Contribution to the knowledge of the geological evolution of the Riacho do Pontal orogenic system – PE, BA, PI

Abstract

The Riacho do Pontal System (RPS) is just the central part of a long Neoproterozoic orogenic development placed at the northern periphery of the São Francisco Craton (SFC), from northeast (Sergipano belt) to southwest (Rio Preto belt). The basement of the foreland (SFC) and the backland (“Morro do Estreito-Icaiçara”) is mostly formed by Archean rocks, with some insertions of supracrustal and orthogneisses of Tonian age (Cariris Velhos cycle?). RPS exhibits a remarkable tectonic-metamorphic zoning from north to the south: the backland; the internal zone (supracrustal terrigenous assemblages); the central zone (ocean floor rock assemblages); the external zone (supracrustal terrigenous assemblages); and the cratonic domain (SFC), remarked by the presence of a wide (> 200 km) foreland thrust and fold belt (to the south), formed by the Neoproterozoic pelitic-carbonatic cratonic cover. The processes of deformation were intense, presenting vergence towards south and southwest, resulting in reclined and overturned folds, reverse and thrust faults, nappes, and specially the occurrences of some klippen over the basement of the northern part of the CSF. The regional metamorphism was undertaken under amphibolite conditions, with some local retrogression to the greenschists facies. This paper presents a series of new geochronological determinations (Rb-Sr, Sm- Nd and U-Pb methods) of practically all rock types of the basement, supracrustals, and of the associated syncollisional granites. It was possible to discriminate some Archean nuclei (2740 – 2624 Ma), as basement (backland), and two different set of Neoproterozoic volcano-sedimentary rock units associated with granites, the first one of Eotonian age (980 – 940 Ma) and the second one positioned in the Cryogenian-Ediacaran boundary (ca. 635 Ma). A preliminary essay of “wilsonian” tectonic evolution is here introduced for this south-western portion of the Borborema Province (the regional branching system of orogens).