GEOLOGY OF THE HUAUTLA VOLCANIC FIELD AND ITS SIGNIFICANCE FOR UNDERSTANDING THE ORIGIN AND TEMPORAL EVOLUTION OF SILICIC MAGMATISM IN THE NORTH-CENTRAL SIERRA MADRE DEL SUR OF MEXICO
The northern central region of the SMS is formed by a discontinuous inland paleoarc (late Eocene to early Oligocene) which consists of ~300 km of volcanic successions along the strike of the arc with a total preserved volume of ~2,500 km3. It is composed of nine Paleogene silicic and intermediate volcanic centers. The SMS has been considered as the southern extension of the Cenozoic ignimbrites of the Sierra Madre Occidental (SMO) province. However, contrasting details in the age trends and associated tectonic features are now emerging.
In the HVF several volcanic units have been recognized and grouped into four main episodes based on their ages: 1) a porphyritic granodiorite ranging in age from 37 to 36 Ma, corresponding to the subvolcanic basement; 2) a ~3 Ma hiatus characterized by the deposition of ignimbrite units derived from the adjacent Tilzapotla Caldera (~34.5 Ma); 3) an andesitic- siliceous event, with ages ranging from ~33.5 to 31 Ma; and 4) a final andesitic episode at ~30-29 Ma.
From the integration of new dates for the HVF and other silicic centers, we recognized an ignimbrite flare-up with major pulses in its temporal evolution: ~ 36.5-35.5, 34.5-34 and 33.5-32 Ma. These pulses produced minimum preserved volumes of ~920, 630 and 710 km3, respectively. The volcanic geochronology reveals an apparent cyclicity in the silicic volcanism, and a slight decrease in age from west to east. These pulses do not coincide with those of the flare-up of the SMO in its southern sector (~24-20 Ma).
Using the new U-Pb dates as the ages of magmatic crystallization within the chamber, and combining them with previously reported Ar-Ar ages interpreted as eruption ages, we estimated magma residence times for several silicic centers: 0.44, 0.58, 0.69 and <1Ma for the HVF, the Tilzapotla caldera, Valle de Bravo and La Goleta, respectively.
New trace element and isotope data obtained for the HVF show that the silicic and intermediate products were generated through fractional crystallization of a mafic end-member over a wide range of pressures, coupled with crustal contamination at different crustal levels.