THE SIGNIFICANCE OF CONTEMPORANEOUS OLIVINE BASALTS AND BASANITES TO COLUMBIA RIVER BASALT GROUP GENESIS – EVIDENCE FOR LITHOSPHERIC DELAMINATION?

The compositions and distribution of small volume olivine basalt and basanite flows that erupted contemporaneously and co-spatially with later Miocene flows of the Columbia River Basalt Group (CRBG) may resolve an ongoing controversy associated with the genesis of this large igneous province. Though the ca. 13.8 Ma olivine basalts erupted significantly after the extensive Imnaha and Grande Ronde stages of the CRB, chemically these flows have major element and trace element signatures (n=6; SiO₂=48.2-51.1 wt%, MgO=7.4-8.7, Cr=250-480 ppm) indicative of high degrees of partial melting and prove equally or more primitive than the most mafic Imnaha lavas. Stratigraphically younger (12-10.8 Ma) basanites are lower in silica (n=6; SiO₂=40.8-48.4, MgO= 6.7-8.2, Na₂O=3.60-5.39) than both the olivine basalts and Imanaha lavas and are surprisingly sodic. The olivine basalts have euhedral Fo_85-82 phenocrysts in equilibrium and that contain chromite inclusions. Oxygen isotopes for these olivines have higher del ¹⁸O than the global average for mantle melts. Euhedral to skeletal basanite olivines are more fayalitic (Fo_80-70) and in equilibrium. Trace element compositions of basanites (Zn/Fe*1000 = 12.9-15.2; Sr/Y=55-80) and high Mn contents of basanite olivines are indicative of an eclogite source whereas the most primitive olivine basalts have a robust peridotite signature (Zn/Fe*1000=9.8-11.1; Sr/Y=14.33). When these source characteristics are considered in tandem with the eruptive positions of the lavas, within the highest density of Chief Joseph dikes and above the edge of the fast seismic Wallowa anomaly seen at mantle depths of <150 km, they support the following interpretion: the CRBG initiates with arrival of a mantle plume beneath NE Oregon and eruption mainly of evolved basaltic andesites (Imnaha and Grande Ronde stages). Underplating and injection of mafic magma into the lower lithosphere during the early CRBG partially transformed the lower lithosphere to eclogite. Eclogite delamination of a drip-like volume of lithosphere seen in the geophysical data allowed a few high-melt fraction plume melts (olivine basalt) to reach the surface. Melting the foundering eclogite would explain the later basanites, though melting edges of a ruptured Farallon slab cannot be ruled out.