DEFORMATION OF HYDROUS MINERALS IN SUBDUCTION CHANNEL: TECTONIC IMPLICATIONS

Dehydration of subducting slabs can generate hydrous fluids to metasomatize mantle wedge and cause the formation of hydrous minerals (serpentine, chlorite, amphibole, etc) in subduction channels. These hydrous minerals are of great importance for understanding the physical mixing and exhumation processes in subduction channels. Here we report the deformation of chlorite, tremolite, and serpentine schists within the ophilolite complex from the Monviso area of Italy, as well as experimental constraints on the deformation of antigorite, chlorite and tremolite. Our results show that all hydrous minerals are strongly shear deformed in the subduction channel with strong preferred orientations. Antigorite develops strong fabrics with the (001) planes parallel to foliation and the [010] axes parallel to lineation. Chlorite develops the strongest fabrics with the (001) planes parallel to foliation and a great girdle for the [100] and [010] axes in the foliation plane. Tremolite develops fabrics of an intermediate strength with the (100) planes parallel to foliation and the [001] axes parallel to lineation. Adverse to their fabric strength contrast, the mechanical strengths of these hydrous minerals are amphibole > serpentine > chlorite. Chlorite has the highest bulk rock and shear wave anisotropy while tremolite has the lowest anisotropies. Our results imply that: 1) The low mechanical strengths of hydrous minerals provide lubrications and channelized network for the exhumation of HP/UHP blocks in subduction zones; 2) The low densities of hydrous minerals provide the buoyancy required for exhumation in the subduction channel; 3) The trench parallel faster shear wave polarization in front of the continental and oceanic subduction zones can be best explained by the deformation of serpentine and chlorite in low temperature subduction channel with high subduction angles (60-90°) and by the deformation of amphibole in high temperature subduction channel with low subduction angles (<60°), respectively.