Implications of Chemical and Isotopic Composition for Petrogenesis of Chilcotin Group Basalts, British Columbia

Abstract

Thin, flat-lying basalt flows of the Mio-Pliocene Chilcotin Group, erupted from central vents, coalesce to form a 50,000 km2 lava plateau in south-central British Columbia. Olivine-bearing transitional basalt is the predominant rock type; alkali basalt and quartz tholeiite are also present. Sr and Pb isotopic ratios indicate a mantle source for the magmas. Based on Mg values of 45–65, and Ni contents of <250 ppm, Chilcotin basalts are differentiated magmas. The genesis of Chilcotin basalt magma can be modelled in two steps, (1) partial melting of spinel lherzolite to produce picritic basalt, and (2) fractionation of olivine from picritic magma to produce transitional basalt. Residual peridotite left over from the partial melting event is similar in mode and composition to ultramafic nodules found in many alkali basalt flows from western North America. Magma was not trapped in the crust long enough to react with it or to produce silicic differentiates. Individual flows of Chilcotin basalt from different vents are isotopically distinct and cannot be related to each other by any reasonable crystal fractionation model, suggesting that the mantle source for Chilcotin basalt is heterogenous on the scale of the basalt plateau, and that many batches of slightly different primary magma were parental to the basalts that make up the volcanic field.