Key patterns of S‐type granitic gneiss to define the baric (low‐ to high‐pressure) nature of Phanerozoic basement terranes

Abstract

AbstractS‐type granitic gneiss is a major rock type in exposed Phanerozoic basement areas. However, it is hardly used to decipher the metamorphic evolution of such areas formed by the collision of tectonic plates. It is demonstrated by thermodynamic modelling of the metamorphism of common to evolved S‐type granite that mineralogical key patterns can elucidate the baric nature of granitic gneiss. Low‐ to medium‐grade granitic gneiss shows increasing and decreasing contents of potassic white mica and biotite, respectively, with rising pressure. Pressures above c. 12 kbar (500°C) result in considerable amounts of Na‐rich clinopyroxene. A relatively precise peak‐pressure estimate can be obtained by the determination of the Si content in potassic white mica but only for water pressures below 13–15 kbar. Thus, eclogite facies granitic gneiss can only be recognized by the presence of Na‐rich clinopyroxene. Silicon contents in biotite might have a geobarometric potential as potassic white mica. Titanium minerals in S‐type granitic gneiss are of limited baric potential.