Differentiation of the great lake dolerite sheet, Tasmania

Abstract

The Great Lake dolerite sheet, which is up to 2,000 feet thick, was emplaced during the Jurassic in an essentially flat‐lying sequence of Permian and Triassic sedimentary rocks. The intrusion is markedly transgressive in the west, and becomes sill‐like to the east. The dolerite is tholeiitic, and chemical and micrometric analyses show that differentiation is marked. Above the floor of the intrusion a Mg‐rich zone is found, which passes upward into progressively more silicic dolerites. An elongated, lenticular body of granophyre, which crops out adjacent to Great Lake, appears to have been concentrated in a structural high in the roof of the intrusion. The granophyre passes gradationally into dolerite vertically and laterally and was formed by differentiation. The silicic dolerites and the granophyre are enriched in iron, silica and alkalies, and impoverished in CaO and MgO. Differentiation has taken place by fractional crystallization and the movement of phases under gravity. The fractionation trend is similar to that found in other Jurassic dolerite intrusions in Tasmania.