Evaluating Metamorphic Pressure-Temperature-Time Paths Along Milford Sound, Fiordland, New Zealand

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Fiordland, New Zealand offers an opportunity to study exposures of mid- to- lower crust that contain evidence for high temperature and pressure processes occurring beneath magmatic arcs. This thesis presents pressure, temperature, and time paths for rocks along Milford Sound to better understand the tectonic history and timing of metamorphism within and adjacent to the Anita Shear Zone. Samples analyzed are from St. Anne Point to near Milford Village here named the Milford Sound Transect. Rocks from the Thurso Gneiss (18MSNZ510a), Milford Geiss (15NZ51), Pembroke Granulite (05NZ12), and Camp Oven Creek Paragneiss (15NZ63), from west to east along the Milford Transect yielded garnet Sm-Nd ages of 99±7, 93±5, 122±2, and 119±8 Ma. Mineral assemblage diagrams and thermobarometry indicate somewhat higher pressures and temperatures in the central and eastern parts of the Milford Transect. Rocks in the Anita Shear Zone (18MSNZ511b; 18MSNZ510a) equilibrated at 9.5 to 11 kbar at temperatures of 650-700 °C and 525 – 625 °C, respectively. East of the Anita Shear Zone (15NZ51) peak metamorphism reached 10.5 to 11 kbar at 625-675 °C. These new P-T-t data, with P-T data from Czertowicz and others (2016) and Klepeis and others (1999) provide a direct temporal link between garnet growth, high pressure metamorphism, and shear zone fabrics. Further east, samples 05NZ12, and 15NZ63 equilibrated at; 11-16 kbar at 640 – 725 °C; and 8.5-13.5 kbar at 525- 625 °C, respectively. Our new data delineates a ~ 7 km wide zone of < 100 Ma 9.5 to 11 kbar metamorphism along the western, and a ~8 km wide zone of > 11 kbar >100 Ma metamorphism along the eastern part of the Milford Transect. Garnet age differences between 15NZ51 and 05NZ12 of ca. 30 m.y., higher pressures of 11-16 kbar for 05NZ12, and the ~ 4 km distance between them is compatible with possible fault separation. I infer a fault or shear zone separating the young (<100 Ma) medium pressure (9.5 to 11 kbar) metamorphism along the eastern Milford Transect and the considerably older (122 to 126 Ma) Pembroke Granulite metamorphism at higher pressure (>11 kbar).

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