GEOCHIMICAL AND METAMORPHIC CHARACTERIZATION OF PROTEROZOIC ROCKS OF THE PAJEU-PARAIBA FOLDBELT, NE BRAZIL

The Pajeu-Paraiba foldbelt, NE Brazil, comprises basement rocks, a supracrustal sequence and intrusive granitoids. The basement rocks were divided, based on their structural relationships, into two groups: one characterized by a metamorphic banding represented predominantly by hornblende orthogneisses and to a lesser extent garnet-biotite-gneisses. The second group includes orthogneisses presenting a fine to medium texture and a prominent foliation. These orthogneisses cut the metamorphic banding observed in the former orthogneisses. The Irajai (CI) and Sertania (CS) Complexes, which outcrops NNW and SSW respectively to the Afogados da Ingazeira shear zone (AISZ), represent the supracrustal sequence. The CI corresponds to a metavolcanosedimentary sequence, whereas the CS represents a metasedimentary sequence. The granitoids are divided into: sin-tangential and sin-transcurrent granitoids.

The structural and P/T data indicate a distinct metamorphic evolution for both sequences during the main deformational event (D₂). The P/T metamorphic conditions, using the garnet-biotite and plagioclase-garnet-Al₂SiO₅-quartz equilibria were: 650° ± 25°C and 6.5 ± 0.5 kbar for the CI, and 600° ± 50°C and 3.5 ± 0.5 kbar for the CS rocks.

The sin-tangential orthogneisses (D₂) intrude the CI, and show an antiformal pattern due to the interference with the third deformational phase. It is characterized, predominantly, by three facies: the major one is composed by microporphyritic hedenbergitic-granodioritic-augen-gneisses. The second one comprises fine to medium grain hedenbergitic orthogneisses, whereas the third one is represented by a coarse grain biotite augen-gneiss, with K-feldspar grains up to 5 cm. The petrographic and field similarities suggest that the first and the second ones were derived from the same magma, the only difference being the absence of augen in the second facies. The whole chemistry corroborates the similarities and differences between the first two and the third one. The geochemical signature clearly shows a collision type environment for the emplacement of these rocks.

The data suggest that Pajeu-Paraiba foldbelt developed during a continental accretion-collision episode at the end of the Mesoproterozoic during the Cariris Velhos Cycle (ca. 1.0Ga).