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Comparison of detrital and metamorphic zircon from metapelites in the Rayner Complex, east Antarctica: provenance and age of deposition investigated via U-Pb and Hf-analysis and interpretation of Th/U ratio, CL zonation and morphology
thesisposted on 2022-03-28, 19:48 authored by K. R. Murray
In East Antarctica’s Archaean-Proterozoic Rayner Complex, metasedimentary gneisses at Stillwell Hills in Kemp Land are interfolded with orthogneisses. At Cape Bruce and Forbes Glacier in MacRobertson Land, metasediments form more continuous exposures that are intruded by the Mawson Charnockite (c.980Ma) (Halpin et al 2005; Young & Black 1991). Detrital and metamorphic zircon grains from six metapelitic samples were analysed for U-Pb age, TDMͨ, εHf, Th-U ratio, length, CL zoning and morphology. This detailed analysis helped to determine their age of deposition as sediments, when they were metamorphosed and the provenance of the grains within each sample. Colbeck Gneiss from Cape Bruce (AC34) recorded a 207/206Pb maximum depositional age of 1258±23 Ma. A metapelitic lens within Painted Gneiss in Forbes Glacier (90024) recorded a maximum depositional age of 1251±27 Ma. These dates correlate with Young and Black (1991) whole rock analysis of these two Proterozoic gneisses. To better understand the regional history of Stillwell Hills, four metapelites were dated to determine their relative age compared with host Stillwell orthogneiss. This work indicates that even the oldest inherited zircons in these four metapelite samples are a minimum 340Ma younger than the youngest dated zircon from Archaean Stillwell orthogneiss (Halpin et al 2005). In order of deposition, Pink Gneiss (FS0214) maximum depositional age is 1317±51 Ma; followed by Green layered sequence (GS01) at 1316±33 Ma, Garnet-sillimanite metapelite (SW54) at 1260±47 Ma and Rusty gneiss (FS0223) the youngest at1178±84 Ma. Petrogenesis of the majority of zircon grains in all six samples could be traced to felsicigneous events. Considering Antarctic basement rock is formed from igneous mafic granitoids (Veevers &Saeed 2011), this felsic trend indicates that most detritus originated not from Archaean basement protoliths, but younger, more crustally evolved, sources. All samples, to a greater or lesser extent,contained zircon grains whose U-Pb age, TDMͨ, εHf, Th-U ratio and CL patterns indicated sharedprovenance. Only two known rock types were identified as possible sources of provenance, whichtrended towards felsic igneous petrogenesis. Metamorphism shown by igneous zircon grains indicated regional peak metamorphism at Kemp or Macrobertson Land; after erosion, transportation and deposition from mostly unknown igneous protoliths.