SILICIC VOLCANISM AS A TRACER FOR COLUMBIA RIVER FLOOD BASALT VOLCANISM

The Pacific Northwest boasts a complex tectonic history resulting in a quilt of terranes accreted to cratonic North America and stitched with Mesozoic plutons. The trifecta of subduction, extension, and a mantle plume lead to a singularly complex distribution of volcanic rocks, including the Columbia River flood basalts (~2.3 X 10⁵ km³). For more than 4 decades Vic Camp has championed mapping and interpretation of the Columbia River Basalt Group (CRBG) with a focus on the role of the Yellowstone plume in PNW volcanism. A long-standing conundrum is how to reconcile the wide distribution of CRBG dikes with plate motion over a mantle point source, regardless whether the plume originated to make Siletzia or if it initiated with onset of the CRBG (17.0 ± 0.1 Ma). Inasmuch as CRBG may have transported magmas hundreds of miles, the distribution of silicic rocks coeval with the main pulse of CRBG volcanism (17-15 Ma) serves to assess where magma traversed the crust and resided in magma reservoirs.

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.