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Greenschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form the typical greenschist facies minerals chlorite, actinolite, and albite. Greenschist facies results from low temperature, moderate pressure metamorphism.
The zeolite facies is the metamorphic facies with the lowest metamorphic grade. At lower temperature and pressure processes in the rock are called diagenesis. The facies is named for zeolites, strongly hydrated tectosilicates. It can have the following mineral assemblages: In meta-igneous rocks and greywackes:
In his pioneering work on metamorphic facies in the Scottish Highlands, G.M. Barrow identified the chlorite zone as the zone of mildest metamorphism. [12] In modern petrology, chlorite is the diagnostic mineral of the greenschist facies. [10] This facies is characterized by temperatures near 450 °C (840 °F) and pressures near 5 kbar. [13]
Formation of a metamorphic core complex. Metamorphic core complexes are exposed areas of deep crust brought to the surface by crustal extension (stretching). [1] They form, and are exhumed, through relatively fast [citation needed] transport of middle and lower continental crust to the Earth's surface [2] in the form of uplifting welts of hot rock and magma. [1]
The alteration occurs during either late-stages of magmatic crystallization at low temperatures (< 500 °C) or during low-grade metamorphic events (sub-greenschist facies metamorphism). The reaction is paramorphic, meaning that the structure of the mineral is modified, but its original chemistry is retained. [2]
Width of the peak at one half of its height is measured and this angle (recorded with units of ∆ °2θ), [1] can be plotted on a chart with metamorphic zones and facies like the one in figure 1. If the illite crystallinity values fall in the 0-0.25 °2θ range, it corresponds with a metamorphic epizone or greenschist facies.
Based on inspection of extreme metamorphism and post-subduction magmatism at convergent plate margins, paired metamorphic belts are further extended to two contrasting metamorphic facies series: [7] one is blueschist to eclogite facies series that was produced by subducting metamorphism at low thermal gradients of <10 °C/km, and the other is ...
Examples include metamorphic overprinting (superimposed metamorphism [1]), in which new structure, [2] texture, or mineral composition is imposed on existing rock. [3] [4] For example, the Tauern window of Alps contains beds that were originally metamorphosed to eclogite but have since been overprinted to the blueschist and then the greenschist ...