Reconnaissance rubidium—strontium geochronology of New Zealand plutonic and metamorphic rocks

A reconnaissance rubidium‐strontium geochronological study of the plutonic and metamorphic rocks of the South Island and Stewart Island, New Zealand, shows that many of these deep‐seated rocks have mineral ages that fall within the narrow limits of 98‐119 million vears. In this grouping are rocks associated with the Separation Point Granite, the Charleston Gneiss, the Fiordland — southern Stewart Island Complex, and the Otago Schists. The metamorphism of the Otago Schists has heretofore been related to the Rangitata Orogeny on geological grounds; the present work suggests that this orogeny is of mid‐Cretaceous age and was associated with the production of plutonic rocks over a wide area of the South Island. Only two of the samples measured in this study show evidence of Paleozoic orogenic activity. A schist in south Westland has an early Carboniferous age of metamorphism. A pegmatite sample associated with the basic and ultrabasic intrusives in Southland is Permian. No rocks were found with Precambrian mineral ages, not even the Charleston Gneiss which had previously been mapped as Precambrian. However, whole‐rock ages on the south Westland schist suggest that the pre‐metamorphic sedimentary rock may have been deposited as long ago as the late Precambrian. Minerals from a pegmatite in the Alpine Schists give discordant ages, supporting the complex history of these schists already deduced by previous workers.