GEOCHEMISTRY AND GEOCHRONOLOGY OF MESOPROTEROZOIC BASEMENT ROCKS FROM THE EASTERN AMERY ICE SHELF AND SOUTHWESTERN PRYDZ BAY, EAST ANTARCTICA: IMPLICATIONS FOR A LONG-LIVED MAGMATIC ACCRETION IN A CONTINENTAL ARC

The high-grade metamorphic rocks from the eastern Amery Ice Shelf and southwestern Prydz Bay of East Antarctica represent reworked Rayner Complex during the late Neoproterozoic/Cambrian metamorphism. These Mesoproterozoic basement rocks can provide important information for the earlier tectonic evolution of the Rayner orogen. Chemical compositions of the mafic granulites from the Sostrene Island and Munro Kerr Mountains area suggest that their protoliths resemble Nb-enriched arc basalts, whereas those from the McKaskle-Mistichelli Hills, Reinbolt Hills and Manning Nunataks show typical characteristics of island arc basalts. Nd isotopic analyses yielded initial Nd isotope compositions, expressed as epsilon(Nd)(T) values, ranging from +4.1 to -0.4 for the former, and mostly from -3.2 to -4.7 for the latter. The felsic orthogneisses have the characteristics of volcanic arc granites, and one fifth of the samples belong to high Sr/Y types. The felsic orthogneisses yielded epsilon(Nd)(T) values of -2.4 to -7.6 and Nd depleted-mantle model ages of 2.2 to 1.9 Ga, implying an important episode of crustal formation in the Paleoproterozoic. The high Sr/Y orthogneisses have high K2O/Na2O ratios (>1), positive Eu anomalies, significant HREE depletion and negative epsilon(Nd)(T) values, suggesting their derivation from partial melting of garnet-bearing K-rich mafic sources at the lower crust of the continental arc. Geochronological data give protolith ages ranging from 1380 to 1020 Ma for these arc-related rocks, indicating a long-lived magmatic accretion in the Rayner continental arc for similar to 360 Ma. In addition, a younger island arc (<1080 Ma) was probably formed in the south of the Fisher oceanic arc. Coupled with the available data for the adjacent areas, we propose that the tectonic process of the Rayner orogen involved the collision of several island arcs with East Antarctica (the Lambert Terrane or the Ruker craton), followed by the closure of ocean and final collision of the Indian craton with the newly accreted Antarctic margin.