GEOCHRONOLOGY OF THE BREALITO BASIN, NORTHWESTERN ARGENTINA

Isotopic analyses of zircons from the Brealito basin, Northwestern Argentina, are combined with new field observations to elucidate the tectonic history of the Andean Eastern Cordillera. The basin is delineated by the local extent of the Pirgua Sub-group, composed of clastic sediments deposited during a Cretaceous episode of intra-cratonal extension known as the Salta Rift. Unroofed by Andean structural inversion, the intrusion of the Famatina Arc into the Pampean Puncoviscana Formation is exposed in the footwalls of the rift graben. Contact metamorphic features are present around the Brealito and Angostura Plutons, in the southern Cachi Range, and in a zone of mixing and assimilation between country rock and the Famatina Batholith at the southern end of the Valle Luracatao. Uranium-lead ages obtained by LA-MC-ICPMS of zircons from Famatina plutonic rocks show a trend of eastward-younging emplacement during the Ocloyic Orogeny. The weighted mean ages are 490.8 ± 2.7 Ma in the batholith; 475.4 ± 2.0 Ma in the southern Cachi Range; 471.9 ± 4.1 Ma in leucogranitic dykes near Cerro Rumio; 464.5 ± 4.9 Ma in the Brealito Pluton; and 452.1 ± 4.5 Ma in the Angostura Pluton. Uranium-thorium-helium ages from the same zircons indicate a period of exhumation in the Carboniferous, consistent with the Acadian-equivalent Chanica Orogeny, and zircons with high uranium concentrations record Salta Rift extension in the early Cretaceous. Field observations of conglomerate facies and U-Pb ages of detrital zircons from the Pirgua Subgroup indicate that Cretaceous syn-rift sediments were almost entirely derived from Famatina-related plutonic rocks to the west of the basin, with a minor component from the Puncoviscana Formation. Compared to the nearby Angostaco and Pucara basins, there is relatively little storage of Neogene sediments in the Brealito basin, and uranium-thorium-helium ages from detrital zircons in the Pirgua have not been reset. These data suggest that the Brealito Basin did not experience significant reburial during the Andean Orogeny, but was rapidly incorporated into the modern cordillera as an intermontane basin and bypassed by foredeep sedimentation.