PROVENANCE OF THE LAHOOD AND NIEHART FORMATIONS OF THE BELT BASIN: RELATIONS TO ARCHEAN AND PROTEROZOIC CRYSTALLINE BASEMENT

The LaHood formation is the basal unit of the Belt Supergroup in southwestern Montana and lies unconformably on Archean basement in most locations. Although known for its distinctive, coarse-grained arkosic sands and conglomerates, finer-grained lithologies have also been assigned to the LaHood. There is, therefore, some ambiguity regarding the extent to which these finer grained lithologies are part of the LaHood or they represent LaHood inter-fingering with younger units (e.g., Greyson). It is also unclear whether the LaHood is equivalent to the Neihart quartzite (Little Belt Mountains), which has been considered basal Belt or even “pre-Belt” in age. We have measured U-Pb ages of zircons from: 1) samples collected from both the Neihart quartzite (quartz arenite) and a range of LaHood lithologies and locations (coarse- and fine-grained), 2) for some cobbles included within the classic LaHood outcrops in Jefferson Canyon (Tobacco Root Mountains), and 3) from Archean and Proterozoic crystalline basement west of the western limit of LaHood exposures. These data suggest that the provenance of the LaHood changes little as the lithology changes from conglomerate to siltstone, but that the conglomerate’s matrix has a more restricted provenance than the siltstones. For the LaHood and Neihart units, detrital zircon data provide limited constraints on depositional ages (youngest zircons are 1.71 Ga, Neihart; 1.78 Ga, LaHood). Despite this similarity, ages of detrital zircons from the Neihart and all LaHood lithologies have very different age-distributions; Niehart is dominantly Paleoproterozoic and LaHood is dominantly Archean. In particular, the dominance of Eoarchean detritus in the LaHood (~3.5 Ga) and its absence in the Neihart clearly indicate distinct and separate provenances for the two units. In addition, ages of pre-Belt basement in the Tendoy Range (1.78, 1.89, and 2.45 Ga) and Armstead anticline (2.7 Ga) are either minor constituents (e.g., 1 grain) or not present in any of the LaHood samples. This suggests that deposition in spatially and sedimentologically restricted basins (fault bounded?) was an important process in the development of the both the LaHood and the Neihart units, as suggested by their contrasting lithologies and detrital zircon populations.