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The Sooke gabbro Mitchell, William Sutherland 1973

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THE SOOKE GABBRO by . WILLIAM SUTHERLAND MITCHELL B.Sc, Un i v e r s i t y of Aberdeen, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the department of GEOLOGICAL SCIENCES We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973. In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced, degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department The University of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT The geology of the East Sooke peninsula comprises a core of o l i v i n e gabbro, p e r i p h e r a l l y u r a l i t i z e d and bounded on the north and east by upper Eocene Metchosin basalts of submarine o r i g i n . The gabbro intrudes the b a s a l t s which are unconformably o v e r l a i n by sediments of the Sooke Formation of Miocene-Oligocene age. The Sooke gabbro i n t r u s i o n i s an e l l i p t i c a l body of s l i g h t l y d i f f e r -e ntiated o l i v i n e gabbro which i s composed of c a l c i c p l a g i o c l a s e and c l i n o -pyroxene with minor o l i v i n e and orthopyroxene. The gabbro does not exhi b i t any obvious c r y p t i c or cumulate l a y e r i n g of the type which characterises many other layered basic igneous i n t r u s i o n s . Instead steeply dipping structures such as weak l a y e r i n g , f o l i a t i o n and l i n e a t i o n are believed to be flow s t r u c t u r e s . Intensity of u r a l i t i z a t i o n of the o l i v i n e gabbro increases near the margin of the i n t r u s i o n and towards fractures which appear to have acted as channelways f o r a convective flow of hydrous f l u i d s within and around the hot i n t r u s i o n . Concentrations of copper sulphides, deposited from these f l u i d s , are found i n s t r u c t u r a l l y favourable areas. The gabbro i n t r u s i o n i s thought to mark the p o s i t i o n of a vol c a n i c neck or feeder, now exposed by erosion of a th i c k sequence of Eocene submarine basalts which b u i l t up from ocean f l o o r i n a manner s i m i l a r to the Hawaiian chain of oceanic i s l a n d s . An hypothesis of oceanic o r i g i n f o r the basal t and gabbro sequence i s complemented by r e i n t e r p r e t a t i o n of the geophysical data from south Vancouver I s l a n d . The Metchosin basalts and gabbro i n t r u s i o n s are thought to represent an oceanic su i t e of rocks emplaced by ohduction on the southern t i p of Vancouver I s l a n d . 11X CONTENTS INTRODUCTION 1 GENERAL STATEMENT 1 LOCATION AND ACCESSIBILITY 1 TOPOGRAPHY AND GLACIATION 1 PREVIOUS WORK 4 PRESENT INVESTIGATION , 5 GEOLOGICAL SETTING 7 THE METCHOSIN FORMATION 11 INTRODUCTION 11 THE METCHOSIN BASALTS OF EAST SOOKE - 12 DISTRIBUTION AND DESCRIPTION 12 CHEMISTRY 14 STRUCTURE 14 METAMORPHISM 15 AGE OF THE BASALTS 16 THE GABBROS OF EAST SOOKE 17 INTRODUCTION 17 OLIVINE GABBRO 17 URALITIZED GABBRO 22 PLAGIOCLASE- AND OLIVINE-RICH PHASES OF THE GABBRO 23 LEUCOCRATIC QUARTZ DIORITES 24 BASALTIC DYKES ' 25 STRUCTURAL RELATIONS OF THE INTRUSIVE ROCKS 27 Internal Structures - Xenollths 27 - F o l i a t i o n l i n e a t i o n and l a y e r i n g 31 - Shear zones 33 External structure 35 DIFFERENTIATION 36 EMPLACEMENT OF THE GABBRO 42 AGE OF THE GABBRO 43 ALTERATION OF THE GABBROS 45 THE MINERAL DEPOSITS OF THE EAST SOOKE PENINSULA 57, GEOPHYSICAL DATA 61 GEOLOGICAL HISTORY OF THE AREA 70 SUMMARY AND CONCLUSIONS 75 LITERATURE CITED 77 APPENDIX 80 FIGURES i v Figure 1 Location of thesis area 2 2 Topographic map - East Sooke peninsula 3 3 The d i s t r i b u t i o n of Eocene marine basalts i n the Olympic peninsula and Vancouver Island 8 4 The geology of S. Vancouver Island 9 5 Paleogeologic map of western Oregon and Washington i n early Eocene time 10 6 Pla g i o c l a s e and o l i v i n e p o i k i l i t i c a l l y enclosed i n clinopyroxene 20 7 Pyroxene compositions 20 8 Metasomatic reaction at dyke contacts 26 9 P o r p h y r i t i c basalt 29 10 P a r t i a l l y r e c r y s t a l l i z e d basalt 29 11 R e c r y s t a l l i z e d basalt 30 12 R e c r y s t a l l i z e d basalt 30 13 Thinly banded gabbro 32 14 S t r i k e frequency diagram of a i r photograph lineaments 34 15 Coexisting pyroxenes 38 16 Coexisting clinopyroxenes, o l i v i n e and p l a g i o c l a s e 38 17 D i s t r i b u t i o n of analysed clinopyroxenes 39 18 D i s t r i b u t i o n of o l i v i n e compositions 39 19 Schematic representation of the. b u i l d up and. subsequent erosion of the vo l c a n i c p i l e 44 20 Incipient a l t e r a t i o n of o l i v i n e 46 21 Replacement of p l a g i o c l a s e by amphibole 48 22 ' Pla g i o c l a s e c r y s t a l - a l b i t i z e d along margins and fractures 48 Figure 23 Relationship between anorthite content of pl a g i o c l a s e and major fractures 50 24 Diagram i l l u s t r a t i n g hornblendic a l t e r a t i o n 50 25 Location of mineralized zones - East Sooke 58 26 Chalcopyrite i n t e r s t i t i a l .to bladed hornblende 59 27 Bouguer anomaly map of south Vancouver Island 62 28 Compiled geologic map of south Vancouver Island and part of the Olympic peninsula 63 29 Coastal Bouguer anomaly p r o f i l e across the Olympic peninsula and Vancouver Island 64 30 Bouguer g r a v i t y anomalies of New Caledonia, Papua and East Sooke 65 31 Travel-time curve, 1956 s e r i e s 67 32 P r o f i l e along the 1956 serie s 67 33 Travel-time curves - three layer i n t e r p r e t a t i o n 68 34 Model f o r oceanic crust 72 35 Schematic model f o r obduction of oceanic crust on to continental crust - S. Vancouver Island 73 v i TABLES. Table I. P l a g i o c l a s e compositions and s t r u c t u r a l s t a t e s 83-86 Table I I . An.% of e a r l y formed p l a g i o c l a s e i n gabbros and An.% of phenocrysts i n b a s a l t . 87 Table I I I . O l i v i n e compositions 87 Table IV. Modal a n a l y s e s 88 Table V. Chemical a n a l y s e s 89 MAPS Map A. G e o l o g i c map - East Sooke p e n i n s u l a 90 Map B. A n o r t h i t e percentages of p l a g i o c l a s e 91 Map C. S t r u c t u r e map - East Sooke gabbro. 92 Map D. Topographic lineaments 93 Map E . Sample l o c a t i o n map 9^ ACKNOWLEDGEMENTS This thesis was undertaken under the supervision of Dr. K.C. McTaggart who i n s t i g a t e d the study and made many useful suggestions. During the time of t h i s study the author was supported by a National Research Council of Canada Postgraduate Scholarship. 1 INTRODUCTION GENERAL STATEMENT The aim of t h i s t h e s i s was to study a gabbro i n t r u s i o n at East Sooke, Vancouver I s l a n d , and to consider the o r i g i n of the body i n the l i g h t of recent advances i n the understanding of layered and massive basic igneous rocks. LOCATION AND ACCESSIBILITY The map area, covering f i f t e e n square miles of the East Sooke peninsula, i s on Vancouver Island about sixteen miles southwest of V i c t o r i a ( f i g . 1 ) . The peninsula i s bounded on the south by Juan de Fuca s t r a i t and on the west and north by Sooke I n l e t and Sooke Basin. The northern part of the area i s r e a d i l y accessible from a secondary road which branches o f f Island Highway No. 14 about f i v e miles east of Sooke v i l l a g e . Numerous logging roads traverse the area but many are overgrown by scrub and s a l a l . A t r a i l constructed i n the summer of 1971 under the Opportunities for Youth Program provides access along the southern coast. TOPOGRAPHY AND GLACIATION R e l i e f i n the area ranges from sea l e v e l to almost 900 feet at Mt. Maguire and Babbington H i l l ( f i g . 2 ) . The area i s cut by precipitous north and north-easterly trending v a l l e y s most of which are Figure I Location of Thesis Area. F i g u r e 2 TOPOGRAPHIC MAP -2A3T SCOKE PENINSULA. 4 the topographic expression of f a u l t s . The c o a s t l i n e along Sooke Harbour and Basin consists of s i l t y beaches with l i t t l e rock outcrop. Much of the area i s covered by v a r i a b l e thicknesses of g l a c i a l d r i f t which consists l a r g e l y of a mixture of yellow sand, gravel and bluish-grey boulder clay with numerous boulders and cobbles. The d r i f t forms a thick blanket i n the northeast part of the area where rock exposure i s generally poor. Elsewhere the proportion of outcrop to d r i f t i s v a r i a b l e . Good exposures of bedrock are found along c l i f f and scarp sections on the south coast and i c e -sculptured exposures of the roche moutonnee type are common on h i l l s and r i d g e s . PREVIOUS WORK Copper m i n e r a l i z a t i o n i n the East Sooke gabbro was reported as e a r l y as 1893 but the geology was not described u n t i l 1912 when the Geological Survey of Canada published a memoir on southern Vancouver Island (Clapp 1912). Clapp described two stocks of basic p l u t o n i c rocks at East Sooke and Rocky P o i n t . He also noted the presence of small gabbroic bosses along the coast northwest of Sooke Harbour. Clapp subdivided the gabbro in t o amphibole gabbro, f i n e grained augite gabbro, o p h i t i c gabbro, o l i v i n e anorthosite, anorthosite and various pegmatites. He noted that the gabbro had suffered from "considerable metamorphism, rendering the rock g n e i s s i c and shearing i t along w e l l marked and often extensive shear zones". The age of the Metchosin basalts was at t h i s time thought to be T r i a s s i c or Lower J u r a s s i c . Consequently Clapp postulated an Upper J u r a s s i c age for the intrusions as they could be 5 seen to intrude the Metchosin rocks and to be much older and unconformably o v e r l a i n by the Sooke and Carmanah formations. H.C. Cooke, i n a Geological Survey of Canada museum b u l l e t i n published i n 1919, described the gabbro body of East Sooke. The b u l l e t i n i s accompanied by a geologic map of the East Sooke peninsula. Cooke subdivided the gabbro into mappable units of o l i v i n e gabbro, augite gabbro, anorthosite and g r a n i t e . He found a s p a t i a l r e l a t i o n -ship between these units i n that the o l i v i n e gabbro occupies the centre and forms the main part of the East Sooke mass while the augite gabbro and granite l i e on the periphery. Cooke believed that the differences i n rock type were caused by "the processes of d i f f e r e n t -i a t i o n acting on an o r i g i n a l l y homogeneous magma whose composition was probably that of a rather basic b a s a l t " . By t h i s time the Metchosin basalts were recognized to be of Upper Eocene age. Therefore Cooke postulated a Lower Oligocene age f o r the Sooke i n t r u s i v e s . Copper deposits of the East Sooke peninsula ensured a continuing i n t e r e s t i n the area and reference i s made to the deposits i n B r i t i s h Columbia M i n i s t e r of Mines Annual Reports of 1893, 1902, 1908, 1915, 1916, 1917, 1918, 1919, 1925 and 1931. In the Annual Report of 1948, Fyles summarised the economic h i s t o r y of the area and described some of the mineralised zones. PRESENT INVESTIGATION F i e l d data for t h i s t hesis was c o l l e c t e d i n the f a l l of 1970 and the spring of 1971. The gabbro i n t r u s i o n and the Metchosin basalts occurring on the East Sooke peninsula were mapped on a scale of 1:12,000. Other bodies of gabbro, although not mapped i n course of the present study, were examined. Since the o l i v i n e and augite gabbros were v i r t u a l l y i n d i s t i n g u i s h a b l e i n the f i e l d , mapping was supplemented by microscopic examination of approximately one hundred specimens of the gabbro. 7 GEOLOGICAL SETTING B a s a l t i c p i l l o w lavas of the Metchosin Formation form a westerly trending b e l t across South Vancouver I s l a n d . These Eocene basalts are part of a voluminous submarine volcanic assemblage of the P a c i f i c North-west ( f i g . 3 ) . Volcanic rocks of the Metchosin Formation are d i f f e r e n t from other T e r t i a r y volcanics on the mainland of B r i t i s h Columbia which are, as a r u l e , subaerial flows s i m i l a r to the Columbia River b a s a l t s . A s e r i e s of dyke- and s t o c k - l i k e gabbroic bodies known as the Sooke I n t r u s i v e s , are i n t r u s i v e i n t o the Metchosin b a s a l t s . The largest of these gabbro masses i s the e l l i p t i c a l body which forms the peninsula of East Sooke. Other gabbroic bodies are noticeably elongate i n a northwesterly d i r e c t i o n ( f i g . 4 ) . Carson (1968) obtained a 43 m i l l i o n year K/Ar age (Upper Eocene) from the gabbro. The gabbro i s unconformably o v e r l a i n by sediments of the Sooke and Carmanah Formations of Upper Oligocene - Lower Miocene age. The Sooke Formation, found i n many places as downfaulted blocks w i t h i n the gabbros or b a s a l t s , consists mainly of conglomerates with minor bands of yellowish sandstone. The conglomerates are derived d i r e c t l y from the Metchosin Formation and the Sooke I n t r u s i v e s . A d i s t i n c t tectonic break, the Leech River F a u l t , separates the b e l t of Metchosin basalts from the s c h i s t s of the Leech River Formation which l i e north of the f a u l t . Sooke gabbro i n t r u s i v e s are not known to intrude the Leech River Formation which i s believed to be of Palaeozoic or Lower Mesozoic age. Thus the geology of Vancouver Island south of the Leech River Fault which shows clear a f f i n i t i e s with the geology of the Olympic province to the south i s d i s t i n c t from that of the rest of the Isl a n d . 8 F i g u r e 3- The d i s t r i b u t i o n of Eocene marine b a s a l t s i n the Olympic p e n i n s u l a and Vancouver I s . a f t e r Danner 1955. Leech River Formation. Work Gneisses. Figure 4. The geology of S. Vancouver Island ( a f t e r Carson and Muller) 10 Hypothetical cross section near latitude 45* North. F i g u r e 5« Paleogeologic map of western Oregon and Washington i n e a r l y Eocene time, ( a f t e r Snavely and Wagner 1965) 11 THE METCHOSIN FORMATION INTRODUCTION The Metchosin b a s a l t s i n the area comprise part of an extensive t r a c t of Eocene p i l l o w lavas exposed on south Vancouver Island and on the Olympic Peninsula ( f i g . 3 ) . The volcanic assemblage consists mainly of submarine, b a s a l t i c lavas erupted on to ocean f l o o r occupying the area which i s now the Olympic Peninsula and south Vancouver Island ( f i g . 5 ) . The lavas are estimated to cover more than 60,000 square m i l e s . In the northeastern Olympic area the lavas are more than 15,000 feet thick (Snavely and Wagner 1963) and Clapp (1917) considered a thickness of 7,500 feet to be a moderate estimate of the Metchosin v o l c a n i c p i l e on south Vancouver I s l a n d . The volume of Eocene basalt erupted i s estimated as being at leas t 40,000 cubic miles (Waters 1955). A f i g u r e of 100,000 cubic miles of Eocene basalts i s quoted by McKee (1972). Volcanism was l a r g e l y submarine and p i l l o w lava-palagonite com-plexes dominate the volcanic p i l e . Some of the b a s a l t i c flows are s p i l i t i z e d (Park 1944). Waters (1955) postulated that "water, a l k a l i s , s i l i c a and other e a s i l y removable constituents stewed from the slowly metamorphosing root of geosynclinal sediments as i t was downbuckled to form a tectogene" caused the " s p i l i t i z a t i o n " or a l b i t i s a t i o n of the b a s a l t s . The basalts i n t e r f i n g e r complexly with cherts and marine tuffaceous sedimentary rocks containing pelecypod and gastropod faunas. 12 Submarine eruptive centres appear to have been i n t e r m i t t e n t l y a c t i v e over most of the Eocene and e a r l y Oligocene (Snavely and Wagner 1963). Sit e s of underwater basalt or sediment accumulation s h i f t e d from place to place, and because of s l i g h t d ifferences i n age, the Eocene basalts have been given many d i f f e r e n t l o c a l formational names. In western Washington, the Eocene lava i s known as the Crescent Formation. The same s t r a t a on Vancouver Island, across the S t r a i t of Juan de Fuca, co n s t i t u t e the Metchosin Formation. Other names are used i n the Coast Range of Oregon - Tillamook Volcanic Series i n the northern part of the range, S i l e t z River Volcanics i n the c e n t r a l part, and the Umpqua Formation toward the south end of the range. THE METCHOSIN BASALTS OF EAST SOOKE - DISTRIBUTION AND DESCRIPTION Metchosin rocks outcrop on the north and northeastern edge of the East Sooke peninsula and are d i s t r i b u t e d p e r i p h e r a l l y to the gabbro which forms the c e n t r a l core of the area. Although there i s no outcrop of Metchosin basalt on the southwest part of the peninsula facing Juan de Fuca S t r a i t , large fragmented screens and xenoliths of baked and p a r t l y remelted b a s a l t i c rocks are there enclosed by the gabbro. I t seems probable therefore that the southerly contact between the gabbro and the basalt l i e s j u s t offshore. The Metchosin basalts exposed i n the north and northeast are dark grey, commonly weathered reddish brown. The basalts exhibit a range i n textures from aphanitic or fine-grained types to d i s t i n c t l y p o r p h y r i t i c v a r i e t i e s . 13 M i c r o s c o p i c a l l y , the fine-grained basalts are h o l o c r y s t a l -l i n e and generally e x h i b i t a diabas i c or sub-ophitic texture i n which aggregates of pyroxene c r y s t a l s enclose long laths of p l a g i o c l a s e . The basalts consist of almost equal parts of pla g i o c l a s e and pyroxene with subordinate magnetite, apatite and poss i b l y i l m e n i t e . Various amphiboles, j a r o s i t e , c h l o r i t e , epidote and s c a p o l i t e are secondary a l t e r a t i o n minerals. The pl a g i o c l a s e of the basalt i s la b r a d o r i t e (An 60-70) and i s normally zoned. In some of the more a l t e r e d rocks the margins of the pla g i o c l a s e c r y s t a l s are a l b i t i c , and h a i r l i n e veins which cut the c r y s t a l s are f i l l e d with a l b i t e , epidote and u r a l i t i c amphibole. The pyroxene i s a brownish coloured augite which i s s l i g h t l y pleochroic and i s generally a l t e r e d , at lea s t i n p a r t , to u r a l i t i c amphibole. The a l t e r a t i o n , where not pervasive, generally takes place along c r y s t a l margins or along fractures and cleavage planes within the c r y s t a l . Fresh augite contains d u s t - l i k e i n c l u s i o n s of magnetite or ilmenite arranged i n p a r a l l e l l i n e s nearly perpendicular to the prismatic cleavage. Phenocrysts i n the p o r p h y r i t i c v a r i e t i e s of the Metchosin basalt are bytownitic p l a g i o c l a s e , the anorthite content ranging from An 86 to An 78. The plagioclase phenocrysts exhibit normal zoning. O l i v i n e , though usually absent from the Metchosin b a s a l t s , i s a minor constituent i n some of the more p o r p h y r i t i c v a r i e t i e s . Aggregates of o l i v i n e a l t e r a t i o n products, mainly serpentine and c h l o r i t e , commonly pseudomorph the o r i g i n a l mineral. CHEMISTRY The chemical composition of four Eocene Metchosin basalts from the Olympic Peninsula i s shown i n column 6 of Table V. Although the analyses of the Metchosin basalts are not i d e n t i c a l to those of the average ocean t h o l e i i t e s , they show generally low values of K2O, T i02 and ?2®5' T ^ e Metchosin basalts also exhibit high AI2O3 and CaO values and a high Na/K r a t i o . These features are supposedly c h a r a c t e r i s t i c of ocean t h o l e i i t e s (Cann 1971). The analyses of the Metchosin basalts are also characterised by r e l a t i v e l y high s i l i c a and lime and low a l k a l i e s as are the t h o l e i i t i c basalts of the Hawaiian area (column 7). STRUCTURE There i s no obvious f o l d i n g of the basalts i n the East Sooke area other' than a l o c a l steepening of dip near the gabbro contacts. Basalts close to the contact dip northeasterly at approximately f i f t y degrees and the dip decreases r a p i d l y away from the gabbro. Pillowed sequences are found within the b a s a l t s . The pillows are i r r e g u l a r bulbous and tubular masses commonly broken by subsequent f r a c t u r i n g . P o r p h y r i t i c v a r i e t i e s of the b a s a l t , ( f i g . 9 ) , generally found close to the gabbro i n t r u s i o n , i n t e r d i g i t a t e complexly with the f i n e grained b a s a l t s . Immediately north of the Sooke Basin, s i l l - l i k e bodies of medium-grained rocks which are gabbroic i n aspect grade d i s t a l l y into f i n e grained v o l c a n i c rocks. The basalts are extensively fractured on a l l scales from 15 minor fractures to wide shear zones. There i s a dominant north-e a s t e r l y trend of shears and f r a c t u r e s , many of which extend i n t o the gabbro. The contact between the gabbro and basalts i s a sharp i n t r u s i v e contact with no evidence of f a u l t i n g . A steeply dipping f o l i a t i o n developed i n the basalts close to the gabbro contact i s p a r a l l e l to the gabbro contact and i s formed by a p a r a l l e l alignment of metamorphic amphibole, pl a g i o c l a s e laths and by t r a i n s of magnetite g r a i n s . The f o l i a t i o n i s probably caused by r e o r i e n t a t i o n and -. r e c r y s t a l l i z a t i o n of the o r i g i n a l basalt f a b r i c to correspond with the stress system imposed by the i n t r u s i o n . METAMORPHISM The Metchosin basalts have been subjected to thermal meta-morphism at the contacts with the Sooke Gabbro. The t y p i c a l d o l e r i t i c texture of the basalts gradually disappears towards the gabbro contact. The mineralogy i s unchanged but feldspars become more equant and, along with pyroxene, form an i n t e r l o c k i n g equigranular mosaic i n d i c a t i v e of r e c r y s t a l l i z a t i o n . Hydrothermal a l t e r a t i o n of the basalts i s most extensive along fractures near the contact with the gabbro. Hydrated phases are common. Pyroxene i s progressively replaced by hornblende. Veins of c h l o r i t e , epidote and a c t i n o l i t e are common. Pla g i o c l a s e i s s a u s s u r i -t i z e d e s p e c i a l l y along fractures i n the c r y s t a l s . Near the veins and fractures i n the rock the pla g i o c l a s e laths are strongly zoned and margins of some c r y s t a l s show low or negative r e l i e f i n d i c a t i n g an 16 a l b i t e - o l i g o c l a s e composition. Scapolite i s seen mainly as a vein mineral although on occasion i t appears to replace p l a g i o c l a s e near the v e i n s . The i n t e n s i t y of hydrothermal a l t e r a t i o n i s always greatest along shear zones and i t i s obvious that the fractures acted as channelways f o r hydrothermal f l u i d s . The hydrothermal a c t i v i t y i s probably caused by a convective flow of heat and f l u i d s around the slowly cooling gabbroic stock ( c f . Norton 1972). This concept i s discussed more f u l l y i n a l a t e r section on hydrothermal a c t i v i t y . AGE OF THE BASALTS The Metchosin b a s a l t s are part of the voluminous sequence of submarine b a s a l t i c lavas i n the P a c i f i c Northwest which are Eocene i n age. A potassium argon age of 44±6 m i l l i o n years from hornblende i n a l t e r e d Metchosin Basalts i s given by Kirkham (G.S.C. 70-63). 17 THE GABBROS OF EAST SOOKE INTRODUCTION The gabbros comprise an e l l i p t i c a l body occupying the major part of the East Sooke peninsula. The major axis of the i n t r u s i o n i s almost f i v e miles i n length; the minor axis i s approximately h a l f t h i s length. The gabbros can be subdivided into d i f f e r e n t members of the gabbro family on the basis of mineralogy. Two main groups are d i s t i n g u i s h e d , an o l i v i n e gabbro which forms the core of the area arid a u r a l i t i z e d gabbro which l i e s on the periphery. The o l i v i n e gabbro grades into the u r a l i t i z e d gabbro by gradual disappearance of o l i v i n e and fresh pyroxene with concomitant increase i n amount of secondary amphibole. Other v a r i e t i e s such as pl a g i o c l a s e - or o l i v i n e - r i c h 'gabbros' are minor i n extent and are believed to represent layered phases of the gabbro. Leucocratic quartz d i o r i t e s are i n t r u s i v e into the gabbros and are thought to represent l a t e d i f f e r e n t i a t e s . OLIVINE GABBRO The o l i v i n e gabbro underlies the major part of the area and forms the e l l i p t i c a l core of the i n t r u s i o n (Map A). The average unaltered o l i v i n e gabbro i s a dark grey or black rock, composed of approximately 45% p l a g i o c l a s e , augite clinopyroxene and normally l e s s than 20 per cent o l i v i n e . Accessory minerals are a p a t i t e , sphene, ilmenite and magnetite. Sulphide minerals are generally absent from the o l i v i n e gabbro. The pl a g i o c l a s e i s p u r p l i s h or black giving r i s e to the dark, 18 almost ultramafic aspect of many of the gabbros. In the fresh o l i v i n e gabbro the anorthite content of the pla g i o c l a s e i s co n s i s t e n t l y high (Table I and Map B). Unzoned cores of the pla g i o c l a s e c r y s t a l s consist of c a l c i c bytownite. Normal zoning towards a s l i g h t l y more sodic p l a g i o c l a s e i s seen at the margins of the c r y s t a l s . The unzoned core generally comprises around 80 to 90 per cent of the area of p l a g i o -clase c r y s t a l and measurements using the Universal stage i n d i c a t e that the anorthite content of the marginal zones of the c r y s t a l s does not d i f f e r g r e a t l y from that of the core. The maximum measured diff e r e n c e s of anorthite content from core to margin are of the order of 10 to 15 per cent An. There i s no noticeable d i f f e r e n c e i n the composition of the cores of the pla g i o c l a s e c r y s t a l s i n the unaltered o l i v i n e gabbro e i t h e r l a t e r a l l y or with e l e v a t i o n . The i n t r u s i o n therefore does not seem to exh i b i t the c r y p t i c layering c h a r a c t e r i s t i c of many basic i n t r u s i o n s . The s t r u c t u r a l state of the plagioclase within the o l i v i n e gabbro i s generally low. Slow cooling of plagioclase a f t e r c r y s t a l l i z a t i o n i s believed necessary to permit s t r u c t u r a l rearrangements which lead to the formation of the more ordered structure of the low temperature f e l d s p a r s . P l a g i o c l a s e c r y s t a l s are i n v a r i a b l y twinned. A l b i t e twinning i s most common. Combined a l b i t e - c a r l s b a d , carlsbad, p e r i c l i n e , manebach and a c l i n e twins occur i n progressively decreasing incidence. Certain gabbros showing evidence of s t r a i n such as deformation lamellae i n o l i v i n e contain p l a g i o c l a s e with a higher incidence of "rare" twin types. In such c r y s t a l s the s t r u c t u r a l state also shows s l i g h t v a r i a t i o n s away from the ordered or low temperature s t a t e . 19 The predominant pyroxene i n the o l i v i n e gabbro i s an a u g i t i c clinopyroxene which has generally c r y s t a l l i z e d l a t e r than both o l i v i n e and p l a g i o c l a s e . Usually the clinopyroxene forms large p o i k i l i t i c c r y s t a l s which enclose p l a g i o c l a s e and o l i v i n e ( f i g . 6). Inclusions and t h i n i r r e g u l a r exsolution lamellae of a calcium-poor pyroxene are common within the augite c r y s t a l s ^ g i v i n g r i s e to a s c h i l l e r texture. Lamellae are generally exsolved p a r a l l e l to the (100) plane of the augite and are probably hypersthene. Unaltered clinopyroxene shows a very s l i g h t l y pink pleochroism. The c r y s t a l s also show a compositional zonation towards the margins. Compositions of a number of augite c r y s t a l s were determined o p t i c a l l y by the Universal Stage, and although o p t i c a l data on the clinopyroxenes are not completely r e l i a b l e , reasonably reproducible r e s u l t s were obtained using unaltered, twinned c r y s t a l s with few exsolution lamellae. The compositional zonation of the augite c r y s t a l s was too small to be determined by o p t i c a l methods. Compositions of the clinopyroxenes are shown on the pyroxene q u a d r i l a t e r a l ( f i g . 7). The clinopyroxenes are mainly d i o p s i d i c augites but an o v e r a l l i r o n enrichment trend can be observed. Calcium-poor pyroxenes are present i n small amounts i n only some of the o l i v i n e gabbros. These normally occur as rims bordering o l i v i n e c r y s t a l s or i n small c r y s t a l aggregates associated with amphibole, serpentine and magnetite which appear to be replacing the o l i v i n e . Orthopyroxene does not appear to have c r y s t a l l i z e d d i r e c t l y from the magma, but i t i s not possible to t e l l whether the orthopyroxene and abundant magnetite represent oxidation of the o l i v i n e or a reaction replacement. O p t i c a l data from a number of orthopyroxenes i n d i c a t e that 20 F i g u r e 7« Pyroxene compositions - trends of Skaergaard pyroxenes are shown by the dashed l i n e s . 21 they vary i n composition from bronzite to a r e l a t i v e l y magnesian r i c h hypersthene ( f i g . 7 ) . O l i v i n e i n the gabbro does not normally constitute more than 15 per cent of the rock. O l i v i n e has c r y s t a l l i z e d early and apparently almost coeval with early formed p l a g i o c l a s e . O l i v i n e o c c a s i o n a l l y encloses small laths of t h i s early p l a g i o c l a s e a l l of which i s a very c a l c i c bytownite (Table I I ) . In the fresh rock the o l i v i n e forms subhedral c r y s t a l s which are commonly embayed at the margins. Deformation lamellae i n o l i v i n e s are common i n gabbros containing complexly twinned p l a g i o c l a s e . Most o l i v i n e present i n the gabbro shows various stages of reac t i o n and a l t e r a t i o n ranging from a s l i g h t exsolution of magnetite and rimming by orthopyroxene to c r y s t a l s which are completely pseudomorphed by aggregates of i d d i n g s i t e , c h l o r i t e , serpentine and various other o l i v i n e a l t e r a t i o n products. The composition of the o l i v i n e i n the gabbros i s f o r s t e r i t i c . Samples analysed by X-ray methods range from Fo.82 to Fo.67 (Table I I I ) . Small amounts of magnetite are present i n the o l i v i n e gabbro. Textural r e l a t i o n s i n d i c a t e that some of the magnetite which occupies i n t e r s t i t i a l spaces has c r y s t a l l i z e d l a t e from the magma. Other c l o t s and blebs of magnetite are derived, as previously noted, from the break-down of o l i v i n e . Ilmenite, recognised i n polished samples, i s present i n very minor amounts. Other accessory minerals found i n the o l i v i n e gabbro are a p a t i t e , z i r c o n and rare sulphide. An analysis of o l i v i n e gabbro i s given i n column 1 of Table V. Microscopic textures i n samples of the East Sooke gabbro c l e a r l y demonstrate that there i s a d e f i n i t e sequence of c r y s t a l l i z a t i o n of the main mineral phases. O l i v i n e i s commonly enclosed by both 22 p l a g i o c l a s e and clinopyroxene. Only i n a few samples does o l i v i n e enclose small laths of extremely c a l c i c bytownite. Pyroxene i s commonly i n t e r s t i t i a l to both pl a g i o c l a s e and o l i v i n e and usu a l l y encloses both these minerals p o i k i l i t i c a l l y ( f i g . 6). O l i v i n e appears to have c r y s t a l l i z e d f i r s t , c l o s e l y followed and somewhat overlapped by the 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 . Clinopyroxene which has c r y s t a l l i z e d contemporaneously with most of the l a t e r formed p l a g i o c l a s e , has p e r s i s t e d l a t e i n the c r y s t a l l i z a t i o n sequence. Magnetite, which has c r y s t a l l i z e d l a t e , occupies i n t e r s t i t i a l spaces between the s i l i c a t e s but seldom grows large enough to produce a p o i k i l i t i c texture. There i s l i t t l e evidence that a cumulus mechanism has operated on a large scale within the magma chamber as true cumulus textures are seldom seen, except i n the o l i v i n e - r i c h gabbro. URALITIZED GABBRO Ol i v i n e gabbro, which forms the core of the i n t r u s i o n , grades in t o u r a l i t i z e d gabbro at the margins by gradual disappearance of o l i v i n e and gradual increase i n the i n t e n s i t y of u r a l i t i z a t i o n . The mapped d i s t r i b u t i o n of u r a l i t i z e d gabbro corresponds i n general with the augite gabbro mapped by Cooke. The change from o l i v i n e gabbro to u r a l i t i z e d gabbro can be distinguished i n hand specimen as a progressive replacement of greenish o l i v i n e by brown a l t e r a t i o n products. Microscopic examination shows that, though the u r a l i t i z e d gabbro i s t e x t u r a l l y s i m i l a r to the o l i v i n e gabbro, the mineralogy exh i b i t s c e r t a i n d i f f e r e n c e s . Plagioclase cores i n the u r a l i t i z e d gabbro are le s s c a l c i c than those of the o l i v i n e gabbro and range between 23 An. 65 and An. 70 i n composition (Table I ) . The p l a g i o c l a s e i s also more strongly zoned i n the u r a l i t i z e d gabbro. Almost a l l the augite i s a l t e r e d to a hornblende amphibole and only r e l i c t cores or blebs of pyroxene remain within the amphibole. The amphibole i s i n v a r i a b l y an a l t e r a t i o n mineral and has not c r y s t a l l i z e d as a primary magmatic mineral. Small c l o t s of c h l o r i t e , amphibole and iddingsite:surrounded by orthopyroxene are common i n many of the lea s t a l t e r e d gabbros. Occasionally small remnant cores of o l i v i n e occupy the centres of the c l o t s . These rounded c l o t s are therefore considered to be pseudomorphs formed by the a l t e r a t i o n of o l i v i n e . The u r a l i t i z e d gabbro therefore appears to be an altere d equivalent of the o l i v i n e gabbro of the core area. The a l t e r a t i o n i s believed due to hydrothermal a c t i v i t y and i s discussed more f u l l y i n a l a t e r s e c t i o n . PLAGIOCLASE- AND OLIVINE-RICH PHASES OF THE GABBRO Plag i o c l a s e - and o l i v i n e - r i c h phases of the normal o l i v i n e gabbro c o n s t i t u t e only a small part of the gabbro i n t r u s i o n and are gradational i n t o the o l i v i n e gabbro. Although most of the p l a g i o c l a s e -r i c h gabbros are i n the v i c i n i t y of Mt. Maguire, they also occur elsewhere as minor bands and pods within the o l i v i n e gabbro. The p l a g i o c l a s e - r i c h gabbro i s not a true anorthosite but i s rather a f e l d s p a t h i c phase of the o l i v i n e gabbro. P l a g i o c l a s e i n the rock seldom exceeds 65 per cent (Table I V ) . Many p l a g i o c l a s e - r i c h v a r i e t i e s of the gabbro are coarse-grained. Large p l a g i o c l a s e 2h c r y s t a l s and minor o l i v i n e are p o i k i l i t i c a l l y enclosed by large i n t e r s t i t i a l clinopyroxenes which are commonly u r a l i t i z e d . A strong f o l i a t i o n caused by a planar alignment of tabular feldspars and occasionally of other minerals i s usually developed i n the p l a g i o c l a s e - r i c h gabbro. The f o l i a t i o n , evident both i n hand specimens and t h i n s e c t i o n , i s i n v a r i a b l y steeply dipping or v e r t i c a l . O l i v i n e - r i c h gabbros or hornblende p e r i d o t i t e s occur near the eastern margin of the i n t r u s i o n . These contain as much as 50 to 60 per cent o l i v i n e . Pyroxene and minor p l a g i o c l a s e make up the remainder of the rock. The pl a g i o c l a s e i s a c a l c i c bytownite approaching anorthite i n composition. O l i v i n e forms the cumulate framework of the rock while p l a g i o c l a s e and pyroxene have c r y s t a l l i z e d i n the i n t e r -s t i t i a l spaces. Pyroxene, as i n the normal o l i v i n e gabbro, has c r y s t a l l i z e d l a s t and p o i k i l i t i c a l l y encloses the other mineral phases. A platy mineral f o l i a t i o n i n the o l i v i n e - r i c h gabbros i s also steeply dipping but i s les s obvious than that seen i n the p l a g i o c l a s e - r i c h gabbro. LEUCOCRATIC QUARTZ DIORITES Small bodies of l e u c o c r a t i c quartz d i o r i t e , previously mapped as granites and a p l i t e s by Cooke, are intruded into the gabbro. Most of these are dykes and pods which i r r e g u l a r l y cut across the gabbro and are too small to be shown on the geological map. The l a r g e s t area of l e u c o c r a t i c quartz d i o r i t e , on the western edge of the peninsula, i s an i r r e g u l a r dyke-like pod which cuts across the f o l i a t i o n i n the gabbro. I t therefore appears to have been intruded a f t e r the main mass of the gabbro had s o l i d i f i e d . 25 The l e u c o c r a t i c quartz d i o r i t e s consist mainly of p l a g i o -clase and quartz with minor amounts of green amphibole. The p l a g i o -clase which makes up approximately h a l f the rock i s strongly zoned from cores of andesine composition to a l b i t e - o l i g o c l a s e margins. Fractures which cut the p l a g i o c l a s e are bordered by a t h i n band of sodic p l a g i o -c l a s e . Quartz forms up to 20 per cent of the rock. The amphibole i s hornblende and i s generally present only i n minor q u a n t i t i e s . In decreasing order of abundance, magnetite, epidote, sphene and z i r c o n are the accessory minerals. The l e u c o c r a t i c quartz d i o r i t e i s thought to represent a l a t e d i f f e r e n t i a t e product of the gabbroic magma. BASALTIC DYKES Many narrow, fine-grained b a s a l t i c dykes intrude the gabbro cutting across both f o l i a t i o n and l a y e r i n g . Although most of these are less than twelve inches wide, are sharply bounded by matching walls and show no c h i l l i n g at the margins, there are exceptions. A number of dykes have i r r e g u l a r embayed contacts with the gabbro and show metasomatic r e l a t i o n s . In these the gabbro i s depleted i n ferromagnesian minerals f o r up to two inches away from the dyke contact ( f i g . 8). 26 F i g u r e 8 Metasomatic r e a c t i o n at dyke c o n t a c t s . 27 STRUCTURAL RELATIONS OF THE INTRUSIVE ROCKS Internal structures - X e n o l i t h s . In the southwestern part of the area the gabbro encloses many xenoliths and screens of b a s a l t i c Metchosin country rock. Some of the inc l u s i o n s are l a r g e , with dimensions i n the order of a few hundred f e e t . Others are small angular fragments with a matrix which i s generally gabbroic but i n c e r t a i n areas i s the l a t e l e u c o c r a t i c quartz d i o r i t e . The mineralogy of the inc l u s i o n s i s s i m i l a r to that of the gabbro except that i n the i n c l u s i o n s , o l i v i n e i s lacking and magnetite i s more abundant. Like the gabbro, the in c l u s i o n s have undergone v a r i a b l e u r a l i t i z a t i o n . There are however d i s t i n c t t e x t u r a l differences between the i n c l u s i o n s and the gabbro. In the leas t r e c r y s t a l l i z e d i n c l u s i o n s the diabasic texture or p o r p h y r i t i c fine-grained texture of the basalts i s r e t a i n e d . These lea s t r e c r y s t a l l i z e d rocks are generally found near the periphery of the i n t r u s i o n as large screens. Some of the x e n o l i t h i c blocks have become extremely coarse-grained at the margins i n contact with the gabbro and show a gradational decrease i n grain s i z e from the margins towards the centres of the x e n o l i t h s . In other i n c l u s i o n s the d i s t i n c t p o r p h y r i t i c texture of the p o r p h y r i t i c volcanics i s preserved. Smaller i n c l u s i o n s further away from the probable gabbro-basalt contact have been more extensively r e c r y s t a l l i z e d and have become almost as coarse-grained as the gabbro. M i c r o s c o p i c a l l y , the r e c r y s t a l l i z e d i n c l u s i o n s show a medium- to fine-grained mosaic of i n t e r l o c k i n g p l a g i o c l a s e and pyroxene c r y s t a l s . In the p o r p h y r i t i c v a r i e t i e s containing large 28 bytownite pl a g i o c l a s e phenocrysts, the matrix consists of s m a l l , equant p l a g i o c l a s e and pyroxene c r y s t a l s ( f i g j . 0 ) . Some of the most completely r e c r y s t a l l i z e d basalt i n c l u s i o n s e x h i b i t unusual graphic p l a g i o c l a s e and pyroxene intergrowth textures ( f i g . 12). A sequence of r e c r y s t a l l i z a t i o n textures from the moderately r e c r y s t a l l i z e d basalts to almost completely reconstituted rocks which are as coarse-grained as the gabbros can be observed m i c r o s c o p i c a l l y ( f i g s . 10- 12). In f i g u r e 11 the pyroxene c r y s t a l s are beginning to coalesce g i v i n g r i s e to a texture which i s believed to be the precursor of the graphic texture shown i n figure 12. The s t r u c t u r a l states of a number of plagioclase phenocrysts from the i n c l u s i o n s were determined o p t i c a l l y . Cores of these pheno-crysts generally e x h i b i t high intermediate s t r u c t u r a l s t a t e s , t y p i c a l of the disordered structure of quickly quenched volcanic rocks while the margins of the phenocrysts show the low s t r u c t u r a l state more t y p i c a l of i n t r u s i v e rocks. It seems strange that the s t r u c t u r a l state of the cores did not change a l s o . It i s p o s s i b l e that a chemical control as w e l l as a temperature di f f e r e n c e i s r e f l e c t e d i n the ordering of domains near the c r y s t a l margins. 29 F i g u r e 10. P a r t i a l l y r e c r y s t a l l i z e d b a s a l t ; u r a l i t i z e d pyroxene - brownish-yellow and p l a g i o c l a s e -grey and white. 30 F i g u r e 11. R e c r y s t a l l i z e d b a s a l t ; p l a g i o c l a s e - g r e y s , c l i n o p y r o x e n e - blue green. F i g u r e 12. R e c r y s t a l l i z e d b a s a l t , g r a p h i c i n t e r g r o w t h of p l a g i o c l a s e and c l i n o p y r o x e n e . 3 1 - F o l i a t i o n , l i n e a t i o n and l a y e r i n g . There i s no obvious layering or banding of cumulate nature with i n the gabbro, but at numerous l o c a l i t i e s a mineral l i n e a t i o n and f o l i a t i o n are wel l developed. A common feature of the gabbro i s a steeply plunging l i n e a t i o n which i s produced by a l i n e a r arrangement of ferromagnesian minerals and occasionally tabular p l a g i o c l a s e . F o l i a t i o n i s shown by a planar o r i e n t a t i o n of i n d i v i d u a l p l a g i o c l a s e and pyroxene c r y s t a l s and i n some areas intergrades with a t h i n banding or layering of the gabbro. The layering i s caused by the p a r t i a l separation of l e u c o c r a t i c and melanocratic c r y s t a l s into t h i n layers which give the rock a f i n e banded appearance ( f i g . 13). F o l i a t i o n and layering are almost everywhere steeply dipping or v e r t i c a l (Map C). In some l o c a l i t i e s the f o l i a t i o n i s p a r a l l e l to the contact between the gabbro and the basalt but t h i s i s not a general r u l e as the f o l i a t i o n swings around i n an i r r e g u l a r manner. Foliation,and i n some places layering are seen to wrap around d i s c r e t e a u t o l i t h i c blocks of gabbro which contain no s t r u c t u r e . Some of these blocks are round and the margins frequently show evidence of a s s i m i l a t i o n by the enclosing gabbros. In other places gabbroic rocks of s l i g h t l y d i f f e r i n g texture and mineralogy exh i b i t complex i n t r u s i v e r e l a t i o n s with each other, implying i n j e c t i o n of magma subsequent to at leas t the p a r t i a l c r y s t a l l i z a t i o n of e a r l i e r gabbro. 3 2 33 - Shear Zones. The gabbros are extensively fractured on a l l scales from minor j o i n t i n g to extensive shear and fracture zones which show up as marked topographic lineaments. Map D, prepared d i r e c t l y from a i r photographs, shows the d i s t r i b u t i o n of these lineaments. A strong north and northeasterly trend i s evident. A s t r i k e frequency diagram ( f i g . 14) based on 739 measurements of s t r i k e s of fractures and shear zones shows a predominant northeasterly trend. An i n d i s t i n c t northerly trend of fractures can also be d i s t i n g u i s h e d . Shear zones are now represented by wide areas of highly a l t e r e d rock. Movement has taken place even a f t e r the a l t e r a t i o n as secondary hornblende i s i n many cases fragmented and granulated. Certain shear zones within the gabbro are not strongly amphibolitized. These may be l a t e r or may not have opened enough to allow the passage of hydrothermal or deuteric s o l u t i o n s . In these shear zones f l a s e r gabbros show extreme microbrecciation and m y l o n i t i z a t i o n of fresh pyroxene and pl a g i o c l a s e which i n some cases are preserved as angular fragments within the m i c r o c r y s t a l l i n e sheared rock. The mylonitic zones vary i n extent from microscopic bands of crush material which cuts across strained but unaltered rock to zones of m y l o n i t i c gabbros up to ten feet wide. J o i n t i n g i s common throughout both gabbro and b a s a l t . J o i n t patterns are varied and complex and no r e g u l a r i t y could be determined even among conjugate j o i n t s e t s . 34 It F i g . 14 Strike frequency diagram of air photograph lineaments. 1739. measurements) 35 - External s t r u c t u r e . The gabbro i s i n i n t r u s i v e contact with the Metchosin volcanics along the north east margin of the body and at H i l l Head and Pim Head on the northern coast. Small occurrences of basalt i n contact with the gabbro are found at Company Point on the west coast and at Creyke Point on the east. Many e a s i l y recognizable screens and xenoliths are found along- the southwest coast facing Juan de Fuca S t r a i t . They are also recognizable near the gabbro basalt contact i n the northeastern part of the area but are d i f f i c u l t to trace because of heavy overburden. The preponderance of screens along the southern coast also suggests that the margin of the i n t r u s i o n i s j u s t o ffshore. The East Sooke gabbro therefore appears to be flanked on a l l sides by the b a s a l t . The steeply dipping contact between the gabbro and Metchosin basalts to the northeast of the area i s a sharp, i n t r u s i v e contact with no evidence of f a u l t i n g . The Metchosin basalt near the contact has, as already described, been r e c r y s t a l l i z e d and the texture r e c o n s t i t u t e d . The gabbro shows no evidence of c h i l l i n g and was probably intruded while the basalts were s t i l l hot. 36 DIFFERENTIATION There i s some evidence of d i f f e r e n t i a t i o n i n the gabbro i n t r u s i o n but i t s e f f e c t s are obscured by a l t e r a t i o n caused by l a t e hydrothermal or deuteric processes which are responsible for a zonal pattern of a l t e r a t i o n towards the periphery and shear zones. Universal stage work on calcium-rich clinopyroxenes shows that there i s a change i n composition which i s p r i m a r i l y a progressive I | replacement of Mg by Fe ( f i g . 7). The c r y s t a l s also show a zoning to more i r o n r i c h margins. The calcium content throughout the samples remains f a i r l y constant between Ca^g and C a ^ . Compositional trends of a u g i t i c clinopyroxenes from the East Sooke i n t r u s i o n are approximately p a r a l l e l to the early part of the Skaergaard trend and augite composition trends i n numerous other d i f f e r e n t i a t e d i n t r u s i o n s of b a s a l t i c composition. Compositions of orthopyroxenes coexisting with the c l i n o -pyroxenes also exhibit a s i m i l a r trend of i r o n enrichment ( f i g . 15). I f locations of clinopyroxenes whose chemical compositions are known are p l o t t e d on the map ( f i g . 17) i t i s seen that there i s a tendency f o r the more i r o n - r i c h pyroxenes to occur near the periphery of the i n t r u s i o n . The gabbros, therefore, seem to exhibit a zonal pattern of d i f f e r e n t i a t i o n and the trend of pyroxene compositions i s presumed to r e f l e c t equilibrium c r y s t a l l i z a t i o n of pyroxene from successively more i r o n - r i c h l i q u i d s formed by f r a c t i o n a l c r y s t a l l i z a t i o n . This zonal pattern of d i f f e r e n t i a t i o n i s , however, d i f f e r e n t from the usual case where the border i s more basic than the core. • 37 O l i v i n e compositions are magnesian r i c h and range from Fo82 to Fo67. Some of the l e s s f o r s t e r i t i c o l i v i n e s occur i n the peripheral zone of the gabbro but the c o r r e l a t i o n of increasing i r o n content with the peripheral gabbros i s much less obvious than even that observed among the clinopyroxenes ( f i g . 18). Although t i e l i n e s between co-existing o l i v i n e s and pyroxenes do not show a consistent c o r r e l a t i o n ( f i g . 1 6 ) , the range i n composition of the o l i v i n e s i s also thought to represent the same d i f f e r e n t i a t i o n trend whereby l a t e r f r a c t i o n s of the magma become progressively enriched i n i r o n . P lagioclase compositions within the o l i v i n e gabbro show a s l i g h t v a r i a t i o n . In fresh rocks containing f o r s t e r i t i c o l i v i n e , the p l a g i o c l a s e i s a c a l c i c bytownite and although there are several exceptions, the more i r o n - r i c h o l i v i n e s are found i n rocks which contain sodic bytownite ( f i g . 16). The p l a g i o c l a s e of the unaltered o l i v i n e gabbro i s only very s l i g h t l y zoned, much of the i n d i v i d u a l c r y s t a l being uniform i n composition with only narrow rims of s l i g h t l y l e s s c a l c i c p l a g i o c l a s e at the c r y s t a l margins. These les s c a l c i c rims are caused, •at l e a s t i n p a r t , by enlargement of the o r i g i n a l p l a g i o c l a s e c r y s t a l by successive growth of material from the cooling magma. D i f f e r e n t i a t i o n of the b a s a l t i c magma which formed the o l i v i n e gabbro i s therefore thought to be the cause of both the v a r i a t i o n i n composition and normal zoning of the p l a g i o c l a s e . The most important mechanism of d i f f e r e n t i a t i o n i n b a s a l t i c magma i s generally considered to be f r a c t i o n a l c r y s t a l l i z a t i o n , whereby early formed c r y s t a l s are e f f e c t i v e l y removed from the magma. In the e a r l y stages of c r y s t a l l i z a t i o n , separation of magnesian o l i v i n e and pyroxene increases the FeO/MgO r a t i o i n the melt. The l i q u i d also becomes 38 F i g u r e 15 C o e x i s t i n g pyroxenes F i g u r e .16 C o e x i s t i n g c l i n o p y r o x e n e , o l i v i n e and p l a g i o c l a s e F i g u r e 17. D i s t r i b u t i o n of analysed c l i n o p y r o x e n e s . c f . f i g . 1 6 f o r c o m p o s i t i o n s . comparatively enriched in silica which may even become concentrated in excess-of the amount corresponding to pyroxene-plagioclase mixtures. In the later stages of crystallization, a filter-pressed residual liquid may become greatly enriched in soda, potash and silica with corresponding impoverishment in alumina and lime. There is ample textural evidence that olivine appeared early in the crystallization of the gabbro and was as a rule not susceptible to reaction with the magma as it was enclosed by pyroxene or plagio-clase. Since the olivine is magnesian, the FeO/MgO ratio in the melt would be expected to increase. Residual leucocratic differentiates, enriched in sodium, potassium and silica and impoverished in alumina and calcium are probably represented by the leucocratic quartz diorites. These leucocratic differentiates are probably filter-pressed residual liquids subsequently injected as dykes and pods into gabbro which had already completed crystallization. Differentiation by fractional crystallization requires that the early formed crystals be effectively separated from the magma to prevent any possible equilibrium readjustments with the magma. Gravity settling is the most frequently invoked mechanism of removing crystals from the melt. Gravity settling aided by both intermittent and steady convection currents is the major mechanism of ;differentiation in the Skaergaard intrusion and there is abundant well documented evidence of gravity stratification and other 'sedimentary' features. (Wager and Brown 1967). The gabbros of East Sooke do not exhibit any obvious large scale g r a v i t y s t r a t i f i c a t i o n . Only l o c a l l y i s there any evidence that g r a v i t y settlement has occurred. F r a c t i o n a t i o n by g r a v i t y s e t t l i n g must therefore have played only a minor part i n the d i f f e r -e n t i a t i o n of the Sooke gabbro. There i s also no detectable c r y p t i c v a r i a t i o n i n composition of any of the minerals over the height of the exposed rocks, further evidence that the mechanism of d i f f e r e n t i a t i o n i n the East Sooke gabbro i n t r u s i o n was fundamentally d i f f e r e n t from that which operated i n the Skaergaard i n t r u s i o n . Other processes of d i f f e r e n t i a t i o n may have operated at East Sooke. The observed outcrop of o l i v i n e - r i c h rocks would seem to f i t the pattern expected for flowage d i f f e r e n t i a t i o n . Flowage d i f f e r -e n t i a t i o n i s an experimentally demonstrable process capable of causing c r y s t a l and chemical f r a c t i o n a t i o n i n nature (Bhattacharji and Smith 1964). It i s believed to be a possible mechanism for forming o l i v i n e - r i c h rocks i n a v e r t i c a l or steeply dipping p o s i t i o n without p r i o r concen-t r a t i o n on a f l a t f l o o r . It seems probable that t h i s type of mechanism rather than cumulate, gravity s e t t l i n g formed the o l i v i n e - r i c h gabbros of East Sooke. I f cumulate s e t t l i n g had occurred, i t i s d i f f i c u l t to explain why a l l the observed lay e r i n g i s almost v e r t i c a l rather than h o r i z o n t a l or sub-horizontal as i s the case i n t y p i c a l cumulate layered i n t r u s i o n s . EMPLACEMENT OF THE GABBRO The abundant in c l u s i o n s and screens of ba s a l t w i t h i n the gabbro suggest that stoping of the vol c a n i c country rock has at leas t played some part i n providing room for the i n t r u s i o n . Some of the basalt i n c l u s i o n s are completely r e c r y s t a l l i z e d . Basalts i n contact with the i n t r u s i o n have also been subject to r e c r y s t a l l i z a t i o n and have developed a l i n e a r f a b r i c p a r a l l e l to the contact. Such complex contact r e l a t i o n s would probably e x i s t i n the lower l e v e l s of a vol c a n i c p i l e , where a conduit supplying magma to the ever growing p i l e would eventually be expected to produce i n t r u s i v e r e l a t i o n s i n the e a r l i e r formed v o l c a n i c s . The observed r e c r y s t a l l i z a t i o n of volcanic rocks i n contact with the gabbro may be explained by t h i s mechanism whereby magma r i s i n g i n the conduit to supply the higher l e v e l flows would probably heat and even p a r t i a l l y remelt the e a r l i e r s o l i d i f i e d but s t i l l hot basalt flows. The inc l u s i o n s of basalt i n the gabbro probably represent blocks of b a s a l t i c country rock stoped from the walls of the conduit by the r i s i n g magma, which i n turn caused the r e c r y s t a l l i z a t i o n of the i n c l u s i o n s . The f o l i a t i o n , l i n e a t i o n and minor laye r i n g observed i n the gabbro are believed to be flow structures developed i n a c r y s t a l 'mush' of magma r i s i n g up the vent. Flow f o l i a t i o n and layering i s a widely recognized phenomenon i n lavas and i n t r u s i v e rocks (Balk 1937) , and i s believed by Thayer (1963) to be the p r i n c i p a l kind found i n alpine peridotite-gabbro complexes. The structures seen i n the.gabbros of East Sooke.are therefore considered to be flow structures formed during the flow of a p a r t i a l l y c r y s t a l l i z e d magma through the volcanic vent. The 43 i r r e g u l a r i t y i n flow f o l i a t i o n over small distances may i n d i c a t e complex v a r i a t i o n s i n the magmatic currents i n the volcanic conduit. Successive pulses of magma surging up the vent have displaced and engulfed blocks of e a r l i e r and at leas t p a r t l y c r y s t a l l i n e gabbro. These are the a u t o l i t h i c blocks around which the flow f o l i a t i o n i s wrapped. The poorly defined pattern of d i f f e r e n t i a t e d rocks within the i n t r u s i o n may also i n d i c a t e that the i n t r u s i o n represents a volcanic vent, p e r i o d i c a l l y recharged by in f l u x e s of parental magma. The Metchosin b a s a l t s , fed from feeders now represented by the gabbro i n t r u s i o n s , are thought to have b u i l t up from the ocean f l o o r i n a manner comparable to the build-up of the Hawaiian Islands. A schematic representation of t h i s i s shown i n fi g u r e 19. AGE OF THE GABBRO An age of t h i r t y - n i n e m i l l i o n years from potassium-argon determinations on hornblende i n the gabbro i s quoted by Carson (1968). A potassium-argon age of 44±6 m i l l i o n years from hornblende i n al t e r e d Metchosin basalts i s given by Kirkham (G.S.C. 70-36). Since amphibol-i t i z a t i o n of both basalts and gabbro i s believed to be related to l a t e hydrothermal c i r c u l a t i o n caused by the gabbro, these dates are considered to approximate the l a t e stages of magmatic a c t i v i t y . Much of the i n t r u s i v e a c t i v i t y seems therefore to have taken.place during the Late Eocene. Stage 1. Extrusion of b a s a l t i c lava as a pillowed sea f l o o r sequence. Stage 2 . Continuing b u i l d up of b a s a l t i c lava flows around f i s s u r e i n a manner s i m i l a r to the Hawaiian Islands. Development of flow f o l i a t i o n i n a p a r t i a l l y c r y s t a l l i n e magma- xenoliths and screens - r e c r y s t a l l i z a t i o n of e a r l i e r extruded basalts during t h i s stage. Stage.3. Erosion to present l e v e l . Figure 19 . Schematic representation of the b u i l d up and subsequent erosion of the volcanic p i l e . 45 ALTERATION OF THE GABBROS Primary magmatic minerals of the gabbro and basalt are pervasively a l t e r e d i n c e r t a i n areas. A l t e r a t i o n i s most intense along fractures and shear zones i n both gabbro and surrounding b a s a l t . U r a l i t i z e d gabbros characterize the peripheral parts of the i n t r u s i o n . The a l t e r a t i o n which generally r e s u l t s i n the formation of hydrous phases i s probably caused by hydrothermal f l u i d s c i r c u l a t i n g along fractures and shear zones. P r i o r to a l t e r a t i o n the i n t r u s i o n was probably a r e l a t i v e l y homogeneous and only s l i g h t l y d i f f e r e n t i a t e d o l i v i n e gabbro c o n s i s t i n g of s l i g h t l y zoned c a l c i c bytownite, c a l c i c augite and subordinate f o r s t e r i t i c o l i v i n e . I t i s maintained t h a t , from t h i s rock, v a r i a b l e hydrothermal action produced progressive amphibolitization towards the periphery of the i n t r u s i o n . Hydrothermal f l u i d s are also responsible for the increasing i n t e n s i t y of a l t e r a t i o n towards shear zones and fractures where the gabbro i s almost t o t a l l y replaced by a hornblende rock containing magnetite, p y r i t e and ch a l c o p y r i t e . A sequence of a l t e r a t i o n from the fresh o l i v i n e gabbro of the core region to the u r a l i t i z e d gabbros of the periphery and fracture zones i s w e l l seen i n thi n s e c t i o n s . O l i v i n e which i n the o l i v i n e gabbro generally appears as an unstable primary phase reacting to form ortho-pyroxene i s very susceptible to hydrothermal a l t e r a t i o n . At f i r s t serpentine and magnetite form i n cracks or fractures within the o l i v i n e c r y s t a l . Consequent expansion i n volume has produced fractures i n adjacent primary magmatic minerals which are f i l l e d with a mixture of serpentine and c h l o r i t e . Iddingsite and bowlingite, common a l t e r a t i o n products of kG Figure 20. I n c i p i e n t a l t e r a t i o n of o l i v i n e . O l i v i n e - high r e l i e f mineral at centre a l t e r i n g to brown i d d i n g s i t e and bowlingite 47 o l i v i n e , occur along fractures and form rims around the c r y s t a l ( f i g . 20). Iddingsite i s reddish brown i n colour and consists of haematite, goethite and an undetermined s i l i c a t e . Bowlingite consists mainly of c h l o r i t e and goe t h i t e . Both of these minerals form i n hydrous co n d i t i o n s . The formation of i d d i n g s i t e from o l i v i n e i s believed to be a continuous transformation i n the s o l i d state brought about by d i f f u s i o n of hydrogen atoms in t o the structure where they become 2+ attached to oxygens and so' release Mg, Fe and S i , and allow t h e i r ~ 3+ replacement by Fe , A l and Ca i o n s . (Deer et a l . 1962 V o l . 1). The pyroxene of the o l i v i n e gabbro shows i n c i p i e n t a l t e r a t i o n to a secondary;brown-green hornblendic amphibole. The a l t e r a t i o n begins at the periphery of the c r y s t a l or along cleavages and fractures and forms patchy areas of c o l o u r l e s s pyroxene flecked with small plates of u r a l i t i c amphibole. In the l e a s t a l t e r e d rocks p l a g i o c l a s e remains v i r t u a l l y unchanged, s l i g h t s a u s s u r i t i z a t i o n being the only i n d i c a t i o n of a l t e r a t i o n . Further hydrothermal a l t e r a t i o n causes increased replacement of o l i v i n e by a v a r i e t y of hydrous phases. I d d i n g s i t e , c h l o r i t e , amphibole and t a l c are more common than serpentine as replacement products. In some specimens rims of orthopyroxene surrounding the a l t e r a t i o n pseudomorphs provide evidence that unstable o l i v i n e within the o l i v i n e gabbro had undergone rea c t i o n to orthopyroxene before being completely replaced by the hydrous a l t e r a t i o n products. At t h i s stage of the a l t e r a t i o n sequence the pyroxene i s progressively replaced by amphibole and the rock, now devoid of fresh o l i v i n e , has the mineralogy of the u r a l i t i z e d gabbro of the pe r i p h e r a l 48 Figure 22, Plagioclase c r y s t a l - a l b i t i z e d along margins and fractures. 49 region. The p l a g i o c l a s e of the u r a l i t i z e d gabbro also shows evidence of increased a l t e r a t i o n . Epidote i s a common a l t e r a t i o n product r e s u l t i n g from the breakdown of the basic p l a g i o c l a s e which i n some of the more al t e r e d rocks i s almost completely s a u s s u r i t i z e d . In highly a l t e r e d rocks containing no o l i v i n e and i n which the pyroxene has completely a l t e r e d to amphibole, pl a g i o c l a s e i s d i r e c t l y replaced by green amphibole ( f i g . 21). The range of compositional zoning within the plagioclase increases i n the more a l t e r e d rocks. The v a r i a t i o n of anorthite content between the cores and margins of p l a g i o c l a s e becomes greater with increasing i n t e n s i t y of a l t e r a t i o n . In extreme cases, p l a g i o c l a s e with lab r a d o r i t e cores are zoned progressively to a l b i t e o l i g o c l a s e margins. The observed progressive zonation from cores to margins i n the p l a g i o c l a s e of the a l t e r e d rocks i s not considered to be only a primary magmatic feature. Since the range i n composition of the zoning i n the plagioclase corresponds d i r e c t l y with the i n t e n s i t y of a l t e r a t i o n , much of the zoning i s believed to be caused by the hydrothermal a c t i v i t y which produced the a l t e r a t i o n . The anorthite content of the cores of the plagioclases i n the a l t e r e d rocks i s s i g n i f i c a n t l y less than that of the p l a g i o c l a s e s i n the unaltered o l i v i n e gabbro (Table I ) . Although i t i s uncertain whether t h i s d i f f e r e n c e i s caused by primary magmatic d i f f e r e n t i a t i o n or.whether i t i s caused by secondary a l t e r a t i o n processes i t i s s i g n i f i c a n t that, even i n the o l i v i n e gabbro of the core area, as i n t e n s i t y of a l t e r a t i o n increases towards f r a c t u r e s , the anorthite content of the p l a g i o c l a s e decreases. This r e l a t i o n s h i p i s demonstrated g r a p h i c a l l y i n figure 23. It seems p o s s i b l e , therefore, that hydrothermal a l t e r a t i o n may have had an e f f e c t on the anorthite content of the p l a g i o c l a s e cores. 5 0 90 T 80 f An.% 70 f 60-50 9 200 400 DISTANCE FROM FRACTURE 600 Feet, F i g u r e 23 R e l a t i o n s h i p b e t w e e n a n o r t h i t e c o n t e n t o f p l a g i o c l a s e a n d m a j o r f r a c t u r e s . r e K , 0 fJo.,0 ? 2o 3 ! i ! i 1 _. i i _ - - 1 i "~ "i - ^  1 t i i • i i | s tf 1 i 1 I i r i i 1 1 .1 V ! ! J 1 ! I 1 I 1 i 1 0 0 ZOO 20OO yooo sooc 300 uoo 500 fcOO K>C Sooqoo\ooo F i g . 2 4 D i a g r a m i l l u s t r a t i n g h o r n b l e n d i c a l t e r a t i o n ( Cooke 1919) 51 The a l b i t i z a t i o n of fractured p l a g i o c l a s e i n the a l t e r e d rocks also provides evidence that hydrothermal f l u i d s could have produced the observed compositional zoning of the p l a g i o c l a s e . The a l b i t i z e d c r y s t a l s show a strong zonation towards margins of low r e l i e f p l a g i o c l a s e . Low r e l i e f a l b i t i c p l a g i o c l a s e i s also seen to border fractures cutting across the c r y s t a l ( f i g . 22). Some c r y s t a l s e x h i b i t a zonation towards the fractures while i n others there i s no zoning towards the a l b i t i c p l a g i o c l a s e bordering the f r a c t u r e s . A l b i t i c p l a g i o c l a s e does not seem to have been added to margins of fractures i n p l a g i o c l a s e c r y s t a l s , rather the part of the plagioclase c r y s t a l adjacent to the fractures appears to have been depleted of the anorthite component by f l u i d s passing through f r a c t u r e s . These f l u i d s are believed to be the same hydrothermal solutions which caused the a l t e r a t i o n of the gabbros. The a l b i t i z a t i o n and p o s s i b l y the strong zoning of the p l a g i o -clase i s apparently caused by a progressive cation exchange mechanism since the p l a g i o c l a s e s e r i e s i s the prime example of s o l i d s o l u t i o n i n v o l v i n g a coupled s u b s t i t u t i o n , Ca A l * Na S i to preserve charge balance. Experimental work ( O r v i l l e 1972) on p l a g i o c l a s e cation exchange e q u i l i b r i a with aqueous chloride solutions demonstrates that a l b i t i c p l a g i o c l a s e can be produced from a n o r t h i t e - r i c h plagioclases by addition of s i l i c a and sodium while calcium and aluminium are removed. The probable hydrothermal or deuteric r e a c t i o n c i t e d by O r v i l l e i s : 2NaAlSi 30g + CaCl2 - CaAl 2Si20 8 + 2NaCl 2 + 6 S i 0 2 . Reaction i n the l e f t hand silica-consuming d i r e c t i o n produces an a l b i t i c feldspar product whose volume i s larger than the o r i g i n a l feldspar and therefore no opportunity e x i s t s for opening up channels i n the structure 52 by pseudomorphic replacement. In other words, once a l b i t i c feldspar has formed at the margins of c r y s t a l s , i t seems that no further r e a c t i o n can take place and the r e a c t i o n ceases. Although reaction of t h i s type could have produced a l b i t i c zones at the margins and along fractures i n p l a g i o c l a s e c r y s t a l s i t i s doubtful whether i t caused the zoning of the c r y s t a l s before the reaction was terminated by the formation of the a l b i t i c margins. Adams (1968) c i t e s a mechanism of d i f f e r e n t i a l s o l u t i o n of p l a g i o c l a s e i n s u p e r c r i t i c a l water whereby p l a g i o c l a s e held i n water at 500°C to 800°C at 2kb. loses a l b i t e components i n s o l u t i o n while the anorthite components remain as a r e l a t i v e l y i n s o l u b l e residue. According to Adams, s o l u t i o n works inward from the surfaces of g r a i n s , producing a "reverse" compositional zoning i n the p l a g i o c l a s e . He also noted that the anorthite component could be removed p r e f e r e n t i a l l y by a c i d i c s o l u t i o n s . This mechanism could p o s s i b l y produce the normal zoning i n p l a g i o c l a s e by removal of anorthite i n a manner s i m i l a r to that which produced the reverse zoning by removal of a l b i t e . Anomalous s t r u c t u r a l states of many of the a l b i t i z e d feldspars i n d i c a t e that the structure has been disturbed by the a l t e r a t i o n . The cores of the c r y s t a l s consist of ordered l a b r a d o r i t i c p l a g i o c l a s e i n low s t r u c t u r a l state but the l e s s c a l c i c margins frequently show a departure from low s t r u c t u r a l state to a s l i g h t l y more disordered intermediate structure (Table I ) . In general, p l a g i o c l a s e , i n which the margins show p a r t i a l l y disordered structures of intermediate s t a t e s , i s common withi n the zone of u r a l i t i z e d gabbro. Most intense a l t e r a t i o n of the gabbros and basalts pccurs 53 along or within shear, or f r a c t u r e , zones. Pyroxene has been completely replaced by pleochroic green amphibole and the pla g i o c l a s e i s strongly s a u s s u r i t i z e d o r , i n more extreme cases, i s replaced d i r e c t l y by amphibole ( f i g . 21). The f i n a l product of the a l t e r a t i o n process i s a hornblendite consisting of a f e l t e d mass of long bladed c r y s t a l s of dark green common hornblende. Only minute i n t e r s t i t i a l grains of a l b i t i c feldspar and s c a p o l i t e remain. Epidote occurs i n veins and more commonly as segregation blebs and pockets. Apatite also occurs as i s o l a t e d blebs surrounded by cl u s t e r s of bladed hornblende. An analysis of the hornblendite i s given i n column 2 of Table V. In comparison to the analysis of the o l i v i n e gabbro (column 1) the hornblendite i s r e l a t i v e l y deplete i n s i l i c a , calcium and alumina but i s enriched i n sodium, i r o n , water and.to a l e s s e r extent i n magnesium and potassium. This i s i n accord with the conclusions of Cooke (1919) who u t i l i z e d a s t r a i g h t l i n e diagram ( f i g . 2 4) as a convenient means of expressing g r a p h i c a l l y the character of the changes that have taken place during a rock a l t e r a t i o n of any kind. The following d e s c r i p t i o n of the a l t e r a t i o n of the gabbros and the u t i l i z a t i o n of the s t r a i g h t l i n e diagram i s taken from Cooke's report of 1919: "The (straight l i n e ) diagram i s made up of a number of ho r i z o n t a l l i n e s , which are subdivided by v e r t i c a l l i n e s according to any convenient method. The diagram f i g u r e d , which has been contrived by Mead, i s so divided that a f i n i t e l i n e represents any quantity up to i n f i n i t y . One of the h o r i z o n t a l l i n e s i s a l l o t t e d to each of the component oxides of the rock. The p o s i t i o n of the point on each l i n e , which represents the change of that oxide during a l t e r a t i o n , i s obtained by d i v i d i n g the percentage of that component i n the chemical analysis of 54 the fresh rock by i t s percentage i n the a l t e r e d rock, and m u l t i -p l y i n g the r e s u l t by 100. The points so obtained on the various h o r i z o n t a l s may then be connected by l i n e s . The r e s u l t shows at a glance the r e l a t i v e gains and losses during a l t e r a t i o n , or the absolute gains or losses i f any factor i s knovm to have remained constant. Thus, i f any constituent has remained constant, then a l l of the constituents whose points f a l l to the r i g h t of the known point have decreased i n absolute amount, and the constituents whose points f a l l to the l e f t have increased. I f weight has remained constant, i . e . , i f 100 grams fresh has yielded 100 grams of a l t e r e d rock, then a l l constituents whose points l i e to the r i g h t of the v e r t i c a l 100-line have decreased i n absolute amount, those whose points l i e to the l e f t of t h i s l i n e have increased. I f an absolute change i n weight can be determined, i . e . , i f 100 grams of fresh are known to have yiel d e d 90 grams of a l t e r e d rock, then the v e r t i c a l l i n e 90 i s the zero l i n e , and points to the r i g h t or l e f t of t h i s represent absolute losses or gains r e s p e c t i v e l y . When lack of information renders i t impossible to f i x any point as constant, the r e l a t i v e gains or losses of the d i f f e r e n t constituents are a l l that can be determined. In the present case no one constituent can be assumed to have remained constant, as a l l were probably very soluble i n the hot s o l u t i o n s . I f alumina, the most insoluble oxide, were supposed constant throughout, the curves show that a large increase both i n weight and volume must have taken p l a c e , of which there i s no f i e l d evidence . . . . . the hornblendite i s about ten to f i f t e e n per cent heavier, volume for volume, than the gabbro. I f therefore the weight had remained constant throughout the 55 a l t e r a t i o n , i t must have been accompanied by decrease of volume. There i s no evidence observed i n the f i e l d that e i t h e r have occurred. The most probable assumption appears to be that volume remained constant or nearly so. I f so, the zero point for the horn-blen d i t e curve would l i e between the v e r t i c a l l i n e s 110-115 Under t h i s assumption, leaving out of consideration the minor components of the s o l u t i o n s , such as K2O, MnO, Ti02, ?2®5> which altogether make up less than one per cent of any of the rocks, i t i s seen that the hornblendite a l t e r a t i o n resulted i n s l i g h t increase of s i l i c a , large increase of i r o n , magnesium, soda and water, with loss of lime and alumina I t may also be seen from Figure 2k, that i n the main these conclusions are c o r r e c t , whether the hypothesis of constancy of volume be accepted or not, as most points are so f a r to the r i g h t or l e f t of the assumed v e r t i c a l zero t h a t , i n order to a l t e r t h e i r s i g n i f i c a n c e m a t e r i a l l y , quite inadmissable assumptions as to weight and volume changes would have to be made." Late me t a l l i f e r o u s solutions have caused s i g n i f i c a n t m i n e r a l -i z a t i o n i n some of the hornblende bearing rocks. P y r i t e , p y r r h o t i t e , marcasite, chalcopyrite and minor magnetite are i n t e r s t i t i a l to bladed hornblende. Shear zones containing hornblende rocks are of v a r i a b l e width up to approximately one hundred feet wide. Since the i n t e n s i t y of a l t e r a t i o n of the o l i v i n e gabbro decreases away from the shear zones at a rate generally proportionate to fracture density, i t appears that the a l t e r a t i o n pattern i s a consequence of the decreasing a c t i v i t y of hydrous f l u i d s away from the main shear zones. Any explanation of the observed a l t e r a t i o n pattern must take i n t o account the peripheral zone of amphibolitized augite gabbros. Rocks of the pe r i p h e r a l zone demonstrate the same c h a r a c t e r i s t i c s as the less intensely a l t e r e d rocks found near shear zones and are believed to have formed s i m i l a r l y by aqueous f l u i d s moving through the rock. Convective c i r c u l a t i o n (Norton 1972) of the f l u i d s i s believed to be caused by the hot i n t r u s i v e mass. Fractures i n the Metchosin country rock and gabbro i n t r u s i v e appear to have considerably aided the flow of f l u i d s and as a consequence the most intense a l t e r a t i o n of both volcanic country rock and gabbro i s found i n the v i c i n i t y of fractures and shear zones. 57 THE MINERAL DEPOSITS OF THE EAST SOOKE PENINSULA Copper m i n e r a l i z a t i o n at East Sooke was discovered i n 1863 and since then various deposits have received sporadic a t t e n t i o n . Detailed descriptions of the deposits and t h e i r h i s t o r y are found i n the Report of the M i n i s t e r of Mines, B.C. (Fyles 1963). The properties on which the mineralized zones occur are shown i n fig u r e 25. Five of the zones of copper m i n e r a l i z a t i o n have been explored by a d i t s , shafts or open cuts and ore has been extracted from two of them. A zone of magnetite m i n e r a l i z a t i o n at Iron Mine H i l l has also been subject tb some underground development. Copper deposits of the type found i n the Cooke, Heustis, G r i f f i t h and Merryth zones are the most common and most important of the area. These deposits are s t r u c t u r a l l y c o n t r o l l e d by shear or f r a c t u r e zones and a l l occur i n the highly a l t e r e d gabbros. The best m i n e r a l i z a t i o n i s found i n hornblende rocks at the i n t e r s e c t i o n of major north and northeasterly trending shears with east-west trending shear or fracture zones. Chalcopyrite, the common copper bearing mineral of the East Sooke copper deposits, i s disseminated throughout the a l t e r e d rock as an i n t e r s t i t i a l mineral between bladed hornblende ( f i g . 26). Many of the h a i r l i n e fractures and veins i n the hornblende rocks are f i l l e d with c h a l c o p y r i t e . Other sulphides present are p y r r h o t i t e , p y r i t e marcasite, and cubanite. The sulphides are almost t o t a l l y confined to the a l t e r e d gabbros and are v i r t u a l l y absent from the unaltered o l i v i n e gabbro. Although textures indi c a t e that most sulphide deposition occurred Figure 26. Chalcopyrite i n t e r s t i t i a l to blades of hornblende. 60 a f t e r the formation of the hornblendites, t h e i r o r i g i n i s also hydrothermal. It appears that hot aqueous f l u i d s c i r c u l a t i n g i n both the gabbro and the Metchosin country rock produced, i n the l a t e r stages of hydrothermal a c t i v i t y , concentrations of sulphides. The c i r c u l a t i o n of f l u i d s , as previously described, i s possibly caused by convective flow around the hot i n t r u s i v e . Fractures have acted as passage-ways f o r the hydrothermal f l u i d s and zones of extensively broken rock between cross cutting fractures or f a u l t s have allowed the passage of the l a t e sulphide r i c h s o l u t i o n s . These areas, therefore, are the s i t e s of the most extensive copper m i n e r a l i z a t i o n . Since the f a u l t s are generally steeply dipping or v e r t i c a l , the ore bodies, although i r r e g u l a r , are contained i n steeply plunging ore shoots which l i e along fracture i n t e r s e c t i o n s . Magnetite m i n e r a l i z a t i o n occurs i n veins and i s also disseminated through the a l t e r e d gabbro and i n some places through i n c l u s i o n s of country rock. Magnetite at Iron Mine H i l l i s concentrated i n f r actures w i t h i n and close to xenoliths and screens of basalt enclosed within the gabbro. At t h i s l o c a l i t y magnetite i s abundant and i n places forms veins up to s i x inches wide. Microscopic examination of p a r t i a l l y r e c r y s t a l l i z e d basalt in c l u s i o n s from the Iron Mine H i l l area i n d i c a t e that magnetite was highly mobile during the r e c r y s t a l l i z a t i o n process. Abundant magnetite i s disseminated through the rock but i s also concentrated i n t o veins and s t r i n g e r s which cross-cut the r e c r y s t a l l i z e d rock. Since many specimens of the Metchosin basalt examined are extremely r i c h i n magnetite, i t i s possible that the magnetite i n the basalts close to the i n t r u s i o n was mobilized and l o c a l l y concentrated by the hot but cooling magma of the i n t r u s i o n . 61 GEOPHYSICAL DATA A p o s i t i v e Bouguer anomaly, the Sooke High, which reaches a maximum i n the v i c i n i t y of Sooke i s shown i n the Bouguer Anomaly Map of Southwestern B r i t i s h Columbia (Walcott 1967). Over the basalts and gabbros of the Sooke area the Bouguer anomaly i s around +60 m i l l i g a l s . This anomaly decreases towards the south and reaches a minimum of -95 m i l l i g a l s over the Olympic Peninsula ( f i g s . 27 and 28). Over the Metchosin basalts of the Olympic Peninsula the anomaly i s around -20 m i l l i g a l s . The Sooke High also decreases northwards across the Leech River F a u l t . The change from the denser rocks south of the f a u l t to the l e s s dense metasediments of the Leech River Formation to the north i s believed to cause the rapid decrease i n gradient of the gra v i t y anomaly ( f i g . 29). I f the Leech River Fault has any e f f e c t upon the Bouguer anomaly, the observed change i n gra v i t y over the f a u l t i s inconsistent with a northward dipping f a u l t (Walcott 1967). This evidence may i n d i c a t e t h a t , although the f a u l t dips northwards on the surface, i t may, at depth, dip to the south and be a reverse or thrust type f a u l t . The Olympic Low i s a t t r i b u t e d i n part to the thick greywacke and a r g i l l i t e sequence (Walcott 1967) and though the Sooke High can be p a r t l y accounted for by the dense gabbros and basalts of the area, the rocks are not of s u f f i c i e n t density to produce the strong p o s i t i v e anomaly. The anomaly i s thought to be caused by an underlying layer of dense p e r i d o t i t i c rocks. S i m i l a r , though more intense, Bouguer g r a v i t y anomalies exist i n New Caledonia and Papua ( f i g . 30) where obduction of 62 contour i n t e r v a l 10 mgals. F i g u r e 2 7 . Bouguer anomaly map of South Vancouver I s l a n d ( a f t e r W a l c o t t 1967) 63 V V V • ITT1 DID QUATERNARY OLIGOCENE EOCENE (Metchosin Volcanics) EOCENE / OLIGOCENE (Soleduck) morinc sediments and volconics UPPER CRETACEOUS (Nonoimo group) sandstones , shale , conglomerates, coal. MIDDLE JURASSIC (Island Intrusivos) LOWER JURASSIC (Bonanza Volcanics) CARBONIFEROUS (Sicker Volcanics) PALEOZOIC ? L.MESOZOIC ? ( Loech River Schlsis) 4 PALEOZOIC (Work-Colqultz Complex) F i g u r e 28 Compiled g e o l o g i c map of South Vancouver I s l a n d and p a r t of the Olympic P e n i n s u l a . Olympic Peninsula Juan dt Fuca Strait Vancouver Island Fig. 29 C o a s t a l B o u g u e r Anomaly Prof i le a c r o s s the O l y m p i c P e n i n s u l a and V a n c o u v e r i s l a n d . N E W CALEDONIA / s 1-160 -80Mgals •10 Km r l 6 0 80Mgals -10 Km H60 SOOKE -80Mgals crust basalt gabbro LO -0 p e r i d o t i t e .lOKrn _20 20 I 4 0 I _60_ 80 I F i g u r e 30. Bouguer g r a v i t y anomalies C a l e d o n i a , Papua and East of New Sooke. 66 oceanic crust and mantle on to continental margins i s reported to have taken place (Coleman 1971). Seismic data includes a p r o f i l e crossing the Leech River Fault (Milne and White 1960). The seismic p r o f i l e l i e s east of the g r a v i t y maximum and was c a r r i e d out by exploding a number of charges i n Juan de Fuca S t r a i t and recording at Albert Head, the Dominion Astrophysical Observatory and the P a c i f i c Naval laboratory. The travel-time curves obtained are shown i n f i g u r e 31. Milne and White consider the Albert Head and D.A.O. records as two separate groups and ignore the P.N.L. readings. Thus they consider the travel-time curve to i n d i c a t e two d i f f e r e n t v e l o c i t i e s . Using a l e a s t squares method, the c l o s e r s t a t i o n at Albert Head y i e l d s a v e l o c i t y of 5.41 Km/s with an intercept time of 0.12 s whereas the D.A.O. records f i t a travel-time curve of v e l o c i t y 8.7 Km/s. Milne and White discount the 8.7 Km/s v e l o c i t y as being obviously i n c o r r e c t since they believe the area to be underlain by g r a n i t i c basement. They i n t e r p r e t the 8.7 Km/s v e l o c i t y as being a high apparent v e l o c i t y obtained because of a dipping top bed and propose that the Metchosin basalts o v e r l i e a granite basement and dip north at 20 degrees r e s u l t i n g i n a depth of 14,000 feet of basalts at the Leech River Fault ( f i g . 32). Granitic rocks are, however, not seen south of the Leech River Fault and g e o l o g i c a l evidence indicates that gabbroic rather than granitic rocks underlie the b a s a l t s . An a l t e r n a t i v e i n t e r p r e t a t i o n , though grossly o v e r s i m p l i f i e d , does provide some i n t e r e s t i n g r e s u l t s . A travel-time curve of v e l o c i t y 7.1 Km/s with an intercept time of 0.48 s can be drawn through the P.N.L. readings which were omitted by Milne and White i n t h e i r i n t e r p r e t a t i o n . 67 4 3 1956 SERIES • D.A.O. READINGS o ALB X P.NJ ERT HEAD • • t ''•25 + -A-8-70 X 0 / ?^ -X—0 A / « t»C 0 5 10 15 20 25 30 L' KILOMETRES Figure 51 Travel-time curve, 1956 series Milne and WWe. Seo level • SEISMIC STNS. X SHOTS 6 -Bosemtnt r-.-gnitic Roc* V , i 6 29 Km/sec. Figure 3 2. Profile along \v56 series (plane-layer interpretation Milne and White) 68 F i g u r e 33 T r a v e l - t i m e curves - three l a y e r i n t e r p r e t a t i o n . 69 The three travel-time curves ( f i g . 33) i f interpreted on the basis of a three layer model in d i c a t e that a layer of v e l o c i t y 5.41 Km/s o v e r l i e s a layer of v e l o c i t y 7.1 Km/s which i n turn o v e r l i e s a high v e l o c i t y layer of v e l o c i t y 8.7 Km/s. The upper layer comprises the basalts with a seismic v e l o c i t y of 5.41 Km/s. The seismic v e l o c i t i e s from the second and t h i r d l a y e r s , though s l i g h t l y high, are about equivalent to v e l o c i t i e s obtained from gabbroic and mantle rocks r e s p e c t i v e l y . The apparent high v e l o c i t i e s even for gabbroic and mantle rocks possibly i n d i c a t e that the lower i n t e r f a c e dips north but at a shallower angle than i n Milne and White's i n t e r p r e t a t i o n . According to t h i s model the thickness of the layer of basalts i s approximately 1.6 Km. and the thickness of the underlying gabbroic layer i s of the order of 6 Km. In other words the depth to the surface of the high v e l o c i t y layer i s between 7 and 8 Km., which i s equivalent to the thickness of oceanic c r u s t . The high v e l o c i t y layer may therefore represent the upper mantle. Though t h i s model i s obviously o v e r s i m p l i f i e d i n that planar layers probably do not e x i s t as the gabbro intrudes the b a s a l t i c l a y e r , i t does f i t the strong p o s i t i v e Bouguer g r a v i t y anomaly and i n general f i t s a hypothesis that the gabbros and basalts represent a s l i c e of oceanic c r u s t . 70 GEOLOGICAL HISTORY OF THE AREA The Sooke gabbro i n t r u s i o n i s the largest of a number of elongate gabbroic i n t r u s i o n s which occur w i t h i n the Metchosin Formation on the south-west coast of Vancouver Island ( f i g . 4 ) . S i g n i f i c a n t l y , no i n t r u s i o n s of Sooke gabbro are found north of the Leech River Fault which separates the T e r t i a r y basalts of the Metchosin Formation on the south from the Mesozoic metasediments on the north ( f i g . 28). The Metchosin basalts and gabbros on Vancouver Island therefore comprise part of a g e o l o g i c a l province d i s t i n c t from the C o r d i l l e r a n Insular B e l t . The fundamental d i f f e r e n c e i s that the basalts and gabbros are part of a "eugeosynclinal" assemblage st r e t c h i n g southwards through the Olympic Peninsula i n north west Washington (Snavely and Wagner 1963), whereas the rocks of the Insular Belt are mainly much older and of quite d i f f e r e n t types. Extensive melting of a s u i t a b l e parent rock must have taken place to produce the vast volume of up to 100,000 cubic miles of Eocene b a s a l t . I f the assumption that the mantle i s composed l a r g e l y of dense s i l i c a t e s r i c h i n i r o n and magnesium i s c o r r e c t , then the b a s a l t i c magma could be derived from mantle ma t e r i a l by d i f f e r e n t i a l f u s i o n . McKee (1972) states t h a t , "The lava probably came from the mantle; chemical analyses of the b a s a l t strongly resemble those from recent b a s a l t i c eruptions i n oceanic areas where a subcrustal o r i g i n i s c e r t a i n " . The basalts and gabbros which are c l o s e l y related i n time are probably g e n e t i c a l l y r e l a t e d . The gabbro i s thought to have been intruded during the l a t e Eocene and since much of the v o l c a n i c a c t i v i t y also took place during the l a t e Eocene, the basalts and gabbros are considered 71 contemporary. Contemporaneous o r i g i n i s reasonable i n the l i g h t of the hypothesis that the gabbro represents intermediate l e v e l s of what was presumably a volcanic feeder vent, now exposed by erosion of part of the volcanic p i l e . Feeder vents and surrounding basalts appear to have grown in t o volcanic islands as the nature and d i s t r i b u t i o n of sediments interbedded with the lava suggests that they were deposited on or near vo l c a n i c islands (McKee 1972). McKee suggests that the extensive Eocene v o l c a n i c i t y o r i g i n a t e d i n a manner comparable to the Hawaiian eruptions and that the Eocene p i l l o w lavas b u i l t up from the ocean f l o o r u n t i l the p i l e eventually b u i l t above sea l e v e l . This seems a reasonable suggestion since there i s evidence that some of the l a t e r Eocene lavas were erupted subareally because old s o i l zones formed by weathering can be seen between the flows (Waters 1955). I f the Eocene lava was erupted to form oceanic i s l a n d s , the gabbro and p i l l o w basalts of the Metchosin Formation must be considered to represent an ocean f l o o r sequence. Geophysical information on the Sooke area substantiates t h i s hypothesis that the gabbro and basalts are part of an oceanic sequence. The Bouguer gra v i t y anomaly over the Sooke area i s one of the most pronounced i n North America and though les s intense i s s i m i l a r to g r a v i t y anomalies i n areas such as New Caledonia and Papua where there i s obduction of ocean crust on to continental edges (Coleman 1971). An a l t e r n a t i v e i n t e r p r e t a t i o n of the seismic r e s u l t s of Milne and White (1950) indicates that 1.6 Km. of basalts are underlain by approximately 6 Km. of gabbroic rocks. Beneath these layers i s a high v e l o c i t y layer situated at a depth which corresponds to the depth Figured MODEL FOR THE OCEANIC CRUST After Dewey and Bird 1970. Leech R i v e r F a u l t '^sHxsts 1 i i — East Sooke // *-+- II r i II •*• [/1v *• ( w +- — S t r a i t of Juan de Fuca V V v Gabbro' * v -? 1 — ^ <- »- J- 4 - * -, i x _ ' / ^ ~ - - ? v \ \ i \ > P e r i d o t i t e s ? 1> / Mono— — . - — -C o n t i n e n t a l c r u s t \ ^ z Oceanic c r u s t . \. / — Km. F i g u r e 35. Schematic model f o r obduction of oceanic c r u s t on to c o n t i n e n t a l c r u s t - South Vancouver I s l a n d . ^ 3 7<+ of oceanic c r u s t . The high v e l o c i t y l a y e r i s therefore thought to represent p e r i d o t i t e rocks of the upper mantle. I f t h i s seismic model i s reasonable then the sequence of rocks i n the Sooke area i s s i m i l a r to that proposed by Dewey and B i r d (1970) i n t h e i r model for ocean crust ( f i g . 34). The Leech River Fault i s believed to mark the i n t e r f a c e between oceanic crust and continental c r u s t , as does the Queen Charlotte F a u l t . Although the Leech River Fault dips to the north at the surface, there i s evidence from the g r a v i t y i n t e r p r e t a t i o n that the dip may be reversed at depth. Since the f a u l t which marks the i n t e r f a c e between oceanic crust and continental crust appears to be a reverse or thrust type f a u l t , i t i s proposed that the oceanic assemblage was obducted on to Vancouver Island ( f i g . 35). Obduction of the large s l i c e of oceanic material on to southern Vancouver Island must have involved large scale tectonic forces which only the mechanism of ocean f l o o r spreading could provide and probably resulted from the r e l a t i v e motion of the Juan de Fuca and North American p l a t e s . 75 CONCLUSIONS AND SUMMARY The Sooke gabbro i n t r u s i o n i s an e l l i p t i c a l body of s l i g h t l y d i f f e r e n t i a t e d o l i v i n e gabbro which i s composed of c a l c i c p l a g i o c l a s e and clinopyroxene with minor o l i v i n e and orthopyroxene. The gabbro does not ex h i b i t any obvious c r y p t i c of cumulate layering of the type which characterizes many other layered basic igneous rocks. Instead steeply dipping structures such as weak l a y e r i n g , f o l i a t i o n and l i n e a t i o n are believed to be flow structures of the type described by Balk 1937. U r a l i t i z a t i o n of the gabbro i s a hydrothermal or deuteric a l t e r a t i o n , the i n t e n s i t y of which i s everywhere s p a t i a l l y r e l a t e d to fractures or to the margin of the i n t r u s i o n . Fractures i n both gabbro and surrounding basalt appear to have acted as channelways f o r the migrating solutions which, i n the l a t e r stages, deposited copper sulphides i n s t r u c t u r a l l y favourable areas. The hot i n t r u s i o n i s believed to have acted as a "heat pump" causing a convective flow of hydrothermal or l a t e stage deuteric solutions around the peripheral regions of the i n t r u s i o n and along f r a c t u r e s . The gabbro i s bounded on a l l sides by upper Eocene Metchosin basalts which are believed to be comagmatic with the gabbro. Though the gabbro causes some r e c r y s t a l l i z a t i o n of the basalts near the contact and though i t i s generally i n t r u s i v e into the b a s a l t s , i t i s believed to represent a lower l e v e l of a volcanic neck or feeder now exposed by erosion of a thick sequence of b a s a l t s . The Metchosin pillowed basalts which are l o c a l l y interbedded with tuffaceous marine sediments (Snavely and Wagner 1963) are c l e a r l y submarine i n o r i g i n . The nature and d i s t r i b u t i o n of the sediments 76 interbedded with the lava suggests that they were deposited on or near v o l c a n i c islands (McKee 1972). There i s evidence that some of the lava flows highest i n the volcanic p i l e were erupted subaerally as old s o i l zones formed by weathering can be seen between flows (Waters 1955). The build-up of the Eocene b a s a l t i c p i l e can be compared to the Hawaiian chain which grew from the ocean f l o o r by the successive build-up of b a s a l t i c lava flows u n t i l f i n a l l y the p i l e b u i l t above sea l e v e l . An hypothesis of oceanic o r i g i n f or the basalt and gabbro sequence i s further complemented by the geophysical information on the area. The high Bouguer gra v i t y anomaly i s compared to s i m i l a r anomalies i n areas where obduction of oceanic crust on to continental crust i s considered to have taken p l a c e . An a l t e r n a t i v e i n t e r p r e t a t i o n of the seismic r e s u l t s of Milne and White y i e l d s a three layer model with seismic v e l o c i t i e s of each layer corresponding to those of b a s a l t i c , gabbroic and mantle rocks. The nature of these layers and t h e i r r e l a t i v e thicknesses corresponds w e l l with other models proposed for oceanic c r u s t . If Eocene lava was indeed erupted to form oceanic i s l a n d s , then subsequent spreading of the sea f l o o r seems to have r a f t e d them against the edge of the continent. The oceanic su i t e has been obducted on to the southern end of Vancouver Island by the large scale t e c t o n i c forces r e s u l t i n g from the r e l a t i v e motion of the Juan de Fuca and North American p l a t e s . 77 LITERATURE CITED. Adams, J.B., 1968. D i f f e r e n t i a l s o l u t i o n of pl a g i o c l a s e i n super-c r i t i c a l water. American M i n e r a l o g i s t , 53, 1603-1613. Balk, R. , 1937. S t r u c t u r a l behaviour of igneous rocks, Geol. Soc. Amer., Memoir 5, 177p. B h a t t a c h a r j i , S. and Smith, C.H., 1964. Flowage d i f f e r e n t i a t i o n . Science, 145, 150-153. Cann, J.R., 1971. Major element v a r i a t i o n s i n ocean-floor b a s a l t s . P h i l . Trans. Roy. Soc. Lond., A268, 495-505. Carson, D.J.T., 1968. Metallogenic study of Vancouver Island with emphasis on the r e l a t i o n s h i p s of mineral deposits to plutonic rocks. Unpublished PhD. Thesis, Carleton U n i v e r s i t y , Ottawa. Clapp, CH., 1912. Southern Vancouver I s l a n d , Geol. Surv. Can., Memoir 13. , 1917. Sooke and Duncan map-areas, Vancouver Isla n d . Geol. Surv. Can., Memoir 96. Coleman, R.G., 1971. Plate tectonic emplacement of upper mantle perido-t i t e s along continental edges. J . Geophys. Research, V o l . 76, No. 5, 1212-1222. Cooke, H.C., 1919. The gabbros of East Sooke and Rocky P o i n t . Geol. Surv. Can., Museum B u l l e t i n No.'30. Danner, W.R., 1955. Geology of Olympic National Park, Univ. of Washington Press. Deer, W.A., Howie, R.A. and Zussman, J . , 1962. Rock-Forming Minerals. V o l . 1, Ortho- and Ring S i l i c a t e s . Longmans, London. Dewey, J.F. and B i r d , J.M., 1970. Mountain b e l t s and the .new g l o b a l t e c t o n i c s . J . Geophys. Research, V o l . 75, No. 14, 2625-3206. Engel, A.E.J., Engel, C.G. and Havens, R.G., 1965. Chemical c h a r a c t e r i s t i c s of oceanic basalts and the upper mantle. Geol. Soc. Amer. , B u l l . 76, p. 719. 7 8 F y l e s , J.T., 1949. Copper deposits of the East Sooke peninsula. M i n i s t e r of Mines, B.C., Ann. Rept., 1948, A162-A170. Kirkham, R.V., 1971. Age determinations and g e o l o g i c a l s t u d i e s , K. Ar. Isotope Ages Report 10 Geol. Surv. Can., Paper 71-2, p. 22. McKee, B., 1972. Cascadia, the geologic evolution of the P a c i f i c Northwest, McGraw-Hill, 1972. M i l n e , W.G., and White, W.R.H., 1961. A seismic survey i n the v i c i n i t y of Vancouver I s l a n d , B r i t i s h Columbia. Dom. Obs. Publns. 24, 145-154. Moores, E.M., and Vine, F . J . , 1971. The Troodos Massif, Cyprus and other o p h i o l i t e s as oceanic crust - evaluation and i m p l i c a t i o n s , P h i l . Trans. Roy. Soc. Lond., A268, 443-466. Norton, D., 1972. Concepts r e l a t i n g anhydrite deposition to s o l u t i o n flow i n hydrothermal systems. International Geological Congress, 24th s e s s i o n , Section 10, Geochemistry, p 237. O r v i l l e , P.M., 1972. P l a g i o c l a s e cation exchange e q u i l i b r i a with aqueous chloride s o l u t i o n : Results at 700 C. and 2000 bars i n the presence of quartz. Amer. Jour. S c i . , 272, 234-272. Park, C F . , 1944. The s p i l i t e and manganese problems of the Olympic Peninsula, Washington. Amer. Jour. S c i . , 244, 305-323. Ruegg, N.R., 1964. Use of the angle A^C i n o p t i c a l determination of the composition of augite. Amer. Mi n e r a l . , 49, 599-606. Slemmons, D.B., 1962. Determination of volcanic and p l u t o n i c plagioclases using a three- or four-axis universal stage. Geol. Soc. Amer., Special Paper No. 69. Snavely, P.D. and Wagner, H.C, 1963. T e r t i a r y geologic h i s t o r y of western Oregon and Washington. D i v i s i o n of Mines and Geology, State of Washington, Report of I n v e s t i g a t i o n s , No. 22. Thayer, T.P., 1963. Flow-layering i n alpine peridotite-gabbro complexes. Mineral Soc. Amer., Special Paper 1, 55-61. Wager, L.R. and Brown, G.M., 1967. Layered igneous rocks, 588p. O l i v e r and Boyd, Edinburgh and London, 1968. Walcott, R.I., 1967. The Bouguer anomaly map of southwestern B.C., Univ. of B.C. I n s t i t u t e of Earth Sciences, S c i e n t i f i c Report No. 15. Waters, A . C , 1955. Volcanic rocks and the t e c t o n i c c y c l e . Geol. Soc. Amer., Special Paper 62, 703-722. Wentworth, C.K. and Winchell, H., 1947. Koolau basalt s e r i e s , Oahu, Hawaii. Geol. Soc. Amer., B u l l e t i n 58, p. 71. Yoder, H.S. J r . , and Sahama, Th. G., 1957. O l i v i n e X-ray determination curve. American M i n e r a l o g i s t , 42, 475-491. 80 APPENDIX DETERMINATIVE METHODS FOR MINERAL COMPOSITIONS A. ) P l a g i o c l a s e . P l a g i o c l a s e compositions were determined on a four axis universal stage. O p t i c a l and c r y s t a l l o g r a p h i c elements i n twinned p l a g i o c l a s e were measured i n order to determine the twinning law, anorthite percentage and the s t r u c t u r a l s t a t e . The procedure used i n the measurement of these elements i s described by Slemmons (1962) and i s e s s e n t i a l l y a r e v i s i o n of the Turner method. O p t i c a l curves for both volcanic and plutonic plagioclases are used to determine the s t r u c t u r a l state of the p l a g i o c l a s e . Slemmons also gives various t e s t s and procedures for eliminating any ambiguous r e s u l t s . The composition of d i f f e r e n t zones i n zoned pl a g i o c l a s e were determined by the same method using o p t i c a l and c r y s t a l l o g r a p h i c data from each zone. Compositions of p l a g i o c l a s e derived using Slemmon's method seem to be accurate and r e s u l t s are normally reproductable to within two per cent a n o r t h i t e . B. ) Pyroxenes. Compositions of the orthopyroxenes were determined using 2V. The percentage of calcium i n the orthopyroxenes cannot be determined by t h i s means and an average value of three weight per cent calcium was assumed when p l o t t i n g compositions. Clinopyroxene compositions were determined using a combination 81 A A of 2V, Z C and A C angles and p l o t t i n g the values on the appropriate determination chart given by Ruegg (1964). This method was found to be of low accuracy because of a number of f a c t o r s . S l i g h t errors i n the measurement of o p t i c a l data can lead to s i g n i f i c a n t errors i n composition. A l s o , exsolution lamellae and the patchy development of u r a l i t i c amphibole within many pyroxene c r y s t a l s influence the o p t i c a l data to such an extent that any compositions so derived are subject to high e r r o r . To minimise t h i s source of er r o r fresh pyroxene was used where pos s i b l e f o r o p t i c a l measurements. The r e f r a c t i v e index n^ s was determined i n some cases to narrow the margin of e r r o r . C. ) Amphiboles. A 2V and Z C vslues determined on the universal stage were usef u l only i n determining that common hornblende i s the usual a l t e r a t i o n amphibole present i n the gabbros. D. ) O l i v i n e . Compositions of fresh o l i v i n e s were determined using X-ray methods. A slow scan speed on the diffractometer was used to determine the 130 spacing. Potassium Bromide was used as a standard. The 130 spacing can be used to determine the composition of the o l i v i n e by p l o t t i n g values on the o l i v i n e X-ray determination curve given by Yoder and Sahama (1957). E. ) Other Minerals. The presence of s c a p o l i t e i n veins was v e r i f i e d by X-ray d i f f r a c t i o n . Various opaques i n the ore zones were determined using a r e f l e c t i n g microscope. 82 MODAL ANALYSES Several modal analyses of gabbros were made. One thousand points were counted f o r each specimen i n order to achieve an accuracy of about one per cent. CHEMICAL ANALYSES AND AGE DATES A l l chemical analyses were obtained'from previous works. S i m i l a r l y , the K/Ar age dates were obtained from Carson (1968) and Kirkham (1972). 83 TABLE I . Spec. No. Gore An.% margin S t r u c t u r a l .state Rock Type M6 8 7 L O l i v i n e gabbro. M 7 50 L U r a l i t i z e d gabbro. M9 8 6 L O l i v i n e gabbro. M i l 8 2 L-I O l i v i n e gabbro. M15 83 L O l i v i n e gabbro. M 1 6 80 L O l i v i n e gabbro. Ml8 88 L O l i v i n e gabbro. M20 7 0 I-H U r a l i t i z e d o l i v i n e gabbro. M21 59 L-I U r a l i t i z e d gabbro. M 2 3 a 6 5 5 3 I U r a l i t i z e d o l i v i n e gabbro. M 2 3 d 5 8 ? U r a l i t i z e d gabbro. M26 7 2 6 6 L O l i v i n e gabbro. M 2 8 60 20 I U r a l i t i z e d gabbro. M 3 0 8 i f L O l i v i n e gabbro. M33 7 2 I U r a l i t i z e d gabbro. M33a 80 6 7 L O l i v i n e gabbro. M3^ 84 7 7 L O l i v i n e gabbro. M36 7 5 I U r a l i t i z e d o l i v i n e gabbro. M 3 7 b 80 I Gabbro/dyke c o n t a c t . M38a 7 7 L-I O l i v i n e gabbro. M39 78 L O l i v i n e gabbro. M^ fOb 8 5 L O l i v i n e gabbro. S t r u c t u r a l s t a t e s : L= Low, I= I n t e r m e d i a t e , H = H i g h . TABLE I. (co n t . ) Spec. No. core An.% margin S t r u c t u r a l s t a t e Rock Type M44 53 I B a s a l t i n c l u s i o n . M47 5 1 26 I G a b b r o - x e n o l i t h c o n t a c t . M51a 87 L O l i v i n e gabbro. M52 86 L O l i v i n e gabbro M53 77 L O l i v i n e gabbro. M54 81 L O l i v i n e gabbro. M55 78 L O l i v i n e gabbro. M58b 58 I B a s a l t i n c l u s i o n . M60 83 L O l i v i n e gabbro. M63 85 L O l i v i n e gabbro. M64 76 L-I O l i v i n e gabbro. M66 68 79 64 L-I R e c r y s t a l l i z e d b a s a l t . M69 68 60 L-I U r a l i t i z e d gabbro. M70a 54 L-I U r a l i t i z e d gabbro. M72 53 I P o r p h y r i t i c b a s a l t x e n o l i t h . M?4 65 49 I-H B s a a l t i n c l u s i o n . M75 61 L-I O l i v i n e gabbro M76 73 L O l i v i n e gabbro. M84 66 70 L B a s a l t i n c l u s i o n . M84b 28 9 ? L e u c o c r a t i c v e i n M85 75 65 L B a s a l t i n c l u s i o n . M87a 75 L B a s a l t i n c l u s i o n M90 61 I B s a a l t i n c l u s i o n . M92 52 L U r a l i t i z e d gabbro. M93 51 L-I B a s a l t i n c l u s i o n . 85 TABLE I . ( c o n t . ) Spec. No. core An.% margin S t r u c t u r a l s t a t e Rock Type 80 L O l i v i n e gabbro. M95 8 5 7 2 L O l i v i n e gabbro. M96 8 5 70 L O l i v i n e gabbro. M98 8 2 L O l i v i n e gabbro. M100 8 0 7k L O l i v i n e gabbro. M102 7 8 L U r a l i t i z e d o l i v i n e gabbro. M104 63 5 2 I B a s a l t i n c l u s i o n . M106 7 5 k9 L N o r i t e . M109 7 5 L O l i v i n e gabbro. M i l l 6 2 L U r a l i t i z e d o l i v i n e gabbro. M112 8 8 65 L-I B a s a l t i n c l u s i o n . Ml 13 52 5 9 L-I B a s a l t i n c l u s i o n . M i l 7 8 1 L O l i v i n e gabbro. M118 63 ? O l i v i n e gabbro. M120 81 L O l i v i n e gabbro. M121 80 71 L U r a l i t i z e d o l i v i n e gabbro. M122 85 69 L O l i v i n e gabbro. M123 86 L O l i v i n e gabbro. M124 Gk I U r a l i t i z e d gabbro. M128 91 L .Olivine gabbro. M130 60 L-I U r a l i t i z e d gabbro M130a 5k I B a s a l t i n c l u s i o n . M131 8 0 L O l i v i n e gabbro. TABLE I. (cont.) Spec. No. An.% S t r u c t u r a l core margin state M132 88 L M135 86 63 L-I M139 81 L Ml40 56 L-I M142 76 68 L-I Ml42a 64 L M153 82 L M155 84 L MI56 79 L M86 22 10 I M86a 68 54 I Rock Type O l i v i n e - r i c h gabb Basalt i n c l u s i o n . Olivine gabbro. U r a l i t i z e d gabbro U r a l i t i z e d gabbro U r a l i t i z e d gabbro Olivine gabbro. Olivine gabbro. Olivine gabbro. Leucocratic vein. Basalt i n c l u s i o n . TABLE I I 87 Spec. No. An.% of e a r l y formed An. % of phenocrysts p l a g i o c l a s e , p o i k i l i t i c a l l y i n b a s a l t , e n c l o s e d i n l a t e r m i n e r a l s . M33a 81 M39 83 M87a 89 M85 75 M9 88 M i l 87 M51a 90 M139 87 TABLE I I I . OLIVINE COMPOSITIONS. Spec. No. %Fo. Spec. No. %Fo. M9 72 M63 78 M i l 77 M?6 67 M16 76 . M9*+ 68 M26 70 M100 67 M30 79 M112 70 M38a 7k M118 75 M39 7k M120 81 M^Ob 76 M132 82 M52 81 M139 79 M55 73 M153 80 88 TABLE IV. Modal a n a l y s e s . Spec. P I . Cpx. Opx. 01. . Hb. • Ep. C h i . S e r p . A c c . M7 62*1 - - - 34.8 - 2.6 0 . 5 M34 58.4 29 .7 2.0 8 . 9 t r 1.0 M38a 40 . 7 38.4 11.6 7.4 t r 1.6(mag M 5 3 5 8 . 7 2 3 . 5 t r 16.2 t r 1.6(Qtz M63 40.4 3 5 . 5 8.6 9 . 9 4.8 t r 0.8 M64 60 . 5 3 35.46 2.o6 1.4 e . 5 3 M69 47.2 28.1 24 . 5 0.2 M76 46 3 9 . 5 12 .3 1.4 0 . 5 0 . 3 ( c p y M100 64 .9 19.3 12.4 0 . 5 0 . 7 1 . 5 0 . 7 M147 56 .5 3 1 . 3 10.1 1 . 3 0 . 5 0 . 3 ( c p y M7 U r a l i t i z e d p l a g i o c l a s e - r i c h gabbro M34 O l i v i n e gabbro M38a O l i v i n e gabbro M53 O l i v i n e gabbro s l i g h t l y u r a l i t i z e d . M63 O l i v i n e gabbro M64 P l a g i o c l a s e - r i c h o l i v i n e gabbro M69 U r a l i t i z e d gabbro. M?6 O l i v i n e gabbro. M100 P l a g i o c l a s e r i c h o l i v i n e gabbro. M147 Gabbro - p a r t l y u r a l i t i z e d . TABLE V. Chemical analyses. 89 S i 0 2 47.58 45;42 7 7 . 8 9 0 7 . 5 0 45; 70 51.24 5'0.45 4 9 . 6 1 T i 0 2 0.20 0 . 3 0 0.25 0 . 5 5 0 . 3 3 2.01 2 . 3 3 1.43 A 1 2 ° 3 1 8 . 0 3 10.64 1 1 . 9 6 13.20 14.90 13.12 14.94 16.01 Fe 0 1.01 3 . 0 3 0.21 2.20 1 . 8 0 4 . 3 3 3 . 3 8 c- 3 11.49 FeO 3 . 3 ^ 10.22 0 . 3 2 4.10 5 . 8 0 9 . 1 5 7 . 5 5 MnO 0 . 0 6 0 . 0 8 0.02 0 . 0 7 O.15 6.21 0 . 0 8 0 . 1 8 MgO 10.88 1 3 . 8 2 O.29 1.40 8.70 4 . 2 6 7 . 6 9 7.84 CaO 1 6 . 9 2 11.84 1 .92 3 . 0 0 13.40 8 . 6 1 9.17 11 .32 Na20 1.04 2.17 5 . 6 7 5 . 7 0 2 . 3 0 - 2.40 2.84 2 . 7 6 K 20 0 . 3 2 0.45 0 . 3 1 0 . 3 0 0 . 1 6 1.01 0.35 0 . 2 2 P 2 ° 5 0.01 0.01 0.04 0 . 1 3 O.06 0 . 6 1 O.27 0.14 - - - 0 . 9 0 2.50 2 . 6 6 O.96 -co 2 - 0 . 0 3 - 0.10 0 . 1 - - -1 0 0 . 0 8 100.21 9 9 . 6 5 99.10 9 5 . 9 0 . 9 9 . 6 1 100.04 1. Fresh rather fine grained olivine gabbro with approximately 5 per cent olivine,(Cooke 1 9 1 9 ) . 2. Hornblendite, (Cooke 1 9 1 9 ) . 3. Aplite, (Cooke 1 9 1 9 ) . 4 . Quartz diorite, (Carson 1 9 6 8 ) . 5 . Altered gabbro, (Carson 1 9 6 8 ) . 6 . Average of four analyses for Eocene Olympic basalts, ('Waters 1955) . 7 . Average of ten basalts, Koolau basalt series, Hawaii. ('.Ventworth and vVinchell 1 9 4 7 ) . 8 . Mean of 96 oceanic basalts (tholeiites), (Cann 1 9 7 1 ) . 90 LEGEND Sooke format ion (sedimentary) - Miocene /Oligocene Uralitized gabbro • Olivine gobbro Hi Olivine gobbro plagioclase > 60% - Upper Eocene m Olivine gabbro olivine > 4 5 % Metchosin basalt - Upper Eocene Areas of basaltic Inclusions within gobbro Geological contact « Geological contact - gradallonal Mineralogy of tha gabbros * clinopyroxene A orthopyroxene • olivine • amphibole ptogloclase is principal constituent in most samples. Symbols for ferromognesian minerals ore listed (opto bottom in decreasing order of abundance. Scolo - feci 0 2000 4000 f-000 eOOOfeet. MAP A , G E O L O G I C M A P - E A S T S O O K E P E N I N S U L A . M A P . B. Anorthite percentage of plagioclases. Figures in brackets are anorthite percentages of c r y s t a l margins -v V x y y 4 LEGEND —. gobbro / bosalt contact \ strike and dip of bedding (basalt) \ strike ond dip of foliotion strike and dip of layering Scolo - feet 0 2 0 0 0 -1000 6 0 0 0 eOOOfeet. MAP C . STRUCTURE MAP - EAST SOOKE GABBRO. Map D. Topographic lineaments. Map prepared from a i r photos. MAP E . Sample Location Map 

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