THE GONDWANAN ORIGIN OF THE CUYANIA TERRANE, WESTERN ARGENTINA, BASED

PERALTA, SILVIO H.; FINNEY, STANLEY C.; GLEASON, J.D.; GEHRELS, GEORGE; ACEÑOLAZA, GUILLERMO F.

Bahía

Simposio; IV South American Symposium on Isotope Geology; 2003

Asociación Brasilera de Geoquímica

In this paper geochronology and graptolite biostratigraphy of the Early Paleozoic from CuyaniaTerrane, are used to address its geotectonic and paleogeographic evolution. The Cuyania Terrane (Ramos et al., 1986) is a tectono-stratigraphic unit placed in the central-western part of Argentina, which includes the Precordillera of La Rioja, San Juan and Mendoza Provinces (Furque & Cuerda, 1979), the Sanrafaelino-Pampean geological Province (Criado Roque & Ibáñez, 1979), including the San Rafael and Las Mahuídas Blocks, and the Angaco Belt (Vujovich & Ramos, 1994), as part of the western Sierras Pampeanas. This terrane hasspecial significance in the Early Paleozoic geotectonic evolution of Western Gondwana (Ramos et al., 1984, 1986; Astini et al., 1996). In this scenery, the widely accepted model of the Cuyania (=Precordillera) terrane as an exotic Laurentia-derived fragment, is mainly based onstratigraphic and paleobiologic similarities of their correlative cambrian until early ordovician carbonate bank (Astini et al., 1995; Thomas & Astini, 1996; Astini et al., 1996), besides its basement, though as Grenvillian type (Kay et al., 1996), also it is considered as argumentto support the Laurentian connection of Precordillera. Since the Penrose Conference held in 1995 at San Juan, many workers have provided additional geological evidence in order to support such model, known as the ?funeral ship? model. Other alternative models to explainthe allochtonous character of Precordillera were suggested by Dalziel (1997) and Dalla Salda et al. (1992). On the other hand, formerly Baldis et al. (1989) suggested a para-autochthonous model for the Precordillera, as a displaced terrane from Gondwana. Additional data were provided by Aceñolaza et al. (1999, 2002) to support this para-autochthonous model.Chemostratigraphic data combining neodymium isotope studies and graptolite stratigraphy from Middle-Upper Ordovician from Precordillera at San Juan province (Gleasson et al., 2001) provided some evidence about the connection of Precordillera with western Gondwana on that time. Recently, new geochronologic data from U-Pb age obtained from individual detritalzircons (Finney et al., 2002, 2003), from Cambrian and Upper Ordovician rocks, and paleobiologic study of Upper Ordovician graptolite fauna from Precordillera and San Rafael Block (Peralta and Finney, in press), have provided strong evidences to support Gondwananaffinities for the Cuyania Terrane, challenging the widely accepted Laurentian origin. U-Pb ages (Finney et al., 2003) were acquired from quartz sandstone beds of the Early Cambrian Cerro Totora Fm., and Late Ordovician Las Vacas Fm. in the Argentine Precordillera, at La RiojaProvince, and from the lowest Ordovician La Cébila Fm., in the Sierra de Ambato, northwestern Sierra Pampeanas, at Catamarca Province. Paleobiologic data were obtained by mean of thestudy of graptolite faunas of several Upper Ordovician (Caradocian), lithostratigraphic units of Precordillera and San Rafael Block. The analyzed graptolite faunas belong to N. gracilis and C. bicornis Zone showing a warm Pacific province signature, like that of the Laurentia continent. In this case, sampled sandstones for U-Pb age data are associated with one other of these faunas. In this paper, geochronological data are from Finney et al. (2003), Nd isotope data from Gleason et al. (2001) and graptolite biostratigraphy from Peralta & Finney (in press).