Felsite–nanogranite inclusions and three Al₂SiO₅ polymorphs in the same garnet in ultrahigh–temperature granulites from Rundvågshetta, Lützow–Holm Complex, East Antarctica

Abstract

Ultrahigh–temperature (UHT) granulites characterized by the parageneses of sapphirine + quartz and orthopyroxene + sillimanite + garnet + quartz are found in Rundvågshetta, Lützow–Holm Complex (LHC), East Antarctica. In this study, we report previously undocumented features of the UHT pelitic granulites, including the presence of felsite–nanogranite inclusions (FNIs), which are interpreted to have formed by the supercooling of melt inclusions in garnet, and the presence of three Al₂SiO₅ polymorphs within the same garnet porphyroblast. Whereas sillimanite is present as inclusions in garnet and in the matrix, kyanite occurs as inclusions in garnet and andalusite appears in FNIs. Porphyroblastic garnet is compositionally zoned with Mg–rich cores surrounded by Fe– and Mn–rich rims, indicating homogenization and later modification of Fe, Mg, and Mn. Ca contents oscillate from core to rim, which may be a remnant of growth zoning and suggests compression before high–temperature decompression. Integration of these new data with those previously documented leads to the inference of a rapid change in pressure–temperature conditions during regional UHT metamorphism in the late Neoproterozoic to early Paleozoic Gondwana–forming collisional orogen. Locally occurring intimate intergrowths of kyanite/sillimanite + sapphirine + garnet ± quartz in garnet may be a breakdown product of Mg–rich staurolite and thus may be a link between high–pressure and UHT metamorphism.