GARNET SM-ND AGES FOR MULTIPLE GRAIN SIZE FRACTIONS: A TOOL FOR ESTIMATING THE DURATION OF HIGH-GRADE METAMORPHISM
We present new garnet Sm-Nd ages for 2-4 mm grains from two migmatitic garnet-granulite samples (samples 22a and 36) in the Western Fiordland Orthogneiss (WFO), Fiordland, NZ. These samples contain abundant garnet ranging from 2 to 10 mm in diameter, which occur in multiple textural domains. Both 22a and 36 contain leucosomes bordered by selvages of ca. 1 cm garnet and garnet reaction zones (GRZ) of fine-grained garnet, garnet porphyroblasts, and clinopyroxene. Garnet from each texture have been dated. Prior work on 10 mm selvage garnet in 22a yielded an age of 112±2.2 Ma (10 points) and the 2-4 mm GRZ garnet dated in this study produced a 103.6±2.3 Ma age (5 points). Thus far, only 2-4 mm selvage garnets have been analyzed from sample 36, yielding an age of 108.1±1.8 Ma (5 points). Cooling rates, estimated from combined U-Pb zircon, titanite, apatite, and K-Ar hornblende data range from 21°C/m.y. at 116-108 Ma to 7°C/m.y. at 108-106 Ma. These cooling rates, input into closure temperature estimates, result in closure temperatures of 923-877°C and 799-762°C for 10 mm and 2 mm grains, respectively. Therefore, the ca. 112 Ma age corresponds with peak garnet-granulite conditions of 12-14 kbar and ca. 900°C, while the ca. 108 and 103 Ma ages correspond to cooling of the WFO below 800°C. These data illustrate that the duration of high-temperature metamorphism can be determined from dating multiple garnet size fractions and incorporating closure temperatures with cooling rate and history.