Provenance and metamorphism of conglomerates within the Otago schist, Hawea

Brown, Kirstin

The New Zealand micro-continent has been described as an amalgamation of various tectonostratigraphic terranes which were accreted on to the Gondwana margin. The origin of the accreted terranes is important in the understanding of Phanerozic Pacific tectonics. This study aims to help in the understanding of the provenance of the Torlesse terrane, which has been subject to extensive geological research for decades. A conglomerate horizo11; located on the southeast side of Lake Hawea was examined with the aim of using the conglomerate clasts to provide provenance information on the Torlesse terrane, in particular the Rakaia terrane. Conglomerate clasts are coarsegrained hand specimen size samples of the source rock, which have thought to have travelled a shorter distance than the fine-grained detrital material which make up the Otago Schist and therefore are able to be used to trace proximal sources. The size of the clasts also allowed for much more extensive geochronological, geochemical, petrographical and geothermometry analysis than that of the fine-grained lithologies within the schist and therefore provide a more confident indicator of provenance. An additional goal of the studywas to understand the effect of metamorphism and deformation on the clasts. Based on geochemical and mineralogical observations, the majority of the clasts can be classified as fractionated I-type tonalite. Petrographic observations show that the clasts have been subject to low grade metamorphism, with the presence of recrystallised quartz and microfractured feldspar, pressure shadows and growth of stilpnomelane. Petrographic and geochemical observations indicate that the clasts have been subject to metasomatic additions ofNa20, Si02 and C02 and the removal ofK20, Rb and Ba during progressive metamorphism and deformation. Titanium (Ti) concentration in quartz grains for the three localities was determined by LA-ICP-MS. These data were combined with the revised calibration of Thomas et al. (2010) to estimate crystallization temperatures. Results show that at low metamorphic grades the original igneous Ti-in-quartz temperature was preserved, but with increasing metamorphic grade and textural zone the temperatures re-equilibrate. 111 U-Pb zircon ages for the five selected clasts and a schist sample were determined by LA-ICP-MS. Four igneous clasts yield crystallisation ages, 253±3 Ma, 298±3 Ma, 332±7 Ma and 339±6 Ma. A quartzite sample has detrital zircons that range in age from 440 to 2650 Ma. Detrital zircons within a schist sample showed a prominent Permian to Triassic age peak and correlate well with the ages of the clasts. The crystallisation ages of the clasts and the detrital ages of the schist and quartzite broadly correlate best with crystallisation ages of igneous complexes located within Western Antarctica.

vii, 137 : ill. (some col.), maps ; 30 cm

2010Brown