Abstract
Detrital zircons in metasedimentary rocks attributed to three different complexes in the Central and Pernambuco-Alagoas domains of the Borborema Province, northeastern Brazil, were dated by LA-ICP-MS. Two samples from each of the following complexes were analyzed: Sertania and Surubim, in the Central Domain, and Rio Una, in the Pernambuco-Alagoas Domain. One sample of the Sertania Complex yielded only Paleoproterozoic zircons, whereas Neoproterozoic zircons predominate in the other, with the youngest zircon indicating deposition after 642 Ma. One of the samples of the Surubim Complex yielded, again, only Paleoproterozoic concordant zircon ages. In the other sample, although Paleoproterozoic zircons predominate, ages vary from the Mesoarchean to the Neoproterozoic. The age of the youngest zircon in this latter sample indicates deposition after 850 Ma and an overgrowth on an Archean grain yielded an age of 623 +/- 6 Ma (2 sigma), interpreted as the age of peak metamorphism. The two samples of the Rio Una Complex yielded predominantly detrital zircons with ages in the ranges 2.2-2.1 Ga and 1.7-1.6 Ga. In this complex, the youngest detrital zircon suggests deposition after 917 Ma and metamorphic zircons indicate peak metamorphic conditions between 600 Ma and 617 Ma.;The results of this study, together with available data from the literature, allow the following conclusions: (a) The Sertania Complex is a Neoproterozoic sequence, and not Paleoproterozoic, as previously thought; (b) The similarity in lithological associations, detrital zircon populations and C isotope signature between the Sertania and Surubim complexes suggest that they belong to the same lithostratigraphic unit; (c) Although the Rio Una Complex can be older than the Sertania/Surubim Complex, the lack of zircons younger than 900 Ma can also result from insufficient sampling or absence of young rocks alongside the drainage system that collected detritus to this complex: (d) The regional metamorphism related to the Brasiliano Orogeny attained its climax at 630-600 Ma; (e) The age spectra of detrital zircons require their derivation from erosion of rocks formed not only during known geological events in the Borborema Province and in the nearby Sao Francisco/Congo Craton, but also from the Amazonian Craton, suggesting accumulation in an intracratonic setting within a preexisting large continent: (f) The short timespan between deposition and orogenic deformation may explain the overall medium- to high-temperature metamorphism due to maintenance, in the subsequent compressional event, of the elevated geothermal gradients produced during broad-scale lithosphere extension.