Osumilite-sapphirine-quartz granulites from Enderby Land,Antarctica: P-T conditions of metamorphism,implications for garnet-cordierite equilibria and the evolution of the deep crust

Osumilite-sapphirine-quartz granulites from Enderby Land, Antarctica (Ellis et al. 1980) were metamorphosed at 8–10 kb pressure, 900°-980° C under very low conditions. Retrograde mineral coronas in these rocks record subsequent cooling from the peak of metamorphism at approximately constant pressure. The inferredP-T cooling-uplift path differs markedly from that evident in many other granulite terrains.Present garnet-cordierite geothermometers imply equilibration at temperatures of 500°–600° C, well within the kyanite stability field. These temperatures are inconsistent with the presence of sillimanite and the high temperature stability fields of the actual mineral assemblages. Examination of available garnetcordierite experimental data suggests a possible large increase in the Gt-Cd Fe-MgK _D with increasingX _Mg of the cordierite (and pressure). New experiments designed to test this possibility were inconclusive because of the failure to attain satisfactory equilibrium, even at 1,000° C.Possible reasons for the exposure of these unusual granulites in Enderby Land are considered. Although they formed at much higher temperatures than other granulites exposed on a regional scale, suchP-T conditions are not exceptional for the base of the crust. Instead, the unusual isobaric cooling to low temperatures followed by uplift to the surface which these granulites are inferred to have undergone is considered of importance. The unusual tectonic conditions are reflected in the disctinctive type of mineral reaction coronas found in these rocks. The common occurrence elsewhere of mineral reaction during uplift, and the role of anatexis during uplift in obliterating such high temperature assemblages elsewhere in the world are considered.