Petrology and geochronology of granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and implications for the evolution of the Prydz Belt

Abstract

A combined petrological and geochronological study was carried out on mafic granulites and associated felsic gneisses from the McKaskle Hills, eastern Amery Ice Shelf, East Antarctica. Garnet-bearing mafic granulites exhibit reaction textures and exsolution textures that indicate two-stage metamorphic evolution. Thermobarometric estimates from matrix and symplectite assemblages yield peak and retrograde P-T conditions of 9.0-9.5 kbar and 880-950°C and 6.6-7.2 kbar and 700-750°C, respectively. Similar but slightly scattered peak P-T estimates of 7.9-10.1 kbar and 820-980°C are obtained from the core compositions of minerals from felsic para- and orthogneisses. Evidence for the prograde history is provided by muscovite inclusions in garnet from a paragneiss. Sensitive high-resolution ion microprobe U-Pb zircon dating reveals an evolutionary history for the granulites, including a mafic and felsic igneous intrusion at 1174-1019 Ma, sedimentation after 932-916 Ma, and a high-grade metamorphism at 533-529 Ma. In contrast, Sm-Nd mineral-whole-rock dating mainly yields a single age population at ∼500 Ma. This suggests that the McKaskle Hills form part of the Prydz Belt, and that the relatively high peak P-T conditions and a decompression-dominated P-T path for the rocks resulted from a single Cambrian metamorphic cycle, rather than two distinct metamorphic events as formerly inferred for the granulites from Prydz Bay. The age data also indicate that the Precambrian history of the McKaskle Hills is not only distinct from that of the early Neoproterozoic terrane in the northern Prince Charles Mountains, but also different from that of other parts of the Prydz Belt. The existence of multiple basement terranes, together with considerable crustal thickening followed by tectonic uplift and unroofing indicated by the clockwise P-T-t evolution, suggests that the Prydz Belt may represent a collisional orogen that resulted in the assembly of Gondwana during the Cambrian period. © The Author 2007. Published by Oxford University Press. All rights reserved.