THERMOMECHANICAL MATURATION OF THE CONTINENTAL CRUST AND ITS EFFECTS ON THE LATE EOCENE-EARLY OLIGOCENE VOLCANIC RECORD OF THE SIERRA MADRE DEL SUR PROVINCE OF MEXICO

The voluminous Late Eocene-Early Oligocene intermediate-silicic successions of the north-central Sierra Madre del Sur are interpreted as the volcanic manifestation of a progressive thermomechanical maturation of the crust, driven by the sustained igneous activity that affected the region since Early Eocene times. Abundant Eocene magmatism and injection of mantle-derived magmas 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 centers were generated through fractional crystallization, crustal contamination and anatexis. In particular, decreasing Sr and Eu concentrations and progressively lower Nd isotope ratios with increasing SiO₂ 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 rocks; in contrast, marked Eu, Sr and Ba depletions coupled with high Rb/Nd ratios at constant ¹⁴³Nd/¹⁴⁴Nd in the Goleta rhyolites suggest derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. The rising of the thermal anomaly induced by magma emplacement and accumulation at shallow depth shifted the brittle-ductile boundary close to the surface, and produced a massive ignimbrite flare-up through caldera-forming and fissure eruptions. A different petrogenetic-volcanological scenario developed in northwestern Oaxaca, where less profuse Early-Middle Eocene igneous activity and the presence of a refractory granulitic lower crust inhibited the expansion of the hot zone to shallow levels, and constrained the processes of 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 ratios with increasing silica contents in the Oaxaca suite, and negative correlations of Nd isotopes with SiO₂ at low Rb/Nd ratios, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crust.