Brittle-ductile deformation of granulitic clinopyroxene during incipient eclogitisation

In the mafic lower crust, clinopyroxene is among the main rock-forming minerals. Based on experimental investigations, clinopyroxene is considered to be strong and to deform in a brittle manner at dry lower crustal conditions. 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 adjacent to eclogitic shear zones.

This study focusses on the strain accommodating processes of the granulitic clinopyroxene during incipient eclogitisation. Representative samples of deformed weakly eclogitised granulite were analysed via scanning electron microscopy, electron back-scattered diffraction mapping, electron probe micro analysis and Fourier-transform infrared spectroscopy.

Microstructural analysis reveals the formation of garnet lamellae along the {010} planes of the diopsidic clinopyroxene. Initial bending of this anisotropic clinopyroxene is accommodated by the development of en échelon microcracks at a high angle to the {010} planes. The micro-cracks are traced by garnet with similar composition as the lamellae, suggesting that both formed at similar pressure-temperature conditions. With ongoing strain, the cracks start to link and evolve into micro-shear zones, which systematically widen with strain and eventually connect forming networks. This widening is accompanied by the nucleation of amphibole and a second clinopyroxene with higher magnesium and lower aluminium concentration when compared to the host clinopyroxene, facilitating further macroscopic bending of the granulitic foliation. Increased intracrystalline misorientation and formation of subgrains adjacent to the micro-shear zone indicate that the diopsidic clinopyroxene host grain deforms by crystal plastic processes. In contrast, shape-preferred orientation and minor chemical zoning of the newly crystallised grains related to the micro-shear zone suggest that diffusion-related processes predominately accommodated the strain in the micro-shear zones.

In recent literature, low-permeable granulite has been described as dry. The observed deformation style as well as the formation of amphibole in the micro-shear zones indicate the presence of water, either in form of external fluids, infiltrating through en échelon microcracks, or as minor amounts of OH-groups occurring in the nominally anhydrous clinopyroxene. First Fourier-transform infrared spectroscopy results suggest that the nucleation of amphibole might be facilitated by the incorporated OH in the diopsidic clinopyroxene.

The observed microstructures and mineral compositions suggest that the micro-shear zones form at an early deformation stage throughout the eclogitisation process on Holsnøy. Our investigations show the complex interplay of brittle and ductile processes on a microscopic scale during macroscopically ductile flow.