Detrital zircon ages and geochronological constraints on the Neoproterozoic Puga diamictites and associated BIFs in the southern Paraguay Belt, Brazil

Abstract

The Paraguay belt comprises a thick sedimentary succession deposited on the southwestern border of the Amazonian Craton and the Rio Apa Block. The base of the succession in the southern Paraguay belt is marked by a level of glacially derived deposits from the Puga Formation associated with banded iron formations, which has been assumed to be end-Cryogenian in age (635Ma) by previous authors is spite of the lack of geochronological data. Here we present the first U-Pb SHRIMP ages on detrital zircon grains separated from the matrix of six samples of these diamictites two different localities (Puga Hill and Bodoquena area). U-Pb ages determined from two samples (ca. 130 grains) of Puga Hill show a large variation between 970Ma and 2100Ma. Rocks with these ages can be found in the Amazonian Craton suggesting that it is the most probable source of the sediments. Detrital zircons (ca. 230 grains) from the Bodoquena area (about 200km south of Puga Hill) range from 706 to 1990Ma. The 1760Ma source is significantly more important in these samples, comprising more than 70% of analyzed grains, and indicates provenance from the adjacent Rio Apa Block. The youngest zircon was dated at 706±9Ma, thus constraining the maximum depositional age for the Puga Formation. Possible sources for this younger population could be either the juvenile Mara Rosa magmatic arc in the Brasilia belt, or the rocks from the Laurentian external fold belts located to the west of the sampled area in Neoproterozoic paleogeographic reconstructions. The maximum depositional age of the diamictites (and associated BIFs), together with cap carbonate carbon and strontium isotope data (δ13C=-5.0 and 87Sr/86Sr=0.7077) in Puga Hill, indicate that they were deposited after 700Ma, suggesting that they may represent the end-Cryogenian event.