THE MESOPROTEROZOIC LAHOOD FORMATION: PROVENANCE AND TECTONIC ENVIRONMENT OF DEPOSITION

The LaHood Formation plays a critical role in constraining the sources and tectonic setting during the initial stages of formation of the Mesoproterozic Belt Basin, particularly the Helena Embayment of the eastern Basin. In the Bridger Range the LaHood Formation comprises coarse conglomerates, lithic arkoses, carbonaceous shales, and interbedded limestones. Twelve measured stratigraphic sections show a general fining to the northeast with limestone beds exposed only in the northern half of the Range. The sandstones are lithic arkoses with plagioclase > microcline and significant abundances of biotite and white mica. U-Pb ages of detrital zircons (LA-ICP-MS) from four sandstones show: 1) The basal conglomerate and coarse arkosic facies in the lower and southern two sections have a primary concentration at ~3.2 Ga with a minor ~3.5 Ga component; 2) zircons from an arkosic sandstone inter-fingered with limestone has a dominant ~3.2 Ga signal, but includes a significant secondary concentration near 2.8 Ga, which corresponds to the age of the Beartooth-Bighorn Magmatic Zone; 3) the northernmost section exhibits a concentration near 3.2 Ga, a more significant ~2.8 Ga signal, and the first occurrence of ~1.8 Ga grains, indicative of the Great Falls Tectonic Zone. These detrital zircon ages confirm the results of Guerrero et al. (2015) in the Bridger Range and from LaHood exposures in the Horseshoe Hills to the west, but differ significantly from the dominant 3.4-3.5 Ga signal from LaHood exposures in Jefferson Canyon and north of Cardwell, MT (Ross and Villeneuve, 2013; Mueller et al., 2016). Collectively, the detrital zircon ages demonstrate a) occurrences of the LaHood Formation across the Belt Basin received sediment from temporally distinct sources that do not necessarily include proximal Archean basement, and b) the stratigraphic succession reflects contributions from progressively younger source areas. The significant variety of ages and abrupt facies changes in the LaHood support syn-depositional extension within multiple localized basins. The definition of the LaHood Formation should be restricted to the coarse clastic units (conglomerates and arkosic sandstone), and the intercalated calcareous shales and limestones should be assigned to the Newland Formation.