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A petrological study of intrusive rocks along the Fraser Canyon near Hell's Gate, British Columbia Morris, Peter Gerald 1955

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A PBTROLOOICAL ST0DY OF H?TBUSI?B BOOKS ALOWO THE FBASBR CAIfOi WEAR HELL*S GATE, BRITISH COLUMBIA PETE1 QEftaU* MORRIS A THESIS StJSMlTfiU IK PAifJAL PHLFILMEIT OF THE JtSQIJlBBKIXS FOR 'THE ©10111 OF MASTER OF SCIESCB i n th® Dapartsiant of ©EOGiAPHf AID GEOLOGY W® atoopt this thosl* as conforming to th® standards roquirod from candidates f o r tha »@j3?®# of KASTBB OF SCXHWCS Me®6®fs o f tfe@'uB»f*fl55nl^of"" THE OHIVEHSm OF BRITISH COLOMBIA A p r i l , 1955 ABSTRACT A study was aad© of rocks outcropping along a road section i n the v i c i n i t y of Hell's Gat®, SOB© 12 miles south of Boston Bar, B r i t i s h Columbia. B i o t i t e schists belonging to th® Hoxaeeen group (Carboniferous or Permian i n ago)* were the oldest rocks found. The Custer gr&no-d i o r l t e , a heterogeneous igneous body having the average composition of a hornblende granodlorite, Intrudes the b i o t i t e sehlutt* The age of the Caster granodiorit® Is believed t© be Upper Jurassic* Large numbers of pegmatite and a p l i t a dykes Intrude the Custer granodiorite. Two types of pegmatites # sagmatle and eetasoifiatie were d l s t l n * guished. The Hell's date gra n o i i o r i t e , a tmlfora igneous body of trondjhemitic composition, Intrudes th® Custer granodiorite and also post dates the pegmatite and aplit® dykes. The age of th® Hell's Qate granodiorite Is believed to b® Upper harassle or possibly Lower Cretaceous. Sight plagioclase porphyry dykes, c l a s s i f i e d into one of three groups, (staple, multiple, aad protoclastle types) were found. Th® plagioclase porphyry dykes are believed to be genetically related to the Hell's Gate granodiorite. Six dykes having an andesltic composition were found j the dykes have been c l a s s i f i e d into l i g h t colored and dark colored andesltes, fb© took* along th® road s e c t i o n have been intensively fractured. F a u l t i n g has taken place close to the southern contact between the two granodiorites. fhe narrow N»S trending outcrop of Custer granodiorite i n the area could p o s s i b l y have been produced by f a u l t i n g , and i t s shape suggests a hers* s t r u c t u r e , A study, using a method suggested by 0©ral, was made of the p l a g i o c l a s e twins occurring i n the Hell's Gat® rooks* The conclusions drawn ffron t h i s mfk l a r g e l y confirms the conclusions drawn from the f i e l d evidence. ©ttPTBB I Introduction * 1 Location • 1 F i e l d Work , . , . , , , • . . » . . . • • • • . » 1 Acknowledgments . . . . . . . . . . . . . 2 Physiography . . . . . . 2 CHAPTBR 11 Seneral Geology .... 4 Metamorphosed Sedimentary Boeks • • 4 Igneous l o c k s 4 CHAPTER III Custer Qrmnodiorltt . . . . 8 (km«*4l features • • * „ • • • « « • • » * • • » • 8 Petrography . . . . . , . . . • • • . . • » . . . . 8 Hornblende 0r«no41orit# . * • • » * * 10 B i o t l t e S » a o d i o r l t « , • • « • « • • • • • • • « 12 Hornblendlt® . . . . . . . . . . . . . . . . . . 12 I n t e r n a l Structures » » . * . . * » « . . * » . * 13 External s t r u c t u r e s . * , » . , # . » » « « . * « * 1* Origin . . . . . . . . . . . . . . . . . . . . . . 14 Age , 16 CH&Pf Fa IT Pegmatites and Aplitea . . . . . . . . . 16 Oceurr@nc# . . . . . . # . . . . . . . ia Petr©graphy « 19 Aplitss . . . . . 22 Ag« a»S Origin of th© Pagoatltts and AplitM # . 22 CHAPTER V Tha H«H*a Gate Grano<Uo*it« . . . . . . 24 and Shape . . . . . . . . . . . . 24 Petrography • 28 Internal structure* , . * * 31 External Structures . . . . . . . . . . . . . . . 32 Origin « » * * * , < * * * . « « * • • » * * » * • 33 Age * 35 CHAPTER f I Plagioclase Porphyry Dykes • , , . , . 39 General Features 39 Simple Type 39 Occurrence . * . • • « • • • • • 39 Petrography . . . . . . 39 Multiple Type • • • • . * • • • • • • » • » • • • 4 ? Occurrence • • • • • • « * ' « » • • • . . . . • 45 flagf. Petrography . . , » , • . , , • . « . » . . « » » 45 Protoolastle Type • 46 . « , . . . . . « . . . 46 Petrography « » • » * . * « * • . . • • * • • » • 50 Chilled Marginal Zone . , . . . • « • • , • » • 50 Central Zona • . • 51 Contorted Fhaaooryat Zona . « . # « , # » * * # 52 Origin • » . . , • • . , » ' » • • • « , » » » « • « 53 R«latloa«hlp of the Plagioclase Porphyry 'Dykes to the Sell's Gate OrsBodiorlte 55 CH&PTB8 Vn 0th#r Bykss « « „ » , « » . » . * „ # # * # • < > » * • 57 light Colored Andaftitaft . « • • # * » « * • • • « 57 Dark Colored andasltas . , * • « • • • • • « « • • 58 OHAPTIR VIII Plagioclase Twinning of the Intrusive Bocks » • * • * 60 Coral 1s Work on Plagioclase Twinning . . . . . . . 60 Volcanic Bocks . . . . » « . . . . . . » # * # 60 Plutonic locks forming BathoXlths and stocks , 61 Ooblasts aad Gneisses . * * . * * , . * » • • • 61 Horafelsic locks 61 Laboratory Toehnlque , * • • • • • « « * « • • • • 64 Results • • • * • • • • • • « * * « 64 CHAPTER 1% Suaaary and Conclusions » • * * • , . « « • * . • • « 69 nmmmzm I General view of the road suction looking north * 3 IX Custer Granodlerlte . . , * • • • • , « . » * * 11 III Folded sedimentary xenoltth in th© Custer Graaodlorite • • * « • * » « • » « • • * • « , If If Pegnatitea intruding Custer Cranooiorlto . . • . *K> ? Sell* a Oat® Cranodiorlte , . * . . 2*> VI Closely epaoed fraaturaa developed .in th® Holloa Oate Granodtorlte • » « • « * » • • » • » • • 34 VII Southern eontaet of th© S e l l * a 0ate and Custer &?anodlarltes » • » » « • • » * • « » • « , « 36 VIII Southern contact of the Hell*a Sate and Cuetor G r a n o d l o r l t a t eloae ap v i a * • « » • , « • * » 37 IX. Thruet faults developed close to the aouthem contact of the dranodloritee » » * * « • » » 38 I Plagloelafte porphyry dyke i n t r u d i n g Cuater Oranodiorite • • * • « « • « * • • • « * • * 4a II P r o t e e l a a t i e dyka' * » * • • , , , , * • • • • * 49 XII Photomicrograph Cneter.Granodiorite Croeaod aicols 1 45 * • • « • • • * • • • • * 71 XIII Phot osier ©graph Custer Granodlerlte P.P^II I 45 • 72 XIV Photeaieregraph H a l l * a Gate granodlorlte Crossed nleols X 4 5 * • * * » « « « • » • • • 73 XT Pfa©t«io»grapti P r o t o o l a s t l e Oytee, Cent m l 3tonef P«P#&* X 4 5 * , 74 W I Photomicrograph Protoplastic fcyka, Contorted Pheaoorfst Zot*« X45 » » * • 75 4 1. fable of Formations . . . . . . . . 7 2* Caster (Iranon* lor It at Mineral Percentages . . » 9 3, ©nst« Granoaiorlte- . . . 15 4» Pegmatite and Aplite Bykas in the Caster Grojaodiorlto * » » , . » * * . , » » » . # » 21 5* ftelatlonshlp between the Custer 6raac4iorite ami the Hell*a Gate Greuodlorlte at th© southern contact » » « • . * • • • . • • » 26 6* Hell's Gate ctranodiorita:- Mineral Percentages « 27 7. Zoned Plagioclase Crystal frost a Plagioclase Porphyry Dyke * • » « « • • * • « * » * • • #3 8. Plagioclase Porphyry Dykes* Mineral Percentages 44 9* P r o t e o l y t i c Dyke . . , . « . . • , . » » * , . 47 10# Textures of t h e Proteolytic Dyke 46 11. Bi&graaatlc Illustration of the Cooling History of the Protoelostic Dj*e . . . . . 54 12* Example* of Plagioclase Twinning (After Coral). 63 13. 0#A.C« ratios of th® Igneous M i a Metamorphic Bocks. (After Serai) . « . , » # • # . . * » • 6€ 14* £F.a»C# ratios of Socks from th© Hell's Gate area 67 15# Table of 0.A.C,Percentages of the E#ll fs Gate rocks* * « * « . * . » • » # « . » » # • « # 68 Xm Q*9*.C* Map fro* 73?A» lope Sheet (In Pocket) 2 « Geological amp of the H e l l * s Cata are® 1 * 3 0 f 0 0 0 scale* 76 % Geological map of the roafl s e c t i o n 77 4, Geeleglaal ma© of th© road s e c t i o n showing also rock spacl&an l o c a t i o n s 78 location fhe rocks which were studied outcrop In the Fraser l i v e r Canyon i n the v i c i n i t y of Be l l ' s Gate, some IS miles south of Boston Bar, B r i t i s h Columbia* Jk series of fresh .road cats along a five mile road section were examined. Close to th® middle of the road section a path leads down from the roa4 to the Hell* s date flshways. F i e l d work fhe f i e l d area was v i s i t e d twice during 1953. During October, five days were spent In sapping the read section and co l l e c t i n g specimens. During December a v i s i t lasting eight days was n d e to the f i e l d area* in which time the nor® interesting features of the read section wara examined, fhe road section was mapped topographically and geologically on a scale of Is 10,000, k It 30,000 scale map (Map Ho,3) was prepared fro® the Is 10,000 f i e l d map. 2 Acknowledgments The writer would l i k e to acknowledge, with grate-f u l thanks, Dr. K.C.McTaggart, Professor of Petrology at the University of B r i t i s h Columbia, for his encouragement and help i n th® preparation of this thesis. Thanks ar© du© to Mr. J . A , Soles for his helpful criticisms of th® rough draft of th® -thesis and also to the B r i t i s h Columbia Academy of Science, for f i n a n c i a l aid received during th© 1 9 5 3 - 5 4 University session. F i n a l l y the writer would l i k e to acknowledge Mrs, J , Donnao for the time sh© has spent deciphering my writing and typing this thesis. Physiography The terrain around Hell's Gat© i s extremely rugged and mountainous (Plat© I ) . The Fraser River at Hell's Gat® flows south i n a steep-sided youthful v a l l e y , along the eastern side of which the Trans-Canada Highway has been constructed, The steepness of the va l l e y sides and th® intensely fractured bed rock combine to make rock slides a common feature of the area. SECTION, L0OKIN6 NORTH . 4 CHAPTER II Met«©rth©ied sedimentary Books I© eadlsentary recks outcrop along th* section of -the road, u n d e r study, hut j s e ta&orphlc rocks of sedimentary or volcanic o r i g i n outcrop at i t s northern and. Xenoliths of K e t a s i o r p h o s e d s e d i m e n t a r y rocks occur i n th® Custer gr&no-d i o r i t e . The ©etaaorphic rocks outcropping: at the northern end of the road section have been mapped by of f i c e r s of the Canadian Geological Survey as part ©f th® Hoiaaeen group, (Hope Sheet-Ho, ?37A), ftxplanatory notes attached t o the lope Sheet describe the Boawaaan group aa p r i n c i p a l l y an intlaate association of andeeltlo lavas (green®tones) and r i b b o n chert with argillaceous partings. Ho f o s s i l s have been found i n It but the rooks are llthologieally s t e l l a r to the Cache Creek series, from which, i n recent years, many collections of Permian marine f o s s i l s have been made. Igneous locks Intrusive igneous rocks ware the noat coaaoo rocks found along the road section. The oldest Igneous rock body i s the Custer pranodiorlte, which outcrops on both sides of 5 the Fraser ?alley. Outcrops of Custer gr&nodiorite form a narrow bait two to four miles v i d e which trends N»Sf p a r a l l e l i n g the Fraser B i t e r at H e l l ' s Gate, The Custer graaodiorlte has Intruded and metamorphosed rocks of the Sosaneen group. Explanatory notes attached to the Sop® Sheet ( l o . 737A) describe the Ouster Intrusive as mainly a l i g h t c o l o r e d t medium to coarse grained rock that ranges In In composition from granite to granodlorlte* For the most part i t i s 'noticeably crushed, and f o l i a t e d . A c i d rocks younger than the Custer granedlorlto outcrop mat wide areas of country vest of the Fraser V a l l e y , s i m i l a r rocks ar® found In the middle of the studied road Seetlon* The awe f e l l ' s Qate granodlorlte w i l l be used i n t h i s report f o r th#se younger a c i d rocks to d i s t i n g u i s h then f r o a the Caster gvanoftiorlte* fhe H e l l ' s Oat® g*anodlevltt and a c i d Igneous rocks west of the Fraser V a l l e y hate been described by the Canadian Oeologleal fmrtey as a series of c h i e f l y a c i d i n t r u s i o n s * Only the r e l a t i v e sges of the two granodiorites could be determined f r o a a study of the Igneous contacts along the road s e c t i o n , Pegmatites9 a p l i t e s , p l a g i o c l a s e porphyry, and basic dykes intrude the granodlorltes s. the f i r s t two buing r e s t r i c t a d to the Custer grmnodiorlte. The pegmatite dykes 6 are noticeable for tholr complex Intrusive relations (Plate IV), Plagioclase porphyry dykes, believed by the writer to be g e n e t i c a l l y related to the Bell's Gate graao-d i o r i t e , intrude both g r a n o d l o r l t e s , AndesIt8 dykes intrude both granodlorltea* F I G U R E Z , — z T A B L E O F F O R M A T I O N S P E R I O D N A M E C H A R A C T E R PLEISTOCENE AND RECENT STREAM AND GLACIAL DEPOSITS U N C O N F O R M I T Y POST UPPER AND POSS/BLY POST LOWER CRETACEOUS. PLAGIOCLASE PORTWYP} AND BASIC DYKES PLAGIOCLASE PORPHYRY AND BASIC DYKES. I N T R U S I V E C O N T A C T POST UPPER JURASSIC AND POSS/BL Y POST LOWER CRETACEOUS HELLS GATE GPANOD/OP/TE" UNIPORM POTASSIUM DEF/CfENT GRANOOIOR/TE I N T R U S I V E A N D F A U L T E D C O N T A C T POST LOWER JURASSIC CUSTER GRANOD/ORITE GNE/SSOSE HORNBLENDE GPANOOIORtTB - LARGE NUMBERS OF PBGMAT/TE AND APLITE DYMES INTRUDE THE GRANOD/ORfTJ? . I N T R U S I V E C O N T A C T PERMIAN f HOZAMEEN GROUP BIOTITE SCHISTS 8 CHAPTER III General feature® The Custer granodlerlte contains a variety of roek types ranging l a ooiapoaitlon fro» ultrabaaie to acid, Along the road a eat ion studied the moat eomaon rook type, napped a® Caster granodiorlta» in a gnalaaoaa hornblende granodiorlta* Blotlta granodlorite, d i o r i t e , and hornblend* it© are also facies of the Caatar granodlerlte, A horn-alandlta body, 20 feet wide and dipping v e r t i c a l l y , ontoropa close to M*P» 135* fhe horablendlte gradei into less basic d i o r i t e * Snail lenses and bands of b i o t i t e granodiorlta are intimately associated with the mora ooanon hornblende grano-diorlta (Figure 3)# Diorite ontoropa cl©t« to M»*» 135 and also at the northern end of the road section* Pegmatite and aallte dykes intrude the Custer granodiorlta* but are older than the Hall's Gate granodiorlta. Petrography The average apmi&m of Custer granodiorlta i s composed of approximately equal sisounts of l i g h t and dark minerals, which are llneated producing a recognisable gneissic U3S8LZ GIISIEE GRABODIOStTE'r M18KBA£ $paei»en So* 3 5 e 22 31 33 41 44 Plagloelaii© * m 45 40 40 50 50 55 50 Orthoelaaa % Qnart* fl 10 5 4 4 4 5 Bornblanda £ I f 45 30 50 40 35 35 40 Biotlta £ «•*« 15 4 5 10 5 5 Cnlorita % 5 Magna t i t a * •patlta * — Averaga d r a i n las) 3 2 2 1 2 3 4 a Ho* 3 . * . * Horaalanda Biotlta Granodiorlta l?o# 8 , . . . Sornbl®Bd« Granodiorlta I n . 5, 2af 31 33i 41, 44 Quart* Dior1to 10 texture. The g r a i n slms of various specimens were found to l i ® w i t h i n th* H a l t s o f one and four m i l l i t a e t e r a . Hornblende Granodiorlta * Thin Section Hornblende c r y s t a l s found i n the hornblende g r a n o d l e r l t e are green i n c o l o r , anhedral and f a i n t l y pie©* c h r o i c . fhe pieoohrisa formula and e x t i n c t i o n angle* ere H a t e d bel@*r* S 1 X Srean Brownish ®mm Mght Brownish Srean 0 £ e x t i n c t i o n angles varied between 7° sad 16°. fhe average angle of e x t i n c t i o n vaa found to be 0 Z a 13 * M u l t i p l e twinning, at w e l l as ateple twinning ©f the horn-* blend® c r y s t a l s is e«sea, A nuaber of th® hornblende c r y s t a l s ware found to contain small rounded Inclusions of quart® and plagloalaaa* Biotite and hornblende ©flea occur together and In soae of these lntergrovths the born* blende apptars to be r e p l a c i n g the b i o t i t e . The b i o t i t e i i a s t r o n g l y pleochrole brown v a r i e t y having undnlosa e x t i n c t i o n , P l a g i o c l a s s c r y s t a l s ar© generally anhedral and show only a f a i n t progressive xoning* fhe average composition of the p l a g i o c l a s e i s Abg£ An-^. Haltlpla twinning of the p l a g i o -c l a s e crystals- i s eoa»on, Quarts c r y s t a l s a r t anhedral and i n sous aaaaa lenticular i n shapa, ttoder crossed nlcola the l e n t i c u l a r Quarts grains have a mosaic appearance. P L A T E . II C U S T E R 6 R A N ODI O R I T E . 12 Magnetite, sph«»«| and apatite occur as accessory minerals* B i o t i t e g r a n o d l o r i t e Minor aaoanta of the Custer granodlorite eontain biotite i n exaese at hornblende (Figure 3 ) . The textare and g r a i n s i z e of I b i s facias i s similar to thos* of the born* blend® $raa©diorit© t bat tba name b i o t i t e granodlorlt® Is a p p l i c a b l e * i o r a b l s n d i t ® The hornbleadlte Is a dark green rook made up e s s e n t i a l l y of large (1,5 em a**Uwj* ilaa) I n t e r l o c k i n g c r y s t a l s of hornblende* Swill grains of p y r r h e t l t e occur* soattered at random taroagh the hornblendlta* A t h i n aeatIon study was »ad® and tna minerals listed below ware found. Iiaet*l^J!tii§al JL Hornblende % % B i o t l t e IQ$ Pyrrhotlte 3^ Magnetite 2% fhe hornblende c r y s t a l s a r t anhedralt green i n c o l o r and f a i n t l y pleoohroie. The o p t i c a l properties of th® hornblende are Hated b@low» -13 2 T 1 Olive Green Olive Green Light Green C 2 a 14° 2V * 50° • 60° A light brown b i o t i t e occurs In th© hornblendite* fhe biotite has a small ZV angle and. son® crystals give . a psuedo-uniaxal optic figure* Crystals of biotite tend to be elongated bat they do not pmsms a eosaton direction of orientation. Magnetite is a e«s©« aocesaory adneral and ecwaonly occurs as orientsi rods la the hornblende crystals. It i s probable that cleavage plants In the hornblende have aontrolled the crystallisation! of the aagnetita* Iwegel&rly shaped grains of magnetite are found scattered at random through the roefc* Magnetite also occurs filling thin irregular fractures in the homhlmA® crystals. A minor amount of pyrrhotite occurs m irregular grains intergrovn with the hornblende crystals. Internal structures fhe Custer granodiorite has a wall developed gneissie banding, fhe orientation of the gnelsslc bands, even over short distances, la extreaaljr variable, k large, banded quartalte xenollth outcrops near the southern contact of the two granodlorltaa. (Plate III). Th© xenollth has been laoellnally folded with limbs of the fold dipping 6 5° south-east. 14 External structures The poorly exposed contact between the Custer granodiorite and the losameen rocks appears to be gradatlenal* fhe boundary zone includes both Igneous and metamorphic rocks, with the metamorphle rocks Increasing In abundance towards the north, the contact between th® two granodioritts is described i a a later chapter» as are the pegmatite and aplite dykes that intrude the Custer granodiorlta # Origin The Custer granodiorite i s a nonuniform rock body* A study of the plagioclase twin frequencies (page 60) suggests that the body is of magmatlc rather than ©etaaomatie origin* fhe irregular shape of the rook units forming the Custer granodiorite suggests that d i f f e r e n t i a t i o n of the Custer granodiorite took place before emplacement, fhe gradatlonal contact ©f the Custer granodiorite soald have been produced by either magmatle or metasomatic processes* fhe pegmatite dykes that Intrude the granodiorite • hate no gneissose structure> hence the gaeissosity of the granodiorite i s a pr©~peg»atlte feature* Forces acting during the emplacement of the granodiorite eoald have pro-duced th® gneissic texture. F I G U R E 3 C U S T E R G R A N O D I O R I T E S c a l e ; 1 w e k = I f o o t . 16 Age The Custer granodiorlta intrudes the Hozameen rocks whose stratlgraphic age i s Missippian to Permian, (Explanatory notes Hope sheet Ro* 737A), fhe Custer gran©* d i o r i t e i s older than any other igneous body outcropping along the .studied road auction, but from the writer's studies i t s age cannot be defined more closely than post<*Bo*aa&een .and pre-Hell's Gate granodiorlta. Cairnes (C.O.S. Men* 139, pp. 88) makes the following statement on th© age of th© Custer granodiorite* The age of these batholithic rocks cannot be fixed with any great degree of accuracy fran observations within the map area (Coquihalla area). Certain of these d e f i n i t e l y intrudes the Cache Creek rocks and as d e f i n i t e l y underlie the Cretaceous rocks. They appear to forus a small o u t l i e r from the great body of rocks of the Coast aange, which is regarded for the greater part as Upper Jurassic, mvxv:"rv\ Kerth of the Coquihalla area these presumably Jurassic intrusives are continuous with exposures of sheared granodlorite through which the Fraser River has cut a narrow and d i f f i c u l t passage. PLATE. Il l FOLDED QUARTZITE XENOLITH IN THE CUSTER GRANODI ORJTE . THE HELLS GATE GRAMODIORfTE CAN BE SEEN ON THE LEFT HAND SIDE OF THE PHOTOGRAPH . 18 CSAWEB I f Occurrence fhe best exposures of pegmatite and a p l i t e are found north of the Ball*a Sate gr a n o d i o r i t e where these dykes Intrude the Custer g r a n o d i o r i t e ( P l a t e I V ) , Only a small number of aplite dykes and no pegmatite dykes intrude the H e l l * a 8&te granodiorite* Xanaxith* ©f (teste? gran®* d i o r l t e I n the Hall*a Sate f r a n o d l o r i t e contain pre*R«ll*e Gate granodiorite a p l i t e and pegmatite dykes, fhe w r i t e r considers that most of the pegmatite and a p l i t e dykee i n t r u d i n g the Custer g r a n o d i o r i t e were eaplaeed p r i o r to the Hell's Gate granodiorite emplacesent, Pegmatite and aplite dyke emplacement In the Ouster granodiorite was extremely complex. Two types of pegmatite can be di s t i n g u i s h e d * One type of pOfmatite has a gradations! contact to country rock, and the dyke boundaries tend t© be sinuous (figure 4.A), fhe second type of pegmatits.dyke i s u s u a l l y gonad and has a sharp contact with the country rock ( f i g u r e 4.1)* fhe widest pegmatite dyke found did not exceed three f e e t in width. Dilation ©f the country rock by pegmatite dykes of t h i s second type Is In some places evident* Pegmatites belonging 19 to th© f i r s t group war® i n general emplaced before pegmatites of the second group. A tm pegmatite dykes that could not be clas s if.lad i n e i t h e r of the above categories* are t i s l l a r to the f i r s t type along part of t h e i r l e n g t h , and along another p o r t i o n of t h e i r length have the character Is t i e s of the second type, (Figure 4,1). Petrography fhe ptgmmtito* are m l n e r a i o g l c a l l y simple and made up of the following minerals i F l a g i o e l a s e (AB^Q An^t mieroelineg b i o t l t e , m«se@vitef and quart** Klnarmiogleml toalng found I n the second type of pegmatite I® cession f ag i s i l l u s t r a t e d l a the f o l l o w i n g examples* Boundary zone •* mailta 3 W Intermediate gone Medium grained pegmatite y mainly p l a g i o c l a s e and quart* M n C e n t r a l none Coarse pegmatite w i t h abundant maseovlta 4" l a m i n g Boundary son« • S i o t i t e » r i c h pegmatite Intermediate gone * Blotitaf plmgloelase, quarts, pegmatite. Central son® * P l a g i o c l a s e * quarts, Muscovite, pegmatite d e f i c i e n t In b i o t i t e . P L A T E . IV. PEGMATITE DYKES INTRUDING CUSTER GRANODIORI TE . M E T A S O M A T I C PEGMATITES CAN BE S E E N IN T H E RIGHT H A N D SIDE OF THE PIC T U R E . M A G M A T I C P E G M A T I T E S C A N BE S E E N IN THE B O T T O M L E F T HAND CORNER OF T H E P I C T U R E . F I G U R E A METASOMATIC AND MAGMATIC P E G M A T I T E S E M P L A C E D IN CuSTER GRANODIORITE. ScaJ,e: 1 inch = 2..5 £ce.V. L E G E N D Coarse Csnlnaf Z^o/7<? B/o///e Muscoi/z/e e/c. S/rnp/e PJa^/oc/ase Pe^maP/e ApJ/k Gne/ssose Mb Gr<nnoc//hrr/e +- + + 22 AplitCS A p l l t e dykes at® l a s s numerous than pegmatite dykes* fhey ar® white-weathering, even-grained dykes that have sharp contacts w i t h the country rock, there i s no obvious s i n g l e r e l a t i o n s h i p between the pegaatlte and the a p i l t e dykas, A p l l t e dykes are found to be c a t t i n g or being cut by pegmatite dykes. Age and O r i g i n of the Pegmatites and Aplltaa fhe f i r s t type of pegmatite described above, i*#* those having i r r e g u l a r and grmdatlonal contacts w i t h the country rock, were In the writer's opinion* forated by metasomatism, fhe matmaomatlo pegmatites i n general appear to have been amplmood before the magmatle p t g s & t i t e s , t he second type of pegmatite described abev® was. In the w r i t e r ' s o p i n i o n , formed from a pogmatito magma* D i l a t i o n ©f the country rook has oooarred i n some places where the mags&tic pegmatites cut older dykes, the small number of pegmatites that seen to be nei t h e r e n t i r e l y magmati© or metasomatie are eoasldered to represent a t r a n s i t i o n be-tween the two types* fhe S*U*e Gate granodiorite contains only a few aplite dykes, fhe bulk of the aplite and pegmatite dyke© emplacement preceded the amplaeemeat of the B e l l f s Gate •1 23 granodlorlte, therefor® they can he dated mostly as post-Custer fjranodlorlte, pre*Hell»s gat® granodiorlta, fhe w r i t e r was not able to determine any genetid r e l a t i o n s h i p between the pegmatites and e i t h e r of the g r a n o d l o r i t e s . I f the e l a s s i e a i view i s taken that pegmatites are l a t e stage features o f g r a n i t e or granodiorlta c o n s o l i d a t i o n , i t would appear that the pegmatites and aplita dykes are g e n t t i e a l l y r e l a t e d to the Custer granodiorlta* On the other hand, work l a the Massif Oaatrala CJung, J« and ftoqttae M, lf3S5 has shown that g r a n i t e or granodierlt® raplaoanant oan he preceded by dyfca esplaeeseiitf i f I t i s so i n t h i s ease the pegmatites and a p l i t e s i n tha Ball*a Qate area would fee r e l a t e d to the I e l lfs f a t e granodlorita* i CHAPTER V $im and Snap© goat ©f the Fraser River the Sail*® (late gran©** d i o r l t e forms a semicircular body emplaeed i s Cos tar gra»o« d l o r i t o aad i t outcrops nearly continuously im three miles along the road aeetloa (Map 3)* Tho southern l i m i t of th© outcrop i s 2500* aouth»«mst of mile post 132* Clos® to mils post 132 g l a c i a l d r i f t and talus bava eaysred the Igneous rocks, but at soma dlstmnoe baok off the road the Halloa Gate granodiorite forms steep c l i f f s p a r a l l e l i n g the road* i o r t h of mile post 132 the cliff« swing towards the road and for® a buttress through which a road tunnel has been constructed* From this point ©awards, for two miles the Roll*a Gate grmne* d i o r l t e outcrops continuously* The granodiorite is a l i o found along a path leading down from the road to the Hell's 0ate flsbw&ys* Vest of the Fraser River outcrops of acid Intro* sives, approximately of the sam© age as the Bell's Gate grano-d i o r i t e occur* One mile east of the main mass, an Isolated -stock of Hall's Sate granodiorite,'one mile in dlaaeter has been emplaeed In rocks of the Hesameen and Ladner groups* PLATE . V 25 HELL'S GATE GRAN ODIOPJTE. F I G U R E S R O A D S E C T I O N , OVIAPIH. P O I N T B ) . I L L U S T R A T I N G T H E C O N T A C T R E L A T I O N S H I P S B E T W E E N T H E T W O G R A N O D I O R I T E S H o r i y n f a l S o a l e : 1 inch ^ aOO feek Fhutt&d arrfrcl/ne of sec//'rrriS'rr/s frr Custer L E G END Thrusf Zones. /Cone ofRock Breccias si My/or?//e. Plag/ocUse Porphyry. He Ik Gads Granocf/'orife. Cusfsr GrarjoJ/or/fe. T S I H*I&»S GATE nmmmmim nwrnm mmmfmn $9a*iaan 1©* 16 1? 18 55 ftagioeiast 55 45 60 Orthoalaaa *»** »*• 5 Quarts 10 10 10 20 5 5 4a 5 Blotlta 15 15 5 10 Avaraga grata Biz® (jam). 2 2 2 3 Petrography the Hall'a Gate granodiorite i s muoh aor® uniform i n composition than the Custer granodiorita* The fresh granodiorite i s white i n colour, the weathered surfaces are grey. It has an even-grained, random texture* Since the granodiorite i s deficient in. potassium feldspar, the nam® trondhjoaite (Ooldeobmidt i f16) could he applied to i t . The granodiorite wan found to contain between A% and 6 ( £ plagioelaae feldspar of the average composition ^90*85 * B 1 0 » 1 5 * plagloclaao crystals show strong oscillatory aoalng which resemhles that observed in the plagioclase crystals of the plagioclase porphyry dyksa;. The s i m i l a r i t y of soalng, aad other evidence given l a t e r , suggests a genetic relationship between the Se l l ' s Cats granodiorite ®nd the plagioclase porphyry dykes,. The M i l l ' s Gate granodiorite contains betweon 15 and 2$f quart*• Tho quartz has c r y s t a l l i z e d late and is generally I n t e r s t i t i a l to the plagioclase. The granodiorite was found t© contain less than 5^ orthoelase, B i o t l t e and hornblende ar® present, the b i o t i t e being th© more abundant, Where blotite-hornblande Intergrowths occur, the b i o t i t e appears to be replacing the hornblende. The general pleochrolsm scheme of the hornblende 29 crystals i s as followst % 1 X Dark Green Green l i g h t Green Five C Z angles were measured .and these were 11°, 15°, 16°, 19°, 20#» •Oscillatory zoning i s a common feature of plagio-clase crystals i n t h e granodiorite* fhe inner zones are wide and Irregular sided: while the boundary zones are narrow with p a r a l l e l sides, (Photomicrograph Plat© XIV). Os c i l l a t o r y z o n i n g Is generally considered to have been caused by d i f f e r e n t i a l movement between the l i q u i d and soli<S fractions o f a magma. It i s th© writer's belief that soning can also be produced In a s t a t i c environment, fhe width of a zone produced in a dynamic environment should be proportional to the time that a crystal remained in a l i q u i d o f constant composition, f h i s being th© case In the Hell's Sate granodiorite i t would appear, from th® relative width of the zones, that the crystals had a greater mobility at the end o f c r y s t a l l i z a t i o n than at the beginning. This conclusion Is not l o g i c a l . The following explanation provides a more l o g i c a l explanation o f the zoning that occurs. I n i t i a l l y , crystal movement throughout the magma was possible and the core seaae w f « formed, As eryatalllaatlon proceeded, crystal movement became less and the cuter acnes of the crystals were produced i n a static envir©i»ent« Temperature fluotua* tlons, superimposed on the uniform drop of temperature of the magna could produce sonlng* temperature changes of the magna apart from those due to heat losses could fee produced by chemical reaction mad/or loss of v o l a t i l a s . A riae In tanporatura would produce a calcic r i c h gone while a drop i n temperature would produce a sore sodic rich stone. & aaall amount of a#to»etasomatiam has taken place within tha granodiorite s forming narrow C 5 cm) blaaohad gone* in the rock*. Theas blaaohad eonea have baas oallad by the writer * ghost valae»$ soma of the bast exampiee of which outcrop eloae to M«P. 152* la soma places Joints hate exarted a structural c o n t r o l over the metasomatism and tha bleached grasodlorite occurs on both aides of a Joint* In most eases, however, there has bean no obvious structural control, and the ghost veins- form a *8toefcvark,»« .4 s n a i l number of quarts-feldspar veins ( 3 cm width) cat the tel l » s Gats granodiorlta and soma of these are bounded by a sons of bleached granodiorlta. In tha writer*s opinion, the metasomatism of the granodiorlta and tha intrusion of tha quarts«feldspar veins were peneconteaporaneous. A thin section study of the matosomatised grano* diorite showed that the intensity of metasomatism wars closely 31 related to the number of orthoelase veinlets catting the g r a n o d i o r i t e . The large valalets parallel the sides of the ghost v e i n s . Smaller ramifying vel&lets*. offshoots from the l a r g e r vel&lets, are found throughout the altered graao* diorlte. The plagioclase crystals within the altered *one have become opaquey the turbidity being oaused by large numbers of small inclusion*» The inclusions are anisotropic, with first order polarisation colours and are probably either aerioite or kaolin* In some eases replacement anti** p e r t h l t e hat boon formed* Mafic mineral* ia the altered %®m have been bleaehed and ehloritl»«d* Occasionally the boundary of a ghost vein would pass through a plagioclase crystal} that portion of tho crystal lying within tho 1 ghost vein* Is opaque* while the rest of- the crystal would be fresh aad unaltered* Internal Structures The Soil's aote granodiorite contains very few menolifhs* A xooollth of Cuater granodiorite six feet wide Is found eloae to the southern eon tact between the two graoodiorltes* Veins of Hell** Gate granodiorite intrude the xo&ollth* k few small xenollths of dark qua* tal to are found Immediately north of the stcond road tunnel. The long axes of the aooolitha su?© vertical* Two sets of Jointsf one primary and one secondary, can be distinguished, Tim primary jointss davelopsd auring the consolidation of the Rail' s Gate granodlorlte, have for the most part been destroyed by th* later development of secondary Joints, Primary Joints bav* in some cases formed conduits for tha matasomatie solutions «iaanatiag from the Hall* a (Jata granodiorlta* In such oases a primary Joint can ha distinguished by a aona of bleached granodiorlta on either side of the -joint* Primary Joints form a rectangular bloc* pattern, •Secondary Jointing has baan produced by external forces and is discussed below* External Structures , The H«ll»s Gat a granodiorlta intrudes tha Caster granodiorlta* Tha sontham contact between th© two graao* diorite* is wall exposed half a mils south*aast of tha Hell's (Sata Cafe. The northern contact between tha two granodiorites has bean obscured by talus, but its approximate position is indicated on Map 3. Faulting close to tha southern contact has produced a series of fault blocks and has brought tha two granodiorites into juxtaposition, (figure §j# a sedimentary xenollth in the Custer granodlorlte close to the southern contact has been folded into a recumbent anticline, fhe axial plane of the anticline dips In the 33 same d i r e c t i o n as the f a u l t ©lasts f ^ - f1. fKf® (Figure 5 ) , which suggests that the f a u l t i n g Is thrust f a u l t l a g . fw© igneous contacts are present i n a d d i t i o n to the f a u l t c o n t a c t s . Along one of the contacts a p l a g i o c l a s e porphyry dyke has been intruded (Figure 5 ) . l e n o l l t h s of H e l l * s Gate g r a n o d i o r i t e i n the p l a g i o c l a s e porphyry dyke i n d i c a t e that the dyke i s yoaager than th® S e l l * a Gate g r a n o d i o r i t e aad not a marginal phase of th® g r a n o d i o r i t e , fhe second of the igneous contacts between the gr a n o d i o r i t e i t sharp. Contact metasorphlsrn of the Custer g r a n o d i o r i t e at the southern contact I s s l i g h t » close to the ecntttet i t has been Indurated over a width of one inch and has become more r e s i s t a n t to weathering. £«»fO numbers of c l o s e l y spaced t s t e e p l y dipping J o i n t s have developed i n the S o i l ' * ©ate g r a n o d i o r i t e , I n some cases c l o s e l y spaced J o i n t s have developed between the steeply dipping J o i n t s ( P l a t e f l ) # S i m i l a r Joints have also developed l a dykes Intruding the H e l l ' s Gate g r a n o d i o r i t e , fhe J o i n t i n g Is thus secondary and was formed a f t e r the emplacement and co n s o l i d a t i o n of the H e l l ' s Rate grano-d i o r i t e . O r i g i n fhe H e l l ' s Gate granodiorite was formed by eon-PLATE. VI. CLOSELY IN THE SPACED HELL'S JOINTING DEVELOPED GATE GRANODIORITE . 35 solid®tIon of a magma that had Intruded the Custer grano-d i o r i t e . fhe f i e l d evidence supporting t h i s statement i s as follows: (1) fhe southern contact Between the two grano-d i o r i t e s Is sharp* (2) fhe plagioclase porphyry dykes believed by the w r i t e r to he g e n e t i c a l l y r e l a t e d to the B a l l * a Oate g r a n o d i o r i t e were Intruded as magmas and consolidated a f t e r I n t r u s i o n * Evidence of o r i g i n obtained fro® work i n the laboratory i s given on page 60 et seg* Age fhe M l * a Sate g r a n o d i o r i t e i s younger than the Custer g r a n o d i o r i t e as has been shown by the contact r e l a t i o n , fhe Hell* s Gate granodiorite i s also younger than the pegmatite and a p l i t e dykes that Intrude the Custer g r a n o d i o r i t e , but i s o l d e r than the p l a g i o c l a s e porphyry and basic dykes* k granodiorite stock, east of the area s t u d i e d , has been napped and described by the e c o l o g i c a l ©urvey of Ganada as being the same age as the Hell's ©ate granodiorite* fhe stock i n -trudes rocks of the Ladner group, which has been dated as Upper <itara*8ie or p o s s i b l y Lower Cretaceous* fhe age of the Hall's Gate granodiorite i s therefore post-Upper Jurassic or possibly post-Lower Cretaceous* PLATE . VII HELL'S GATE GRAN0D40RITE INTRUDING. C.USTER GRANODIORITE . PLATE . VIII . • SAME CONTACT AS SEEN IN P L A T E . VII. PLAT E IX. THRUST FAULTS DEVELOPED CLOSE TO THE SOUTHERN CONTACT OF THE Q R AN ODI OR I TES . 3f CRaPSER VI General Features Bight p l a g i o c l a s e porphyry dykes were observed along the road section* f i v e of the dykes nave Intruded the Ouster granodlorite, two .have intruded the H e l l ' s date granodlorlte| and one has been intruded along the southern Igneous eontact between'the two granodioritss, ( f i g u r e 5 ) . the dykes have been c l a s s i f i e d under three headlngat simple type, multiple type, and prot©clastic type, Simple Type Occurrence . S i x of the eight plagioclase porphyry dykes have been classed at simple type dykes* Four of the dyfees i n * trwde the Custer granodiorlta, one intrudes tha S e l l ' s Gate granodiorlta, and another intrudes alone th® southern igneous contact of the two granodiorltea* 411 the simple type dykes are vertical or near v e r t i c a l but the s t r i d e v a r i e s from dyke to dyke* Petrography The s i x pl a g i o c l a s e porphyry dykes c l a s s i f i e d as 4 0 simple types nave ess e n t i a l l y tha same mineralogy and texture, fhey are porphyrltle, w i t h euhedral or ttubbedval plagioclase phenoerysts ranging ap to 5 mm. i n l e n g t h , fhe nam® p l a g i o -clase porphyry was used i n preference to a more r i g i d l y defined rook ter® because the mineral composition of the groundaass eould not be determined. A few quarts and b i o t i t e phenoorysts a l s o are prosant* One simple p l a g i o c l a s e porphyry - dyke outcropping c l o s e to the protcolastie dyke shows protoalastle textures* Forces have contorted and caused re-c r y s t a l l i z a t i o n of the quarts phenoerysts of tha simple dyke, and a few p l a g i o c l a s e phenocryate have been brecclated. fhe braeeiated phenoerysts are commonly c l o s e l y associated w i t h non*br«eeiat#i phenoerysts, a r e l a t i o n s h i p which suggests that the breceiation took place during the emplacement of the dyke* The p l a g i o c l a s e phenoerysts commonly show a strong o s c i l l a t o r y z o n i n g , s i m i l a r to that shown by tha p l a g l o * clase c r y s t a l s i n the Custer granodlorlta« s e v e r a l types of complex aonlng were observed* One type was produced fey the j o i n i n g together of two c r y s t a l s having an independent o r i g i n , ( f i g u r e 7>* P l a g i o c l a s e c r y s t a l s that formed I n -cl u s i o n s i n l a r g e r p l a g i o c l a s e c r y s t a l s were Boned lndspend* e n t i y of the host c r y s t a l * A f a i n t r e a c t i o n sone u s u a l l y occurs batveen th® host and guest c r y s t a l s l a the l a t t e r type* Quart® pasnocrysts occur in the pla g i o c l a s e 41 porphyry dykes but are not m abundant as ts® plagioclase phenocrysts. The quartz c r y s t a l s are anhedral, are u s u a l l y rounded ? and th® majority have undulose e x t i n c t i o n , A t h i s r e a c t i o n rim u s u a l l y surrounds the quarts phenocrysts. The r e a c t i o n rim appears to have the same mineral composition as the grottndmasa* but i s s l i g h t l y coarser i n g r a i n sins, P l a g i o c l a s e phenocrysts form 40]f of the average p l a g i o c l a s e porphyry, Quarts phenocrysts are not so abundant and they form $$ of the average p l a g i o c l a s e porphyry, B i o t i t e and hornblende are common mafic minerals of the simple dykes and together they u s u a l l y make up 10 +> 15$ o f tha mineral assemblage. The general pleochrolsa scheme of the hornblende crystal.was found t o fee S I X Bark Groan Green Light Brown Fiv e datarainatlons of C i were sad® and those were found t o be m f o l l o w s : f % 7 ° , f®t 12° and 1?°, The greundmaes forms about 40# ©f the average p l a g i o c l a s e porphyry* The low r e l i e f and f i r s t order p o l a r -i s a t i o n colours indicate that the groundmaaa i s l a r g e l y f eldspar and quart*. PLATE. X. PLAGIOCLASE PORPHYRY DYKE INTRUDING CUSTER GRANODIORITE. r i i o u K . t / C O M P L E X L Y Z O N E D P L A G I O C L A S E C R Y S T A L O C C U R R I N G IN A P L A G I O C L A S E P O R P H Y R Y D Y K E . X IO / • / , it r-:'L_YV\J \ -•-! ! • ''• ''7 ' A ^ • Mill c!"" 7- ' • -F I G . I F I G . 2. M O pi Average composition ab__ an DETERMINED P R O M R E F R A C T I V E INDICES S E C T I O N A - B F I G The cirysla.1 is osci ll&lory joined &.nd. dlso Ass mu.llz.ple iwiixnlTZ^. Ii is Jjelieved £Ii£il llie double core w<a,s £r>rsrzed drortz Iwo crystals Jomuzg txx£elher during Ine ci'L/st&,Lliz3.tLon per Lod. Nohe liiAt Hie ^ piling occurs independently of Ike Iwznnui^. H S f f i O pxjL0iocuaE pw>mst SHOCKS tmm&i pm&msdm $pmimm No. 6 1 11 14 1? a* P l a g l o o l a s a £ 4© 50 40 40 40 40 40' 40 Orthcttlta* % Quart* £ 5 5 5 10 5 10 10 Sofnalend* % 10 10 5 10 10 . 10 3 B l e t l t a * 5 5 10 5 5 5 ao Orotmteasa I 40 40 40 40 40 20 46 Avaraga a l s a of fhmmmfni twa) 3 6 1 1 2 2 5 A C t a t r a l 2©a# of tae® Dyfca 45 Multiple fypa Occurrence One multiple dyke outcrops Along the road. section. Map 3, Point 0) . the dyke, intruding th© Custer granodiorlta, strikes I I and dips 40® §*1, fhe dyke is made up of a 24-foot wide, eentral aone of nlagtoelaee porphyry, and boundary mmm of andasite* fha andasite en tha hanging wall side is tvo*fsat wide* while that on the foot wall is fonr*feat wide. Petrography fhe central sane of plagioclase porphyry has chiliad marginal contacts with the andasite son**, thus the emplacement of the andasite preceded the emplacement of the plagioclase porphyry* fhe and©site is dark green in colour and eontains a few large rounded crystals of quart** tha » i a e r a l assoc-iation of large quarts crystals i a an andesitic groundnass leads tha writer to believe that they are quarts xanadvysta* fhe andesite has a flow handed texture, Banadval and sub* hadral hornblende erystals are oriented with their long axes parallel* Plagioclase of the composition of Augg kn^4 and quartz have crystallised mnh#drally and interstlally to the hornblende* Ms ted below are the minerals that occur in th® boundary andasite: 4$ m 5% fhe central zont of the multiple iytes i s slssil&r l a mineralogy and tsxturo to the simple plagioclase porphyry dykes. Clos® t© the boundaries ®S tha central ion© the groimdmass of the plagioclase porphyry i s finer grained* but i t s t i l l remains c r y s t a l l i n e . Plagioclase phenoerysts close to the contact of the andasite ana porphyry have their long axes oriented p a r a l l e l to the boundary. Pretoulastle type Occurrence A protoela3tie dyke intrudes the Custer grano-dlorlte close to the southern and of the road section, (flap 3, Point A), fhe northern contact of the dyke with the Custer granodiorlta i s a sharp intrusive contact bat the southern contact has been obscured by the Intrusion of a s i s -foot wide basic dyke. A normal fault dipping south dis-locates the protoelastlc dyke. Along the fault there has been an apparent displacement of twenty feet (Plate XI, Figure 9). llas§ia~Z£i£iB& Hornblende Plagioclase (Ab^An^) Quart* (Xsnocrysts?) Quart* (Groundmass) F I G U R E 9 M A P 3 , POINT A . PROTOCLAST IC D Y K E 5 cade-- 1 mck = 25 feet. NW + T > + I B B -0- + -8- < 6 +• -0- +- I ' B 6 -0- + / '' r + + * + > ' , B B | B B B / + + + h % +• -a . * -a- +• TQ-+ -a + B g; B B I B B &; B B 6! B B 8 " , B B B SE / L E G E N D B&^/c Dyke Sh<aa/~&c/ P/?er7ocrysJ- Zone CusJ-<sr Gr<3f?oc//br-//'e-B a + + -o--0. + -vl '*"-*- - 1 F I G U R E (O T E X T U R E S O F T H E P R O T O C L A S T I C D Y K E CENTRAL ZONE C E N T R A L ZONE X 3.5 P. P L X3 2 CROSSED N1C0LS CONTORTED PHENOCRYST ZONE CHILLED MARGINAL ZONE X3.2 P.P. L. X3.2 CROSSED NICOLS L E G E N D J l a o i o c l a s e . p. : o g ma fic bands A3. PLATE. XI. , PRpTOCLASTIO DYKE 50 Petrography fhre© lithologlcal units vers distinguished. Sharp boundaries do not exist between th® l i t h o l o g l c a l units but for convenience of description they have been given the following ua&est c h i l l e d marginal sor.e, central gone, and contorted phenocryst Chiliad Marginal Zone The c h i l l e d marginal son® has a *axlntni width of two feet and forsss that part of the dyke in contact with the Custer granodiorite* fhe chiliad marginal aone rock is a l i g h t green ®lcr©p©rphyrltie rock containing plagioclase and quarts alcrophenocrysts (2 mm). Foliation has developed in the pTotoalaatlc dyke at i t s contact with the Custer grano-d i o r i t e , the planes of sehlstoslty paralleling the wail of the dyke. fhe average composition of the plagioclase i a the ©hilled Karglnitl son® is Ab^0 An^ Q, fhe largest nlcro* phenceryste are euhedral or subhedrait a**® o s c i l l a t o r y toned* and are occasionally twinned, the smaller nloropbenoerysts tend to b© anhedral and rounded, and are rarely zoned or twinned* In the c h i l l e d marginal gone quarts occurs l a four wayss 5 i 1* As rounded quarts phenoerysts that have undulose extinction* 2* As renrystalllsed phenoorysts s t i l l possessing their o r i g i n a l shape but now having a mosaic texture when viewed between crossed nlcolsf 3* As completely r e c r y s t a l l i z ^ c and contorted phenoorysts which have been attenuated and folded. This third stage i s only incipiently developed i n the c h i l l e d marginal son«| 4, 4s a a&Jor mineral i a the groundless of the c h i l l e d marginal rocks* Chlorite epidote and c a l c i t e occur together i n thin bands throughout the c h i l l e d sarginal sons« Cantral Zone The contact between the c h i l l e d marginal sone end the central sona i s gradatlonal* fhe central sons i s the largest of the three cones, being 100 feat wide at road l e v e l . In this zone the plagioclase phenoorysts attain their maximum size of 2.5 cm. Unlike the c h i l l e d marginal zone, tha central sons does not have a schistose texturej instead, bands of quartz and mafic Minerals occur folded into complex microfolds, (Figure 10, Plate XV). The composition of the plagioclase crystals i s AB^ o They are generally ontvinned. The plagioclase 52 phenocrysts tend to be opaque due to the presence of numer-©as small inclusions, I^rge inclusions of b i o t l t e ana horn-blende, which have their long axes oriented p a r a l l e l to the c r y s t a l boundaries, mm occasionally found i n the plag-ioclase c r y s t a l s , Undulos© extinction of the plagioclase crystals i s common, and a few of the crystals have bean brecclated. Oscillatory aonlng i s to be seen In aoae pheno-cryst s but It i s not a common feature. In contrast to the chilled, marginal son®, the quarts of the central son© occurs i n strongly folded bands and l e n t i c l e s rather than as phenoorysts. Between crossed nicols the quartz bands and lentlcl«s have a mosaic appear-ance. Thin bands of b i o t l t e and chlorite have been folded i n a manner s l a i l a r to that of the quart* bands, (Plate X?)« Snail anhedral crystals of hornblende are associated with the b i o t l t e , Sphene and aircor* occur as accessory minerals i n the rocks ©f the central aona* Sontorted Phanocryst Zom The contorted phenocryst «or?« i s 24 feet wide and dips v e r t i c a l l y . It is g*«dational into th® central zone on the north side and on the south i s In contact with the basic dyke which has Intruded along the southern contact of the protoclastic dyke and Custer granodiorite. The taineralogy and textures of the contorted phenocryst son® are s t e l l a r to 53 that of the central zone bat In contrast to i t the plagio-clase phanoorysts have been strongly breeelated or contorted* Origin fhe protoclastic dyke is younger than both the Custer granodlorlte and the pegmatite dykes cutting the Ouster gr&nodiorite, Th® dyke has the seme mineralogy as the simple- plagioclase dykes and the. central part of tha multiple dyke* Minor protoalastle textures ooeur l a some of th© simple porphyry dykes. The similarities between these rocks suggests that the protoelaatlc dyks is genetically related to the multiple and slapl© plagioclase porphyry dyke® out-cropping along tha road section* fhe writer .ballevaa that the textures found in the prot©elastic dyke was farmed by stresses acting while the dyke was consolidating* At the time of emplacement i t is believed that the dyke was a non-hoaogenous, partially con-solidated magna* Stresses acting at th® tie® of emplacement would affect} 1. plagioclase pnanocrysts existing in the eon-tor ted phanoarysts and eentral sones) 2. quarta phenocrysts of a l l thrs® zones, with those of central and contorted phenocryst aonas being affected the uost? 3. the mafic minerals of all thrse zones. 54 PROTOCLASTIC DYKE: Cooling history. JQ Ox Ll_ u o-V <0 o O. Dyke tinploctd. t o Or m CO >_ «»-o u 4> CD UI • e < -2 a 5 CD u O O N C 0» Q. w O o 2 o o u o i c o o to 55 The einerula reacted to the stress In different ways, Plagio-clase phenocryst3 were aither breceipted or contorted. Qnarta pfeaaoeryats vara reerystallieed and deformed Into highly folded bands. Tho mafia minerals were aligned i n strongly oontorted bands or, i n the cms of the chilled marginal aone, in bands pa r a l l e l i n g the wall of the dyke. After the i n i t i a l period of stress the dyke is bol.ltavad to hay® crystallised uadsr streas-free conditions, The lack of large plagioclase phenocrysts i n the ch i l l e d marginal zone was caused by a greater rate of cooling in this son©* Figure 11 I l l u s t r a t e s graphically what was be-lleved to hava been the cooling history of the protoelastlc dyke* the Relationship of th® Plagioclase Porphyry Dykes to the Hell's Out© Oranodlorlte The Hell* s Gate granodiorite and the plagioclase porphyry dykes are youngar than the pegmatites that cat the Custer granodiorite. three of the plagioclase porphyry dykes are younger than the Hell*s Sate granodiorite» but i t was not possible to determine whether the remaining five -plagioclase porphyry dykes were older or younger than tho granodiorite. fwo pronounced a i B l l a r i t l e s exist between the Ball's Gate granodiorite and the plagioclase porphyry dykes: 56 1, they have the same amoral ogy* 2, ths plagioclases possess a similar o s c i l l a t o r y foiling, Although the porphyry dykes In thvm instances post-date the Hell's Gate granodlorlte» tha nlneralegla and petrographlc evidence suggest that the granodiorit© and porphyry dyks® are genetically related. 57 CHAPTER ? i i Apart from th© dykes described above si* ©thai dykes of a more basic composition outcrop along tat road settle®, Tha widest dyke has a width of s i s feet. Ml are v e r t i c a l or nearly v e r t i c a l , there are two type*, ©»® rather light i n eolor ana the other nearly black. They set* to he of andaaltlc composition* tight Colored AndesItas Four dykes ar© included in this group* two of the dykes intrude tha Cuttar granodlorlte end the other two intrude the Hell* s date granodiorlta* All four dykes have undergone tha sagte intense fracturing as tha two- grartodlor* l i e s , and in one ease cleavage has developed* The andositea which are l i g h t green in color, have a B l o r o p o r p h y r i t i o texture* Sunedral or subhedral alarophanocrysts of hornblandu occur in a ground-mass of chlorite and altered plagioclase feldspar* The hornblende i s s l i g h t l y pleoohrolo sad Is unusual in that some crystal sections are nearly colorless along on© vibration direction* Th® hornblends crystals were found to have a wtxiMm extinction of C 2 * 2 3 ° , p i a f i o c l e s c c r y s t a l s are o p a q o e and th® r*ultlple twinning Is preserved only i n a few cases, l i s t e d below ar© the minerals found and t h e i r percentages? Homblsnde Altered plagioclar.a C h l o r i t e C a l e l t e Dark Colored Andest t e e Two b l u i s h - b l a c k dykes of andenitie composition are present i n th® s e c t i o n . On® dyke Intrudes along the southern contact of the protoelastlc dyke and the Custer granodiorite, th© other intrudes the Custer g r a n o d i o r i t e . Both of the dykes, i n contrast to the rocks thay intrude, are unshuttered* The andesitss havs a ciicroporphyritic texture and although the taicrophenocrysts hsva been eoapletely a l t e r e d to t a l c and carbonata the o r i g i n a l formation habit i s s t i l l preserved. Fresh, brown c o l o r e d , hornblende c r y s t a l s i n t e r -mixed with a c o l o r l e s s c h l o r i t a form most of the groundaass. The hornblende c r y s t a l s have ascicular habit and a f a i n t I , '10:1 59 pleochrolsm. The maximum extinction cycle of the hornblende was found to be C 2 8 SO®, A email amount of altered plagio-clase feldspar Is s t i l l preferred In the groundaase. L i s t e d below are the minerals found and their percentages. Hornblende 3% Plagioclase % Calclte \ % ehlwite 30* falc \%, 60 CHAPTER VIII PLAGIOCTA.^ T I E N I N G 0* T'H'-7 INTRU^TV" HOTifo Qoral's Work on Plagioclase twinning Goral (An,Mltnl9?l) has «ade a quantitative study of plagioclase twins that occur i n lentous and aetaaorphlc rocks* He elates, that f r o a a quantitative study of th© plag-ioclase twins, i t is possible to deter&lne the origin o f a granite. Lilted belov are the types of plagioclase twinning most commonly found in volcanic, p l u t c n i c , and matair*orphic rocks* CI) Volcanic Rocks Plagioclase crystals found in volc&nio rocks were twinned according to th® following laws: a l b i t e , carlsbad, albite-oarlabad, perlcllne or aeline, p e r i c l i n e , a c l l n e , aanebach, alblte«ala 8, ala a, haven©, ala B. albite tvinn-ing was found to be the Boat common type and ala B the least eoRacon type of twinning. In the volcanic rocks there was found to be only a slight difference in the type of twinning or twinning frequencies of plagioolaae phenecryets and of plagioclase crystals occurring in th© groundless. The plagioclase crystals fori-:inr the groundless tended to be richer in perlcllne/acline twins and the rare twins l i k e sanebach. 61 (2) Platonic Rocks Forming Patholiths and Stocks Plagloeiase crystals i n plutonic rocks are commonly twinned according to the following lawst albitc, p e r l c l l n e , a c l i n e , carlabad, and albite-earlsbad. Other types of twinning are rarely found, (3) Schists and Gneisses Plagioclase crystals found i n schists and gneisses are commonly twinned according to the a l b i t t , p e r i c l i n e , and acline laws. Other types of plagioclase twinning are rarely found* (4) Hornfaisle rocks Plagioclase crystals occurring i n hornfaisle rocks are twinned on the same laws as the plagioclase crystals of schists and gneisses. Th© hornfelsic rocks tend to be somewhat richer i n carlsbad and alblte*«arlsbad twins. The twinning found In the four groups of rocks l i s t e d above can be suaaarl2#d as follow*t 1. A l b l t e and p@rlci.ine or acline twins are vary common i n igneous and metamorphle rocks. a . Carlsbad and albite*»earlsbad twins are common only i n igneous rocks, but are occas-ionally found i n hornfelsic rocks* 3* Hare types of twinning l i k e manebaeh are confined almost entirely to igneous rocks. 62 Geral grouped twinned p l a g i o c l a s e i n t o two groups, C-twins and A*twins* C*twins are c h a r a c t e r i s t i c of, and r e s t r i c t e d t o , igneous rocks whereas A»twln® commonly occur i n aetsstorphic as w e l l as igneous r o c k s . I l l u s t r a t e ions of C*.twins and A*twins are shown in Figure 12. M u l t i p l e twins are almost e n t i r e l y twinned according t o a l b i t e , p e r l e l i n e , or a c l i n e laws ( f i g u r e 12, ©roup 1), and belong to the A*»tvin group* Simple twins tPlgur@l2-, Orottp 2) can be e i t h e r e*>twin® or A»tvlna and are not e a s i l y c l a s s i f i e d w i t h a flat stage microscope* Se r a i found that i f , a f t e r p l a c i n g the crystal i n the 4 5 ° p o s i t i o n , the twin u n i t s showed d i f f e r e n t retardation* the twin was a C-twln? i f the twin u n i t s had the same r e t a r d * atiofi, the twin could be either a Otwin or an A*>twln* Complex and intergrown twins ( f i g u r e 12, ©roups 3 and 4), are always C-twins, Gorai c l a s s i f i e d p l a g i o c l a s e c r y s t a l s i n t o one o f three groupst totwinned p l a g i o c l a s e ©*typ® Twinned p l a g i o c l a s e C-type Twinned p l a g i o c l a s e A-type l e measured the frequency of occurrence of the 0, C and *> type i n matamorphie and Igneous rocks and p l o t t e d h i s r e s u l t s on t r i a n g u l a r diagrams (Figure 13). A study of the t r i a n g u l a r diagrams show that there i s a d i s t a n t contrast F I G U R E 12. E X A M P L E L S OF P L A G I O C L A S E Twi NNING a. w a > 1-9% UJ a > Type 1- Poltysynlhelfe 7w/ns A. Type. Type 2,. Simple Tivins A.orC- Typxss. Type 3. Corryo/ex Twir?s" C. Ty, Attar M<3S30 Gons'-~Jyp<s 4-. P&nelr&l/off Tv/rrs yp e-Typz 6 4 i n the 0*A»C, ratios of plagioclase crystals found la «®ta-amorphic and igneous rocks, . laboratory Technique Excepting th® ambiguous case ©f siiaple twine showing equal retardation the writer found that i t was possible to distinguish reasonably quickly and accurately between Otwins and A-twins whan using a f l a t stag© s l c r o * scope, Simple twins that could not be classified wera ignored, for in the thin tactions studied the number of simple,twins was snail and ignoring the® did not affect appreciably the accuracy of the Method, "Even grained rocks wars found to b© the b#st to work with, but i t was also found possible to determine the U*A.*0* ratios of gnelasoao ami porphyrltlo .rocks* A Leltft f l a t stage microscope with an attached integrating stage was used* Traverses 1 w* apart ware made acres a the thin sections under test and the plagioclase crystals sncountared classified as U, A or 0 types* fhe number of counts per s l i d e ranged from 40 to 139* I f a plagioolaso crystal was encountered twice on successive traverses i t was counted twice* aa Ooral suggests. Besults Six u.A.C* determinations ware made on the plagio* 65 ©last; porphyry dykes* The plots were found to correspond with the hornblende andtslte*d«clto plots of Serai, tf*A.C* detefalaationa «ade on the two graaodlorltes» with one exception., agreed with th© tf.A.C. determinations aide by Ooral on aeli plutoaie rocks, A quart* dlorlto faciei of the Ouster granodiorite foil elose to the V«*A line of the t r i a n g l e , an utm&lms plot which the writer 1® unable to explain* fhe !J.A*C* plot of the chilled marginal mm fell midway between the Igneous aad the fcorafolale fields* The U.A*C. ratios of the orotoolftatio dyke were also anomlous. fhe H*A»Ct plots of the central and contorted phenocryst genes of the protoplastic dyke fell eloao to th® 0-apox of the triangular Alagraau Iha 0*a«C* ratios determined era tabulated in Figure 15 and the 0«A»C* ratios are tfaown plotted on a triangular diagram (Figure 1 4 ) , F I G U R E 13 U. A X . RATIOS OF IGNEOUS AND METAMORPHIC ROCKS. U.A.C. RATIOS OF PHEN0CRYST6 U.A.C. RATIOS IN PLAGIOCLASE IN VOLCANIC AND ALLIED ROCKS OF PLUTONIC ROCKS Gor-<3/ F I G U R E . 14 U . A . C . R A T I O S O F R O C K T V P E S C O L L E C T E D F R O M T H E F l E L D A R E A , L £ G E N D Porphyry Dy/<<ss • 6 Me//'s Oafs GnarrocZ/or/r'e + 19 Cusrer Gr&noc/ior/ks ^ 3 Ps^rr?s'//'/<ss ^ p 8 P'ro/'oc/ash'c Dykes 0 10 Numbers refer fa spec/men numbers. mmM Spec,. J U L ' No, Of Counts Custer Oraaodlorlte 3 43* m 6 7 Custer S r i i n o d i o r i t e 4 17* 49* 34* 61 Caster Graaediorlte 5 31* 56* 13* 97 Caster ftrenodlorite 22 275S 50* 2 ^ 51 Custer QraBodiorlte 41 m 651 20* 129 Pegmatite 8 m 59* 6* 68 Kell*s Gate Qranodiorlte 19 25* 46* ^ 29* 64 Sell*a date Oraaodlorite 33 5# 36* m P l a g i o c l a s e Porphyry 6 16* 19* 6»( 64 P l a g i o c l a s e Porphyry 11 31$ 25* 5# P l a g i o c l a s e Porphyry 14 *7* 26{f 47* 6 7 P l a g i o c l a s e Porphyry 24 6^ 7 0 P l a g i o c l a s e Porphyry 2$ 10* 30* 60* 54 P'lAgloalaaa Porphyry 2 6 36* 17* 4?* 43 ?**tool*stic Dyke1 10 74* m Q* 23 Pretoelastle Pyk@g 1© 6Q5C m 100 1 Central Ion® 2 Chiliad Kargloal Zone 69 CKaPTEB IX The Custer granodiorite intrudes and metamorphoses rocks of the Bcwa&oen group* It Is a heterogenous igneous Body whose stratigraphieal age haa been determined as Jurassic or possibly younger. Large numbers of pegai&tite and a p l i t e dykes intrude the Custer granodiorite* Two types of pegmatites, Bag&atle and metaaomatiet were found? in general the aagaatie pegmatites are later than th® metasom* atlc pegmatites* The K a i l 1 s Gate granodiorite intrudes the Ouster granodiorite and is also younger tfaasa the pegmatite and aplite dykes mentioned above. It is a uniform acid igneous body deficient la potassium 'feldspar* the strati* graphical age of the Hell*a Gat© granodiorite has been determined as post tipper Jurassic or potsibly post tower Cretaceous. Geaetieolly related to the Mall*a Gate grano* diorlte are a series of plagloalase porphyry dykts* Three of these dykes intrude tha Hall's Gate granodiorite, banc* the plagioclase porphyry phase i» part post-dates the granodiorite* The rooks along tha road section have bean intensely shattered and i n several places closely spaced 70 j o i n t i n g has been dovolepedU Thrust faulting has taken plane close to th® southern contact between the two gran©-dlorltcs, Tha narrow trending onterop of cueter gr«no*» dlorlte In this area could possibly haw® been produced by f a u l t i n g | and its shape suggests a Horst a-true tare* The conclusions drawn fro® the quantitative analysis of the plagioclase twins found in th© Hell*a Sate rocks agreed with conclusions based on tha field evidence. Plagioclase crystals In the protoolastic dyke tend to be utttwitmed and this suggests that twinning, say be Inhibited, by stress. PLATE. XII. CUSTER GRANODIORITE. crossed nlcols . x45. CUSTER GRAN ODI OR I TE. pp.1, x 45. 73 PLATE. XIV. HELL'S GATE GRANODIORITE. crossed nlcols x 45. 74 PLATE. XV. PROTOCLASTIC DYKE, cenrrol zone. x45. p-p.l. i P L A T E . XVI PROTOCLASTIC DYKE, conrorred phtnocryst zone. p.p.l. x45. M A P 2. GEOLOGICAL. MAP OF THE FIELD AREA MAP. 3 . 77 R O C K R E L A T I O N S H I P S A L O N G T H E S E C T I O N S E C T I O N DETAILS c B A HORN B L E N D IT E M U L T I P L E D Y K E N C O N T A C T BETWEEN T H E TWO Q R A N 0 D I 0 R 1 T E 9 F A U L T E D S. CONTACT OF T H E T W O G R A N O -DIORITES P R O T O C L A S T 1 C DYKE . • S C A L E 1 t 3 0 , 0 0 0 L E G E N D D DYKE BASALT -ANDESITE . P L A Q I Q C L A S E M P P O R P H Y R Y s / ~- H E L L S G A T E G R A N O D I O R I T E - H . H O R N B L E N D I T E + + +- C U S T E R . +• + G R A N O D I O R I T E . B I O T I T E S C H I S T — S E 0 L O S 1 C A U C O N T A C T T T T H R U S T GOLDEN NUGGET AUTO C A M P o PATH TO H E L L S G A T E PISH WAYS MAP. 4. 78 ROCK SPECIMEN LOCATIONS I S P E C I M E N S N ° I GOLDEN N U G G E T A U T O C A M P a Sc me S C A L E I : 3 0 , 0 0 0 L E G E N D D D Y K E B A S A L T . -A N D E S I T E • P P L A G I O C L A S E P O R P H Y R Y X / ~-\ / v . Y O U N G E R . »•/ <3RA.NODlOR.ITE H O R . N B L E W D 1 T E + 1 t t f O L D E R . GR.ANOD10R.ITE, Mil B I O T I T E S C H I S T G E O L O G I C A L C O N T A C T . T ' — T T H R U S T F A U L T . N?4£ N? 15 N».« 13. 39-4-1 N» 13. 57- 38.49 mMmmm Bowen, N.L, (1914)i A geological reconnaisance of the Fraser Valley froa Lytton to Vancouver, B.C., Survey Report for 1912, Oeol. 3urv. Canada. Buerger, K.J., (1945)? The genesis of twin crystals, Anser. Kin, Vol. 30 PP 469*482. Cairnes, C..E,, (1924): Coquihalla Ar@a, B,C,, CJaol. Ssrv. Canada, hem., 139. Etoanay, J.A*, (1940)t Width of Albita Twin La&a31ae, Afcor. Min. Vol. 25i pp» 578-586. Emmons, B.C., and Gates, U?43)s Plagioclase Twinning, Gaol* Soc. Aiser,, l u l l . , vol. 54, pp. 2 6 7 * 2 & f . Coralt K«| (1951): Petrologlc studies on Plagioclase twins, Aiaer, Min,, vol* 36, pp. 884*901, Hatch, F.H., W i s , 4.1., and Wells, K.X., (1952)* fhe Petrology of the Igneous Books* Rills» 1.8., (1936)i Reversed and oscillatory zoning in plagioclase feldstar, (tool. Mag. Vol. 73» pp. 49*56, Jung, and So«p«3# l l * , (1938)? tea Seaistes crystalline du Massif Coat ml* t Bull* Serv. Carte, geol. France, Wo, 197 (XXXIX)t p* 120, Ptosiste*, J», (1936): Zoning in plagioclase feldspar, Min. Mag. (1.XXII) p. 541 et sag, Read, R.H., <1946)t Place in plutonl3mf q.J.S.s, f o l . CIT. Pres. Address. Symposiua, (1948)s Origin of granite, fJeol. Soc* A*er. Kea* 28. Turner, F.J., (195D* Twinning of plagioclase in aetaaorphie rocks, teer. Min. (XXXVI), p 581 et sag. Tuttle, O.F., and Keith, (1954)J Granite problem, Ceol. mag, Bo. 1* Walton, li., (1955): The emplacement of granite, teer. Jour. Science, f o l . 253, PP. 1-16. 


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