HR: 0800h
AN: V41C-0729    [Abstracts]
TI: Eclogite Facies Relicts and Decompression Assemblages; Evidence for the Exhumation of a Large Coherent Metabasite Block From > 40 km Depth; Central Metamorphic Terrane, Eastern Klamath Mountains, Northern California
AU: * Barrow, W M
EM: wendy.barrow-johnson@unlv.edu
AF: University of Nevada, Las Vegas, Department of Geoscience 4505 Maryland Pkwy, Las Vegas, NV 89154-4010, United States
AU: Fairhurst, R J
EM: rob.fairhurst@unlv.edu
AF: University of Nevada, Las Vegas, Department of Geoscience 4505 Maryland Pkwy, Las Vegas, NV 89154-4010, United States
AU: Metcalf, R V
EM: rod.metcalf@unlv.edu
AF: University of Nevada, Las Vegas, Department of Geoscience 4505 Maryland Pkwy, Las Vegas, NV 89154-4010, United States
AB: Recent exhumation models for eclogite terranes have focused on the exhumation of sialic rocks. Exhumed high pressure terranes are typically > 85% − 90% sialic material with only minor amounts of mafic and ultramafic rock. Most known metabasitic eclogites are blocks in mélange rather than large coherent bodies. The Central Metamorphic terrane (CMt) is a large (~300 km³) coherent, fault-bounded package of metabasites thought to represent a remnant of a downing plate subducted in an intra-oceanic convergent margin. Thermochronology indicates that the CMt was metamorphosed and later accreted to the base of the Trinity ophiolite along the Trinity fault during Early Permian extension (Hbl and Musc ⁴⁰Ar/³⁹Ar ages of 275 Ma - 294 Ma). Previous work suggested that the peak metamorphic temperatures and pressures were ~650°C and 0.4 to 0.8 GPa (Peacock and Norris, 1989) which is consistent with the amphibolite facies mineral assemblage. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures, however, suggest that CMt amphibolites record much deeper subduction burial with subsequent decompression exhumation. A decompression sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry (Watson et al., 2006) combined with experimental data for rutile stability in metabasites (Ernst and Lui, 1998) suggests that relict rutile crystals preserve early P-T conditions of ~600°C and > 1.3 GPa consistent with eclogite facies metamorphism. Transition from eclogite facies is further supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of garnet (Bhowmik and Roy, 2003) during decompression. Amphibole compositions vary significantly and reflect lower grade (low Na, Al, Ti actinolite) overprint of earlier amphibolite facies compositions (high Na, Al, Ti magnesio- hornblende). Application of the Al-Ti hornblende thermobarometer (Ernst and Lui, 1998) yields upper P-T estimates of ~600°C and > 1.5 GPa, consistent with hornblende eclogite facies. The dominant mineral assemblages and metamorphic fabrics indicate dynamic recrystallization of metabasites during declining P-T conditions through amphibolite - epidote amphibolite facies (ilmenite - titanite transition). Exhumation via extension along the Trinity fault is suggested by the coplanar relationship between metabasite decompression-related deformation fabrics and the Trinity fault. Present models call on the higher buoyancy of the sialic eclogites as the mechanism for exhumation and to explain the volumetric difference between the mafic and felsic eclogites exhumed. The exhumation of the CMt from depths > 40 km requires new models for the exhumation of subducted oceanic crust from eclogite facies conditions. We speculate that the CMt was exhumed from eclogite facies conditions by tectonic unroofing along the Trinity fault. However, the mechanism is still unclear and requires further investigation.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
DE: 3651 Thermobarometry
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting