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

摘要

The age and tectonic significance of the Barro Alto, Niquelandia, and Cana Brava layered mafic-ultramafic complexes in Goias 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 Niquelandia 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 Niquelandia 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 Niquelandia 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 Sao Francisco-Congo continent, coeval with the development of the Goias Magmatic Arc to the west.