U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma

Journal Article · Thu Nov 01 04:00:00 UTC 1990 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1990)018<1065:UPASNA>2.3.CO;2· OSTI ID:5796842

Premo, W R ^[1]; Helz, R T ^[2]; Zientek, M L ^[3]; Langston, R B ^[4]

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1. U.S. Geological Survey, Denver, CO (USA)
2. U.S. Geological Survey, Reston, VA (USA)
3. U.S. Geological Survey, Spokane, WA (USA)
4. Stillwater Mining Company, Nye, MT (USA)

Seven analyses from two samples of the Lower Banded series of the Stillwater Complex, Montana, yielded a U-Pb zircon-baddeleyite age of 2,705 {plus minus}4 Ma. A more precise age of 2,704 {plus minus}1 Ma is obtained if the authors regress only five of seven analyses. In either case, the age is in very good agreement with the previously reported Sm-Nd mineral isochron age of 2,701 {plus minus}8 Ma for a gabbro from the Lower Banded series. Noncumulate mafic sills and dikes that immediately underlie the Stillwater Complex are separated into five compositional groups. U-Pb zircon ages for a group 1 gabbronorite (2,711 {plus minus}1 Ma) and a group 3 mafic norite (2,703 {plus minus}10 Ma) are comparable to previously reported U-Pb zircon ages for a group 1 diabase (2,713 {plus minus}3 Ma) and a group 4 high-Ti norite (2,712 {plus minus}3 Ma). Although group 2 high-Mg gabbronorite and group 6 olivine gabbro have not yet yielded significant amounts of zircon, baddeleyite, or sphene, mineral separates from group 2 yielded a four-point Pb-Pb internal isochron age of 2,704 {plus minus}25 Ma, and separates from group 6 yielded a Sm-Nd internal isochron age of 2,706 {plus minus}64 Ma, indicating that these groups are at least contemporaneous with intrusion of the Stillwater Complex. These results indicate that groups 1 and 4 of the associated sills and dikes are perhaps slightly older than the complex and that groups 2, 3, and 6 may be coeval with it, supporting the idea that these three groups are the best candidates for a Stillwater parent magma, a conclusion previously presented on the basis of detailed mineralogic and geochemical evidence.

• OSTI ID: 5796842
• Journal Information: Geology; (USA), Journal Name: Geology; (USA) Vol. 18:11; ISSN GLGYB; ISSN 0091-7613
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580000* -- Geosciences
ACTINIDE ISOTOPES
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