THE SAN PEDRO SUMMIT CALDERA, TEMASCALCINGO, MEXICO

The San Pedro caldera (SPC), in the Mexican Volcanic Belt, is located 72 km NW of Toluca, Mexico, in the vicinity of the town of Temascalcingo. The caldera is within the Acambay Graben, and it was affected by E-W faults of the Acambay system. The SPC is a depression 2.5 x 3 km build on top of the dacitic Temascalcingo stratovolcano; thus, it is summit type caldera (Aguirre, 2008). Two main phases are defined in this volcanic complex: 1) construction of a stratovolcano of dacitic composition, 2) an explosive phase of rhyolitic composition, which formed the summit caldera. Basaltic-andesite cinder cones are located along the rim of the caldera and on the volcano’s flanks. One preliminary ⁴⁰Ar/ ³⁹Ar age of 3.77 ± 0.05 Ma is here reported for the stratovolcano’s early building lava stage. Chemically, SiO₂ varies from 64-68 wt.%. The caldera stage includes explosive rhyolitic volcanism in the form of pyroclastic flows and ash fallouts. The SiO₂ contents of the caldera stage varies from 70-73 wt.%. The intracaldera fill consists of stratified sequences as pyroclastic surges, volcanic breccias, reworked tuffs and lake sediments. This rhyolitic phase is apparently associated to ash-fall deposits located on the SE flank of the Temascalcingo volcano that were dated at 1.2 ± 0.13 Ma by Mercer et al. (2002). Basaltic flows are exposed in the SE corner of the study area and their best exposures are east of Pastores. A K/Ar age of 0.4 ± 0.1 was obtained by Suter et al., (1995) for these rocks. All the volcanic rocks of the SPC are calcalkaline, with high K, enriched in LREE, and showing a small Eu anomaly.