Adakite-like magmas from fractional crystallization and melting-assimilation of mafic lower crust (Eocene Macuchi arc, Western Cordillera, Ecuador)

The basement of the Western Cordillera of Ecuador consists of a fragment of oceanic plateau crust (Pallatanga) accreted to the continental margin during the Late Cretaceous. Magmatic arcs were formed upon the oceanic plateau before (Rio Cala arc, Maastrichtian) and after (Silante, Upper Maastrichtian-Paleocene, and Macuchi, Paleocene?-Eocene) accretion, during westward and eastward subductions respectively. Both arc sequences display isotopic and geochemical evidence of involvement of different source reservoirs in the petrogenesis of their rocks. Rio Cala arc basaltic rocks can be interpreted as the result of mixing between variably evolved magmas derived from different mantle domains or of melting and assimilation of oceanic plateau crust by mantle-derived magmas. Crustal thickening through the build-up of previous magmatic arcs and, possibly, increased coupling between subducting and overriding plates favoured the interaction of mantle-derived melts with the lower to mid-levels of the thickened and modified mafic crust of the accreted oceanic plateau during the subsequent eastward subduction event (Macuchi arc). Adakite-like signatures were developed in andesitic to dacitic magmas of the Macuchi arc in response to the evolution of mantle-derived melts within the lower to mid-parts of the thickened juvenile crust. This adakite-like magmatism marked the onset of a long-lived period of adakite-like magmatism in the Western Cordillera, which, although interrupted by several periods of normal calc-alkaline magmatism, continues still nowadays in the active frontal volcanoes of Ecuador. (C) 2009 Elsevier B.V. All rights reserved.