Alkalic basalts and included ultramafic xenoliths from the Geronimo Volcanic Field (GVF), southeast Arizona, have been analyzed for their major and trace element compositions. Chemical, petrographic, and physiographic distinctions define two groups of lavas and a temporal evolution for the suite. Basalts on the uplifted flanks of the San Bernardino Valley (3.5 to 9 Ma) range in composition from alkali olivine basalt to hawaiite and mugearite. Most are slightly to moderately evolved (Mg value, 0.30 to 0.66). Major and trace element concentrations show well-defined linear correlations with Mg value, and incompatible trace element ratios are distinct (Zr/Nb > 4, La/Sm <5). They contain relatively few xenoliths, although megacrysts are common. Younger cinder cones and flows (0.25 to 3.0 Ma), which cover the valley floor, are predominantly basanites with lower Zr/Nb (<4) and greater light rare-earth element enrichment (La/Sm >6) than flank basalts. Valley basanites exhibit generally higher Mg values (0.52 to 0.68), many of which are within the range for liquids calculated to be in equilibrium with mantle olivine compositions. Major and trace element concentrations correlate poorly with Mg values in most cases. Mafic and ultramafic xenoliths, as much as 45 cm in diameter, are abundant.

Flank basalts record the effects of significant polybaric fractional crystallization that occurred as these magmas established conduits to the surface. Fractionation at moderate pressures involved the phases aluminous clinopyroxene + aluminous spinel ± olivine. The compositions and modal proportions of fractionated phases are similar to those observed in Type II (Al-augite series) xenoliths and megacrysts. Younger lavas (<3.5 Ma) were probably generated at greater depths in the mantle, but escaped to the surface through a crust “primed” by previous magma passage, undergoing less extensive fractional crystallization. The distinctive Zr/Nb ratios demand that the source areas of flank and valley basalts be distinct; these sources differ largely in the modal proportion of clinopyroxene relative to garnet and in the proportion of garnet + clinopyroxene relative to olivine and orthopyroxene. The compositional spectrum observed in the younger lavas reflects differences in the degree of partial melting.