Two-sided accretion and polyphase metamorphism in the Haast Schist belt, New Zealand: Constraints from detrital zircon geochronology.

The Haast Schist of South Island, New Zealand, consisting of the Otago, Alpine, and Marlborough Schist components, is the most deeply buried and strongly metamorphosed part of an accretionary complex above a westward-dipping subduction zone on the eastern margin of Gondwana. Conventional wisdom is that the schist formed in the Jurassic (ca. 170 Ma) by collision between Carboniferous to Triassic graywacke sequences of the exotic eastern Torlesse composite terrane and the western Caples terrane. However, extensive areas of Alpine Schist in the west, rich in metabasite, and in part overlying a metamorphosed ophiolitic sequence, have anomalously young, mid-Cretaceous, ca. 108 Ma, detrital zircon ages. Their deposition and subsequent deformation occurred approximately synchronous with the cessation of westward-directed subduction in the Eastern Province of New Zealand, the onset of regional extension, and deposition of terrestrial and volcanic sediments on the exhumed erosion surface of the eastern schist basement. These Cretaceous schist protoliths are interpreted to form the exotic Pounamu terrane, accreted and interfolded with Otago Schist on the western margin of the Haast Schist. The Alpine Schist metamorphism is dated from zircon overgrowths on detrital grains at ca. 70 Ma (although over an extended period, 98-64 Ma, regionally). This thermal event is unknown further east. The Haast Schist is therefore a polygenetic unit formed by the amalgamation of metamorphic components accreted from both the east and west sides of the SE Zealandia microplate. Existence of the Pounamu terrane requires a major revision to the Cretaceous tectonic history of southern New Zealand. [ABSTRACT FROM AUTHOR]