Simple mixing as the major control of the evolution of volcanic suites in the Ecuadorian Andes |
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Authors: | Pierre Schiano M Monzier J-P Eissen H Martin K T Koga |
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Institution: | 1. Laboratoire Magmas et Volcans, Clermont Université, Université Blaise Pascal, BP 10448, 63000, Clermont-Ferrand, France 2. CNRS, UMR 6524, IRD, R 163, 5 rue Kessler, 63038, Clermont-Ferrand Cedex, France 3. Laboratoire Magmas et Volcans, Université Blaise Pascal, 5 rue Kessler, 63038, Clermont-Ferrand Cedex, France
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Abstract: | Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series
of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern
Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations.
The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element
modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive
melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting
that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two
end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after
their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist
efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and
can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary
magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely
originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle
wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust. |
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