A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence

Abstract

Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with those of selected oceanic basalts. Although many recent models for CRBG petrogeneses do not include a plume component, spidergrams for the American Bar flows of the Imnaha Basalt of the CRBG have broad humps at Ba, Th, and Nb (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr/ 86Sr data. This plume may have been identical to that presently beneath Yellowstone. The inferred hot spot track lies almost directly east-west, and requires a vector of motion of the Pacific Northwest part of the North American plate significantly different from that inferred by Minster and Jordan (1978) [31]. Our proposed vector for the North American plate agrees, however, with that suggested by the Anahim Belt, a volcanic lineament in southern British Columbia that has been proposed as a hot spot track. This proposed vector requires alterations in current models for Pacific and North American plate movement during the Cenozoic.