CRUSTAL GROWTH BY TECTONIC ACCRETION OF ISLAND ARC TERRANES: RADIOISOTOPIC PROVENANCE OF LATE PALEOZOIC–MESOZOIC ROCKS OF THE BLUE MOUNTAINS, WESTERN UNITED STATES

Paleozoic–Mesozoic terranes of western North America represent numerous intra-oceanic and peri-cratonic subduction complexes, volcanic arcs, and fragments of ophiolitic crust that were tectonically integrated into the plate margin prior to development of an Andean-type ocean-continent subduction system in Cretaceous time. There is much debate concerning the nature of accreted crust (i.e., juvenile or evolved) and the timing of crustal accretion. Constraining these variables is critical for developing accurate models of continental growth. We used mudrock Sm-Nd and trace-element geochemistry and detrital zircon U-Pb geochronology of deep-marine turbidites in the Blue Mountains Province, Oregon and Idaho, to assess a 105 Ma period of continental growth in the region (ca. 250–145 Ma). Shale in the outboard Wallowa terrane has juvenile Nd isotope compositions (ε_Nd = +8 to +4) and depleted-mantle model ages (T_DM) <1.0 Ga, indicating deposition of juvenile-arc detritus derived predominantly from a depleted mantle source. U-Pb detrital zircon ages from associated sandstones bear no Paleozoic or Precambrian zircons. Age-equivalent deposits of the inboard Olds Ferry-Izee terranes have ε_Nd isotope values -4 to 0 and T_DM 1.6 to 1.2 Ga, indicating more compositionally evolved arc detritus. Associated sandstone samples contain abundant Archean–Mesozoic detrital zircon grains. These contrasting crustal fragments are overlain by ca. 159–145 Ma deep-marine turbidites of the Coon Hollow Formation with ε_Nd isotope values +4 to +2 and T_DM 1.1 to 0.8 Ga—values that are intermediate with respect to the underlying accreted terranes. These results suggest a three-stage process of: (1) magmatic crustal generation within an intra-oceanic juvenile and a continent-fringing evolved arc; (2) tectonic amalgamation of juvenile arc crust to offshore continent-fringing arc crust; and (3) subsequent tectonic accretion of the amalgamated mass of new and reworked crust to the continent. An important implication is that only the mass of juvenile crust within the amalgamated terranes represents true growth of continental crust. The amalgamated terranes then acted as a crustal substrate for future arc growth and magmatic crustal accretion during development of an Andean-type system.