SEDIMENTARY TECTONICS OF THE MESOPROTEROZOIC LAHOOD FORMATION, EASTERN BELT BASIN , SOUTHWEST MONTANA

The Mesoproterozoic LaHood Fm in the Helena Embayment of the eastern Belt Basin includes basal coarse conglomerates, lithic arkoses, carbonaceous shales, and interbedded limestones. This succession plays a critical role in constraining the provenance and tectonic setting of sediments deposited during the initial stages of formation of the eastern Belt Basin. In the Bridger Range, sedimentary structures include graded and cm-scale rhythmic bedding, and minor channel deposits that exhibit rapid facies changes, with a fining upward sequence to the NW. Cobbles in the conglomerates include tonalitic, granitic, and amphibolite gneisses; arkoses contain high modal abundances of K-spar and muscovite. Chondrite-normalized rare earth element (REE) abundances in lithic arkoses show LREEs of ~15-240x with pronounced negative Eu anomalies. U-Pb (LA-ICP-MS) ages of detrital zircons from four arkoses show: 1) the arkosic matrix of the basal conglomerate has a major component at ~3.2 Ga and a minor component at ~3.5 Ga, corresponding to the major crust-forming events in the northern Wyoming Province; 2) stratigraphically higher arkosic sandstones inter-fingered with calcareous shales have zircons of ~3.2 to ~2.8 Ga, corresponding to ages in the Beartooth-Bighorn Magmatic Zone; and 3) zircons from interlayered arkoses and shales in the northernmost section yield ages of 3.2, 2.8, and 1.8 Ga, indicating a source component in the Great Falls Tectonic Zone in addition to those previously noted. Lu/Hf systematics yield a range of εHf values from +3.3 to -12.85 at 3.2 Ga. Tdm (Sm-Nd) ages are 3.2-3.4 Ga for the arkosic matrix of the conglomerate, with ɛNd values ranging from -27.0 to -41.7. Two Paleoproterozoic Tdm were obtained from calcareous shales (2.35 and 2.51 Ga) with ɛNd -21.9 and -22.7. These data indicate that the provenance of the LaHood Formation included a variety of distal sources not limited to proximal Archean basement and that the higher stratigraphic levels reflect incorporation of progressively younger components. The proposed tectonic setting is a series of isolated basins formed in a transtensional environment associated with the break up of Columbia. Coeveal deposition of the arkosic sandstones of the LaHood Formation and calcareous shales of the upper Newland Formation suggest these basins coalesced with time.