Sm-Nd ISOTOPIC DATA FROM UPPER CAMBRIAN - MIDDLE ORDOVICIAN FAMATINIAN SUCCESSIONS SHOWING A PROGRESSIVE INFLUENCE OF THE OCLOYIC ARC

COLLO, GILDA; STANG, ALEJANDRO; ASITNI, RICARDO A.; DO CAMPO, MARGARITA; ESCAYOLA, MÓNICA; PIMENTEL, MARCIO

San Carlos de Bariloche

Simposio; VI Simposio Sudamericano de Geología Isotópica; 2008

The Famatina belt constitutes a complex region within the present-day Andean foreland and has been interpreted as part of an ancient accretionary margin built along western Gondwana during the Early Paleozoic. Upper Proterozoic to Lower Paleozoic rocks within this segment show a complex variety of low to high-grade metamorphism. Low-grade rocks comprise a silisiclastic to volcaniclastic Upper Cambrian to Middle Ordovician succession that experienced a complex post-depositional history related to the Ordovician Ocloyic Orogeny. Within the sequences fine-grained rocks are abundant. In this contribution we present isotopic Sm-Nd analysis that allows interpreting provenance areas and their distribution within the Early Paleozoic. Samples of the Negro Peinado and Achavil Formations present rather homogeneous eNd(500) values, ranging from -6.3 to -9.8, with TDM values between 1.81 and 1.58 Ga (n=12). The isotopic distribution of these Upper Cambrian units suggest derivation from Paleoproterozoic crustal sources which were probably recycled during Grenville to Pampean orogenies in the central Andean region of Argentina. Volcancito, Bordo Atravesado, Suri and Molles Formations show more heterogeneous data, with TDM ages between 1.87 and 1.27 Ga (n=18) and eNd(t) between -3.1 and -12.2, consistent with progressive isotopic contribution from Ordovician juvenile material related to the Ocloyic magmatic arc. As younger the sequences, higher the contribution from younguer igneous material and hence, the lower TDM ages as demonstrated for the Molles Formation. Nevertheless, the first record of volcanic contribution is probably associated with rocks from the Upper Cambrian (~500 Ma) Negro Peinado Formation (TDM age 1.58 Ga) that allows considering the alternative of an earlier arc activity.