Blueschist 1238181 204857802 2008-04-11T04:54:00Z 68.40.169.236 /* Blueschist facies */ {{Unreferenced|date=October 2007}} [[Image:Sivrihisar blueschist2.jpg|thumb|right|Photomicrograph of blueschist facies basalt, Sivrihisar, Turkey]] '''Blueschist''' ({{pronEng|ˈbluːʃɪst}}) is a [[Rock (geology)|rock]] that forms by the [[metamorphism]] of [[basalt]] and rocks with similar composition at high [[pressure]]s and low [[temperature]]s, 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 [[orogeny|orogenic belts]] as [[terrane]]s 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 group|chlorite]] +/- [[garnet]] +/- [[white mica|muscovite]] in a rock of roughly basaltic composition.<br> Blueschist often has a [[lepidoblastic]], [[nematoblastic]] or [[schist]]ose [[Texture (crystalline)|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==<!-- This section is linked from [[Talc]] --> [[Image:Sivrihisar blueschist1.jpg|thumb|right|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 [[Pelite|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 + sphene +/- garnet +/- quartz'' * ''jadeite + quartz'' - diagnostic of pressures ~> 10 kbar * [[Ultramafic]] rocks: ''serpentine/lizardite +/- talc +/- zoisite'' * [[Pelite]]s: ''[[kyanite]] + zoisite +/- pargasite or phengite +/- albite +/- quartz +/- talc +/- garnet'' * [[Granite]]s: ''kyanite +/- paragonite +/- chlorite +/- albite +/- quartz +/- pargasite or phengite'' * Calc-silicates: Various * [[Limestone]]s 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 [[gigapascal|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. [[Image:Sivrihisar blueschist3.jpg|thumb|right|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 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. 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== * [[Metamorphism]] * [[List of rock types]] * [[List of minerals]] {{Metamorphic Facies}} [[Category:Metamorphic rocks]] [[de:Blauschiefer]] [[et:Sinikilt]] [[hu:Kékpala]]