INTERMEDIATE-COMPOSITION LAVAS, GOLDFIELD-SUPERSTITION VOLCANIC FIELD, CENTRAL ARIZONA: HYBRIDS OF MIXED BASALTIC AND SILICIC MAGMAS

Intermediate-composition lavas compose a portion of the Miocene, ~20.5 to 18 Ma, Goldfield-Superstition volcanic province (G-SVP) east of Phoenix. These lavas have compositions that cluster as andesite, trachyandesite, dacite, and trachydacite in a SiO₂-total alkalis plot. They erupted after rhyolite domes and alkalic basalt lavas initiated G-SVP volcanism, but prior to voluminous silicic pyroclastic and lava outpourings that form most of the province. Their exposures cover ~100 km². Modally, most andesitic-dacitic lavas have distinctive plagioclase phenocrysts (2-6 mm; ~15-30 vol%) with spongy and resorbed oligoclase-andesine (An_25-42) cores surrounded by thin, labradorite (An_50-65) rims. Other phenocrysts in varying amounts (≤6 vol%) are amphibole, biotite, clinopyroxene (Mg# 87-72), and olivine (iddingsite); quartz occurs as xenocrysts. The lavas have SiO₂ ~57-68 wt.%; MgO ~1.5-4.5%; Fe₂O₃ ~4-7%; K₂O ~2.5-3.5%; LREE modestly enriched, La_(n) 80-200 and La/Yb_(n) 10-24; Sr ~500-1300 ppm, Nb ~10-25, and Zr ~80-260. Trace element abundances define two subgroups: at a given MgO, one has higher Zr, Nb, Hf, P₂O₅, and lower Sr and La/Nb than the other. Compared to G-SVP most primitive basalts (~10-12% MgO), the intermediate lavas have lower abundances of incompatible elements La, Ti, P, Nb, Th.

Important for interpreting origins of the andesitic-dacitic lavas are (1) their stratigraphic position above the rhyolite domes and basalt; (2) calcic plagioclase rims on reacted sodic cores; and (3) the abundances of some incompatible elements lower than those in the basalts. All are consistent with early G-SVP basalt magmas and silicic melts mixing to yield intermediate hybrid magmas. The silicic components of the hybrids were partial melts of lower crust now represented by the early rhyolite domes, as some domes have appropriately low La, Ti, Zr, Nb, P, Th, and sodic plagioclase. The compositional ranges among the collective intermediate hybrid lavas (e.g., SiO₂ 57-68%) are due more to different proportions of silicic and basaltic melts that mixed than due to differentiation. The two trace-element subgroups suggest more than one source composition for the silicic melts and/or for the basaltic magmas. The silicic melts represent the first expression of lower crust melting, which marked the onset of G-SVP magmatism.