GRANULITE FACIES METAMORPHISM AND THE INITIATION OF INTRAPLATE EXTENSION IN THE LOWER CRUST OF A CONTINENTAL MAGMATIC ARC, FIORDLAND, NEW ZEALAND

Mid- to lower-crustal plutons of the Western Fiordland Orthogneiss (WFO) have heterogeneously developed magmatic flow and subsolidus fabrics. Reconnaisance mapping in central Fiordland indicates that the subsolidus fabrics are well-developed around pluton margins. Garnet Sm-Nd ages from granulite facies fabrics near the northeastern margin of the Malaspina Pluton constrain the timing for initiation of crustal extension. The Malaspina was emplaced at depths of ca. 40 km and garnet granulite metamorphism occurred at P=11±1 kbar and T=850±50°C. Emplacement began by ca. 120 Ma on the west and likely continued until ca. 116 Ma on the east side of the pluton. Granulite facies extensional LS fabrics in the eastern Malaspina (Crooked Arm) are gently to moderately dipping and record top down-to-the-WSW and down-to-ENE senses of shear consistent with extension recorded outside Fiordland. The Crooked Arm fabrics developed during replacement of igneous orthopyroxene and pargasitic amphibole with garnet, clinopyroxene, and diffuse trondhjemite melts. Granulite-facies metamorphism culminated in additional partial melting which produced crosscutting veins of trondhjemite lined with selvages of coarse peritectic garnet (112.8±2.2 Ma - garnet Sm-Nd). Preliminary work suggests near identical granulite textures in the western Misty Pluton north of the Malaspina, compatible with a similar deformational and metamorphic history. However, intervening parts of the WFO contain little evidence for subsolidus ductile deformation indicating heterogeneous strain. The age of fabrics in the Misty is poorly known, but peritectic garnet from a selvage adjacent to an undeformed cross-cutting trondhjemite vein (Bradshaw Sound) suggests that this pervasive deformation in parts of the Misty had ceased by 109.2±2.0 Ma. The high temperature ductile shearing in both plutons is inferred to indicate initial crustal collapse and predates development of discrete (but still ductile) shear zones (e.g., Doubtful Sound and Resolution Island shear zones) which formed at amphibolite facies conditions prior to initial crustal breakup and opening of the Tasman Sea at ca. 85 Ma. The 112 Ma garnet age for Crooked Arm fabrics provides the oldest known age for intraplate extension in Fiordland.