Petrogenesis of the Holocene Tacambaro Monogenetic Volcano Cluster in the Michoacan-Guanajuato Volcanic Field, Mexico

The Trans-Mexican Volcanic Belt (TMVB) comprises several monogenetic volcanic fields, including the Michoacán-Guanajuato Volcanic Field (MGVF), located in the central TMVB and considered one of the most active volcanic fields in the world. Within the MGVF many monogenetic volcanoes erupt in clusters that occur closely in space and time (Siebe, 2017), yet the relationship among volcanoes within such clusters is not well understood. The Tacámbaro cluster consists of four Holocene volcanoes that erupted between 3650 to 320 BC, with an average recurrence interval of ~1000 years (Mahgoub et al., 2017). These eruptions became progressively more silicic with time, ranging from basaltic andesite to andesite. We are conducting a detailed petrologic and isotopic study to determine if these magmas are petrogenetically related via a long-lived magma reservoir, or instead reflect multiple episodes of magma generation from one or more sources. Within the Tacámbaro cluster, highly incompatible trace element abundances (e.g., Th, Zr) increase progressively with time and silica, consistent with closed-system fractional crystallization of a single magma. However, increasing La/Yb ratios are inconsistent with this model, and require either multiple magma batches produced by variable degrees of partial melting of a single mantle source, crustal assimilation, or a heterogeneous mantle source. A progressive decrease in Nb/U, and increase in Sr and Pb isotopes ratios with silica and with time, are inconsistent with variable degrees of melting of a single source, and instead require a heterogeneous mantle source or crustal assimilation. Trends of Sr and Pb isotope ratios vs. silica are qualitatively consistent with assimilation of upper crust as represented by crustal xenoliths from nearby localities. However, the Pb and Sr isotope ratios of samples from the Tacámbaro cluster overlap with those of the nearby Jorullo and Paricutin volcanoes, which have been previously interpreted to reflect mantle source compositions. As such, the data from the Tacámbaro cluster may indicate progressive melting of a heterogeneous mantle source that has been variably enriched by slab fluid and subjected sediment. These scenarios will be tested further using Os isotopes in olivine-matrix pairs.