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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