Intralithospheric differentiation and crustal growth: Evidence from the Borborema province, northeastern Brazil

Thousands of cubic kilometers of high-K calc-alkalic magmas intruded the Bo rborema Province (northeastern Brazil) during the Neoproterozoic Brasiliano orogeny. They make up large batholiths in which mantle-derived mafic to in termediate rocks coexist with a larger amount of granitoids, The relatively low silica contents (61-70 wt% SiO2) and moderate to high compatible eleme nt concentrations (0.3-3.5 wt% MgO, 1.5-3.8 wt% CaO, as much as 150 ppm of Cr) of the granitoids indicate that they contain an appreciable mantle comp onent. The similar trace element geochemical (high contents of incompatible trace elements) and isotopic (strongly negative epsilon(Nd) values) signat ures of mafic and felsic rocks combined with geochemical modeling suggest t hat (1) the mafic and felsic rocks are genetically Linked, (2) the granitic magmas were produced by 20%-30% partial melting from a source having geoch emical characteristics similar to the mafic rocks, and (3) mingling and mix ing of felsic magmas with subsequent batches of mafic magmas yielded the si lica-poor granitoids. Isotopic data preclude involvement of the asthenosphe re in the genesis of the mafic melts and instead indicate their derivation from an old, enriched lithospheric mantle. Therefore, addition of mantle ma terial to the crust occurred through internal Lithospheric differentiation, in contrast with conventional crustal-growth models.