Mafic and ultramafic crustal fragments of the southwestern Precordillera terrane and their bearing on tectonic models of the early Paleozoic in western Argentina

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. Ro ck 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 e xtensional settings, Late Ordovician (450 +/- 20 Ma) layered gabbro has a d istinct initial Nd isotope value of +0.6. It intrudes quartzofeldspathic gn eiss and tectonized ultramafic rocks; this assemblage represents the deep p art of a continental margin volcanic are 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 drift ed from southeastern Laurentia in the Cambrian to Early Ordovician and that the western margin of the Precordillera terrane represents a suture betwee n two fragments of continental crust.