Abstract
The volcanic rocks of the Cretaceous Celica Formation of southern Ecuador are affected by a weak although widespread alteration. The chemical study of the secondary chemical phases present in andesitic and basaltic lava flows reveals that this alteration corresponds to very low-grade metamorphism comprising the zeolite and the prehnite-pumpellyite facies. Main features of this metamorphism are: weak lithostatic pressure, moderate to steep thermal gradient, high ƒ O 2 , low value of the seawater/rock ratio and total absence of deformation. These characteristics are typically present in other volcanic suites of similar age and composition along the Andes and correspond to the pattern of metamorphism developed in extensional settings (diastathermal metamorphism) linked to various degrees of thinning of the continental crust. Based on this metamorphic pattern, a geodynamic model is proposed in which the Celica Formation is interpreted as an ensialic, aborted, marginal basin developed on strongly attenuated continental crust at the border of the South American plate. The relationship between the Ecuadorian and Colombian volcanic suites of Cretaceous age present along the Western Cordillera is discussed in the light of the model suggested.
Published Version
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