Insights on Variscan geodynamics from the structural and geochemical characterization of a Devonian-Carboniferous gabbro from the Austroalpine Domain (Western Alps)

Abstract

Ivozio kyanite-bearing eclogites represent one of the main pre-Alpine mafic complexes of the Sesia Lanzo Zone (Austroalpine Domain) whose protoliths have been dated at Late Devonian to Early Carboniferous. The complex consists of various HP mafic-ultramafic rocks such as amphibole-bearing eclogites, zoisite-bearing eclogites, amphibole-epidote-bearing eclogites, quartz-bearing eclogites, chlorite-bearing amphibolites and ultramafites surrounded by eclogitic micaschists and jadeite-bearing metagranitoids. Multiscale structural analyses, integrated with geochemical investigation, allowed to distinguish heterogeneities of eclogite types consequent to strain partitioning and different degrees of metamorphic re-equilibration during Alpine polyphase deformation history or to pristine magmatic differences. Five groups of Alpine structures (D₁ to D₅) have been detected: they consist of folds, foliations and shear zones developed during the Late Cretaceous to Eocene subduction-exhumation path. The metamorphic mineral assemblages related to the deformation stages indicate a polyphase deformation (pre-D₁ to D₃) under eclogite-facies conditions followed by a blueschist re-equilibration (during D₄), localized shearing and successive folding and discrete shearing (D₅) under greenschist-facies conditions. Four veining stages, developed under the eclogite- and blueschist-facies conditions have been documented.

Whole rock major and trace element composition of main igneous lithotypes suggest a subduction-related context for the emplaced of the Ivozio mafic Complex. It also suggests that all rocks are not co-genetic and that they likely emplaced in the continental crust.

Igneous products with arc and back-arc geochemical affinity (from 380 to 345 Ma) are described in the European Variscan Belt from French Central Massif to Bohemian Massif, supporting the interpretation that the Ivozio mafic Complex protoliths are the witness of such Devonian-Carboniferous arc magmatism in the Alpine chain.

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