The Tancítaro–Nueva Italia region (∼4400 km2) is located within the west-central Mexican volcanic belt and spans ∼80 km of arc length along the volcanic front. On the basis of 65 40Ar/39Ar ages, it is shown that a total volume of ∼326 ± 57 km3 of magma has erupted from >200 vents since 1.2 Ma. Nearly a third of this volume (103 ± 5 km3) was erupted as crystal-rich andesite from a large stratovolcano, Volcán Tancítaro, whereas the remaining two-thirds (∼223 ± 52 km3) span the complete range from 51 to 63 wt% SiO2, are relatively crystal-poor, and were erupted from peripheral vents. The total proportions of erupted lava types are ∼6%–7% basalt, ∼18% basaltic andesite, and ∼74%–76% andesite; there is no rhyolite and only a trace of dacite.
There is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment. Based on phenocryst abundances and modes, major- and trace-element data, as well as phase-equilibrium experiments from the literature, the following mechanisms for formation of andesite in this region can be ruled out: (1) crystal fractionation of basalt/basaltic andesite in the upper crust, (2) magma mixing of basalt/basaltic andesite with dacite/rhyolite, and (3) assimilation of granitoid by basalt/basaltic andesite. Instead, the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting (>20%) of hornblende-rich (∼40%) gabbronorite. The basaltic andesites are required, owing to their elevated Ni concentrations, to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two-pyroxene fractionation in the deep crust.