U-Pb SHRIMP geochronological constraints on the age of the Ischilín dextral shear zone (Sierras Pampeanas de Córdoba, Argentina)

IANNIZZOTTO, N.F.; RAPELA, C.W.; BALDO, E.; FANNING, C.M

Santiago, Chile

Congreso; GEOSUR: 2007: An International Congress on the Geology and Geophysics of the Southern Hemisphere; 2007

Departamento de Geología, Universidad de Chile

The age of the milonitic deformation in the Sierra Norte de Córdoba has been constrained from field intrusive relations and U-Pb SHRIMP ages of the plutonic rocks that crops out in the southern sector of the Sierra Norte-Ambargasta batholiths. The igneous rocks are the roots of the Lower Cambrian Pampean magmatic arc (Rapela et al., 1998). The dominant plutonic units are suites of metaluminous granodiorites and porphyritic granites, with abundant roof pendants of a low to medium grade metamorphic basement. Both the metaluminous suites and the metamorphic rocks were affected by a non-coaxial deformation that developed large transcurrent, dextral shear zones (Ischilín and  Sauce Punco shear zones, Gordillo et al., 1997;  Martino, 2003). In the Sierra de Ischilín, the Juan García granodiorite (JGG) is a pre-milonitic pluton with a mineral assemblage of  Qtz – Pl (An34-39) – Kfs – Bt  – Hbl – Ttn - All – Ep – Ap – Zrn, representative of the I-type metaluminuos event (ASI = 0.93-1.01, %SiO2= 63-67, #Mg= 2.3-4.2). Circular and elongated tonalitic enclaves are very common,  while the dominant foliation follows a NW-SE direction. An U-Pb SHRIMP age of 538 ± 3 Ma was obtained on zircons of the JGG, that is interpreted as representative of the crystallization age of the pre-milonitic plutons.  The Villa Albertina granite (VAG) is a peraluminous pluton (ASI=1.09-1.36%, SiO2 = 73-75, #Mg=0.6-1.2),  representative of a post-milonitic magmatic event. The VAG is a non foliated body, with a mineral assemblage of  Qtz – Pl (An16-23) – Kfs – Ms ±Bt - Grt - Ap – Zrn, carrying abundant milonitic xenolites of the Ischilín shear zone. A crystallization age of 528 ± 4 Ma has been determined in zircons of the VAG, that show also some inherited zircons grains of ~ 630 Ma. The inferred Lower Cambrian age inferred here for the Ischilín and the Sauce Punco shear  zones confirmed previous assumptions made by Martino (2003), while this milonitic event has been associated with the dextral oblique collision of the Pampean terrane against the western edge of the Río de la Plata craton (Rapela et al., 2007).   REFERENCES - Gordillo D. E., Kirschbaum A. M. y Baldo E., 1997. Magmatismo del Borde Oriental de las Sierras Pampeanas: Los Granitoides de la Sierra de Ischilin, Pcia de Córdoba, Argentina. VIII Congreso Geológico Chileno.  Actas Vol. II: 1290-1294,  Antofagasta, 13 de Octubre de 1997. - Martino, 2003. Las fajas de desformación dúctil en las Sierras Pampeanas de Córdoba: Una reseña general. Revista de la Asociación Geológica Argentina, 58 (4): 549 – 571. - Rapela, C.W., Pankhurst, R.J., Casquet C., Baldo E., Saavedra J., Galindo C., Fanning C.M., 1998. The Pampean Orogeny of the Southern proto-Andes: Cambrian continental collision in the Sierras de Córdoba. In: Pankhurst , R.J. and Rapela C.W. (eds.). The Proto-Andean Margin of Gondwana. Geological Society, London, Special Publications, 142, 181-217. - Rapela C.W., Pankhurst R.J., Casquet C., Fanning C.M., Baldo E.G., González-Casado J.M., Galindo C., Dalquist J., 2007. Earth Science Reviews, 83 (1-2): 49-82.