The Quaternary Moctezuma volcanic field: A tholeiitic to alkali basaltic episode in the central Sonoran Basin and Range Province, México

The Quaternary Moctezuma volcanic field at the foothills of the Sierra Madre Occidental, México, is characterized by a close association of tholeiitic and alkaline magmas. The vents of the tholeiitic magmatism (1.7 Ma), which forms the overall mesa morphology of the volcanic field, lie along the major faults that define the eastern margin of the Moctezuma basin. This half-graben formed during the early Miocene as indicated by the emplacement of basaltic flows (22.3 Ma), which were intercalated with fanconglomerates of the Báucarit Formation. The youngest volcanics are alkaline lavas (0.53 Ma), which erupted from scoria cones located in the center of the basin. The main mineralogical difference between the Quaternary tholeiitic and alkaline lavas is the composition of the clinopyroxenes: they are Ca-rich in the alkaline lavas, whereas they are subcalcic with orthopyroxene or pigeonite in the tholeiitic lavas. The geochemical data show a gradual change in the characteristics of the lavas. The Sr and Nd isotopic compositions, and the shape of the mid-oceanic-ridge basalt (MORB)-normalized patterns, are similar to oceanic island basalt suites, suggesting that the source of the Moctezuma volcanic field lavas was dominated by asthenospheric mantle. The absence of mantle or granulite xenoliths in the Moctezuma volcanic field lavas, compared to the neighboring Geronimo volcanic field, reflects slower ascent rates. The Southern Cordillera Basaltic Andesite (SCORBA)-type geochemical signature of the early Miocene synextensional basalts of Moctezuma compared to the Quaternary Moctezuma volcanic field lavas can be correlated with a progressive thinning of the mantle lithosphere during the Neogene.