Los complejos metamórficos del retro-arco Famatiniano (noroeste de Argentina): caracterización geoquímica e isotópica de sus protolitos e implicancias geotectónicas

Abstract

Geochemical and isotopic studies of two Ordovician metasedimentary units of the back-arc of the Famatinian orogenic belt (NW Argentina), the El Portezuelo metamorphic-igneous complex (EPMIC) and the La Cébila metamorphic complex (LCMC), in combination with published geochronology, show that their protoliths were diachronically developed in different sedimentary basins. Lithologic studies and whole-rock major and trace element geochemical analysis determine that the protoliths of both complexes belonged to sequences of clastic sedimentary rocks made up mainly of greywackes (metapsammites) and shales (metapelites). The metapelites have higher concentrations of Al2O3, Fe2O3(T), MgO, K2O, TiO2, Rb, Nb, V and Cr, and lower concentrations of SiO2 than the metapsammites. The negative correlation of SiO2 with these elements reveals higher contents of clay minerals in the metapelites. For both complexes, Al2O3/TiO2, La/Sc, Th/Sc, La/Co and Th/Cr ratios, REE patterns, Cr, Ni and V values, and La/Th-Hf, K-Rb and F1-F2 geochemical diagrams indicate that the metasediments derived from felsic to intermediate source rocks. Several tectonic setting discrimination diagrams suggest that the protoliths of the EPMIC and the LCMC were related to a magmatic arc tectonic setting. The greater lithological and geochemical variations observed in the LCMC indicate a more unstable depositional regime than for the EPMIC. Crustal residence ages obtained from Sm-Nd isotopic analysis indicate that the protoliths of the LCMC (~1.3–1.6 Ga) were younger than the protoliths of the EPMIC (~1.8–1.9 Ga). The geochemical variations and the isotopic data, together with previously published geochronology, allow relating the protoliths of each metamorphic complex to different depositional events. In this context, the possible precursor rocks of the EPMIC would be the Pampean low-grade metamorphic rocks of the Puncoviscana Formation, while the protoliths of the LCMC were Lower-Middle Ordovician sedimentary rocks. The westwards rejuvenation of the protoliths of the Famatinian back-arc metamorphic complexes such as the LCMC is consistent with the development of less stable basins associated with the emplacement to the west of the new orogenic front on the Gondwana margin.