Geology of the Cocks Glacier area, Antarctica : a study of neo-proterozoic metamorphism, deformation and magmatism during the Ross Orogeny in South Victoria land


Wynyard, Matthew John.


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The Cocks Syenite/Granite Suite (CSGS) occurs over an area of 4km2 north ofthe confluence Cocks and Skelton Glacier confluence in South Victoria Land Antarctica. The CSGS represents the earliest Ross Orogeny plutonism and the oldest intrusion of the Koettlitz Glacier Alkaline Province (KGAP) at 551 ± 4 Ma. New geochemical data show the CSGS comprises a complete fractional crystallisation spectrum from silica-oversaturated syenite to granite. The CSGS is an A-type syenite/granite which shows high Fe/Mg, K +Na, Zr, Nb, Ga/Al, LREE, Y and Zn with low Ca, Eu, Sr, Ba, Cr, V and Ni. The parental syenite magma was metaluminous, anhydrous and relatively hot. Upon emplacement it became hydrated. Evolution to a peraluminous composition was controlled by the crystallisation of alkali feldspar. Sr-Nd isotopic analysis shows anomalous values of 87Sr/86Sri = 0.7201 relative to other A-type granitoids from the KGAP and has a ENd= -3.35. The CSGS is inferred to have been produced from melting of an upper crustal source rock with an inherited negative Eu anomaly at low pressure and high temperature due to mafic underplating during neoProterozoic crustal extension. The Skelton Group represents the basement rocks of the area and comprises: l)Anthill Limestone which consists of limestone, calcareous shale, calc-silicate and quartzite and was deposited in a shallow continental shelf setting. 2) The Cocks Formation which consists of shale, calc-silicate, meta-basite and minor limestone and was deposited by turbidity currents in a deeper marine setting. The Anthill Formation shows at least three folding events, Fl which transposed bedding, F2 produced a tight recumbent phase of folding and F3 a tight to open upright foldingphase. The Cocks Formation shows at least two deformational events, F 1 which transposed bedding and F2 a tight to open upright phase. During the recumbent deformational phase the two formation have been juxtaposed by a major tectonic shear. This Shear has possibly been refolded during the upright folding event. The Skelton Group has been metamorphosed to greenschist facies which has been locally overprinted by hornblendehornfels facies metamorphism during intrusion of the GSGS. Mineralogy and calcite-dolomite geothermometry suggest regional metamorphism of ~450-500°C at 2-3kbar overprinted by contact temperatures to ~550-600°C. One area of the Cocks Formation has contact metamorphism grade of pyroxene-hornfels facies indicating temperatures of~ 700°C. Dolerite dikes which cross-cut the Skelton Group and xenoliths within the CSGS show correlation through geochemical data. This correlation coupled with field relationships shows they are pre-551Ma and post deformation of the Skelton Group and are geochemically similar to the Skelton Dolerite cropping out on the southern confluence of the Cocks and Skelton Glaciers. New geochemical data shows they have high-K calc-alkaline affinities and may represent the dikes and sills emplaced during the initial stages of the extensional event which produced the CSGS. Deformation of the Skelton Group in the Skelton-Koettlitz region followed by extension and alkaline magmatism of the KGAP may be a result of the collision of seamounts and associated

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156 leaves : col. ill., col. maps ; 30 cm.


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POLYGON ((162.063848854016754 -78.677619506797697,161.75403942069795 -78.675689520625539,161.75582920769719 -78.667524071138914,161.770888177259337 -78.598358380476753,161.777037122366607 -78.5984070832762,161.778897448952762 -78.598421792332886,162.073656435276433 -78.600604425062073,162.064066843191824 -78.67591888426044,162.063848854016754 -78.677619506797697))

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Wynyard, Matthew John., “Geology of the Cocks Glacier area, Antarctica : a study of neo-proterozoic metamorphism, deformation and magmatism during the Ross Orogeny in South Victoria land,” Otago Geology Theses, accessed January 22, 2018,