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

BOEDO, F.L., VUJOVICH, G.I.

Mar del Plata, Argentina

Congreso; International Geological Congress of the Southern Hemisphere GEOSUR 2010; 2010

Cordón del Peñasco area is located in western Precordillera, northern Mendozaprovince, Argentina (Fig. 1). Low grade metamorphic, slope and deep marinemetasiltstones and metasandstones of Eopaleozoic age, associated with mafic andultramafic rocks are the most widely exposed rock types. Mafic and ultramafic rockscorrespond to an ophiolitic belt composed of discontinuous exposures along westernPrecordillera (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 tosouth at: río Bonete area (La Rioja province), Rodeo and Calingasta (San Juanprovince), Cordón del Peñasco, Sierra de las Cortaderas and Cordón de Bonilla(Mendoza province) (Fig. 1). Particularly, Cordón del Peñasco and Cortaderas localitiesshow 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 greenschistfacies, and homogeneous gabbro dikes or sills, amigdaloid metabasalts andmetahialoclastic rocks with greenschist facies metamorphism. Cortaderas mafic andultramafic bodies comprise serpentinized ultramafic rocks (wehrlites, harzburgites,lherzolite websterites and dunites), layered gabbros, gabbros, microgabbros anddiabases. In both areas, these bodies are heavily deformed and in tectonic contact withslope and deep marine metasedimentary rocks. In addition, the petrography ofserpentinites and mafic granulites are similar. In Cortaderas, Davis et al. (1999) haverecognized that a low-grade regional metamorphism has partially replaced the igneousand high temperature assemblages in mafic and ultramafic rocks. In Cordón delPeñasco, we have recognized the same process and retrogradation evidences such asreaction rims on garnets, recristalization and formation of a fine-grained clinopiroxeneplagioclase-quartz mosaic and lizardite-chrysotile-talc folded veins on an antigoritemesh texture. In both localities, a similar low-grade metamorphism has affectedmetavolcanic and metasedimentary rocks, where primary textures are better preserved.Additionally, Davis et al. (1999) calculated metamorphic P and T conditions on garnetsof mafic granulites obtaining temperatures of approximately 850-1000ºC and aminimum pressure of 9 kbar. As mafic granulites have very similar mineralogy andtextures in both localities, we assume that this PT condition, that fall into high pressuregranulite facies, could be similar in our study area.Geochemical analyses on mafic and ultramafic rocks of different localities plot inthe E-MORB (Enriched Mid Ocean Ridge Basalt)/within plate basalt field (Haller yRamos, 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 oceaniccharacter (Kay et al., 2005).Western Precordillera Ophiolite Belt has been interpreted as a suture zone betweenChilenia and Cuyania terranes (Ramos et al., 1984). Davis et al. (1999, 2000) suggestedthe occurrence of two ophiolite assemblages along the suture based on different ages forthe mafic and ultramafic rocks. We propose that the mechanism of exhumation in adipping eastward subduction channel could explain the association of low and highgrademetamorphic rocks, rocks with retrogradation evidences and the east and weststructural vergence reported by many authors (Ramos et al., 1984; Davis et al., 1999;Von Gosen, 1997) along the ophiolite belt.