An archean tholeiitic layered sill from Mt Kilkenny, Western Australia

Abstract

A small, metamorphosed, layered gabbroic sill at Mt Kilkenny near Leonora in the Eastern Goldfields of Western Australia intrudes a greenstone sequence and attains a thickness of 600 m. The simple layered sequence with only minor phase and rhythmic layering consists of gabbroic rocks with well developed cumulate textures and igneous lamination. Early crystallization of plagioclase and a paucity of orthopyroxene have led to a layered sequence in which Fo-olivine—Ca-plagioclase cumulates are overlain by plagioclase-augite cumulates, a sequence significantly different from most layered intrusions previously described from the Eastern Gold-fields. Differentiation by crystal settling resulted in liquid enrichment in SiO2, Fe, TiO2, P2O3, and Na2O and produced a small amount of granophyre as an end-stage differentiate. Agreement between the estimated bulk composition and a chilled margin dolerite, and the simple layered sequnce with no apparent reversals, suggest fractional crystallization from a single pulse of basaltic magma. The parent magma is considered to be a high-alumina tholeiite similar to abyssal tholeiites, which was intruded and fractionated at a shallow level. Show more