PETROGENESIS OF THE ALKALINE MAFIC UPPER MIOCENE-PLIOCENE VOLCANISM FROM THE SAN FRANCISCO DE BATUC BASALTIC FIELD, SONORA, MEXICO

The San Francisco de Batuc basaltic field is located 30 km east of Hermosillo, Sonora, Mexico. These lavas were emplaced in a southern Basin and Range tectonic paleo-basin limited by NNW-SSE normal faults and occurred in two pulses: The initial pulse (8 Ma) was followed by a period of terrigenous deposition, covered by the second pulse (4 Ma). Both volcanic pulses had fissural and strombolian eruptions, scattered evidences of hydrovulcanism occurred in the second pulse. Abundant plagioclase with deformed twinning, olivine kink banding, clinopyroxene and spinel xeno-megacrysts, as well as gabbroic and ultramafic xenoliths, suggest a fast ascent of the magma with no magmatic chamber justified. The lavas have a porphyric texture with oriented groundmass that varies from intergranular to interstitial, and rare presence of doleritic and ophitic-mottled textures.

These lavas are classified as mildly alkalic basalts and hawaiites, with moderate silica subsaturation, normative nepheline and TiO₂ > 2%, characteristics of the alkaline-sodic series. Multielemental diagrams show OIB-type and E-MORB-like patterns. The low Ni (50-150 ppm), Cr (75-300 ppm) and the relatively low #mg (56-65), show that these rocks are evolved respect to primary magmas, but little fractionated. The Th/La ratios are similar to primitive mantle (~0.1), and indicates a low crustal assimilation, while the Nd/La (> 1) suggest an asthenospheric mantle source. The Sr and Nd isotopes suggest a PREMA mantle reservoir. Trace elements petrogenetic models indicate a low partial melting (1-4%) of a lherzolitic source with spinel and garnet (3 Gpa). The presence of anorogenic alkaline volcanism in the San Francisco de Batuc basaltic field reflects that upwelling of asthenosphere was already established in this region in the upper Miocene.