A microstructural record of brittle-ductile clinopyroxene deformation in the lower crust

In the mafic lower crust, clinopyroxene and plagioclase are among the main rock-forming minerals. Both are considered to be strong and to deform by brittle mechanisms at dry lower crustal conditions. However, at fluid-present high-pressure conditions both are highly reactive, transforming to omphacite, (clino-)zoisite and garnet, thus forming weaker eclogite.

Recent studies have focussed on plagioclase breakdown reactions and their effect on rock weakening. However, field observations on Holsnøy, Norway, indicate ductile deformation of coarse-grained clinopyroxene in the mafic lower crust, reflected by bending of the granulitic foliation. This study focusses on the strain accommodating processes of the granulitic clinopyroxene at an early stage of eclogitisation. Representative samples of deformed weakly eclogitised granulite were analysed using scanning electron microscope, electron back-scattered diffraction, and electron probe micro analysis.

Microstructural analysis revealed the formation of garnet-lamellae along the clinopyroxene-cleavage. Initial bending of this anisotropic clinopyroxene is accommodated by the development of micro-en-echelon cracks at a high angle to the cleavage. Micro-cracks are traced by garnet with similar composition as the lamellae, suggesting that both formed at identical conditions. With ongoing strain, cracks start to link and evolve into micro-shear zones, systematically widening with strain. This leads to the nucleation of amphibole and a second clinopyroxene with higher magnesium-concentration, facilitating further macroscopic bending of the clinopyroxene. Late static eclogitic overprint suggests that micro-shear zones form at an early deformation stage. Our investigations show the complex interplay of brittle and ductile processes on a microscopic scale during macroscopically ductile flow.