Photomicrograph of blueschist facies basalt, Sivrihisar, Turkey

Blueschist (pronounced /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. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere 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 Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface 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 The Celsius Temperature scale was previously known as the centigrade scale. The blue color of the rock comes from the presence of the mineral glaucophane. Glaucophane is a Mineral belonging to the Amphibole group chemical formula Na 2( Mg, Fe)3 Al 2

Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks. Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and A terrane in Geology is a fragment of crustal material formed on or broken off from one Tectonic plate and accreted — " sutured " Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock Eclogite (ˈɛklədʒaɪt is a coarse-grained Mafic ( Basaltic in composition Metamorphic rock.

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. Glaucophane is a Mineral belonging to the Amphibole group chemical formula Na 2( Mg, Fe)3 Al 2 Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH2·H2O Epidote is a Calcium Aluminium Iron sorosilicate Mineral, Ca2Al2(Fe3+Al(SiO4(Si2O7O(OH Jadeite is a Pyroxene mineral with composition Na[[aluminum Al]] Si 2 O 6 Albite is a plagioclase Feldspar Mineral. It is the sodium endmember of the Plagioclase Solid solution series The chlorites are a group of phyllosilicate Minerals Chlorites can be described by the following four endmembers based on their chemistry via substitution The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives Muscovite (also known as Common Mica, Isinglass, or Potash mica) is a phyllosilicate Mineral of Aluminium
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. The schists form a group of medium-grade Metamorphic rocks chiefly notable for the preponderance of lamellar Minerals such as Micas chlorite In Materials science, texture is the distribution of crystallographic orientations of a sample The word "mica" is thought to be derived from the Latin word la micare, "glitteren" in reference to the brilliant appearance of this mineral (especially

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. Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH2·H2O

Blueschist facies

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. Felsic is a term used in Geology to refer to Silicate minerals, Magma, and rocks which are enriched in the lighter elements such as Silicon Pelite ( Greek Pelos, Clay is a descriptive name for a Clastic rock with a grain size of less than 1/16mm (originally sand or silt Examples include

Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are:

• Basalts: glaucophane + lawsonite and/or epidote + albite + sphene +/- 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. Ultramafic (also referred to as ultrabasic) rocks are igneous and meta -igneous rocks with very low Silica content (less than 45% generally Pelite ( Greek Pelos, Clay is a descriptive name for a Clastic rock with a grain size of less than 1/16mm (originally sand or silt Examples include Kyanite, whose name derives from the Greek word kyanos, meaning blue is a typically blue Silicate Mineral, commonly found in Aluminium -rich Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Marble is a nonfoliated Metamorphic rock resulting from the Metamorphism of Limestone, composed mostly of Calcite (a crystalline form of Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 Aragonite is a Carbonate mineral, one of the two common naturally occurring polymorphs of Calcium carbonate, Ca[[carbon C]] O 3 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.

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). In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Eclogite (ˈɛklədʒaɪt is a coarse-grained Mafic ( Basaltic in composition Metamorphic rock. Rocks which have been subjected to blueschist 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. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock Eclogite (ˈɛklədʒaɪt is a coarse-grained Mafic ( Basaltic in composition Metamorphic rock.

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. The geothermal gradient is the rate of increase in temperature per unit depth in the Earth.

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. The United States Geological Survey ( USGS) is a scientific agency of the United States government. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.

See also

• Metamorphism
• List of rock types
• List of minerals