Geochemical window into subduction and accretion processes: Raspas metamorphic complex, Ecuador

The high-pressure, low-temperature (P = 1.3-2 GPa; T ≤600 °C) Raspas metamorphic complex is an exhumed fragment of the partially accreted, partially subducted Amotape-Chaucha terrane in southwest Ecuador. Comparative analysis of major and trace elements plus Sr, Nd, and Pb isotopes in bulk lithologies and individual crystalline phases shows that the complex includes one to three layers of ordinary oceanic crust and underlying mantle lithosphere together with oceanic plateau fragments. Subduction (and exhumation) of oceanic lithosphere resulted in selective bulk trace element geochemical changes: Rb, Ba, and Sr have been lost (in amounts from approximately 85%-50%) from the high-P, low-T metamorphosed pelites and basalts, whereas Pb is enriched in mafic rocks. During formation of the eclogite, U, Pb, and rare earth elements (REEs) were immobile. High-P, low-T metamorphosed terranes form the basement of active Ecuadorian arc volcanoes; partial melting of this basement by mantle-wedge-derived basalt is a likely source of adakitic components.