Evidences of Rhyacian extensional event in the Central Domain of the Borborema Province, NE Brazil

Abstract

Granitic gneisses (Fazenda Salvador) and metamafic to meta-ultramafic rocks occur as intrusions in granodioritic to granitic orthogneisses, in the Central Domain of the Borborema Province, NE Brazil. The Fazenda Salvador orthogneisses are coarse to very coarse grained, porphyroclastic to equigranular and locally migmatized. Enclaves of dioritic compositions are common, usually showing crenulated to lobate contacts with the augen gneisses. Metabasite and meta-ultramafic rocks including amphibolite, garnet amphibolite and garnet pyroxenite, were recorded in the studied area. They occur as lens and pods within the migmatized granodioritic orthogneisses. The Fazenda Salvador orthogneisses are slightly peraluminous, ferroan and show geochemical signatures of alkaline granitoids. They show REE patterns characterized by (Ce/Yb)<inf>N</inf> ratios ranging from 111.84 to 8.65 and Eu negative anomalies (Eu\Eu* = 0.38 – 0.81). The spidergram patterns are characterized by trough at Sr, Nb, P and Ti. The metamafic rocks are tholeiitic basalts metamorphosed under high-P granulitic conditions. They show high TiO<inf>2</inf>/K<inf>2</inf>O ratios, similar to those recorded in ocean tholeiitic basalts. The metabasites and Fazenda Salvador orthogneisses have similar U-Pb zircon SHRIMP ages, 2095 ± 4 Ma and 2094 ± 8 Ma respectively, which are interpreted as the protholith crystallization ages. Our data suggest an extensional event of Rhyacian age in the Central Domain of the Borborema Province. Show more