1 Distributed volcanic fields of the southwestern U.S.

This session invites submissions from various disciplines (e.g., petrology, tectonics, geophysics, geochronology) that explore distributed volcanism in the southwestern U.S. Topics of interest include spatio-temporal trends, magma compositions, and mantle and/or crustal controls on volcanism.

Monday, 18 May 2026
8:00 AM - 12:00 PM
Alvarado A

Unraveling the Geochemical Evolution of a Monogenetic Andesitic Cluster: The case of the Holocene Zacapu cluster (Michoacan, Mexico)

Paper number: 1-7
Program: Technical Sessions
Time: 10:00 AM – 10:20 AM

Authors

Abstract

The origin of intermediate magmas in the Michoacán–Guanajuato Volcanic Field (MGVF) remains incompletely understood, despite their prevalence in the region. While the genesis of andesitic compositions is well-studied in polygenetic systems, fewer studies have addressed monogenetic volcanic fields, where proposed mechanisms include fractional crystallization (FC), crustal assimilation coupled with fractional crystallization (AFC), and mantle metasomatism. In this context, the Zacapu Holocene Cluster (ZHC), in the west-central MGVF, provides a good case to evaluate these processes, representing one of the youngest evolved (andesitic) monogenetic clusters in the region. The ZHC comprises five eruptions within 50 km2, spanning a transition from Strombolian activity at ~3.2 ka to purely effusive eruptions by ~AD 900. Magma compositions evolved from low-to-high SiO₂ andesites (56–64 wt.% SiO₂), offering a well-characterized sequence to test petrogenetic models.

We investigated the relative contributions of FC, AFC, and mantle source heterogeneity using new whole-rock Sr–Nd–Pb–Hf isotope data across the complete eruptive sequence, integrated with existing major and trace element analyses.

Closed-system FC modeling using MELTS v.1.2.0, combined with trace element systematics, demonstrates that major and trace element compositions of the ZHC cannot be reproduced by fractional crystallization of mafic parental magmas alone. AFC modeling, constrained using the most mafic and least Sr-radiogenic samples in the cluster along with four potential crustal assimilants, likewise fails to reproduce the observed Sr–Nd–Pb isotopic systematics, indicating that crustal contamination did not play a significant role.

Given the limited influence of FC and AFC, mantle metasomatism models were carried out to explain the chemical characteristics of the ZHC. Mixing models involving a mantle metasomatized by slab-derived components (altered oceanic crust and sediment-derived fluids) successfully reproduce the ZHC isotopic signatures. Partial melting (3–10%) of this metasomatized mantle also generates the observed trace element abundances.

These results highlight fine-scale mantle heterogeneities beneath the MGVF and provide key insights into the formation of andesitic monogenetic volcanoes. This study addresses a critical gap in understanding intermediate magmas in short-lived, clustered volcanic systems and emphasizes the dominant role of mantle variability over crustal processes in generating andesitic elemental and isotopic signatures.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410613

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