Zircon Geochronology and Petrology of Alkaline-Potassic Syenites, Southwestern Serrinha Nucleus, East São Francisco Craton, Brazil

Abundant syenitic magmatism occurred on the São Francisco Cráton (Bahia State, Brazil) during the Paleoproterozoic period. Two syenitic stocks (Serra do Pintado and Morro das Agulhas-Bananas) from the eastern part of the craton have been dated at 2098 ± 2 Ma (U-Pb) and 2086 ± 17 to 2067 ± 22 Ma (Pb-Pb evaporation), respectively, suggesting that these are contemporaneous intrusions correlated with a post-Transamazonian episode of magmatism. In both of these stocks, hypersolvus alkali-feldspar syenites predominate. Subordinate hornblende-cumulates also occur, as well as dikes of alkali-feldspar syenite and alkali-granite. These syenitic plutons are alkaline, potassic, strongly enriched in LILE, and HFSE depleted. They also show moderate Al2O3, high La/Nb and Ba/La ratios, and low Ce/Pb ratios. Petrographic and geochemical data suggest that these syenitic rocks are the result of fractional crystallization from potassic mafic magmas. These data indicate that the magmas were produced during subduction, and that garnet was a stable constituent of the residue of partial melting, indicating a depth of at least 80 km. These potassic magmas did not crystallize plagioclase which, along with other chemical and mineralogical characteristics, indicates a lamprophyric affinity.