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 approximately 500 MPa and temperatures reaching 600-700-degrees-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 approximately 1.52 Ga (at 516 Ma, epsilonNd -11; Sr-87/Sr-86 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 (epsilonNd +5.1; initial Sr-87/Sr-86 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 I-type Wando Granodiorite (which yields a preliminary apatite-whole rock Nd/Sm isochron of 516 Ma) reflect hornblende-plagioclase dominated fractionation combined with assimilation of the local metasediment. Rb/Sr and Nd/Sm isotope data (epsilonNd -3.2 to 8.6; initial Sr-87/Sr-86 0.706-0.712) fit this model and show a continuous gradation through to the migmatitic Harrow S-type pluton (epsilonNd -8.7; initial Sr-87/Sr-86 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 (epsilonNd 2.9; initial Sr-87/Sr-86 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.