Types of Metamorphic Zones
The type of metamorphic zones in a terrane are also determined by the form of metamorphism. This is dependent on the geodynamic (tectonic and magmatic) setting in which metamorphism took place. The sequence of metamorphic zones is called a metamorphic facies series, and the most common of these is Barrovian (called after George Barrow who first mentioned it in 1912). In this series of zones, both pressure and temperature increase gradually along the metamorphic gradient. Barrovian metamorphism takes place during regional metamorphism, caused by crustal thickening in the roots of an orogenic belt (under mountain chains). Barrovian zones are especially easy to recognize in pelitic rocks. The prograde sequence of Barrovian zones is:
- chlorite - biotite - garnet - staurolite - kyanite - sillimanite
Often only part of the series can be found. Another metamorphic facies series is the Buchan series, that sees a fast increase in temperature but a relatively small increase in pressure. Characteristic minerals include andalusite, biotite and cordierite. Buchan metamorphism occurs often in extensional settings, for example at rift basins. At contact metamorphism (metamorphism caused by high temperatures at low pressure in the vicinity of an igneous intrusion) a local contact aureole of zones is formed around a heat source.
In rocks in subduction zones, that are transported to great depths in relatively low temperatures, rare types of metamorphic zones can develop. Two facies series are the Franciscan and Sanbagawa types. The rocks are characterized by prehnite-pumpellyite, blueschist or eclogite facies minerals.
Read more about this topic: Metamorphic Zone
Famous quotes containing the words types of, types and/or zones:
“... there are two types of happiness and I have chosen that of the murderers. For I am happy. There was a time when I thought I had reached the limit of distress. Beyond that limit, there is a sterile and magnificent happiness.”
—Albert Camus (1913–1960)
“Our children evaluate themselves based on the opinions we have of them. When we use harsh words, biting comments, and a sarcastic tone of voice, we plant the seeds of self-doubt in their developing minds.... Children who receive a steady diet of these types of messages end up feeling powerless, inadequate, and unimportant. They start to believe that they are bad, and that they can never do enough.”
—Stephanie Martson (20th century)
“The technological landscape of the present day has enfranchised its own electorates—the inhabitants of marketing zones in the consumer goods society, television audiences and news magazine readerships... vote with money at the cash counter rather than with the ballot paper at the polling booth.”
—J.G. (James Graham)