Abstract
Provenance of ancient and Cenozoic East Otago and Canterbury Basin sedimentary rocks, modern Haast River and Wairau River bedload river sediment have been investigated using titanium-in-quartz geothermometry grain size analysis coupled with petrography and detrital zircon geochronology. Titanium-in-quartz geothermometry allows determination of crystallisation temperature of quartz on the basis of measured titanium (Ti) concentrations at known pressure and titanium activity ( ). Calculated using the Ti-in-quartz concentration, temperatures can be precise (often better than ±15˚C) for quartz in igneous and metamorphic rocks. Temperature estimates for hydrothermal veins and detrital grains involve additional uncertainties. Analysis of quartz grains is relatively simple and rapid, with data obtained using LA-ICP-MS. Where loose sediment was sampled, grain size splits of 125 – 250, 250 – 500, and 500 – 1000 μm were analysed. To ascertain the possibility of variation in detrital quartz temperatures between grain sizes in sedimentary rocks. Within sediments inferred to be sourced from metamorphic rocks, higher temperature quartz is observed to occur with the smallest 125 – 250 μm split.
Modern Haast River sediments represent detritus eroded from the Southern Alps through pumpellyite-actinolite greenschist to garnet-oligoclase amphibolite facies metamorphic basement rocks of the Alpine Schist. Detrital quartz temperatures were observed to vary between grain sizes analysed, with higher temperature quartz typically found within the 125 – 250 μm split. Comparatively the catchment of the Wairau River is composed dominantly of Torlesse Supergroup Greywacke and chlorite greenschist facies Marlborough Schist. A dominantly bimodal distribution was observed throughout all grain sizes analysed with no grain size partitioning of temperatures. Ancient sedimentary lithologies of the East Otago sedimentary sequence and Canterbury Basin reflect widespread marine transgression associated with separation and submersion of the New Zealand continent from Gondwana during the late Cretaceous to late Oligocene. Alpine Fault inception occurred during the Miocene and correlates with deposition of marine regression sedimentary sequences. Additional zircon geochronology was applied to the Green Island Sand to confirm Ti-in-quartz results and provenance interpretations.
Ti-in-quartz geothermometry and zircon geochronology provenance interpretations have been found to agree with previous work on ancient sedimentary lithologies and inferred provenance of modern river sediments. Sediments analysed were able to be effectively correlated to basement rocks from around the New Zealand continent. Ti-in-quartz geothermometry has been found to enable provenance determinations from quartz-rich sediments which are geologically reasonable and in keeping with interpretations from studies using other provenance methods.