U-Pb zircon ages of orthogneisses and supracrustal rocks of the Cariris Velhos belt: Onset of Neoproterozoic rifting in the Borborema Province, NE Brazil

Abstract

New geochronological and geochemical constraints on Precambrian sedimentary and volcanic successions exposed in the western part of the Central Domain of the Borborema Province, NE Brazil, indicate the presence of two distinct tectono-stratigraphic complexes: Riacho Gravatá and São Caetano. Both complexes and associated orthogneisses are referred in the literature as the Cariris Velhos belt, having depositional, extrusive, or intrusive ages within the interval 985-913Ma. The Riacho Gravatá complex consists of bimodal (but mostly felsic) volcanic and volcanoclastic rocks, muscovite±graphite schists, quartzites, and marble with local occurrences of banded-iron-formation. The São Caetano complex mainly consists of metagreywackes, marbles, calc-silicate rocks, and rare meta-mafic rocks. Meta-mafic rocks from both complexes have geochemical signatures similar to those of continental flood basalts, with e{open}Nd (1.0Ga) values ranging from -1.0 to -2.8. Felsic volcanic rocks from the Riacho Gravatá complex show e{open}Nd (1.0Ga) values ranging from -1.0 to -7.4 and geochemical signatures similar to A 2-type granitoids. New SHRIMP U-Pb zircon data from felsic volcanic rocks within the Riacho Gravatá complex yielded ages of 1091±13Ma and 996±13Ma. In contrast, meta-graywackes from the São Caetano complex show a maximum deposition age of ca. 806Ma in the northern part and ca. 862Ma in the southern part of the outcrop area. The orthogneisses show e{open}Nd (1.0Ga) values ranging from 1.0 to -4.2 with U/Pb TIMS and SHRIMP ages ranging from 960 to 926Ma and geochemical signatures of A 2-type granitoids.The data reported in this paper suggest at least two periods of extension within the Central Domain of the Borborema Province, the first starts ca. 1091. Ma with magmatism and deposition, creating the Riacho Gravatá basin and continued intrusion of A-type granites to 920. Ma. A second rift event, which reactivated old faults, generated a basin with a maximum deposition age of ca. 806. Ma. Furthermore, the oldest granitoids cutting these metasedimentary rocks have crystallization ages of ca. 600. Ma. This suggests that the second rift event could be early Brasiliano in age. The resulting São Caetano basin received detritus from a variety of sources, although detritus from the Riacho Gravatá complex dominated. Deposition ages of the Riacho Gravatá and the São Caetano complexes are coeval with deposits in other basins of the Borborema Province (Riacho do Tigre in the Central Domain; Macururé and Marancó in the Sergipano Belt of the Southern domain). The Macaúbas Group from SE Brazil and its counterparts in Africa, the Zadanian and Mayumbian Groups, in the western edge of the Congo Craton are also coeval.Closure of the Riacho Gravatá and São Caetano basins occurred during the Brasiliano convergence (705-600. Ma). During the last stage of convergence, ca. 612. Ma, pull-apart basins were created and filled; final basin closure took place 605-592. Ma, after deposition ceased.