Moctezuma

The Moctezuma volcanic field of central Sonora, Mexico is located at the western margin of the Sierra Madre Occidental. The field covers the central part of the half-graben which forms the Moctezuma Valley, overlying limestones, granites, and volcanics of Sierra La Madera comprising the footwall. The field covers ~380 km2, representing a total volume of <2 km3 of lava, two-thirds of which are early tholeiitic flows and the remainder late alkaline. Alkaline volcanism is focussed around five monogenetic scoria cones located in the centre of the graben. These rocks are mainly hawaiites containing <5% normative nepheline, with some transitional alkaline rocks plotting as alkaline on a TAS plot without containing normative nepheline. All lavas are xenolith free suggesting a slower ascent rate to surface, and geochemistry suggests a metasomatized garnet-bearing upper mantle source for the melts. Petrography and geochemical data, including mineral chemical, Sr-Nd isotopic, and whole rock major and trace element can be found in Paz Moreno et al. (2003).