Detrital zircon ages and Nd isotope compositions of the Seridó and Lavras da Mangabeira basins (Borborema Province, NE Brazil): Evidence for exhumation and recycling associated with a major shift in sedimentary provenance

Abstract

U–Pb ages of detrital zircons and neodymium (Nd) isotope compositions from two Neoproterozoic schist belts (Seridó and Lavras da Mangabeira) in the Borborema Province (NE Brazil) indicate a significant change of sediment provenance during the deposition of sands and conglomerates and the overlying graywacke-pelite sequences. The shift in provenance defines an unconformity in the Seridó Group characterized by a metadiamictite horizon that contains pebbles and cobbles of the Paleoproterozoic to Archean basement and recycled quartzites of the underlying Equador Formation. In Lavras da Mangabeira basin the change in provenance is defined by a cryptic internal disconformity that is characterized, as in the Seridó basin, by a time gap of c. 1.0Ga in the age of the detrital zircons. Nd compositions across these unconformities indicate that the pelites are very radiogenic (ɛNdpresent-day=−4 to −11) compared with the sandy deposits (ɛNdpresent-day=−22 to −31). The provenance of the metapelites at the top of the metasedimentary succession includes a prominent population of Cryogenian zircons with the youngest grains dated to between 0.64 and 0.62Ga. These zircons were deposited in a basin developed on continental basement that preserves remnants of near-shore to terrestrial deposits with no detrital zircons younger than 1.8Ga. This unconformity defines a useful stratigraphic horizon to correlate with other metasedimentary successions preserved in the Borborema Province. In the Seridó belt the unconformity documents the uplift of the orogenic hinterland, possibly during the continental collision, which preceded the deposition of the deep-water pelites.