Geochemical Evidence for a Lithospheric Source for Magmas from Acoculco Caldera, Eastern Mexican Volcanic Belt

Abstract

This study reports new geochemical and Sr-Nd-Pb radiogenic isotope data on eight samples of hynormative Quaternary basaltic-to-rhyolitic volcanic rocks from Acoculco caldera, located in the state of Puebla, Mexico, at the eastern part of the Mexican Volcanic Belt (MVB). The isotopic ratios of basaltic rocks are as follows: 87Sr/86Sr ∼ 0.70383, 143Nd/144Nd ∼ 0.51283, 206Pb/204Pb ∼ 18.78, 207Pb/204Pb ∼ 15.60, and 208Pb/204Pb ∼ 38.49. These isotopic ratios are somewhat similar to the rift-related basic rocks from Los Humeros caldera, eastern MVB, as well as from the Sierra de Chichinautzin monogenetic field along the volcanic front of the central MVB. For comparison, all samples from basalt to rhyolite in the Acoculco area show the following ranges: 87Sr/86Sr 0.703824-0.706413, 143Nd/144Nd 0.512685-0.512837, 206Pb/204Pb 18.778-18.810, 207Pb/204Pb 15.599-15.628, and 208Pb/204Pb 38.487-38.608. The basaltic rocks from the Acoculco caldera do not show the characteristic negative Nb anomaly with respect to Ba and Ce, as is typical of such magmas in volcanic arcs. Available geochemical and isotopic evidence does not support the generation of these basic magmas by direct (slab melting) or indirect (fluid transport to the mantle) participation of the subducted Cocos plate. They could be generated in the upper mantle from a lithospheric source, similar to that proposed recently for the nearby Los Humeros caldera. The evolved basaltic trachyandesitic, basaltic andesitic, and rhyolitic magmas could have originated from such basaltic magmas through assimilation coupled with fractional crystallization. All available geological, geochemical, and geophysical evidence also supports the recently proposed tectonic-petrogenetic model for the origin of magmas in the MVB.