Abstract
The La Horqueta Shear Zone (LHSZ) is the sixth shear zone recognized in the Sierra de Velasco, Sierras Pampeanas, NW Argentina. This shear zone separates two types of metagranitoids of contrasting petrographical and geochemical characteristics: metagranodiorites, outcropping towards the SW, consist in weakly peraluminous biotite-allanite-titanite granodiorites similar to the I-type granitoids of southern Sierra de Velasco and of neighboring ranges that are part of the Famatinian magmatic arc, while metagranites, outcropping towards the NE, consist in strongly peraluminous biotite-muscovite monzogranites identical to the Antinaco Orthogneis of northern Sierra de Velasco, which is part of an inner S-type granitoid belt. The LHSZ, developed from these two protoliths, has a thickness of 1 to 2 km and is composed of protomylonites and mylonites with strong NNW trending and E dipping mylonitic foliation. Cinematic analyses indicates a main transpressive deformational regime with an inverse-sinistral component, accompanied by a subordinate dextral transcurrent component, which possibly placed the two tectonic settings in contact.
Highlights
the sixth shear zone recognized in the Sierra de Velasco
in weakly peraluminous biotite-allanite-titanite granodiorites similar to the I-type granitoids of southern Sierra de Velasco
U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin
Summary
La sierra de Velasco es una unidad orográfica de rumbo general N-S perteneciente a la provincia geológica de las Sierras Pampeanas (fig. 1a). En estas sierras, Pieters et al (1997), Pankhurst et al (1998 y 2000) y Dahlquist et al (2005), han obtenido edades, por los métodos U-Pb SHRIMP y U-Pb convencional sobre circones, que abarcan el Ordovícico inferior (490 a 463 Ma), coetáneas con las edades de los granitoides tipo S del sector central de la sierra de Velasco. Las fajas de cizalla en la sierra de Velasco corresponden a la continuación hacia el sur de las fajas de la sierra de Copacabana (i.e. Lopez y Toselli, 1993; Söllner et al, 2003) (fig. 1a), por lo que estas edades pueden considerarse válidas para las fajas de la sierra de Velasco
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