Geochemical and Sr-Nd isotope signatures of pristine slab melts and their residues (Sierra del Convento melange, eastern Cuba)

Major and trace element geochemical signatures and Sr-Nd isotope data of MORB-like amphibolite blocks and associated leucocratic low-K trondhjemitic rocks from the Sierra del Convento melange (Cuba) indicate that oceanic crust underwent partial melting processes during subduction. Theoretical melting models suggest that the trondhjemites are not oceanic plagiogranites formed after differentiation or partial melting of MORB at oceanic ridges, but instead they formed by water-saturated melting of an oceanic crust under hot subduction conditions. REE patterns of the trondhjemites are fractionated (1 <(La/Yb)(n) < 16.8), with LREE enrichment and flat or slightly depleted HREE patterns which compare well with adakitic melts formed by partial melting of mafic material at moderate to high pressure. Indeed, trondhjemites from Sierra del Convento have some of the geochemical features of Cenozoic adakites, including SiO2 > 56 wt.%, 3.5 wt.%< Na2O < 7.5 wt.%, (K2O/Na2O) approximate to 0.42, Mg# approximate to 50, Ni = 20-40 ppm, Cr = 30-50 ppm, LREE enrichment, HREE depletion, and high La/Yb and Sr/Y. However, the trondhjemites from Sierra del Convento are more comparable to acid rocks from Catalina Schist melange (California) considered to be primary slab melts which did not react with the mantle wedge. In agreement with major and trace element geochemistry, the Sr-Nd isotope systematic of selected amphibolites and trondhjemites can be explained by a two stage model involving a) generation of MORB-like basalts and b) partial melting of subducted MORB. (C) 2008 Elsevier B.V. All rights reserved.