El Chichon Volcano (Chiapas Volcanic Belt, Mexico) Transitional Calc-Alkaline to Adakitic-Like Magmatism: Petrologic and Tectonic Implications

摘要

The rocks of the 1982 eruption of El Chichon volcano (Chiapas, Mexico) display a series of geochemical and mineralogical features that make them a special case within the NW-trending Chiapas volcanic belt. The rocks are transitional between normal arc and adakitic-like trends. They are anhydrite-rich, and were derived from a water-rich, highly oxidized sulfur-rich magma, thus very much resembling adakitic magmas (e.g., the 1991 Pinatubo eruption). We propose that these rocks were generated within a complex plate tectonic scenario involving a torn Cocos plate (Tehuantepec fracture zone) and the ascent of hot asthenospheric mantle. The latter is supported by an oustanding negative S-wave anomaly widely extending beneath the zone, from 70 to 200 km in depth. The adakitic-like trend would be derived from the direct melting of subducting Cocos plate, whereas the transitional rocks would have resulted from the mixing of two poles, one reflecting a mantle source, and the other, the already mentioned adakitic melts. The basaltic source would also account fir the high sulfur content and delta~(34)S values of the El Chichon system (about + 5.8), as result of a contribution of SO_2 in fluids released from an underlying mafic magma.