Sm-Nd Garnet Ages For Granulite And Eclogite In The Breaksea Orthogneiss And Widespread Granulite Facies Metamorphism Of The Lower Crust, Fiordland Magmatic Arc, New Zealand
Sm-Nd garnet and U-Pb zircon ages for eclogite and granulite from the Breaksea Orthogneiss provide a detailed chronology for pluton emplacement and subsequent thermal history of the lower arc crust exposed in Fiordland, New Zealand. The 147Sm-143Nd ages for ∼1 cm garnet grains in eclogite yield a 108.2 ± 1.8 Ma (7 points) age and similar sized grains of garnet from granulite interlayered with eclogite yield a ca. 110.5 ± 1.6 Ma (8 points) age. Both samples retain sparse domains with older ages of 123–121 Ma. Distinct Ca, Lu, and Hf zoning in garnet indicate that eclogite and granulite cooled rapidly enough to negate significant diffusion. The Ca zoning is interpreted to indicate significant garnet recrystallization during the granulite facies event, ca. 110 Ma. The older garnet ages are indistinguishable from the oldest 206U/238Pb zircon ages, ca. 123 and 120 Ma, in granulite orthogneiss that yielded two age populations; these granulites have younger age populations of 111.1 ± 1.4 and 115.2 ± 1.3 Ma, respectively. Zircon from orthogneiss samples nearby yield single age populations indicating additional intrusions ca. 115 and late metamorphic zircon growth ca. 95 Ma. The zircon and garnet ages combined with pressure-temperature-time paths document magma intrusion into the lowermost arc crust, near isothermal exhumation of Breaksea rocks at ∼2.2 km/m.y. from ∼65 km to 40–45 km depths, followed by continued high heat flow with granulite facies metamorphism. The latter high temperatures were synchronous with granulite facies metamorphism in the adjacent Malaspina pluton, indicating that high-temperature metamorphism affected >600 km2 of lower crust in the continental magmatic arc.