GRAIN-BY-GRAIN COMPARISON OF SEM-CATHODOLUMINESCENCE AND PETROGRAPHIC FEATURES: AN EXAMPLE FROM THE PAPAROA TROUGH, NEW ZEALAND

An integrated approach using grain-by-grain comparison of SEM-cathodoluminecscence (CL) and petrographic characteristics can generate detailed data for quartz, often a relatively uninformative mineral for provenance analysis. SEM-CL shows CL characteristics and features such as homogeneous, patchy, mottled, zoned, dark or light luminescence, microcracks, light or dark linear features and intragrain boundaries. Petrography shows extinction behaviour, polycrystallinity, grain roundness, shape and size. This integrated approach was used to investigate the provenance of fluvial/lacustrine quartz-rich lithic arenites deposited in the Late Cretaceous extensional Paparoa Trough, New Zealand. Results show that nearly all polycrystalline quartz grains display either an indistinct patchy/mottled or dark CL appearance indicating recrystallization during metamorphism. Quartz grains with weak undulose extinction and light homogeneous CL appearance indicate that many grains were only slightly metamorphosed or deformed. These follow a similar trend as grains with dark linear CL features, indicating fracturing and subsequent healing during diagenesis. Together, they show a source that has minor metamorphic overprinting of original diagenetic features. Fine, angular grains with straight extinction, normally indicating a plutonic or volcanic source, follow the same trend as grains with dark CL appearance indicating that, in this case, straight extinction behaviour is from disaggregation of polycrystalline quartz. Grains with straight extinction, light CL appearance, and microcracks typical of plutonic quartz are also present. Analysis of the spatial and stratigraphic distribution of quartz types shows that the quartz in the Paparoa Trough was derived from a relatively low-grade metamorphic source increasing in grade upward, most likely recording the unroofing of the Greenland Group off the Paparoa metamorphic complex to the northeast. On the west side of the basin about 15% of quartz grains are from a plutonic source, probably the Barrytown Granite pluton. This provenance information, along with the influx of detrital micas, provides the first concrete evidence for sinistral strike-slip fault movement predicted by tectonic reconstructions.