Provenance study of the Torlesse Terranes and implications for the origin of the continental crust of eastern New Zealand.
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The Torlesse terranes - part of the New Zealand Eastern Province - are accretionary complexes that comprise an enormous volume of quartzofeldspathic sandstones and mudstones with subsidiary conglomerates plus minor oceanic assemblages. Two terranes are recognised in the South Island, the Permian to Late Triassic Rakaia terrane and the Late Jurassic to Early Cretaceous Pahau terrane. Recent studies in detrital petrology and geochemistry have been important in establishing the broad type of source for these two terranes. All studies point to a continental arc/cratonic provenance and various source areas have been proposed. This thesis provides the best evidence yet that the Pahau terrane is locally derived and that an Antarctic source for the Rakaia sediments must be (re)considered. A detailed sampling program and geochronological, geochemical and Sr-Nd isotope analyses of igneous clasts from seven Torlesse terrane conglomerates, in conjunction with SHRIMP U-Pb detrital zircon ages from three Torlesse sandstones, have helped to broadly characterise the igneous protosources for the Pahau and Rakaia terranes. The conglomerate locations were chosen to represent the full stratigraphic range of both terranes, and the geographical distribution of the conglomerates mimics an approximate inboard to outboard transect of the two terranes with respect to the Panthalassan margin of Gondwana. Igneous clasts from the Aptian (Mount Saul and Ethelton) and Albian (Kekerengu) Pahau terrane conglomerates are predominantly volcanic and hypersolvus, calc-alkaline, metaluminous to weakly peraluminous and display a general geochemical concordance that suggests a similar petrogenesis. SHRIMP U-Pb zircon ages of these clasts range from 128-123 Ma and from 147-135 Ma. These clasts are indistinguishable in age (except for the younger group), chemical composition, and petrogenesis from the felsic members of the calc-alkaline I-type granitoids of the Darran Suite, whereas alkaline rhyolitic clasts correlate best with the Electric Granite. The age range of all clasts overlaps with detrital zircon ages of the conglomerate matrix from Ethelton, indicating that Late Jurassic to Early Cretaceous magmatism was penecontemporaneous with the sedimentation of the Pahau terrane. The youngest detrital zircon from the Ethelton conglomerate matrix gives an age of 112±2 Ma that constrains the minimum age of magmatism in the source region. An Early Jurassic calc-alkaline, weakly peraluminous rhyolite clast from Kekerengu (188±3 Ma) correlates with the calc-alkaline, weakly peraluminous to peraluminous Bounty Island Granite. SHRIMP U-Pb zircon ages of Rakaia terrane igneous clasts define three distinct groups. The first group, Permian to Middle Triassic, ranges in age from 292-243 Ma with two subgroups recognisable, a minor one ranging in age from 292-277 Ma and a major one from 258-243 Ma. All these clasts are confined to the Kazanian Te Moana, the Dorashamian McKenzie Pass, and the Carnian Lake Hill conglomerates. The calc-alkaline to high-K calcalkaline, metaluminous to peraluminous clasts range in composition from andesites to rhyolites and their intrusive equivalents. Adakitic, mylonitic and gneissic clasts are especially common at Lake Hill. Carboniferous, calc-alkaline, metaluminous to weakly peraluminous clasts are confined to the (?)Permian Boundary Creek conglomerate, ranging in age from 356-325 Ma, constituting the second group. Pooled individual zircon ages from igneous' clasts from the Boundary Creek conglomerate point towards a possible presence of Carboniferous sediments within the Haast Schist. The third group consists of two Cambrian clasts, a monzogranite from Te Moana (497±8 Ma) and a dacite from Lake Hill (c. 517 Ma). These two clasts indicate that Cambrian plutons and volcanics were a protosource that provided detritus to the Rakaia depocentres. Cambrian magmatism was confined to the New Zealand Western Province and its Australian and Antarctic correlatives as well as the Transantarctic Mountains and their Australian correlatives. The presence of the Cambrian clasts indicates an autochthonous setting of the Rakaia depocentres with respect to the Gondwana margin as early as the Kazanian. Detrital zircon age distributions from the Anisian Kurow Hill and Balmacaan Stream Rakaia sandstones identify a Permian to Triassic arc source as the main contributor of detritus to the Rakaia sedimentary basin. Geochronology, geochemistry and Sr-Nd isotopes of Rakaia igneous clasts correlate broadly with those of Permian to Triassic plutons and volcanics from the Antarctic sector of the Panthalassan margin of Gondwana. Sandstone clasts from two Rakaia and two Pahau conglomerates were collected to investigate the recycling of the older Rakaia rocks. Petrography and geochemistry of Pahau terrane clasts indicate that at the time of the Pahau sedimentation Permian to early Late Triassic Rakaia rocks were exposed and recycled into the Pahau basin. Recycling of the Rakaia sediments into the Pahau terrane is also supported by the detrital zircon age data from this and other studies. Furthermore, the similarities of petrographic and geochemical data between sandstone clasts from the Rakaia terrane and Rakaia sandstones suggest that clasts were derived by autocannibalistic reworking of older, consolidated, Rakaia sediments. Geochronology, geochemistry and Sr-Nd isotopes of igneous clasts from the Pahau terrane identify the Median Tectonic Zone (Darran Suite and Electric Granite) as a detritus contributor to the Pahau depositional basin. Based on sandstone and sandstone clast geochronology, geochemistry and Sr-Nd isotopes, the recycling of the older inboard Rakaia and Caples terranes into the Pahau basin is demonstrated. A multi- source model is proposed in which the uplifted Rakaia and Caples terranes as well as an active volcanic arc contributed detritus to the Pahau sedimentary basin.