Geochronology and pressure-temperature conditions of mid-to-lower crustal processes in a Cretaceous magmatic arc, Fiordland, New Zealand

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Exposures of mid-to lower-crust in Fiordland contain evidence of high temperature and pressure processes occurring beneath magmatic arcs. This thesis presents 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 contact metamorphic zircon rim age of 114.8 ± 2.4 Ma for the 119.8 ± 2.9 Ma Misty Pluton. From west to east across DS, Sm-Nd garnet ages are 113.6 ± 2.5, 113.2 ± 2.8, 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. 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 Amp + Cpx + Grt + Qtz + Pl ± Bt + Rt + Ilm ± relict OPx ± relict Czo. Euhedral, peritectic garnet occurring in leucocratic veins and reaction zones, has little zoning in composition and ranges from Alm_53 Prp_27 Sps_3 Grs_17 in the west to Alm_44 Prp_36 Sps_1.5 Grs_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 for melted WFO and thermobarometry using garnet, clinopyroxene, and plagioclase compositions indicate 700-840°C and 10.6-14.2 kbar for metamorphism on DS. Similar methods for Pembroke Granulite of >750°C at 12-16 kbar indicate diachronous widespread high P metamorphism and partial melting for this ca. 4,375 km^2 section of mid-to lower-crust with these processes occurring early in the north and later in the south (126-105) Ma.

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