MORPHOSTRUCTURAL PARTITIONAL SITES OF SHOSHONITIC VOLCANISM IN THE CAHULOTE GRABEN, MICHOACÁN GUANAJUATO VOLCANIC FIELD, MÉXICO
The shoshonitic-type rocks are scarce in volcanic arcs (<3 vol.%), typically associated with mafic volcanism, and both suites are commonly connected to extensional tectonics and deep lithospheric magmatic source zones. The Michoacán Guanajuato Volcanic Field (MGVF), located at the central sector of the Transmexican Volcanic Belt (TMVB), has over 2000 volcanic vents, mainly mafic-andesitic calc-alkaline type. Through a holistic approach, including morphostructural, volcano morphometry, petrography, geochemistry and geochronology data, this work presents a detailed study of Cahulote Graben (CG) in the Jorullo-Tacámbaro volcanic region. We have divided the CG into three morphostructural domains containing 116 volcanoes of several types, mainly spatter cones. Morphostructural domains were developed by the interaction of four extensional and transtensional brittle deformation systems: NE-SW, NW-SE, E-W and NNW-SSE. CG is an extensional-transtensional NE-SW tectonic system. Three magmatic calc-alkaline, shoshonitic and intra-plate-like plumbing systems were arranged during the Quaternary (<2 Ma). Petrographic, major and trace elements support relatively fast ascent and different stages of magma stagnation from deep reservoirs. Comparison of Zr/Al and Ti/ Al ratios and Nd–Sr isotopic signatures of CG shoshonites with others reported in the TMVB supports the hypothesis of the contrasting depth of source magmas and strongly suggests an extensive tectonic regime for these shoshonites. A volcanotectonic model is proposed to explain the emplacement mechanism of shoshonitic magmatism.