Middle Proterozoic and Lower Paleozoic of the Sierra de Umango, Andean Foreland (29°S), Argentina: U-Pb ages and isotopic characteristics.

Abstract

Igneous and metamorphic units formed mainly during the Middle Proterozoic and Early Paleozoic times have been distinguished in the Sierra de Umango crystalline basement. These include granitic orthogneisses, a sequence of schists and marbles, amphibolites, and three small granitic plutons. Geochemical analyses of granitoids and amphibolites, as well as age determinations using U-Pb and K-Ar methods were carried out. The U-Pb age in zircón of the Juchi Orthogneiss (1108±13 Ma) confirms the previous Rb-Sr Mesoproterozoic dating (1030±30 Ma). The U-Pb in zircón age of El Peñón Granite (473±17 Ma) is slighty older than the Rb-Sr age (469±9 Ma). The K-Ar mouscovite dates from El Peñón Granite (409±14 and 381±11 Ma) are related to a Devonian deformation and metamorphism. On the basis of Nd and Sr isotopic characteristics, two different sets of units can be identified, one related to the Grenvillian cycle and the second to the Pampean-Famatinian cycles. According to the data obtained for the Sierra de Umango, we interpret that in this part of the Western Sierras Pampeanas there are relics of a Mesoproterozoic basement (Grenville cycle), together with more important exposures of a schists and marble succession, with amphibolites and small granitic plutons, that were formed during Late Proterozoic to Devonian times (Pampean and Famatinian cycles). The major basement structure, formed during Ordovician to Devonian, shows a clear tectonic vergence toward the west, and consists of thrust sheets (nappes) typical of collisional tectonics. These sheets are transported from the inner parts of the orogen (Sierras Pampeanas Orientales) toward the contiguous foreland of the interacting terrane. The Upper Ordovician to Devonian sedimentary rocks of the Precordillera (foreland basin) could complete the geodynamic environment.