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Neoproterozoic backarc basin: Sensitive high-resolution ion microprobe U-Pb and Sm-Nd isotopic evidence from the Eastern Pampean Ranges, Argentina

¹ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Universidad Nacional de Salta, Buenos Aires 177, 4400 Salta, Argentina
² Instituto de Geociências, Universidade de Brasília, Brasília DF 71910-900, Brazil
³ Research School of Earth Sciences, Australian National University, Canberra, 0200 ACT, Australia

The Eastern Pampean Ranges comprise 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 ca. 640 Ma with the deposition of supracrustal sequences in a backarc basin between a Neoproterozoic magmatic arc to the east and the Pampia terrane to the west. Ophiolite remnants of this backarc basin yielded a whole-rock isochron indicating the age of 647 ± 77 Ma (2) and _Nd (initial time [T]) of +5.2. Sensitive high-resolution ion microprobe U-Pb data for detrital metasediments show provenance patterns with two main age populations: the older 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 becomes less abundant toward the west. Depleted mantle ages show a similar pattern, with ages generally increasing from east (1.42 Ga) to west (1.76 Ga), suggesting the presence of Neo-proterozoic 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 Río 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 metaluminous and peraluminous granitic magmatism ca. 530–514 Ma. 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).

Key Words: Pampean Ranges • Neoproterozoic subduction • SHRIMP • provenance • Sm-Nd ages

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