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UBC Theses and Dissertations

Petrological studies within the iron mask batholith, south central British Columbia Snyder, Lori D.


The alkalic Early Jurassic Iron Mask batholith (204 ± 5 Ma) intrudes Late Triassic Nicola Group volcanic rocks of the Quesnellia terrane in south-central British Columbia. The batholith consists of three intrusive rock units which are, from oldest to youngest, Pothook diorite, Cherty Creek monzodiorite to monzonite and Sugarloaf diorite and one ‘hybrid’ unit (Iron Mask hybrid). Pothook and Cherty Creek rocks are locally gradational, contain clinopyroxene or biotite as the major mafic phase and define a chemical trend in major element space. Sugarloaf rocks intrude all other Iron Mask units, contain abundant hornblende and sparse clinopyroxene and do not follow the linear major-element trends defined by the other units. Primary quartz is rarely found in a few samples from the Cheny Creek and Sugarloaf units. REE profiles from the intrusive units are very similar, showing slight enrichment of LREE’s and no evidence of plagioclase fractionation. These intrusive units are part of a complex magmatic system and appear not to be related through simple fractionation processes. The Iron Mask hybrid unit is highly variable in texture and composition; it is divided into three types. Type I and II are characterized by abundant (partially digested) xenoliths, many of which are derived from Nicola Group rocks. Type III is xenolith-poor and exhibits extreme textual variation on an outcrop scale. The Iron Mask hybrid is postulated to represent the effects of progressive assimilation of Nicola Group rocks by Pothook magma. This postulate is supported by REE data. One consequence of selective assimilation processes is the potential for increasing the volatile content of the melt. It is shown that thermal dehydration and partial assimilation of Nicola Group rocks by Pothook magma is a valid method for promoting early volatile saturation in the melt. Xenoliths and blocks of serpentinized picritic basalt are found within the Iron Mask batholith. These exposures are correlated with relatively well-preserved picritic basalt from three localities outside of the Iron Mask batholith on the basis of texture, mineralogy and chemistry. The Kamloops Lake picritic basalts represent an episode of ultramafic volcanism during the latter stages of Nicola volcanism. The rocks are olivine ± clinopyroxene porphyntic. Olivine compositions range from Fo[sub 89.5] to Fo[sub 92.9]. Clinopyroxene MG#’s [100•Mg/(Mg + Fe²⁺] range from 87.1 to 99.7 with calculated Fe203 contents of 1.44 to 6.66 wt% and Cr-spinel is also highly oxidized (100•Fe³⁺/(Fe³⁺ + Al + Cr) from 13.7 to 82.9). Mineral compositions, as well as incompatible element patterns for bulk rocks indicate crystallization from an island-arc primaxy magma under oxidizing conditions. REE profiles indicated olivine control on the pattern and abundances. Calculation of T-1nfO₂ paths, primary melt compositions and olivine crystallization suggest that the Kamloops Lake rocks contain a significant cumulate component. The results support crystallization under high oxygen fugacity from a magma with 14.9 to 20.5 wt% MgO.

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