Back-arc convection model for Columbia river basalt genesis

Abstract

Genesis of the Columbia River Basalt Group (CRBG) may be accounted for by the progressive thermal erosion of an attenuated, heterogenous subcontinental lithosphere (SCL) by back-arc convection behind the Cascade arc. Assembly of the source components of the CRBG began during the Early Mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin. Subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small-degree partial melts from the asthenosphere which solidified in the SCL as pyroxenite veins. The SCL source of the CRBG underwent a further episode of shearing accompanied by attenuation during the Eocene and Oligocene when it was caught between contrasting right-lateral terrane translation and E-W extension regimes in the northern Cordillera and Basin and Range province, respectively. The attenuation resulted in the raising of the geotherm close to that of the pyroxenite and Iherzolite solidi. Melting was triggered by back-arc convection behind the Cascade arc. The pyroxenite was the first component mobilised, constituting the principal source of the Imnaha Formation. Continued back-arc convection resulted in the failure of the SCL and the generation of the voluminous Grande Ronde Formation from pyroxenite and the surrounding SCL. The change to Wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of Proterozoic SCL from the east and asthenospheric material by back-arc convection. The Saddle Mountains Formation was derived entirely from Proterozoic SCL. Small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the Saddle Mountains Formation comprise three mixing lines and do not have any age significance.