Aluminium-phosphates and borosilicates in muscovite-kyanite metaquartzites near Diamantina (Minas Gerais, Brazil): Petrogenetic implications

Within the muscovite-dumortierite-kyanite metaquartzites of the Middle Proterozoic Banderinha formation near Diamantina (State of Minas Gerais, Brazil), five rock types, Al through A4, and B are distinguished on the basis of whole-rock composition. In addition to quartz, muscovite and Ti-bearing hematite (0.2 to 5.0 wt% TiO₂), the five rock types show the following typical mineral assemblages: A1: lazulite, kyanite, andalusite; A2: lazulite, kyanite, augelite; A3: lazulite, amblygonite, berlinite, tourmaline; A4: lazulite, goyazite, tourmaline, kyanite, and B: dumortierite, kyanite. From lithostratigraphical considerations, microscopic investigations and microprobe mineral analyses, as well as stable oxygen isotope thermometry, these parageneses formed at a maximum temperature of 440°C and pressure of at least 3.4 kbar during the multistage Brasiliano tectonothermal episode between 680 and 450 Ma. Considerable solid solution series exist between lazulite and scorzalite (Mg ↔ Fe), goyazite and svanbergite (P ↔ S) and goyazite and crandallite (Sr ↔ Ca). The MgO/FeO ratio of lazulite increases with the increasing MgO/FeO ratio of the bulk rock. A contact between lazulite and dumortierite was never observed. The protolith of the lazulite-bearing layer was probably a phosphorus- and locally also boron-rich clay-bearing sandstone deposited in a sabkha-like environment in originally highly alkaline conditions. In the study area, the lazulite-bearing muscovite-kyanite metaquartzites mark the strongly tectonised contact between the Middle Proterozoic Rio Paraúna and Espinhaço supergroups.