PROVENANCE HISTORY OF A CARBONIFEROUS GONDWANA MARGIN FORE-ARC BASIN, NEW-ENGLAND FOLD BELT, EASTERN AUSTRALIA - MODAL AND GEOCHEMICAL CONSTRAINTS

The Visean-?Stephanian Myall Trough, eastern Australia, formed part of a forearc basin to the east of a magmatic are situated along the palaeo-Pacific margin of Gondwana. Petrographic, modal, whole-rock geochemistry and mineral phase chemistry of sandstones, siltstones, conglomerates and associated igneous rocks provide a consistent and coherent data set which indicate sediment derivation from a magmatic are of predominantly silicic volcanic composition. The low-quartz modes of the sandstones, the largely volcanic origin of lithic fragments and the overall high SiO2 content of the siliciclastic sediments and igneous rocks require the are source to be largely dacitic to rhyolitic in composition. The predominance of vitric type-a (highly siliceous volcanic rocks) lithic fragments suggests a source composed largely of silicic pyroclastics. The presence of potassium feldspar, as well as the presence of biotite as the principal detrital ferromagnesian phase in the sandstones, suggest a source of medium- to high-K tholeiite composition. The high trace element abundances of relatively immobile elements such as Zr, La, Ce, V, Nb, Y and Th and corresponding high Th/U, Zr/Y, La/Y La/Sm ratios and low Zr/Th, Zr/Nb, La/Th and Ti/Zr ratios for both the igneous rocks and derived siliciclastic sediments in the Myall Trough also require a high-K and/or strongly medium-K are source. A lack of temporal or spatial trends in the data indicate that the magmatic are remained of uniform composition throughout the 25 Ma interval of strata preserved within the trough. The provenance history of the Myall Trough requires accumulation along an Andean-type continental margin, rather than an intra-oceanic are outboard of the Gondwana margin.