Polymetamorphism,polyphase metamorphism and metamorphic p-T-t paths

Monometamorphism is resulted from one metamorphic event,whereas polymetamorphism resulted from two or more metamorphic events.If a metamorphic event involves only one phase of heating or pressing and then cooling or depressing,it is called monophase metamorphism.If a metamorphic process involves two or more climaxes in the changes of temperature and/or pressure,it is called a polyphase metamorphism.Although criteria are quite rare,it is very important to determine if a metamorphism belongs to a monometamorphism or a polymetamorphism.However,metamorphic processes preserved in one geologic entity with different geodynamic backgrounds are definitely of different metamorphic events.To accurately retrieve metamorphic p-T-t paths,metamorphic events should be discriminated by inspecting metamorphic reaction textures,p-T conditions should be determined by valid geothermometers and geobarometers,and finally,the geologic time should be determined by in-situ geochronology.Since uncertainties exist in all of these three aspects,refining and developing the fundamental theory and technology are much needed in the near future.