Coexisting of ductile and brittle behaviors at depth during continental crust eclogitization (Mt. Emilius klippe, Western Alps)

Abstract:

Eclogitic rocks provide key constraints on both the evolution (P-T-t-ε paths) and the deformation modes of the crust along the subduction interface and therefore are crucial for the understanding of tectonics at large scale. Here we present some preliminary results of a microstructural study on eclogitized mafic dykes exposed within granulites from the continental basement silver of the Mt. Emilius klippe (Western Internal Alps, Italy). In this region, highly deformed eclogites characterized by a strong layering between garnetite and clinopyroxenite bands is the most predominant feature. These different levels present very heterogeneous deformation patterns, and while the garnet-rich levels tend to have a brittle behavior, the deformation within clinopyroxene-rich levels is possibly accommodated by creep. This is evidenced by the presence of elongated grains, subgrain boundaries and intense grain size reduction close to rigid garnets. Crystallographic preferred orientation (CPO) measurements in garnets indicate a quasi-random distribution. In most of the clinopyroxenes levels nevertheless, the CPO is relatively strong, with multiples of uniform distribution varying from 4 to 5.5 (value of 1 is random texture). This CPO in the clinopyroxenes is characterized by a strong alignment of poles (001) parallel to the lineation and (100) and [010] distributed along girdles cross-cutting the foliation plane. These mylonitic rocks are sometimes found as clasts within meter-thick brecciated fault rocks formed at, or nearly close to, the metamorphic peak in eclogite facies. Therefore, the materials along the subduction interface at these P-T conditions (i.e., ~2.0-2.5 GPa; 500-550°C) can locally be brittle where deformation is classically envisioned as ductile. We propose a model that involves creep deformation, heterogeneous fluid circulation, and local brittle behavior to explain the co-existence of ductile and brittle features developed in the same depth region.