Evolution of the late Pleistocene Mojanda–Fuya Fuy

Authors: Claude Robin JeanPhilippe Eissen Pablo Samaniego Hervé Martin Minard Hall Joseph Cotten

Publish Date: 2008/06/19

Volume: 71, Issue: 3, Pages: 233-

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Abstract

The Mojanda–Fuya Fuya Volcanic Complex consists of two nearby volcanoes Mojanda and Fuya Fuya The older one Mojanda volcano 06 to 02 Ma was first constructed by andesites and highsilica andesites forming a large stratovolcano Lower Mojanda This edifice was capped by a basaltic andesite and andesitic cone Upper Mojanda which collapsed later to form a 3kmwide summit caldera after large phreatomagmatic eruptions The Lower Fuya Fuya edifice was constructed by the extrusion of viscous Sirich andesitic lavas and dacitic domes and the emission of a thick sequence of pyroclasticflow and fallout deposits which include two voluminous rhyolitic layers An intermediate construction phase at Fuya Fuya is represented by a mainly effusive cone andesitic in composition San Bartolo edifice the construction of which was interrupted by a major sector collapse in the Late Pleistocene Finally a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre forming the Upper Fuya Fuya volcanic centre This paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source Although all the rocks of the complex are included in the mediumK field of continental arcs the Fuya Fuya suite 61–75 wt SiO2 shows depletion in Y and HREE and high Sr/Y and La/Yb values compared to the less silicic Mojanda suite 55–665 wt SiO2 The Mojanda calcalkaline suite was generated by partial melting of an adakitemetasomatised mantle source that left a residue with 2 garnet followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow intracrustal depths For Fuya Fuya geochemical and mineralogical data suggest either 1 partial melting of a similar metasomatised mantle with more garnet in the residue 4 followed by fractional crystallization involving plagioclase amphibole and pyroxene or 2 mixing of mafic mantlederived magma from the Mojanda suite and slab melts followed by the same fractional crystallization processWe acknowledge Erwan Bourdon for stimulating discussions We are also very grateful for the comments on a preliminary version of this paper by Suzanne M Kay and Nick Petford We thank Marcel Bohn CNRS UMR 6538 Brest France for his help in performing the microprobe analyses and Thierry Pilorge IRD Bondy France for his careful preparation of the thin sections This research was supported by the French IRD Institut de Recherche pour le Développement and the Instituto Geofisico de la Escuela Politecnica Nacional of Quito in the framework of their cooperation agreement The careful reviews and suggestions of James Luhr and Leonid Danuyshevsky are greatly acknowledged

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