Fractionation modelling of segregations in the Toomba Basalt, north Queensland

Abstract

Segregations that are darker than their host basalt are locally common in the Late Pleistocene Toomba Basalt flow northwest of Charters Towers. At Basalt Corner this flow has been inflated to a thickness of 17 m and segregations are displayed in pipes, sheets, veins and vesicles. Their chemical compositions are distinct from the host hawaiite and least-square mixing solutions provide close models involving crystallisation of olivine, plagioclase, augite and magnetite in the cooling host basalt and gas-pressure extraction of residual melt to form the segregations. Trace-element concentrations are consistent with this mechanism and can, in general, be modelled using mineral partition coefficients from the literature. Segregations may concentrate near the edges of the inflation structures