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A petrographic description of a plutonic mass on Gambier Island, Howe Sound, British Columbia Wilson, George Alexander


A granodiorite pluton near the southwest boundary of the Coast Range of British Columbia was examined by field and petrographic methods, with the object of determining the origin of the pluton. Field and petrographic characteristics are presented and descriptions of 42 of 65 thin sections studied are included. There are no field structures which can be explained solely by magmatic processes. Several field characteristics which should be associated with magmatic intrusion are absent. Petrographic examination revealed no features which can be explained only by magmatic processes. On the other hand, the replacement textures found by microscopic examination cannot be attributed to crystallization from a melt. The physical chemical system which produced the present form of the pluton was an open one. Potash was added and soda and lime were removed in the early stages of recognizable replacement. Later silica was added and potash, alumina, soda and lime were removed. The processes which developed the granodiorite pluton are most accurately described as granitization.

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