Grain boundary microstructures and their control on deformation mechanisms in high-grade quartz-rich rocks

High-angle misorientations, including grain boundary domains within quartz aggregates, may exert significant control on how strain is accommodated in quartz-bearing domains of the continental lithosphere. Among the high-angle boundaries in quartz, Dauphiné twin boundaries (DTBs) are the most prominent type that displays coincident site lattice (CSL) relationships, a special grain boundary geometry commonly found in metals. In metals, it is known that CSL boundaries stabilize the microstructure by reducing the overall grain boundary surface energy, and thereby impart certain special properties to the material. The present study explores CSL relationships of DTBs and how they control strain accommodation and deformation mechanisms in quartz-rich rocks by combining optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM). This investigation was carried out on thin sections of granulite facies quartzo-feldspathic gneisses prepared following a standard protocol, culminating with chemical mechanical polishing (CMP) using colloidal silica. The CMP procedure causes preferential material removal along less-compact random high-angle grain boundaries (RHAGBs) and forms indented channels that are prominent in high-resolution, nanoscale AFM images. The absence of corresponding channels along DTBs and the presence of 'bridge-like' structures at RHAGB-DTB intersections suggest greater compactness of DTBs in quartz, which is compatible with CSL relations. Importantly, there is a demonstrable change in misorientation across adjacent quartz grains between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° and 81–84° with parallel rhombohedral faces. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments are modified by DTBs into low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Contextually, these quartzofeldspathic gneisses record multiple deformations of the previous ultra-high-temperature (UHT) fabric under granulite facies conditions, with grain boundary migration recrystallization (GBM) as the dominant dynamic recrystallization process. Owing to the GBM recrystallization, the interaction of RHAGBs with DTBs increases, thereby producing more intersection-induced twin boundaries. The relict vs. recrystallized quartz grain maps with DTBs indicate a higher frequency of DTBs in the relict grains. Crystallographic preferred orientations (CPOs) of these relict grains do not define any slip system pattern. In contrast, the recrystallized grains show the operation of prominent slip system patterns, suggesting differences in strain accommodation. Therefore, the formation of DTBs and DTB-RHAGB intersections in quartz can cause strain partitioning due to the higher compactness of DTBs, which will eventually control the response to the deformation of high-grade quartz aggregates. As a result, interpreting the timing of DTB formation becomes important while deciphering the deformation history of rocks based on quartz CPOs from a poly-deformed high-grade terrane.