Granitic rocks of the southern Coast Plutonic Complex and northern Cascades of British Columbia T. A. RICHARDS Geological Survey of Canada, Vancouver, British Columbia, Canada V6B I RS K. C. McTAGGART Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T I WS ABSTRACT The mapped area, about 170 km east of Vancouver, British Co- northern Cascades and of the adjacent part of the southern Coast lumbia, lies at the intersection of the northwest-trending Mountains, between Hope, British Columbia, and the international Mesozoic-early Cenozoic Coast Plutonic Complex and the north- boundary (Fig. 1). The mapped area, about 1,000 km2, lies 170 km trending late Cenozoic Cascade belt. The Late Cretaceous meso- east of Vancouver, British Columbia. The Trans-Canada Highway zonal Spuzzum intrusions (90 to 80 m.y. old) of the Coast Crystal- and many side roads give easy access to within about 10 km of any line Complex are made up of a central diorite complex and a mar- part of the area. The terrain is rugged, with local relief of more than ginal tonalite. Modal variation in the diorite, which is pyroxenic in 1,800 m (Fig. 2). Outcrops are abundant except in valley bottoms. the central parts and hornblendic in the marginal, was controlled n by Ph2o i the magma. The Yale intrusions (59 to 35 m.y. old), of Previous Work tonalite, granodiorite, and quartz monzonite, are stocks and sills that may represent the latest intrusions of the Coast Plutonic Com- R. A. Daly (1912) provided the first account of the geology of the plex. The Chilliwack composite batholith (40 to 16 m.y. old) is rep- southern part of the area. He named the Chilliwack batholith and resented by the Chilliwack batholith, the Mount Barr batholith, assigned to it a Miocene age. Cairnes (1924) mapped the northern and the Silver Creek stock; these epizonal batholiths consist largely part of the area and adjacent regions and summarized the regional of tonalite, granodiorite, and quartz monzonite. Variation in the geology of the Hope map-area (1944). The adjacent area, south of Chilliwack composite batholith is due mainly to differentiation at the international border as well as some parts on the Canadian side, depth, followed by minor evolution both as the various phases rose have been mapped and described in detail by Misch (1966) and his and also after they were emplaced. The Fraser River-Straight students. McTaggart and Thompson (1967) mapped along the Creek fault zone may have controlled the emplacement of many of eastern and northern borders of the area. Roddick and Hutchison the late Cenozoic plutons. During the past 40 m.y., intrusion and (1969) provided new information about the region to the north of volcanism may have been nearly continuous in southwestern the area. Monger mapped along the western side of the area (1966) British Columbia and Washington. Key words: igneous petrology, and prepared an updated edition of the Hope map-area (1970). granitic rocks, geochronology. McTaggart (1970) has written a tectonic history of the region. INTRODUCTION General Geology We describe here the origin, differentiation, and sequence of The area lies at the intersection of the Coast Mountains and the emplacement of the granitic rocks of the northern part of the Cascade Range (Figs. 1, 3). The Coast Mountains are carved into Figure 2. View northeast across northern half of study area. Geological Society of America Bulletin, v. 87, p. 935-9J3, 15 figs., June 1976, Doc. no. 60614. 935 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/935/3429356/i0016-7606-87-6-935.pdf by guest on 01 October 2021 936 RICHARDS AND McTAGGART the northwest-trending Coast Plutonic Complex (Fig. 3), which is Washington (Misch, 1966, 1968), consists mainly of biotite gneiss, composed largely of tonalitic plutons and extends along the coast schist, amphibolite, and marble. It contains Precambrian zircons, of British Columbia and into Alaska a distance of nearly 1,700 km. possibly of detrital origin, and the age of the beds from which the The plutons appear to be mainly of Cretaceous age, but along the gneiss was derived is uncertain (Mattinson, 1972); widespread eastern flank they may be largely early Cenozoic in age (Hutchison, metamorphism and migmatization affected the unit in middle to 1970; Douglas, 1970). The Cascade Range consists of a north- Late Cretaceous time. The Skagit Volcanic Formation (Daly, trending belt of late Cenozoic volcanic and intrusive rocks that ex- 1912), or the Skagit Volcanics (Staatz and others, 1972), equivalent tends from California through Oregon and Washington and into to the Hannegan Volcanics (Misch, 1966), consists of andesitic and British Columbia (Waters, 1955). It is superimposed on the Coast rhyolitic flows and pyroclastic rocks some 1,500 m thick. These are Plutonic Complex; the intersection lies mostly in northern younger than some phases of the Chilliwack composite batholith Washington (Fig. 3). and have been considered to be Oligocene in age (Misch, 1966). The granitic plutons are flanked or overlain by various units. The Eocene conglomerate and sandstone occur in patches in a north- Old Settler Schist (Lowes, 1972) in the north and the Darrington trending belt through the middle of the area. They may be correla- Phyllite (Misch, 1966) in the south are among the oldest rock units tive with the Chuckanut Formation of northern Washington in the map-area (Fig. 4). The Old Settler Schist is mainly kyanite (Monger, 1970). and staurolite schist and amphibolite. Along the Chilliwack River, Two major structural features cross the area. In the west, the the Darrington Phyllite is composed mainly of foliated pelite and Chilliwack Group and Cultus Formation are involved in a series of graywacke metamorphosed in the greenschist facies. To the south, thrusts (Fig. 4), among them the Shuksan thrust, and in recumbent the Darrington Phyllite may attain a thickness on the order of folds of middle Cretaceous age (Misch, 1966; Monger, 1966, 3,000 m (Misch, 1966). The age of these two units is uncertain, but 1970). The Fraser River fault zone (Duffell and McTaggart, 1952; they probably predate the Chilliwack Group of late Paleozoic age. Read, 1960; McTaggart and Thompson, 1967; McTaggart, 1970), Rocks of the Chilliwack Group (Fig. 4) underlie large areas west represented in the area by the Hope and Yale faults, extends from of the granitic bodies. The Chilliwack Group (Misch, 1966; the international boundary northward for about 250 km (Fig. 15) Monger, 1966, 1970) is a few thousand metres thick and ranges in and may form part of a major tectonic boundary extending to the age from Early Pennsylvanian to Early Permian. The lower beds Yukon (Price and Douglas, 1972). It extends southward into consist of clastic sedimentary rocks and limestone, and the upper Washington and is probably continuous with the Straight Creek beds are pyroclastic rocks and chert. In the northern part of the fault (Misch, 1966) and the Evergreen fault (Yeats and Engels, area, the Chilliwack assemblage has undergone high-grade regional 1971). The faults are steep, and in the study area a graben may metamorphism (Read, 1960; Lowes, 1972). Rocks of the Triassic- have formed along this zone between the Hope and Yale faults (Fig. Jurassic Cultus Formation, mainly pelite, are associated with the 4), in which Eocene beds accumulated. Strike-slip movement has Chilliwack Group in complex thrusts and folds. The Hozameen been suggested by Misch (1966) and Monger (1970). Group (McTaggart and Thompson, 1967), exposed along the east- ern side of the area, consists of more than 6,000 m of ribbon chert, SPUZZUM INTRUSIONS basic to intermediate lava, limestone, and argillite. Its age is un- known but has been considered to be late Paleozoic or possibly The northern part of the area (Figs. 4, 5) is underlain by diorite Triassic. The Custer Gneiss (McTaggart and Thompson, 1967; and tonalite of the Spuzzum intrusions (McTaggart and Staatz and others, 1972), generally called the "Skagit Gneiss" in Thompson, 1967). These are part of the Spuzzum batholith that ex- tends many kilometres to the north and northwest of the area and U5° forms one of the large plutons of the Coast Plutonic Complex. Two main units are distinguished: a central zoned dioritic complex and a surrounding, probably younger, tonalite. Diorite The diorite is a fresh, medium-grained rock consisting of brown hypersthene (En65) and black augite (Wo45En35Fs20, by optical properties) and (or) hornblende and white, gray, or pink plagio- clase. Biotite is a minor constituent, and quartz is rarely more than 10 percent. K-feldspar is absent in both diorite and tonalite. The rock generally shows an alignment of plagioclase and aggregates of dark minerals that is probably due to flow. In thin section the rock shows little sign of cataclasis. The diorite complex is crudely zoned, with hypersthene-augite diorite (rarely norite) in its core regions and hypersthene- hornblende diorite (rarely tonalite) at its margin. The mineralogical variation appears continuous, but three varieties have been defined: hypersthene-augite diorite in two core regions; an intermediate zone of augite-hypersthene-hornblende diorite; and a marginal zone of biotite-hypersthene-hornblende diorite. Only small chemi- cal differences accompany the pronounced mineralogical variation (Fig. 6, Table 1; sample locations are shown in Richards, 1971). Plagioclase and augite decrease markedly in abundance from the core to the margin as quartz and hornblende increase. Hypersthene abundance is nearly constant, although it is progressively replaced from the core to the margin by a fibrous mineral (anthophyllite?) and colorless pargasitic hornblende. Much of the increase in am- phibole can be accounted for by replacement of augite, but large Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/935/3429356/i0016-7606-87-6-935.pdf by guest on 01 October 2021 a * r t.