Abstract

GARCIA-TOVAR, Gloria P. et al. Geology, geochronology and geochemestry of the Plio-Quaternary volcanism of the Apán-Tecocomulco Volcanic Field, Transmexican Volcanic Belt. Rev. mex. cienc. geol [online]. 2015, vol.32, n.1, pp.100-122. ISSN 2007-2902.

The Apan-Tecocomulco Volcanic field is located ~85 km to the north of the present-day volcanic front, belonging to the eastern part of the Trans-Mexican Volcanic Belt. Geological, stratigraphic, geochronological and geochemical studies were used to establish the plio-quaternary volcanic sequence. This sequence was emplaced after a magmatic hiatus of ~7 Ma on Middle Miocene andesitic-dacitic rocks. The magmatism restarted ~3 Ma ago with the emplacement of some dacitic domes, lava flows, and several scoria cones, mostly of basaltic andesite composition. Later, some basaltic andesite shield volcanoes and scoria cones formed between 1 and ~0.5 Ma. Finally, several basaltic scoria cones and associated lava flows were emplaced between ~0.5 to ~0.2 Ma. The K-Ar age determinations suggest that the plio-quaternary volcanism started at the northern part of the Trans-Mexican Volcanic Belt and later moved towards the Middle-America trench, to the present-day volcanic front, at which time it spreads throughout the study area and the Sierra Nevada. The plio-quaternary volcanic structures seem to follow NE-SW alignments, in the same way as the regional faults. The mafic rocks display similar petrographic features, such as fluidal microlitic textures with some olivine, plagioclase and pyroxene phenocrysts, whereas the dacitic rocks show porphyric textures with relatively abundant quartz, plagioclase, sanidine, amphibole and pyroxene phenocrysts. Volcanic rocks were mainly classified as basaltic andesites and basalts, with minor andesites and dacites, most of them following a calc-alkaline trend in the SiO₂ vs. alkalis diagram. However, some basalts, trachy-basalts and basaltic trachy-andesites follow an alkaline trend in the same diagram. The chondrite-normalized REE patterns for mafic and dacitic rocks suggest heterogeneous magmatic sources in the mantle. These patterns display light rare earth elements enrichment (La-Sm) with respect to the heavy rare earth elements (Eu-Lu), which have flat patterns. The trace element patterns for most basaltic andesites, andesites and dacites, that were emplaced between ~3 and ~0.5 Ma, display enrichment in the large-ion lithophile elements (LILE: Ba, K) and Pb relatively to the high-field-strength elements (HFSE: Nb, Ta), that are typical of subduction-related volcanic arcs. However, basalts, trachybasalts and some basaltic andesites, emplaced more recently (<0.5 Ma), display trace element patterns that suggest transitional magma type. The plio-quaternary volcanic rocks from this field were probably produced by tectono-magmatic processes related to the slab roll-back, and migration of the arc toward the trench at its present position. These tectonic processes produced the partial melting of a heterogeneous mantle source composed of enriched asthenospheric mantle products and subduction related components of the mantle wedge.

Keywords : geochronology; transitional magma; geochemistry; Sierra de Apan; K-Ar ages; Faja Volcánica Trans-Mexicana; obsidian.