Browsing by Subject "Metamorphic petrology"

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    Metamorphic petrology of glacial clasts from the byrd glacier drainage:implications for the crustal history of East Antarctica
    (2013-01) Radakovich, Amy Laureen
    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.
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    New evidence of Proterozoic high P-T metamorphism in East Antarctica from thermobarometry and in-situ U-Pb age dating of monazite in metamorphic glacial clasts, central Transantarctic Mountains, Antarctica
    (2014-11) Nissen, Chelsea I.
    The East Antarctic shield (EAS) is a key component in the study of early crustal evolution due to its ancient geologic history and involvement in the amalgamation and break-up of major supercontinents. The EAS has documented affinities with the cratons of Africa, India, and Australia based on limited coastal outcrop, but an ice cap up to 4 km thick prevents direct access to the interior bedrock of the EAS. Additionally, thermomechanical effects of the Ross Orogeny (~500 Ma) obscure the Precambrian history in rarely-exposed crystalline basement. Metamorphic rock clasts from glacial moraines near the central Transantarctic Mountains were studied for petrologic, geochemical, and isotopic characteristics in order to further understand the geologic history of the EAS. These clasts were presumably eroded from the interior of the EAS and may provide unique natural samples of the ice-covered basement. Metamorphic rock clasts selected in this study are semi-pelitic gneisses with high-pressure mineral assemblages, as well as accessory minerals including monazite and zircon. New in-situ SHRIMP U-Pb analysis of monazite yielded Paleoproterozoic to Neoproterozoic ages in six clasts. Among these, two clasts from Lonewolf Nunataks, at the head of Byrd Glacier, gave previously unrecognized ages of ~1900-1700 Ma, with one having a Mesoproterozoic overprint at ~1200 Ma. Lonewolf Nunataks clasts preserve evidence of high-pressure granulite-facies metamorphic conditions associated with Proterozoic crustal convergence and thickening during orogenic activity. Clasts sampled from moraines near the Miller Range yielded Neoproterozoic U-Pb ages; one clast yielded ages of ~660 and ~590 Ma, whereas other clasts gave ages of ~570- 545 Ma. Neoproterozoic samples conducive to thermobarometric analysis record high P-T conditions comparable to previously documented Ross Orogen activity in reactivated Precambrian crystalline rocks of the Nimrod Group. Together, the clast ages coupled with P-T analysis record previously unknown Paleoproterozoic tectonometamorphic events in central East Antarctica, overprinted by younger Mesoproterozoic metamorphism. Clasts from Lonewolf Nunataks may reflect a Paleoproterozoic event within the EAS related to development of the Nuna supercontinent (~1870-1900 Ma), overprinted by a Mesoproterozoic orogenic event, possibly related to the final amalgamation of Rodinia (~1200 Ma). One clast with a metamorphic age of ~660 Ma may record rift-margin activity associated with supercontinent breakup. Neoproterozoic ages of ~590-545 Ma from some of the clasts are demonstrably older than Ross Orogen ages known from Nimrod Group metamorphic basement; these older ages may therefore provide evidence that Ross orogenic activity was initiated earlier than previously thought, and that its metamorphic overprint extends farther inboard of the Transantarctic Mountains.