Award Date

1-1-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Rodney Metcalf

Number of Pages

194

Abstract

The Central Metamorphic terrane (CMt) is a fault-bounded package of metabasites that underlies the Eastern Klamath terrane along the Trinity fault. Interpretation of the CMt as the lower oceanic plate of a Silurian-Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the Eastern Klamath province. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures and amphibole compositions, however, suggest significantly greater subduction burial and subsequent decompressional history during exhumation for CMt metabasites. New thermobarometry yields upper P-T estimates of ∼600°C and >1.5GPa, consistent with eclogite facies. Predominate mineral assemblages and metamorphic fabrics indicate recrystallization of metabasites during declining P-T conditions through amphibolite → epidote amphibolite facies (ilmenite → titanite transition). Exhumation via extensional reactivation of the Trinity fault is suggested by the coplanar relationship between metabasite decompression-related deformation fabrics and the Trinity fault. New 40Ar/39Ar thermochronology indicates an Early Permian (∼274--294 Ma) tectonic exhumation; We propose a modification of the Mid-Paleozoic convergent margin model for the eastern Klamath Mountains. Our findings indicate that the CMt was subducted prior to 290 Ma to eclogite facies conditions of >1.5GPa at 600°C. The CMt was subsequently exhumed from depth by Early Permian extension along the Trinity fault followed by a greenschist overprint at the base of the CMt in the Late Triassic (∼219 Ma). There is no evidence of the prograde grade path; however, the most likely path is blueschist → eclogite facies. Therefore, we suggest a clockwise P-T path. Such a path is typical for subduction zone metamorphism; Present models call on the higher buoyancy of the sialic eclogites as the mechanism for exhumation. Most known metabasitic eclogites are as blocks in melange rather than large coherent bodies, which are rafted along with the buoyant sialic rocks. However, the CMt was exhumed from >40 km depth as a large coherent body. The exhumation of the CMt from depths >40 km requires new models for the exhumation of subducted oceanic crust from eclogite facies conditions.

Keywords

California; Central; Central Metamorphic Terrane; Eclogites; Klamath Mountains; Metamorphic; Northern; Paleotectonic; Significance; Terrane

Controlled Subject

Geology; Geochemistry

File Format

pdf

File Size

4945.92 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

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