Nd data from the Blue Mountains province, eastern Oregon and western Idaho, clarify terrane correlations and regional evolution of the western Laurentian plate margin during Mesozoic time. We report an Early Jurassic age for a red tuff unit at Pittsburg Landing, Idaho, which is 25 m.yr. older than previous Middle Jurassic estimates. In the Coon Hollow Formation at Pittsburg Landing and at the type location on the Snake River, chemical abrasion thermal ionization mass spectrometry U-Pb zircon ages on interbedded tuff and detrital zircon U-Pb maximum depositional ages indicate that deposition spanned ca. 160-150 Ma, entirely during Late Jurassic time. Detrital zircon U-Pb ages represent local Wallowa arc basement and regional magmatic sources spanning ca. 290-140 Ma. Mudrock Nd isotope compositions of the Coon Hollow Formation record an increase in juvenile magmatism consistent with regional Late Jurassic trends in western North American magmatic systems. These data show that the Coon Hollow Formation is not part of a Middle Jurassic overlap assemblage, as has been historically interpreted. Instead, we propose that the Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest Jurassic time due to a well-documented period of trench retreat in the western United States. Our new data require that the underlying Wallowa terrane was accreted to and received detritus from western North America by ca. 160 Ma (early Late Jurassic). This minimum estimate for the age of terrane accretion in western Idaho and eastern Oregon is substantially earlier than previous estimates (∼135-118 Ma). In the Blue Mountains region, westward expansion of Laurentia was accomplished by accretion of arc terranes to the North American craton prior to Late Jurassic time.

Recent mapping, U-Pb zircon geochronology, trace-element geochemistry, and tracer isotope geochemistry of plutonic and volcanic rocks in the Wallowa and Olds Ferry terranes of the Blue Mountains Province yield new insights into their tectonic evolution and pre-accretion history. Igneous rocks of the Wallowa arc terrane formed in two magmatic episodes of contrasting duration and geochemical characteristics. Magmatism in the first episode lasted for at least 20 Ma (ca. 268–248 Ma), spanning the Middle Permian to the Early Triassic and was of generally calc-alkaline affinity. Rock units associated with this episode include the Hunsaker Creek and Windy Ridge formations of the Wallowa terrane, as well as potentially equivalent tonalite and diorite plutonic rocks in the Cougar Creek Complex and related basement exposures, which show midcrustal levels of the terrane. The second episode of magmatism in the Wallowa arc was remarkably brief (U-Pb zircon dates range from 229.43 ± 0.08 Ma to 229.13 ± 0.45 Ma) and dominated by mafic to intermediate compositions of tholeiitic affinity. Rock units associated with the second episode may include the Wild Sheep Creek and Doyle Creek formations, as well as ubiquitous dikes and plutons in the Cougar Creek Complex and similar basement exposures. After 229 Ma, the Wallowa arc apparently became dormant. The record of igneous activity in the Olds Ferry arc contrasts with that of the Wallowa in its age range and the continuity of calc-alkaline magmatism. Radiometric ages and stratigraphic field relationships allow the magmatic history of the Olds Ferry terrane to be divided into at least three cycles separated by brief hiatuses and collectively spanning the late Middle Triassic through the Early Jurassic (ca. 237– 187 Ma). Rock units related to these episodes are divided by unconformities, and they include the Brownlee pluton, lower Huntington Formation, and upper Huntington Formation. Magmatic activity in the Olds Ferry arc may have persisted until at least 174 Ma, based on the presence of volcanic ash horizons in the lower portion of the overlying Weatherby Formation of the Izee basin. All cycles of Olds Ferry magmatism display generally calc-alkaline affinity. The contrasting magmatic histories of the Wallowa and Olds Ferry arc terranes provide the basis for at least two conclusions. First, these arcs formed as separate tectonic entities, rather than as a single composite arc. Second, progressive closure of the ocean basin between the arcs in the Late Triassic and Early Jurassic was related to continued subduction beneath the Olds Ferry arc, but the Wallowa arc was apparently dormant during much of that interval.

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The Olds Ferry terrane is the more inboard of two accreted volcanic arc terranes in the late Paleozoic−early Mesozoic Blue Mountains province of the northern U.S. Cordillera. We present geologic, geochronologic, and geochemical data from the volcano-sedimentary Huntington Formation of the Olds Ferry arc that place the terrane within a firm temporal and tectonomagmatic context, and establish its identity as a fringing arc terrane along the Triassic to Early Jurassic Cordilleran margin. The Huntington Formation is divided into two unconformity-bounded informal members: a Norian (ca. 220 Ma) lower member comprising a sequence of mafic-intermediate volcanics, massive volcaniclastic breccias, and minor carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through Pleinsbachian (<210−187.0 Ma) upper member composed of massive conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic flows, and interlayered sandstone turbidites, deposited with angular unconformity onto the lower member. An erosional hiatus and regional tilting produced an angular unconformity separating the Huntington Formation from the overlying basal conglomerates of the late Early to Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive onlap sequence. Huntington Formation volcanic rocks are isotopically enriched relative to depleted mantle and coeval igneous rocks in the outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036−0.7057) and εNd values (+5.4 to +3.1) in the upper member volcanics suggests increasing involvement of continental-derived material in their petrogenesis. Precambrian xenocrystic zircons in both lower and upper member volcaniclastic rocks strongly support a proximal location of the Olds Ferry terrane to cratonal North America during much of its history. The chronology and tectonostratigraphic architecture of the Olds Ferry terrane allows its robust correlation to other fringing-arc terranes along the U.S. and Canadian Cordillera.