Abstract

Andalusite has been identified as the aluminosilicate polymorph intergrown with orthopyroxene in fine-grained symplectites replacing cordierite in granulites from the Strangways Range, Arunta block, central Australia. The reaction cordierite = orthopyroxene + andalusite + quartz, which has never been described before, is possible under extremely low water activity (aH2O < 0.1–0.2) conditions at ∼500 °C and ∼4 kbar. Incursion of low a(H2O) fluids during exhumation of the granulites during the Alice Springs (c. 350 Ma) orogeny resulted in formation of orthopyroxene in the andalusite stability field. This interpretation is radically different from those of earlier workers, who suggested that the orthopyroxene-bearing symplectites formed at high temperature, in response to isobaric cooling, or even during a second high-grade event. The granulites, preserved in low strain domains between retrograde amphibolite facies shear zones, have previously been considered essentially unaffected by the lower-grade metamorphic conditions because they remained anhydrous. Our data show that localized low a(H2O) fluid access during exhumation produced the anomalous orthopyroxene-andalusite assemblage.

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