NEW CONSTRAINTS ON THE TIMING OF HIGH PRESSURE GRANULITE FACIES METAMORPHISM, PARTIAL MELTING, AND THE DURATION OF GARNET GROWTH, FIORDLAND, NEW ZEALAND

Large euhedral garnet porphyroblasts in Pembroke granulite grew during partial melting of intermediate to mafic gneiss. These ≥1 cm garnet crystals lack significant zoning of major and most trace elements (including Sm & Nd). Crystal size and closure temperature estimates indicate that major and trace element homogeneity cannot be readily attributed to volume diffusion. Garnet - rock isochron ages for partly melted gneiss and apparently unmelted gneiss constrain the timing and duration of metamorphism and melting. Garnet core and rim fractions yield progressive growth ages, indicate that growth lasted ca. 1.0 m.y. and 0.5 m.y., and result in growth rates of 9 and 12 mm/m.y. for the two samples. Ages calculated from all garnet fractions are 122.6±2.0 Ma [5 points] and 126.0±2.1 Ma [6 points], respectively. Melt was produced by amphibole breakdown at 730-830 °C and 14.8-16.7 kbar. Garnet ages indicate that peak metamorphism and partial melting, which produced distinctive high Sr/Y [HiSY] melts, occurred over <1 m.y. Garnet grew during emplacement of HiSY Western Fiordland Orthogneiss [WFO] magmas and likely records crustal heating by these intrusions at high pressure. The timing of WFO emplacement and production of low volumes of magma in granulite suggests multiple pulses of HiSY magma production during construction of the Median batholith along the margin of Gondwanaland. A single garnet rim fraction and bulk fine grained garnet adjacent to leucocratic veins yield two point isochron ages of ca. 94 and 109 Ma, respectively. These two ages are compatible with slow cooling and isotopic re-equilibration or late garnet growth. The youngest garnet indicates that the lower crust remained hot for 20-30 m.y. (compatible with published ages), prior to significant cooling and exhumation.