GEOCHEMISTRY, PETROGENESIS AND TECTONIC SETTING OF IGNEOUS ROCKS OF THE HARTVILLE UPLIFT, EASTERN WYOMING

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. We have studied igneous rocks in the Laramide-age Hartville Uplift (HU) that exposed a Precambrian terrane. At least part of the NNE-trending HU is bisected along its length by the Hartville Fault (HF) that juxtaposes high-grade rocks on its eastern side against lower-grade rocks on its western side. Our objective is to use the geochemical features of the igneous rocks to infer the tectonic settings in which they formed.

The oldest magmatic rocks are 2.6 Ga (Day et al., 1999) 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 Muskrat Canyon metabasalt on the western side of the HF and the Mother Featherlegs metabasalt on the eastern side. The ages of the metabasalts are unknown, thus it is not certain if they are coeval. However, they may correspond to the ~2 Ga Kennedy dike swarm in the Laramie Range and amphibolites in the Black Hills.

In the southern HU, Proterozoic, 1.74 Ga (Chamberlain, pers. comm.) Twin Hills diorite and Haystack Range granite crop out on the eastern side of the HF. The latter appears to be younger as its dikes cut the diorite. SiO₂ of ~55 wt % and K₂O /Na₂O ratios of < 1 suggest a lithospheric mantle origin for the Twin Hills diorite, whereas SiO₂ >69 wt %, K₂O/Na₂O > 1, and peraluminous composition indicate crustal origin for the Haystack Range granite.

We suggest that the the Archaean granitoids may be related to the building of the continent Kenorland that was later rifted in an episode corresponding to the basaltic suites. The Twin Hills diorite and the Haystack Range granite appear to be related to the Black Hills collisional orogeny based on coincident ages in the Black Hills. On the east side of the HF, an unrecognized lithospheric break may separate the Archean-dominated block in the northern HU from Proterozoic rocks in the southern HU.