THE ROLE OF SUBDUCTED SEDIMENT DIAPIRS IN THE FORMATION OF MANTLE-DERIVED ANDESITES FROM THE NORTHERN COLOMBIAN ANDES

The andesitic stratovolcanoes of the Bravo-Machin Volcanic Complex (BMVC) represent the northernmost expression of Quaternary volcanic activity in the Andean chain. Volcanoes along the complex traverse a ~45 km thick uniform Paleozoic basement architecture, but have been influenced by a subducted slab containing carbonate and hemipelagic sediments that gradually dips to the SE forming a southward gradual increase in arc front depth (120-180 km). In order to elucidate the petrotectonic context of these volcanoes, here we report new whole-rock geochemical data and olivine trace element compositions of andesites from Nevado Santa Isabel (NSI), located in the northern segment of the BMVC, and compare this information to published geochemical data on the southernmost volcano of the region: Cerro Machín (CM; Laeger, et al., 2013, J.S. Am. Earth Sci). The NSI and other eruptive centers of the northern segment of the BMVC are dominated by high constant Mg# (~60) andesites with unusually high Th/La (~0.6), low Nb/Ta (­~12), spoon-like REE patterns and a relatively depleted Sr-Nd isotopic compositions when compared to those of CM, which overall display adakitic geochemical features, lower Th/La ratios and more enriched isotopes. NSI andesites also display the highly radiogenic Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = ~18.96) that are typical of the Northern Andean volcanic products but differ from those of the Paleozoic crustal basement (²⁰⁶Pb/²⁰⁴Pb = ~18,10). Olivine phenocrysts are relatively evolved (Fo < 86 %) but have NiO contents up to 0.54 wt% indicative of a pyroxenitic mantle source. The geochemical characteristics within and among volcanoes are difficult to explain by differentiation or crustal contamination of a common parental basaltic magma, and instead suggest the participation of different kinds of subducted sedimentary materials that interact with the mantle wedge in the form of buoyantly rising diapirs that melt at variable depths. We suggest that NSI and the volcanoes of the northern segment incorporate a larger proportion of subducted hemiplegic sediments that melt in the presence of amphibole. In contrast, rocks of CM appear to be influenced by a larger proportion of carbonate sediments that melt under deeper and dryer eclogite facies conditions.