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

The anorogenic, alkaline silica-oversaturated Serra do Meio suite is locate d within the Riacho do Pontal fold belt, northeast Brazil. This suite, assu med to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional ev ent. Preserved late-magmatic to subsolidus amphiboles belong to the riebeck ite-arfvedsonite and riebeckite-winchite solid solutions. Riebeckite-winchi te is frequently rimmed by Ti-aegirine. Ti-aegirine cores are strongly enri ched 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 m ineral assemblages was associated with deformation although some original g rains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe + Mg) ratios. Recrystallised amphibole is pure r iebeckite. 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-m afic minerals recrystallised under metamorphic oxidising conditions corresp onding to temperatures of 600 degreesC between the NiNiO and HM buffers. (C ) 2001 Elsevier Science Ltd. All rights reserved.