title: Deformation mechanisms in micas and mica-bearing mylonites in regional scale shear zones
Creators: name: Aslin, Joseph
Publisher: name: University of Liverpool Repository
Year: 2020

description: Mica minerals are ubiquitous constituents of the Earth’s crust, occurring commonly within faults and shear zones. They are widely understood to be weaker than most other silicate minerals across a range of pressure, temperature and strain rate conditions and, as a result, accommodate a disproportionately large amount of strain within lithospheric faults and shear zones. Despite this, the mechanisms and processes which govern mica deformation are not fully understood. This is due primarily to the strong crystallographic anisotropy of micas, which sets them apart from the majority of other minerals. This thesis reports microstructural work carried out on naturally deformed mica-bearing and mica-dominated mylonites from the upper-greenschist to amphibolite facies Cossato-Mergozzo-Brissago (CMB) line and Pogallo line shear zones in North West Italy. The principal objectives are to identify the mechanisms of intracrystalline deformation within micas, investigate the interaction of stress, chemistry and fluids within mica-bearing mylonites and examine how the degree of mica content affects the distribution of strain within polyphase rocks. Transmission electron microscope (TEM) images reveal evidence of ripplocations, a novel defect proposed for layered materials, in naturally deformed biotite. Nanoscale lenticular delaminations and bending of biotite lattice planes are observed in en-echelon arrays. These features cannot be explained within the framework of existing understanding of intracrystalline defects within micas as basal dislocations contain no component of c-axis parallel strain. The features closely resemble those resulting from ripplocations, modelled and observed within layered engineering materials, that are theoretically applicable to phyllosilicates. The existence of ripplocations within phyllosilicates opens up ripplocation motion as a potential deformation mechanism in geological phenomena which is able to explain many of the existing ambiguities relating to non-brittle intracrystalline deformation of phyllosilicates. Biotite within the studied mylonites derived from granitic orthogneiss protoliths underwent a dramatic grain size reduction with increasing strain, forming 1-5 μm aggregates. The breakdown was focussed initially at sites of high strain, such as kink band boundaries, grain tips and grain surfaces, but eventually entirely replaced coarse grains. Electron probe microanalysis (EPMA) data suggests the fine grains represent a subtly different compositional population to the parent grains, centred on a reduction in Ti. TEM images reveal irregular boundaries between fine-grained biotite and quartz in the matrix, with the transition from coarse to fine grains having occurred in a single stage, with no evidence of subgrain formation. Muscovite in the same thin sections did not undergo the same process. It is suggested that the grain-size reduction occurred by means of a stress-induced, fluid facilitated dissolution-precipitation reaction rather than a purely mechanical recrystallisation mechanism. This process facilitated the formation of an interconnected weak network of fine-grained polyphase material which likely deformed by grain-size sensitive creep, ultimately leading to rheological weakening of the mylonite. While the grain size reduction of biotite is critical to forming interconnected weak networks in orthogneiss mylonites, its occurrence is distinctly less pronounced in mylonites with higher mica content. In these rocks (with mica content up to and over 50%) the mica phase forms an interconnected weak network from the outset of deformation. This enables strain to be distributed more evenly, as the microstructure is dominated by a weak framework, and contrasts with the high strain C’ shear bands formed in orthogneiss rocks where biotite grain size has been dramatically reduced. Such a pattern is also evident at the scale of hundreds of metres, with mica-rich rocks forming a 200 m to 300 m wide region of distributed deformation and orthogneiss rocks producing ~50 m wide zones characterised by heterogeneous deformation, high strain shear zones and ultramylonites. It is proposed that strain localisation is most important in rocks with mica content between around 5% - 25%, whereas deformation of rocks with mica content above around 25% promotes more distributed accommodation of strain. In granitic orthogneiss ultramylonites, K-feldspar porphyroclasts are decorated along high stress interfaces by fine grained plagioclase feldspar and a small amount of quartz. The reverse occurs at low stress sites of plagioclase porphyroclasts such as fractures and surfaces in contact with pressure shadows. Microstructural observations and electron backscatter diffraction (EBSD) data suggest the fine grains have grown topotactically at the expense of the porphyroclasts, indicating that this is a form of mineral replacement reaction controlled by local stress heterogeneities. Molar volume calculations show that the driving force for the reaction may be a re-distribution of volume from sites of high stress to those of low stress, making this process a form of incongruent pressure solution as it involves a phase change. These reactions are facilitated by ionic exchange and local chemical gradients within an aqueous phase and serve to accelerate the transformation of feldspars from coarse, rigid porphyroclasts to fine-grained components of a polyphase matrix.