Western Precordillera Ophiolite Belt: correlations between Cordón del Peñasco and Cortaderas localities (Mendoza province, Argentina)

BOEDO, FLORENCIA L.; VUJOVICH, GRACIELA I.

Mar del Plata

Congreso; International Geological Congress on the Southern Hemisphere-GEOSUR; 2010

Cordón del Peñasco area is located in western Precordillera, northern Mendoza province, Argentina (Fig. 1). Low grade metamorphic, slope and deep marine metasiltstones and metasandstones of Eopaleozoic age, associated with mafic and ultramafic rocks are the most widely exposed rock types. Mafic and ultramafic rocks correspond to an ophiolitic belt composed of discontinuous exposures along western Precordillera (Ramos et al., 1984, 2000; Haller y Ramos, 1984; Cortés y Kay, 1994; Davis et al., 1999; Fauqué y Villar, 2003). This belt can be recognized, from north to south at: río Bonete area (La Rioja province), Rodeo and Calingasta (San Juan province), Cordón del Peñasco, Sierra de las Cortaderas and Cordón de Bonilla (Mendoza province) (Fig. 1). Particularly, Cordón del Peñasco and Cortaderas localities show great similarities (Fig 2 and 3). Cordón del Peñasco mafic and ultramafic rocks comprise serpentinized peridotites (probably dunites and harzburgites) and mafic granulites retrograded to greenschist facies, and homogeneous gabbro dikes or sills, amigdaloid metabasalts and metahialoclastic rocks with greenschist facies metamorphism. Cortaderas mafic and ultramafic bodies comprise serpentinized ultramafic rocks (wehrlites, harzburgites, lherzolite websterites and dunites), layered gabbros, gabbros, microgabbros and diabases. In both areas, these bodies are heavily deformed and in tectonic contact with slope and deep marine metasedimentary rocks. In addition, the petrography of serpentinites and mafic granulites are similar. In Cortaderas, Davis et al. (1999) have recognized that a low-grade regional metamorphism has partially replaced the igneous and high temperature assemblages in mafic and ultramafic rocks. In Cordón del Peñasco, we have recognized the same process and retrogradation evidences such as reaction rims on garnets, recristalization and formation of a fine-grained clinopiroxene-plagioclase-quartz mosaic and lizardite-chrysotile-talc folded veins on an antigorite mesh texture. In both localities, a similar low-grade metamorphism has affected metavolcanic and metasedimentary rocks, where primary textures are better preserved. Additionally, Davis et al. (1999) calculated metamorphic P and T conditions on garnets of mafic granulites obtaining temperatures of approximately 850-1000ºC and a minimum pressure of 9 kbar. As mafic granulites have very similar mineralogy and textures in both localities, we assume that this PT condition, that fall into high pressure granulite facies, could be similar in our study area. Geochemical analyses on mafic and ultramafic rocks of different localities plot in the E-MORB (Enriched Mid Ocean Ridge Basalt)/within plate basalt field (Haller y Ramos, 1984; Kay et al., 1984; Cortés y Kay, 1994; Fauqué y Villar, 2003). Besides, these rocks show positive values of εNd (+6 to +9.3) that confirm their oceanic character (Kay et al., 2005).Western Precordillera Ophiolite Belt has been interpreted as a suture zone between Chilenia and Cuyania terranes (Ramos et al., 1984). Davis et al. (1999, 2000) suggested the occurrence of two ophiolite assemblages along the suture based on different ages for the mafic and ultramafic rocks. We propose that the mechanism of exhumation in a dipping eastward subduction channel could explain the association of low and high-grade metamorphic rocks, rocks with retrogradation evidences and the east and west structural vergence reported by many authors (Ramos et al., 1984; Davis et al., 1999; Von Gosen, 1997) along the ophiolite belt.