Geochemistry, petrogenesis and tectonic setting of igneous rocks of the Hartville uplift, Eastern Wyoming

Abstract

The location of the eastern margin of the Wyoming Archaean Province and its Proterozoic evolution are still debated. Previous studies have attributed north-south and east-west directed structures to the Proterozoic Black Hills and Central Plains orogenies, respectively, but the tectonic details of these orogenies are unclear. I have studied igneous rocks in the Laramide-age Hartville Uplift (HU), which exposed Precambrian rocks. At least part of the NNE-trending HU is bisected along its length by the Hartville Fault (HF) that juxtaposes high-grade metamorphic rocks on its eastern side against lower-grade metamorphic rocks on its western side. The objective is to use the geochemical features of the igneous rocks to infer the tectonic settings in which they formed. The oldest dated magmatic rocks are 2.6 Ga Archaean Rawhide Buttes and Flattop Butte granites (SiO2 > 67 wt%; K2O /Na2O wt% > 1; ASI > 1.05). They crop out only in the northern part of the HU and appear to be of crustal origin. The next magmatic episodes involved basaltic volcanism. They are represented by the Mother Featherlegs metabasalt on the eastern side of the HF and the Muskrat Canyon metabasalt on the western side. Compositions of these basalts are attributed to rifting and a mantle plume, respetively. The ages of the metabasalts are unknown, thus it is not certain if they are coeval. However, they may correspond to the [about]2 Ga Kennedy dike swarm in the Laramie Range and amphibolites in the Black Hills that show similar extension and plume-related chemical characteristics.