SHRIMP U-Pb and Sm-Nd ages of the Niquelândia layered complex: Meso- (1.25 Ga) and Neoproterozoic (0.79 Ga) extensional events in central Brazil

The Niquelândia mafic-ultramafic layered complex, in the central part of the Brasília Belt, is one of three large complexes forming an approximately 350 km long linear NNE belt. The complexes are metamorphosed under granulite to amphibolite facies and have been traditionally interpreted either as part of an important Paleoproterozoic (ca. 2.0-1.8 Ga) rift system or as the exposure of ancient (Archean or Paleoproterozoic) oceanic crust. In both cases, they have been always interpreted as part of the basement of the Neoproterozoic rocks of the Brasília Belt. New SHRIMP-RG U-Pb data and Sm-Nd mineral isochron age do not support these models. The results indicate that the eastern part of the complex (Lower Series) is a Neoproterozoic (797±10 Ma) continental intrusion, heavily contaminated with older sialic material (ε_Nd(T) of -5.8, and inherited zircon grains with ages between ca. 1.0 and 1.9 Ga). The western part (Upper Series) represents a rift-related Mesoproterozoic layered body (1248±23 Ma). These two different bodies have been tectonically juxtaposed along a major shear zone. The complex was metamorphosed at 765±8 Ma as indicated by the age of metamorphic zircon overgrowths in rocks of the Upper Series. These data confirm previous studies suggesting that the Niquelândia Complex is made of two distinct intrusions with different ages and isotopic characteristics. The new data forced the re-interpretation of previous U-Pb and Re-Os geochronological data for the Niquelândia Complex which have indicated Paleoproterozoic (ca. 2.0 Ga) crystallisation ages and suggested that the Mesoproterozoic age of ca. 1.25 Ga represents a metamorphic event. Extensional events at approximately 1.25 and 0.8 Ga, although well documented in other continents (e.g. Laurentia, Baltica), are not usual in the Precambrian terrains of South America, suggesting that the continental block including these complexes and associated volcano-sedimentary sequences may be allochthonous, being accreted to the Brasília orogen during the Neoproterozoic.