Recycled metaigneous crustal sources for S- and I-type Variscan granitoids from the Spanish Central System batholith : constraints from Hf isotope zircon composition

摘要

The Spanish Central System (SCS) is one of the largest granite batholiths in the European Variscan Belt. Zircons from five granitic intrusions from the eastern SCS have been separated and analysed for Hf isotopes by laser ablation MC-ICPMS, in order to evaluate the nature of granitic magma source. Two samples are cordierite-bearing S-type granites, the rest are amphibole-bearing I-type monzogranites. Hf-isotope composition of zircons from these granites defines a narrow range of negative ℇHf values (-. 1.1 to -. 5.8), typical of crustal protoliths. The within-sample variation of zircon Hf isotope composition is slightly above the analytical uncertainty (±. 1.5 ℇ units), suggesting the mixing of different magma batches during pluton assembly. The S- and I-type granites display the same range of initial ℇHf values, that is in agreement with previous data showing that both SCS granite types have similar (Sr, Nd, O, Pb) isotope signatures. These isotopic data suggest that they derived from similar sources, while the variation in peraluminosity could be related to different partial melting conditions. The presence of a Lower Ordovician inherited zircon population (478-462. Ma), at least in I-type granites, suggests the involvement of Cambro-Ordovician orthogneisses as components in the source of some SCS granites. Inherited zircon population of similar age has also been identified in the SCS lower crustal granulites. The Hf-isotope composition of the analysed granite zircons is within that of zircons from the above granulites. Moreover, some evidence of a juvenile input at 560-595. Ma is recorded by Hf-isotope data in zircons from both the SCS granites and granulite xenoliths. Hf-isotope signature, in conjunction with other chemical, isotopic (Sr, Nd, O, Pb) and geochronological evidences, suggests that SCS granites were probably derived from metaigneous lower crustal sources. The Variscan magmatism in central Spain is dominantly a crustal reworking event.