UBC Theses and Dissertations

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UBC Theses and Dissertations

Petrogenetic study of the Guichon Creek Batholith, B.C. Westerman, Christopher John


The Guichon Creek batholith is exposed approximately 200 miles N.E. of Vancouver, B.C. and is a zoned 'granitic' pluton of Lower Jurassic age (198±8my). The batholith consists of seven major intrusive phases. The predominant rock typo is granodiorite with lessor amounts of quartz diorite and quartz monzonite. Study of rock and mineral compositions has revealed a gradual variation in the relative proportions of mineral phases as crystallisation of the batholith proceeded. The chemical compositions, however, of the individual mineral phases show very little change. The relative roles of assimilation and magma convection in producing compositional variations in the early phases of the batholith are discussed. Alkali feldspars are microperthitic with bulk compositions in the range 71-86 Wt% Or and the compositions of the porthitic components approach pure end members. The potassic phases of the perthites have structural states equivalent to that of orthoclase. The plagioclase feldspars are oligoclases with low to intermediate structural states. Plagioclase from the early phases of the batholith (Hybrid and Highland Valley phases) show normal zoning whereas those from the later phases (Bethlehem and Bethsaida) show oscillatory zoning. Biotites from the major phases of the batholith have progressively lower Fe/(Fe+Mg) ratios with progressively increasing silica content of the host rocks. Plagioclase crystallized early in the older phases of the batholith whereas quartz was the early mineral to crystallize from the younger major phases. Evidence suggests that magma convectod during crystallisation of the older phases of the batholith but that the younger phases crystallised from a stationary magma. Comparisons with experimental systems suggest that the early magma crystallised under conditions of relatively low total pressures in the order of 1 or 2 kb but that the later phases may have been subjected to total pressures in the order of 4 or 5 kb during crystallisation. This increase in total pressures was most probably due to increasing volatile pressures during crystallisation

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