U-Pb SHRIMP geochronological constrains on the age of the Ischilin dextral shear zone. Sierras Pampeanas de Cordoba, Argentina.

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

Santiago de Chile

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

Departamento de Geologia, FCFM, Universidad de Chile / Proyecto ARTG-04 Conecciones Antartica-Patagonia, PBCT-CONICYT / Departamento de Ciencias Ecologicas, Pontificia Universidad Catolica de Chile.

The age of the mylonitic deformation in the Sierra Norte de Cordoba has been constrained from field intrusive relations and U-Pb SHRIMP age of the plutonic rocks that crop out in the southern sector of the Sierra Norte-Ambargasta batholith. The igneous rocks are the roots of 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 (Ischilin and Sauce Punco shear zones, Gordillo et al., 1997; Martino, 2003). In the Sierra de Ischilin, the Juan Garcia granodiorite (JGG) is a pre-mylonitic pluton with a mineral assemblage of Qtz - Pl (An 34-39) - Kfs - Bt - Hbl - Ttn - All - Ep - Ap - Zrn, representative of the I-type metaluminous 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 +- Ma was obtained on zircons of the JGG, that is interpreted as representative of the crystallization age of the pre-mylonitic 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-mylonitic magmatic event. The VAG is a non foliated body, with a mineral assemblage of Qtz - Pl (An 16-23) - Kfs - Ms - Bt - Grt - Ap - Zrn, carryng abundant mylonitic xenoliths of the Ischillin shear zone. A crystallization age of 528 +- 4 Ma has been determined on zircons of the VAG, that show also some inherited zircons grains of ca. 630 Ma. The Lower Cambrain age inferred here for the Ischilin and the Sauce Punco shear zones corfirmed previous assumptions made by Martino (2003), while this mylonitic event has been associated with the dextral oblique collision of the Pampean terrane against the western edge of the Rio de la Plata craton (Rapela et al., 2007).