The Preservation of Intragranular and Intergranular Coesite in Metasedimentary Rocks: Insights From a TEM‐Based Structure Analysis

ABSTRACT

Coesite is a key indicator mineral of ultrahigh‐pressure (UHP) metamorphism. A “dry” environment is widely recognized as a prerequisite for coesite preservation during exhumation. However, the deformation behaviour of coesite, its retrograde replacement by quartz and their host and surrounding minerals remain poorly constrained at the micro‐ to nanoscale, especially for coesite included in or surrounded by nonrigid minerals. Here, we employed electron backscatter diffraction (EBSD), focused ion beam (FIB) and transmission electron microscopy (TEM) to characterize the micro‐nanostructure of intragranular coesite (within dolomite) and intergranular coesite (surrounded by dolomite and apatite) in a UHP metasedimentary rock from the Ganjialing area, Dabie Shan region in east‐central China. Our observations reveal that both intragranular and intergranular coesite grains are essentially undeformed and dislocation‐free, whereas the palisade quartz rim enveloping coesite exhibits abundant dislocations and tangles. Based on the dislocation densities within these quartz rims, the differential stress is estimated at 347–663 MPa for intragranular coesite and 320–741 MPa for intergranular coesite grains, respectively. Combining this differential stress estimate with the regional P–T–t path constrains the corresponding metamorphic conditions to 715°C–720°C and 1.7–2.0 GPa. Furthermore, dolomite and apatite domains adjacent to coesite show higher dislocation densities than those distal to coesite. Collectively, the palisade quartz rim, dolomite and apatite form a composite buffer zone that not only maintains a closed, dry system but also accommodates the high differential stress generated by phase transformation. This buffer system effectively suppresses the replacement of coesite by quartz during decompression. Our findings demonstrate that transformation‐induced differential stress plays a pivotal role in the preservation of coesite within the mechanically weak, nonrigid matrix of UHP metasedimentary rocks during exhumation.