Geochemical and Sr-Nd isotopic characterization of the Miocene volcanic events in the Sierra Madre del Sur, central and southeastern Oaxaca, Mexico
The Etla, Mitla-Tlacolula and Nejapa volcanic regions in central and southeastern Oaxaca comprise the southeastern part of a wide Cenozoic magmatic arc in the Sierra Madre del Sur. Most volcanic events in these regions ocurred between 22 to 15 Ma, almost contemporaneously with the initial volcanic events of the Trans-Mexican Volcanic Belt. Petrographic, geochemical, and isotopic characteristics were determined for representative volcanic samples from the three regions, where ignimbrites, volcaniclastic and epiclastic deposits, lava flows and minor lacustrine deposits are found. In a SiO2 vs. alkalis diagram, chemical classification of volcanic products for the study area indicate variations from basaltic andesites to rhyolites, following a subalkaline trend, but with a bimodal pattern. Components with SiO2 concentrations between 58 to 67 wt. % are absent. The trace-element patterns for andesites and rhyolites are similar, with enrichment in the large-ion lithophile elements relative to the high- field-strength elements. Chondrite-normalized REE patterns display light rare earth element enrichment (La-Sm) with respect to the heavy rare earth elements (Eu-Lu), which show flat patterns. These chemical characteristics are typical of volcanic arc rocks. Initial Sr and Nd isotopic data show certain differences between samples from the Etla and Mitla-Tlacolula regions (87Sr/86Sr: 0.7047 to 0.7066 and εNd: -1.15 to 1.75) and the Nejapa region (87Sr/86Sr: 0.7035 to 0.7048 and εNd: +0.52 to +1.42). These data and those reported for the basement rocks suggest greater involvement of continental crust for the magmas of the two first regions in comparison to magmas of the Nejapa region. The isotopic compositions are similar to those observed in other volcanic regions of the Sierra Madre del Sur. Early to middle Miocene volcanic events in central and southeastern Oaxaca, together with the contemporaneous initial magmatic events in the Trans-Mexican Volcanic Belt, could conform a magmatic arc with an anomalous orientation before attaining its present position.
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