Geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of Borborema Province, NE Brazil: Geodynamic implications

The Borborema Province, northeastern Brazil, occupies a central position in pre-drift reconstructions of western Gondwana, making an understanding of its geological evolution crucial for Neoproterozoic reconstructions. In recent years, it has been proposed that the Borborema Province grew by accretion of distinct tectonic terranes. In order to test this hypothesis, we compare here the geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of the province. Orthogneiss samples show smooth trends in Harker diagrams and similar rare earth element (REE) patterns, characterized by sharp decreases from La to Sm (chondrite-normalized La/Sm = 3–6) and flat heavy REE profiles (chondrite-normalized Tb/Yb = 1.5–2.5), with small or no Eu anomalies. In primitive mantle-normalized multi-element diagrams, all samples show parallel patterns characterized by sharp negative anomalies of U, Ta, Nb, P and Ti. The metasedimentary samples show little scatter of the major elements in Harker diagrams, suggesting that their chemistry was little affected by post-depositional diagenesis and metamorphism. They have indistinguishable chondrite-normalized REE patterns, characterized by light REE enrichment, flat heavy REE (normalized Tb/Y = 1–2) and small or no negative Eu anomalies, and similar ratios of immobile trace elements (e.g., Th/Sc, Zr/Sc). The geochemistry of the metasedimentary samples is comparable in many ways to those of the orthogneisses, suggesting that these may have been an important source of the precursor sedimentary rocks. These data do not support the terrane accretion hypothesis, rather suggesting the existence of a continuous basement that became available for erosion during intraplate continental extension in the late Neoproterozoic. Comparisons of the studied sequences with those present in the Northern Domain suggest that most, if not all of the Neoproterozoic geodynamic evolution of Borborema Province, occurred in an intracontinental setting.