THE PETROGRAPHY OF BLUESCHIST ROCKS FROM OUEGOA, NEW CALEDONIA

Between the townships of Koumac and Ouegoa, in the very north of New Caledonia, there are a series of blue schist metamorphosed rocks (here in referred to as the Ouegoa lavas) which were collected as part of a larger study of Cretaceous volcanism in New Caledonia. It is believed that these rocks form part of an in-situ late Cretaceous suite of lavas and sediments that are related to the Noumea Basin in the southern part of the island. Recent work has shown that the Noumea Basin represents a sequence of late Cretaceous continental-arc lavas overlain by within plate volcanic rocks. The Noumea Basin lavas are weakly altered and metamorphosed to prehnite-pumpellyite lower greenschist facies.

The relationship between the Noumea Basin and the Ouegoa lavas has been unclear as the Ouegoa lavas appear highly fractured, metamorphosed and weathered, and have not been studied in the past. However these lavas are important as there are few on-land rock suites preserved in the region, and hence the tectonic evolution of this portion of the southwest Pacific is poorly constrained. Understanding the nature and formation of the Ouegoa lavas could provide valuable information about this region.

Questions that remain to be answered are: the age of the lavas, the timing of metamorphism, and the formation of the lavas (arc, back-arc, continental arc, withinplate etc). Our results support the hypothesis that the lavas are in-situ and were partially subducted during the Eocene. The results of this work show the Ouegoa lavas have been metamorphosed to blueschist facies. Primary minerals including orthopyroxene, clinopyroxene and plagioclase are preserved but are pitted, embayed and unstable. Hornblende may also occur as a remnant primary phase. The preserved primary minerals suggest that the Oeugoa lavas fall into the basaltic or andesitic compositional fields. Metamorphic minerals observed include quartz, biotite, glaucophane, chlorite, pink garnet, prehnite, and pumpellyite. This mineral assemblage suggests the Ouegoa lavas reached a maximum metamorphic facies of blueschist then retrograded back through greenschist and prehnite-pumpelyite facies. Finally we observed alteration/weathering minerals including illite, smectite and calcite.