HR: 0800h
AN: V41C-0718    [Abstracts]
TI: Sm--Nd Garnet Geochronology Demonstrates Wholesale Transformation of Continental Crust During UHP Subduction---Western Gneiss Region, Norway
AU: * Peterman, E M
EM: epeterman@umail.ucsb.edu
AF: UC Santa Barbara, Dept. of Earth Science, Santa Barbara, CA 93106, United States
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: UC Santa Barbara, Dept. of Earth Science, Santa Barbara, CA 93106, United States
AU: Baxter, E F
EM: efb@bu.edu
AF: Boston University, Dept. of Earth Sciences; 685 Commonwealth Avenue, Boston, MA 02215, United States
AB: Determining the extent to which ultrahigh-pressure (UHP) terranes transform to high-pressure (HP) minerals during subduction is central to understanding the processes attending the subduction of continental margins. The Western Gneiss Region (WGR) of Norway is one of two giant UHP terranes on Earth, and as such constitutes an important natural laboratory for investigating these processes. The distribution of UHP and HP eclogites shows that a 60,000 km² area was subducted, but these eclogites are sparse (1--2 vol%) blocks in a sea of chiefly quartzofeldspathic, amphibolite-facies orthogneiss. Aside from the eclogites, the only widespread indicator of (U)HP metamorphism in this enclosing orthogneiss is garnet. This study uses high- precision Sm–-Nd geochronology of these garnets to evaluate the degree of transformation of the WGR to high- pressure minerals during subduction and back to low-pressure minerals during exhumation. We compare these garnet ages to the 420–-400 Ma Sm-–Nd ages reported for WGR eclogites (e.g., Kylander-Clark et al., 2007). Eight garnet samples dated using the garnet–-whole-rock isochron technique yielded the following results: 1) 921.7 ± 1.3 Ma for a sample from the easternmost section of the WGR; 2) 418.1 ± 1.7 Ma, 417.3 ± 1.2 Ma and 403.9 ± 0.8 Ma for three UHP samples; 3) 410.3 ± 2.5, 406.9 ± 1.5 Ma and 398.5 ± 0.8 Ma for three HP samples; 4) 587.3 ± 4.3 Ma for a fourth HP sample. These ages are interpreted to represent 1) the Proterozoic granulite-facies metamorphism of ca. 950 Ma; 2 & 3) (U)HP prograde metamorphism of the continental crust; and 4) a mixed age derived from the granulite-facies and (U)HP metamorphic events. The Proterozoic garnet age implies that the easternmost WGR did not transform to eclogite-facies minerals. In contrast, the good agreement between the 420–-400 Ma eclogite ages (e.g., Kylander-Clark et al., 2007) and the 418–-398 Ma gneiss ages reported here, indicates that the bulk of the WGR continental crust underwent wholesale transformation at (U)HP conditions. That the garnets are the only HP silicate mineral (other than quartz) surviving within a quartzofeldspathic gneiss composed of otherwise amphibolite-facies minerals indicates wholesale retrogression during exhumation.
DE: 1115 Radioisotope geochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3652 Pressure-temperature-time paths
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting