Genetic models for granitoid formation in orogenic environments commonly entail varied mantle participation, with differentiation of mafic magmas typifying an end-member model if mantle input dominates over crustal anatexis. By departing from this model, diverse petrogenetic processes and combination thereof could also produce weakly peraluminous compositions in such domains, deriving in opposite implications in crustal evolution (crustal growth vs. reworking). The interpretation may be further complicated if mafic terms or characteristic mineral phases are absent in a certain granitoid suite, as in the case of the Variscan La Bazana Pluton (Ossa–Morena Zone of the Iberian Massif). This pluton, composed by magnesian, alkali-calcic, and weakly- to strongly-peraluminous biotite-bearing monzogranites and magnesian-ferroan, alkali-calcic, and strongly peraluminous two-mica leucogranites, emplaced at 336.3 ± 0.7 Ma (UPb in zircon; quadrupole LA-ICP-MS) in an intra-orogenic transcurrent setting where mantle-derived magmatism was predominant. Whole-rock major- and trace-element data (ICP-MS), together with SrNd isotopes (MC-ICP-MS; 87Sr/86Sri = 0.7077–0.7134; εNdi = −8.6 ± 0.2) and zircon inheritances (Neoarchean to Ordovician) indicate that the weakly-peraluminous monzogranite parental magmas derived from a heterogeneous crustal source (metagreywackes and felsic metaigneous rocks). The asthenospheric upwelling and intrusion of mantle-derived magmas, together with shear heating, likely caused melting of such crustal lithologies at mid-crustal levels, with the subsequent emplacement and crystallization at 2–3 kbar and 730–780 °C (Ti-in-zircon). The geochemical evolution of these monzogranites can be modelled by 29% fractional crystallization of Pl (~53%) + Bt (~32%) + Qz (~15%) + Ap (0.80%) + Zrn (0.06%) + Mnz (0.04%). By contrast, the coeval strongly peraluminous two-mica leucogranites are not cogenetic with the biotite-bearing monzogranites. Their geochemical composition, with different εNdi values (from −9.9 to −10.5) for the same SiO2 contents, suggest that these leucogranites would represent discrete magma pulses derived from partial melting of Ca-poor metapelites. The weakly peraluminous parental magmas of the studied biotite-bearing monzogranites may be considered as S-type, yet not in the sense of exclusively sediment-derived, but as supracrustal, contrary to the nearby I-type (infracrustal) weakly peraluminous counterparts. The obtained results demonstrate that in transcurrent intra-orogenic mantle-dominated settings both addition of new continental crust and reworking of older crustal lithologies may occur concurrently, and that crustal melting may be broader than expected in such environments.
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Errandonea-Martin et al. (2024) studied this question.