Early Miocene arc volcanism in the Mexico City Basin: Inception of the Trans-Mexican Volcanic Belt

Abstract

In central Mexico, remnants of the early volcanic activity in the Trans-Mexican Volcanic Belt (TMVB) are exposed at Sierra de Guadalupe in the northern part of the Mexico City Basin and to the south in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas. A few published studies indicate a general early to middle Miocene age for some of these volcanic centers. We present new geologic, geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character, shedding light on the geodynamic evolution during this period. The Sierra de Guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase, ortho- and clinopyroxene plus amphibole phenocrysts, as well as dacitic lava flows and domes with plagioclase, orthopyroxene, amphibole and/or biotite phenocrysts. Pyroclastic and epiclastic (lahars) deposits are also observed in the northern slopes of this volcanic complex. The chemical composition of the Sierra de Guadalupe rocks is calc-alkaline andesitic to dacitic. New 40Ar/39Ar and U-Pb zircon ages demonstrate that volcanic activity at Sierra de Guadalupe lasted ca. 5.0 Ma from ca. 20.1 Ma to 14.8 Ma. Remnants of volcanic centers found in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas ~80 km to the south show similar compositions and eruptive styles, with 40Ar/39Ar ages ranging between 21 and 16 Ma. Rocks with similar ages and lithologies were also described in deep wells drilled in the Mexico City Basin. As a whole, the age distribution shows that early to middle Miocene volcanism, making up the initial activity of the TMVB, migrated toward the north from Tepoztlán (310 km from the trench) up to Pachuca (445 km from the trench), and is likely associated with a shallowing the subducting Cocos slab underneath this region. Our results confirm that magmatism in the TMVB began in the early Miocene by dehydration of the Cocos slab as attested by high Ba/La and Th/Yb ratios.