Geochemical evidence for a lithospheric source for magmas from Acoculco caldera, eastern Mexican Volcanic Belt

This study reports new geochemical and Sr-Nd-Pb radiogenic isotope data on eight samples of hy-nonnative Quaternary basaltic-to-rhyolitic volcanic roc ks from Acoculco caldera, located in the state of Puebla, Mexico, at the ea stern part of the Mexican Volcanic Belt (MVB). The isotopic ratios of basal tic rocks are as follows: Sr-87/Sr-86 similar to 0.70383, Nd-143/Nd-144 sim ilar to 0.51283, Pb-206/Pb-204 similar to 18.78, Pb-207/Pb-204 similar to 1 5.60, and Pb-208/Pb-204 similar to 38.119. These isotopic ratios are somewh at similar to the rift-related basic rocks from Los Humeros caldera, easter n MVB, as well as from the Sierra de Chich-inautzin monogenetic field along the volcanic front of the central MVB. For comparison, all samples from ba salt to rhyolite in the Acoculco area show the following ranges: S-87/S-86, 0.703824-0.706413, Nd-143/Nd-144 0.512685-0.512837, Pb-206/Pb-204 18.778-1 8.810, Pb-207/Pb-204 15.599-15.628, and Pb-208/Pb-204 38.487-38.608. The ba saltic rocks from the Acoculco caldera do not show the characteristic negat ive Nh anomaly with respect to Ba and Ce, as is Epical of such magmas in vo lcanic arcs. Available geochemical and isotopic evidence does not support t he generation of these basic magmas by direct (slab melting) or indirect (f luid transport to the mantle) participation of the subducted Cocos plate. T hey could be generated in the upper mantle from a lithospheric source, simi lar to that proposed recently for the nearby Los Humeros caldera. The evolv ed basaltic trachyandesitic, basaltic andesitic, and rhyolitic magmas could have originated from such basaltic magmas through assimilation coupled wit h fractional crystallization.,All available geological, geochemical, and ge ophysical evidence also supports the recently proposed tectonic-petrogeneti c model for the origin of magmas in the MVB.