provenance, and derivation from the Rio de la Plata craton is highly unlikely. considerations, previously zircon data, palaeocurrent and structural data, suggest that these rocks must have had an origin within Gondwana-forming blocks, for which the closest ide ntifiable somces are 'Brazilian' and 'African' (Namaqua Natal). Consequently, the model for the orogeny is that the Rio de la which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa Antofalla and Rio Protracted relative displacement of the RPC after the Orogeny 1ed to its fina1 !-'U.>lU'UH.

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The evolution of the Gondwana margin proposed here is based on new geochemical, isotopic, petrological, and sedimentological data from a 500 km traverse across the Eastern Sierras Pampeanas and Precordillera (Fig. 1). Pre-Silurian metamorphic and magmatic history is inferred from (1) dating by conventional U-Pb on abraded zircons, U-Pb SHRIMP analyses, and whole-rock Rb-Sr and K-Ar, (2) thermo-barometry based on microprobe mineral analyses, and (3) Nd and Sr isotopes, and major and trace element geochemistry of the magmatic suites. Detailed data and interpretation have been presented for the Sierras de Córdoba (Rapela et al., 1998) and Sierras de La Rioja (Pankhurst et al., 1998). THE GONDWANA MARGIN Geological and paleontological evidence indicating that the Argentine Precordillera is a Laurentian terrane has revolutionized ideas about the proto-Andean margin of South America (Dalziel, 1997, and references therein). The exotic origin of the Precordillera is now widely accepted, although the timing of accretion and the associated geotectonic models remain controversial (e.g.

New data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature (P/T) type metamorphism, reaching high-grade migmatitic conditions in places (686 40 MPa, 790 17 C), comparable to the Grenvillian M 2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 100 Mpa, 600 50 C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane.

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A multidisciplinary study (U-Pb SHRIMP geochronology, Hf and O isotopes in zircon, Sr and Nd isotopes in whole-rocks, as well as major and trace element geochemistry) has been carried out on granitoid samples from the area west of Valcheta, North Patagonian Massif, Argentina. These confirm the Cambrian age of the Tardugno Granodiorite (528 ± 4 Ma) and the Late Permian age of granites in the central part of the Yaminué complex (250 Ma). Together with petrological and structural information for the area, we consider a previously suggested idea that the Cambrian and Ordovician granites of northeastern Patagonia represent continuation of the Pampean and Famatinian orogenic belts of the Sierras Pampeanas, respectively. Our interpretation does not support the hypothesis that Patagonia was accreted in Late Palaeozoic times as a far-travelled terrane, originating in the Central Transantarctic Mountains, and the arguments for and against this idea are reviewed. A parautochthonous origin is preferred with no major ocean closure between the North Patagonian Massif and the Sierra de la Ventana fold belt.Supplementary material: [U-Pb SHRIMP Analytical Data, Geochemical Analyses and Sample GPS locations] is available at ww.geolsoc.org.uk/SUP00000 3 The hypothesis that Patagonia originated as a far-travelled terrane that became accreted to the rest of South America in Late Palaeozoic times (Ramos 1984) has proved remarkably resilient. It proposes collision of a Patagonian terrane with the rest of South America along a hypothetical suture now buried beneath the Río Colorado basin (Fig. 1), causing NEvergent deformation in the Palaeozoic sedimentary succession of the Sierra de la Ventana (Sierras Australes) and SW-vergent thrusting in northern Patagonia (von Gosen 2003 and references therein). Nevertheless, despite years of further investigation, relevant evidence is still scarce and sometimes inconclusive. Whereas some geophysical surveys have been claimed to show a major crustal discontinuity along the proposed suture zone (e.g., Chernicoff & Zappettini 2004), others have emphasized the complexity of the underlying block structure and the importance of east-west displacements, but do not indicate an overall difference in gravimetric signature across the boundary (Kostadinoff et al. 2005;Gregori et al. 2008). Palaeomagnetic data permit, but do not require, a small separation of the landmasses on either side in Devonian times (Rapalini 2005). Other authors have argued that Patagonia was an autochthonous part of South America (e.g., Varela et al. 1991; Dalla Salda et al. 1992). Rapela & Pankhurst (2002) and Pankhurst et al. (2003) supported the idea that the Early Palaeozoic Pampean and Famatinian magmatic belts of the Sierras Pampeanas might be traced continuously into the basement of the Sierra de la Ventana and northeastern Patagonia, respectively. Pankhurst et al. (2006) emphasized the idea that the North Patagonian Massif at least was an autochthonous part of South America, with a Carboniferous collision zone farth...

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