1 CONICET - Centro de Investigaciones Geológicas (CIG). Calle nº 644. (1900) La Plata. Pcia de Buenos Aires.

Argentina. E-mail: ccingola@cig.museo.unlp.edu.ar 

The ophiolitic belts in the pre-Andean region of central Argentina have received increasing attention in recent years as they have been interpreted as possible sutures of a collage of terranes accreted to the proto-Andean margin of Gondwana during Lower Paleozoic (Ramos et al., 1984, 1986, 2000). The Precordillera ophiolitic belts were described in the pioneer works of Borrello (1963, 1969) and in several thesis and papers (Cingolani, 1970; Cucchi, 1971; Villar, 1975; Cortés and Kay, 1994; Davis et al., 1999).

On the western side of the Precordillera (see Figure) the ophiolite complex is cropping out, extending over 900 km (26º LS to 33ºLS, from La Rioja-San Juan-Mendoza provinces) and was recognized by Haller and Ramos (1984) as the "Famatinian Ophiolites" emplaced during the early Paleozoic. This belt is represented by a series of discontinuous exposures of mafic and ultramafic rocks preserved in the western slope facies of the carbonate platform of the Precordillera. The ultramafic rocks are abundant toward the south, whereas pillow lavas and mafic sills are preserved in the northern sector. In the later at the Rodeo and Calingasta localities (San Juan Province) are interlayered with black shales, siltstones and sandstones, bearing Caradoc (to Ashgill?) graptolites (Aparicio and Cuerda, 1976; Blasco and Ramos, 1976)known as Alcaparrosa, Yerba Loca Formations. The graptolite fauna is composed of the following genera: Dicellograptus sp., Dicranograptus sp., Climacograptus aff. antiquus, Orthograptus sp., Diplograptus sp., Glyptograptus sp.

Towards the north of Uspallata (Mendoza Province) the Cortaderas section, is the type locality of these ophiolites. Rock types include serpentinized peridotites, ultramafic cumulates, layered gabbros, and diabases. All these unit are in tectonic contact and heavily deformed. Country rocks consist of metasedimentary and metavolcanic rocks consisting of medium-bedded metasandstones and metasiltstones. The depositional age of the metasedimentary rocks is unknown. K-Ar whole rock ages of 370 to 358 Ma have been interpreted as representing the age of metamorphism (Cucchi, 1971) and 353 to 399 Ma on the basis of K-Ar dating of fine fraction interlayered illite-muscovite separated from the metasedimentary rocks (Buggish et al., 1994) interpreted as dating the neocrystallized phase of syndeformational metamorphism. Ar39-Ar40 ages obtained by Davis et al. (1999) interpreted the metamorphic age of the sequence as 384.5 ±0.5 Ma (Early Devonian).

The geochemical analysis of the mafic and ultramafic rocks from western Precordillera is presented by Kay et al. (1984) and Haller and Ramos (1984). The basalts from all the studied localities represent evolved oceanic tholeiites that formed in a similar tectonic environment. Possible origins consistent with the chemical data include an oceanic ridge in an early stage of development, a transitional ridge segment, or a backarc basin. Chemical analysis from the Cortaderas gabbros show the E-MORB signatures or within plate origin, others indicated anomalous oceanic crust formed in a plume or plateau setting (Haller and Ramos, 1993).

The western Precordillera mafic rocks are a pseudostratigraphic sequence as a disrupted ophiolite emplaced along a suture zone between the Chilenia and Precordillera terranes (Ramos, 1988; Ramos et al., 1984, 1986; Rapela et al., 1998).

Aparicio, E.P. & Cuerda, A.J., 1976. Nuevos hallazgos de graptolitos en la vertiente occidental de la Precordillera de San Juan (Calingasta). Ameghiniana 13 (2):159-168.

Blasco, G. and Ramos, V.A., 1976. Graptolitos caradocianos de la Formación Yerba Loca y del Cerro La Chilca, Departamento de Jáchal, Provincia de San Juan. Ameghiniana, 13:312-329.

Borrello, A.V., 1963. Elementos del magmatismo simaico inicial en la evolución de la secuencia geosinclinal de la Precordillera. Instituto Nac. Inv. Ciencias Naturales y Museo Argentino de Ciencias Naturales "B. Rivadavia". Revista, Serie Ciencias Geológicas, 1:1-19. Buenos Aires.

Borrello, A.V., 1969. Los geosinclinales de la Argentina. Dirección Nacional de Geología y Minería, Anales, v. 14:1-188. Buenos Aires.

Buggish, W., von Gosen, W., Henjes-Kunst, E., and Krumm, S., 1994. The age of early Paleozoic deformation and metamorphism in the Argentine Precordillera: Evidence from K-Ar data. Zentralblatt für Geologie und Paläontologie, teil 1:275-286.

Cingolani, C.A., 1970. Geología de la Sierra de la Cortadera y adyacencias, Nordeste de Uspallata, Mendoza. Tesis inédita n. 318, Fac. Cienc. Nat. y Museo, Universidad Nacional de La Plata, 170 pp.

Cortés, J.M., and Kay, S.M., 1994. Una dorsal oceánica como origen de las lavas almohadilladas del Grupo Ciénaga del Medio (Silúrico-Devónico) de la Precordillera de Mendoza. 7º Congreso Geológico Chileno (Concepción), Actas, 2:1005-1009.

Cucchi, R.J., 1971. Edades radimétricass y correlación de metamorfitas de la Precordillera de San Juan y Mendoza, República Argentina. Asociación Geológica Argentina, Revista 26:503-515.

Davis, J.S., Roeske, S.M., and Mc Clelland, W.C., 1999. Closing the ocean between the Precordillera Terrane and Chilenia: Early Devonian ophiolite emplacement and deformation in the southwest Precordillera. In Ramos, V.A. and Keppie, J.D., eds., Laurentia-Gondwana connections before Pangea. Geological Society of America, Special Paper 336:115-138.

Haller, M.A. and Ramos, V.A., 1984. Las ofiolitas famatinianas (Eopaleozoico) de las provincias de San Juan y Mendoza. 9º Congreso Geológico Argentino (Bariloche), Actas 2:66-83.

Haller, M.A. and Ramos, V.A., 1993. Las ofiolitas y otras rocas afines. In Ramos, V.A. ed., Geologúia y Recursos Naturales de Mendoza. 12º Congreso Geológico Argentino y 2º Congreso de Exploración de Hidrocarburos (Mendoza), Relatorio 1:31-40.

Kay, S.M., Ramos, V.A., and Kay, R., 1984. Elementos mayoritarios y trazas de las vulcanitas ordovícicas en la Precordillera Occidental: Basaltos de rift oceánicos tempranos (¿) próximos al margen continental. 9º Congreso Geológico Argentino (Bariloche), Actas, 2:48-65.

Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., and Galindo, C. 1998. Early evolution fo the Proto - Andean margin of South America. Geology, 26:707-710.

Ramos, V.A., 1988. Tectonics of the Late Proterozoic-Early Paleozoic: A collisional history of southern South America. Episodes, 11:168-174.

Ramos, V.A., Jordan, T., Allmendinger, R., Kay, S.M., Cortés, J.M. and Palma, M.A., 1984. Chilenia: un terreno alóctono en la evolución paleozoica de los Andes Centrales. 9º Congreso Geológico Argentino (Bariloche), Actas,2:84-106.

Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Mpodozis, C., Kay, S., Cortés, J.M., and Palma, M.A., 1986. Paleozoic terranes of the Central Argentine-Chilean Andes. Tectonics, 5: 855-880.

Ramos, V.A., Escayola, M., Mutti, D. & Vujovich, G.I., 2000. Proterozoic-early Paleozoic ophiolites of the Andean basement of southern South America. In Dilek, Y, Moores, E.M., Elthon, D. & Nicolas, A. eds. Ophiolites and Oceanic Crust: New insights from field studies and the Ocean Drilling Program: Boulder, Colorado, Geological Society of America, Special Paper 349:331-349.

Villar, L.M., 1975. Las fajas y otras manifestaciones ultrabásicas en la República Argentina y su significado metalogenético. 2º Congreso Ibero-Americano de Geología Económica (Buenos Aires), Actas 3:135-155.

Villar, L.M., 1985. Las fajas ultrabásicas argentinas, tipos de ultramáficas. Metalogenia. 4º Congreso Geológico Chileno (Antofagasta), Actas 3 (4):610-633.