Metamorphic petrology of glacial clasts from the byrd glacier drainage:implications for the crustal history of East Antarctica

Abstract

The geology of the Precambrian East Antarctic shield remains enigmatic due to extensive ice-cover in continental East Antarctica. Understanding the nature of this large Precambrian shield is essential for understanding its assembly, which has bearing on past supercontinent cycles as well as modern ice sheet growth. Optical petrography and mineral chemical analysis of metamorphic glacial clasts collected from the Lonewolf Nunataks, Antarctica, reveal a complex metamorphic history representative of bedrock in the East Antarctic shield beneath the Byrd Glacier drainage. Three lithologic groups were identified: (a) felsic gneisses, (b) mafic gneisses and amphibolites, and (c) schists. Both felsic gneisses and mafic gneisses and amphibolites include garnet-bearing and garnet-free varieties. Typical mineral assemblages consist of quartz + plagioclase ± microcline + biotite ± muscovite ± garnet ± scapolite in felsic gneisses, and quartz + plagioclase ± microcline + biotite ± muscovite ± garnet ± tschermakite ± hypersthene ± scapolite in mafic gneisses and amphibolites. Petrologic evidence, including the presence of garnet, tschermakitic Ca-amphibole, hypersthene and scapolite in relatively anhydrous mineral assemblages, indicate amphibolite- to granulitefacies metamorphism. Of 16 suitable garnet-bearing samples, eight show compositional zoning in garnet indicative of prograde metamorphism, 12 show evidence of retrograde metamorphism, and two display no discernable compositional zoning. P-T calculations give prograde to peak temperatures ranging from ~500 to ~800 °C, with all samples reaching peak metamorphic conditions of at least 600 °C. Metamorphic pressures are less well constrained due to a lack of pelitic mineralogy. In one pelitic gneiss sample, GASP (garnet-aluminosilicate-silica-plagioclase) barometry indicates pressures of ~8-9 kbar. In five other samples of mafic gneiss and amphibolite, pressures determined from the Al content in calcic amphiboles give pressures of ~10-20 kbar. Clasts from Lonewolf Nunataks exhibit petrographic and P-T similarities with Archean to Proterozoic metamorphic rocks in the Terre Adélie craton in East Antarctica, the Gawler craton in southern Australia, and the Nimrod Group in the Transantarctic Mountains (TAM). These correlations are consistent with continuation of the Proterozoic Mawson Continent into the vast area of East Antarctica underlying the Byrd Glacier drainage. Specifically, it is possible that a geographically widespread metamorphic/magmatic tectonic event produced metamorphism in all of these areas, but that P-T variations between them is the result of differing local tectonic environments. Previous studies indicate that metamorphism documented in the Terre Adélie craton, Gawler craton, and Nimrod Group resulted from the ~1.7 Ga Nimrod-Kimban orogenies. Although geochronologic data are lacking to confirm a correlation, this study indicates that similar high-grade metamorphic rocks extend well into the East Antarctic shield beneath the Byrd Glacier drainage. Combined with recent age-dating of igneous glacial clasts that confirm the presence of heterogeneous Proterozoic basement underlying the Byrd Glacier drainage, this study also provides further evidence consistent with an interpretation of a connection between East Antarctica and Laurentia during the time of Rodinia.