The alkaline silica-saturated ultrapotassic magmatism of the Riacho do Pontal Fold Belt, NE Brazil: an example of syenite–granite Neoproterozoic association

Abstract

The Neoproterozoic of northeastern Brazil was marked by the development of collisional fold belts, mainly surrounding the São Francisco Craton, and an associated widespread granitic magmatism. The Casa Nova (555±10Ma, Sri=0.7068 and Engraçadinha syenites and granites, intrusive in the Riacho do Pontal Fold Belt, are related to the late stages of this collisional event. Melanocratic syenites, probably generated by magmatic-flow cumulate processes, and mesocratic and leucocratic syenites, representing magmatic liquids, are associated with granites, pegmatites, and syenite–granite dykes. Homogeneous or perthitic alkali feldspar, quartz, aegirine–augite, diopside, titanite, apatite, magnesian biotite, winchite–richterite, and magnetite are the dominant mineral phases. It is suggested that these magmas belong to an ultrapotassic silica-saturated alkaline series, defined on the basis of its alkaline, silica-saturated character and by a K2O/Na2O ratio of >3.0 — that is, intermediate. Major- and trace-element evolution is consistent with mineral fractionation processes, controlled by magmatic flow, and dominated by apatite–titanite–pyroxene in the less differentiated terms and by alkali feldspar in the more evolved. The source of primary magmas is a previously subduction-metasomatised mantle, probably with anomalous enrichment in LREE and LILE elements. Barite–ilmenite mineralisations are related to the more differentiated Engraçadinha granites.