Geochemical and geochronological constraints on the glenelg river complex, western victoria

During the Cambro-Ordovician the muddy sandstone and mudstone of the Glenelg River Complex underwent low grade (greenschist) metamorphism involving one principal phase of deformation (D₁), which produced open style folds. K-Ar total-rock ages of well-cleaved slate are 550-570 Ma suggesting a minimum Cambrian age of metamorphism. However, around syn-metamorphic plutons the metasediment has reached upper amphibolite grade and shows at least three phases of deformation including isoclinal F₂ folds. In these rocks the onset of partial melting and development of andalusite-biotite-muscovite-feldspar-garnet assemblages records pressures ˜ 500 MPa and temperatures reaching 600-700°C. K-Ar biotite and total-rock ages on schist and gneiss in this sequence fall in the range 480-500 Ma indicating the time of postmetamorphic closure and, therefore, a minimum Late Cambrian or Early Ordovician age for the metamorphism. Nd (depleted mantle) model ages on the metasediment are ˜ 1.5-2 Ga (at 516 Ma, εNd -11;⁸⁷Sr/⁸⁶Sr 0.723) consistent with derivation from the Proterozoic craton to the west. Deformed metagabbro within the metasediment has chemical and isotopic characteristics indicating a mantle origin (εNd +5.1; initial⁸⁷Sr/⁸⁶Sr 0.704) while an outcrop of serpentinized harzburgite is thrust emplaced, has Nd (depleted mantle) model ages of 650-700 Ma and is interpreted as the basal part of a fragment of obducted Late Proterozoic oceanic crust. Data on the syn-metamorphic Itype Wando Granodiorite (which yields a preliminary apatite-whole rock Nd/Sm isochron of 516 Ma) reflect homblende-plagioclase dominated fractionation combined with assimilation of the local metasediment. Rb/Sr and Nd/Sm isotope data (εNd -3.2 to -8.6; initial⁸⁷Sr/⁸⁶Sr 0.706-0.712) fit this model and show a continuous gradation through to the migmatitic Harrow S-type pluton (εNd -8.7; initial⁸⁷Sr/⁸⁶Sr 0.717), which was derived almost purely by melting of the Glenelg River Complex metasediment during peak metamorphism. The final magmatic events in the Glenelg River Complex involved gabbro and the undeformed A-type Dergholm Granite (εNd -2.9; initial⁸⁷Sr/⁸⁶Sr 0.703) which yielded a K-Ar biotite age of 485 Ma. This places a lower age limit on the orogenic event which, in general, correlates well with the Delamerian Orogeny in South Australia.