The lithospheric mantle as a source of magmas during orogenic processes: insights from high-K diorites in the Borborema Province and implications for continental dynamics

Sérgio Pacheco Neves, and Gorki Mariano

Incompatible-element-rich dioritic rocks are common in several Brasiliano/Pan-African belts. In the Borborema Province, northeastern Brazil, these rocks were emplaced in an intracontinental setting during regional deformation and intruded high-grade metamorphic rocks. The diorites have relatively high MgO (up to 7 wt %), FeOt (up to 10.6 wt %) and CaO (up to 8 wt %) together with high K2O (up to 5.8 wt %) and large ion lithophile elements (LILE) (e.g., 700-3000 ppm Ba, 30-150 ppm La), and are characterized isotopically by negative &egr;Nd(t) values (-6.9 to -14.9) and high initial 87Sr/86Sr ratios (0.7058 - 0.7102). These geochemical and isotopic features are similar to those of shoshonites erupted in the Tibetan plateau in the advanced stages of the India/Asia collision, for which a subcontinental lithospheric mantle source is attributed. The diorites can thus be considered as intrusive analogs of modern shoshonites, pointing to a significant role of the mantle lithosphere as a magma source.