Coeval high Ba-Sr arc-related and OIB Neoproterozoic rocks linking pre-collisional magmatism of the Ribeira and Araçuaí orogenic belts, SE-Brazil

Abstract

Two arc systems have been described within the Neoproterozoic Ribeira Belt: the inner continental magmatic arc and the outer intra-ocean magmatic arc. New geochemical and U-Pb and Lu-Hf (LA-ICPMS) data from zircons of the Marceleza-Leopoldina stocks, belonging to the inner magmatic arc system, in the boundary region between the Ribeira and Araçuaí orogenic belts are presented. The intermediate to felsic magmatic rocks have high Ba, Sr, K/Rb, LILE and LREE contents. Low Rb, Th, U, Nb, and very low Y and HREE contents point to their generation in a supra-subduction tectonic scenario, from melting of the asthenospheric wedge above a subducting oceanic plate. Zircon U-Pb (LA-ICPMS) data indicates crystallization ages between ca. 650 and 600 Ma, with high-grade metamorphic overprint at ca. 582 Ma. Tonian inherited zircons were found in the older rocks, suggesting that the development of the continental magmatic arc might have begun earlier. Associated basic (gabbros and monzogabbros) to intermediate (diorites and monzodiorites) rocks are geochemically heterogeneous. Some represent less evolved calc-alkaline basaltic magmas, while relatively more enriched varieties are intraplate basalts with OIB components. Together with lithogeochemical data, Lu-Hf signatures showing TDM model ages between ca. 1.7 to 1.3 Ga, with initial ɛHf values ranging from slightly negative values (−0.9 to −3.75) to strongly negative (up to −10.4) indicate a magmatic arc developed on continental margin environment (i.e., continental-type arc). The studied rocks represent the Rio Doce arc in the studied region and are correlated with other arc segments located from the north, in the Araçuaí Orogen, to the southernmost Ribeira Orogen, defining a c. 1000 km long continental-type magmatic arc. This inner arc system first collided against the São Francisco craton during the Neoproterozoic Brasiliano orogeny around 580 Ma. The geochemical diversity of coeval intermediate to felsic rocks and the variability of associated basic rocks are suggestive of a complex evolution of the subduction zone system. Possible mechanisms for the enrichment in incompatible elements of magmatic products may have involved slab roll-back and subduction of fore-arc rocks, and the influx of ascending asthenosphere associated with slab windows (slab break off, ridge subduction) prior to collision.