GARNET SM-ND AGES FOR MULTIPLE GRAIN SIZE FRACTIONS: A TOOL FOR ESTIMATING THE DURATION OF HIGH-GRADE METAMORPHISM

Garnet Sm-Nd geochronology is an important tool for understanding high temperature tectonic processes and events, such as the timing and duration of lower crustal heating. However, interpretation of the ages is complicated because high temperature may facilitate diffusion. This results in younger ages when daughter isotope diffusion dominates and may result in older ages when parent isotope diffusion dominates. Slow cooling rates allow more time for diffusion and small grains are affected more than larger grains due to short diffusion length scales. If garnet Sm-Nd ages are reset to younger ages by diffusion, then dating small and large grains in the same samples can be used to calculate the duration of high-temperature events.

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.