Andesitic-dacitic volcanism at the Sierra Chichinautzin Volcanic Field, and its influence in the construction of the Xochicalco pre-Hispanic site, central Mexico

Abstract

Jumento (~2.0 kyr) and Tabaquillo (~7.8 kyr) are examples of basaltic andesitic and dacitic volcanic structures, respectively, in the Sierra Chichinautzin Volcanic Field (SCVF). This volcanic field is situated at the southern limit of the Mexico basin and the Metropolitan area of Mexico City, comprises more than 220 monogenetic volcanoes of heterogeneous chemical composition. The stratigraphic sequence of the Jumento volcano records a sequence from bottom to top: (1) Wet pyroclastic surge deposits (within a >5 km radius) that have liquefaction structures (seismites), covered by a scoria fallout deposit, which built the 32o slopes of the Jumento’s cone; (2) Lava flows that breached the southern part of the cone and flowed southwards up to 2.5 km from the vent. This sequence suggests that Jumento initiated its activity with hydromagmatic eruptions, followed by Strombolian explosions and ended with effusive events. Charcoal fragments from the base of the sequence provided an average 14C age of ~2 ky BP. Tabaquillo volcano comprises a lava dome and lava flows with steep margins with dacitic composition. These lavas have porphyritic texture, containing large plagioclase, quartz, and biotite phenocrysts in a microlithic and glassy groundmass. The presence of equigranular enclaves is common, as well as rare metasandstone and quartzite xenoliths. Lava flows rest on a paleosoil rich in charcoal fragments that yielded a radiocarbon age of ~ 7.8 kyr BP. The presence of crustal xenoliths attest for crustal assimilation of the Sierra Chichinautzin magmas producing a heterogeneous composition. Volcanic eruptions from the SCVF have not been recorded yet in the Xochicalco Archaeological Site (southern SCVF); however, it has not been discarded. Xochicalco’s buildings are made of lava blocks, with different texture and composition, likely coming from different volcanic sources (monogenetic and polygenetic volcanoes). The question about the source of these materials remains unanswered since there are no quarries or a place from where these materials could have been taken. Epiclastic deposits deposits of the Cuernavaca Formation, located near the archaeological site, might be the most probable source of these blocks.