METAPELITIC GRANULITES FROM JETTY PENINSULA, EAST ANTARCTICA - FORMATION DURING A SINGLE EVENT OR BY POLYMETAMORPHISM

Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, ea st Antarctica, contains assemblages involving limanite-biotite-cordier ite-spinel-ilmenite-rutile and garnet-orthopyroxene-cordierite-biotite , as well as quartz and K-feldspar. Peak assemblages involve garnet sillimanite + ilmenite (+/- rutile) and garnet + orthopyroxene. P-T ca lculations suggest formation conditions of approximately 800-degrees-C at 7-7.5 kbar. Cooling from peak conditions is suggested by biotite garnet (+/-sillimanite) overprinting some peak assemblages. A subsequ ent increase in temperature is inferred from the formation of cordieri te + garnet + biotite + ilmenite, garnet + sillimanite + cordierite ilmenite and cordierite + orthopyroxene assemblages during D2. In slig htly zincian bulk compositions, hercynitic spinel + cordierite + silli manite constitutes the peak D2 assemblage. Average pressure calculatio ns indicate peak pressures of 5.9 +/- 0.4 kbar at 700-degrees-C for th e cordierite-bearing D2 assemblages. Available radiometric data sugges t that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 +/- 20 Ma. The following two possibilities exist for the metamorph ic evolution. (1) The formation of the lower pressure cordierite-beari ng assemblages is associated with a separate metamorphic event (M2), u nrelated to the peak assemblage (M1), and the lower pressure assemblag es have no relevance in terms of a single tectonothermal event. (2) Th e cordierite-bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a br oadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P-T paths from granulites of similar age in adjacent areas suggests that the second possibility should be prefe rred. The cooling interval between peak conditions and the development of cordierite-bearing coronas and symplectites suggests affinities wi th isobarically cooled granulites of similar age immediately to the we st, and the low-P/high-T post-peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica .