Subduction-related volcanism: Field and experimental studies of hydrous lavas in central Mexico

This study provides a comprehensive perspective on the processes involved in generation and eruption of subduction-related volcanism in the central part of the Mexican Volcanic Belt, near Zitácuaro, Michoacán. Although the tectonic setting is one of subduction, there is abundant evidence that large-scale extension is superimposed on the subduction regime. This on-going extension is manifest by the ∼E-W striking normal faults that are prevalent to the north of Zitácuaro, but also occur in the Zitácuaro and Valle de Bravo regions. Many of the regional volcanic features are associated with these E-W normal faults, such as the Irimbo cinder cones (Chapter 1), the Zitácuaro plagioclase-free andesites (Chapter 2), and the El Peñon xenolith-bearing andesite (Chapter 3), indicating that the faults act as conduits for eruption. Other volcanic features in the region, including plateau-forming lava flows, broad (10 km₃) shield volcanoes, and large dome complexes with explosive and pyroclastic deposits (Chapter 1), are not associated with obvious E-W normal faults.; The diverse compositions of the Middle Miocene to Pleistocene Zitácuaro lavas include highly postassic shoshonites and calc-alkaline basalts, basaltic andesites, andesites and dacites, all of which have a decidedly subduction-related chemical signature. These subduction-related lavas, coexist with basalts and basaltic andesites that have the chemical characteristics of ocean island-type lavas. Water saturated and water-CO₂ phase equilibria experiments have been done on the plagioclase-free andesites to provide constraints for their pre-eruptive equilibration conditions. Data from this study and other phase equilibria data has been combined with crystal fractionation and magma mixing models to show that the compositional diversity of the volcanism in the region reflects melting of a remarkably heterogeneous mantle. A discovery of spinel lherzolite and chromite websterite xenoliths in an andesite flow near El Peñon, Mexico supports the hypothesis that the mantle source material beneath this part of central Mexico has a variety of compositions and oxygen fugacities.