870-850 Ma-old magmatic event in eastern Borborema Province, NE Brazil: Another Tonian failed attempt to break up the São Francisco Paleoplate?

Abstract

The Central and Southern subprovinces of the Borborema Province, northeastern Brazil, were the locus of an important early Tonian (c. 1000-920 Ma) magmatic event called Cariris Velhos. Here we report results of a geochronological study conducted in eastern Borborema showing that another magmatic event took place c. 50 Ma after this event had ceased. We dated three small orthogneiss units: one in the Rio Capibaribe Domain (RCD, Central Subprovince) and two in eastern Pernambuco-Alagoas Domain (PEALD, Southern Subprovince). We obtained LA-ICP-MS U–Pb zircon ages of 863 ± 7 Ma (RCD), and 851 ± 8 Ma and 869 ± 4 Ma (PEALD), which are interpreted as crystallization ages. The results indicate that the RCD and PEALD were part of the same crustal block at c. 870-850 Ma and thus that the East Pernambuco shear zone, which separates them, is not a suture. Rocks with ages corresponding to the Cariris Velhos event and those reported in this study also occur in the São Francisco Craton. These similarities suggest that the Borborema Province and the São Francisco Craton were part of the same plate, which was subject to several extensional episodes before the convergence that led to the Brasiliano Orogeny.