Microstructural evolution and stepwise recrystallization across the coesite–quartz transition in UHP whiteschists from the Dora-Maira Massif (Western Alps)

The occurrence of coesite has been observed to be contingent upon the presence of ultrahigh-pressure (UHP) metamorphic conditions. Nevertheless, its presence is limited due to reaction bound to UHP conditions, inherent instability, and tendency for immediate recrystallization to quartz. This study aims to better constrain the relationship between coesite, quartz, and garnet in whiteschists from the Brossasco–Isasca Unit of the Dora Maira Massif (Western Alps) based on detailed microstructural observations, electron backscatter diffraction (EBSD), and cathodoluminescence (CL) analyses. The microstructural observations have enabled the differentiation of the four discrete microstructural stages of coesite to coesite-to-quartz transformation: (I) preserved primary coesite porphyroblasts, (II) the initial generation of palisade quartz, (III) the second generation of palisade quartz exhibiting indications of subsequent recrystallisation, which resulted in (IV) complete recrystallised matrix quartz. Based on the EBSD analysis, it is evident that the orientation of the quartz grains within all microstructures was solely driven by pressure relaxation, as they do not exhibit any preferred crystallographic orientation. After thoroughly examining the misorientation relationships of coesite, quartz, and garnet, it was confirmed that no crystallographic relationships were identified between the investigated phases. The CL images of the various quartz microstructures display homogeneous patterns. In contrast, coesite is distinctly more luminous. However, after a brief time interval, the high luminescence of coesite disappears and becomes indistinguishable from quartz. The observed luminosity may be associated with the elevated defect density of the coesite. Our results suggest that the recrystallisation process of exhumed UHP complexes is not uniform, but comprises several definable steps that can be identified through detailed investigation.