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Microstructural evolution of metabasalts within a contact metamorphic aureole : a preliminary quantitative bi-dimensional approach

¹ UMR CNRS 7072, Département des Sciences de la Terre, Université de Cergy Pontoise, 8 Le Campus, Bâtiment I, 95031 Cergy cedex, France.
² Géosciences Rennes UMR CNRS 6118, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France.

Abstract

The emplacement of the Saint-Brieuc diorite (533 ± 12 Ma ; Brittany, France) developed a narrow contact aureole in metabasaltic host-rocks which were previously submitted to a regional-scale deformation and metamorphism at around 590-570 Ma. This study aims to qualify and quantify the microstructural changes of rocks that occur within the contact aureole and that are the result of the static recrystallization due to the thermal effect of the diorite. In order to quantify the textural evolution of rocks, an image analysis has been performed on thin sections. It focused on the measurements of morphological features of hornblende and opaque phases. They are anisotropy shape factors (stretching and elongation) of minerals, the preferred orientation of minerals and the distribution size (areas) of minerals. The quantification of these different parameters shows that the static recrystallization, which increases when approaching the contact with the intrusive, (i) is responsible of a reduction of the anisotropy shape and elongation parameters of grains, (ii) causes coarsening and (iii) is responsible for the disappearance of preferred orientation of minerals. Both together, the qualitative description and quantitative measurements show that the solid-state transformations due to contact metamorphism tend to make the rocks isotropic and equigranular.

Key Words: Microstructure • Texture • Contact metamorphism • Metabasic hornfelses • Image analysis • St-Brieuc diorite • Cadomian orogeny