Geochemistry of the Mamil Choique Granitoids at Río Chico, Río Negro, Argentina: Pre-Permian crustal melting process in the North Patagonian Massif.

LÓPEZ DE LUCHI, M. G.,; CERREDO, M.E.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2008 vol. 24 p. 526 - 546

0895-9811

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). On the basis of their major and trace element relations as well as the 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, indicating minimum P of 9-10 kbar during melting. CMG would have formed from partial melting of supracrustal fertile lithologies (biotite-bearing) in the presence of H2O-bearing volatile phase which 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 (»5kbar). 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 still upper crustal levels. Keywords :  Mamil Choique Granitoids, North Patagonian Massif, Upper Paleozoic, partial melting, geochemistry