Major Rock-Forming Minerals in UHP Metamorphic Rocks

This paper reviews chemical characteristics of common minerals in ultrahigh-pressure (UHP) metamorphic rocks. Garnet in UHP metamorphic rocks belongs to the almandine-pyrope-grossular series and has a wide compositional range in X_alm (up to 0.73), X_prp (0.98), and X_grs (0.92). Garnet with pyroxene exsolution microstructures reported from the Western Gneiss Region, Norway, shows relicts of majoritic garnet formed under P = 6 to 7 GPa. Most clinopyroxenes are Na-Ca groups and their Xjd ranges from 0.0 to 0.89. Members of the clinoenstatite-clinohypersthene (P2₁/c) were reported from peridotites in UHP terranes, and are considered to have been originally the high-pressure polymorph (C2/c) formed under P > 7 to 8 GPa. The presence of sodium in garnet and of potassium in clinopyroxene characterizes UHP equilibrium. Attention is drawn to minerals of the epidote group as strontium and REE containers, to amphibole as a sensor of metamorphic fluid composition, and to phengite as a geobarometer.