PRE-BRASILIANO OROGENIC EVOLUTION IN THE SERIDÓ BELT, NE BRAZIL: CONFLICTING GEOCHRONOLOGICAL AND STRUCTURAL DATA

Abstract

The Serido Belt (Borborema Province, NE Brazil) comprises Proterozoic supracrustal formations (the Serido Group), its gneissic basement and granitoid plutons. The older intrusions (the G (sub 2) orthogneisses) were affected by penetrative tangential deformation (D (sub 2) event). The Brasiliano transcurrent deformation (D (sub 3) ) overprints structures ascribed to the older (D (sub 2) ) tangential regime, whose absolute age is still under debate. A 1.99+ or -0.01 Ga Pb/Pb zircon evaporation date was obtained from a G (sub 2) augen gneiss pluton, thought to be intrusive in a flysch-type formation of the Serido Group. Syn-D (sub 2) metapegmatites are at least 1.80+ or -0.03 Ga old, as defined by a Rb-Sr isochron. Such data are consistent with a late Paleoproterozoic collisional orogeny (1.95+ or -0.05 Ga) in the region (Jardim de Sa 1994). On the other hand, U-Pb zircon and Nd model dates from Serido Group supracrustals point to a much younger, Meso- to Neoproterozoic age for this unit (Van Schmus et al. 1995a,b), allowing to relate the collisional orogeny to the Brasiliano Cycle. Both interpretations bear conflicting points and still unsolved questions, which will have to be addressed by future investigations in the region. Notwithstanding, the Serido Belt is regarded as a continental microplate preserving an older thermotectonic imprint, welded to adjacent terranes in a Meso or Neoproterozoic orogenic event.