- Metamorphism occurs when rocks recrystallize in response to changes in physical conditions, notably temperature, but also pressure and the fugacities of volatile components such as H2O and CO2.
- Contact metamorphism may occur adjacent to magmatic intrusions due to temperature changes in the host rocks caused by heat from the cooling intrusion.
- The scale of contact metamorphism is likely to be local, with significant changes over distances that are less than the width/radius of the intrusion.
- Regional metamorphism is normally accompanied by ductile deformation and occurs in belts that may be a thousand kilometers or more in length.
- Crustal thickening due to continent-continent collision is the principal cause of medium P/T (Barrovian) regional metamorphism. Because continental crust is rich in heat-producing radioactive elements, an increasing the thickness of continental crust is like putting an electric blanket on the rocks in the lower parts of the thickened crust.
- Temperatures in subduction zones are lower than in continental or ocean crust at the same depth. However, by subducting rocks to greater depths, high P/T (blueschist facies) metamorphic rocks and even ultra-high pressure (UHP) metamorphic rocks are formed.
- The conditions of low P/T (Buchan) metamorphism require the addition of magmatic heat to thickened crust or more complicated tectonic processes to boost crustal temperatures. Continental arc settings can lead to juxtaposed high P/T and low P/T metamorphic belts.
- Because regional metamorphic rocks are formed where tectonic plates collide or subduct, the temperature and pressures they experience may may change in complicated ways that can cause metamorphic overprinting.
- By deciphering the T-P history of metamorphic rocks, petrologic studies provide important data to help geologists understand plate tectonic processes.
2.8 Summary
