REVISING AGE AND DISTRIBUTION OF THE PICTURE GORGE BASALT AND THEIR IMPLICATION FOR EVOLUTION MODELS OF COLUMBIA RIVER BASALT VOLCANISM
Temporally, PGB has been considered coeval with Grande Ronde volcanism. Existing PGB ages are poor and conflict with recent 40Ar/39Ar dates of stratigraphically younger basalts, U-Pb dates of intercalated tuffs, and magnetic reversals within the CRBG. We report the first 40Ar/39Ar ages obtained on PGB lavas and dikes. These new dates range between 17.23 and 15.76 Ma, and indicate PGB is older and erupted longer than all other main-phase CRBG units. Initiation of CRBG volcanism falls in the center of this flood basalt province at Picture Gorge. The earlier concept of a south-to-north migration of main-phase CRBG, that many recent models propose to explain CRBG magmatism have explored, is now untenable given our new high precision 40Ar/39Ar ages.
Geochemically, PGB samples display low 87Sr/86Sr, elevated Ba and K, and overall low concentrations of high-field strength elements (HFSE), specifically Nb and Ta. These geochemical characteristics have been interpreted to represent a back-arc like mantle and a magmatic source component unrelated to the other main-phase CRBG units. Combining our ages with the strongest arc-like but depleted geochemical signal of PGB among CRBG units, indicates shallowest metasomatized back arc-like mantle was tapped first with later units (Steens, Imnaha) showing increased influence of a deeper plume-like source. This newly identified temporal and geochemical signal offers an exciting new constraint for CRBG evolution models.