DID CRETACEOUS ACCRETION OF EASTERN FIORDLAND TO THE GONDWANA MARGIN CAUSE HIGH-P METAMORPHISM AND PARTIAL MELTING?

In the Early Cretaceous, the paleo-Pacific continental arc margin of Gondwana experienced a progressive sequence of voluminous magmatism, compression and high-P (≤ 16 kbar) granulite facies metamorphism, and extensional exhumation. However, the driving forces (i.e. tectonic versus magmatic) for high-P metamorphism remain debated. Recent field and laboratory studies imply that collision of a para-autochthonous block (eastern Fiordland, New Zealand) with Gondwana (western Fiordland) caused rapid tectonic thickening.

The Grebe Shear Zone, a mylonitic zone separating these blocks, has been traced for over 100km. Distinctive magmatic, metamorphic and structural histories characterise differences on either side of the Grebe Shear Zone prior to juxtaposition. U-Pb isotope ages from pre-, syn- and post-kinematic plutonic rocks constrain compressive ductile deformation on this structure to 128-116 Ma. Rotated K-feldspar porphyroclasts combined with mineral lineations suggest an oblique continental-wards directed reverse sense of movement.

As a result of tectonic loading, a hot, mafic intrusive complex (Western Fiordland Orthogneiss) in the lower crust was partially melted to produce voluminous granitic and dioritic magmas with distinctive high Sr, low Y, and elevated alkali and alumina chemical signatures. Several of the larger granitic plutons occur in belts parallel to the shear zone and the arc margin, consistent with structurally controlled emplacement during compression. Subsequent extensional exhumation of the deepest levels of the arc margin had begun by 108 Ma (Scott and Cooper, 2006, Tectonics).