This diagram shows shale being metamorphosed.
Chlorite and muscovite form at relatively low temperatures. Garnet forms at higher temperatures and pressures. Sillimanite indicates the highest level of temperatures and pressures.
Shale is metamorphosed to slate and then to phyllite. Schist is next to form, then gneiss when high-grade metamorphism is reached. Beyond 800°C, the rock may completely deform by melting.
Another version showing kyanite.
Would you call the Blueschist facies a higher grade of metamorphism than the Amphibolite facies? This sequence only works for pelitic rocks going through Greenschist and Amphibolite facies of metamorphism, doesn't it?
ReplyDeleteBlueschist facies occurs at low T & high P, while amphibolite facies occurs at medium to high T & medium to high P. They also occur in different tectonic settings. Blueschist facies is associated with subduction zone, and amphibolite facies are associated with regional & contact metamorphism. As to which is higher grade, based on this I infer the Amphibolite? Disclaimer: I am not a metamorphic geologist.
ReplyDeleteThe sequence I referenced is the greenschist-amphibolite-granulite Barrovian metamorphism sequence.