Geology and insights into volcanotectonic interactions of small-shield volcanoes south of Penjamillo Graben, Michoacán-Guanajuato Volcanic Field

Abstract

The Michoacán-Guanajuato Volcanic Field, the largest volcanic field in Mexico, is located in the central part of the Trans-Mexican Volcanic Belt. In this volcanic field, more than 1200 monogenetic volcanoes have been recognized, from which at least 378 are small-shields. Small-shield volcanoes are not the most abundant within this monogenetic field; however, they are important because they emplace large volumes of lava flows. Little work has been done on these volcanoes, so many unanswered questions exist about their origin, distribution, morphology, volume, and tectonic control. Based on geological mapping, stratigraphy aided by new ⁴⁰Ar/³⁹Ar radiometric dates, petrographic, chemical, and structural analyses, we characterized five small-shield volcanoes south of the Penjamillo graben to establish the eruptive chronology of the study area and evaluate their tectonic control. Stratigraphy and radiometric ages obtained from the literature and the five new ⁴⁰Ar/³⁹Ar ages reveal that more than a dozen small-shield volcanoes were emplaced in the region throughout the Pleistocene between 2.09 and 0.11 Ma. The rocks associated with these volcanoes have andesitic calc-alkaline composition, with a mineral assemblage composed of plagioclase, pyroxene, olivine, and Fe-Ti oxides. The region is characterized by being structurally complex, suggesting that tectonic structures favor the ascent and emplacement of mafic magmas to form small-shield volcanoes. Moreover, the local tectonic structures controlled the morphology, volume, and spatio-temporal distribution of small-shield volcanoes south of Penjamillo Graben.