MAGMATIC EVOLUTION IN THE EASTERN SECTOR OF THE TRANS-MEXICAN VOLCANIC BELT: CHICHICUAUTLA VOLCANIC FIELD, HIDALGO, MEXICO

In the eastern sector of the Trans-Mexican Volcanic Belt we found several volcanic structures related to the subduction of Rivera and Cocos plate under the North-American plate. The Chichicuautla Volcanic Field (CVF) present many volcanic structures such as scoria cones, lava flows, pyroclastic deposits, domes and a collapse structure that were emplaced during two periods: Late Miocene and Pleistocene-Holocene. The volcanic activity in this area was separated by a hiatus of about less than 9 Ma.

According to petrographic features and major element data, the rocks found in the CVF can be classified into two main groups: The first group is represented by a collapse structure named Chichicuautla andesite and by Francisco Sarabia lava flows, which compositions range from andesite to dacite. These rocks display LILE enrichment with respect to HFSE indicating a magmatic source that was modified by fluids derived from subduction processes. The Chichicuautla andesite was dated in 9.82 ± 0.21 (K-Ar method) and display particular geochemical signatures such as high-Sr concentrations (from 400 to 1 000 ppm), low-HREE concentrations (Yb ≤ 2 ppm, Y ≤ 24 ppm) and high Sr/Y and La/Yb relations. These evidences suggest the influence of the slab melting in the magmatic source.

Basaltic-andesite lava flows and scoria cones, emplaced during the Pleistocene-Holocene, represent the second group. These structures are related to a NE-SW normal fault system. The rocks show a minor LILE enrichment with respect to HFSE. In addition, the REE patterns show LREE enrichment with respect to HREE indicating a magma origin related to subduction processes. However, the REE concentrations observed in rocks of this second group are more important than in the first group suggesting different magmatic sources for these events.

Sr and Nd isotopic compositions for the first group show two ranges: the Chichicuautla andesite displays values less radiogenic (⁸⁷Sr/⁸⁶Sr from 0.7035 to 0.7036 and εnd from 2.75 to 4.15) in comparison to the Francisco Sarabia lava flows (⁸⁷Sr/⁸⁶Sr from 0.7046 to 0.7050 and εnd from -1.35 to 0.45), indicating different interaction of these magmas with the continental crust. Isotopic compositions for the second group indicate a more important interaction with the continental crust (⁸⁷Sr/⁸⁶Sr from 0.7040 to 0.7048 and εnd from 0 to 3).