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Geology and geochronology of the Guichon Creek Batholith, B.C. Northcote, Kenneth Eugene

Abstract

The Guichon Creek batholith is exposed within an elongate area of 400 square miles. Sedimentary rocks of Cache Creek group (Permian) and Nicola group (Karnian of Upper Triassic) are intruded by the batholith. In the few places where outer intrusive contacts are exposed, the older rocks have been metamorphosed to albite-epidote and hornblende hornfels and to epidote-chlorite skarn. Middle and Upper Jurassic sediments, Lower Cretaceous and Tertiary volcanic rocks and sediments unconformably overlie intrusive rocks. Pleistocene glacial and interglacial deposits mantle the batholith leaving less than 3% of the surface of the batholith exposed. The Guichon Creek batholith is a composite, upper mesozonal to epizonal, intrusive pluton consisting of seven major, nearly concentric phases. In shape it is an elongate, semi-concordant dome. The magma was emplaced as a crystal mush in a series of pulses and crystallized during a short period of geologic time. It is suggested that erosion of overlying sedimentary and volcanic rocks, as indicated by absence of Lower Jurassic strata, accompanied emplacement and crystallization of successive phases of the batholith. This may explain the association of mesozonal features of older phases at the batholith margin with epizonal features of younger inner phases. The batholith was emplaced by a combination of sidewall and roof stoping, forceful intrusion and assimilation of older rock. There is, therefore, a variety of contacts between phases including sharp intrusive contacts, intrusive contacts of dyke-like bodies and brecciated contacts. Contacts between two phases, although generally intrusive, may be gradational in some parts of the batholith. Contacts between varieties of a phase are gradational and were not observed in intrusive contact. Effects of assimilation are evident in outer contaminated margins of Hybrid and Highland Valley phases. Textures of these rocks are extremely varied and the rocks range in composition from diorite to quartz monzonite. Inner uncontaminated phases have orderly compositional and textural variations. The outermost uncontaminated rock is granodiorite and is thought to represent closely the composition of the original magma. Compositional differences within the inner phases are the result of differentiation within the magma chamber. The most effective process of differentiation was upward and outward movement of alkalis and silica accompanying diffusion of volatiles to regions of lower temperature and pressure. The differential material collected in cupolas and along the walls of the magma chamber. Pressure buildup at the roof of the magma chamber may have exceeded the confining pressure and resulted in fracturing which allowed emplacement of magma into older crystalline phases and into surrounding country rock. The number of phases may have been largely determined by rate of diffusion of volatiles. Emplacement of differential magma from the margin of the chamber into cooler wall rock produced dyke-like bodies of various textures and compositions. Undifferentiated magma remaining in the magma chamber crystallized more slowly and produced rock of more uniform texture and composition. Twenty-six potassium-argon age determinations for the various phases of the batholith are centered around 198 ± 8 my. It is not possible to differentiate among phases on the basis of potassium-argon apparent ages because variations are within analytical limits of uncertainty of techniques used. No interval of time can be given for period of emplacement. All phases began retaining argon at approximately the same time 198 ± my. ago. The batholith has undergone no significant metamorphism since that time. On the basis of isotopic and geologic considerations, it is probable that the Guichon Creek batholith was emplaced approximately 200 my. ago, after Karnian stage of Upper Triassic but prior to Middle Jurassic.

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