Isotopic strontium, carbon and oxygen study on neoproterozoic marbles from Sierra de Umango, Andean Foreland, Argentina

The Umango Hill (La Rioja Province, 29o00'S-68o40'W) is one of the mountain blocks of Sierras Pampeanas Occidentales (Caminos, 1979), bounded by thrust faults and surrounded by Upper Devonian to Tertiary marine and continental sedimentites. The exposed crystalline basement is composed of basic igneous rocks and a siliciclastic-limestone sequence, both affected by amphibolite facies metamorphic peak. In the southern area (Juchi creek), the metamorphic complex carries relics of granitic orthogneisses, with Rb/Sr and U/Pb dates of ∼1000 Ma (Varela et al., 1996). These ancient inliers were asigned to a Mesoproterozoic Grenville Orogenic Cycle. Granitic bodies, intrusives at different ages in the Metamorphic Complex, have also been distinguished. The most ancient is El Penon Granite, with 469±9 Ma Rb/Sr age (Varela et al., 2000) and 523±26 Ma U/Pb zircon age (unpublished data). In this way it is possible to point out broadly that the siliciclastic-limestone sequence belongs to the Neoproterozoic-Early Palaeozoic times. It was a platform cover over grenvillian cratonic basement. The metamorphism and deformation, we understand, took place in the Early Palaeozoic, related to the Pampean-Famatinian Orogenic Cycle. In this work, compositional and isotopic data of Strontium (87Sr/86Sr), Carbon (δ13CvPDB) and Oxygen (δ18OvPDB) of the marbles derived from the siliciclastic-limestone sequence are presented. The results are interpreted and correlated with the temporal variation curves of 87Sr/86Sr and δ13C from Neoproterozoic marine carbonates (Jacobsen and Kaufman, 1999) (au)