Serra Negra granites as a record of Tonian rifting within the Sergipano Orogenic System, NE Brazil

Abstract

The Serra Negra batholith (940 ± 7 Ma) is one of the most important intrusions related to the Cariris Velhos event (0.9–1.0 Ga) in the southern Borborema Province, NE Brazil. This body crops out as discontinuous slices tectonically intercalated with metasedimentary sequences of the Marancó Domain in the northwestern segment of the Sergipano Orogenic System along the Sergipe and Bahia states. Here, we report new whole-rock geochemical and U-Pb geochronological data on the Tonian Serra Negra granites to elucidate the mechanisms of magma generation and differentiation and advance the understanding of the Cariris Velhos event in the Borborema Province. The Serra Negra intrusion is composed of granites with a monotonous major mineralogy of K-feldspar, plagioclase, quartz, biotite, and rare muscovite. The accessory phases include titanite, zircon, apatite, allanite, fluorite, thorite, xenotime, ilmenite, and magnetite. Most of the exposed area is now converted into augen-gneisses, with localized occurrences of isotropic, porphyritic granites preserving almost their entire igneous features. Granites from both facies are weakly metaluminous to peraluminous and show calc-alkalic to alkalic affinities, with high FeO∗/(FeO∗+MgO), SiO₂, and Na₂O + K₂O. The augen-gneisses have high HFSE and REE abundances, with significant Eu, Ba, and Sr depletions, akin to A-type granite suites worldwide. Otherwise, porphyritic granites are characterized by relatively Ba and Sr-enriched compositions, along with lower contents of HFSE and REE. The complementary geochemical behavior between augen-gneisses and porphyritic granites suggests that the latter represents partial cumulates of the parental rhyolitic magma, whose extraction led to the highly evolved composition recorded by augen-gneisses. Geological and geochemical evidence indicates that Serra Negra A-type magmas were generated by the partial melting of calc-alkaline granitoids under low-pressure conditions triggered by underplated basaltic magmas in an extensional environment. Taking into account that granite emplacement was contemporary with well-documented rifting episodes in the São Francisco-Congo craton, we interpret the Serra Negra batholith as a record of an attempt to break up the basement of the Borborema Province.