Origin and crustal contamination of magmas of the Chiapanecan Volcanic Arc in Southern Mexico: Constraints from new geochemical, isotopic, and geochronologic data

The processes related to the heterogeneous magma compositions of the Chiapanecan Volcanic Arc (CVA) are still poorly understood. Here we present new field descriptions combined with U-Pb zircon geochronology, petrography, and geochemistry data on poorly studied volcanic structures that surround the active El Chichon volcano in the northwestern part of this arc. These consist of the Santa Fe and La Mina intrusive bodies, volcaniclastic fans (La Catedral, and newly discovered Juarez and El Azufre) as well as Miocene volcanics. The mineralogy of the studied rocks consists of plagioclase + orthopyroxene + clinopyroxene + amphibole + biotite, +/- Fe-Ti oxides, +/- olivine, +/- quartz (quartz only in the Santa Fe intrusive), and zircon as accessory phase. Young populations of zircons from El Azufre and Ju ' arez volcaniclastic deposits were dated by U-Pb at similar to 2.1 Ma, being among the oldest yet-found magmatic products of the CVA. Strikingly, the Miocene volcanics have similar chemical characteristics as the products of the modern arc. Based on geochemical data, we deduce that magma genesis in the CVA takes place in the mantle wedge, where it is constantly modified by aqueous fluids from the slab, causing relatively high Ba/La and Th/Yb ratios. The Sr-87/Sr-86 and Nd-143/Nd-144 ratios, zircon geochronology, and trace elements strongly suggest that crustal contamination was the dominant mechanism of differentiation of the magmas, supported also by Ce/Pb and Nb/U ratios. Furthermore, inherited zircons suggest the presence of Grenvillian and Permian (Chiapas Massif) rocks underneath the CVA. The contamination of the magmas by these basement rocks may be responsible for the K2O-enrichment in all CVA rocks.