Early Evolution of the Proto-Andean Andean Margin of South America

RAPELA, C.W.; PANKHURST, R.J; CASQUET, C.; BALDO, E.G.; SAAVEDRA, J.; GALINDO, C.

Barcelona

Seminario; I Seminario Iberoamericano de Cuencas de Antepaís en los Andes los Pirineos y los Varíscides; 1999

Universidad de Barcelona

The crustal framework of the Southern Andes was formed Paleozoic continental collisions. From detailed study of a 500 the Sierras Pampeanas, central-west Argentina, tectonomagmatic episodes are recognized, each culminating continental collisions against the proto-Andean margin Gondwana (Pankhurst et al., in press; Rapela et al., in press). The earliest recognized event in the Eastern Cordillera and northwestern Sierras Pampeanas is deposition of a turbiditic passive margin sequence with trace fossils indicating a Vendian to Tommotian age (Puncoviscana Formation and metamorphic equivalents, Durand, 1996). The Pampean orogeny started ¡n early Cambrian times with a short subduction phase, indicated by ca. 530+/- 3. My calc-alkaline granitoids (conventional U-Pb on abraded zircons), partially emplaced along the recently developed supercontinent passive margin. After the Pampean terrane collision, the margin was conditions (M2 event, 8.6 +/-0.8 kbar, 810+/5O0 relaxation to pressures ca. 4 kbar (M3 event) and migmatites and highly peraluminous granites Pampeanas. According to concordant results from migmatites monazite, Rb-Sr isochron and conventional anatectic granites and associated cordierites, synchronous at ca. 525 My.  After brief quiescence during the late Cambrian, a second major episode, the Famatinian orogeny, started with subduction at ca. 490 My, forming a wide continental arc and ensialic back-arc basin to the west of (he accreted Pampean terrane. Conventional and SHRIMP U-Pb dating of the main ho + bi granodiorite phase of the batolith yielded a combined age of 490+1-5 My. Younger monzogranite gave Rb-Sr whole-rock ages of 470-450 My, suggesting that intrusive activity lasted until the late Ordovician.