GEOCHRONOLOGICAL EVIDENCE FOR MESO- AND NEOPROTEROZOIC MAFIC MAGMATISM ALONG THE WESTERN MARGIN OF THE WYOMING CRATON

Isotopic dating of mafic dikes in the southern Tobacco Root Mountains of southwestern Montana provides evidence for two distinct episodes of subparallel Proterozoic mafic magmatism separated by about 700 m.y. Previously published geochemical data from dikes in the southern Tobacco Root Mountains had identified three geochemical groups (termed groups A, B, and C) with apparent Rb-Sr ages of 1455 Ma (group A) and 1100-1120 Ma (group B and C). Sm-Nd dating of geochemical group A/C dikes yields a mineral isochron age of 1448 ± 49 Ma that is interpreted to represent the emplacement age of the dikes. This age is similar to published U-Pb dates from mafic sills that intrude the nearby Mesoproterozoic Belt Supergroup and dikes that cut the Laramide basement-cored uplifts of the Archean Wyoming Province. We suggest that the mafic magmatism recorded by the southern Tobacco Root Mountains group A/C dikes probably corresponds with continental extension that accompanied development of the Mesoproterozoic Belt Basin. U-Pb dating of baddeleyite from the group B dikes provides evidence for a younger magmatic event with an age of 782.4 ± 4.9 Ma (95% confidence; ± 7.1 Ma incorporating uranium decay constant errors). This date is similar to isotopic dates from Neoproterozoic mafic dikes and sills exposed elsewhere in uplifts of the Rocky Mountain foreland and that intrude the Mesoproterozoic strata of the Belt Supergroup. ⁴⁰Ar/³⁹Ar dates from hornblende in the group B dikes and biotite from a baked contact zone from one of the dikes yield similar isotopic dates indicating that they have not been disturbed by subsequent (Laramide) tectonothermal events. Geochronologic data from elsewhere in the northern Cordillera and Canadian Shield indicate that the group B dikes in the southern Tobacco Root Mountains are part of the regional 780 Ma Gunbarrel magmatic event that extended for greater than 2400 km along the western margin of Laurentia. The origin of the magmatic event is not clear, but it may be related to a mantle plume and/or crustal extension accompanying or perhaps slightly predating initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. No evidence of ca. 1100 Ma mafic magmatism in this part of the northern Cordillera was found.