Age, Origin and Cooling History of the Coronel Joao Sa Pluton, Bahia, Brazil

In north-east Brazil, Archean and Paleoproterozoic cratonic blocks are enclosed within a network of Brasiliano-age (0·7–0·55 Ga) metasedimentary foldbelts. The unfoliated Coronel João Sá granodiorite pluton, which contains magmatic epidote and strongly resorbed clinopyroxene, intrudes the Sergipano Foldbelt. Zircons yield a concordant U–Pb crystallization age of 625 ± 2 Ma; titanite ages are approximately 621 Ma. Discordant zircons suggest inheritance from at least two magma sources of ages <1·8 and >2·2 Ga. Model calculations based on diffusion parameters and Rb–Sr isotope data from separated minerals indicate that the pluton cooled at a rate of ∼36°C/Myr. Whole-rock element compositions and initial Sr–Nd isotopic compositions that are heterogeneous on all length scales suggest magma mixing. Trace-element concentrations and Nd isotope data argue against a contribution from a contemporaneous mantle-derived magma. Values of magmatic εNd (at 625 Ma) resemble contemporary εNd for local supracrustal rocks and basement, compatible with anatexis of a crustal source. In north-east Brazil, cratonic blocks could have amalgamated with foldbelts that originated as: (1) a mosaic of island arcs and arc basins (traditional allochthonous model), or as (2) extensional continental sedimentary basins (but not oceanic crust) later involved in collision (autochthonous model). The Coronel João Sá isotopic and chemical data support an autochthonous origin.