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| Titel |
Exhumation of an unusually large, ~3000 km3 coherent block of oceanic crust from >40 km depth |
| VerfasserIn |
Wendy Barrow, Rodney Metcalf, Robert Fairhurst |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044168
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| Zusammenfassung |
| The Central Metamorphic terrane (CMt) is an unusually large (~3000 km3) coherent block
of mid-ocean ridge (MOR) metabasites; the first one of this scale reported with eclogite facies
relicts, decompression assemblages, and thermobarometry indicating exhumation of the
entire block from >40 km depth. The CMt is exposed in the eastern Klamath Mountains
of northern California and is dominantly an amphibolite facies metabasite which
represents remnant oceanic crust subducted in a mid-Paleozoic Pacific-type margin.
Thermochronology indicates that the CMt was subsequently exhumed along the
Trinity fault during an early Permian extensional event. Newly discovered relict
textures with new thermobarometry results suggest the CMt metabasites record
the retrograde segment of the P-T-deformation-time path during exhumation from
hornblende eclogite facies P-T conditions. A decompression and cooling sequence
consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt
amphibolite samples. Zr-in-rutile thermometry combined with experimental data for
rutile stability in metabasites suggests that relict rutile crystals preserve early P-T
conditions of ~600Ë C and >1.3 GPa. Transition from eclogite facies is further
supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of
garnet or omphacite during decompression. The dominant mineral assemblages and
metamorphic fabrics indicate dynamic recrystallization of metabasites during declining P-T
conditions through amphibolite - epidote amphibolite facies. Exhumation via extension
along the Trinity fault is suggested by the coplanar relationship between metabasite
decompression-related deformation fabrics and the Trinity fault. We propose that subducted
oceanic crust (CMt) was subsequently exhumed as a large coherent block from
depths >40 km. This is significant because the conversion of mafic oceanic crust to
eclogite produces the negative buoyancy (relative to mantle peridotite) that drives
subduction. Exhumed HP and UHP terranes are dominantly low density material,
either sialic crustal material or serpentinite, with high density metabasite typically
representing only about 15% exhumed terranes. Exhumed metabasite generally
occurs either as meter-scale blocks in shaley mélange or as kilometer-scale sheets
in serpentinite. The largest masses of exhumed eclogitic metabasite are found in
Alpine massifs preserved as sheets (~1 km by 10-20 km, |
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