SILICIC VOLCANISM AS A TRACER FOR COLUMBIA RIVER FLOOD BASALT VOLCANISM
CRBG-related silicic volcanic rocks amount to about 5500 km3 dominated by ignimbrites. Calderas and dome-flow complexes define a footprint of > 25,000 km3, concentrated near the craton boundary with A-type varieties at the height of the mafic pulse. Silicic centers are absent or scarce around Steens Mountain, near the postulated sites of long-lived Steens and Imnaha-Grande Ronde mafic magma reservoirs. Silicic volcanism typically postdates local CRBG, which is mimicked by crustal contamination in Grande Ronde basaltic andesites relative to underlying, parental Imnaha basalt as well as increased crustal contamination upsection in Steens Basalts. Estimates of as much as 10 to 25% assimilation of crustal melt for CRBG basaltic andesites (more than 60 % of the CRBG volume) yields a conservative estimate of 17,000 km3 of silicic melt.
The eastward march of silicic magmatism defining the Yellowstone plume track is not clear until after ~12 Ma. Impingement and flattening of the Yellowstone plume against the thick lithosphere beneath (now) SW Idaho and upflow along lithospheric topography between the craton and accreted terranes does much to account for wide distribution of CRBG-aged silicic volcanism near the craton margin and by inference the basalt drivers. With apologies to the many not referenced in this summary.