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

January 1, 1990

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??10Ma) 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). Mineral separates from group 2 yielded 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. 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. -from Authors