Information AboutGreenschist |
| CATEGORIES ABOUT GREENSCHIST | |
| metamorphic rocks | |
| petrology | |
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PETROLOGY Geenschist, as a rock type, is defined by the presence of the minerals chlorite + actinolite +/- albite +/- epidote. Greenschist often has a lepidoblastic, nematoblastic or schistose texure defined primarily by chlorite and actinolite. Greenschists often have some Foliation resulting in mineral alignment, especially of chlorite and actinolite. Grain size is rarely coarse, due primarily to the mineral assemblage. Chlorite and to a lesser extent actinolite prefer small, flat or acicular crystal habits. GREENSCHIST FACIES Greenschist Facies is determined by the particular T-P 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. Metamorphic conditions which create typical greenschist facies assemblages are called the ''Barrovian Facies Sequence'', and the lower-pressure ''Abukuma Facies Series''. Temperatures of approximately 400 to 500 °C and depths of about 8 to 50 kilometers are the typical envelope of greenschist facies rocks. The equilibrium mineral assemblage of rocks subjected to Greenschist Facies conditions depends on primary rock composition.
In greater detail the greenschist facies is subdivided into subgreenschist, lower and upper greenschist. Lower temperatures are transitional with and overlap the Prehnite - Pumpellyite facies and higher temperatures overlap with and include sub- Amphibolite facies. If burial continues along Barrovian Sequence metamorphic trajectories, greenschist facies gives rise to Amphibolite Facies assemblages, dominated by amphibole and eventually to Granulite Facies . Lower pressure, normally Contact Metamorphism produces Albite -epidote Hornfels while higher pressures at great depth produces Eclogite . Oceanic basalts in the vicinity of Mid-ocean Ridge s typically exhibit sub-greenschist alteration. The Greenstone Belt s of the various Archean Craton s are commonly altered to the greenschist facies. These ancient rocks are noted as host rocks for a variety of ore deposits in Australia , Namibia and Canada . See also: REFERENCE
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