Decoding crystallographic orientations: how grain-scale textures can be used to infer UHP conditions in felsic rocks

At plate boundaries where continents collide, felsic continental crust can be buried to depths of  > 100 km resulting in the formation of ultra-high pressure (UHP) minerals such as coesite, a high-pressure polymorph of SiO₂. While the burial and subsequent exhumation of buoyant continental crust poses interesting questions for large-scale tectonics, the identification of such UHP terranes is difficult as few petrological barometers are suitable for dominantly felsic lithologies. In such cases, burial to extreme depths is commonly identified through the preservation of coesite or from parallel or radiating columnar grains of quartz assumed to have formed as quartz transforms from coesite—a microstructure termed 'palisades'. However, coesite readily transforms to quartz upon exhumation, while palisade microstructures can easily be modified by annealing during exhumation, meaning that UHP metamorphism of felsic lithologies may often be overlooked. Recent studies proposed that the former presence of coesite could be identified through an orientation signature inherited by quartz, providing a crucial and relatively simple test of deep subduction. However, debate exists within the literature as to whether the quartz↔coesite transformations involve specific crystallographic relationships. Before using crystallography to identify UHP terranes in nature, a better understanding of the coesite-to-quartz crystallographic signature and the conditions under which it forms is required. 

We collected crystallographic data using electron backscatter diffraction (EBSD) on quartz in rocks from the Tso Morari Complex (NW Himalaya) and the Dora Maira Massif (Western Alps), two areas known to reach UHP conditions. We demonstrate that neighbouring domains of quartz commonly feature an 84 ± 4° rotation of [c] axes around the pole of a common {m} plane, matching the rotation axis and angle of a Japan Twin. This orientation relationship is a product of epitaxy, whereby the Japan twin plane in quartz nucleates on the (b) plane in coesite. In supercell simulations, the nucleation of Japan twins can be explained by the energetically favourable alignment of quartz tetrahedra on parental coesite tetrahedra. Through subsequent high-pressure, high-temperature experiments, we demonstrate that this microstructural signature emerges over a broad range of conditions, regardless of the availability of nucleation sites (e.g., grain boundaries) or the density of crystal lattice defects (e.g., dislocations). In addition, Japan twins are present in all experimental specimens that traversed the quartz↔coesite phase boundary, whereas palisade microstructures are largely absent. Our crystallographic method of identifying UHP terranes is therefore more robust, and remains applicable even in the absence of palisade quartz. Overall, this work provides a new tool to quantitatively and unambiguously identify UHP terranes, even when all coesite has transformed to quartz.