LITHOSTRATIGRAPHY AND PETROSTRUCTURAL CHARACTERIZATION OF SUPRACRUSTAL UNITS IN THE BRASILIANO BELT OF NORTHEAST BRAZIL - GEODYNAMIC IMPLICATIONS

The Proterozoic lithostratigraphic sequences of the Borborema Province north of the Patos shear zone are revised in light of new petrostruct ural and isotopic data. Key areas where an indisputable stratigraphic unconformity between the pre-Brasiliano basement and the Proterozoic c over exists are described in detail. Few supracrustal sequences in the pre-Brasiliano metaplutonic basement are reported from the basement i n southern Ceara. The Proterozoic cover comprises a cratonic to peri-c ratonic quartzite-carbonate association, locally with evaporitic beds, deposited throughout the province. Dating of volcanics interlayered w ith the metasedimentary sequences suggests that rifting and sedimentat ion began around 1.8 Ga following the Transamazonian orogeny. A second unconformity, well characterized in the Serido belt, separates this e arly rift sequence from an overlying mainly terrigenous, turbidite-lik e succession which may have been deposited in an extensional setting a round 1 Ga. Petrostructural investigations and pressure-temperature da ta show that in many places the rock units share the same metamorphic fabric, i.e., one main foliation and one main lineation developed unde r high-temperature conditions. The intensive low-pressure migmatisatio n that affected the basement over large areas coupled with the widespr ead granitic and mafic magmatism suggest that asthenospheric mantle wa s involved during the Neoproterozoic Brasiliano orogeny in this part o f the Borborema Province.