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Arc plutonism following regional thrusting: Petrology and geochemistry of syn- and post-Nevadan plutons in the Siskiyou Mountains, Klamath Mountains province, California

By
Calvin G. Barnes
Calvin G. Barnes
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
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Arthur W. Snoke
Arthur W. Snoke
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Gregory D. Harper
Gregory D. Harper
Department of Earth and Atmospheric Sciences, State University of New York, Albany, New York 12222, USA
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Carol D. Frost
Carol D. Frost
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Rory R. McFadden
Rory R. McFadden
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Jonathan C. Bushey
Jonathan C. Bushey
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Melanie A.W. Barnes
Melanie A.W. Barnes
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
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Published:
January 01, 2006

A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase.

Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H2O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization processes. The latter magmas were parental to the gabbroic and dioritic units. Many of the felsic rocks show evidence of origins by partial melting of metabasaltic crustal rocks, particularly their low heavy rare-earth element concentrations and high Sr/Y ratios. Mixing of crustal melts with primitive basaltic magmas was locally important (e.g., Pony Peak pluton). The mafic parental magmas show trace element features typical of an origin by partial melting of a subduction-modified mantle wedge. It is unclear whether subduction was coeval with western Klamath magmatism or whether the subduction signature developed as the result of Middle Jurassic subduction.

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GSA Special Papers

Geological Studies in the Klamath Mountains Province, California and Oregon: A volume in honor of William P. Irwin

Arthur W. Snoke
Arthur W. Snoke
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Calvin G. Barnes
Calvin G. Barnes
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Geological Society of America
Volume
410
ISBN print:
9780813724102
Publication date:
January 01, 2006

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