Metamorphic Rocks

Three general trends

  • Lo T, Hi P: blueschist, at the subduction zone where the cold downgoing plate undergoes very distinctive metamorphism.  When it returns to the surface, it is often mixed with fragments of ophiolite in a rock called melange. 
  • Regional metamorphism: both T and P go up in tandem, and we see a progression such as: Shale--> slate  --> schist  --> gneiss  --> melting (igneous rock)
  • Contact metamorphism: Hi T, Lo P, adjacent to large igneous intrusions.

Names in the diagram are metamorphic facies, which correspond only partially with the rock names.



Image from Wikipedia

The concept of paired metamorphic belts was originally theorized by the Japanese geologist, Akiho Miyashiro in 1961. The parallel arrangement between the metamorphic belts and the similar ages of each belt, led Miyashiro to the idea that metamorphic belts formed together as pairs. The introduction of the paradigm of plate tectonics in the late 1960s, led to a better understanding of regional metamorphism and permitted the association between paired metamorphic belts and subduction zones. Low temperature, high pressure conditions are generated in the areas along the oceanic trench. High temperature, low pressure conditions are generated beneath the arc region.



last revision 10/28/2016