Variscan granitoid magmatism in Northwest Iberia (Europe): Age and nature of the melting events and implications for continental orogenic evolution

The Iberian Variscan orogen is notorious for its important granitic magmatism. New field data, petrography, U-Pb zircon geochronology (sensitive high-resolution ion microprobe and chemical abrasion−isotope dilution−thermal ionization mass spectrometry), mineral and whole-rock geochemistry, and Sr-Nd isotope geochemistry were used to define two magmatic pulses in Northwest Iberia (Europe). The first magmatic event (324−315 Ma), caused by radiogenic heat production in a thickened crust, produced syntectonic G1 granitoids formed by I- ± S-type granites−granodiorites containing mafic enclaves. Lower crust metaigneous ± metasedimentary ± upper-mantle sources are inferred for these magmas. Simultaneously, important volumes of syntectonic S-type granites (G2) were produced. Neoproterozoic and early Paleozoic metasedimentary rocks and Cambrian−Ordovician orthogneisses were the likely protoliths. This initial melting event reduced the fertility of the middle−lower crust and conditioned the subsequent magmatic pulse (294−287 Ma) that produced post-tectonic G3 and G4 granitoids in a thinner crust. Lithospheric delamination and asthenospheric upwelling are the proposed heat sources in this case. G3 includes S-type granites derived from sources that preserved fertile components. Simultaneously, a significant volume of G4, including mainly I-type granitoids, was produced by the melting of residual and unmelted sources left behind after the first melting event. These sources comprised restitic metasedimentary rocks and juvenile mafic, intermediate metaigneous rocks. Small mafic enclaves attest to the minor involvement of mantle-derived melts. This model of Variscan magmatism shows that most S-type granites initially occurred due to the presence of fertile lithologies. The depletion of these fertile protoliths conditioned the subsequent melting event, resulting in the formation of a majority of I-type granites.