Late Cenozoic basalts of southwestern New Mexico

Chemical analyses of 208 basaltic rocks from 13 occurrences in southern New Mexico show the existence of two differing basalt types. The group I basalt occurrences contain both alkalic and tholeiitic varieties with low SiO₂ and high MgO and TiO₂ concentrations. Group II basalts are 100% tholeiitic and contain high SiO₂ and low MgO and TiO₂ concentrations. Differences in Y, Zr, Rb, Sr and Ba concentrations also exist between the two groups. Spatially, the group I basalts occur in the center of the area and are flanked on the west and east by the group II basalts; this distribution is roughly parallel to the proposed boundaries of the Rio Grande rift.