GEOCHEMISTRY OF GRANITIC CLASTS ASSOCIATED WITH THE FRONTIER FORMATION (UPPER CRETACEOUS), NORTH-CENTRAL WYOMING: PROVENANCE AND TECTONIC INTERPRETATION

Granite clasts occur in the Torchlight polymictic conglomerate belonging to the Upper Cretaceous Frontier Formation. From a sample of 30 granite clasts, two types have been recognized. Most of the clasts are between 1.5 to 8.5 cm in diameter. The most distinctive granite type is a muscovite-bearing biotite granite. The second type has a more euhedral, aplitic texture, with zoned alkali feldspar present. Accessory minerals include zircon, magnetite, augite, and trace hornblende. Concentrations of Ba, Rb, and Sr in both types scatter widely, suggesting that the detritus was derived from a different area or different level in a similar crustal block. Geochemical comparison between the exposed Precambrian granitic rocks from the Beartooth region with the Torchlight granitic clasts shows a similar geochemical as well as petrological characteristics. Our study suggests that the Beartooth region could have provided some, if not all of the igneous detritus to the foreland basin. This implies that sufficient Paleozoic unroofing must have occurred before the Frontier time (ca. 85-90 Ma) in response to the incipient Laramide Uplift.