Unravelling the origin of A-type granites in an extensional arc setting: Insights from the Permian El Fierro pluton, Colangüil batholith (Frontal Cordillera, Argentina)

The El Fierro pluton represents one of the late-stage plutons of the Choiyoi Magmatic Province in the Colangüil batholith (Frontal Cordillera, Argentina). New U Pb zircon dating yields a crystallisation age of 262 ± 2 Ma, with zircon antecrysts (~273 Ma) indicating prolonged magmatic activity that overlaps with the early-stage Choiyoi I-type magmatism (ca. 270–290 Ma). This pluton is ferroan, alkali-calcic to calc-alkalic, and essentially weakly peraluminous (av. 1.05 ± 0.06), exhibiting high Ga/Al ratios, elevated HFSE concentrations (Ce + Zr + Nb + Y; av. 310 ± 147 ppm; range: 150–584 ppm), extreme F enrichment (6101–31,442 ppm) and relatively high zircon-saturation (775 ± 45 °C) and apatite-saturation temperatures (887 ± 78 °C), consistent with an A-type affinity. Isotope data (εHf t = −7.0 to +1.25; εNd i ≈ −3), together with geochemical compositions, reflect a hybrid crustal source involving metasedimentary components likely metasomatized by mantle fluids. Accordingly, we interpret the El Fierro pluton as the product of crustal melting during lithospheric thinning and slab retreat, marking the onset of incipient rifting in southwestern Gondwana. This study highlights that A-type granites can be generated in extensional regimes within arc settings.