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Geology of the Strachan Creek area, British Columbia Lee, Randolph

Abstract

The Strachan Creek area is on the east shore of Howe Sound about three miles north of the town of Horseshoe Bay, B.C. The rocks of the area consist of migmatite of the Bowen Island group, volcanic rocks of the Gambier group, plutonic rocks of the Coast Intrusions, and late basic and acidic dykes. These rocks are described and their relationships discussed, A striking feature of the Strachan Creek area is the banding in the diorite, one of the units of the Coast Intrusions. Each complete band is a couplet composed of one light- and one dark-coloured layer, one layer grading into the other. The light-coloured layer is composed mostly of plagioclase, whereas the dark-coloured layer is composed mostly of hornblende and magnetite. Generally, the ratio of hornblende (plus magnetite) to plagioclase decreases downward from a sharp contact, the couplets thus resembling inverted "graded-bedding". The author tentatively concludes that the banding in the diorite originated by a process of differentiation and crystal rising within a cooling diorite magma.

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