SHRIMP U–Pb zircon geochronology and thermal modeling of multilayer granitoid intrusions: Implications for the building and thermal evolution of the Central System batholith, Iberian Massif, Spain

Abstract

This work shows the results of a U–Pb SHRIMP zircon geochronological study of the central part of the Gredos massif (Spanish Central System batholith). The studied batholith is composed of several granodiorite and monzogranite tabular bodies, around 1km thick each, intruded into partially molten pelitic metasediments. Granodiorites and monzogranites, belonging to three distinct intrusive bodies, and samples of anatectic leucogranites have been selected for SHRIMP U–Pb zircon geochronology. Distinct age groups, separated by up to 20Ma, have been distinguished in each sample. Important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies: 312.6±2.8Ma for the Circo de Gredos Bt-granodiorites (floor intrusive layer), 306.9±1.5Ma for the Barbellido-Plataforma granitoids (top intrusive layer) and 303.5±2.8Ma for Las Pozas Crd-monzogranites (middle intrusive layer). These age differences are interpreted in terms of sequential emplacement of the three intrusive bodies, contemporary with the Late Paleozoic D3 deformation phase. The anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites (305.4±1.6Ma for Refugio del Rey leucogranites and 303±2Ma for migmatitic hornfelses). It is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions. Thermal modeling with COMSOL Multiphysics® reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts (around 750°C) for several million of years, favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions.