Detrital zircon geochronology (U-Pb LA-ICP-MS) of syn-orogenic basins in SW Gondwana: New insights into the Cryogenian-Ediacaran of Porongos Complex, Dom Feliciano Belt, southern Brazil

Abstract

The Dom Feliciano Belt represents the major geotectonic unit in southeastern South America, being part of the Neoproterozoic western Gondwana. The Porongos Complex comprises the southernmost Brazilian supracrustal sequence of this belt, overlying the Paleoproterozoic Encantadas gneiss (c. 2.2–2.0 Ga). Until recently, these rocks were traditionally interpreted as a single early Neoproterozoic (Tonian, c. 790 Ma) basin.Here we present new U-Pb detrital zircon ages for the Capané Antiform, located in the northern part of the Porongos Complex. The central sector, interpreted as a basement domain, shows both pronounced Paleoproterozoic (2.2–2.0 Ga) and Neoproterozoic (Tonian, 830–780 Ma) age peaks. The latter accounted for the rift-related Porongos succession that is tectonically imbricated with basement rocks. In contrast, metavolcano-sedimentary units from the eastern and western sectors reflect predominant Cryogenian-Ediacaran provenance (peaks at 615–580 Ma).This new detrital zircon signature challenges the traditional view of a single Neoproterozoic depositional episode for the Porongos Complex by implying two different sequences termed Porongos I and II of Tonian and Ediacaran ages, respectively. The Porongos I sequence is made up of mica schist from the central sector, showing Tonian maximum depositional ages. On the contrary, the younger volcano-sedimentary units of the two peripheral sectors display the Porongos II sequence deposited in a Cryogenian-Ediacaran basin. The Porongos I sequence may have evolved in an extensional setting as a marginal basin with rift-related units, whereas Porongos II formed during compression related to active thrusting, which resulted from the collision with the Pelotas Batholith.Therefore, the present age data combined with previously published geochronological, stratigraphic, and tectonic data sets identify the Capané Antiform as a key area to understand the tectonic evolution of the Porongos Complex. Its protracted evolution has recorded a tectonic history of more than 200 million years, lasting from the break-up of Rodinia to the Gondwana amalgamation.