STABLE COEXISTENCE OF GRANDIDIERITE AND KORNERUPINE DURING MEDIUM PRESSURE GRANULITE-FACIES METAMORPHISM

Petrological and mineral chemical data are presented for two new occur rences of co-existing borosilicate minerals in the Larsemann Hills, Ea st Antarctica. The assemblages contain kornerupine and the rare borosi licate, grandidierite (Mg,Fe)Al3BSiO9. Two distinct associations occur : (1) At McCarthy Point, 1-10 mm thick tourmaline-kornerupine-grandidi erite layers are hosted within quartzofeldspathic gneiss; and (2) Seal Cove, where coexisting kornerupine and grandidierite occur within coa rse-grained, metamorphic segregations with Mg-rich cores of cordierite -garnet-spinel-biotite-ilmenite and variably developed plagioclase hal os. The segregations are hosted within biotite-bearing, plagio-feldspa thic gneiss. Textural relationships from these localities indicate the stability of co-existing kornerupine and grandidierite. The grandidie rite- and kornerupine-bearing segregation from Seal Cove largely postd ate structures developed during a crustal thickening event (D2) which was coeval with peak metamorphism. At McCarthy Point, grandidierite, k ornerupine and late-tourmaline growth predates, or is synchronous, wit h F3 fold structures developed during a extensive granulite grade, nor mal shearing event (D3) which occurred prior to, and synchronous with, near-isothermal decompression. Average pressure calculations on assem blages that coexist with the borosilicates at Seal Cove, indicate the prevailing conditions were 5.2-5.5 kbar at approximately 750-degrees-C for formation of the grandidierite-kornerupine assemblage.