MAGMATIC EVOLUTION IN THE APAN-TLÁLOC REGION, EASTERN SECTOR OF THE TRANS-MEXICAN VOLCANIC BELT: SR, ND AND PB ISOTOPIC AND GEOCHEMICAL EVIDENCES

The Apan-Tláloc volcanic region (ATVR) is located in the eastern sector of the Trans-Mexican Volcanic Belt. In this region exists domes, cinder and scoria cones with lava flows associated, shield volcanoes and pyroclastic deposits. Almost all structures are related to a NE-SW normal fault system. The rocks were grouped into two volcanic events: a Lower Volcanic Event (≈13.4 to 9.82 Ma) with andesites to porphyric dacites and rhyolitic ignimbrites, and an Upper Volcanic Event (≈2.1 to 0.188 Ma) composed of basalts, basaltic andesites and trachy-basaltic andesites that were erupted from monogenetic volcanoes.

All volcanic rocks are of calc-alkaline nature but their SiO₂ contents are contrasting: more than 65 wt. % in the Lower Event and less than 57 wt. % in the Upper Event. Trace element patterns are relatively heterogeneous for both volcanic events. Lower volcanic rocks show LILE enrichment with respect to HFSE as well as negative anomalies of P and Ti, and positive anomalies of Pb, suggesting a depleted mantle source origin modified by subduction fluids. In contrast, Upper volcanic rocks do not show a clear enrichment of LILE with respect to HFSE and negative Nb and Ta anomalies are absent. Negative P and Ti anomalies and positive Ba and Pb patterns are also observed. These patterns suggest a less depleted mantle source, which was also affected by fluids derived from subduction processes. REE patterns for both volcanic events show enrichment of LREE with respect to HREE, confirming a magma origin related to a subducted slab. Isotopic compositions range as follow for rocks of the Lower Volcanic Event: ⁸⁷Sr/⁸⁶Sr from 0.7033 to 0.7063, epsilon-Nd from -4.04 to +4.29, ²⁰⁶Pb/²⁰⁴Pb from 18.61 to 18.76, ²⁰⁷Pb/²⁰⁴Pb from 15.53 to 15.60 and ²⁰⁸Pb/²⁰⁴Pb from 38.24 to 38.59. For rocks of the Upper Event isotopic values range: ⁸⁷Sr/⁸⁶Sr from 0.7040 to 0.7048, epsilon-Nd from -0.55 to +3.57, ²⁰⁶Pb/²⁰⁴Pb from 18.64 to 18.79, ²⁰⁷Pb/²⁰⁴Pb from 15.53 to 15.61 and ²⁰⁸Pb/²⁰⁴Pb from 38.26 to 38.59.

All geochemical results suggest a depleted heterogeneous magmatic source with a strong influence of subduction products for rocks of the Lower event, whereas rocks of the Upper Volcanic Event were generated from a relatively enriched mantle source affected by subduction fluids. The last volcanic events could represent a back-arc magmatic activity.