Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

摘要: In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting accreted oceanic and terranes. centers occur, from west to east, along Western Cordillera (frontal arc), in Inter-Andean Depression Eastern (main Sub-Andean Zone (back-arc). The adakite-like signatures frontal main volcanoes have been interpreted either as result slab melting plus subsequent melt–mantle interactions or lower crustal melting, fractional crystallization, assimilation processes. this paper, we present petrographic, geochemical, isotopic (Sr, Nd, Pb) data on dominantly andesitic dacitic rocks well xenolith cumulate samples five (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming NW–SE transect at about 0° latitude encompassing Pichincha), (Ilalo, Chacana), back-arc (Sumaco) chains. All display typical subduction-related signatures, such Nb Ta negative anomalies LILE enrichment. They show relative depletion fluid-mobile elements general increase incompatible front suggesting derivation progressively degrees partial mantle wedge induced by decreasing amounts fluids released slab. We observe widespread petrographic evidence interaction primary melts mafic xenoliths clinopyroxene- and/or amphibole-bearing cumulates magma mixing all centers. Within each center, correlations between evolution indices radiogenic isotopes, although absolute variations isotopes are small their values overall rather primitive (e.g., eNd = +1.5 +6, 87Sr/86Sr 0.7040–0.70435). Rare earth element patterns characterized variably fractionated light heavy REE (La/YbN 5.7–34) absence Eu these limited plagioclase fractionation. interpret indicating open-system crystallization processes different lower- mid-crustal levels its melts. Thus, propose that Ecuadorian high Sr/Y La/Yb values) primarily processing mantle-derived melts, than interactions. least evolved active recent same those Tertiary ”normal” calc-alkaline magmatic Ecuador source did not change through time. What changed was depth evolution, probably consequence increased compression stronger coupling subducting overriding plates associated subduction aseismic Carnegie Ridge.