LU/HF AND SM/ND AGES OF ECLOGITE-FACIES METAMORPHISM IN THE WESTERN GNEISS REGION, NORWAY

Six samples of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks from the Western Gneiss Region (WGR), Norway, dated using Lu/Hf and Sm/Nd methods on identical garnet-omphacite-whole rock fractions yield surprising results. Three HP eclogites (700°C and ~2.0–2.5 GPa) yielded indistinguishable Lu/Hf ages of 411.5 ± 4.1 Ma (2s), 412.0 ± 4.7 Ma, and 420.6 ± 4.7 Ma (decay constant of 1.865 x 10-11 yr-1); the two samples with the younger Lu/Hf ages gave Sm/Nd ages of 398.3 ± 5.5 Ma and 402.7 ± 4.6 Ma. Two UHP eclogites (750–800°C and ~3 GPa) yielded significantly younger, but equivalent Lu/Hf and Sm/Nd ages: 380 ± 14 Ma and 369 ± 11 Ma for Lu/Hf and 388 ± 10 Ma and 384 ± 11 Ma for Sm/Nd, respectively. Following Lapen et al., (2003), the older Lu/Hf of the HP eclogites likely reflects prograde garnet growth during early stages of burial; the younger Sm/Nd ages are likely to represent an ‘average' age of garnet growth. The similarity between the Lu/Hf and Sm/Nd age data of the UHP eclogites and the surprisingly young age they represent, most likely indicate garnet growth at conditions that superceded the closure of those two systems; the ages reflect passage through closure to Sm/Nd and Lu/Hf. The old Lu/Hf ages from HP rocks are equivalent to the 415–410 Ma zircon ages reported by Krogh et al. (2003) from HP eclogites, and the older Sm/Nd ages overlap with the 410 ± 16 Ma age reported by Mørk and Mearns (1986). All our ages on the UHP rocks are younger than zircon ages (401.6 ± 1.6 Ma, Carswell et al., 2003; and 405–400 Ma, Root et al., 2004) and Sm/Nd ages (408 ± 7 Ma, Carswell et al., 2003) reported for UHP rocks. In aggregate, these data suggest that i) HP subduction began in the Western Gneiss region as early as ~421 Ma; ii) these HP rocks remained at eclogite-facies conditions until at least ~398 Ma; and iii) the UHP rocks remained at sufficiently high temperatures for Sm-Nd and Lu-Hf diffusion for another 10 Myr, until ~388 Ma.