Precambrian Felsic and Mafic Dikes Along the Northwestern Margin of the Wyoming Province: Tectonic Constraints from the Southern Highland Mountains, Madison County, Montana

Abstract

The Precambrian rocks of the southern Highland Mountains in Montana record the geologic history of the northwestern Wyoming craton. Multiple generations of mafic and felsic dikes cut the Archean gneissic fabric and are useful for constraining the Proterozoic tectonic evolution. This study provides mapping of the Nez Perce Hollow and Twin Bridges SW 7.5’ quadrangles with a focus on dike characterization. Also presented here is U-Pb geochronology on two suites of granitic dikes and geochemical data on two suites of mafic dikes. Trace element data shows that amphibolite dikes in the mapping area intruded during a back-arc setting which is constrained in time between 2450 Ma and 1880 Ma. Analyses done here shows that no significant mobilization of elements occurred due to metamorphism, hydrothermal alteration, or chemical weathering and the geochemical signatures can be inferred to be the same for the protolith rock. Samples of diabase dikes have geochemistry similar to the Ramshorn Creek dikes recognized in the Tobacco Root Mountains nearby. This constrains this generation to the Moyie-Purcell large igneous province event ~1460 Ma. Geochronology of the granitic dikes show there are two separate tectonic events separated by ~100 million years. The youngest granite has a best age of 1791±8.1 Ma which corresponds to the timing of the Big Sky Orogeny. The oldest granite has an age of ~1880 Ma which is interpreted to be a previously unrecognized event prior to the Big Sky Orogeny. This result suggests that a new model is needed for the amalgamation of the Wyoming Province and the Medicine Hat block.