Geochemical evidence for crustal anatexis during intra-orogenic transcurrent tectonics: Insights from Variscan peraluminous granites from the La Bazana Pluton (Ossa–Morena Zone, Iberian Massif)

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 evo-
lution (crustal growth vs. reworking). The interpretation may be further complicated if mafic terms or charac-
teristic 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 per-
aluminous two-mica leucogranites, emplaced at 336.3 ± 0.7 Ma (U-Pb 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 Sr-Nd isotopes (MC-ICP-MS; $^{87}$Sr/$^{86}$Sri = 0.7077–0.7134; εNd$_i$ =
... mehr
8.6 ± 0.2) and zircon inheritances (Neoarchean to Ordovician) indicate that the weakly-peraluminous mon-
zogranite parental magmas derived from a heterogeneous crustal source (metagreywackes and felsic metaig-
neous 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 SiO$_2$ 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 conti-
nental crust and reworking of older crustal lithologies may occur concurrently, and that crustal melting may be
broader than expected in such environments.