A STABLE ISOTOPE AND PETROGENETIC STUDY OF THE ALKALINE PENTICTON GROUP VOLCANIC SUITE: THE WHITE LAKE BASIN, SOUTH-CENTRAL BRITISH COLUMBIA, CANADA

A well-exposed suite of Eocene alkaline volcanic rocks associated with continental extension occurs at the White Lake Basin, south-central BC. Stable isotopes, petrography, IR-spectometry and XRD identified a complex mineral assemblage formed during primary crystallization, post-solidus hydrothermal activity, and contact metamorphism. "Sieve" textures in feldspar phenocrysts suggest rapid magma ascent. Absence of quartz and abundance of sanidine, albite, biotite along with abundant acmite indicate an alkaline origin for the Yellow Lake Formation. Phenocryst assemblages from the more siliceous Kitley Lake Formation include glomeroporphyritic plagioclase, K-feldspar, biotite, augite, hornblende, and rare quartz. Within single crystals, feldspar-series end-members trend from sodic to potassic, indicating that Na-rich primary source magmas became more K-rich during ascent and fractional crystallization. Volcanic rocks were later altered to a zeolite-rich assemblage and cut by calcite veins during detachment faulting very soon after eruption. Biotite and amphibole δD values (­–76‰ to –161‰) and water contents (0.7% to 6.1%) are variable. A negative correlation between δD and wt.% H₂O indicates post-solidus exchange with meteoric-hydrothermal water with an increased degree of alteration observed at the upper parts of the section. Biotite and amphibole δ¹⁸O values (+3.5‰ to +6.0‰) from Yellow Lake and Kitley Lake samples having dD values > –120‰ and wt.% H₂O < 2% were within the range of deep-crustal sourced alkaline magmas, but below values recorded for the Eocene Ladybird suite (δ¹⁸O = +9.0‰ to +10.0‰; Holk and Taylor, 2000). Magmatic quartz and feldspar δ¹⁸O values are not preserved and are the result of meteoric-hydrothermal alteration. A match between the Penticton Group biotite and amphibole δ¹⁸O values and those of the Coryell suite (+3.5‰ to +8.0‰; Holk and Taylor, 2008) and phenocryst crystallation trends both suggest a cogenetic relationship between the lower part of the Penticton Group and the Coryell Intrusive Suite. Hence, these magmas are the product of crustal contamination of a magma formed by decompression melting of the lower continental lithosphere during extension. The significant degree of meteoric-hydrothermal interaction is consistent with syn-extensional eruption.