Columbia River flood basalts from a centralized crustal magmatic system

Abstract

The Columbia River Basalt Group in the northwestern United States, derived from flood basalt eruptions that occurred 16 million years ago, exhibits variability in geography and trace element geochemistry that has led to a number of proposed magma origins. However, the geochemical variability can be explained by a relatively simple model in which magma is derived from a mantle plume that assimilated continental crust in a centralized magma system. The Columbia River Basalt Group in the northwestern United States1, comprising about 230,000 cubic kilometres of rock, exhibits unusual patterns in lava distribution, geochemistry and its apparent relationship to regional tectonics. Consequently, there is little consensus on the origin of its magmas2,3,4,5,6,7,8,9,10,11,12. Here, we examine the isotopic ratios of Sr, Nd, Pb and Os and trace-element abundances in Columbia River basalts. The results suggest that most of the lava was produced when magma derived from a mantle plume assimilated continental crust in a central magma chamber system located at the boundary between the North American craton and the accreted terranes of Idaho and Oregon. Other, related basalts are the product of mixing between the mantle plume and different types of regional upper mantle. Magma was then transported over a wide region by an extensive network of dykes, a process that has been identified in other flood basalt provinces as well13. Interactions of the plume with surrounding upper mantle, and of mantle-derived magmas with regional crust, provide a relatively simple6,7,9,14,15 model to explain the more unusual features of the main-phase Columbia River Basalts.