Work

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

Year: 2026

Type: article

Abstract: 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 reactions bound to UHP conditions, inherent instability and tendency for immediate recrystallization to quartz. This study aimed 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 enabled the differentiation of the four discrete microstructural stages of coesite to coesite-to-quartz transformation: (1) preserved primary coesite porphyroblasts, (2) the initial generation of palisade quartz, (3) the second generation of palisade quartz exhibiting indications of subsequent recrystallization, which resulted in (4) complete recrystallized matrix quartz. Based on the EBSD analysis, it is evident that the orientation of the quartz grains within all microstructures was driven solely 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. 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 recrystallization process of exhumed UHP complexes is not uniform, but comprises several definable steps that can be identified through detailed investigation.

Source: Journal of the Geological Society

DOI: 10.1144/jgs2025-243

Authors: Tomáš Potočný, Karolina Kośmińska, Jarosław Majka, Katarzyna Berent, Mattia Gilio, Magdalena Pańczyk