The Ahualulco volcanic complex: Geochemistry and temporal evolution of Oligocene felsic volcanism in the eastern Mesa Central, San Luis Potosí, Mexico

Abstract

The Ahualulco Volcanic Complex (AVC), in the southern region of the Mesa Central of Mexico, consists of a set of Cenozoic volcanic rocks, which includes andesites. The AVC was emplaced in the middle Eocene, followed by dacitic-rhyolitic lavas, as well as felsic pyroclastic sequences emitted during the Oligocene, some of which contain garnet. This work emphasizes, at least, a second Oligocene andesitic event (Petronilas andesite) that was linked to the genesis of the Cortes ignimbrite, thus forming a bimodal volcanism evidenced by field relations, mineralogy, the correlation of incompatible elements such as Th and Nb and rare earth elements patterns. In addition, new U-Pb crystallization ages were obtained for the Zapatero Rhyodacite (31.7 Ma), the Los Cuervos Rhyolite (30.8 Ma), the garnet-bearing rhyolite El Arenal (31.4 Ma) and the El Negro rhyolitic porphyry (31.1 Ma). Possibly, the emplacement of the bimodal suite, like that of the rocks found to the north, was fissure fed, associated with NW-SE faults originated in the extensional regime between the Oligocene and the Miocene. However, in the central area of the AVC, explosive or caldera-related volcanism might be required to explain certain lithological and stratigraphic features, such as large lithic breccias with abundant granitic clasts and Mesozoic sedimentary basement lithics, in the Palos Colorados and Hienera tuffs.