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Title:
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New U-Pb radiometric dates of the Bear Mountain intrusive complex,Klamath Mountains, California
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Reference Number:
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2046
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ISSN:
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0072-1077
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Publication Year:
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2006
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Book Editors:
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Snoke, Arthur W; Barnes, Calvin G
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GeoREF Number:
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2007-028096 (View Original Data File)
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Authors:
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Chamberlain, Kevin R; Snoke, Arthur W; Barnes, Calvin G; Bushey,Jonathan C
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Descriptors:
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absolute age; Bear Mountain Complex; Bear Mountain Pluton; California;crystallization; dates; dikes; diorites; faults; igneous rocks;intrusions; Jurassic; Klamath Mountains; magmas; magmatism; Mesozoic;nesosilicates; Nevadan Orogeny; oblique-slip faults; Orleans Fault;orthosilicates; Paleoproterozoic; patterns; plutonic rocks; plutons;Precambrian; precision; Proterozoic; silicates; South Siskiyou ForkFault; tear faults; thrust faults; titanite; titanite group; tonalite;U/Pb; United States; upper Precambrian; variations; zircon; zircongroup
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ISBN:
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0813724104; 978081372410
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Source:
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Special Paper - Geological Society of America, vol.410, pp.317-332,2006
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Abstract:
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New, high-precision U-Pb titanite (sphene) and zircon dates from fivesamples of the Bear Mountain intrusive complex establish the timingand duration of magmatism. The oldest, magmatic date (150.5+ or -0.6Ma) comes from dark-colored titanite from a biotite-hornblendetonalite that is part of a composite pluton that intrudes the BlueRidge ultramafic-gabbroic intrusion. Pale titanite and zircon fromthis sample yielded a distinctly younger date of 149.3+ or -0.3 Ma. Asimilar pattern of mineral dates is also apparent in two samples ofthe areally extensive Punchbowl unit of the Bear Mountain pluton.Dark-colored titanite in one of these samples yielded a date of 149.5+or -0.6 Ma, whereas the dates of pale titanite and zircon are 147.4+or -0.3 Ma. The second sample of the Punchbowl unit only contained asingle morphology of pale titanite, which yielded the same date aszircon (148.2+ or -0.3 Ma). The U-Pb zircon date of the Buck Lake unitof the Bear Mountain pluton, 148.2+ or -0.2 Ma, supports fieldevidence that the Buck Lake unit was emplaced synchronously with thePunchbowl unit. A lower age limit on magmatism in the Bear Mountainintrusive complex comes from a 145.4+ or -0.4-Ma zircon date from alate crosscutting mafic dike. All samples exhibit slight inheritancein the zircon data, with 152- to 150-Ma minimum ages. The mafic dikecontains inherited components that are at least 264 Ma and possiblyPaleo-proterozoic in age. The new dates constrain magmatism in theBear Mountain intrusive complex to the period from 151 to 147 Ma, witha minimum duration of 1.5 m.y. and a maximum of 6 m.y. The datesestablish that the emplacement and crystallization of the BearMountain intrusive complex post-dated regional thrust faulting(Orleans fault) associated with the Nevadan orogeny, including theSouth Siskiyou Fork fault, which is interpreted as an oblique-sliptear fault associated with the Orleans (thrust) fault system. Thepattern of mineral dates from the composite pluton intruded into theBlue Ridge intrusion as well as the areally more extensive Punchbowlunit indicate that crystallization of these bodies occurred over 1.5-2m.y., due to either insulating effects of the intrusive complex and/ormagma recharge.
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Copyright:
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GeoRef, Copyright 2007, American Geological Institute.
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