PETROLOGY OF THE NEOPROTEROZOIC ITARANTIM NEPHELINE SYENITE BATHOLITH, SAO FRANCISCO CRATON, BAHIA, BRAZIL

Abstract

The Itarantim nepheline syenite batholith, with an area of 230 km2, is located at the extreme south of the Southern Bahia Alkaline Province, in Brazil. It has a Pb–Pb zircon age of 721 ± 3 Ma, and consists of the Serra do Rancho Queimado facies of pyroxene-bearing nepheline syenite, and the Serra do Felissimo facies of biotite-bearing nepheline syenite, surrounded by fenite. The two facies have diffuse contacts, and occur along oriented NE–SW bands. The mineralogical and geochemical evolution of the pyroxene syenites was controlled by fractional crystallization, but the biotite-bearing nepheline syenites, whose evolution involves decreasing SiO2 and strong increases in Na2O (up to 9.6%) and Al2O3 (up to 23.5%) contents, probably evolved by open-system behavior with peralkaline fluids. This is indicated by the presence of fenites, and by the strong enrichment in Zr (up to 5100 ppm), Hf (up to 141 ppm), Th (up to 117 ppm), U (up to 74 ppm), Ta (up to 169 ppm) and Nb (up to 1372 ppm) in dyke rocks. The eNd(T) values of the rocks, between +0.6 and +3.0, and eSr(T) values, between − 6.3 and − 12.7, lie along the mantle array, whereas geochemical characteristics are similar to those of OIB. The δ13C values, between − 6.37 and − 6.83‰, and δ18O values, between 6.48 and 6.98‰ in interstitial carbonates, show that even the late magmatic fluids retain a mantle signature. The anorogenic character, the mantle signatures, and the age of the batholith show that it was emplaced during the breakup of Rodinia.