Origem, significado petrogenético e idade dos ortognaisses Terra Nova: registro de magmatismo alcalino transtensional neoproterozoico no domínio da zona transversal da província Borborema

Abstract

The Terra Nova syenitic orthogneiss is intrusive into the Feira Nova fold belt of the Vertentes Complex, eastern Rio Capibaribe Terrane of the Borborema Province, northeastern Brazil. The orthogneisses occurs as tabular intrusions that have been folded by transcurrent tectonics related to the Glória do Goitá shear zone, which limits the paleoproterozoic basement and the Feira Nova fold belt. The rock is banded alkali-feldspar syenite to alkali-feldspar granite, in which mafic bands are composed of aegirine-augite and magnetite, while the felsic ones are composed of orthoclase and minor albite. The rock is chemically peralkalic to slightly peraluminous, showing high FeO/MgO and Ga/ Al ratios that together with concentrations of Zr, Nb, Ce e Y suggest it is lithospheric mantle-derived A1-type anorogenic granitoids. Chondrite-normalized rare earth element (REE) patterns show enrichment of light REE compared with the heavy REE suggests LREE enrichment in relation to HREE with pronounced negative Eu anomaly, suggestive of feldspar fractionation. This is also compatible by interelemental variations of Rb, Sr and Ba, as well as Rb/Sr vs. Sr/Ba. Tectonic setting discriminating diagrams indicate that the syenites have been emplaced in a post-collisional to within-plate tectonic setting. Isotopic data U-Pb zircon dating an indicated rystallization age the syenogranitc orthogneiss of around 617 ± 8.8 Ma, suggesting an transtensional ediacaran event in the Transversal Zone Domain of the Borborema Province. Key words: Alkalic syenite; A-type granites; Transtensional regime.