Age and Isotopic Investigations of the Olds Ferry Terrane and its Relations to Other Terranes of the Blue Mountains Province, Eastern Oregon and West-Central Idaho

摘要

The Olds Ferry terrane of the Blue Mountains province is one of the numerous accreted terranes that comprise the western North American Cordillera. The Blue Mountains province is located in central and eastern Oregon, western Idaho, and extreme southeastern Washington, and is a crucial link in reconstructions of the North American Cordillera due to its position in an area with few visible terranes between the more extensively exposed terranes to the south in California and Nevada and to the north in Canada and Alaska.New field evidence and U-Pb zircon geochronology for volcanics within the sedimentary onlap assemblages overlying the Wallowa and Olds Ferry volcanic arc terranes provide evidence for an earlier connection between the terranes than has previously been recognized. The boundary between the Izee and Olds Ferry strata is an angular unconformity based on geochronological data, the consistent angular discordance between the Olds Ferry and Izee strata, and the presence of locally derived volcanic and plutonic clasts of the Olds Ferry arc in the basal Weatherby conglomerate of the Izee terrane. I correlate this unconformity with that present at the base of the transgressive fluvial-deltaic and marine sequence of the Coon Hollow Formation of the Wallowa terrane based on new ages from units bracketing this unconformity. Ages for the basal Coon Hollow and Weatherby Formations allow these to be correlative sedimentary onlap packages and thus demonstrate the connection between the Wallowa and Olds Ferry terranes by Early Jurassic time.Detailed field mapping and geochronology was used to split the Huntington Formation into two members, and establish their relationships to underlying plutonic rocks and the overlying sediments of the Izee basin onlap sequence. Geochemical data support splitting the Huntington Formation into two members, and support a model of eruption in an island-arc environment with a combination of mantle and crustal magma sources distinct from the island arc that is the Wallowa terrane. Geochronologic and structural constraints also provide evidence that Olds Ferry volcanism continued well into the Early Jurassic, and that the lower and upper members of the Huntington Formation are separated by an angular unconformity and rest with nonconformity on the underlying plutonic units. Precambrian xenocrystic zircons in lower and upper Huntington Formation volcanics indicate that the Olds Ferry terrane was proximal to cratonal North America during much of its history. Age correlations made possible by new U-Pb geochronology indicate that the Olds Ferry and Wallowa arcs may have been active concurrently in the early Late Triassic, followed by a period in the Late Triassic when the Olds Ferry arc was volcanically active while the Wallowa arc was quiescent.