Origin of bimodal rear-arc volcanism, Trans-Mexican Volcanic Belt eastern sector: Geochemical and isotopic evidence from the Quaternary Xihuingo-La Paila Volcanic Field

The Quaternary Xihuingo-La Paila Volcanic Field comprises basalts and basaltic andesites (0.8-0.5 Ma) that built many scoria cones, shield volcanoes, and contemporaneous (similar to 0.4 Ma) dacitic dome structures, emplaced in the rear-arc area of the Trans-Mexican Volcanic Belt eastern sector. Most basalts and basaltic andesites exhibit LILE/HFS ratios related to subduction processes. Nevertheless, some basaltic cones are alkaline in composition, with trace-element patterns that indicate an enriched mantle source, but Ba and Pb positive anomalies are also present. Although dacitic rocks also display typical trace-element patterns for subduction settings, a simple magmatic liquid line of descent cannot relate the basalts to dacites. The Sr and Nd isotopic values (Sr-87/Sr-86 = 0.70399 to 0.70522 and epsilon Nd = +0.25 to +4.80) for felsic and mafic rocks suggest a small interaction of magmas with continental crust components, and the Pb isotopic ratios ((206)pb/Pb-204 = 18.68-18.78; (207)pb/Pb-204 = 15.58-15.63 and (208)pb/(204)pb = 38.44-38.62) show that all volcanic products form a mixing line between a mantle-like component and a radiogenic component like the Palaeozoic Acatlan complex. A slab rollback process probably produced the magmatic migration of the volcanic front to the Middle American trench during the Late Miocene-Quaternary, which favoured the ascent of the asthenosphere to the mantle wedge. Melting of this heterogeneous mantle then produced mafic magmas that intruded the lower continental crust, and dacitic products could be formed by the partial melting of these mafic intrusions emplaced by frequent injections at the base of the continental crust.