Microfabrics of quartz formed from coesite (Dora-Maira Massif, Western Alps)

The microfabrics of quartz formed from coesite during exhumation of ultrahigh pressure (UHP) metamorphic rocks of the Dora Maira Massif (western Alps) are investigated using polarizing microscopy with universal (U)-stage, electron back-scattered diffraction (EBSD), and cathodoluminescence (CL) microscopy. Three types of microstructures are distinguished: aggregates of small elongate quartz grains with radial orientation (palisades) replacing coesite single crystals included in other minerals (type I); platy quartz single crystals showing tapered margins and midribs observed by CL-microscopy occurring associated with type I microstructures in the same inclusions (type II); radial aggregates of large elongate quartz grains in the rock matrix, where coesite is not preserved (type III). The radial shape preferred orientation (SPO) in the type I and III palisade structures is not combined with a crystallographic preferred orientation (CPO). There is no systematic orientation relation between quartz grains in type I aggregates and relict coesite or the host mineral of the inclusion. This indicates that the coesite-quartz transformation started with a distinct random nucleation stage along the original interphase boundary between coesite and the host mineral. The elongate grain shape reflects the progress of the reaction front, which is not influenced by crystallographic orientation. The concentric discontinuities in grain shape frequently observed in type I quartz palisades are attributed to distinct events during transformation, presumably brittle failure of the host mineral. By analogy, coarse-grained quartz palisades (type III) in the rock matrix originated by random nucleation of quartz along the high angle grain boundaries of coesite. Each aggregate of radially oriented elongate grains (type I and III) impinging at a centerline represents the size and shape of a single original coesite grain, with a grain size of up to several millimeters in the matrix of pyrope quartzites (type III), and significantly smaller in metagranites. Type II platy quartz crystals with tapered margins are interpreted to have grown along cracks in coesite, which formed in several generations with distinct orientation during progressive transformation, indicating a very low nucleation probability of quartz for that stage.