NEOPROTEROZOIC LITHOSPHERIC GRANITE SYSTEMS IN THE PERNAMBUCO-ALAGOAS COMPLEX, AND THEIR SIGNIFICANCE FOR EVOLUTION OF NE BRAZIL

The Borborema Province (BP) is the western part of the 600 Ma Brasiliano-Pan African orogenic belt that extends from NE Brazil through central Africa in pre-drift reconstruction of Gondwanaland. The Pernambuco–Alagoas complex is a large area within the BP that consists of high-grade gneisses and granitic batholiths; it includes many different types of protoliths or plutons The protoliths for many gneisses are difficult to decipher due to deformation and metamorphism. Sm-Nd crustal residence ages or initial Nd isotopic compositions from gneisses and plutons in the five major granitic batholiths that intrude the complex can help define its crustal history. There are two major domains: Garanhuns and Agua Branca. Rocks of the Garanhuns domain have Sm-Nd model ages of 1.8 to 2.4 Ga, showing that most of it is underlain by Paleoproterozoic crust and does not include significant younger juvenile crust. Rocks of the Agua Branca domain show model ages of 0.90 to 2.10 Ga, with most between 1.5 and 1.0 Ga. The pre-Brasiliano crust in this domain thus includes substantial amounts of juvenile Mesoproterozoic to Neoproterozoic additions. The presence of some older model ages suggests that this domain may have some vertical crustal zonation, with Paleoproterozoic and Neoproterozoic crustal segments tectonically interleaved during successive accretional or collisional episodes. The Mesoproterozoic Nd model ages therefore represent mixing of different crustal or mantle sources, rather than a discrete episode of Mesoproterozoic crustal formation. Various metaluminous or peraluminous granitoids of Brasiliano age yield model ages of 0.90 Ga to 1.10 Ga, suggesting that they either were wholly derived from juvenile material of the 1.0 Ga Cariris Velhos orogeny or contain substantial contributions of juvenile material from the 0.6 Ga Brasiliano cycle. Correlations of these domains from NE Brazil eastward into western Africa will provide additional data for identification of individual terranes and associated sutures in the tectonic collage that comprises the Brasiliano-Pan African fold belt.