Jurassic volcanic succession in the Charcas area, San Luis Potosi: contribution to understanding the Nazas Arc in northeastern Mexico [Sucesión volcánica Jurásica en el área de Charcas, San Luis Potosí: Contribución al entendimiento del Arco Nazas en el noreste de México]
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This paper reports field, petrographic, geochemical and zircon U-Pb geochronological data for a volcanic succession several tens to hundreds of meters thick, unconformably overlying Triassic marine sediments of the Zacatecas Formation and covered in erosional unconformity by clastic deposits of La Joya Formation of Callovian-Oxfordian age, west of Charcas, San Luis Potosi. This volcanic succession has been correlated by lithological similarity and stratigraphic position with small comparable outcrops, distributed from the northern states of Durango, Zacatecas and southern Coahuila, to southern Nuevo Leon, Tamaulipas, and the western part of the state of San Luis Potosí. The studied rocks correspond geochemically to andesites, dacites and rhyolites, showing a wide compositional range (SiO2 = 53-82%25). Trace element geochemistry was used to determine the tectonic setting, with rare earth element diagrams chacterized by enrichments in light rare earth elements and a small negative Eu anomaly, while multi-element diagrams show large ion lithophile element (LILE) enrichment, with a clear negative anomaly in Nb and Ta. Similarly, tectonomagmatic discrimination diagrams indicate a volcanic arc tectonic environment. We report the first isotopic dating for this locality, from the U-Pb analysis of 29 zircons with laser ablation multi-collector ICP-MS, which yielded an age of 179 Ma for a rhyodacitic pyroclastic flow. The interpretation of these data supports the correlation of this volcanic succession, in both geotectonic setting and age, with rocks exposed in other localities of the Lower Jurassic continental volcanic arc in northeastern Mexico. These rocks have been considered part of the Lower Jurassic continental volcanic arc located along the paleo-Pacific margin of Pangea.
