Ti-in-quartz geothermometry and sediment provenance in the Western Southland basins

Abstract

The provenance of sediments in the Western Southland Basins has been investigated using Ti-in-quartz geothermometry in conjunction with cathodoluminescence imaging, petrography and zircon geochronology. Data from the Fiordland basement along with the Te Anau, Waiau, Balleny and Winton basins are reported and used to interpret basin and source correlations, history, and regional tectonic evolution.

Ti-in-quartz analysis yields unimodal peaks for all samples and equilibration temperatures that fall within a relatively restricted range. The data are useful in making empirical comparisons between: basement rocks and sediment, the different basins sampled and sediments within these basins. Differences between samples allows for discrimination between individual samples and basins. Similarities between samples are consistent with previous correlations. The data suggests a shift in Ti-in-quartz temperatures in relation to the stratigraphic sequence. Absolute Ti-in-quartz temperatures of Eastern Fiordland granite and numerous basin samples systematically yield equilibration temperatures below the wet granite solidus. This limits the resolution of the technique.

Detrital zircons from five of the six samples analysed show an almost absolute dominance of Early Cretaceous U-Pb ages. This is consistent with the interpretation of an Eastern Fiordland provenance for these five samples. The Blackmount Formation is strikingly different and appears not derived from Fiordland but from rocks of New Zealand’s Eastern Province.

Subtle features displayed in the Ti-in-quartz geothermometry and zircon geochronology results suggest migration of the sediment sources from the Eocene through to the Miocene. A combination of all information acquired is used to consider basin and regional tectonic evolution.