COMPOSITIONAL HETEROGENEITY IN COLUMBIA RIVER BASALT GROUP FLOWS

Columbia River Basalt Group flows always have been thought to be homogeneous because of the relatively narrow compositional range observed in many of the eruptions. Detailed studies over the past several years, however, have shown that many flows are highly variable, both laterally and vertically through a flow. The source of the heterogeneity has been suggested to result from initial magma compositional variability, in situ fractional crystallization and, more recently, compaction. The compositional variation of several Grande Ronde Basalt flows is examined from their dike systems to distal edges. The least variability was found in the Umtanum flow, which consists of approximately 5,000 km³ of lava. The 10,000 km³ Sentinel Bluffs Member has greater variability, especially in the Cohassett flow in central Washington. The Sentinel Bluffs Member has six regional compositional types. The Cohassett flow has compositional layers that are identical to four of the regional compositional types. The order of compositional zoning reflects the sequence of eruptions. Numerical modeling results suggest that in situ fractional crystallization or compaction can not account for the variations observed in the Cohassett flow. The compositional types and the field relations are best explained by rapid changes in magma composition feeding regional flows. The compositional layering developed as new compositions were injected to form the Cohassett flow.