Petrogenesis of the neoproterozoic alkaline ultrapotassic suites of northeastern Brazil: Major- and trace-element evidence from pyroxene chemistry and numerical modeling

Syenites, quartz syenites, and granites sharing ultrapotassic alkaline affi nities were emplaced during the late stages of Brasiliano collision (555 +/ - 10 Ma) along the northern border of the Sao Francisco craton, in the Riac ho do Pontal fold belt, northeastern Brazil. The mafic and accessory parage nesis includes diopside evolving to aegirine-augite and eventually to aegir ine, titanite, apatite, riebeckite, biotite, winchite, zircon, and magnetit e, whereas barite and ilmenite are abundant in the more evolved aegirine-be aring granites. Differentiation was controlled by now segregation and miner al fractionation. Trace-element patterns of aegirine-augite reflect the ear ly crystallization of apatite + titanite + aegirine + augite, A mantle-enri ched source affected by Paleoproterozoic subduction-related metasomatism is suggested.