Case study of a cluster of simple and complex monogenetic volcanoes in the north-east part of the Michoacan–Guanajuato Volcanic Field, Central Mexico: Nomenclature implications

Abstract

With the advent of new terminologies to categorize and characterize the simple and complex monogenetic volcanoes, also came the semantic issues, which caused a predicament for the usage of terms like simple, complex, polycyclic, polymagmatic, complex monogenetic volcanoes with polygenetic inheritance. To analyse and validate this nomenclature, we studied an overlapping volcanic structure located south of the present-day town of Irapuato, Central Mexico, that appears to be a monogenetic complex at first sight. Field observations, tephra stratigraphy, petrography and geochemistry of the tephra deposits confirms that the structure is in fact a cluster of three simple (San Joaquin tuff ring and two scoria mounds) and one complex, polycyclic, polymagmatic (La Sanabria-San Roque tuff ring) monogenetic volcanoes formed by independent events, governed by distinct conduits and magma bodies of different origin (subduction-related, OIB and E-MORB origin) and separated by different tephra sequences of dissimilar components and depositional characteristics. We estimate the magma volumes (using the juvenile content and their vesicularity percentage) to be at 0.40–1.31 × 10⁸, 0.25 × 10⁸ and 0.42–0.90 × 10⁸ m³ for San Joaquin, La Sanabria and San Roque, reckoning an eruption duration of 77 and 48 and 81 days, respectively (considering an average eruption rate of 6 m³/s from the well-documented shallow crater (<30 m), Ukinrek maars in Alaska) occurring within the age range of 40–70 k years (crater diameter and depth ratio). This study not only aided to validate the above-mentioned terms for monogenetic volcanoes, but also reconsider a few of them and avoid confusion with the polygenetic counterparts.