New U-Pb zircon ages and initial Nd isotope data from mafic and ultramafic rocks that have been tectonically imbricated on the southwest margin of the Precordillera terrane, western Argentina, indicate that these rocks do not constitute a single ophiolite pseudostratigraphy as previously thought. Rock units include Late Proterozoic (576 ± 17 Ma) microgabbro and Silurian (418 ± 10 Ma) sills, both of which have initial Nd isotope values of +6.0 to +7.3 and identical trace element geochemistry indicating formation in extensional settings. Late Ordovician (450 ± 20 Ma) layered gabbro has a distinct initial Nd isotope value of +0.6. It intrudes quartzofeldspathic gneiss and tectonized ultramafic rocks; this assemblage represents the deep part of a continental margin volcanic arc that formed on the eastern margin of Chilenia, as the ocean basin west of the Precordillera terrane began to close in the Middle Ordovician. The new tectonic interpretations generally support existing hypotheses that the Precordillera terrane rifted and drifted from southeastern Laurentia in the Cambrian to Early Ordovician and that the western margin of the Precordillera terrane represents a suture between two fragments of continental crust.

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