Stable Coexistence of Grandidierite and Kornerupine During Medium Pressure Granulite Facies Metamorphism

C. J. Carson, M. Hand and P. H. G. M. Dirks
School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3052, Australia
Department of Earth Sciences, University of Utrecht, P.O. Box 80.021, 3508TA, Utrecht, The Netherlands

Abstract: Petrological and mineral chemical data are presented for two new occurrences of co-existing borosilicate minerals in the Larsemann Hills, East Antarctica. The assemblages contain kornerupine and the rare borosilicate, grandidierite (Mg,Fe)A13BSiO9. Two distinct associations occur: (1) At McCarthy Point, 1–10 mm thick tourmaline-kornerupine-grandidierite layers are hosted within quartzofeldspathic gneiss; and (2) Seal Cove, where coexisting kornerupine and grandidierite occur within coarse-grained, metamorphic segregations with Mg-rich cores of cordierite-garnet-spinel-biotite-ilmenite and variably developed plagioclase halos. The segregations are hosted within biotite-bearing, plagio-feldspathic gneiss. Textural relationships from these localities indicate the stability of co-existing kornerupine and grandidierite.

The grandidierite- and kornerupine-bearing segregations from Seal Cove largely postdate structures developed during a crustal thickening event (D2) which was coeval with peak metamorphism. At McCarthy Point, grandidierite, kornerupine and late-tourmaline growth predates, or is synchronous, with F3 fold structures developed during a extensive granulite grade, normal shearing event (D3) which occurred prior to, and synchronous with, near-isothermal decompression. Average pressure calculations on assemblages that coexist with the borosilicates at Seal Cove, indicate the prevailing conditions were 5.2–5.5 kbar at ∼ 750°C for formation of the grandidierite-kornerupine assemblage.

Keywords: grandidierite • kornerupine • decompression • coexisting borosilicates • Larsemann Hills • East Antarctica

Mineralogical Magazine; June 1995 v. 59; no. 395; p. 327-339; DOI: 10.1180/minmag.1995.059.395.16
© 1995, The Mineralogical Society
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