Geochemical evidence for a lithospheric source for magmas from Los Humeros Caldera, Puebla, Mexico.

Abstract

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 MVB. The isotopic ratios of mafic basaltic rocks range as follows: super 87 Sr/ super 86 Sr = 0.703350-0.703387, super 143 Nd/ super 144 Nd = 0.512867-0.512888, super 206 Pb/ super 204 Pb = 18.849-18.866, super 207 Pb/ super 204 Pb = 15.593-15.598, and super 208 Pb/ super 204 Pb = 38.490-38.491. These values are similar to the rift-related mafic rocks from the volcanic front of the central MVB. For comparison, the entire suite from basalt B to rhyolite R in the LHC shows the following ranges. super 87 Sr/ super 86 Sr = 0.703350-0.704678, super 143 Nd/ super 144 Nd = 0.512702-0.512888, super 206 Pb/ super 204 Pb = 18.710-18.866, super 207 Pb/ super 204 Pb = 15.593-15.631, and super 208 Pb/ super 204 Pb = 38.444-38.594. The basaltic rocks from Los Humeros do not show the characteristic negative Nb anomaly with respect to Ba and Ce, as is typical of such magmas in volcanic arcs. The available geochemical and isotopic evidence does not support the generation of the LHC mafic magmas by direct slab melting, nor by indirect fluid transport to the mantle participation of the subducted Cocos plate. Instead, they could be generated in the upper mantle from a lithospheric source. The evolved andesitic to rhyolitic magmas could have been originated from such mafic magmas through assimilation coupled with extensive fractional crystallisation. Two different compositions representing lower LC and upper crusts UC as well as a hypothetical crust HA were tested as plausible assimilants for the assimilation coupled with fractional crystallisation process.

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