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

Abstract

The rocks of the 1982 eruption of El Chich6n 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-liketrends. 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 asthenosphericmantle. The latter is supported by an outstanding negative 5-wave anomaly widely extending beneath the zone, from 70 to 200 km in depth. The ada kitic-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 for the high sulfur content and 834S values of the El Chich6n system (about +5.8), as result of a contribution of 502 in fluids released from an underlying mafic magma.

Evidence of an upper mantle seismic anomaly opposing theCocos slab beneath theIsthmus ofTehuantepec,Mexico

“…Our images along with the previously published seismic images suggest that the Cocos plate may be missing beneath these volcanic centers. In particular, the magma from the very young El Chichón volcano is both mineralogically and geochemically distinct from the volcanoes of TMVB and CAVA [ De Ignacio et al ., ]. The magma contains magmatic anhydrite, and high water content (4–10 wt%) and oxygen fugacities, which are absent in the normal‐arc rocks of the TMVB and CAVA [ De Ignacio et al ., ].…”

“…In particular, the magma from the very young El Chichón volcano is both mineralogically and geochemically distinct from the volcanoes of TMVB and CAVA [ De Ignacio et al ., ]. The magma contains magmatic anhydrite, and high water content (4–10 wt%) and oxygen fugacities, which are absent in the normal‐arc rocks of the TMVB and CAVA [ De Ignacio et al ., ]. A significant amount of water in the magma is difficult to explain by typical dehydration of the subducted sediment and basaltic oceanic crust alone.…”

“…A significant amount of water in the magma is difficult to explain by typical dehydration of the subducted sediment and basaltic oceanic crust alone. In fact, such features are common in adakites, and require partial melting of a subducting plate under high water fugacity conditions and partial melting of the mantle wedge [ De Ignacio et al ., ].…”

“…However, volcanism associated with El Chichón (age of 0.2 Myr) is suggested to be related to the subduction of the TR [ Manea and Manea , ] although the slab is located 200–220 km beneath El Chichón [ Bravo et al ., ], which might be too deep to be the source of the volcanism [ Wada and Wang , ]. Geochemical characteristics in the volcanics from El Chichón suggest transitional calc‐alkaline to adakitic magma [ De Ignacio et al ., ], distinctively different from those in TMVB to the northwest and also trench‐parallel Central American Volcanic Arc (CAVA; Figure a) to the southeast. Although the TMVB sits oblique to the trench (at ∼17° to the strike of the trench), the top interface of the subducting Cocos slab at the start of the TMVB along the MASE line is at no more than ∼100 km depth based on seismic images [ Kim et al ., , 2013].…”

Palaeomagnetic results from the Chiapanecan Volcanic Arc, Chiapas, Southern Mexico: geomagnetic and geodynamic significance

“…This transitional calc-alkaline to adakite magmatism probably was generated from a mantle-wedge source, slightly metasomatized by slab-derived melts. De Ignacio et al (2003) proposed an asthenospheric inflow through major faults in the subducting Cocos slab. This inflow would have melted the oceanic slab and produced adakitic magmas.…”