Abstract

The mapped area, about 170 km east of Vancouver, British Columbia, lies at the intersection of the northwest-trending Mesozoic—early Cenozoic Coast Plutonic Complex and the north-trending late Cenozoic Cascade belt. The Late Cretaceous mesozonal Spuzzum intrusions (90 to 80 m.y. old) of the Coast Crystalline Complex are made up of a central diorite complex and a marginal tonalite. Modal variation in the diorite, which is pyroxenic in the central parts and hornblendic in the marginal, was controlled by PH2O in the magma. The Yale intrusions (59 to 35 m.y. old), of tonalite, granodiorite, and quartz monzonite, are stocks and sills that may represent the latest intrusions of the Coast Plutonic Complex. The Chilliwack composite batholith (40 to 16 m.y. old) is represented by the Chilliwack batholith, the Mount Barr batholith, and the Silver Creek stock; these epizonal batholiths consist largely of tonalite, granodiorite, and quartz monzonite. Variation in the Chilliwack composite batholith is due mainly to differentiation at depth, followed by minor evolution both as the various phases rose and also after they were emplaced. The Fraser River—Straight Creek fault zone may have controlled the emplacement of many of the late Cenozoic plutons. During the past 40 m.y., intrusion and volcanism may have been nearly continuous in southwestern British Columbia and Washington.

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