Differentiation in Ferrar dolerites, Antarctica

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Abstract
Chemical and modal data from three sills, a dike, and a lava flow of Ferrar Dolerite from near McMurdo Sound, Antarctica, show that marked differentiation took place during crystallisation of the magma. Differentiation was controlled by the crystallisation and gravitational accumulation of orthopyroxene, resulting in the regular variation of magnesia from 22% to less than 1% and of silica from 47% to 67% in samples of rock from different heights in. the sills. The final products of differentiation are magnesia-poor, alkali-rich, and silica-rich pegmatoids and granophyres. Phase layering and progressive change of mineral composition are demonstrated by means of modal diagrams and optical determinations, and the Ferrar Dolerite differentiation trends are compared with those of other circum-Antarctic dolerites.

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