Records of Mesoproterozoic taphrogenic events in the eastern basement of the Araçuaí Orogen, southeast Brazil
ABSTRACT: The history of palaeocontinents alternates long fragmentation to drift periods with relatively short agglutination intervals. One of the products of a Rhyacian-Orosirian orogeny was a palaeocontinent that brought together the basement of the Araçuaí-West Congo orogen (AWCO) with regions now located in the São Francisco and Congo cratons. From ca. 2 Ga to ca. 0.7 Ga, this large region of the São Francisco-Congo palaeocontinent was spared of orogenic events, but underwent at least five taphrogenic events recorded by anorogenic magmatism and/or sedimentation. The taphrogenic events are well documented in the AWCO proximal portions and neighboring cratonic regions, but lack evidence in the AWCO high-grade core. Our studies on amphibolites intercalated in the Rhyacian Pocrane complex, basement of the Rio Doce magmatic arc, allowed to the recognition of two Mesoproterozoic taphrogenic episodes. The oldest one, a Calymmian episode, is recorded by amphibolites with a zircon magmatic crystallization age at 1529 ± 37 Ma (U-Pb SHRIMP), and lithochemical signature of basaltic magmatism related to continental intraplate settings. Another set of amphibolite bodies records the youngest taphrogenic episode, a Stenian event, with a zircon magmatic crystallization age at 1096 ± 20 Ma (U-Pb SHRIMP), and lithochemical signature similar to mature magmatism of continental rift setting. The Calymmian episode (ca. 1.5 Ga) correlates to the Espinhaço II basin stage and mafic dikes of the northern Espinhaço, Chapada Diamantina and Curaçá domains, while the Stenian episode (ca. 1.1 Ga) correlates to the Espinhaço III basin stage. We also present U-Pb data for 87 detrital zircon grains from a quartzite lens intercalated in the Pocrane complex, the Córrego Ubá quartzite. Its age spectrum shows main peaks at 1176 ± 21 Ma (35%), 1371 ± 30 Ma (18%), 1536 ± 22 Ma (19%), 1803 ± 36 Ma (17%) and 1977 ± 38 Ma (12%), suggesting a Stenian (ca. 1176 Ma) maximum depositional age (although only one zircon with low discordance shows an age of 955 ± 66 Ma). Comparing with data from the western sector of the Araçuaí orogen and São Francisco craton, it is noteworthy that no igneous zircon from the three samples yielded an age older than early Orosirian (~2.05 Ga), showing age spectra essentially limited in the range of ca. 1-2 Ga; i.e., younger than the Late Rhyacian orogeny that amalgamated the basement, and older than the main anorogenic event (930-870 Ma) associated with the Early Tonian precursor basin of AWCO. All together, these continental taphrogenic events testify the several unsuccessful fragmentation attempts that affected the long-lived São Francisco-Congo palaeocontinent, which remained preserved from a complete break-up associated with ocean spreading from the Early Orosirian to the Atlantic opening in Cretaceous time.