GEOCHRONOLOGY AND MID-TO LOWER CRUSTAL PARTIAL MELTING IN A CRETACEOUS MAGMATIC ARC, FIORDLAND, NEW ZEALAND

Exposures of mid-to lower-crust in Fiordland contain evidence of high T and P processes occurring beneath magmatic arcs. We present new U-Pb zircon ages that constrain emplacement of the Malaspina pluton, Western Fiordland Orthogneiss (WFO) to 116-114 Ma and new Sm-Nd garnet ages of 115-110 Ma for partial melting of the Malaspina near the Doubtful Sound Shear Zone (DSSZ), central Fiordland. U-Pb igneous zircon ages, from west to east across Doubtful Sound (DS), are 114.2 ± 2.2, 115.4 ± 2.2, and 116.1 ± 2.4 Ma. This compares to a metamorphic zircon age of 114.8 ± 2.4 Ma for the 119.8 ± 2.9 Ma Misty pluton north of DS in Bradshaw Sound. We infer zircon rim growth due to emplacement of the adjacent Malaspina pluton and/or subsequent metamorphism affecting both plutons. From west to east across DS, Sm-Nd garnet ages are 113.6 ± 2.5, 115.6 ± 2.6, 111.6 ± 3.1, 110.6 ± 1.9, 113.3 ± 2.6, and 111.9 ± 3.0 Ma. Hence partial melting and garnet granulite-facies metamorphism lasted ≥5 m.y. in DS and occurred within 5.5 m.y. of pluton emplacement. Unmelted garnet-pyroxene gneiss near the DSSZ yields a younger Sm-Nd garnet age of 105.6 ± 3.6 Ma implying continued metamorphism or slow cooling. These ages compare to Sm-Nd garnet ages for garnet granulite of 126-109 Ma in northern Fiordland (Pembroke) and ca. 110 Ma in southern Fiordland (Resolution Island). Mineral assemblages in DS migmatite include Hbl + Cpx + Grt + Qtz + Pl ± Bt + Rt + Ilm ± relict OPx ± relict Czo. Euhedral, peritectic garnet occurring in leucocratic veins and reaction zones, has little to no zoning in composition and ranges from Alm₅₃Prp₂₇Sps₃Grs₁₇ in the west to Alm₄₄Prp₃₆Sps_1.5Grs_18.5 in the east. Ilmenite with exsolved rutile and oriented exsolution needles of rutile in garnet indicate re-equilibration after high T and P. Phase diagram sections (THERIAKDOMINO) for unmelted WFO and thermobarometry using garnet, clinopyroxene, and plagioclase compositions for melted WFO, indicate 700-750°C and 9.5-15 kbar for metamorphism on DS. Similar methods for Pembroke Granulite of >750°C @ 12-16 kbar indicates diachronous widespread high P metamorphism and partial melting for this ~4,375 km² section of mid-to lower-crust with these processes occurring early in the north and later in the south (126-105) Ma.