Provenance of the sedimentary Rakaia sub-terrane, Torlesse Terrane, South Island, New Zealand: The use of igneous clast compositions to define the source

The Permian to Late Triassic Rakaia sub-terrane-part of the Torlesse superterrane of the New Zealand Eastern Province-is an accretionary complex that comprises an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages. The field of provenance analysis has undergone a revolution with the development of single-crystal isotope dating techniques using mainly silt- to sand-sized single mineral grains. Fine sand and mud may be transported over thousands of kilometres with potential problems for provenance studies, reinforcing the need for caution when interpreting provenance evidence from single heavy mineral grains. This study focuses on coarser-grained rocks, notably conglomerates, which involve much shorter transport distances and therefore may be used to trace proximal sources. A detailed rock sampling programme and geochronological, geochemical and Sr-Nd isotope analysis of igneous clasts from four conglomerates (Boundary Creek, Te Moana, McKenzie Pass and Lake Hill) have broadly characterised the igneous source for the Rakaia sub-terrane. SHRIMP U-Pb zircon ages of Rakaia sub-terrane igneous clasts define three distinct periods of magmatic crystallisation. The first period ranges in age from 292 to 243 Ma (Permian to Middle Triassic) with two clusters recognisable: a minor Early Permian one ranging in age from 292 to 277 Ma, and a major Late Permian to Middle Triassic one from 258 to 243 Ma. All these clasts are confined to the Kazanian (Permian) Te Moana, the Dorashamian (Permian) McKenzie Pass and the Carnian (Late Triassic) Lake Hill conglomerates. The clasts, which are subduction-related calc-alkaline to high-K calc-alkaline, and metaluminous to peraluminous, range in lithology from andesite to rhyolite and their plutonic equivalents. Many clasts at Lake Hill have chemical compositions characteristic of partial melts of a source of basaltic composition in equilibrium with amphibole±pyroxene±garnet (adakites), indicative of a thick crust in the clast source area. The second period comprises Carboniferous, calc-alkaline, metaluminous to weakly peraluminous clasts of the Permian Boundary Creek conglomerate, ranging in age from 356 to 325 Ma. The third group consists of two Cambrian clasts, a monzogranite from Te Moana (c. 497±8 Ma) and a dacite from Lake Hill (c. 517 Ma). Geochronology, geochemistry and Sr-Nd isotopes of Rakaia igneous clasts correlate broadly with those of Permian to Triassic plutons and volcanics from the Amundsen and Ross provinces of Marie Byrd Land. Thus, the Antarctic sector of the Panthalassan margin of Gondwana is the probable source for the Rakaia sub-terranes as opposed to the other postulated sources in Eastern Australia.