DETRITAL ZIRCON DATA SUPPORTS A NEOPROTEROZOIC PHASE OF TILTING WITHIN THE LEMHI ARCH IN EAST-CENTRAL IDAHO

East of the Salmon River in the Lemhi Range and Beaverhead Mountains of Idaho is a >10 km thick succession of Mesoproterozoic strata correlative with the Belt Supergroup. This southwestern extent of the ~1450 to ~1380 Ma Belt basin is spatially coincident with a westward deflection of the Neoproterozoic to Cambrian rift margin of western Laurentia. In contrast to along-strike localities in SE Idaho and NE Washington where Neoproterozoic to Cambrian rocks are >5 km thick, an ~15-20° angular unconformity separating Mesoproterozoic rocks from overlying Middle Ordovician quartzites is exposed in E-C Idaho, defining the Lemhi arch unconformity. Prior work documents this unconformity was formed, in part, during late Cambrian-Early Ordovician time, but missing stratigraphic record beneath the Lemhi arch unconformity precludes further constraints on the tectonic history of the region. However, in the southernmost Lemhi Range near South Creek in Howe, ID, recent work correlated exposed strata with the Neoproterozoic and Cambrian upper Wilbert Formation. Additionally, these strata overlie an unidentified quartzite unit in an angular unconformity. In the Lost River Range to the west, the Wilbert Formation is ~1 km thick, and includes Neoproterozoic strata that is difficult to differentiate from Belt strata. Thus, the identity of a previously unclassified quartzite underlying Neoproterozoic to Cambrian strata in angular unconformity at South Creek was uncertain. The unclassified quartzite ranges from very fine to fine-grained with colors ranging from reds, purples, to grays, with partial recrystallization in fractures containing pervasive hematite. Samples were collected from three locations and dated using LA-ICPMS. Preliminary DZ results indicate that the quartzite has bimodal age distributions with dominant modes of ~1700 ± 50 Ma (interpreted as deriving from Mazatzal-Yavapai provinces) and ~1440 Ma (enigmatic source of Belt age). DZ data are consistent with inclusion of the unclassified quartzite as Belt Supergroup. This suggests that the lower, Neoproterozoic Wilbert Formation is missing beneath the southern Lemhi Range, and that an early phase of tilting and exhumation of the Lemhi arch occurred after Mesoproterozoic sedimentation and before Ordovician, likely during Neoproterozoic time.