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The Sooke gabbro Mitchell, William Sutherland

Abstract

The geology of the East Sooke peninsula comprises a core of olivine gabbro, peripherally uralitized and bounded on the north and east by upper Eocene Metchosin basalts of submarine origin. The gabbro intrudes the basalts which are unconformably overlain by sediments of the Sooke Formation of Miocene-Oligocene age. The Sooke gabbro intrusion is an elliptical body of slightly differentiated olivine gabbro which is composed of calcic plagioclase and clino-pyroxene with minor olivine and orthopyroxene. The gabbro does not exhibit any obvious cryptic or cumulate layering of the type which characterises many other layered basic igneous intrusions. Instead steeply dipping structures such as weak layering, foliation and lineation are believed to be flow structures. Intensity of uralitization of the olivine gabbro increases near the margin of the intrusion and towards fractures which appear to have acted as channelways for a convective flow of hydrous fluids within and around the hot intrusion. Concentrations of copper sulphides, deposited from these fluids, are found in structurally favourable areas. The gabbro intrusion is thought to mark the position of a volcanic neck or feeder, now exposed by erosion of a thick sequence of Eocene submarine basalts which built up from ocean floor in a manner similar to the Hawaiian chain of oceanic islands. An hypothesis of oceanic origin for the basalt and gabbro sequence is complemented by reinterpretation of the geophysical data from south Vancouver Island. The Metchosin basalts and gabbro intrusions are thought to represent an oceanic suite of rocks emplaced by ohduction on the southern tip of Vancouver Island.

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