Early to late Neoproterozoic subduction-accretion episodes in the Cariris Velhos Belt of the Borborema Province, Brazil: Insights from isotope and whole-rock geochemical data of supracrustal and granitic rocks

摘要

The Cariris Velhos Belt is an unique domain of the Borborema Province, Brazil, because it presents magmatic and metamorphic lithotectonic associations spanning all the Neoproterozoic. In this study, we investigate the main sources of its major supracrustal sequence (i.e. Sao Caetano Complex) as well as genetic aspects of a later intrusive granitic body, the Sao Pedro Stock. Detrital U-Pb zircon determinations for the supracrustal rocks are characterized by a major peak in the range of 1.0-0.9 Ga, with maximum deposition age at around 0.86 Ga. Minor components are aged at 1.8-1.6 Ga and 2.5 Ga. The Nd isotopic composition for this unit suggest crustal reworking of Meso-to Paleoproterozoic sources as well as inheritance of juvenile mantle material. Whole-rock geochemical data points out to derivation from magmatic arc-related andesites and rhyolites as well as foreland material, and we interpret this sequence as remnant of the Cariris Velhos (1.0-0.9 Ga) post-orogenic basin. The Sao Pedro Stock corresponds to a mostly isotropic intrusion of dominantly granitic composition. U-Pb zircon data from a monzogranitic and a tonalitic samples suggest crystallization ages of ca. 615 +/- 02 Ma and 596 +/- 22 Ma, respectively. Zircon crystals with ages of ca. 981 +/- 29 Ma are interpreted as inherited crystals. Sm-Nd isotopic behavior is similar from samples of the Sao Caetano Complex, whereas overall geochemistry suggests crystallization from high-K talc-alkaline and peraluminous magmas, in addition to trace-element distribution compatible with Cordilleran granites. It is proposed that the Sao Pedro stock were formed in a modern-style subduction zone in later stages of the Brasiliano-Pan African orogeny (0.6-0.5 Ga). Accordingly, we conclude that systematic subduction-accretion episodes took place in this part of West Gondwana from early to late Neoproterozoic times.