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.

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