DISTRIBUTION, SIGNATURE AND ORIGIN OF PERALUMINOUS RHYOLITES FROM SYN-EXTENSIONAL OLIGOCENE (RUPELIAN) VOLCANISM AT THE MERIDIONAL SECTOR OF THE MESA CENTRAL, MEXICO

Silicic volcanism is ubiquitous within the Paleogene of central and northwest Mexico. An example of this subduction-related volcanism is the Sierra Madre Occidental (SMO), which extends southeast to the Mesa Central of Mexico. Both provinces are contemporaneous to the felsic magmatism that extends towards North America, forming the Mid-Cenozoic Southwest volcanic province (U.S.A.), characterized by numerous caldera-forming flare ups, further inland, away from the trench. A similar case is the southern sector of the SMO, with the southeast tip merging (in space and time) with the felsic volcanic cover of the Mesa Central. This, far inland (~600 km), mainly rhyolitic volcanism has been interpreted as the arrival of a slab window within the Cenozoic Farallón slab underneath the continental plate, and to the transition of subduction dominated arc volcanism to extension-related magmas ascending in a thinner crust, forming smaller ignimbrite flare-ups in the central and southern sectors of the Mesa Central. The Mesa Central consists, from base to top of a metamorphic and Mesozoic carbonate basement. Above an unconformity, there are Eocene continental sediments, overlain by andesite and a succession of Oligocene alkaline rocks including lavas, ignimbrites and volcaniclastic sediments. Coeval with this volcanism, rifting tectonics formed grabens of the southern Basin and Range. Very few calderas had been strictly recognized in this province. Our study comprises three volcanic complexes: the Ahualulco, Loreto-Villa García and La Herradura. The database includes updated field mapping and stratigraphy, geochronology, whole-rock and mineral chemistry and isotopes, from the suite of calc-alkaline, peraluminous rocks. Most of this Rupelian volcanism was emplaced during a short timespan (33 to 29 Ma), which coincides with voluminous flare ups at the SMO, hundreds of kilometers northwest. Here we discuss new and published data, to contribute to the knowledge of this part of the Mesa Central. We envisage a combination of processes during this period which enabled the formation of large magma volumes ascending through the crust in a subduction to extension regime, and the possibility of a slab tear. Nevertheless, not all is clear, regarding the processes that favored the emplacement of these successions.