MINERALOGICAL AND GEOCHEMICAL COMPOSITION OF SAMPLES OF POMONA MEMBER, SADDLE MOUNTAINS BASALT

We present and discuss preliminary routine electron probe microanalysis and X-ray fluorescence analysis data obtained from a group of selected samples of the 12 Ma Pomona Member of the Saddle Mountains Basalt (SMB) formation of the Columbia River Basalt Group (CRBG) of north-western USA. We compare and contrast these data to those of representative samples of other units of the CRBG from our database. Much has been written of the homogeneity of the CRBG, but detailed investigations have shown that careful selection of chemical parameters provides distinctive criteria and insight with regard to petrological processes. The Pomona basalts have consistently high (up to 8.11 wt%) MgO concentrations. Compared to representative Grande Ronde Basalt (the highest volume CRBG formation) and Imnaha Basalt samples, the Pomona basalts have the lowest normative apatite and the highest values of normative diopside. The trace and rare earth element patterns are similar to those of Imnaha Basalt, rather than to other, more enriched, members of the SMB. The Pomona Member samples we studied have no glass and contain plagioclase and augitic clinopyroxene as dominant matrix and groundmass phases. The compositions of plagioclase microcrysts are labradoritic to bytownitic. Whole rock compositions were taken as proxies of the liquid composition, so we could apply principles of Ca-Na plagioclase - liquid exchange to estimate pre-eruptive magmatic water contents to be less than 2.4 wt%. Preliminary pressures and temperatures of the magmas were estimated from pyroxene – liquid equilibria to be 0.27 to 0.56 GPa and 1175 to 1201°C, respectively. These values, and the positive correlation between calculated P and T, suggest that magma storage occurred at mid-crustal levels or deeper, which is consistent with our conclusions from a previous study of CRBG Grande Ronde basalts. Significantly, the temperatures calculated from the Pomona magmas are higher than those of all other selected samples across the CRBG entire section. The temperatures and geochemical character of the Pomona rocks suggest that considerable clarification about the nature and evolution of CRBG primary magmas will be obtained by future studies of the Pomona Member.