THE SANTA MARTA RIDGE, NEW INSIGHTS ABOUT THE GEOLOGICAL EVOLUTION OF THE LOS TUXTLAS VOLCANIC COMPLEX, VERACRUZ, MEXICO
One of the several volcanic areas along the eastern coast of Mexico in the South of the Veracruz state is the Los Tuxtlas Volcanic Complex (LTVC), which stands out as an isolated volcanic massif within the Veracruz basin. This basin constitutes a gap between the Neogene Trans-Mexican Volcanic Belt (TMVB) to the NW and the Chiapanecan Volcanic Belt to the SE. The LTVC includes multiple vents and volcano landforms within an area of 2550 km2. The NW sector of the complex concentrates a high density of young scoria cones and maar craters grouped in the Los Tuxtlas Monogenetic Field (LTMF), which also includes the active San Martín Tuxtla volcano. To the SE, the other half of the LTVC lacks of monogenetic manifestations, but is formed by at least four major NW-SE aligned volcanic edifices, which from NW to SE are: Encanto, Yohualtajapan, Santa Marta and Pajapan, which constitute the Sierra de Santa Marta (SSM). The magmatic and tectonic origin of the LTVC has been a controversial topic since the earliest studies, when the occurrence of alkaline rocks were reported. Nelson and González-Caver (1992) grouped the volcanic rocks of the LTVC into two series: “older” and “younger”. According to these authors, lavas located along the coast, in the Montepío area, Cerro El Vigía lavas, as well as the volcanic edifices that constitute the SSM, form the older series, and the volcanic products emitted by the monogenetic activity form the younger ones. Based on geochemical, petrographical, and 40Ar/39Ar data, we propose three different magmatic stages: The older Monte Pío-El Vigía, formed mainly by fisural lavas (7 to 1.4 Ma); the intermediate Santa Marta, formed mostly by massive lava flows emitted by the SSM (1 to 0.5 Ma) and the younger San Martín Tuxtla, formed by the LTMF (50 ka to present). The Monte Pío-El Vigía stage is predominantly alkaline with trachy-basalts and basaltic trachy-andesites, the Santa Marta stage is considered as a period of transition characterized by the coexistence of alkaline and subalkaline magmas, and the younger San Martín Tuxtla stage culminates with the emission of alkaline lavas from the scoria cones and the 1793 eruption of the San Martín Tuxtla volcano.