AGE AND ORIGIN OF THE BRIDGER GNEISSES: IMPLICATIONS FOR PROVENANCE OF THE LAHOOD FORMATION

The Bridger Range occupies a critical tectonic location in the Precambrian crystalline basement of SW Montana because it lies near the eastern limit of the Belt Basin and the western limit of the Great Falls tectonic zone. Six basement samples were taken from the Bridger Range near Bozeman, MT. The range is comprised predominantly of metaigneous lithologies; mostly granitic gneisses with some amphibolites. Although it is widely considered to be the deepest basin on the planet, the Belt Basin lacks the significant research that would allow for its history to be thoroughly unraveled. This project aims to understand the basement rocks that underlie the layers of sedimentary rocks in the basin. Dating of these rocks will bring a greater understanding as to the creation of the basin and the source of the sediments (i.e., local crystalline basement or more distal sources). These data will also help us to relate the Bridger basement to other crystalline basement in the Wyoming province (e.g., relation to the Beartooth Mountains [2.8 Ga] or the Tobacco Root Mountains [3.2 Ga]). REE analyses of the Bridger gneisses show that La is near 100 times chondritic values and Yb is 1 to 10 times chondritic values suggesting derivation from a garnet-bearing source. There are a range of Eu-anomalies, both positive and negative, throughout the suite of samples. Sm-Nd analysis give ε(0) values from -49.1 to -23.6 and T_dm values from 2.56 to 3.78 Ga. Zircons extracted from these samples were ablated in an ICP-MS to determine U and Pb isotopes and Lu and Hf isotopes. Other studies being performed at the University of Florida on the earliest sedimentary rock layers in the Belt Basin (Guerrero, 2015) will aid in the understanding of the formation of the basin.