Geochemistry of Eocene felsic volcanic rocks from the Mesa Virgen‐Calerilla, Zacatecas, Mexico: Implications for the magma source and tectonic setting

The Mesa Virgen Calerilla (MVC) is located in the state of Zacatecas, Mexico. The most intense volcanism, which occurred during the Eocene, formed extensive ignimbrite deposits exposed in some parts as lava spills of rhyolitic (felsic) composition. This felsic volcanism may represent much of the MVC. This study describes whole‐rock geochemistry and mineralogy data from felsic volcanic rocks in the MVC to address their petrogenesis and tectonic setting. The MVC covers a compositional spectrum ranging from trachyte, dacite, to high‐silica rhyolite. The petrography and mineral assemblages indicate that the felsic rocks are composed of K‐feldspar (sanidine), quartz, plagioclase, and biotite. The felsic volcanic rocks have a composition of 64.08–78.17 wt% (SiO2)adj, 0.14–0.69 wt% (TiO2)adj, and 0.11–0.62 wt% (MgO)adj with 12–54 Mg number [Mg# = 100 × (Mg2+/[Mg2+ + Fe2+])]. These felsic volcanic rocks showed enrichment in light rare earth elements (LREE; [La/Sm]N = 3.80–7.19), and are depleted in heavy rare earth elements [HREE; (Tb/Yb)N ratios 0.35–1.84], along with negative Ba, Nb, Sr, P, Eu, and Ti anomalies. The geochemical characteristics and petrogenetic modelling indicate that felsic volcanic rocks are derived from partial melting process of an upper‐middle continental crust. The tectono‐magmatic model and multidimensional tectonic discrimination diagram indicate that an extensional‐related setting prevails for the MVC. The graphical presents a diagrammatic illustration of the subduction zone of the Farallon Plate, which is mostly below the North American Plate. This observation supports the argument that that the young and small Farallon Plate has torn and the remaining shallow steepening slab is rolling back that between the Farallon and North America slabs exists a gap that increases in size at depth. This gap allows magma from the deep mantle (asthenosphere), below the subducted Farallon slab, to rise through the mantle wedge, resulting in extensional‐like felsic magma in the Mesa Virgen‐Calerilla (MVC) during Eocene.