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An interpretation of lithogeochemical data of the Guichon Creek batholith

University of British Columbia

The Guichon Creek batholith magma was emplaced along pre-existing north-trending structures, still largely molten. The emplacement of about 2000 km³ molten rock material led to a surging inwards of material at the centre of the batholith as evidenced by the saucer-shape of the batholith. The contact of hot rock material with cooler rocks led to crystallization of magma from the margins inwards forming a crystalline cover (Border phase) under which magma was still molten. Presence of the crystalline cover reduced subsequent crystallization rate, as evidenced by increase in grain size towards younger phases. The inwards surging of magma towards the batholith centre created two dome-like structures with that of higher elevation to the north. Due to elevation differences crystallization proceeded fairly rapidly at the northern end of the batholith. Magma saturation occurred after crystallization of Highland Valley phase (about 72 percent crystallization). An evolving volatile phase collected beneath the northern dome structure as evidenced by prevalence of biotite over hornblende in Guichon variety prevalent in the northern part of the batholith compared to Chataway variety in the south. Internal pressure soon exceeded confining pressure resulting in release of volatile fluids through fracturing of cover rocks. Release of volatile fluids created lower internal pressures resulting in increased crystallization rate at the northern end with the volatile centre migrating southwards towards the second dome structure where a second period of volatile release took place. ally zoned batholith ranging from mafic units at the margins to acidic units at the core, took place through crystal fractionation characterized by the mineral assemblage plagioclase- biotite-hornblende. Crystal fractionation also played an important role in the distribution of the alkali and alkali earth metals Rb, Ba and Sr, in older units of the batholith. Bulk distribution coefficients (D) show that Ba and Sr were preferentially partitioned into early formed solids since their D values exceeded one, whereas Rb with D values less than one led to concentration in residual melts. However, an increase in K/Rb ratio in younger phases indicates that in general, Rb decreased with progressive crystallization. This K/Rb ratio trend is attributed to preferential partitioning of Rb towards volatile fluids, in contrast with potassium. interesting distribution patterns. D values for these elements are more than one in older phase and fall below one in younger phases. Crystal field theory predicts that these elements would be partitioned to early formed solids during crystallization of silicate magmas. D values of these elements in older phases suggest that actual trace element distribution is in agreement with theoretical predictions. Low D values (less than one) in younger phase conditions indicates that in the presence of a volatile phase these elements are partitioned preferentially towards these fluids, than coexisting solids. Linear correlation coefficients and scatter diagrams for copper and chlorine in all phases of the batholith show that these elements are partitioned preferentially into a volatile phase. Distribution patterns of trace elements can be used to assess which of the elements are preferentially transferred towards an evolving volatile phase which cool to form mineralizing solutions. The timing at which volatile fluids separate from the crystallizing silicate magma can also be estimated from trace element distribution patterns, which together with the knowledge of the affected elements form the basis for ore potential evaluation of granitoid plutons.

Text

2010-05-16

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http://hdl.handle.net/2429/24785

Geological Sciences

University of British Columbia

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