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AAN-01 Antarctic geodynamic evolution and paleogeography

Granulite-facies rocks, Larsemann Hills, Prydz Bay, East Antarctica: New interpretations based on zircon U-Pb-Hf and whole rock Sm-Nd isotopes

Edward Grew, University of Maine (United States)
Christopher Carson, Australian National University (Australia)
Roland Maas, University of Melbourne (Australia)
Mark Fanning, Australian National University (Australia)
Greg Yaxley, Australian National University (Australia)
 

Gneisses of the Larsemann Hills have been customarily divided into a basement igneous complex (Søstrene Orthogneiss) and overlying metasediments (Brattstrand Paragneiss), with mappable units containing boron and phosphate minerals. To better constrain the depositional age and provenance of the Brattstrand Paragneiss, we obtained SHRIMP U-Pb and LA-ICPMS Hf zircon and whole-rock Sm-Nd data on (1) the Tassie Tarn Metaquartzite (TTM), a unit in the Brattstrand Paragneiss; (2) the Søstrene Orthogneiss and (3) the Blundell Orthogneiss, a composite unit that may also represent basement, but we cannot exclude an intrusive origin into the Brattstrand Paragneiss on field relationships alone. The Søstrene and Blundell Orthogneiss samples yield zircon emplacement ages of 1120 ± 10 Ma and 967 ± 17 Ma, respectively. Detrital igneous zircon cores from the TTM sample are dominantly 1100-1150 Ma, with scattered younger ages to ca. 956 Ma. Low Th/U metamorphic rims on the TTM zircons define a broad discordia array between ca. 530 Ma and ca. 900 Ma. Zircon Hf isotope signatures in the Søstrene Blundell Orthogneiss samples, and in the ca. 1100-1150 Ma detrital zircon population within the TTM sample, are all similar (^HfT_DM 1.4 to 1.6 Ga, ε_Hfi -5 to +5). A second detrital zircon population in the TTM sample with U-Pb ages between 1100-1800 Ma are characterised by significantly older ^HfT_DM model ages (2.0-2.2 Ga). Whole rock Nd model ages (^NdT_DM) for 2 samples of TTM are near 2.0 Ga, with ε_Nd956 near -6, whereas two samples of Søstrene Orthogneiss have somewhat younger ^NdT_DM model ages (ca. 1.7 Ga) with less evolved ε_Nd1120 of ca. -0.5. Two samples of Blundell Orthogneiss have ^NdT_DM model ages of 1.8-2.0 Ga (ε_Nd967 = -3.0 and -5.9). Based on the new zircon data, the maximum constraint on deposition of the TTM, and by inference the Brattstrand Paragneiss sequence, is best represented by the youngest detrital igneous zircon cores at ca. 956 Ma. This maximum deposition age, given analytical uncertainties, does not preclude the possibility that the Brattstrand Paragneiss is older than the Blundell Orthogneiss. Sediments were metamorphosed at ca. 900 Ma and again at ca. 530 Ma, the regionally recognised granulite-facies event in southern Prydz Bay. Similarities between the dominant detrital zircon age population within the TTM and the igneous protolith age in Søstrene Orthogneiss, and similar zircon-Hf isotopic signatures, suggest a major role of Søstrene-type basement as a source of clastic detritus in the TTM. However, whole rock ε_Nd values for TTM are lower than in the Søstrene Orthogneiss, implying additional sedimentary contributions from low-ε_Nd sources. Such sources may be represented in our TTM sample by the zircon subpopulation with older ^HfT_DM model ages. The Blundell Orthogneiss apparently preserves Hf zircon and Nd whole rock isotopic signatures indicating derivation from partial melting or assimilation of both Søstrene Orthogneiss and Brattstrand Paragneiss.