta-morphosed, d i f f e r , g r e a t l y from b r e c c i a s .derived i n the ways mentioned above.' I f the metamorphism were s t a t i c r a t h e r than d i s l o c a t i o n , such a t a l u s b r e c c i a would remain undeformed, the fragments h a v i n g j t h e i r o r i g i n a l a n g u l a r shapes and a s s o r t e d s i z e s . The m a t r i x " w o u l d tend to be h o r n f e l s i c and p l e n t i f u l , depending on the n a t u r e of the s e d i m e n t a r y b r e c c i a and i t s - h i s t o r y . V. INTERPRETATION OF BRECCIA FORMATION In a d d i t i o n to., the d a t a summarized on the geology of t h i s b r e c c i a , i t . i s n e c e s s a r y to have an u n d e r s t a n d i n g of the g e n e r a l p r e - b r e c c i a g e o l o g y , a s - d e r i v e d -from g e o l o g i c a l r e p o r t s , i n o r d e r to u n d e r s t a n d any i n t e r p r e t a t i o n of b r e c c i a f o r m a t i o n . P r e - B r e c c i a Geology - S t e e p l y d i p p i n g g n e i s s , p o s s i b l y d e r i v e d from s t r o n g l y f o l d e d , metamorphosed, and g r a n i t i z e d i n t e r b e d d e d t u f f s and sediments (Armstrong, 1954) i s thought to have comprised the g e o l o g i c a l s e t t i n g of the a r e a . Emplacement of the v a r i o u s g r a n i t i c Coast i n t r u s i o n s of the Coast Mountains may have been the cause of f o l d i n g and metamorphism observed i n the r o c k s . Mag-m a t i c a c t i v i t y a t t e n d a n t w i t h the emplacement of d i o r i t e - g r a n o d i o r i t e 2 9 . of the Coast Intrusives i s thought to have supplied heat, gases, and magma that were active in breccia formation. Origin of Pasco Gneiss-Breccia Pipe - The chara c t e r i s t i c s of the breccia summarized previously should be explicable by a theory of breccia emplacement.- Realizing t h i s , one may f i r s t l y propose the mechanism of emplacement and then ascertain how each of the char a c t e r i s t i c s may be related to the theory. The emplacement of\the breccia must be envisaged as having taken place during the lat_p.r.-S_tages of one of the; periods of a c t i v i t y of the Coast intrusions. The schi s t s , steeply dipping due to previous folding, have already been metamorphosed to gneiss-migmatites and jointed.. With these basic assumptions in. mind one must then imagine the action of gases r i s i n g from a heat source at depth ahead of'' advancing magma. Explosive action and pulsating pressures due to the plutonic a c t i v i t y shatters parts of the roof rock especially along pronounced gneissosity and j o i n t i n g . Intersections of major fractures provide channel ways for the advancing gas ahead of the magma. This shattering of the country rock, followed by in-streaming of the gas into the fractures ultimately results in a state of f l u i d i z a t i o n . During f l u i d i z a t i o n the fragments, large and small, grain sized to block sized, are envisaged as being in a state of suspension i n the agitated mixture of frag-ments and gas. If this process continues, abrasion by sand blast-ing of the larger fragments occurs; i n this case the sc h i s t o s i t y and the close j o i n t i n g allow the fragments to break along those 30. l i n e s of weakness, t y p i c a l l y r e s u l t i n g i n r e c t a n g u l a r shapes. I n the case of homophanous r o c k s such as m i g m a t i t e i n which t h e r e i s no s t r u c t u r e l e f t , fragments t h a t o r i g i n a l l y s h a t t e r e d a l o n g j o i n t s and f r a c t u r e s i n l a r g e r p i e c e s would abrade a l o n g g r a i n b o u n d a r i e s i n s m a l l e r ones, r e s u l t i n g e v e n t u a l l y i n rounded shapes. The abraded p a r t i c l e s , m o s t l y g r a i n s i z e d and s m a l l e r , are to the most p a r t f l u s h e d out by the gases i n t o apophyses or d i k e s , a c c o r d -i n g to t h e o r y . S i z e - s o r t i n g by r e l a t i v e s i n k i n g depending on the buoyancy of the fragments c o u l d be e n v i s a g e d . I f the heavy p i e c e s sank f a r enough, they would presumably be i n c o r p o r a t e d i n the m e l t , thus g r a d u a l l y changing i t s c o m p o s i t i o n . I f on the o t h e r hand, f l u i d i z a t i o n had not o c c u r r e d l o n g enough f o r a b r a s i o n to produce f i n e p a r t i c l e s , one must imagine t h a t fragments of the s i z e produced by s h a t t e r i n g would be i n dominant numbers. Extreme p r e s s u r e s d u r i n g or a f t e r the o c c u r r e n c e of f l u i d i z a t i o n would compress the m i x t u r e , r e s u l t i n g on c o o l i n g i n a v e r y compact roc k i n which the fragments would impinge upon one a n o t h e r on a l l s i d e s . On the o u t s i d e of t h i s s h a t t e r e d - f l u i d i z e d zone, fragments would impinge upon the w a l l r o c k s . The i n t e r i o r 'mosaic' t h r e e -d i m e n s i o n a l p a c k i n g i s thought of as b e i n g much more competent than the p h y s i c a l c o n t a c t w i t h the w a l l r o c k . On r e l a x a t i o n of the p r e s s u r e , p o s s i b l y due to the escape of most of the gas a l o n g f r a c t u r e s v e r t i c a l l y and l a t e r a l l y , t h i s t i g h t l y packed t h r e e - d i m e n s i o n a l mosaic, not b e i n g f o r c e d a g a i n s t the r o o f and the S i d e s of the r o o f would slump downward somewhat so t h a t a zone of low p r e s s u r e , i f not even a gap would occur be-31. tween w a l l and r o o f r o c k s . Should magma be p r e s e n t below, i t would be f o r c e d upward around t h i s p l u g , and c r y s t a l l i z e as a i -s k i n around i t . As t h e r e c o n c e i v a b l y was a f i e l d of s t a t i c p r e s s u r e i n t h i s magma s u r r o u n d i n g the p l u g , the g r a i n s on c r y s t a l -l i z a t i o n would a l i g n themselves a c c o r d i n g to the s t r e s s . T h i s s h o u l d i n a l l cases be i n s i m i l a r r e l a t i o n to the o u t l i n e of the p l u g . C o n c e r n i n g n a t u r a l p r o c e s s e s which have o c c u r r e d d u r i n g times p a s t , i t i s c o n c e i v a b l e t h a t t h e r e w i l l be s e v e r a l i n t e r p r e t a t i o n s of f i e l d d a t a , depending t o a g r e a t e x t e n t on the e x p e r i e n c e of the o b s e r v e r . The p r i m a r y o b j e c t i v e of a t h e o r y of a mode of f o r m a t i o n however, i s not to be c o r r e c t t o the e x c l u s i o n of a l l o t h e r proposed t h e o r i e s i but to i n c l u d e and s a t i s f a c t o r i l y e x p l a i n the g e o l o g i c a l c h a r a c t e r i s t i c s Of the f o r m a t i o n i n q u e s t i o n . A p p l i c a t i o n of Data to Theory - I t i s i n defence of the t h e o r y p r o f f e r e d h e r e , t h a t the a v a i l a b l e d a t a l i s t e d e a r l i e r i n t h i s t h e s i s , w i l l be e x p l a i n e d as r e s u l t i n g from p r o c e s s e s a c t i v e d u r i n g the f o r m a t i o n of the b r e c c i a . I n t h i s manner i t w i l l be p o s s i b l e t o r e l a t e f i e l d d a t a to the proposed manner of f o r m a t i o n , thus p r o v i d i n g a t e s t of the t h e o r y . The p r o x i m i t y of the b r e c c i a to a g r a n i t i c body, i n t r u s i v e i n t o c o u n t r y r o c k l e a d s to the b e l i e f t h a t the g r a n i t i c i n t r u s i o n was the so u r c e of the b r e c c i a t i n g f o r c e . Gases and heat d e r i v e d from the i n t r u d i n g body a re f a c t o r s i m p o r t a n t i n t h i s t h e o r y . The shape of both the b r e c c i a p l u g and the fragments can be d i r e c t l y connected to the g n e i s s o s i t y and f a u l t s t r u c t u r e s i n the c o u n t r y r o c k as f o l l o w s . C o n t a c t s i n the exposed p l a c e s to the n o r t h and south are p a r a l l e l to g n e i s s o s i t y , and to the e a s t a very g o u g e - r i c h shear zone forms the c o n t a c t . Here i t i s not known whether the f a u l t f o l l o w e d a l o n g the i n i t i a l l y •.weak con-^ t a c t of the b r e c c i a or whether the f a u l t i s p r e - b r e c c i a and p r o -v i d e d a l a y e r impermeable f o r gases, thus not p e r m i t t i n g b r e c c i a t i o by e x p l o s i v e f o r c e s of them. G e n e i s s o s i t y g r e a t l y f a c i l i t a t e d f r a c t u r i n g of the ro c k a l o n g t h i s s t r u c t u r e . The e x i s t e n c e of c o u n t r y r o c k fragments to the e x c l u s i o n of n o t i c e a b l e q u a n t i t i e s of o t h e r r o c k types i s noteworthy. T h i s tends to c o n f i r m the p o s s i b i l i t y of a s c a r c i t y of o t h e r r o c k types a t d e p t h , as w e l l as the i m p r o b a b i l i t y of s i g n i f i c a n t t r a n s p o r t of m a t e r i a l . The c l o s e p a c k i n g and l a c k of m a t r i x near the c e n t r a l p a r t of the b r e c c i a body i s b e l i e v e d to r e f l e c t the s m a l l e f f e c t of a t t r i t i o n caused by the c h u r n i n g a c t i o n of f l u i d i z a t i o n . Thus, i t i s thought t h a t fluidization„did not o c c u r l o n g enough to produce the f i n e r , p a r t i c l e s but merely a good d e a l of m i x i n g . Subsequent p r e s s u r e from above and below and l a c k of the a d v a n c i n g gas, c a u s i n g a decrease, of f l u i d i z a t i o n , would r e s u l t i n a t i g h t l y packed t h r e e - d i m e n s i o n a l mosaic of fr a g m e n t s , w i t h v e r y l i t t l e m a t r i x . Loose p a c k i n g a t the p e r i p h e r y i s a p r o d u c t of i n s u r g e n c e of d i o r i t e l a t e r i n the sequence. At t h i s time l o o s e fragments a t the p e r i p h e r y of the t i g h t b r e c c i a body were e n g u l f e d by the d i o r i t e , thus p r o d u c i n g the d i f f e r e n t type of b r e c c i a , and the g r a d a t i o n a l c o n t a c t . D i o r i t e d i k e s w i t h i n the b r e c c i a are s i m p l y e x p l a i n e d by r e -33. c a l l i n g t h a t d i o r i t e was pushed around the b r e c c i a p l u g as a s k i n . As i t was under p r e s s u r e i t f i l l e d any c r a c k s . H o r i z o n t a l a l i g n m e n t and p l a s t i c f l o w are thought to be r e s u l t s of i n t e r n a l f a c t o r s . The d e f o r m a t i o n of fragments i s due to s t r e s s e s s e t up i n fragments due to impingement a g a i n s t one a n o t h e r . The t h r e e d i m e n s i o n a l framework i s not merely caused by a f i t t i n g of fragments a c c o r d i n g to s i z e and shape but by f o r c i b l e p a c k i n g which i s demonstrated by the bent fragments. The g r a n i t i c 'pebbles' have aroused c u r i o s i t y at t h i s l o c a l i t y because they^whow g r e a t c o l o u r c o n t r a s t w i t h the drab background <> of the o u t c r o p . The fragments are u s u a l l y e l o n g a t e and e l l i p t i c a l i n shape, n e a r l y c y l i n d r i c a l . P a r a l l e l to t h e i r l o n g a x i s i s the g e n e r a l o r i e n t a t i o n of m a f i c s , hence the s c h i s t o s i t y . The rock p a r t s e a s i l y i n t h i s d i r e c t i o n , and because t h i s a l i g nment i s a c t u a l l y a l i n e a t i o n the fragments a c q u i r e rounded c r o s s s e c t i o n s . Yet the shapes i n a r o c k f a c e l o o k rounded l i k e p e b b l e s . They are thought to be d i s l o d g e d p i e c e s from b o r d e r phase d i o r i t e a l o n g the lower p e r i p h e r i e s of the b r e c c i a body, presumably c a r r i e d up from below, a l t h o u g h t h e r e i s no i n d i c a t i o n of the d i r e c t i o n of t r a n s -p o r t . The w h i t e p e g m a t i t i c areas p r o b a b l y are space f i l l i n g s by q u a r t z and f e l d s p a r d u r i n g the l a s t s t a g e s of compaction of the b r e c c i a . V a r i a t i o n of fragment s i z e i s l i m i t e d . Most fragments are of one s i z e , a l t h o u g h t h e r e are some t h a t are b l o c k s i z e d . The s m a l l most p l e n t i f u l s i z e d e r i v e d i t s dominance through, the v e r y c l o s e p a r t i t i o n Of the r o c k through s h a t t e r i n g . T h i n s l i v e r s were broken up as a consequence of j o i n t i n g and the i n s t a b i l i t y of 34. t h i n s l i v e r s i n an a g i t a t e d environment. Thus, i t i s seen how the rock was broken up to such s m a l l s i z e . I n a l l p r o b a b i l i t y i t was shredded i n t o s m a l l s l i v e r s too but they were too f r a g i l e f o r t h i s c h u r n i n g environment and were reduced to g r a i n s i z e d p a r t i c l e now found between the l a r g e r f r a g m e n t s , and l a r g e l y r e c r y s t a l l i z e d I t has thus been i n d i c a t e d t h a t the proposed method of b r e c c i a f o r m a t i o n i s i n agreement w i t h the f a c t s o b s e r v e d ; t h a t i t thus i s a t h e o r y of emplacement of t h i s b r e c c i a . The a f o r e m e n t i o n e d l i s t of modes of f o r m a t i o n of b r e c c i a s i s a l s o a l i s t of types of b r e c c i a o c c u r r e n c e s . I t i s u n d e r s t a n d a b l e then t h a t i n normal cases one would f i n d , r a t h e r than a t y p i c a l r o c k f o r m a t i o n , one t h a t i n c o r p o r a t e s s e v e r a l methods--a m i x t u r e of p r o c e s s e s . T h i s i s the case i n t h i s s i t u a t i o n . T h e o r i e s , m e n t i o n e d . b e f o r e , t h a t can be ex c l u d e d w i t h c e r t a i n t y are se d i m e n t a r y b r e c c i a t i o n and b r e c c i a t i o n due to s h r i n k a g e , whereas the o t h e r t h e o r i e s may a p p l y to some e x t e n t , as f o l l o w s . Gases s t r e a m i n g upward ahead of an i n t r u d i n g magma mass would cause e x p l o s i v e a c t i o n ; f l u i d i z a t i o n would o c c u r as e x p l a i n e d b e f o r e ; t e c t o n i c b r e e c c i a t i o n may have been the cause f o r b e t t e r c o n d u i t s f o r the r i s i n g ' m a t e r i a l s ; igneous i n t r u s i o n may f o r c i b l y f r a c t u r e zones of ro c k above the p a r t i a l l y congealed magma; f l u i d i n t r u s i o n , a p r o c e s s s i m i l a r i n many ways to the e x p l o s i o n t h e o r y , would c o n s i s t of gases o r l i q u i d s r i s i n g from depths b r i n g i n g up fragments of c o n d u i t r o c k which then are f o r c e d upward i n t o the ro c k s by a t h i c k mud, t h i s i s p o s s i b l y how g r a n i t i c 'pebbles' were i n t r o d u c e d ; s o l u t i o n and replacement may have o c c u r r e d on a s m a l l s c a l e ; m i n e r a l i z a t i o n s t o p i n g as such i s not i n d i c a t e d , ' 3 5, a l t h o u g h s i g n s of b l o c k c a v i n g have been noted i n c o n n e c t i o n w i t h the b l o c k s of c o u n t r y rock p a r t i a l l y r o t a t e d w i t h i n the b r e c c i a mass. CONCLUSION The Pasco b r e c c i a has been s t u d i e d and the d a t a found to i n d i c a t e a c o m b i n a t i o n of s e v e r a l p r o c e s s e s a c t i v e i n the f o r -mation of the b r e c c i a . Among these e x p l o s i v e a c t i o n of gases ahead of an i n t r u s i v e body, t o g e t h e r w i t h f l u i d i z a t i o n of a m i x t u r e of these gases and s h a t t e r e d r o c k are thought to be of prime importance i n the f o r m a t i o n of the b r e c c i a . . EXTENDED BIBLIOGRAPHY Armstrong, J.E. , 1954 B r y n e r , L., 1961 B u t l e r , B.S., 1913 Emmons, W.H., 1938 F a i r b a i r n , W.H. and Robson, G.M., 1942 F a r m i n , R o l l i n , 1934 F i s h e r , R.V., I 9 6 0 Gates, 0., 1959 Goodspeed, G.E. P r e l i m i n a r y Map, Vancouver N o r t h , B.C. G e o l o g i c a l Survey of Canada, Paper 53-28. B r e c c i a and pebble columns a s s o c i a t e d w i t h e p i g e n e t i c ore d e p o s i t s : Econ.. Geology, v. 56, p. 488-508. Geology and ore d e p o s i t s of the San F r a n c i s c o r e g i o n , Utah: U.S. Ge o l . Survey P r o f . Paper 80 i Diatremes and c e r t a i n o r e - b e a r i n g p i p e s : A.I.M.E. Tech, P u b l . 891, M i n i n g Technology, volume 2, no. 3. B r e c c i a a t Sudbury, O n t a r i o : J o u r . G e o l . , v o l . 50, p. 1-33. "Pebble d i k e s " and a s s o c i a t e d m i n e r a l -i z a t i o n a t T i n t i c , Utah: Econ. G e o l . , v. 29, pp. 356. C l a s s i f i c a t i o n of v o l c a n i c b r e c c i a s : B u l l . G.S.A., v. 71, p. 973-981. B r e c c i a p i p e s i n the Shoshone Range, Nevada, Econ. G e o l . , v. 54* p. 790-815. Rheomorphic b r e c c i a s : Am, J . Sc., v. 251, PP. 453-464. ' Grout, F.F., 1937 C r i t e r i a of o r i g i n of I n c l u s i o n s i n P l u t o n i c r o c k s , B u l l . G.S.A., v. 4.8, pp. 1521. H o w e l l , P.H. and M o l l o y , J.S., I960 Geology of the Braden Orebody, C h i l e , S. Am.: Econ. G e o l . , V. 55, pp.. 863-905. H u l i n , C.D., 1948 F a c t o r s i n the l o c a l i z a t i o n of m i n e r a l -i z e d d i s t r i c t s : A.I.M.E. Tr a n s . , v . 178 pp. 36. J o h n s t o n , ¥.P., L o w e l l , J.D. , 1961 Geology and O r i g i n of M i n e r a l i z e d B r e c c i a P i p e s i n Copper B a s i n , A r i z o n a : Econ. G e o l . , v. 56, pp. 916-940. Kuhn, T.H., 1941 P i p e d e p o s i t s of the Copper Creek a r e a , A r i z . : Econ. G e o l . , v. 36, pp. 512-538, Locke, Augustus, 1928 The f o r m a t i o n of c e r t a i n ore b o d i e s by m i n e r a l i z a t i o n s t o p i n g : Econ. Geology, v . 21, pp. 431. L o v e r i n g , T.S., 1942 P h y s i c a l f a c t o r s , i n the l o c a l i z a t i o n of o r e : Ore d e p o s i t s as r e l a t e d t o s t r u c t u r a l f e a t u r e s : P r i n c e t o n U n i v e r s i t y p r e s s , PP. 5-9. M c K i n s t r y , H.E., 1955 S t r u c t u r e of the hy d r o t h e r m a l ore d e p o s i t s F i f t i e t h A n n i v. Volume, P t . 1, Economic P u b l i s h i n g Company, pp. 207-214. Re y n o l d s , D o r i s , 1954 F l u i d i z a t i o n as a g e o l o g i c a l p r o c e s s and i t s b e a r i n g on the problem of I n t r u s i v e G r a n i t e s : Am. J o u r n a l of S c i e n c e , v . 251, no. 1, pp. 577-613. R i c h a r d , K., and C o u r t w r i g h t , J.H., 1958 Geology of Toquepala, P e r u : M i n i n g E n g i n e e r i n g J o u r n a l , F e b r u a r y 1958, pp. 262-266. R o d d i c k , J.A., 1965 Vancouver N o r t h , C o q u i t l a m , and P i t t Lake Map A r e a s , B r i t . Columbia: Men, 335» G e o l o g i c a l Survey of Canada. Ross, J.V., 1957 Combination t w i n n i n g i n P l a g i o c l a s e F e l d s p a r s : Am. J . Sc., V. 255, pp. 650-655. Walker, R.T., 1928 M i n e r a l i z e d v o l c a n i c e x p l o s i o n p i p e s : Eng. and M i n i n g J o u r . , v. 126, pp. 895. Waters, A.C. and K r a u s k o p f , K. 1941. P r o t o c l a s t i c b o r d e r of the C o l v i l l e B a t h o l i t h : B u l l , G.S.A.,:' v. 52, np. 1355-1418. W i l l i a m s , Howel, 1936 P l i o c e n e v o l c a n o e s of the Navajo-Hopi C o u n t r y : B u l l . G.S.A:, v. 47, pp. 111-172. W r i g h t , A.E. and Bowes, D.R., 1963 C l a s s i f i c a t i o n of v o l c a n i c b r e c c i a s : a d i s c u s s i o n : B u l l . G.S.A., v. 74, pp. 79-86. OUTCROP DISTRIBUTION M A P Pasco Creek, Horseshoe Bay y B.C. L e g e n d : E 3 mlgmotltlo hornbltnd* gn«l»«, hornbl»ndl1« g amphibollt* gntltt brcoel a ft border phata* dike g n « l t t o i l t y inferred breccia contact b e d d i n g . OF B R E C C I A B O D Y Pasco Creek, Horseshoe Bay, RC1 Scale' /"--250 apf o ao no 'So §M Q g . A . * . R o l « r i ' 6 P HIGHWAY S E C T I O N RAILROAD SECTION Legend 0 20 40 60 mam 80ft S H E A R Z O N E G N E I S S B R E C C I A G N E I S S , AMPMiBOL I TE amzmt H O R N B L E N C E D I O R I T E tt P L A G P O R P H Y R T EE ' ' "133 of BORDER PHASES Fig.3 D I A G R A M M A T I C C R O S S - S E C T I O N S Pasco C r e e k , Horseshoe Bay, B . C 

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