Isotopic constraints for continental accretion in the NW Sierras Pampeanas of Argentina.

PORCHER, C.C., FERNANDES, L.A.D., VUJOVICH, G.I.

Mendoza

Conferencia; Gondwana 12 "Geological and Biological Heritage of Gondwana"; 2005

Academia Nacional de Ciencias

Metamorphic rocks exposed in the NW Sierras Pampeanas (Umango, Maz–Espinal and Las Ramaditas), in La Rioja Province, Argentina, were included in the Maz, El Taco, El Zaino and Tambillos complexes. These rocks are interpreted either as (i) belonging to the northern end of the Cuyania Terrane, a micro-continent derived from Laurentia that was amalgamated to the proto-margin of Gondwana during the Palaeozoic, or as (ii) part of the western margin of Gondwana. To distinguish between these tectonic settings and to understand the history of the physical conditions of terrane accretion, isotopic studies were carried out as part of a multidisciplinary investigation, and revealed the existence of several of crustal accretion events and metamorphic episodes in the rocks of this area (Porcher et al., 2004). New results from Sm–Nd, Pb/Pb and Rb–Sr isotopes confirmed ages of crustal accretion events and metamorphic episodes previously reported (Porcher et al., 2005; Vujovich et al., 2005). In addition to revealing tectonic affinities of rock units, these data also indicate that different crustal segments are present in the sierras of Maz-Espinal. Rocks of the El Zaino Complex cropping out along the western limb of the Maz Antiform show Mesoproterozoic ages (TDM <1.6 Ga) while rocks of the Maz Complex, cropping out to the east, yielded Palaeoproterozoic ages (TDM >2.0 Ga). Palaeoproterozoic model ages obtained for the host rocks of anorthosites cropping out in the Sierra del Espinal and interpreted as belonging to an intraplate igneous province of late Mesoproterozoic age (of which Laurentia was part - Casquet et al., 2004) are more characteristic of a tectonic setting related to the Amazon Craton. Our results provide additional constraints to locate the suture between the Cuyania Terrane and the western margin of Gondwana with fundamental implications for palaeogeographic reconstructions of Rodinia.   Casquet, C., Pankhurst, R.J., Rapela, C.W., Galindo, C., Saavedra, J., González Casado, J.M., and Fanning, C.M., 2004, Genvillian massif-type anorthosites in the Sierras Pampeanas: Journal of the Geological Society, v. 162, p. 9-12. Porcher, C.C., Fernandes, L.A.D, Vujovich, G.I., and Chernicoff, C.J., 2004, Thermobarometry, Sm/Nd ages and geophysical evidence for the location of the suture zone between Cuyania and Pampia terranes: Gondwana Research, v. 7, p. 1057-1076. Varela, R., Sato, A.M., Basei, M.A.S., and Siga Jr, O., 2003, Proterozoico medio y Paleozoico inferior de la sierra de Umango, antepaís andino (29°S), Argentina: edades U-Pb y caracterizaciones isotópicas: Revista Geológica de Chile, v. 30, p. 265-284. Vujovich, G.I., Porcher, C.C., Chernicoff, C.J., Fernandes, L.A.D., and Pérez, D.J., 2005, Extremo norte del basamento del terreno Cuyania: Nuevos aportes multidisciplinarios para su identificación, in Dahlquist, J.A., Baldo, E.G., and Alasino P.H., eds., Geología de la provincia de La Rioja, Precámbrico-Paleozoico Inferior: Asociación Geológica Argentina, Serie D, Publicación Especial 8, p. 15-38.