Prism‐ and basal‐plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer

Empirical data on quartz subgrain patterns from various metamorphic rocks show that, at least up to 10 kbar in the stability field of low‐quartz, prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed. In the high‐quartz stability field, both prismatic and basal subgrain boundaries occur and form typical rectangular (‘chessboard’) patterns. The likely reason behind the different occurrence of these subgrain patterns is that in high‐quartz prismatic glide becomes as easy as, or probably even easier than, basal glide. The two types of subgrain patterns can be clearly distinguished by optical microscopy. Consequently, the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer. The possibilities of its application are far reaching and include the specification of deformation conditions at high‐grade metamorphism, the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels.