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