2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 14
Presentation Time: 4:45 PM


KYLANDER-CLARK, Andrew1, HACKER, Brad2, JOHNSON, Clark3, BEARD, Brian4, MAHLEN, Nancy3 and LAPEN, Tom3, (1)Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, (2)Geological Sciences, UC, Santa Barbara, Department of Geological Sciences, UC Santa Barbara—Building 526, Santa Barbara, CA 93106-9630, (3)Geology & Geophysics, Univ of Wisconsin, Madison, WI 53706, (4)Geology and Geophysics, Univ of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706, kylander@geol.ucsb.edu

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the UHP eclogites and the surprisingly young age they represent, most likely indicate garnet growth at conditions that superceded the closure of those two systems; the ages reflect passage through closure to Sm/Nd and Lu/Hf.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.