The Columbia River Flood Basalt Province: Current Status

Abstract

The Columbia River flood basalt province is smaller by an order of magnitude than the Deccan, Karoo, Parana, and Siberian continental flood basalt provinces. Its smaller size, relative youth (17-6 Ma), excellent exposure, and easy accessibility have allowed development of a flow-by-flow stratigraphy in which ' many flows can be traced across the Columbia Plateau, often linked directly to their strongly oriented feeder dikes in the southeast quadrant. The detailed stratigraphy provides a precise record of the changes in magma composition and volume with time and demonstrates more clearly here than in other provinces that single fissure eruptions had volumes in excess of 2,000 km 3 and flowed across the plateau for distances up to 600 km with negligible changes in chemical or mineralogical composition. Current evidence suggests that the Columbia River flood basalts resulted from impingement of a small mantle plume, the Yellowstone hotspot, on the base of the lithosphere near the Nevada-Oregon-Idaho border at 16.5 Ma and that the main focus of eruption then moved rapidly north to the Washington-Oregon-Idaho border from where the main eruptions occurred. The rapid northerly translation of the main eruptive activity may have been controlled by weakened or thinned zones in the lithosphere. The few earliest flows have typical mantle plume compositions and the last, small-volume flows are contaminated by continental crust. In between, the great majority of flows carry a strong lithospheric signature, the source of which remains controversial-either an enriched continental lithospheric mantle or assimilated continental crust. The physical nature and rate of magma eruption are also controversial. Recent work suggests flows grew by internal injection rather than by turbulent surface flow and this has been used to imply significantly lower eruption rates than previously envisaged. However, the chemical and mineralogical homogeneity of single Columbia River basalt flows across many hundreds of kilometers implies that eruption and flow rates were still exceptionally high.