GEOCHEMICAL AND ISOTOPIC EVIDENCES OF THE MAGMATIC SOURCES IN THE XIHUINGO-LA PAILA VOLCANIC FIELD, EASTERN SECTOR OF THE TRANS-MEXICAN VOLCANIC BELT

The Xihuingo-La Paila Volcanic Field (XPVF) is located in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB); a Miocene-Quaternary continental volcanic arc related to the subduction of the Cocos Plate beneath the North American Plate. The distribution of volcanism in the area is controlled by NE-SW fault and fractures systems, as it is evidenced by the alignment of most volcanic structures. The XPVF is composed of different volcanic structures formed during two main events. In the first event andesite lava flows and rhyolitic pyroclastic deposits were emplaced during the Miocene (~13.6-9.8 Ma). After a hiatus of c.a. 10 Ma, the second event produced cinder cones and shield volcanoes of basaltic and basaltic andesite compositions (872 ± 22 Ka; K-Ar), as well as dacitic domes (403 ± 11 Ka; K-Ar).

The Miocene rocks display homogeneous trace-element patterns typical of magmatic arcs (LILE enrichment with respect to HFSE and positive anomalies of Pb and Sr). In contrast, the Pleistocene rocks show heterogeneous trace-element patterns: basalts and basaltic andesites do not show a clear enrichment of LILE respect to HSFE, and Nb and Ta anomalies are absent, whereas the dacitic domes present a pronounced subduction signal (Ba/Nb= 75-80). The pleistocenic mafic rocks have the higher REE concentrations respect to dacitic domes, suggesting different magmatic sources. The SiO₂ vs La/Yb diagram shows a positive correlation for basalts and basaltic andesites indicating the presence of garnet in the magmatic source.

Sr and Nd isotopic data indicate variable crust assimilation processes. Thus, the Miocene rocks display variable and radiogenic values (⁸⁷Sr/⁸⁶Sr from 0.7033 to 0.7047 and εNd from -0.23 to 5); the mafic rocks also show variable radiogenic values (⁸⁷Sr/⁸⁶Sr from 0.7040 to 0.7052 and εNd from 1.3 to 4.8), and dacitic domes present homogeneous values (⁸⁷Sr/⁸⁶Sr from 0.7046 to 0.7047 and εNd from 2.2 to 2.8). Pb isotopic data display for the volcanic products a step-mixing trend between a mantle component and ²⁰⁷Pb-rich reservoir.

According to these evidences we can establish that the magmas emplaced in the study area were produced from a heterogeneous mantle source with complex magmatic processes combined with different interaction degrees between the magmas and continental crust.