In Chapter 1, it was stated that the feldspars are the most common group of minerals in the Earth’s mid to lower crust, and so control crustal rheology, but their deformation behaviour remains relatively poorly understood. Complex interactions between metamorphism and deformation occur in plagioclase feldspar, so multiple processes can work consecutively or in tandem to reduce plagioclase grain size in deforming rocks, and promote strain localisation into discrete zones of shear. Grain size reduction can lead to a switch in the dominant deformation mechanism in shear zones, promoting grain size-sensitive creep by (fluid-assisted) grain boundary diffusion creep. This deformation mechanism is relatively poorly understood, as it leaves few microstructural indicators.
In Chapter 3, the relationship between metamorphism and deformation in plagioclase was explored using naturally-deformed, greenschist-facies, albite-rich metamorphosed gabbros from the Gressoney Shear Zone, NW Italian Alps. The plagioclase in the original gabbros would have been Ca-bearing, but a complete lack of Ca was recorded in all chemical analyses of the albite. This indicates a greenschist-facies metamorphic transition from Ca-plagioclase to albite went to completion in the samples, generating both porphyroclasts of albite, and a population of small, new albite grains. For this to have occurred in mm-scale porphyroclasts, the transition must have been very efficient, implying fluid-enhanced kinetics. Initial fracturing of the feldspar opened fluid pathways. Microstructural analysis suggests different amounts of fluid availability, which varied with different degrees of fracturing, influenced whether porphyroclasts or new grains formed. EBSD was used to extract crystallographic orientation data in low- strain samples from both albitic porphyroclasts and associated populations of small grains. The EBSD data showed that small grains that neighbour porphyroclasts tend to share common orientations with the porphyroclasts. Preserved twin orientations in altered porphyroclasts suggest that crystallographic orientation was inherited from parent Ca-bearing plagioclase. These observations, that a fluid-assisted metamorphic
183 reaction, which preserved parent crystallographic information, occurred, suggest that the porphyroclasts underwent interface-coupled replacement reactions (Putnis, 2009). Replacement reactions have been shown to be common in feldspar (Engvik et al., 2008; Plümper and Putnis, 2009). These types of reactions occur in a nm-thin film of fluid (Hövelmann et al., 2009), and thus it was concluded that they were the dominant transformation mechanism in plagioclase porphyroclasts, where fracturing, and therefore fluid availability, was limited.
As energy dispersive spectroscopy (EDS) showed a complete lack of Ca in both grain populations in all studied areas, the system lacked a chemical driving force for ‘neocrystallisation’ of fine-grained albite from the porphyroclasts. Where fracturing (and therefore fluid availability) was more abundant, sub-μm-scale Ca-plagioclase fragments may have acted as nuclei upon which albite could precipitate to generate the bands of small grains that are observed to run through porphyroclasts. However, the orientation relationship between porphyroclasts and small grains must also be explained, the frictional sliding and grain rotations associated with cataclastic grain size reduction would be unlikely to have preserved an orientation. EBSD processing revealed porphyroclast lattices were highly distorted, whereas small grains were observed to be nominally strain free. Replacement reactions can load product grains with growth dislocations and other defects during imperfect topotactic growth, due to mismatches between lattice parameters (Jain et al., 1997; van der Merwe, 1991), resulting in grains with a high dislocation density produced by a chemical, rather than mechanical, process. Thus, although the exact mechanism of small grain nucleation remains unclear, the differences in internal distortion between the porphyroclasts (high dislocation density) and the new, μm-scale grains (low dislocation density) suggest that the small grains are likely to have grown at the expense of the porphyroclasts. Grain boundary segments that separate the porphyroclastic and μm-scale grains require growth of small grains into porphyroclasts by grain boundary migration, which implies boundary migration occurred as a response to the stored strain energy in the porphyroclasts. The mechanism of topotactic replacement in feldspar during fluid-assisted, interface-coupled replacement reactions, has been observed to produce grains with a high defect density (Hövelmann et al., 2009). The results of this study suggest this mechanism of defect production could provide a driving force for recrystallization (i.e. a grain size reduction and associated changes in strength) in response to stored strain energy, even in the absence of crystal plastic deformation.
184 Chapter 3 focuses on low strain metagabbros, where abundant porphyroclasts are preserved. In Chapter 4, microstructural data from a suite of metagabbros at five different strain levels are presented, which show how inherited CPO domains become modified during fluid-assisted grain boundary diffusion creep (pressure solution) in the albite-rich matrix. The inherited CPO domains are interpreted to have been produced by the complete replacement of albite porphyroclast remnants by the growth of small strain-free grains, as described in Chapter 3. On grain size reduction, a switch in dominant deformation mechanism, from grain-size-insensitive to grain-size-sensitive creep, can occur. In the middle crust, where fluids are abundant, pressure solution is likely to dominate viscous deformation, and this occurred in the Gressoney metagabbros. In the studied samples, each CPO domain was produced from a single crystal, so originally had a texture strength of 1 on the M-Index (Skemer et al., 2005), which provides an opportunity to assess how pressure solution modified texture in the domains with increasing strain. In the samples, an increase in strain clearly led to texture weakening, as observed qualitatively by maxima in pole figures becoming more diffuse. Texture weakening was quantified using both the M-Index (Table 4.1), which lowered significantly in individual CPO domains with increasing strain, and misorientation angle distribution plots, which became skewed to higher angles with increasing stain and finally followed the theoretical random distribution.
Nearly all rocks are polyphase materials, so understanding how different phases have interacted during deformation is fundamental to our overall picture of the geodynamic behaviour of the Earth. At greenschist facies, Ca-bearing plagioclase breaks down to albite plus new Ca-Al-bearing phase(s), which in mafic rocks, is normally an epidote mineral (clinozoisite in our samples). At the lowest strains, albite and clinozoisite are not well-mixed, but mixing increases with strain (although never becomes completely homogeneous). Strain caps of clinozoisite and pressure shadows of albite suggest solubility of albite was increased in regions of locally high stress. Albite grain size was observed to be smaller with increasing clinozoisite abundance, which may be due to the inhibition of textural annealing at albite-clinozoisite boundaries (albite-albite boundaries tend to be lobate and exhibit 120° triple junctions). Inhibition of grain growth may have extended the time over which grain-size-sensitive creep dominated deformation in the samples. Aspect ratios of albite grains increased with increasing clinozoisite content towards higher strains, which may be due to enhanced dissolution of albite at albite- clinozoisite boundaries, creating more pronounced flattening in albite during pressure solution. At the same strain level, a higher abundance of clinozoisite within an albite
185 CPO domain preserved texture strength, which may have happened because second phases tend to inhibit the grain rotations normally associated with diffusion creep. Many questions remain about how microstructures evolve during diffusion creep, because very few characteristics of diffusion creep have been identified, and it is often the absence of indicators of other mechanisms that has led investigators to suggest a dominance of diffusion creep during deformation. Numerical simulations of diffusion creep can provide insight into its effects on microstructural evolution (Ford et al., 2004; Ford et al., 2002; Wheeler, 2009). In Chapter 5, a numerical simulation of grain boundary diffusion creep, DiffForm, was used to investigate the effects of porphyroclasts on the microstructural evolution of matrix grains during pure and simple shear. A starting geometry in which a large grain is surrounded by small matrix grains was set up to undergo 200 increments of either pure or simple shear, and the finite rotations and angular velocities of all grains were tracked during deformation. In the model runs, rotations of matrix grains are strongly influenced by rotation of the porphyroclast in both shear geometries, but the rotation direction of the porphyroclast changes depending on how the surrounding matrix behaves, showing there is a feedback between the two populations. Although there is no reason why the presence of porphyroclasts may influence CPO formation in rocks deforming by diffusion creep, pre-existing CPOs can be preserved to high strains during grain boundary diffusion creep (Wheeler, 2009; see also Chapter 4). The results of the simple shear model show a pre-existing CPO in a fine-grained matrix may be split into smaller domains as sets of matrix grains undergo different-sense rotations depending on their interactions with a porphyroclast. This may have occurred in the small CPO domains observed in the naturally deformed metagabbros at strain levels 2–4, as presented in Chapter 4. The model results suggest that in both pure and simple shear, changes in the angular velocity of a rotating porphyroclast can lead to the destabilisation of rotations of matrix grains, which is associated with a strength drop. Restabilisation of rotations to a steady-state is concomitant with a restabilisation in material strength, but the new strength of the material is lower than it was prior to destabilisation.
The work presented here is primarily an EBSD investigation into naturally deformed rocks. The information about deformation histories and processes that can be extracted by the EBSD technique depends on the resolution of EBSD data. High-angular resolution (HR-) EBSD processing is a cutting edge method that can be used to increase the angular resolution of EBSD datasets. The Weighted Burgers Vector (WBV) is also a relatively new innovation that was developed to extract 3D information about
186 dislocation populations in deformed materials from 2D EBSD maps. The WBV has never previously been applied to HR-EBSD datasets so, finally, in Chapter 6, the results of a comparison of the information that the WBV can extract from conventional and HR-EBSD processed datasets are presented. Datasets of both feldspar and olivine are compared. In both cases, HR-EBSD processing enhanced WBV output by refining intracrystalline deformation features, allowing fine detail about Burgers vector populations to be extracted in HR-EBSD datasets that is obscured by noise prior to HR- processing. It should be noted, however, that in general the information extracted on Burger’s vector populations is the same, so unless very specific questions, such as the interactions between sets of dislocations, aim to be addressed by EBSD studies, the extra time-consuming and computing-intensive step of HR-processing may be unnecessary.