Neoproterozoic backarc basin: Sensitive high-resolution ion microprobe U-Pb and Sm -Nd isotopic evidence from the eastern Pam,pean ranges, Argentina.

MÓNICA P. ESCAYOLA; MÁRCIO M. PIMENTEL; RICHARD ARMSTRONG

GEOLOGY

Geological Society of America (GSA)

Lugar: Boulder, Colorado USA; Año: 2006 vol. 35 p. 495 - 498

0091-7613

The Eastern Pampean Ranges comprises high-grade supracrustal sequences with linear belts of mafic-ultramafic bodies representing ophiolite remnants. New U-Pb and Nd isotopic data suggest that the tectonic evolution of the Pampean Ranges started at ca. 640 Ma with the deposition of supracrustal sequences in a back-arc basin between a Neoproterozoic magmatic arc to the east, and the Pampia terrane, to the west. Ophiolite remnants of this back arc basin yielded a whole-rock isochron indicating the age of 647 ± 77 Ma (2s) and initial eNd  of +5.2. SHRIMP U-Pb data for detrital metasediments show provenance patterns with two main age populations: the older at 1.1–0.9 Ga, and the younger between ca. 0.7 and 0.6 Ga. The Neoproterozoic population is relatively more abundant in sediments of the easternmost units of the Eastern Pampean Ranges and become less frequent towards the west. TDM ages show a similar pattern, with ages generally increasing from east (1.42Ga) to west (1.76 Ga), suggesting the presence of Neoproterozoic sources to the east of the ranges. The provenance data do not support previous evolution models for the Eastern Pampean Ranges according to which the supracrustal sequence represents the passive margin of the Rio de la Plata Craton. Early Cambrian collision and high-grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc-alkaline meta- and peraluminous granitic magmatism at ca. 530–514Ma. The results suggest that the geological evolution of the Eastern Pampean Ranges took place between ca. 640 and 514 Ma, coeval with other Brasiliano orogens in Brazil (e.g., the Paraguay and Araguaia fold belts).