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Introduction: The San Rafael Block is included as a part of the pre-Andean region, in the southern sector of the Argentine Cuyania terrane, within the western Gondwana margin. This terrane has been the object of several lines of research, attempting to constrain its allochthonous or para-autochthonous origin. The San Rafael Block is a NW-SE trending structural entity located in the south-central part of the Mendoza Province (Fig. 1).The San Rafael Block is mainly composed of isolated outcrops of ?pre-Carboniferous units? (Mesoproterozoic to Devonian), Upper Palaeozoic sedimentary and volcaniclastic rocks, Permian-Triassic volcaniclastic and magmatic complexes and an extended Cenozoic volcanism. The Río Seco de los Castaños Fm (RSC) is one of the above mentioned ?pre-Carboniferous units?. Sedimentology, tectonic setting and provenance aspects of this formation were described by Manassero et al. (2009). The aim of this paper is to present the first detrital zircon ages obtained in the siliciclastic levels of the Cañón del Atuel and Lomitas Negras sections of the RSC and to discuss the stratigraphical age. Geological aspects and age constraints: The siliciclastic unit studied here was first part of the La Horqueta low grade metamorphic unit (Dessanti, 1956). Later was differentiated based on its sedimentary characteristics by González Díaz (1981). It is separated from the Upper Paleozoic El Imperial Fm by an angular unconformity related with the Chanic tectonic phase. A tonalitic body intruded in the RSC, shows U-Pb (in zircons) and K-Ar (biotite) crystallization ages of 401±3 Ma (Lower Devonian) which constrain the sedimentation age of the RSC (Cingolani et al., 2003). The unit is exposed in several outcrops (González Díaz, 1981) dismembered by Mesozoic and Cenozoic tectonism and are rather isolated within the SRB. The unit comprises in the type-section (km 12 of the río Atuel main road) about 600 m of tabular, green sandstones and mudstones with sharp contacts, with regional folding, N40E and dips 50-72º SE. On the Road 144 outcrops characterized by the Nereites (Pazos et al., 2013) ichnofacies, Rubinstein (1997) found acritarchs and other microfossils assigned to the Upper Silurian. The RSC was interpreted by Poiré et al. (2002) and Manassero et al. (2009) as a distal to proximal silty platform-deltaic system. The dominant sedimentary processes were wave and storm action and probably the source areas were located to the East. The rocks are mainly immature arkosic and micaceous sandstones. At the Lomitas Negras outcrop there are some conglomerate channels with rounded and subangular clasts, some of them contain Ordovician invertebrates. Syndepositional sedimentary structures as current and wave ripples at the top of the sandstone beds are also frequent. Provenance of sedimentary detritus was analyzed by Manassero et al. (2009). A close spatial relationship between the depositional basin of the RSC and the source rocks is supported by textural and compositional immaturity of sandstones and the presence of plant debris (Lycophytes) and charcoal bed. Petrographical analyses suggest source rocks from an igneous- metamorphic complex as well as a sedimentary source input. Similar environment as overfeed sedimentary system with great thickness and low textural maturity have been described by other authors for the Villavicencio Fm (Cingolani et al., 2013). U-Pb detrital zircon data: The analysis of detrital zircons has been intensively used as an interesting element to study the sedimentary provenance. Two samples from the RSC unit were analyzed by U-Pb systematic using LA-ICP-MS equipment at the CPGeo, University of Sao Paulo, Brazil. The results are as follow a) Sample A-11-04 (río Atuel section, 34° 57? 47?S; 68° 36? 40?W): Is characterized by the main mode (61,19%) of detrital zircons of Ordovician to Early Silurian ages (433-480 Ma). Zircon grains of Cambrian and Neoarchean ages are less represented due to a minor input of them during the history of the basin. In order of secondary abundance the register shows the following groups: Mesoproterozoic, with 1043 to 1392 Ma (10,54%), Cambrian with ages of 490 and 520 Ma (8,96%), Neoproterozoic with 548 Ma and 731Ma (7,46%), Paleoproterozoic with ages 1686 Ma and 1888 Ma (5,97%) and Neoarchean with ages between 2582Ma and 2628Ma (5,97%). b) Sample LN-10-04 (Lomitas Negras section, 35° 15? 52?S;68° 30? 19?W): Shows also a dominance of Ordovician zircon grains (46,48%), with 435 and 486 Ma. As a second group, we find Cambrian zircons (18,31%) with 493 and 537 Ma. In less proportion we find Mesoproterozoic grains (15,49%) with ages ranging from 1024 to 1352 Ma and Neoproterozoic zircons (12,68%) with ages from 543 Ma to 956 Ma. Finally, we find two a minor groups of Neoarchean zircons (4,22%) with ages of 2619-2686 Ma and Paleoproterozoic zircons (2,82%) with data of 1402-1535 Ma. In both cases we can observe that most of the detrital zircons are coming from Ordovician sources, with less representation of Cambrian, Neo-Meso-Paleoproterozoic and Neoarchean source ages. We underline the fact that the sample (A-11-04) shows more Ordovician signature, while the sample (LN-10-04) located towards the South shows an increase of Cambrian, Mesoproterozoic and Neoproterozoic sources. It is also interesting that Neoarchean zircons are present in both samples with similar proportions of 4 to 6 %. Final Remarks: Both studied RSC samples show dominant source from Pampean and Famatinian cycles. The U-Pb data show that the detritus coming from the Mesoproterozoic basement are scarce. With these isotopic data the maximum sedimentation age of the unit is constrained to the Lower Silurian. In this sense it is a great difference with the Villavicencio unit in Uspallata-Caracoles region (Cingolani et al., 2013) with dominant source from the Mesoproterozoic rocks.