Thermomechanical maturation of the continental crust and its effects on the late Eocene-early Oligocene volcanic record of the Sierra Madre del sur Province, southern Mexico

We interpret the voluminous late Eocene-early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle-upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant ¹⁴³Nd/ ¹⁴⁴Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle-ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic-volcanologic scenario developed in north-western Oaxaca, where less profuse early-middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO ₂ at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.