Differentiation of the Tasmanian Dolerites: Red Hill Dolerite-Granophyre Association

Abstract

The Tasmanian dolerites were intruded during the Jurassic and form thick sheets, large transgressive bodies, and wide vertical dikes in flat-lying Permian and Triassic sedimentary rocks. The magma was tholeiitic. The Red Hill dike is up to 1 mile wide and extends vertically upward for more than 1000 feet from the roof of an extensive sheet about 1300 feet thick. Sedimentary rocks roofed the dike. Within the highest structural parts of the dike a large volume of silicic granophyre is associated with quartz dolerite. Chemical, petrographic, mineralogical, and field studies show a complete gradation between dolerite and granophyre and provide evidence that the rock series has been formed by differentiation. Magnesian pyroxene and calcic plagioclase crystallized early, and both changed progressively in composition with fractionation; the residual magma was continually enriched in less refractory components. Strong enrichment in Fe relative to Mg and moderate absolute Fe enrichment took place during the main stages of fractionation, followed by enrichment in silica and alkalies (particularly potassium) in the later stages. Differentiation resulted from fractional crystallization and relative movement of phases owing to gravity. The volume of granophyre is larger than expected; some residuum from the associated sheet probably migrated into the Red Hill dike. Differentiation is similar to that found by Edwards (1942), but it is more marked. The Red Hill study reconciles the two trends that Edwards found in sheetlike and dike-like intrusions and shows them to be part of the same sequence.