Arc-related basins of the Neoproterozoic Araçuaí orogen, SE Brazil

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Pedrosa-Soares, Antonio Carlos۱; Silva, Luiz Carlos۲; Cordani, Umberto۳; Noce, Carlos۱; Martins-Neto, Marcelo۴; Lima, Sirlene۵; Vieira, Valter۲
Holding Date 21 September 2008

The Araçuaí orogen extends from the eastern edge of the São Francisco craton to the Atlantic coastline in southeastern Brazil. After the identification of a precollisional calc-alkaline plutonic suite of a continental margin magmatic arc developed from 630 Ma to 585 Ma, recent geochemical and geochronological studies disclose records of three Neoproterozoic arc related basins, named Rio Doce, Nova Venécia and Salinas. The Rio Doce Group, closely associated to the magmatic arc, includes a lower volcanic-sedimentary succession with pyroclastic rocks preserved from the regional metamorphism and metamorphosed volcaniclastic rocks. Both pyroclastic and volcaniclastic rocks show medium-K calc-alkaline dacitic composition and other lithochemical attributes of arc volcanism. The pyroclastic dacite yielded a zircon U-Pb (TIMS) age of 584 ± 5 Ma, indicating a late episode of intra-arc volcanism. U-Pb (LA-ICPMS) less discordant ages of detrital zircon grains from metavolcaniclastic dacitic rocks cluster from 643 Ma to 605 Ma, being the youngest concordant age of 585 ± 4 Ma. These rocks are interpreted as fore-arc basin deposits. This volcanic-sedimentary succession is covered by metawackes with lithochemical signature of arc provenance and detrital zircon grains with U-Pb (LA-ICPMS) ages of 594 ± 3 Ma and 643 ± 3 Ma. The metawackes protoliths are interpreted as proximal turbiditic deposits of a back-arc marine basin.
The Nova Venécia Complex occurs to the east of the magmatic arc and mainly consists of high metamorphic grade paragneisses rich in garnet and cordierite. Their geochemical attributes suggest wacke-pelitic protoliths, and together with ages of detrital zircon grains indicate sedimentary sources located in the magmatic arc of the Araçuaí orogen. The U-Pb SHRIMP ages of detrital zircon grains cluster from 640 Ma to 590 Ma, being the youngest concordant value of 589 ± 10 Ma. The Nova Venécia Complex also represents marine sedimentation in the back-arc basin of the Araçuaí orogen, and is considered to be a distal correlative of the Rio Doce wacke succession. The Salinas Formation occurs somewhat far to the northwest of the magmatic arc. This formation comprises a turbiditic wacke-pelite succession metamorphosed from the low greenschist to the low amphibolite facies, with clast-supported conglomerate lenses rich in pebbles and cobbles of intermediate to felsic volcanic rocks. Lithochemical attributes of metawacke samples, as well as U-Pb SHRIMP ages of detrital zircon grains extracted from them, indicate sedimentary sources located in the magmatic arc of the Araçuaí orogen. Zircon crystals from cobbles of the felsic volcanic rocks yielded a U-Pb SHRIMP age of 622 ± 18 Ma. However, the maximum sedimentation age of the Salinas Formation is constrained by the youngest concordant detrital zircon grains dated at 599 ± 24 Ma. These basins underwent regional metamorphism from 580 to 560 Ma.