Geochemical evidence for a lithospheric source for magmas from Los Humeroscaldera, Puebla, Mexico

This study reports new geochemical and Sr-Nd-Pb radiogenic isotope data on hy-normative Quaternary basaltic to rhyolitic volcanic rocks as well as two samples of Mesozoic to Palaeozoic intrusive igneous rocks from Los Humeros caldera (LHC), located in the eastern part of the Mexican Volcanic Belt (M VB). The isotopic ratios of mafic basaltic rocks range as follows: Sr-87/Sr -86 = 0.703350-0.703387, Nd-143/Nd-144 = 0.512867-0.512888, Pb-206/Pb-204 = 18.849-18.866, Pb-207/Pb-204 = 15.593-15.598, and Pb-208/Pb-204 = 38.490-3 8.491. These values are similar to the rift-related mafic rocks from the vo lcanic front of the central MVB. For comparison, the entire suite from basa lt (B) to rhyolite (R) in the LHC shows the following ranges. Sr-87/Sr-86 = 0.703350-0.704678, Nd-143/Nd-144 = 0.512702-0.512888, Pb-206/Pb-204 = 18.7 10-18.866, Pb-207/Pb-204 = 15.593-15.631, and Pb-208/Pb-204 = 38.444-38.594 . The basaltic rocks from Los Humeros do not show the characteristic negati ve Nb anomaly with respect to Ba and Ce, as is typical of such magmas in vo lcanic arcs. The available geochemical and isotopic evidence does not suppo rt the generation of the LHC mafic magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Coc os plate. Instead, they could be generated in the upper mantle from a litho spheric source. The evolved andesitic to rhyolitic magmas could have been o riginated from such mafic magmas through assimilation coupled with extensiv e fractional crystallisation. Two different compositions representing lower (LC) and upper crusts (UC) as well as a hypothetical crust (HA) were teste d as plausible assimilants for the assimilation coupled with fractional cry stallisation process. (C) 2000 Elsevier Science B.V. All rights reserved.