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 petrostructural and isotopic data. Key areas where an indisputable stratigraphic unconformity between the pre-Brasiliano basement and the Proterozoic cover exists are described in detail. Few supracrustal sequences in the pre-Brasiliano metaplutonic basement are reported from the basement in southern Ceara. The Proterozoic cover comprises a cratonic to peri-cratonic quartzite-carbonate association, locally with evaporitic beds, deposited throughout the province. Dating of volcanics interlayered with the metasedimentary sequences suggests that rifting and sedimentation began around 1.8 Ga following the Transamazonian orogeny. A second unconformity, well characterized in the Serido belt, separates this early rift sequence from an overlying mainly terrigenous, turbidite-like succession which may have been deposited in an extensional setting around 1 Ga. Petrostructural investigations and pressure-temperature data show that in many places the rock units share the same metamorphic fabric, i.e., one main foliation and one main lineation developed under high-temperature conditions. The intensive low-pressure migmatisation that affected the basement over large areas coupled with the widespread granitic and mafic magmatism suggest that asthenospheric mantle was involved during the Neoproterozoic Brasiliano orogeny in this part of the Borborema Province.