"变质岩石学" can be translated into English as "metamorphic petrology".
Metamorphic refers to the process of metamorphism, which is the change in the structure, texture, or composition of rocks under the influence of heat, pressure, and chemical activity.
Petrology is the branch of geology that studies the origin, composition, structure, and history of rocks.
Therefore, "metamorphic petrology" specifically focuses on the study of metamorphic rocks.