Neoproterozoic age of the Niquelândia Complex, central Brazil: Further ID-TIMS U–Pb and Sm–Nd isotopic evidence

Abstract

The age and tectonic significance of the Barro Alto, Niquelândia, and Cana Brava layered mafic–ultramafic complexes in Goiás have been a matter of debate and controversy. In many models, they have been considered representative of Paleoproterozoic intrusions metamorphosed in the Neoproterozoic, at approximately 0.79–0.76 Ga. Recent SHRIMP U–Pb data for dioritic rocks of the Niquelândia Complex, however, suggest that they crystallized around 0.80 Ga. Petrologic and field data suggest that these dioritic rocks are late-stage crystallization phases representing intense contamination with sialic country rocks rather than much younger intrusions into the lower layered series of the complex. In this study, igneous zircon grains from a cumulate websterite belonging to the upper part of the lower series of the Niquelândia Complex are investigated by the ID-TIMS U–Pb method and yield an age of 799±6 Ma. Sm–Nd isotopic data for the same sample reveal strong contamination with older crust ( ε Nd( T) of −6.5 and T DM of 1.88 Ga). Additional Sm–Nd data for dioritic and late-stage granitic rocks of the complex have produced a whole-rock isochron with an age of 759±65 Ma and ε Nd( T) of −8.8. The data confirm that the upper mafic unit of the lower series of the Niquelândia Complex is Neoproterozoic and that the original magma is contaminated with Meso- to Paleoproterozoic crust. The layered complex may have been intruded into a continental rift developed at the western limb of the São Francisco-Congo continent, coeval with the development of the Goiás Magmatic Arc to the west.