Columbite-tantalite-bearing granitic pegmatites from the Seridó Belt, northeastern Brazil: Genetic constraints from U-Pb dating and Pb isotopes

The Seridó Belt, located in the Borborema Province, northeastern Brazil, is well known for its pegmatites and granites. The pegmatites intrude the Seridó Group and show no signs of deformation. They were emplaced along a major NE-trending shear zone during a post-Brasiliano extensional event (550-500 Ma). Homogeneous bodies of pegmatite are barren, whereas the heterogeneous ones have been mined for columbite-tantalite, beryl, gem tourmaline (elbaite), and kaolin. Electron-microprobe analyses reveal that columbite-group minerals have a composition between ferrocolumbite and manganocolumbite. Our U-Pb age determinations on these minerals show that the pegmatites were emplaced between 509.5 ± 2.9 Ma and 514.9 ± 1.1 Ma. As the pegmatites are not deformed, these ages are interpreted as a minimum age for the Brasiliano orogeny in the Seridó Belt. Our U-Pb data on monazite from a spatially related coarse-grained peraluminous granite give an age of 528 ± 12 Ma. The pegmatites are clearly younger than that granite. Lead isotopes from K-feldspar show that the coarse-grained granite and the pegmatites are derived from several separate batches of magma that received variable amounts of Pb from the distinct host-rocks. The heterogeneity in the Pb isotopic compositions is due to in situ Pb growth and the geochemically variable host-rocks.