Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling |
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Authors: | Massimo Chiaradia Othmar Müntener Bernardo Beate Denis Fontignie |
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Institution: | 1.Department of Mineralogy,University of Geneva,Geneva,Switzerland;2.Institut de Minéralogie et Géochimie,Université de Lausanne,Lausanne,Switzerland;3.Escuela Politecnica Nacional,Quito,Ecuador |
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Abstract: | In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon
continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to
east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc),
and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted
either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional
crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb)
data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic
centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the
frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related
geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile
elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively
lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe
widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or
amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center,
rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes
are small and their values are overall rather primitive (e.g., εNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation.
We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers
characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as
by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian
rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts,
rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks
of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting
that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a
consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated
with subduction of the aseismic Carnegie Ridge. |
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