Geochemistry of the Mamil Choique granitoids at Rio Chico, Río Negro, Argentina: Late Paleozoic crustal melting in the North Patagonian Massif

The Mamil Choique granitoids located in the SW border of the North Patagonian Massif consist of three peraluminous units: the banded Huenchuquil tonalites and granodiorites (HTG) and the Cerro Mojón biotite-muscovite monzogranites (CMG), which are intruded by the Nahuelfil monzogranites (NG). According to their major and trace element relations, as well as a comparison with experimentally produced melts, HTG are compatible with partial melting of mafic source rocks at variable H2O contents; residual garnet is suggested by HREE depletion that indicates a minimum P of 9-10 kbar during melting. The CMG would have formed from partial melting of supracrustal fertile lithologies (biotite-bearing) in the presence of H2O-bearing volatile phase that resulted in Sr-rich, moderately depressed HREE melts, likely produced within the garnet stability field (7-10 kbar). The intrusive NG display distinct geochemical signatures (low Sr, no HREE depletion) and general major element ratios akin to the experimentally produced melt at low P (≈5 kbar). Regional constraints indicate that coeval HTG and CMG emplacement took place along a decompressing path slightly postdating the climax of regional metamorphism in the host Cushamen Formation. Later NG intrusion would have occurred at upper crustal levels.