SIGNIFICADO DA OCORRÊNCIA DE FOSFATOS E BORATOS DE ALUMÍNIO NO CONTATO ENTRE OS SUPERGRUPOS RIO PARAÚNA E ESPINHAÇO NA REGIÃO DE DIAMANTINA, MINAS GERAIS, BRASIL

Abstract

Within a muscovite-kyanite quartzite and/or muscovite-kyanite schist at the contact between the UpperArchean Barão de Guacuí Formation and the Bandeirinha Formation near Diamantina (State of MinasGerais, Brazil) five rock types defining mineral assemblages can be found: 1: lazulite, kyanite, 2: lazulite,augelite, kyanite; 3: lazulite, amblygonite, tourmaline; 4: lazulite, svanbergite, tourmaline, and 5:dumortierite, kyanite. Additional phases in all rock types are quartz and muscovite/sericite. The wholerock chemistry influences the chemical composition of the individual minerals.These ocurrences are interpreted as precipitation of aluminium, phosphorous and boron-rich sedimentaryrocks in highly alcaline lakes under sabka conditions in the Middle Proterozoic on the Archeanmetasediments of the Baraõ de Guacuí Formation.During the Brasiliano event (680-450Ma) a greenschist metamorphism and associated tectonism withthrusting the actual texture and foliation were formed. Mobilization of lazulite and its deposition intoquartz veins, cutting the over- and underlain units occurred, too.The aluminophosphate minerals get importance because they could help to define more precisely thecontact between the Rio Paraúna and Espinhaço supergroups.