title: Links between metamorphism and deformation in feldspar at mid-crustal conditions
Creators: name: Gardner, JD
Publisher: name: University of Liverpool Repository
Year: 2019

description: Feldspar is the most common mineral in the Earth’s mid-to-lower crust, and therefore exerts fundamental controls on crustal strength. However, due to the relatively complex chemistry of the feldspar series, particularly its temperature dependence on composition, interactions between metamorphism and deformation, and thus overall deformation behaviour, remain poorly understood. Metagabbroic rocks from a crustal-scale shear zone were studied using electron backscatter diffraction and transmission electron microscopy, to investigate the deformation behaviour of plagioclase at greenschist facies. Numerical modelling was subsequently employed to characterise the microstructural evolution of features observed in the natural samples, specifically the effect that large grains have on the evolution of a fine-grained matrix. The results of this study showed that mm–cm-scale albite grains were formed through fluid-assisted, interface-coupled replacement reactions, which produced a high dislocation density in those grains through a chemical, rather than mechanical, mechanism. Small albite grains of the same composition were formed either through classical precipitation mechanisms, or by growth from strain-free nuclei that consumed the large grains, in response to the stored strain energy in the large grains. The reduction in grain size led to fluid-assisted diffusion creep (pressure solution) becoming the dominant deformation mechanism that accommodated high strains in the samples. Despite this type of deformation being expected to destroy crystallographic preferred orientation, or texture, preferred orientation domains are preserved to high strains in the albite-rich matrix of the rocks. Metagabbroic samples were studied at five different strain levels to characterise how albite grain size, aspect ratio, and texture strength, and second phase abundance, evolved and interacted during deformation. Although the secondary phases produced during metamorphic reactions have generally been reported to inhibit grain growth, which promotes GSS creep, the results of this study show their presence may also limit texture modification by inhibiting the rotations normally associated with diffusion creep and grain boundary sliding, to preserve CPO domains to high strain. The modelling results showed that large grains can have a significant effect on the microstructural evolution of a fine-grained matrix undergoing diffusion creep. In particular, rotations of matrix grains were strongly influenced by the rotation direction and velocity of large grains. The study also showed that the rotation direction of large grains does not have to be simply either synthetic or antithetic; rotation directions changed due to interactions with the matrix. The model output also showed that large grains create stress heterogeneities that focus the effects of diffusion creep. The stress heterogeneities resulted in the alignment of grain boundaries, and led to a profound drop in strength of the deforming material. These results indicate the microstructural evolution of rocks undergoing (fluid-assisted) diffusion creep is more complex than previously realised, and the mechanisms and processes at work during deformation by diffusion creep require further study.