Granitic Rocks of the Southern Coast Plutonic Complex and Northern Cascades of British Columbia

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

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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 eastern 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 extends 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 plagioclase. 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 chemical 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

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® K/Ar age(million years)

Figure 4. General geology of mapped area. KILOMETERS

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poikilitic hornblendes appear to be primary, particularly in the zones and hornblendite to the marginal parts. Such bodies are

outer zone. Plagioclase is unzoned (An45) in the core regions and blocks, small lenses, "veins," or "dikes" as much as 1.5 m across weakly zoned (An50_40) in the marginal regions. that are prominent in areas as much as 30 m across. Quartz and

Small, irregular bodies of pyroxenite and hornblendite are found plagioclase (An80_50) are accessories; in some places the latter are so in the diorite. Pyroxenite is confined to the cores and intermediate abundant that the rock is a gabbro. These ultramafic rocks show

Eocene conglomerate, etc. Tonalité

r Coquihalla stock Spúzzum^ I Biotite-hypersthene-hornblende diorite Yale Intrusions cm. Ogilvie Peak stock Intrusions Augite-hypersthene-hornblende diorite Gabbroic complex Hypersthene-augite diorite

ki V < >'

Figure 5. Geologic map showing Spuzzum and Yale intrusions.

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both sharp and gradational contacts with diorite and, in some laminae in the core and along the southwestern margins. The gen- examples, following contacts or fractures, appear to be structurally eral form of the body as suggested by the foliation is a north- controlled. plunging, tonguelike body with pyroxene-rich rocks confined to the Lenticular pegmatites, composed of quartz, oligoclase, biotite, central part of the complex. The pattern of foliation in tonalite is tourmaline, and occasionally hornblende, are found in both diorite compatible with a magmatic-protoclastic origin, although locally it and tonalite. has been modified by shearing, perhaps associated with the Shuk- san and Fraser River fault systems. East-west faulting along Fraser Tonalite River (Fig. 5) is suggested by minor mismatching of the northern and southern terranes. The tonalite, rimming the diorite, is medium-grained, with a pla- nar alignment of hornblende and biotite crystals. Plagioclase Contact Metamorphism

(An 32), quartz, hornblende, and biotite occur in nearly constant proportions throughout. Regionally, it is homogeneous, but lo- The tonalite is flanked by staurolite-kyanite-garnet schist and is cally, mafic schlieren give the rock a heterogeneous appearance. associated with them over a wide region extending beyond the Deformation twins in plagioclase, bent biotite crystals, undulatory study area (Lowes, 1972). Along certain stretches of the tonalite extinction in quartz, and mortar texture are found throughout the contact, sillimanite has formed, and along others, andalusite tonalite. pseudomorphs are found (Read, 1960; Lowes, 1972; Pigage, 1973), and the contact metamorphism may have taken place at Structure slightly higher temperatures or lower pressures than the regional metamorphism. The staurolite-kyanite-garnet schist may be of In a few places the foliation of the tonalite seems to truncate that mid-Cretaceous age, about contemporaneous with the Spuzzum of the diorite. In addition, xenoliths of gneiss and metalimestone tonalite (Read, 1960; McTaggart and Thompson, 1967; Pigage, occur along the margins of the diorite, suggesting that it intruded 1973). metamorphic rocks rather than tonalite. The tonalite is inferred to be younger than the diorite. Composition Foliation, locally well developed in the diorite, dips 20° to 35° in hypersthene-augite diorite. In more hornblendic varieties, the folia- Chemically, the diorites are nearly homogeneous (Table 1, Fig. tion strikes northerly and generally dips more steeply. An inward 6). CaO, MgO, and iron oxide decrease slightly from the core areas dip is well developed along the western margin of the pluton, south to the margins. The petrographic uniformity of the tonalite is con- of the Fraser River. Layering occurs rarely as pyroxene-rich sistent with the small variation in the few chemical data. Specimen

TABLE 1. CHEMICAL ANALYSES AND CIPW NORMS OF THE SPUZZUM INTRUSIONS

Diorite Tonalite Core zone Intermediate Marginal 265 Spec. no. 034 1270 262 126

Si02 53.7 55.2 55.6 55.4 57.1 56.7 60.6 62.0 AI2O3 19.4 19.1 20.4 18.4 19.0 17.9 17.1 17.2 MgO 5.4 5.8 4.8 5.2 4.1 4.9 3.2 4.3 Fe203 0.9 1.0 1.2 0.7 1.4 0.9 1.0 1.4 FeO 6.9 5.8 4.9 5.4 4.1 4.8 4.0 4.3 MnO 0.13 0.12 0.08 0.11 0.09 0.09 0.09 0.09 CaO 8.4 8.0 7.1 7.4 7.1 6.7 5.9 5.6 Na20 3.6 3.6 4.2 3.8 4.1 4.0 3.9 3.6 K2O 0.3 0.4 0.5 0.7 0.6 0.8 0.9 1.2 Ti02 1.28 0.76 0.81 0.72 0.67 0.77 0.67 0.71 P2O5 0.23 0.18 0.21 0.06 0.18 0.19 0.19 0.17 co2 0.1 0.1* 0.1* 0.1* 0.1 0.1* 0.1 0.1 H2O 0.4 0.6 0.8 0.6 0.1 1.0 1.0 1.3 s 0.03 0.01* 0.01 0.03 0.01 0.01 0.02 0.01 Total 100.77 100.67 100.71 98.62 99.55 98.86 98.67 101.98

Qz 2.70 4.40 4.66 4.80 8.45 7.13 15.90 16.10 Or 1.77 2.36 2.96 3.89 3.60 4.83 5.45 7.04 Ab 30.35 30.44 35.57 33.01 35.20 34.58 33.79 30.25 An 35.75 34.74 33.25 31.69 32.16 29.14 27.12 25.87 Cor 0.77 Di 2.99 2.09 4.08 1.46 2.37 0.76 Hy 21.90 21.94 18.76 19.66 15.14 18.40 13.46 16.29 Mt 1.30 1.45 1.74 0.93 2.06 1.33 1.48 2.02 11 2.43 1.44 1.54 1.37 1.29 1.50 1.31 1.34 Ap 0.55 0.43 0.50 0.20 0.44 0.47 0.47 0.40 Cc 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 Total 99.97 100.05 99.98 99.86 99.97 99.98 99.97 99.97

Note: Rapid analyses by the Geological Survey of Canada by x-ray fluorescence, except for'FeO, Na20, P205, C02, and H20 analyzed by chemical methods. * Amount is less than that shown in the analysis.

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004 (Fig. 6), which differs slighdy from the other two, was col- culated viscosity of the diorite magma of the core zone (Shaw, lected only 120 m from the diorite and may be contaminated. 1972) support Burnham's objections. A third hypothesis is that water distribution was controlled in an underlying magma Age chamber. It is suggested that the marginal diorite magma, first to be tapped from a high point in this magma chamber, had been en- K-Ar ages from the type area, 31 km north of Hope (Monger, riched in water by separation and rise of a hydrous phase, whereas 1970), are 73 m.y. (hornblende) and 74 m.y. (biotite). Ages from the diorite magmas that followed originated deeper in the chamber Sawmill Creek, 25 km north of Hope, are 76 ± 3 m.y. (biotite) and and were drier. It has also been suggested (K. F. Fox, Jr., 1975, per- 76 ± 4 m.y. (hornblende; McTaggart and Thompson, 1967). Ages sonal commun.) that if magmas were formed by partial melting, from the study area (Fig. 4, Table 2) range from 79 to 103 m.y., early magma (marginal part) would be water rich, whereas later with most between 79 and 89 m.y.; thus, the Spuzzum intrusions magma (central part) would be water poor and somewhat richer in may be of Late Cretaceous age. CaO, MgO, and iron oxide. Pyroxene-hornblende stability relations in Spuzzum diorite are il- Crystallization of the Zoned Diorite lustrated in Figure 7. Figure 7a is based on diagrams for basalt (Yoder and Tilley, 1962, p. 449), and pseudobinary diagram Figure Surprisingly little chemical variation accompanies the mineralog- 7b emphasizes the reaction pyroxene + hydrous melt = hornblende ical variation in the diorite, and the zonation is considered to be (Holloway and Burnham, 1972). Both diagrams suggest that dry mainly a result of increase in PH20 from the core regions to the mar- melts yield only pyroxene, whereas those with low water content gin of the intrusion. Such a distribution of water in the crystallizing yield pyroxene and hornblende, and those rich in water yield only diorite (Kennedy, 1955) has been accounted for by diffusion of hornblende, with complete crystallization. magmatic water down T or P gradients in the magma or diffusion The decrease in plagioclase from core regions to the margins is of water into the magma from country rocks. Indeed, because the due mainly to crystallization of hornblende, which uses up consid-

Spuzzum intrusions were accompanied by contact and possibly re- erable A1203. Increase in quartz is partly due to crystallization of gional metamorphism, the hypothesis that water from the wall hornblende and biotite, both relatively low in silica compared to rocks was taken up by the dry dioritic magma seems attractive. pyroxene. Burnham (1967) argued, however, that diffusion of water in felsic magmas is so sluggish that its effect would be seen only a few tens Crystallization of the Tonalite of metres from the contact. The viscosities of andesites and the cal- The tonalite may represent further differentiation and a younger phase of the parent magma from which the diorite magma was ear- so MODES lier extracted. The fact that the marginal tonalite is more acidic than the diorite core, the reverse of the usual relationship in dif- 75 75 ferentiated plutons, suggests that this hypothetical differentiation 70 - 70 occurred at some deeper level. Zoned cores in plagioclase and wormy quartz and pargasite cores in hornblende may be relicts of 65 • 65 earlier pyroxene, most of which may have been removed by fractional crystallization. Graphical tests show that if, by fractional 60 60 • PLAGIOCLASE crystallization, 54 percent of the diorite magma were removed as

plagiocase An52 (74 percent), hypersthene and diopside (13 per- Intermediate Margin cent), and olivine (13 percent), the remainder would have the com- Zone position of the tonalite.

Emplacement of the Spuzzum Intrusions 20 - 20

15 - 15 The diorite appears to have been emplaced in the mesozone as a tonguelike body. Possibly, the earliest diorite magma was relatively 10 - 10 hydrous and the following diorite magma drier, the later one inflat- ing the earlier one and producing the prominent foliation in the 5 - 5 diorite in the manner described by White (1973). As the diorite

OXIDES

•Si02 - 55 Pxt L \ L DIORITE TONALITE 50 Px+Hb< l_\ 20 Fk' •AI2O3 - 15 Px+ Hb

10 .Ca O IRON OXIDE J • MgO 5 Hb+ ? Na20 •H?Q Px Hb H20- rich L—s I "l— T T -r~ -1— 034 262 126 265 211 506 a b Specimen No. Figure 7. Hypothetical relations of pyroxene and hornblende; Px Figure 6. Modal and chemical compositions of Spuzzum intrusions. pyroxene, Hb = hornblende, L = liquid (see text for explanation).

TABLE 2. K-AR AGES FROM THE HOPE AREA

> -5 Spec. Lat Long Mineral Rock K Ar^/Ai*0 Ar"" :10 Age no. (N) (W) (%) cm3(stp)/g (m.y.)

Spuzzum intrusions la 49°20.9' 121°34.8' Biotite Quartz diorite 5.73 ± 0.07 0.82 2.404 103 ± 5 lb 49°20.9' 121°34.8' Biotite Quartz diorite 5.73 ± 0.07 0.79 2.392 103 ± 5 2a 49°23.5' 121°28' Biotite Quartz diorite 6.10 ± 0.06 0.89 1.930 79 ± 4 2b 49°23.5' 121°28' Hornblende Quartz diorite 0.54 ± 0.007 0.65 0.176 81 ± 4 3 49°21.7' 121°28.7' Biotite Quartz diorite 6.02 ± 0.02 0.79 2.010 83 + 4 8H 49°22.6' 121°29.8' Hornblende Diorite 0.536 ± 0.002 0.58 0.1945 89.5 ± 2. Yale intrusions 4a 49°19' 121°20.8' Biotite Quartz diorite 6.75 ± 0.04 0.81 1.598 59 ± 3 4b 49°19' 121°20.8' Biotite Quartz diorite 6.75 ± 0.04 0.53 1.588 59 ± 3 5 49°22' 121°22.2' Biotite Quartz monzonite 5.17 ± 0.02 0.53 0.851 41 ± 2 6 49°23' 121°19.6' Biotite Quartz diorite 7.65 ± 0.08 0.41 1.054 35 ± 2 7 49°06.8' 121°30.1' Whole rock Quartz diorite 1.109 0.31 0.105 24 ± 1 Chilliwack composite batholith Silver Creek stock 9 49°20.3' 121°26.7' Hornblende Quartz diorite 0.433 ± 0.012 0.39 0.06 35 ± 2 Chilliwack batholith 10 49°02.5' 121°18.3' Biotite Diorite 6.24 ± 0.06 0.30 0.723 29 ± 1 11 49°05.3' 121°26' Biotite Quartz diorite 7.51 ± 0.05 0.67 0.824 28 ± 1 12 49°06.3' 121°27.6' Biotite Quartz monzonite 6.83 ± 0.07 0.33 0.718 26 ± 1 13 49°00.8' 121°30.9' Biotite Quartz monzonite 6.97 ± 0.04 0.77 0.732 26 ± 1 14 49°01.2' 121°18.3' Biotite Quartz monzonite 5.90 ± 0.06 0.50 0.609 26 ± 1 Mount Barr batholith 15a 49°19.5' 121°39.1' Biotite Quartz diorite 6.87 ± 0.05 0.50 0.643 24 ± 1 15b 49°19.5' 121°39.1' Biotite Quartz diorite 6.87 ± 0.05 0.57 0.647 24 ± 1 16 49°18.7' 121°27.3' Biotite Granodiorite 4.85 ± 0.03 0.61 0.411 21 ± 1 17 49°13.8' 121°34.5' Biotite Quartz diorite 6.58 ± 0.06 0.22 0.464 18 ± 1 18 49°14.6' 121°33.r Biotite Quartz monzonite 6.91 ± 0.03 0.25 0.438 16 ± 1 Note: After Richards and White (1970), added to and revised by Richards and McTaggart.

magma chamber expanded, the foliated country rock was pushed TABLE 3. MODES OF THE YALE INTRUSIONS back. The dry interior part of the diorite complex probably crystal- lized more quickly than the more hydrous outer zone, so that at- Coquihalla stock Ogilvie stock Williams tachment to the adjacent schist was perhaps weak, and the diorite Peak stock mass, denser than the host rock, subsided. Tonalite magma was 176 220 581 132 082 096 730 1443 then emplaced as a crude ring dike as the block of nearly solid dio- Quartz 38.1 29.3 26.8 25.1 31.2 18.1 16.2 22.2 rite sank. Foliation in the tonalite is perhaps due partly to shearing Orthoclase 26.1 34.8 32.3 4.3 of a pasty magma as the central block subsided and due partly to Plagioclase 31.9 33.5 36.9 58.4 52.8 67.8 60.0 60.1 passage of a partly crystalline magma along a relatively restricted Hornblende tr. 2.0 3.2 3.9 17.1 opening. Biotite 3.3 1.4 3.2 7.5 9.5 8.9 0.5 1.8 Chlorite 0.8 2.6 5.1 5.3 YALE INTRUSIONS Epidote tr. 0.6 1.1 Tremolite 6.3 Prehnite 0.6 0.4 The Yale intrusions (McTaggart and Thompson, 1967) are a Accessories 0.4 tr. tr. 0.9 0.7 1 0.5 2.1 group of stocks and sills that lie along a belt extending from 5 km north of Yale (Fig. 2) southward to near the head of Silver Creek Total 99.8 99.0 99.2 99.0 100 100.3 100.6 99.3 (Fig. 5), a distance of 80 km. Three units are distinguished in the Note: 800 to 1,000 points counted per specimen. Precision checked by map-area (Figs. 4, 5). The Ogilvie Peak stock, along the eastern repeated counts of single thin sections and by counts of six thin sections of a margin of the map-area, is a tonalite except south of Berkey Creek, single specimen (Richards, 1971). where it appears to grade into granodiorite. The Coquihalla stock, leucocratic quartz monzonite, lying immediately east of Hope, has a gneissic extension to the south. It encloses small bodies of The Coquihalla quartz monzonite is relatively homogeneous. gabbro-diabase. The Williams Peak stock (Figs. 4, 9) lies 8 km Plagioclase shows oscillatory zoning in its core and normally zoned northwest of Chilliwack Lake. This tonalite stock, although not rims. K-feldspar, commonly poikilitic, shows patchy to incipient within the main belt of Yale intrusions, has been included because development of microcline twinning. Structural states (Wright, of its petrographic and chemical similarities to the Ogilvie tonalite. 1968) are transitional between microcline and orthoclase (Fig. 8). These three units were earlier (Richards and White, 1970) assigned The Coquihalla stock (Fig. 5) encloses and intrudes scattered to the Hope Plutonic Complex, a name now abandoned. bodies of gabbroic rock over a 5-km2 area centered just east of The Ogilvie Peak tonalite typically contains complexly zoned Kakawa Lake. Gabbro contains phenocrysts of olivine and zoned

coarse-grained ovoid plagioclase (0.5 to 2 cm) surrounded by plagioclase (An70-5o) clouded with dust-sized inclusions. Augite and medium-grained quartz, hornblende, and biotite. A cataclastic foli- hypersthene are partly replaced by hornblende. Some of the gabbro ation is developed in the western exposures and at contacts with is strikingly diabasic. older rocks. Modes are listed in Table 3. All units of the Yale intrusions display some degree of cataclastic

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foliation, the intensity of which increases near the Fraser River garnet-staurolite-kyanite gneiss in the Williams Peak tonalite can- fault zone. The Williams Peak tonalite has been converted to an not be fragments of the adjacent low-grade Darrington Phyllite but augen gneiss. resemble the Old Settler Schist. Ogilvie tonalite and its granodioritic southern extension are cut Granodiorite dikes correlated with the Yale intrusions invade the by dikes of the Coquihalla phase. The Williams Peak tonalite, iso- Spuzzum intrusions north of Yale (McTaggart and Thompson, lated from the other plutons of the Yale intrusions, resembles the 1967), and other members of the Yale intrusions appear to be over- Ogilvie tonalite. Inclusions, although not common, are large and lain by weakly indurated Eocene conglomerate 0.8 km north of include the previously mentioned gabbro. A metachert and Hope. Sheared parts of the Coquihalla stock have been intruded by metavolcanic inclusion in the Ogilvie stock, traced for more than the unsheared 35-m.y.-old Silver Creek tonalite, which intruded the 300 m, may be a roof pendant, for the roof of the stock is exposed Eocene conglomerate. K-Ar ages for the Yale intrusions, listed in along the south slopes of Mount Ogilvie. Large inclusions of Table 2, are not entirely consistent with relative ages determined from geological relations. The 59-m.y. age from the relatively unsheared Berkey Creek part of the Ogilvie phase is accepted. The Ogilvie K-Ar age (35 m.y.) may be a reset age, a result of heating by the adjacent Coquihalla stock and by shearing. The K-Ar age for the Coquihalla stock seems reasonable. The age of the partly cataclastic Williams Peak tonalite is almost certainly greater than that of adjacent unsheared Eocene conglomerate. Biotite (dated at 24 m.y.) from the tonalite is probably metamorphic, produced by the adjacent Chilliwack and Mount Barr batholiths. Analyses (Table 4) of three samples from the Coquihalla stock are almost identical. They show different degrees of cataclasis: sample 176 is undeformed, sample 220 is weakly sheared, and sample 581 is an augen gneiss. The similarity of the analyses of the Ogilvie tonalite (sample 096) and of the Williams Peak tonalite (1443) supports their correlation. Most of the Yale intrusions have been emplaced along or near an east-dipping shear zone that separates the Hozameen Group from the Custer Gneiss (McTaggart and Thompson, 1967). Large Figure 8. Structural states of feldspars. xenoliths in the Coquihalla and Williams Peak stocks show that

TABLE 4. CHEMICAL ANALYSES AND CIPW NORMS TABLE 5. MODES OF THE SILVER CREEK STOCK OF THE YALE INTRUSIONS 525 052 596 1355 Coquihalla stock Ogilvie Williams stock Peak Plagioclase 55.2 54.1 58.5 61.8 stock Quartz 21.0 21.6 17.8 16.0 1.4 tr. 176 220 581 096 1443 Orthoclase 3.2 6.2 Clinopyroxene tr. tr. tr. tr. Hornblende 9.0 5.9 9.1 10.1 Si02 73.7 73.7 74.2 62.6 68.8 Biotite 8.1 7.9 11.0 10.8 AI2O3 14.8 14.6 14.3 16.6 15.2 MgO 0.5* 0.5* 0.5* 1.7 1.5 Chlorite 0.8 1.6 0.9 tr. FeO 0.7 0.9 0.8 3.3 1.7 Epidote 0.3 tr. tr. Sphene tr. 0.2 tr. tr. Fe203 2.9 0.2 0.3 1.9 1.1 CaO 1.1 1.2 1.3 4.8 3.2 Apatite tr. 0.3 0.2 tr. Magnetite 2.0 0.8 0.8 1.4 Na20 4.0 3.9 3.5 4.5 4.7 K2O 3.8 4.0 4.0 0.9 0.7 Total 99.6 98.6 99.7 100.1 MnO 0.04 0.03 0.03 0.07 0.06 Ti02 0.19 0.17 0.17 0.67 0.36 0.03 0.04 0.35 P2O5 0.04 0.11 TABLE 6. CHEMICAL ANALYSES AND CIPW NORMS 0.1 0.2 co2 0.1* 0.1* 0.1* OF THE SILVER CREEK STOCK H2O 0.4 0.3 0.3 1.0 1.3 S 0.02 0.02 0.03 0.05 0.01 525 1355 525 1355 Total 102.29 99.65 99.57 98.54 98.94 15.30 Qz 32.54 32.39 35.21 20.34 30.49 Si02 62.7 59.8 Qz 20.09 Or 9.66 8.37 Or 22.04 23.80 23.82 5.46 4.24 Ti02 0.61 0.65 Ab 31.99 31.67 Ab 33.22 33.22 29.84 39.05 40.73 AI2O3 16.1 17.1 An 23.08 26.21 An 4.48 5.16 5.60 21.44 14.23 Fe203 1.8 2.8 Cor 2.39 1.99 2.19 0.58 1.66 FeO 3.7 3.9 Di 0.62 1.79 10.12 10.45 Di 0.0 0.0 0.0 0.0 0.0 MgO 2.3 2.9 Hy 2.67 4.11 Hy 1.28 2.51 2.25 7.91 5.57 MnO 0.11 0.13 Mt Mt 1.67 0.29 0.44 2.83 1.63 CaO 5.0 6.1 11 1.17 1.25 3.7 3.7 0.32 0.34 Hm 1.69 Na20 Ap 0.46 11 0.35 0.33 0.33 1.31 0.70 K2O 1.6 1.4 Cc 0.23 Ap 0.09 0.07 0.10 0.86 0.27 PA, 0.13 0.14 Total 99.97 99.96 Cc 0.22 0.23 0.23 0.23 0.47 co2 0.1 0.2 H2O 0.6 0.9 Total 99.98 99.99 99.99 100.00 99.98 S 0.01 0.11 * Amount is less than that shown in the analysis. Total 98.46 99.83

sloping may have been important. The Yale intrusions may be re- CHILLIWACK COMPOSITE BATHOLITH lated to the Hell's Gate intrusion (Monger, 1970) 50 km north of Hope. Mineralogically similar to the Coquihalla phase, it shows a The Chilliwack composite batholith (Misch, 1966) includes the distinct cataclastic foliation and has been cut by the Yale fault. Its Silver Creek stock, the Mount Barr batholith (Richards and White, radiometric ages of 35 to 44 m.y. (Baadsgaard and others, 1961; 1970, refer to the Mount Barr Plutonic Complex) and the Chil- Monger, 1970) are similar to those of the Coquihalla stock. liwack batholith (Daly, 1912), which extends south of the interna-

V5 S S

WILLIAMS^»} S U PEAK , V S ^ STOCK' % s , s /-tVtVtVt CREEK: s /tVtVtVA 70 S , ^ ^tVtVtVtVt

STOCK \80 -j. 1 a. 1 u. 1 -tV J. + + + c* A 75 ^rT^r t -,—r-r~ Klesikwa Peak + + + if VÒÓ- O °J -TtWtVtV^ri^M- + ^VtVtVAVtVtVAW t tj t t.t t." 1111 +1,t tj- + + + + V1" v vA^-F + + + + t t t^st 11 t t t t ; + + + + "T + V' +\ 4 +-*•:• + + ^tftVtVt^V-î-^+ + + + + + + V ' A- + + + + + + + + vVAV't + + + •V»- ¡*-.V + + \ + + . + + + + — 'S* S + + + • O + + + + + f • + * +/ + +: 1 + + \ / - ,• T ^ • - ' \ -I - I - - + + + + ¡."t + +'•

" J- + +V- + +REXFORD ; + AMount Rexford H s •.••: + +•<• + + V+ + - X. -L ' T + + + + '+ 4KW-f??, Vi tVt> . t t t t ••'•:••+ + +-.+ + +X + -QUARTZ::::^, H + + vlfxl I'.•••? + 4- 4:, .. H • + + + + + + + + + + ++M- + + V + + + vVj /AMount Slesse.+ + > + + + ",¡0 S '.'>•''•• :'.''i+ + + -.+ + +'A + + + y:-./ + + + v + :MÔNZONÎTE:H + +-=1 - + + + + + + + + +•.'-.f. + S\ + + + + + + + + + + + + + i + + + + + + + + + + - - + +/45 - { +. +: U.S.A. j0a Middle Peak

Alaskite Diorite 3 A >3 0 V Cenozoic sedimentary or volcanic rocks Quartz monzonite . 'V <

Granodiorite S S 5 Custer gneiss - + + + + Tonalite Darrington phyllite E33 Basic tonalite KILOMETERS 4

Figure 9. Geologic map of Chilliwack batholith.

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tional boundary and includes the Perry Creek phase. These late River faujt zone, and contains large xenoliths of local derivation, Tertiary plutons truncate most north- and northwest-trending stoping was probably the main mechanism of emplacement. structures and are related to the north-trending Eocene-Holocene Cascade belt (Waters, 1955; Erikson, 1969). Chilliwack Batholith

Silver Creek Stock The Chilliwack batholith (Figs. 4, 9), north of the 49th parallel, consists of several intrusive phases, which range from hypersthene The Silver Creek stock (Figs. 4, 5), 5 km south of Hope, is about diorite to granite. The oldest phase is diorite, which crops out in 25 km2 in area. It was previously included in the Hope Plutonic three small areas. Tonalite, the dominant rock type, makes up 50 Complex (Richards and White, 1970), a name that has been to 60 percent of the batholith. The third phase, the Paleface dropped. It is composed of homogeneous and unfoliated medium- Granodiorite, intrudes the tonalite near Chilliwack Lake. The grained tonalite (Tables 5, 6). The interior parts of subhedral fourth phase comprises three stocks of granodiorite to quartz mon- plagioclase crystals show strong oscillatory zoning and range from zonite, the Radium, the Rexford, and the Post Creek stocks, which

An 56 to An40. In the cores of many of the crystals there is a central, cut through the tonalite. Two plugs of alaskite appear to be among highly corroded zone of composition An90 to An75. Hornblende, the latest phases. augite, augite rimmed by hornblende, biotite, and hypersthene Diorite. The main body of diorite, covering an area of less than form inclusions in plagioclase. Alkali feldspars show orthoclase 3 km2, is found near the eastern edge of the batholith. Smaller structural states (Fig. 8). Hornblende, some of it with cores of bodies occur at the north end of Chilliwack Lake and north of pyroxene, and biotite are the dominant mafic minerals. the Rexford stock, the latter too small to show in Figure 9. Diorite The stock intruded and metamorphosed Eocene conglomerate is even-grained, massive, and weakly porphyritic. Where it is in and has been intruded by the Miocene Mount Barr batholith. The contact with Custer Gneiss, it contains phenocrysts of pyroxene

walls of the stock appear to be vertical. A single K-Ar determina- and plagioclase. Plagioclase (average, An range, Ango_4o), augite tion on hornblende gave an age of 35 m.y. (Table 2), which is con- (Wo46En39Fs15), and hypersthene (En6)) are the main minerals. sidered to be the time of emplacement of the stock. That the stock is Needles of rutile are common in some plagioclase. In the eastern epizonal is suggested by the high-temperature structural state of the body, hornblende forms thin rims on pyroxene, but in the west it alkali feldspar (Fig. 8), fine-grained margins, adjacent hornfels, and appears to have replaced most of the pyroxene. A mode of a typical the mid-Tertiary age. Since the stock is discordant, lies in the Fraser diorite is given in Table 7 and the chemical analysis in Table 8.

TABLE 7. MODES OF THE CHILLIWACK BATHOLITH

Diorite (CR) and Chilliwack tonalite CR Nes* 748 Lindt 1053 679 478 900 484 891

Quartz 3.9 9.5 17.2 19.1 15.7 15.8 16.2 20.0 22.7 19.6 Orthoclase 2.4 tr. 1.6 2.0 0.4 1.2 tr. 7.0 1.3 6.7 Plagioclase 65.2 58.0 64.4 55.7 60.6 61.0 57.5 53.0 57.4 56.2 Hypersthene 13.9 1.6 0.3 Augite 7.8 4.3 tr. tr. 7.2 1.9 1.0 1.1 1.4 Hornblende 1.3 12.8 11.2 11.1 10.0 1.4 11.0 8.0 7.6 5.8 Biotite 2.9 12.5 4.1 10.3 12.7 11.4 12.2 10.0 7.1 8.6 Apatite 0.4 tr. tr. tr. tr. tr. 0.3 tr. tr. tr. Opaques 2.3 0.8 0.8 1.3 0.4 0.5 0.6 04 1.1 1.0 Total 100.1 98.4 99.3 99.5 99.8 100.1 99.8 99.4 98.6 99.9

Paleface Granodiorite Radium Peak granodiorite 792 1402 1109 789 1144 427 782

Quartz 19.4 26.8 28.9 20.6 33.2 23.3 36.1 Orthoclase 8.3 4.4 14.4 20.6 23.2 25.1 16.4 Plagioclase 56.6 52.6 39.7 50.2 37.3 42.0 41.7 Hornblende 7.2 6.0 10.1 2.1 1.0 3.7 0.6 Biotite 8.3 9.3 5.2 6.0 5.0 4.7 4.7 Apatite tr. tr. tr. tr. tr. tr. tr. Opaques 0.3 0.9 0.8 0.4 0.6 JL0 0.7 Total 100.1 100.0 99.1 99.9 100.3 99.8 100.2

Aplitic alaskite Rexford quartz monzonite Alaskite Post Creek quartz monzonite 769 840 1036 1051 1023 M182 465 432 463 59-23

Quartz 37 35.4 24.9 40.3 26.9 29.8 20.8 36 27 24 Orthoclase 33 34.5 42.9 21.3 22.0 43.6 14.9 26 22 20 Plagioclase 28 28.7 27.8 37.4 49.1 24.6 58.2 34 44 49 Biotite 2 1.1 4.4 tr. 1.8 2.0 5.0 3 6 6 Apatite tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. Opaques tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. Total 100 99.7 100.0 99.0 99.8 100.0 98.9 99.0 99 99 1 Daly (1912); to be added: 1.5 percent pyrite, 0.1 percent sphene and zircon, f Daly (1912); to be added: 0.5 percent apatite and zircon.

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Diorite (CR) and tonalité Paleface Granodiorite Radium Peak quartz monzonite Aplitic Rexford Post alaskite quartz Creek monzonite quartz CR Nes* 748 Lindf 1053 679 478 900 484 891 792 1402 1109 789 1144 427 782 769 1036 monzonite§

Si02 55.3 56.90 60.2 60.36 60.5 60.8 61.2 63.6 63.9 64.2 64.0 67.4 68.6 68.4 71.0 72.9 74.0 77.1 73.4 71.41 Ti02 1.3 0.84 0.4 0.70 0.5 0.6 0.5 0.5 0.5 0.6 0.6 0.5 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.34 AI2O3 17.4 18.17 18.2 17.23 17.9 18.6 17.4 16.4 17.6 18.2 15.0 15.5 15.9 15.1 14.4 14.8 15.3 14.1 12.7 14.38 Fe203 2.4 1.23 2.1 1.93 0.1 1.3 1.6 1.6 1.3 1.5 1.3 1.4 1.2 1.4 1.2 1.0 0.8 0.1 2.5 1.33 FeO 6.1 5.88 2.8 3.74 4.4 3.3 3.6 3.1 2.7 3.3 3.3 2.7 2.2 1.8 1.2 1.5 0.9 0.2 0.9 1.17 MgO 5.2 4.36 2.9 3.66 3.2 2.2 2.2 2.2 1.0 2.2 3.2 2.0 1.6 1.8 1.0 1.0 0.9 0.8 0.9 1.13 MnO 0.16 0.21 0.10 0.14 0.09 0.08 0.10 0.09 0.08 0.09 0.10 0.08 0.06 0.06 0.03 0.05 0.04 0.01 0.02 0.04 CaO 7.5 6.51 6.6 6.07 6.4 6.0 6.6 5.1 5.5 5.0 4.6 3.8 3.8 3.0 2.2 2.6 2.2 0.9 1.3 2.51 Na20 3.3 3.23 3.8 3.58 3.8 3.8 3.9 3.6 3.7 3.7 3.2 3.2 3.4 3.3 3.2 3.3 3.7 2.9 3.7 4.12 K2O 0.7 1.57 0.9 1.74 1.1 1.3 1.1 1.8 1.6 2.1 2.4 2.7 2.4 3.2 3.6 2.9 3.0 4.8 2.7 2.97 P2O5 0.37 0.10 0.16 0.11 0.15 0.15 0.16 0.14 0.14 0.13 0.13 0.10 0.09 0.07 0.06 0.06 0.04 0.02 0.03 0.13 co2 0.10 0.08 0.10 0.08 0.12 H2O 0.60 0.77 1.00 0.55 0.80 0.80 0.70 0.80 0.05 1.00 0.9 0.6 0.8 0.7 0.7 0.6 0.4 0.4 0.3 0.30 s 0.02 0.01 0.02 0.01 0.03 0.02 0.03 0.03 0.12 0.03 0.01 0.03 0.01 0.02 0.02 0.02 0.02 Total 100.45 99.85 99.27 99.89 98.96 98.94 99.09 98.95 98.10 102.05 98.85 100.01 100.46 99.26 98.90 101.03 101.50 101.55 98.67 99.95 Qz 8.36 8.48 16.11 13.74 13.53 16.43 16.57 20.91 22.36 19.0 21.04 26.36 28.33 27.73 32.82 34.88 34.35 38.36 38.12 29.57 Or 4.14 9.55 5.41 10.11 6.62 7.83 6.61 10.84 9.65 12.29 14.50 16.06 14.23 19.19 21.67 17.07 17.54 28.05 16.22 17.80 Ab 27.96 27.35 32.72 30.66 32.76 32.76 33.54 31.04 31.93 30.98 27.67 27.24 28.86 28.33 27.57 27.80 30.97 24.26 31.83 34.83 An 30.64 30.87 30.46 25.91 29.06 29.34 27.16 23.71 26.91 23.72 19.90 18.31 18.33 14.64 10.72 12.46 10.54 4.29 6.36 10.84 Cor 0.39 0.11 1.05 0.64 1.02 0.93 1.40 1.64 2.04 2.51 1.42 0.45 Di 2.91 0.44 0.92 2.68 1.89 4.18 0.94 2.07 Hy 18.87 19.51 9.84 12.28 14.63 9.77 8.20 8.87 5.78 9.34 11.37 8.12 6.48 6.15 3.31 3.98 2.92 1.98 2.31 3.45 Mt 3.49 1.76 3.10 2.77 0.15 1.92 2.36 2.36 1.92 2.15 1.93 2.04 1.75 2.06 1.77 1.44 1.15 0.06 2.36 1.89 11 2.48 1.54 0.77 1.34 0.97 1.16 0.97 0.97 0.97 1.13 1.17 0.96 0.76 0.77 0.58 0.57 0.38 0.38 0.39 0.57 Ap 0.89 0.24 0.39 0.27 0.37 0.37 0.39 0.34 0.34 0.31 0.32 0.24 0.22 0.17 0.15 0.14 0.09 0.05 0.07 0.31 Cc 0.23 0.18 0.23 0.18 HM 0.05 0.91 Total 99.96 99.92 99.96 99.95 99.97 99.97 99.97 99.97 99.97 99.97 99.97 99.97 99.97 99.97 99.99 99.98 99.98 100.00 99.99 99.72 * Daly (1912); includes 0.18 percent SrO. f Daly (1912). S Daly (1912; to be added: 0.03 percent BaO.

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Mineralogically, the rock is a quartz-bearing gabbro, but chemi- margins of the eastern tonalite. Elsewhere, hornblende, with cores cally, especially in its high Si02, it is close to diorite and similar to of augite, is the dominant mafic mineral. Hornblende and biotite, some of the tonalites. interstitial to plagioclase and pyroxene in the marginal tonalite, Tonalite. Tonalite, east of Chilliwack Lake, ranges from two- occur as subhedral crystals in the more central tonalite and pyroxene tonalite ("basic tonalite" of Fig. 9) to hornblende-biotite granodiorite. The assemblage of mineral inclusions in plagioclase tonalite, grading into mafic granodiorite, in a crudely concentric varies in a systematic manner from the hypersthene-augite tonalite disposition of rock types. Suggestions of similar gradation in the (dominantly pyroxenes) to granodiorite (hornblende and biotite).

central part are found west of Chilliwack Lake. There, however, Orthoclase (Or82_87) and quartz are interstitial to plagioclase and pyroxene is uncommon and the dominant mafic minerals are mafic minerals. hornblende (with cores of augite) and biotite. West of Chilliwack Paleface Granodiorite. The Paleface Granodiorite (Fig. 9) con- Lake, tonalite includes Daly's (1912) Slesse Diorite, a name sists mainly of isotropic medium-grained biotite-hornblende herewith abandoned. granodiorite (Table 7). Fine- to medium-grained porphyritic

In pyroxene tonalite, plagioclase (zoned, An50_30) shows little os- granodiorite occurs along the northern margin of the body. Plagio- cillatory zoning, but in the granodiorite, it is pronounced (An42_27). clase commonly shows cores with oscillatory zoning (An50_38) and

Some crystals contain corroded cores of bytownite. Augite normally zoned rims (An35_2o). (Wo43En34Fs23) and hypersthene (En56) are abundant only along the Radium Granodiorite. The Radium pluton (Fig. 9) consists of leucocratic, medium-grained, homogeneous hornblende-biotite r 1» granodiorite and quartz monzonite. Miarolitic quartz monzonite 18 aplites are both intrusive into and gradational with the main body. 17 • Anhedral plagioclase shows oscillatory zoning (An46_38) and nor- 16 mally zoned rims (An32_¡7). Orthoclase (Or80) appears to corrode --0-- - 15 plagioclase. • 14 Alaskite Plugs. The most eastern alaskite plug (Fig. 9) is a white 13 ® to buff medium-grained albite granite, with a fine-grained margin AI2O3 12 against gneiss and tonalite. The other plug, a pink, aplitic, miarolitic biotite quartz monzonite, is central to the Radium granodio- 6 6 4- rite. Scarce phenocrysts of plagioclase resemble plagioclase in the 5 5 surrounding Radium granodiorite. 4 v 4 >, * Rexford Quartz Monzonite. The Rexford stock (Fig. 9) consists 3 - 3 MgO mainly of isotropic, medium-grained leucocratic quartz monzonite. 2 2 It includes a small body of oligoclase-quartz-orthoclase porphyry. 1 - A*- • 1 Plagioclase shows oscillatory zoning (An25_17); orthoclase is mark-

+ edly perthitic. Post Creek Stock. The Post Creek stock (Fig. 9) is composed largely of leucocratic biotite quartz monzonite that is similar to that of the Rexford stock. V V Structure. The sequence of emplacement of the principal phases ^O. ® of the Chilliwack batholith started with diorite and ended with the Rexford and Post Creek quartz monzonites. Tonalite clearly intrudes the diorite. Paleface Granodiorite contains abundant 2- to Fe 0 tot • 15-m inclusions of tonalite and is, in turn, intruded by the Radium stock. The alaskite plug on the west shore of Chilliwack Lake is similar to aplite dikes in the Radium stock and is probably related + to that stock. Foliation, shown by preferred orientation of mafic • V/ ^ minerals and plagioclase, is steep and parallel with the contacts. In the central parts, where tonalite becomes granodioritic, foliation is indistinct. Randomly oriented poikilitic biotite flakes that contain

A aligned plagioclase suggest that this alignment is magmatic. A A- A layered zone in the eastern diorite consists of 10 steeply dip- CoO E • ping layers, 15 to 30 cm thick, parallel to contacts with the gneiss. Within layers, plagioclase and pyroxene are aligned perpendicular X 4 — to the layering. In the Paleface phase, each of 3 layers, exposed over * % -0-- 0--A°- -A ^ 3 • 3 3 vertical m, consisting of a 2.5-cm layer rich in hornblende and 2 2 biotite overlain by 1 m of normal granodiorite, shows graded struc- 1 • NajO 1 ture and shallow dip, suggesting formation by gravity settling. The • Radium, Rexford, and Post Creek plutons show little or no internal 4 - A - 4 structure. 3 - K2O X A "2 3 -O ---O—O" ® 2 - - 2 In the east, structures in the competent Custer Gneiss have been 1 - 1 little disrupted by emplacement of the batholith, but in the west, I T I foliations in the Darrington Phyllite have been deflected to conform 60 65 70 75 to the outlines of the batholith. The roof of the batholith is locally °/o S ¡ O2 preserved. A cap of hornfels, diorite, and migmatite as much as 300 • Diorite A Radium m thick overlies the tonalite at the northern edge of the Rexford • Tonalite 8 Rexford pluton. To the south, at the international border, Cenozoic con- O Palefoce Granodiorite x Post Creek glomerate and pyroclastic rocks form a roof over quartz monzonite • Alaskite and granophyre. One kilometre to the west, in an inclusion-rich Figure 10. Variation diagram for phases of Chilliwack batholith. zone in the Rexford stock, 2- to 8-m slabs of metavolcanic rocks,

veined by the Rexford quartz monzonite, were probably derived depth (see below), where cooling was relatively rapid, the K-Ar from a nearby roof. At the eastern edge of the area, about 12 km ages probably mark closely the time of emplacement. north of the border, 70-m blocks of gneiss are engulfed by Differentiation and Emplacement. Chemical analyses and heterogeneous tonalite, and a narrow tongue of country rock seems norms for the Chilliwack batholith are shown in Table 8. Specimen to extend over the tonalite. CR, from the eastern body of diorite, collected within 30 m of a The Darrington Phyllite has been changed to hornfels and mig- chilled contact, is taken to represent the composition of a diorite matized within a contact aureole some 300 m wide within which parental magma. the metamorphic grade reached that of the K-feldspar-cordierite Figure 10 is the conventional silica variation diagram for all hornfels facies. phases of the Chilliwack batholith. It is unusual in that the varia- Age. The batholith intrudes Eocene conglomerate and the tion curves are drawn as three discontinuous segments. The points Oligocene(P) Skagit-Hannegan volcanic rocks. K-Ar age determina- for the various oxides could be connected with smooth curves in tions from the various phases of the batholith (Fig. 4) fall between the usual way; the quality of the fit so obtained would be accept- 29 and 26 m.y. (Table 2). As the pluton was emplaced at a shallow able, and the variation illustrated would agree with the general

POST/, tvo°o'i CREEK. o°\ STOCKÌ

Wahleach Lake Tonalite Eocene sedimentary rocks Quartz monzonite 5 n Custer gneiss Mount Barr phase Chilliwack group + + + + + Conway phase Darrington phyllite Figure 11. Geologic map of Mount Barr batholith.

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TABLE 9. MODES OF THE MOUNT BARR BATHOLITH

Conway phase Mount Barr phase Leucocratic Wahleach Lake phase Tonalite 114 292 274 379 M76 134 SC 1248 185 144

Quartz 23.1 18.0 20.9 18.9 22 19.7 26 21 36.9 24.5 Orthoclase 6.5 11.2 12.1 10.2 10 22.5 9 15 21.5 5.8 Plagioclase 55.5 54.4 55.7 51.2 52 43.6 55 51 38.0 45.6 Clinopyroxene 0.6 tr. 0.6 Hornblende 5.1 7.9 3.5 7.2 8 8.1 5 8 14.3 Biotite 7.3 6.1 6.9 11.1 6 4.1 4 5 2.5 9.7 Sphene tr. tr. tr. tr. tr. tr. tr. tr. tr. 0.5 Apatite tr. tr. tr. tr. tr. tr. tr. tr. tr. tr. Magnetite 1.0 0.8 0.6 0.8 J_ 1.2 tr. tr. tr. tr. Total 98.5 99.0 99.7 99.9 99 99.2 99 100 98.9 100.4

trends seen in differentiated granitic complexes. Nevertheless, al- magma existed at depth, perhaps having accumulated above a though a single cycle of differentiation is believed responsible for partly melted slab in a subduction zone, and that the various phases the various phases of the batholith, it is suggested that as each of the batholith were derived from this. Early injections from this phase was isolated, it continued to differentiate. For this reason, parental magma formed the small diorite bodies peripheral to tona- values for each individual phase were connected without regard for lite and possibly fed andesitic volcanoes, perhaps now seen as the general variation. Although the difference between a general 01igocene(?) Skagit Volcanics or Hannegan Volcanics. curve and the short curves is not great for some oxides, it is obvious Differentiation of the average tonalite magma from diorite

for A1203, MgO (in part), and K20. magma would involve removal of CaO, MgO, total Fe, and Si02, It is assumed that at an early stage, a large volume of diorite and it seems possible that this could have been accomplished by early separation (by settling?) of pyroxenes and magnetite in relatively greater amounts than plagioclase, which probably did not 17 - - 17 16 -i O - 16 settle as quickly as the other minerals. It is hypothesized that after • - - 15 the tonalite magma was evolved, occupying perhaps the apices of a 15 - AI2Oj U - - 14 parental magma chamber and containing some suspended plagioclase, it freed itself from the remaining somewhat denser magma and, differentiating slightly as it rose, was emplaced near the sur- - 2 2 - o face of the crust, forming the tonalite now present. Minor variation 1 - MgO - 1 within the tonalite (Fig. 10) is partly a result of the central parts of the magma having arrived later, after precipitating plagioclase and >• pyroxene as they rose, and partly of differentiation in place by con- - 4 4 - vection and diffusion. The western tonalite magma, crystallizing 3 - - 3 hornblende and biotite rather than pyroxene, was relatively hy- 2 - - 2 FeO _ i drous, and it is possible that it acquired water from adjacent 1 — tot phyllites. In the meantime, the magma left below continued to differentiate, 5 - - 5 and the more evolved Paleface magma freed itself and rose from the 4 - * v - 4 3 - - 3 parental chamber. It too probably differentiated as it rose (Fig. 10), - 2 apparently mainly by the separation of mafic minerals. The crystal- 2 - CaO 1 - - 1 lization and separation of hornblende would be highly effective in driving the composition toward quartz monzonite and granite 4 - - 4 (Holloway and Burnham, 1972). The Radium quartz monzonite is 3 - • - 3 representative of the last major episode of differentiation, which re- 2 - Na20 - 2 sulted also in the formation of the Rexford and Post Creek quartz monzonites. The small alaskite plug at Chilliwack Lake (769), the 4 - • - 4 most evolved differentiate, plots almost at the ternary "granite" 3 - Û - 3 * • minimum in the system Ab-0r-An-Si02 at 1,000 b Ptt20 (James 2 - •• I ° - 2 KjO and Hamilton, 1969). 1 - - 1 Because the Chilliwack tonalite and the Paleface and Radium Peak granodiorites are symmetrically disposed across the Fraser + * River fault zone (Figs. 4, 9), and the Post Creek stock lies mainly 0.4 - - 0.4 within it, it is speculated that the emplacement of these plutons was 0.3 - - 0.3 to some extent controlled by this zone. Emplacement of the tonalite - 0.2 0.2 - magma, which was partly crystalline, probably began near the 0.1 - Ti02 -0.1 northern end of Chilliwack Lake and spread east and west. Possibly i I the northeastward-extending tongue (Fig. 4) made room for itself 50 80 60 70 as an . elongate laccolith. As the magma chamber enlarged, the °/oSi02 peripheral, partly crystalline magma was stretched, and a foliation, Conway phase O Wahleach Lake Tonalité generally conformable to the outer contact, was produced. Further • Mount Barr phase A Quartz Monzonite distension may have accompanied emplacement of the Paleface Figure 12. Variation diagram for Mount Barr batholith. Granodiorite. The Radium quartz monzonite may have, in its turn,

inflated the unfoliated (largely liquid?) Paleface Granodiorite. The and even-grained but are locally granophyric, miarolitic, or chilled

Rexford quartz monzonite, judged by its abundant inclusions, and at contacts. Zoned plagioclase with cores of An44 to An38 are the Post Creek quartz monzonite, lying in a major fault zone, were rimmed by plagioclase zoned from An29 to An20. Tonalite of the probably emplaced largely by stoping. Wahleach Lake phase is massive, equigranular, and fine grained.

Plagioclase has cores with oscillatory zoning (An45_38) and rims of Mount Barr Batholith An27 to An20. Hornblende and biotite are about equal in amount. Scarcity of chilling or diking at contacts between phases is be- The Mount Barr batholith (Figs. 4,11), previously mapped as the lieved to be due to rapid succession of intrusions, so that certain Mount Barr Plutonic Complex (Richards and White, 1970) is com- units were still hot and possibly plastic or miscible at the time of the posed of four phases. The oldest, the Conway phase, is mostly basic following injection. The Conway phase, judged by its mainly granodiorite and tonalité. Central and eastern parts of the batholith peripheral location, may be the oldest, followed by the more acidic consist of granodiorite and quartz monzonite of the Mount Barr Mount Barr phase. The leucocratic quartz monzonite stocks phase. Three small stocks, one near the eastern edge of the clearly intrude and are chilled against the Mount Barr. The relative batholith and others near Wahleach Lake, are leucocratic quartz age of the Wahleach Lake stock is uncertain. Its uniform fine grain monzonite. The Wahleach Lake Tonalité forms a 16-km2 stock at suggests emplacement after the other three units. Chemical data Wahleach Lake. Modes of these phases are listed in Table 9. (Fig. 12), however, can be taken to suggest that the phase is older The Conway phase is composed of medium-grained massive than the quartz monzonite stocks, and possibly the fine grain is due granodiorite and minor tonalité, locally porphyritic. Plagioclase, to the loss of PH2o during crystallization.

common as phenocrysts, shows cores of An48 to An40, with well- In the Conway phase, 10 gently dipping layered zones, 15 cm to developed oscillatory zoning and rims with normal zoning from 1 m thick, are exposed over 50 vertical m, just north of Mount

An 35 to An25. Hornblende and biotite are about equal in amount; Conway (Fig. 11). Layers are graded and resemble cumulates. Min- the former has cores of augite. The Mount Barr phase, mainly a erals in layered and unlayered rocks are identical. Similar layering subporphyritic granodiorite with phenocrysts of plagioclase and in the Mount Barr phase is less well developed than in the Conway hornblende, is homogeneous except for thin layers and patches of phase but is more common. hornblende-rich rock. It differs from the Conway phase in its con- At elevations near 750 m, hornblende phenocrysts may form as spicuous hornblende phenocrysts and in its more abundant quartz much as 50 percent of the Mount Barr granodiorite, and the propor- and K-feldspar. Plagioclase is strongly zoned, with as many as 100 tion of hornblende at such low levels may mark the floor of the individual zones. Phenocrysts of partly biotitized hornblende, 1 to Mount Barr phase. In the northeastern part of the batholith, a 5 cm long, are abundant in exposures at low elevation. A dike some patch of the Conway granodiorite appears to overlie the Mount 70 to 100 m wide, with phenocrysts of quartz, hornblende, and Barr, suggesting that the latter may be a 1,000-m-thick sill-like plagioclase in a granophyric groundmass, lying between Eocene body. conglomerate and the Silver Creek stock, appears to be an offshoot K-Ar ages from the Mount Barr batholith (Fig. 3) are as follows: of the Mount Barr phase. Conway phase, 18 m.y., 18 m.y. (Baadsgaard and others, 1961); Three leucocratic quartz monzonite stocks are generally massive Mount Barr phase, 24 m.y., 24 m.y., 21 m.y., 16 m.y. (Table 2);

TABLE 10. CHEMICAL ANALYSES AND CIPW NORMS OF THE MOUNT BARR BATHOLITH

Conway phase Mount Barr phase Quartz Wahleach monzonite Lake 114 379 292 274 1248 134 M76 SC 185 144

Si02 65.8 64.3 64.0 63.8 65.3 69.1 67.4 67.2 77.3 69.9 AI2O3 16.7 17.4 17.4 16.9 16.3 14.8 16.5 15.6 14.2 16.0 Fe2Oa 1.6 1.8 2.1 1.4 1.7 1.6 1.6 1.2 0.8 1.3 FeO 2.3 2.6 2.2 2.5 2.8 1.8 2.3 2.1 0.7 2.0 MgO 2.0 2.4 1.8 1.9 2.1 1.7 1.7 1.5 0.5* 1.8 MnO 0.08 0.08 0.08 0.08 0.08 0.07 0.08 0.06 0.03 0.08 CaO 3.2 4.9 4.9 4.6 4.5 3.4 4.1 3.2 1.6 3.1 Na20 3.6 3.6 3.6 3.5 3.6 3.1 3.5 4.0 3.2 3.5 K2O 1.3 2.1 2.3 2.6 2.6 3.9 2.2 2.0 3.0 2.3 Ti02 0.41 0.48 0.45 0.49 0.47 0.38 0.42 0.35 0.17 0.38 P2O5 0.17 0.13 0.14 0.12 0.12 0.11 0.15 0.06 0.05 0.10 co2 0.1* 0.1* 0.1 0.1 0.3 0.1* 0.1* 0.2 0.1* 0.1 H2O 1.3 0.8 0.6 0.4 0.7 0.5 0.8 0.7 0.4 1.0 Total 98.6 100.70 99.69 98.42 100.59 100.57 100.87 98.18 102.06 101.58

Qz 29.40 20.71 21.17 20.57 21.47 26.34 26.92 27.51 42.57 30.84 Or 7.78 12.42 13.72 15.68 15.39 23.03 12.99 12.13 17.44 13.52 Ab 30.39 30.49 30.74 30.22 30.49 26.21 29.59 34.72 26.63 29.44 An 15.01 22.85 22.98 21.85 19.67 14.93 18.72 14.59 6.87 14.02 Cor 3.88 0.83 0.65 0.49 0.36 1.49 1.68 3.08 2.57 Di 0.46 Hy 7.24 8.60 6.62 7.62 8.31 5.46 6.55 6.27 1.61 6.53 Mt 2.32 2.61 3.07 2.07 2.47 2.32 2.32 1.79 1.14 1.87 11 0.76 0.91 0.86 0.95 0.90 0.72 0.80 0.68 0.32 0.72 Ap 0.34 0.31 0.34 0.29 0.29 0.26 0.36 0.15 0.12 0.24 Cc 0.23 0.23 0.23 0.23 0.68 0.23 0.23 0.47 0.22 0.23 Total 97.35 99.97 99.97 99.97 99.97 99.97 99.97 99.97 99.98 99.97 * Amount is less than that shown in the analysis.

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Wahleach Lake Tonalite, 18 m.y. (Table 2). The 21-m.y. age from sion, gaining access to the epizone partly along the Fraser River the Mount Barr phase was obtained from the granophyre dike just fault zone, and that volcanism accompanied intrusion (Fig. 13). east of Silver Peak. This age probably dates the time of emplace- The Conway granodiorite and tonalite magmas were extruded as ment of the batholith. Other ages, from the interior of the dacitic lava, probably accumulating to a thickness of a few batholith, probably mark the times when the large mass cooled to thousand metres (compare the Oligocene(P) Skagit Volcanics and temperatures at which argon was retained. Hannegan Volcanics). The Conway magma then spread laterally as Chemical analyses are listed in Table 10 and are shown in Figure a thick sheet, perhaps along the unconformity at the base of the 12. The suggested sequence of emplacement of the three phases — lava pile. After an interval, during which differentiation continued Conway, Mount Barr, and quartz monzonite — inferred partly below, the Mount Barr magma was injected as a sheet into the still from structural relations, agrees with the sequence (Fig. 12) from hot (and partly liquid?) Conway phase and, well insulated, cooled basic to acidic. The variation diagram (Fig. 12) shows the usual slowly, allowing settling and accumulation of hornblende in the trends. The variation lines fit neatly to the ends of the variation lower parts. After a relatively longer interval, during which dif-

lines (at about 64 percent Si02) for the Chilliwack tonalite (Fig. ferentiation continued below, the leucocratic quartz monzonite 10), but this fit cannot be taken to show that the Mount Barr stocks and the Wahleach Lake Tonalite were emplaced. batholith is directly related to the Chilliwack batholith, which seems to be some 10 m.y. older. Apparently the courses of early dif- DISCUSSION AND CONCLUSIONS ferentiation were nearly identical. We conjecture that the Mount Barr magmas were the product of Plate Tectonics and Magmatism differentiation in a magma chamber below the present level of ero- The Spuzzum intrusions, of Late Cretaceous age, form a part of the Coast Plutonic Complex; the Yale intrusions, because of their ages (59 to 35 m.y.), northwesterly alignment, and cataclastic nature, may represent the latest intrusions of the same belt. The Chil- liwack composite batholith, however, is clearly part of the Cascade belt of late Cenozoic age. These belts represent enormous volumes of magma, the origin of which presents a fundamental problem. Larson and Pitman (1972) concluded that in Early Cretaceous time, the Farallon plate was being subducted along the west coast of North America. Souther (1972) suggested that the Coast Plutonic Complex is related to such a Late Cretaceous to Eocene subduction zone. A model for the geologic evolution of British Co- lumbia (Monger and others, 1972) shows a subduction zone along the coast from Early Jurassic to middle Cenozoic time, at which time the northern segment of the trench disappeared, and the now-active strike-slip boundary between the Pacific and North American plates came into play. From mid-Cenozoic time on, Cascade magmatism seems to be related to short segments of a trench (Fig. 3) that extends southward from the latitude of the north end of Vancouver Island (Dickinson, 1970; Silver, 1971; Barr, 1972). Yeats and Engels (1971) have pointed out that in northwestern Washington, plutonism may not have occurred between 90 and 48 m.y. B.P., and that Paleocene time was free of volcanism and plutonism. They and Misch (1966) suggested that during this interval, plutonic trends were reoriented from northwesterly to northerly. This interval, which may have been shorter in southern British Columbia (Fig. 14), could be related to a discontinuity in the plate tectonics regime about 40 m.y. B.P., which has been postulated on irregular displacements of the North American plate (Coney, 1971), and on the geometry of the Hawaiian-Emperor seamount chain (Clague and Jarrard, 1973). The north-trending Cascade magmatic belt overlaps for some 210 km the southern part of the Coast Plutonic Complex (Fig. 3). This change in trend and position may signify that the late Cenozoic subduction zone is not merely a surviving segment of the older one but represents a new subduction zone developed to the west of the Cretaceous—early Cenozoic trench about 50 m.y. ago.

Evolution of the Plutons Wahleach Lake tonalite Late Cenozoic vol. rocks Diorite magma of the mesozonal Spuzzum intrusions was in- Quartz monzonite Eocene sed. rocks truded largely by shouldering aside upper Paleozoic bedded rocks. After crystallization of the diorite, an annular zone of tonalite was Mt. Barr phase Custer gneiss emplaced around the possibly subsiding diorite. Figures 14 and 15 allow comparison of the intrusive and volcanic activity. Although Conway phase Chilliwack group and Darrington phyllite one would not expect contemporary volcanic rocks to be preserved Figure 13. Emplacement of Mount Barr batholith, through stages A, B, near the mesozonal Spuzzum intrusions, the Midnight Peak ande- and C to D, which shows inferred present structure. site (Barksdale, 1975), some 80 km to the southeast, may be about

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 PLUTONS VOLCANIC ROCKS SOUTHWEST BRITISH COLUMBIA SOUTHWEST BRITISH COLUMBIA NORTHWEST WASHINGTON M.Y. and NORTHWEST WASHINGTON WASHINGTON and OREGON 0 Garibaldi (K) Baker (L) Quaternary .Glacier Peak (M), Valley Basalt (N) Mt. Rainier (P) High-alumina PI iocene basalt

IO Plateau basalt (J)

Coquihalla group (F) 30 Chilliwack Chilliwack batholith batholith (17) (Perry Creek) (16) 01igocene o o Index batholith (15) Q. E Silver Creek Squire Creek stock (14) _ stock Needle Peak più ton (13) Golden Horn pluton (12) 40 Hell's Gate pluton (11) Chilliwack batholith Coquihalla stock in part (10) Granite Falls stock (9) Eocene o o ui O Mt. Pi 1 chuck stock (8) +-> —' CL

60 Paleocene Ogilvie Peak stock

Scuzzy batholith (4) Upper Cretaceous 80 Midnight Peak andesite (A)

Mount Stuart batholith (2)

Beckler Peak stock (1) 90

Figure 14. Ages of intrusive and volcanic units in southwestern British Columbia and (1966); A — Barksdale (1975); B, I, P — Fiske and others (1963); C, N — Rice (1947); D, J — Washington. Numbers and letters refer to locations in Figure 15. References: 1,2 — Engels and Cockfield (1948), Souther (1972); E — Waters (1961); F — Cairnes (1924); C, H —Misch (1966), Crowder (1971); 3, 5, 17, 18 — this work; 6, 7 — Tabor and others (1968); 8, 9, 14, 15, 19, 20, Staatz and others (1972); K — Mathews (1958); L — Coombs (1939). 21, 23, 24 — Yeats and Engels (1971); 4, 11, 13 — Monger (1970); 10, 12, 16, 22 — Misch

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the same age (Fig. 14). The Yale intrusions probably represent the tension southward (Yeats and Engels, 1971) suggest that the zone last of the magmatism associated with the Spuzzum episode. was an important magmatic conduit. Thirty million years ago it Hopson and others (1965) suggested that magmas that formed may have been marked by a line of active volcanoes. the middle and late Cenozoic epizonal plutons of the northern Cas- The Chilliwack composite batholith was emplaced in at least cades were also extruded to form voluminous andesites and dacites. four stages (Fig. 14): a late Eocene phase in northern Washington Indeed, the magmas of the Tatoosh pluton (Fiske and others, 1963) (Misch, 1966), the Silver Creek stock (35 m.y. ago), the Chilliwack broke through to the surface at several places. Intrusions that were batholith (29 to 26 m.y. ago), and the Mount Barr batholith (21 to probably contemporaneous with the eruption of lavas of the west- 16 m.y. ago). Each of these intrusive masses, except for the Silver ern Cascades of Oregon have been described by Peck and others Creek stock, consists in turn of several successive intrusive phases (1964). The clustering (Fig. 15) of late Cenozoic intrusions along that differ texturally and chemically from one another, suggesting and close to the Fraser River—Straight Creek fault zone and its ex- that most of this differentiation occurred below the present level. Many phases, however, differentiated further during and after emplacement.

ACKNOWLEDGMENTS

J. Harakal and the late W. H. White supervised K-Ar age deter- minations. Peter Misch, H. J. Greenwood, R. L. Armstrong, K. F. Fox, Jr., J. A. Roddick, W. W. Hutchison, G. H. Woodsworth, and colleagues and friends made useful suggestions. Chemical analyses were obtained through the Geological Survey of Canada. S. Schu- man, K. More, A. Bentzen, R. Richards, M. Schau, Leo Fox, and S. Richards assisted in the field or in the laboratory. The work was supported by the Geological Survey of Canada.

REFERENCES CITED

Baadsgaard, H., Folinsbee, R. E., and Lipson, J., 1961, Potassium-argon dates of biotites from Cordilleran granites: Geol. Soc. America Bull., v. 72, p. 689-702. Barksdale, J. D., 1975, Geology of the Methow Valley, Okanogen County, Washington: Washington Dept. Nat. Resources, Div. Geology and Earth Resources, Bull. 68, 72 p. Barr, S. M., 1972, Geology of the northern end of Juan de Fuca Ridge and adjacent continental slopes [Ph.D. thesis]: Vancouver, Canada, Univ. British Columbia, 286 p. Burnham, C. W., 1967, Hydrothermal fluids at the magmatic stage, in Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits: New York, Holt, Rinehart and Winston, p. 34-76. Cairnes, C. E., 1924, Coquihalla area, British Columbia: Canada Geol. Survey Mem. 139, 187 p. 1944, Hope area: Canada Geol. Survey Map 737A, scale 1 in. = 4 mi, 1 sheet. Clague, D. A., and Jarrard, R. D., 1973, Tertiary Pacific plate motion de- duced from the Hawaiian-Emperor chain: Geol. Soc. America Bull., v. 84, p. 1135-1154. Cockfield, W. E., 1948, Geology and mineral deposits of Nicola map-area, British Columbia: Canada Geol. Survey Mem. 249, 164 p. Coney, P. J., 1971, Cordilleran tectonic transitions and motion of the North American plate: Nature, v. 233, p. 462-465. Coombs, H. A., 1939, Mt. Baker, a Cascade volcano: Geol. Soc. America Bull., v. 50, p. 1493-1510. Daly, R. A., 1912, Geology of the North American Cordillera at the 49th parallel: Canada Geol. Survey Mem. 38, 857 p. Dickinson, W. R., 1970, Relations of andesites, granites, and derivative sandstones to arc-trench tectonics: Rev. Geophysics and Space Physics, v. 8, p. 817-860. Douglas, R.J.W., 1970, Geology and economic minerals of Canada: Canada Dept. Energy, Mines and Resources Econ. Geology Rept. 1, « ¿M 838 p. Duffell, S., and McTaggart, K. C., 1952, Ashcroft map-area, British Co- lumbia: Canada Geol. Survey Mem. 262, 122 p. Plutons Volcanic Rocks Engels, J. C., and Crowder, D. F., 1971, Late Cretaceous fission-track and Late Cenozoic potassium-argon ages of the Mount Stuart granodiorite and Beckler Peak stock, north Cascades, Washington: U.S. Geol. Survey Prof. Early Cenozoic frxfoj Paper 750-D, p. D39-D43. Erikson, E. H., 1969, Petrology of the composite Snoqualmie batholith, Cretaceous [/»r,V» central Cascade Mountains, Washington: Geol. Soc. America Bull., v. KI LOMETERS 80, p. 2213-2236. 5.0 100 Fiske, R. S., Hopson, C. A., and Waters, A. C., 1963, Geology of Mount Rainier National Park: U.S. Geol. Survey Prof. Paper 144, 93 p. Figure 15. Igneous units of southern British Columbia and Washington. Holloway, J. R., and Burnham, C. W., 1972, Melting relations of basalt in

equilibrium water pressure less than total pressure: Jour. Petrology, v. Read, P. B., 1960, Geology of the Fraser River valley between Hope and 13, p. 1-29. Emory Creek, British Columbia [M.A.Sc. thesis]: Vancouver, Canada, Hopson, C. A., Crowder, D. F., Tabor, R. W., and Cater, F. W., 1965, As- Univ. British Columbia, 145 p. sociation of andesitic volcanoes in the Cascade Mountains with late Rice, H.M.A., 1947, Geology and mineral deposits of the Princeton map- Tertiary epizonal plutons [abs.]: Geol. Soc. America Spec. Paper 87, area, British Columbia: Canada Geol. Survey Mem. 243, 136 p. p. 80. Richards, T. A., 1971, Plutonic rocks between Hope, B.C., and the 49th Hutchison, W. W., 1970, Metamorphic framework and plutonic styles in parallel [Ph.D. thesis]: Vancouver, Canada, Univ. British Columbia, the Prince Rupert region of the central Coast Mountains, British Co- 178 p. lumbia: Canadian Jour. Earth Sci., v. 7, p. 376-405. Richards, T. A., and White, W. H., 1970, K-Ar ages of plutonic rocks be- James, R. S., and Hamilton, D. L., 1969, Phase relations in the system tween Hope, British Columbia, and the 49th parallel: Canadian Jour. NaAlSi308-KAlSi308-CaAl2Si208-Si02 at 1 kilobar water vapour Earth Sci., v. 7, p. 1203-1207. pressure: Contr. Mineralogy and Petrology, v. 21, p. 111-141. Roddick, J. A., and Hutchison, W. W., 1969, Northwestern part of Hope Kennedy, G. C, 1955, Some aspects of the role of water in rock melts: map-area, British Columbia: Canada Geol. Survey Paper 69-1A, Geol. Soc. America Spec. Paper 62, p. 489-504. p. 29-38. Larson, R. L., and Pitman, W. C., Ill, 1972, World-wide correlation of Shaw, H. R., 1972, Viscosities of magmatic silicate liquids: An empirical Mesozoic magnetic anomalies, and its implications: Geol. Soc. method of prediction: Am. Jour. Sci., v. 272, p. 870-893. America Bull., v. 83, p. 3645-3662. Silver, E. A., 1971, Small plate tectonics in the northeastern Pacific: Geol. Lowes, B. E., 1972, Metamorphic petrology and structural geology of the Soc. America Bull., v. 82, p. 3491-3496. area east of Harrison Lake, British Columbia [Ph.D. thesis]: Seattle, Souther, J. G., 1972, Mesozoic and Tertiary volcanism of the western Univ. Washington, 162 p. Canadian Cordillera, in Irving, E., ed., The ancient oceanic litho- Mathews, W. H., 1958, Geology of the Mount Garibaldi map-area, south- sphere: Canada Dept. Energy, Mines and Resources, Earth Physics Br. western British Columbia, Canada: Geol. Soc. America Bull., v. 69, Pub., v. 42, no. 3, p. 55-58. p. 161-178. Staatz, M. H., Tabor, R. W., Weis, P. L., Robertson, J. F., Van Noy, R. M., Mattinson, J. M., 1972, Ages of zircons from the northern Cascade Moun- and Pattee, E. C., 1972, Geology and mineral resources of the north- tains, Washington: Geol. Soc. America Bull., v. 83, p. 3769-3784. ern part of the North Cascades National Park, Washington: U.S. Geol. McTaggart, K. C., 1970, Tectonic history of the northern Cascade Moun- Survey Bull. 1359, 132 p. tains: Geol. Assoc. Canada Spec. Paper 6, p. 137-148. Tabor, R. W., Engels, J. C., and Staatz, M. H., 1968, Quartz diorite- McTaggart, K. C., and Thompson, R. M., 1967, Geology of part of the quartz monzonite and granite plutons of the Pasayten River Area, northern Cascades in southern British Columbia: Canadian Jour. Washington — Petrology, age and emplacement: U.S. Geol. Survey Earth Sci., v. 4, p. 1199-1228. Prof. Paper 600-C, p. C45-C52. Misch, P., 1966, Tectonic evolution of the northern Cascades of Waters, A. C., 1955, Volcanic rocks and the tectonic cycle, in Poldervaart, Washington State, in Tectonic history and mineral deposits of the A., ed., Crust of the Earth (a symposium): Geol. Soc. America Spec. Western Cordillera: Canadian Inst. Mining and Metallurgy Spec. Vol. Paper 62, p. 703-722. 8, p. 101-148. 1961, Keechelus problem, Cascade Mountains, Washington: North- 1968, Plagioclase compositions and non-anatectic origin of migmatitic west Sci., v. 35, no. 2, p. 39-57. gneisses in northern Cascade Mountains of Washington State: Contr. White, W. H., 1973, Flow structure and form of the Deep Creek stock, Mineralogy and Petrology, v. 17, p. 1-70. southern Seven Devils Mountains, Idaho: Geol. Soc. America Bull., v. Monger, J.W.H., 1966, The stratigraphy and structure of the type-area of 84, p. 199-210. the Chilliwack Group, southwestern British Columbia [PH.D. thesis]: Wright, T. L., 1968, X-ray and optical study of alkali feldspars II; An x-ray Vancouver, Canada, Univ. British Columbia, 173 p. method for determining the composition and structural state from 1970, Hope map-area, west half, British Columbia: Canada Geol. Sur- measurement of 20 values for three reflections: Am. Mineralogist, v. vey Paper 69-47, 75 p. 53, p. 88-104. Monger, J.W.H., Souther, J. G., and Gabrielse, H., 1972, Evolution of the Yeats, R. S., and Engels, J. C., 1971, Potassium-argon ages of plutons in the Canadian Cordillera: A plate tectonic model: Am. Jour. Sci., v. 272, p. Skykomish-Stillaguamish areas, north Cascades, Washington: U.S. 577-602. Geol. Survey Prof. Paper 750-D, p. D34-D38. Peck, D. L., Griggs, A. B., Schlicker, H. G., Wells, F. G., and Dole, H. M., Yoder, H. S., and Tilley, C. E., 1962, Origin of basalt magmas: An experi- 1964, Geology of the central and northern parts of the western Cas- mental study of natural and synthetic rock systems: Jour. Petrology, v. cade Range in Oregon: U.S. Geol. Survey Prof. Paper 449, 56 p. 3, p. 342-532. Pigage, L. C., 1973, Metamorphism southwest of Yale, British Columbia [Master's thesis]: Vancouver, Canada, British Columbia, 95 p. MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 12, 1974 Price, R. A., and Douglas, R.J.W., 1972, Variations in tectonic styles in REVISED MANUSCRIPT RECEIVED OCTOBER 13, 1975 Canada: Geol. Assoc. Canada Spec. Paper 11, 688 p. MANUSCRIPT ACCEPTED DECEMBER 4, 1975

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