Strength Reversal in Recrystallisation: an EBSD-based Study in a Naturally Deformed Granitic Vein

Strain partitioning in quartzo-feldspathic rock is closely related to the degree of phase mixing. Both quartz and feldspar tend to form the load-bearing framework (LBF) in naturally deformed rocks, provided there are softer phases such as mica, which behaves as the interconnected weak layer (IWL). In cases where mica is absent, the scenario becomes complicated. Quartz in augen gneisses often behaves as the IWL and feldspar takes the role of the LBF. However, the relative degree of weakening in deformed quartz and feldspar depends on their respective deformation mechanisms. As the mechanisms are different, there is a possibility of dissimilar weaking, followed by a strength reversal.

We have studied a deformed quartzo-feldspathic vein from the Bundelkhand Craton in central India. Despite being Archean, this craton experienced long hiatuses between deformation events, which makes the delineation between different events simpler. The sample we collected from this craton is the result of the latest stage of deformation. A high-temperature fluid entered through fractures and softened the granitic country rock. The fluid, being syn-tectonic, allowed the granitic vein to facilitate different deformation mechanisms in quartz and feldspars.

We investigated the crystal-plastic behaviour of quartz and two feldspars in the deformed vein via electron backscatter diffraction. The quartz crystallographic preferred orientation (CPO) and misorientation index (M) is strongest when quartz grains are adjacent to each other. There is no significant difference in CPO strength in feldspars when the proportion of similar neighbouring phases changes. Additionally, a monomineralic quartz layer exhibits a class 3 buckling fold, implying a higher competency than the adjacent matrix, which contains recrystallised feldspar grains. However, the microstructural evidence suggests that the parent feldspar porphyroclasts are stronger than the recrystallised monomineralic quartz bands. From the inverse pole figure of low-angle (2–10°) misorientation axes in quartz, prism <a> activity is observed which is dominant in the temperature range of 500–650°C. Hence, we infer a deformation temperature of at most 650°C, although it can be lower depending upon the water weakening as such weakening activates prism <a> at lower temperatures. Randomised CPO in feldspar suggests strain accommodation via diffusion creep, followed by grain boundary sliding mechanism might have operated in feldspars. These processes could result in greater softening than that in quartz, which deformed by dislocation creep. Isolated quartz grains existing in the triple junctions of feldspars are not part of such pure dislocation creep; rather, it is more likely that they are byproducts of albitic transformation reactions. Hence, higher strength in quartz is limited to the monomineralic bands, which are purely affected by dislocation creep in the deformed quartzo-feldspathic vein of the Bundelkhand Craton.