Brittle/Ductile deformation at depth during continental crust eclogitization (Mont-Emilius klippe,Western Internal Alps)

International audience; Eclogitic rocks are important for understanding tectonics at large scale as they provide key constraints on boththe evolution (P-T-t-" paths) and the deformation modes of the crust along the subduction interface. We hereinfocus our study on eclogitized mafic dykes remnants exposed within granulites from the continental basementsilver of the Mt. Emilius klippe (Western Internal Alps, Italy). These eclogites exhibit highly deformed garnetiteand clinopyroxenite levels. In some places, these rocks with a mylonitic aspect can be found as clasts withinmeter-thick brecciated fault rocks formed close to metamorphic peak conditions in eclogite facies. Especially, thegarnet-rich levels tend to behave in a brittle fashion while deformation within clinopyroxene-rich levels is mostlyaccommodated by creep. This is evidenced by the presence of elongated grains, subgrain boundaries and intensegrain size reduction close to rigid garnets. Crystallographic preferred orientation (CPO) measurements in garnetsindicate a quasi-random distribution. In most of the clinopyroxenes levels nevertheless, the CPO is relativelystrong, with multiples of uniform distribution varying from 4 to 5.5 (value of 1 is random texture). This CPO ischaracterized by a strong alignment of poles (001) parallel to the lineation and (100) and [010] distributed alonggirdles cross-cutting the foliation plane. Our study thus attests that the materials along the subduction interfaceat P2.0-2.5 GPa and T500-550C can locally be brittle where deformation is classically envisioned as ductile.In addition to this deformation analysis, we present a petrological study of these eclogites, from the outcrop tothe microscopic scale, tracking the chemical evolution associated to the observed deformation. Based on all thesedata, we finally propose a tectono-metamorphic history for these rocks allowing to explain the co-existence ofductile and brittle features developed in the same metamorphic facies, and closely associated to the circulation ofmetamorphic fluids.