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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 Oligocene to Quaternary) occur scattered north of the Trans-Mexican Volcanic Belt; their location is independent of older volcanic provinces and boundaries between today's geologic/tectonic provinces. Intraplate-type rocks have high TiO2, Nb, and Ta contents, and host mantle and/or lower crust xenoliths ± megacrysts. Many fields are in the southern Basin and Range and the most extensive and voluminous were contemporaneous with normal faulting. The locations of some fields suggest that magma ascent was influenced by regional structures. However, normal faulting is minor or absent in many other Mexican fields.

The petrogenetic processes in late Oligocene to Miocene magmas differ from those in Plio-Quaternary magmas. Slow ascent of magmas formed during early stages of extension-favored assimilation-fractional crystallization and gravitational settling of xenoliths. Plio-Quaternary xenolith-bearing magmas traveled faster through cooler crust where brittle structures caused by extension were able to propagate deeper. Geochemical evidence of assimilation is more subtle in the younger volcanic rocks.

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