Reshuffling the Columbia River Basalt chronology-Picture Gorge Basalt, the earliest- and longest-erupting formation

摘要

The Columbia River Basalt Group (CRBG) is the world's youngest continental flood basalt province, presumably sourced from the deep-seated plume that currently resides underneath Yellowstone National Park in the northwestern United States. The earliest-erupted basalts from this province aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. We explore the Picture Gorge Basalt (PGB) formation of the CRBG, and discuss the location and geochemical significance in a temporal context of early CRBG magmatism. We report new ARGUS-VI multicollector Ar-40/Ar-39 incremental heating ages from known PGB localities and additional outcrops that we can geochemically classify as PGB. These Ar-40/Ar-39 ages range between 17.23 +/- 0.04 Ma and 16.06 +/- 0.14 Ma, indicating that PGB erupted earlier and for longer than other CRBG main-phase units. These ages illustrate that volcanism initiated over a broad area in the center of the province, and the geochemistry of these early lavas reflects a mantle source that is distinct both spatially and temporally. Combining ages with the strongest arc-like (but depleted) geochemical signal of PGB among CRBG units indicates that the shallowest metasomatized backarc-like mantle was tapped first and concurrently, with later units (Steens and Imnaha Basalts) showing increased influence of a plume-like source.