U/Pb geochronology of detrital zircons from Upper Ordovician Las Vacas, La Cantera, and Empozada formations, NW Argentina

FINNEY, , S. C; GLEASON, J.D.; GEHRELS, G.E.; PERALTA, S.H.; VERVOORT, J.D.

San Juan

Congreso; 9th International Symposium on the Ordovician System, 6th International Graptolite Conference and 2003 Field Meeting Subcommission on Silurian Stratigraphy, San Juan; 2003

Univ. Nac. San Juan-Univ. Nac. Tucumán

RESUMEN: Considerable geological evidence indicates that the Precordillera terrane of northwest Argentina is exotic to Gondwana and that it rifted from the Ouachita embayment of Laurentia in early Cambrian time, drifted as a microcontinent across the Iapetus Ocean, and then collided with the proto?Andean margin of Gondwana in middle to late Ordovician time (Astini et al., 1995; Dalziel et al., 1996; Thomas and Astini, 1996). When the Precordillera terrane docked with Gondwana, according to this model (Astini et al., 1995; Thomas and Astini, in press), it collided with the Famatina system, which had formed as a westward?facing subduction complex and volcanic arc along the proto?Andean margin between 490 and 470 Ma, as a result of the convergence of the Precordillera terrane. In contrast, however, are the interpretations of Rapela et al. (1998) and Keller (1999) that the Precordillera terrane did not arrive at its position adjacent to the Famatina system until after late Silurian time, which raise questions about the history of the Precordillera during Cambrian and Ordovician time. In fact, Finney et al. (2003) concluded, on the basis of U/Pb age populations of detrital zircons in the upper Lower Cambrian Cerro Totora Formation and the lower Upper Ordovician Las Vacas Formation, that the Precordillera terrane is of Gondwanan affinity, not Laurentian, and that it was not adjacent to the Famatina system until after the Late Ordovician. In this paper, we present detrital zircon analyses from additional samples in the Precordillera, from the Upper Ordovician La Cantera and Empozada formations, that further support these conclusions.