Geochemical Evidence for Slab Melting in theTrans-MexicanVolcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo^Zita ¤ cuaro volcanic field (VBZ) in central Mexico indi-cate that slab melting plays a key role in the petrogenesis of theTrans-Mexican Volcanic Belt. Rocks from the VBZ are typical arc-related high-Mg andesites, but two different rock suites with distinct trace element patterns and isotopic compositions erupted concurrently in the area, with a trace element character that is also distinct from that of other Mexican volcanoes. The geochemical differences between the VBZ suites cannot be explained by simple crystal fractionation and/or crustal assimilation of a common primitive magma, but can be reconciled by the participation of different proportions of melts derived from the subducted basalt and sediments interacting with the mantle wedge. Sr/Yand Sr/Pb ratios of theVBZ rocks correlate inversely with Pb and Sr isotopic compositions, indicating that the Sr and Pb budgets are strongly controlled by mel