Geochemical and petrological constraints on rear-arc magma genesis processes in Ecuador: The Puyo cones and Mera lavas volcanic formations |
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| Authors: | Géraldine Hoffer Jean-Philippe Eissen Bernardo Beate Erwan Bourdon Michel Fornari Jo Cotten |
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| Institution: | 1. Laboratoire Magmas et Volcans, OPGC, CNRS, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France;2. IRD, UMR 163, LMV, 5 rue Kessler, 63038 Clermont-Ferrand, France;3. Departamento de Geología, Minas y Petróleos, Escuela Politécnica Nacional, Quito, Ecuador;4. Institut de Géologie et d''Hydrogéologie, Université de Neuchâtel, rue Emile Argand 11, 2009 Neuchâtel, Switzerland;5. IRD, Géosciences Azur, UMR 6526, Université de Nice, Parc Valrose, 06108 Nice Cedex 2, France;6. UMR 6538, Domaines Océaniques, Université de Bretagne Occidentale, BP 809, 29285 Brest cedex, France |
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| Abstract: | The Puyo scoria cones and the Mera lava flows, two newly recognized volcanic formations dated between Late Pliocene to Middle Pleistocene, extend the limits of the Ecuadorian rear-arc volcanic province some 100 km to the south. The Puyo scoria cones have erupted K-rich absarokites containing olivine, diopside and phlogopite, whereas the Mera lava flows display a basic andesite composition, with olivine and minor augite phenocrysts. In addition to high contents in LILE, LREE and HFSE, the Puyo absarokites exhibit many characteristics of primitive melts, namely high Cr (590–310 ppm) and Ni (330–154 ppm) contents, high Mg# (64–70) and they contain forsteritic olivine (Fo82–89). The composition of the most primary Puyo absarokite was used in petrogenetic models, in order to constrain the genesis of these high-K magmas. Major and trace elements models, as well as isotopic data, indicate that the source of Puyo magmas is a hydrated phlogopite- and garnet-bearing lherzolite. Phlogopite crystallization in the mantle wedge is triggered by the metasomatism by 3–5% of a SiO2-, H2O-rich liquid generated by slab melting. Partial melting of the subducted oceanic crust beneath Ecuador is allowed by the subduction of the young and warm Carnegie Ridge, which modifies the thermal regime of the Benioff zone. A low degree (1–4%) of partial melting of the metasomatized mantle wedge, leaving a variable garnet (4–7%) ± phlogopite (0–4%) lherzolitic residual assemblage, leads to the compositions of the entire Puyo absarokite series and is consistent with previous petrogenetic models developed for the Ecuadorian volcanic arc. Indeed, the homogeneity of isotopic data across the arc suggests a similar source for the whole Ecuadorian magmas. |
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| Keywords: | absarokite subduction rear-arc Ecuador Andes phlogopite |
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