NEW GARNET SM-ND EVIDENCE FOR PALEOZOIC METAMORPHISM, WET JACKET ARM FIORDLAND, NEW ZEALAND

The eastern region of Fiordland contains Permian and Mesozoic arc magmatic rocks, including the Median Batholith, which is inferred to have formed along the eastern margin of Gondwana. West of the Median Batholith, Paleozoic meta-igneous and less common meta-sedimentary rocks were intruded by the Late Cretaceous Western Fiordland Orthogneiss. Subsequently, high temperature and pressure metamorphism caused partial melting. Ages for metamorphism in central Fiordland are controversial. In part, due to a paucity of geochronological data and variable overprinting of early minerals by the high temperature event.

A large sample of amphibolite collected from the south shore of Wet Jacket Arm yielded subhedral garnet up to ca. 20 mm in diameter. A ca. 14 mm crystal was sampled for major element zoning and a second 20 mm crystal was sampled for geochronology. Major element garnet chemistry (14 mm grain) will be used for understanding the P-T history. Core and rim samples (20 mm grain) were dissolved and analyzed for Sm and Nd isotope ratios at UNC Chapel Hill.

Preliminary Sm and Nd isotope data yield two point garnet Sm-Nd ages of 312±10 (core – whole rock) and 320±5.7 (rim – rock matrix) Ma. Due to the inconsistent age order and lack of additional data points, a four point isochron of 316±17 (MSWD = 1.9) is preferred. This age is similar to U-Pb monazite ages for the Mt. Parker and Mt. Gladstone Schists ca. 50 km NE (Ireland & Gibson 1998). In summary, new garnet ages from Wet Jacket Arm indicate that these rocks were once part of the Paleozoic eastern margin of Gondwana. Additional data are needed to better elucidate the early history of these polymetamorphic rocks.