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New U-Pb radiometric dates of the Bear Mountain intrusive complex, Klamath Mountains, California

By
Kevin R. Chamberlain
Kevin R. Chamberlain
1
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Arthur W. Snoke
Arthur W. Snoke
1
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Calvin G. Barnes
Calvin G. Barnes
2
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA
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Jonathan C. Bushey
Jonathan C. Bushey
3
Department of Geology and Geophysics, Department 3006, 1000 East University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
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Published:
January 01, 2006

New, high-precision U-Pb titanite (sphene) and zircon dates from five samples of the Bear Mountain intrusive complex establish the timing and duration of magmatism. The oldest, magmatic date (150.5 ± 0.6 Ma) comes from dark-colored titanite from a biotite-hornblende tonalite that is part of a composite pluton that intrudes the Blue Ridge ultramafic-gabbroic intrusion. Pale titanite and zircon from this sample yielded a distinctly younger date of 149.3 ± 0.3 Ma. A similar pattern of mineral dates is also apparent in two samples of the areally extensive Punchbowl unit of the Bear Mountain pluton. Dark-colored titanite in one of these samples yielded a date of 149.5 ± 0.6 Ma, whereas the dates of pale titanite and zircon are 147.4 ± 0.3 Ma. The second sample of the Punchbowl unit only contained a single morphology of pale titanite, which yielded the same date as zircon (148.2 ± 0.3 Ma). The U-Pb zircon date of the Buck Lake unit of the Bear Mountain pluton, 148.2 ± 0.2 Ma, supports field evidence that the Buck Lake unit was emplaced synchronously with the Punchbowl unit. A lower age limit on magmatism in the Bear Mountain intrusive complex comes from a 145.4 ± 0.4-Ma zircon date from a late crosscutting mafic dike. All samples exhibit slight inheritance in the zircon data, with 152- to 150-Ma minimum ages. The mafic dike contains inherited components that are at least 264 Ma and possibly Paleoproterozoic in age.

The new dates constrain magmatism in the Bear Mountain intrusive complex to the period from 151 to 147 Ma, with a minimum duration of 1.5 m.y. and a maximum of 6 m.y. The dates establish that the emplacement and crystallization of the Bear Mountain intrusive complex post-dated regional thrust faulting (Orleans fault) associated with the Nevadan orogeny, including the South Siskiyou Fork fault, which is interpreted as an oblique-slip tear fault associated with the Orleans (thrust) fault system. The pattern of mineral dates from the composite pluton intruded into the Blue Ridge intrusion as well as the areally more extensive Punchbowl unit indicate that crystallization of these bodies occurred over 1.5–2 m.y., due to either insulating effects of the intrusive complex and/or magma recharge.

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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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