The role of subducted sediments in the formation of intermediate mantle-derived magmas from the Northern Colombian Andes

Unraveling the sources and processes that produce intermediate continental arc volcanoes is still a challenge for geoscientists. To address this problem, here we use comprehensive geochemical and isotopic data from Nevado del Santa Isabel and Cerro Machin volcanoes in the North Volcanic Province of Colombia, and from oceanic sediments sampled outboard the Colombian continental margin. Volcanoes along this province have been influenced by the subduction of a compositionally contrasting sedimentary column constituted by a carbonate-rich pelagic layer overlain by a clay-and-apatite-rich hemipelagic unit. The studied volcanoes exhibit the high Mg# (similar to 60) and calc-alkaline affinities that are typical of continental arcs but display unusually high and contrasting Th(U)/La, Nb/Ta and Dy/Yb ratios and isotopic compositions. We argue that the geochemical variations within and among these volcanoes are not controlled by differentiation or crustal contamination of a parental basaltic magma but formed by melting of different kinds of subducted sedimentary materials detached from the slab as buoyantly rising diapirs at various depths. This model accounts for the reworking of refractory carbonates into arc magmatism and suggests that the geochemical diversity of the North Volcanic Province is mainly controlled by the nature of the subduction inputs and their exhumation pathways within the mantle wedge. (C) 2019 Published by Elsevier B.V.