Blueschist

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Photomicrograph of blueschist facies basalt, Sivrihisar, Turkey
Blueschist (IPA: /ˈbluːʃɪst/) is a rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius. The blue color of the rock comes from the presence of the mineral glaucophane.

Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks.

Petrology

Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + ( lawsonite or epidote ) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition.
Blueschist often has a lepidoblastic, nematoblastic or schistose rock microstructure defined primarily by chlorite, phengitic white mica, glaucophane, and other minerals with an elongate or platy shape.

Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, coarse varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop. When lawsonite occurs in blueschists, it appears as white tabular crystals.

Blueschist facies

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Photomicrograph of garnet-lawsonite-glaucophane blueschist from Sivrihisar, Turkey. Field of view = 3 mm.
Blueschist facies is determined by the particular Temperature-Pressure conditions required to metamorphose basalt to form blueschist. Felsic rocks and pelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt.

Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are:
  • Basalts: glaucophane + lawsonite and/or epidote + albite +/- garnet +/- quartz
  • jadeite + quartz - diagnostic of pressures ~> 10 kbar
  • Ultramafic rocks: serpentine/lizardite +/- talc +/- zoisite
  • Pelites: kyanite + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnet
  • Granites: kyanite +/- paragonite +/- chlorite +/- albite +/- quartz +/- pargasite or phengite
  • Calc-silicates: Various
  • Limestones and marble: calcite transforms to aragonite at high pressure, but typically reverts to calcite when exhumed
Blueschist facies generally is considered to form under pressures of >0.6 GPa, equivalent to depth of burial in excess of 15-18 km, and at temperatures of between 200 to 500 °C. This is a 'low temperature, high pressure' prograde metamorphic path and is also known as the Franciscan facies series, after the west coast of the United States where these rocks are exposed. Well-exposed blueschists also occur in Greece, Turkey, Japan, and New Caledonia.
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Photomicrograph of blueschist facies quartz sediment, Sivrihisar, Turkey
Continued subduction of blueschist facies oceanic crust will produce eclogite facies assemblages in metamorphosed basalt (garnet + omphacitic clinopyroxene). Rocks which have been subjected to bluescist conditions during a prograde trajectory will gain heat by conduction with hotter lower crustal rocks if they remain at the 15-18km depth. Blueschist which heats up to greater than 500 °C via this fashion will enter greenschist or eclogite facies temperature-pressure conditions, and the mineral assemblages will metamorphose to reflect the new facies conditions.

Thus in order for blueschist facies assemblages to be seen at the Earth's surface, the rock must be exhumed swiftly enough to prevent total thermal equilibration of the rocks which are under blueschist facies conditions with the typical geothermal gradient.

Blueschists and other high-pressure subduction zone rocks are thought to be exhumed rapidly by flow and/or faulting in accretionary wedges or the upper parts of subducted crust, or may return to the Earth's surface in part owing to buoyancy if the metabasaltic rocks are associated with low-density continental crust (marble, metapelite, and other rocks of continental margins).

History and etymology

In 1962, Edgar Bailey of the U.S. Geological Survey introduced the concept of "blueschist" into the world of metamorphic geology. His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.

See also

Metamorphic Facies - [ edit]
Prehnite-pumpellyite | Zeolite | Greenschist | Blueschist | Eclogite | Amphibolite | Granulite
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IPA for English The
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Balanced Rock stands in Garden of the Gods park in Colorado Springs, CO]] A rock is a naturally occurring aggregate of minerals and/or mineraloids. The Earth's lithosphere is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic.
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Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i.e without melting. There will be mineralogical, chemical and crystallographic changes.
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Basalt (IPA: /ˈbæsɒlt, bəˈsɒlt/) is a common gray to black extrusive volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface.
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Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.

Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
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trillion fold).]]

Temperature is a physical property of a system that underlies the common notions of hot and cold; something that is hotter generally has the greater temperature. Temperature is one of the principal parameters of thermodynamics.
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Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale
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Glaucophane is a mineral belonging to the amphibole group, chemical formula Na2(Mg,Fe)3Al2Si8O22(OH)2. It is named from its typical blue color (in Greek, glaucophane means "blue appearing").
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Orogeny (Greek for "mountain generating") is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity,
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A terrane in paleogeography is an accretion that has collided with a continental nucleus, or "craton" but can be recognized by the foreign origin of its rock strata. The crustal block or fragment preserves its own distinctive geologic history, which is different from the
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Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered mafic volcanic rock. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the rock.
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Eclogite (IPA: /ˈɛklədʒʌɪt/) is a coarse-grained mafic (basaltic in composition) metamorphic rock. Eclogite is of special interest for at least two reasons.
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Glaucophane is a mineral belonging to the amphibole group, chemical formula Na2(Mg,Fe)3Al2Si8O22(OH)2. It is named from its typical blue color (in Greek, glaucophane means "blue appearing").
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Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common.
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Epidote is a calcium aluminium iron sorosilicate mineral, Ca2(Al, Fe)3(SiO4)3(OH), crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of
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Jadeite is a pyroxene mineral with composition NaAlSi2O6. It is monoclinic. It has a Mohs hardness of about 6.5 to 7.0 depending on the composition. The mineral is dense, with a specific gravity of about 3.4.
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Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate.
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The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.
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Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Garnets are most often seen in red, but are available in a wide variety of colors spanning the entire spectrum.
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Muscovite (also known as Common mica, Isinglass, or Potash mica[3]) is a phyllosilicate mineral of aluminium and potassium with formula: KAl2(AlSi3O10)(F,OH)2.
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The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others.
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In materials science, texture is the distribution of crystallographic orientations of a sample. A sample in which these orientations are fully random is said to have no texture.
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Mica may refer to:
  • Mica, a silicate mineral group
  • The biblical prophet Micah
  • The book of Micah in the Tanakh
  • Mica is a song by Danish indie rock band Mew.

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Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common.
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Felsic is a term used in geology to refer to silicate minerals, magmas, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. The term combines the words "feldspar" and "silica.
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Pelite (Greek Pelos, Clay) is an alternative name for any argillaceous rock or lithified mudstone. Pelitic rock also refers to a metamorphosed argillaceous sediment.
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Ultramafic (or ultrabasic) rocks are igneous and meta-igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content).
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Pelite (Greek Pelos, Clay) is an alternative name for any argillaceous rock or lithified mudstone. Pelitic rock also refers to a metamorphosed argillaceous sediment.
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Kyanite, whose name derives from the Greek word kyanos, meaning blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock.
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Granite (IPA: /ˈɡrænɪt/) is a common and widely occurring type of intrusive, felsic, igneous rock. Granites are usually medium to coarsely crystalline, occasionally with some individual crystals larger than the
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