U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma?
Seven analyses from two samples of the Lower Banded series of the Stillwater Complex, Montana, yielded a U-Pb zircon-baddeleyite age of 2705 ±4 Ma. A more precise age of 2704 ±1 Ma is obtained if we 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 2701 ±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 (2711 ±1 Ma) and a group 3 mafic norite (2703 ±10 Ma) are comparable to previously reported U-Pb zircon ages for a group 1 diabase (2713 ±3 Ma) and a group 4 high-Ti norite (2712 ±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 ielded a four-point Pb-Pb internal isochron age of 2704 ±25 Ma, and separates from group 6 yielded a Sm-Nd internal isochron age of 2706 ±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.
Citation Information
Publication Year | 1990 |
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Title | U-Pb and Sm-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: Identification of a parent magma? |
DOI | 10.1130/0091-7613(1990)018<1065:UPASNA>2.3.CO;2 |
Authors | W. R. Premo, Rosalind Tuthill Helz, M. L. Zientek, R.B. Langston |
Publication Type | Article |
Publication Subtype | Journal Article |
Series Title | Geology |
Index ID | 70016058 |
Record Source | USGS Publications Warehouse |