New U-Pb data for Sierra de Umango, Andean foreland at 29°S, and geodynamic implications

VARELA, R.; BASEI, M.A.S.; SATO, A.M.; GONZÁLEZ, P.D.; CINGOLANI, C.A.

San Carlos de Bariloche

Simposio; VI Symposium on South American Isotope Geology; 2008

Umango Range, 29º00’S-68º40’W, is part of the Sierras Pampeanas, a set of igneous-metamorphic basement blocks rising in the Andean foreland during the Tertiary orogeny. They are bounded by reverse faults and surrounded by Andean synorogenic sediments. Based on lithological composition, Umango Range is part of the Sierras Pampeanas Occidentales, characterized by the presence of schists and gneisses along with a significant amount of amphibolites and marbles, and the absence of granitic batholiths. To the east there are the Famatina System and the Sierras Pampeanas Orientales, characterized by Lower Paleozoic volcano-sedimentary successions, schists, gneisses and granitic batholiths. The geological composition of the Sierra de Umango is revised with a brief description of the known lithological units and six new zircon U-Pb conventional ages. Its particular feature is the Middle-Proterozoic granitic gneisses grouped in the Juchi Orthogneiss and cropping out in the southern sector of the range. An additional zircon dating from an amphibolite extends this age to a sequence of phyllites, schists and amphibolites (Tambillito Metamorphite) exposed in a narrow eastern belt. Other lithological units are considered the cover of the complex substratum. They include a clastic-calcareous sequence intruded by basic igneous rocks (Tambillo Metamorphite). During Mid-Ordovician, a metamorphic event took place at upper amphibolite facies, together with synorogenic plutonism (El Peñón Granite). A second metamorphic event registered in Mid-Devonian and this was coeval with the intrusion of gabbros (El Cordobés Metabasite) and numerous foliated pegmatites in garnetiferous schists. Finally, granitic stock intrusion took place during Early Carboniferous (Los Guandacolinos and Cerro Veladero Granites).