HR: 1340h
AN: V43A-1559    [Abstracts]
TI: Mineralogical Evidence for the Bulk Transformation of Continental Crust to Ultrahigh-Pressure Conditions in Subduction Zones
AU: * Peterman, E M
EM: epeterman@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences; Webb Hall--Building 526, Santa Barbara, CA 93106
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences; Webb Hall--Building 526, Santa Barbara, CA 93106
AU: Kylander-Clark, A R
EM: akylander@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences; Webb Hall--Building 526, Santa Barbara, CA 93106
AB: Evidence for (ultra)high-pressure --(U)HP-- metamorphism in modern orogenic belts and the preservation of exhumed (U)HP terranes around the world suggest that subduction and exhumation of continental crust plays an important role in Phanerozoic plate tectonics. The Western Gneiss region (WGR) of Norway, a major (U)HP province extending over 60,000 km², provides an excellent opportunity to study how subduction to depths >100 km affects continental crust. By studying a ~60 km wide transect bounded to the north by Vartdalsfjorden and Rovdefjorden and the south by the Möre og Romsdal county boundary, we are able to examine mineralogical changes that occurred during subduction and exhumation within a rock composed predominantly of orthogneiss and variably transformed mafic bodies, which indicate the depths to which these rocks were subducted. Previous studies (e.g. Hacker et al., 2005) have suggested that Caledonian deformation in WGR host gneisses is primarily limited to brittle-ductile fabrics characterized by greenschist to lower-amphibolite facies metamorphism; the majority of the deformation in the rocks, including the pervasive foliation and foliation-parallel isoclinal folds, occurred between 1200 and 900 Ma. On the northern half of our study area, however, locally occurring neoblastic garnet crosscuts the foliation in the gneiss. The boundary of this garnet zone coincides with the local HP-UHP boundary, as determined by the presence of coesite in eclogite. Because garnet can retain information about changes in pressure and temperature, as well as the availability of water within the crust to catalyze chemical reactions, our findings suggest that 1) portions of the orthogneiss did transform at high pressures, 2) the presence of garnet within the orthogneiss may indicate conditions that approximate UHP and can therefore be useful in defining the boundaries between UHP and HP conditions, and 3) the growth of garnet during (U)HP metamorphism may be controlled by hydration of the crust, thus explaining the partial transformation to (U)HP mineral assemblages throughout the WGR.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
DE: 8108 Continental tectonics: compressional
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005