REVISING AGE AND DISTRIBUTION OF THE PICTURE GORGE BASALT AND THEIR IMPLICATION FOR EVOLUTION MODELS OF COLUMBIA RIVER BASALT VOLCANISM

The Picture Gorge Basalt (PGB) of the Columbia River Basalt Group (CRBG) has been limited in its spatial distribution and volume (2,400 km³) and thought to not extend far from its type locality at Picture Gorge. In our study however, we observe lavas with PGB composition among previously mapped but unnamed, or newly discovered basalt lavas and dikes across a wide swath of the Malheur and Ochoco Forest, between John Day and Burns and eastward in eastern Oregon. Based on these field and geochemical data, PBG lavas covered a significantly greater area than the previously published maps. These findings indicate PGB’s magmatic volume was much larger.

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.