Intraplate-type volcanism (late Oligocene to Quaternary) occurs in México on both the North American and the Pacific plates. Oceanic localities are voluminous shield volcanoes on or near fossil-spreading ridges. Subaerial rocks of these volcanoes form either geochemically continuous and coherent rock series or bimodal suites. Low-pressure crystal fractionation of alkali basalt and assimilation of hydrothermally altered rocks from the volcanic pile determined the compositions of the mafic-intermediate rocks at Socorro. Trachytes at Socorro were formed by partial melting of alkali basalt, and rhyolites through crystal fractionation of parental trachytes. These felsic rocks also assimilated hydrothermally altered rocks.

Continental localities (late...