Anorogenic alkaline granites from northeastern Brazil: major, trace, and rare earth elements in magmatic and metamorphic biotite and Na-mafic minerals

Abstract

The anorogenic, alkaline silica-oversaturated Serra do Meio suite is located within the Riacho do Pontal fold belt, northeast Brazil. This suite, assumed to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional event. Preserved late-magmatic to subsolidus amphiboles belong to the riebeckite-arfvedsonite and riebeckite-winchite solid solutions. Riebeckite-winchite is frequently rimmed by Ti- aegirine. Ti-aegirine cores are strongly enriched in Nb, Y, Hf, and REE, which significantly decrease in concentrations towards the rims. REE patterns of Ti-aegirine are strikingly similar to Ti-pyroxenes from the Ilimaussaq peralkaline intrusion. Recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe + Mg) ratios. Recrystallised amphibole is pure riebeckite. Magmatic Ti-Na-bearing pyroxene was converted to low-Ti aegirine + titanite + astrophyllite/aenigmatite. The reaction riebeckite + quartz aegirine + magnetite + quartz + fluid is also observed. Biotite and Na-mafic minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 600 degreesC between the NiNiO and HM buffers.