A North-South Transect across the Central Mexican Volcanic Belt at  100 W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Within the Zita ¤ cuaro^Valle de Bravo (ZVB) region of the central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary: (1) high-K2O basaltic andesites and andesites; (2) medium-K2O basaltic andesites, andesites and dacites; (3) high-TiO2 basalts and basaltic andesites.The dominant feature of the first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but the dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine.The spatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280^330 km from the Middle AmericaTrench (MAT), is