Abstract

At Red Hill in southern Tasmania a large near vertical dike-like intrusion of tholeiitic dolerite, one mile in width and about five miles in length crops out. In this body marked differentiation has given rise to a series of rocks varying continuously from undifferentiated dolerite to granophyre; the latter occurring in the highest parts of the intrusion. Five pyroxenes, separated from rocks representative of the differentiation series, have been chemically analyzed and the optical properties determined. These pyroxenes are a coexisting pigeonite and augite, two ferroaugites and a ferrohedenbergite. lo outline the complete fractionation series a number of other pyroxenes have been determined optically. Orthopyroxene occurs as a primary phase in a zone approximately fifty feet wide in dolerites adjacent to, and including, the chilled contacts of the intrusion, but elsewhere pigeonite is the only Ca-poor pyroxene present. Pigeonite also has crystallized in the dolerites adjacent to the contacts, where it has, in some cases, partially inverted to orthopyroxene. A member of the augite series crystallizes in cotectic equilibrium with the pigeonite and during fractionation both pyroxenes become progressively enriched in Fe, primarily at the expense of Mg. At the limit of the two-pyroxene field pigeonite ceases to form but the Ca-rich pyroxene continues to crystallize, steadily increasing in Fe content with fractionation, ultimately to attain the composition of ferrohedenbergite in the more acid grano-phyres. The trend of crystallization with fractionation closely parallels that from the Skaer-gaard intrusion. Exsolution phenomenon occur in both series of pyroxenes throughout much of the sequence, but is virtually absent in the ferrohedenbergite, probably owing to the relatively low temperature of crystallization.

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