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.

A detailed study of the pre-Silurian geology of the Sierras de Córdoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C, c. 525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low- P anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C, c. 520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.

A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87 Sr/ 86 Sr (0.7075–0.7105) and low ɛNd t (−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.

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.

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