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
University of Minnesota M.S. thesis. January 2013. Major: Geological Sciences. Advisor: Dr. John Goodge. 1 computer file (PDF); x, 197 pages, appendices A-C.
Radakovich, Amy Laureen.
Metamorphic petrology of glacial clasts from the byrd glacier drainage:implications for the crustal history of East Antarctica.
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