The Esperanza Granitoids, southern Mexico: apatite and zircon protracted geochronological record during the Rheic evolution

The Esperanza granitoids unit is composed of an assemblage of metamorphic lithologies that belong to a high-pressure suite of the Acatlán Complex, the largest Paleozoic basement in Mexico, that records a continuous tectonic history spanning from the opening and closure of the Rheic ocean. The Esperanza granitoids include megacrystic K-feldspar, amphibolite facies mylonitic augen gneisses. We studied deformed granitoids that vary from protomylonitic to ultramylonitic augen gneisses and strongly foliated pegmatites, obtaining U-Pb zircon and apatite ages and apatite chemistry, intending to understand better the thermotectonic events that affected the Esperanza granitoids from their crystallization to metamorphism. The zircon U-Pb geochronology yielded ages from 466.8 ± 2.7 to 469.1 ± 3.1 Ma for the augen gneisses and 462.9 ± 2.7 Ma for the pegmatitic rocks. These data are interpreted as the protolith crystallization ages in the Middle Ordovician. Based on apatite U-Pb (Tc = 450–550 °C) thermochronology, ages between 338 ± 12 and 363 ± 18 Ma were obtained for the augen gneiss samples (lower intercept of a discordia line), which allowed detecting the first metamorphic stage during retrogression at amphibolite-epidote facies (450–550°C). These apatites have chemical characteristics suggestive of crystallization during the partial melting event that generated the Esperanza granitoids, although few of them record crystallization during the medium and high-grade metamorphism, an event that completely reequilibrated their U-Pb ages.