THE LARGS HIGH-LATITUDE OXYGEN ISOTOPE ANOMALY (NEW-ZEALAND) AND CLIMATIC CONTROLS OF OXYGEN ISOTOPES IN MAGMA

In northern Fiordland the Brook Street terrane of New Zealand consists of two units - the predominantly basaltic Plato and the predominantly andesitic Largs terrane. The Permian Plato terrane has normal to slightly enriched delta-O-18 values, whereas the Largs terrane, which is of similar pre-early Triassic age, has not yielded a single normal delta-O-18SMOW result, with all of 17 total rocks showing less than 3.2 parts-per-thousand, seven less than -4 parts-per-thousand, and two less than -9 parts-per-thousand. These strongly anomalous data confirm an earlier suggested terrestrial character of Largs deposition, and demand the presence of Permo-Triassic geothermal systems running on subAntarctic to Antarctic meteoric water. The skewed data spectrum suggests a relatively immature flow system and likely values for the recharge water are -20 parts-per-thousand delta-O-18 or less. For a climate distribution similar to the present one, including polar ice caps, this would indicate over 70-degrees of southern latitude. Rafts and xenoliths of Largs rocks have been entrained within Mackay Intrusives in the early Triassic. On field evidence the Mackay magmas have also intruded an early Darran Complex, but this complex has been substantially reactivated in the Cretaceous. It has delta-O-18 values near 5.0 parts-per-thousand, which is distinctly low for island arc magmas. Since the complex is isotopically homogeneous, its delta-O-18 is unlikely to be a direct effect of the relatively shallow Largs terrane. More probable is a climate related slight depression of the delta-O-18 of magma sources, in which other high-latitude, low-delta-O-18 sediments and geothermal systems have been involved.