OTAGO-TYPE, HIGH P/T METAMORPHISM IN THE "ALPINE" SCHIST, NEW ZEALAND

The Otago Schist and Alpine Schist of the Haast Schist group, South Island, New Zealand, have different metamorphic records. Garnet-biotite-albite-chlorite rocks are characteristic in the SE-NW trending central arch of the Otago Schist, where they are attributed to an early episode of moderately high P/T metamorphism in the Jurassic. Effects of the lower P/T Alpine Schist metamorphism in the late Cretaceous locally overprinted the Otago Schist. The Alpine Schist is now exposed in an up-tilted section along the east side of the SW-NE trending Alpine Fault in the Southern Alps, as a result of the active oblique continental plate convergence of the Pacific Plate against the Australian Plate, at the Alpine Fault, beginning at c. 8-5 Ma.

The story is not so simple. Not far from Hokitika, some 180 km to the north of the well-studied Franz Josef-Fox Glacier region, a block of the "Alpine Schist" associated with the Pounamu ultramafic belt has an anomalous, Otago Schist-like geological history.

Results of new P-T pseudosection calculations in the 11-component system MnNCKFMASHTO support previous interpretations that these garnet-biotite-albite-chlorite rocks must have a different, and higher P/T metamorphic history compared to the "typical" Alpine Schist in the Franz Josef – Fox Glacier region. A nearly-complete P-T path has been estimated by using the THERMOCALC results and measured garnet compositional zoning data from 4 greyschist rocks with varying MnO contents. The P-T path goes through 380, 460, 490, and 500 °C at 2.5, 8, 8-8.7, and 6.5 kbar, respectively. The results obtained account for observed changes in garnet compositional zoning, which are attributed to changing mineral assemblages, notably the appearance of oligoclase on entering the peristerite zone, rather than to polymetamorphic garnet growth.

Preliminary SHRIMP U-Pb age determinations for detrital zircons may indicate that these Otago-like schists were derived from a source area containing components younger than expected in the Permian to late Triassic Torlesse (Rakaia) Terrane. If the rocks are not part of the Torlesse Terrane, they may represent Otago Schist translated up the west side of the Alpine Fault and later stranded, perhaps by fault reorganisation associated with partitioning of motion onto the developing Marlborough Fault system.