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

Abstract

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.