GEOCHEMICAL ANALYSIS AND PROVENANCE OF THE LAHOOD FORMATION OF THE BRIDGER RANGE, MONTANA

The LaHood Formation in the Bridger Range exceeds 1,000m in thickness and is composed of alternating layers of coarse, pebbly sandstone and fine siltstone. Sandstones are mostly arkosic, typically containing more than 40% feldspar (McMannis, 1963). Exposures of the LaHood along the western flank of the Bridger Range constitute the most easterly outcrops of this formation. The significance of the LaHood to the rest of the Belt stratigraphy is not well defined. Its limited outcrop area along the southern margin of the basin (Helena embayment) makes stratigraphic relations to other “higher” Belt units difficult to define precisely. Consequently, we have undertaken a geochemical and detrital zircon study of the LaHood. Initial REE analyses show typical continental crust Eu-anomalies. La values are split into two sets: one being 80 to 50 times chondritic values and the other being 30 times chondritic values. Respectively, Yb values are 7 times chondritic values and 2 times chondritic values. Sm/Nd model ages (T_dm) range from 2.32 to 3.58 Ga, while Sm-Nd analyses yield ε­_Nd(0) of -33.9 to -21.0. These contrast with previous results from outcrops further west, which show only Archean model ages (Frost and Winston, 1987) and only Archean detrital zircons (Mueller et al., 2008). There seems to be no correlation between grain size and T_dm. U-Pb geochronology of detrital zircons of the LaHood Formation and of nearby metaigneous zircons (Fussell, 2015) is currently ongoing and will help define the formation’s provenance and better constrain its stratigraphic position in the Belt Supergroup.