Petrogenesis of xenolith-bearing basalts from southeastern Arizona

The Geronimo lavas consist of Quaternary alkali-olivine basalt and basanite which have been erupted over 850 km² in the San Bernardino valley, southeastern Arizona. The lava field contains flows, cones of cinder and agglutinate, and several large maars up to 1 km in diameter with associated tuff rings. The lavas contain plagioclase, titaniferous clinopyroxene, olivine, titanomagnetite, and rare spinel, ilmenite, and nepheline. Xenoliths in lavas and tuffs include Iherzolite, websterite, gabbro, clinopyroxenite, and a phlogopite-bearing kaersutite peridotite. Megacrysts of olivine, clinopyroxene, kaersutite, potash oligoclase, and spinel occur in abundance. The lavas exhibit considerable chemical diversity and contain more alumina and alkalies than typical alkali-olivine basalts; some varieties contain up to 6.5 percent Na₂O and 3.4 percent K₂O. Barium (600-900 ppm) and strontium (700-1100 ppm) concentrations are substantial.

Thermodynamic calculations for equilibrium between a basanite melt and Iherzolite residual material, using melt components of Al₂O₃, SiO₂, NaAlSi₃O₈, Fe₂SiO₄, and CaMgSi₂O₆, yield a solution of 1400°C and 30 kbar, equivalent to a depth of 96 km. A megacryst assemblage gives a solution of 1290°C and 13 kbar (42 km), and a phenocryst assemblage gives a solution of 1100°C and 5 kbar (16 km). The lavas could be generated by 10 percent partial melting of pyrolite, whereas Iherzolite requires unrealistically small degrees of partial melting (<0.5 percent). The fractionation of high-pressure megacrysts, and phenocrysts at lower pressures, enables the derivation of all analyzed lavas from a single primary basanitic magma.