Relict granulites in the Ross orogen of northern Victoria Land (Antarctica), I. Field occurrence, petrography and metamorphic evolution

Abstract

Granulite-facies rocks occur as mappable relics in the Wilson Terrane (northern Victoria Land), the lithotectonic unit of the Cambro—Ordovician Ross orogen closest to the East Antarctic craton. Despite the widespread amphibolite-facies overprinting of Ross age, large-scale low-strain tectonic lozenges preserve a layered unit of felsic and quartz-poor garnet + orthopyroxene ± cordierite metasedimentary granulite, with minor, metre-thick, layers of mafic two-pyroxene granulite and rare lenses of marble. Large bodies of massive enderbite are also present and locally show discordant, intrusive contacts with respect to the layered metasedimentary sequence. Mineral assemblages, reaction textures and geothermobarometric estimates in granulite rocks point to a pre-Ross decompressional evolution from higher-pressure ( P = 7.7 ± 0.7 kbar at T = 820 ± 100°C) to lower-pressure (6.3 ± 0.4 kbar and 830 ± 50°C) granulite-facies conditions. Geological and petrological data suggest that the granulite-facies rocks of the Wilson Terrane form a distinct tectonometamorphic unit very similar to other Neoproterozoic granulite-facies terrains of the East Antarctic craton. In this aspect, the occurrence of granulite-rocks within the Transantarctic Mountains strongly suggests the reactivation of the palaeo-Pacific margin of East Antarctica during the Ross orogeny.