Regional implications of U-Pb zircon ages from rhyolitic pebbles of Suncho Formation conglomerates, northern Sierras Pampeanas (NW Argentina)

Abstract

Conglomerates are scarce in the Neoproterozoic/Cambrian strata of the South American Central Andean Basin. In NW Argentina, unusual conglomerates within meta-psammites of the Suncho Formation contain rhyolite clasts that yielded 524.9 ± 1.12 Ma and 525.1 ± 1.3 Ma LA-ICP-MS U-Pb zircon ages. These ages coincide with the reported youngest age populations obtained from detrital zircons in the host rocks. The new geochronological data, the regional geology and published data suggest that the source areas of these pebbles were located to the NE and E of the Suncho Formation, which may include some reworked older sedimentary levels of the Puncoviscana Formation. The origin of the Suncho conglomerate is linked to Pre-Tilcaric magmatism which closed the Pampean Cycle, whose ages are in the range 541-517 Ma. The match between the radiometric data and the age provided by the Oldhamia trace fossils with both sedimentary and magmatic events occurring during the Terreneuvian - Series 2 time span ("lower Cambrian") is here emphasized. Considering that the dated rhyolite clasts were derived from magmatic rocks, a Puncoviscana active margin can be proposed as their source, and an active margin setting is proposed for Puncoviscana in this part of the western margin of South America. The knowledge of unusual facies within the Neoproterozoic/Cambrian sequences in the South American Andes provides a better understanding of the geology of little known areas in the western protogondwanan margin.