Mafic microgranular enclaves from polygenetic and monogenetic swarms in the Gredos sector of the Central System batholith (central Spain) were sampled and studied to elucidate field, geochronological, and geochemical relations. Among-enclave variations in terms of major elements demonstrate contamination with the pelitic country rocks, and not with the host granitic magma. Magmatic enclaves from monogenetic swarms in Gredos are interpreted as fragments from the chilled margins generated at the sidewall of ascent conduits. In situ U-Pb zircon age determinations by sensitive high-resolution ion microprobe yielded two sets of ages. One corresponds to the ages of crystallization at 290 ± 3.1 Ma and 300 ± 3.4 Ma of two enclaves from two different monogenetic swarms. These ages are slightly younger than the zircon ages of the host granodiorites (ca. 319–312 Ma). A secondary set of ages present in the two studied samples (at 322 ± 6.7 Ma and 320 ± 8.5 Ma) corresponds to the age of the main Variscan regional migmatization of host Neoproterozoic pelitic metasediments. These older ages are interpreted as xenocrystic cores that were derived from the pelitic host during magma ascent. Together with the geochemical relations, they prove quenching against the regional host in con¬duits and not against the host granodiorite as classically thought. Monogenetic swarms can be seen as high-energy pulses denoting the switching from steady-state to transient flow regimes within magma ascent conduits. By contrast, polygenetic swarms are generated by collapse of accumulated resisters of igneous and metamorphic origin that were previously present in migmatites at the top of subhorizontal, layer-type intrusions.

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