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Blueschist (), also called glaucophane schist, is a
metavolcanic rock Metavolcanic rock is volcanic rock that shows signs of having experienced metamorphism. In other words, the rock was originally produced by a volcano, either as lava or tephra. The rock was then subjected to high pressure, high temperature or both ...
that forms by the metamorphism of
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue color of the rock comes from the presence of the predominant minerals
glaucophane Glaucophane is the name of a mineral and a mineral group belonging to the sodic amphibole supergroup of the double chain inosilicates, with the chemical formula ☐Na2(Mg3Al2)Si8O22(OH)2. Glaucophane crystallizes in the monoclinic system. Name G ...
and lawsonite. Blueschists are schists typically found within
orogenic belts Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...
as terranes of
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
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 Glaucophane is the name of a mineral and a mineral group belonging to the sodic amphibole supergroup of the double chain inosilicates, with the chemical formula ☐Na2(Mg3Al2)Si8O22(OH)2. Glaucophane crystallizes in the monoclinic system. Name G ...
+ ( lawsonite or epidote ) +/-
jadeite Jadeite is a pyroxene mineral with composition Na Al Si2 O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, but is most often found in shades ...
+/-
albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. ...
or
chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
+/- garnet +/-
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...
in a rock of roughly basaltic composition.
Blueschist often has a lepidoblastic, nematoblastic or schistose
rock microstructure Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable ...
defined primarily by
chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
, 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, porphyritic varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop.


Blueschist facies

Blueschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form blueschist.
Felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
rocks and
pelitic A pelite ( Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary roc ...
sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt. Thereby, these rocks do not appear blue overall in color. Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are: * Basalts: ''glaucophane + lawsonite and/or epidote + albite +
titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals in ...
+/- garnet +/- quartz'' ''jadeite + quartz'' - diagnostic of pressures ~> 10 kbar * Ultramafic rocks: '' serpentinite/
lizardite Lizardite is a mineral from the serpentine group with formula , and the most common type of mineral in the group. It is also a member of the kaolinite-serpentine group. Népouite and lizardite form a series; intermediate compositions are pos ...
+/- talc +/-
zoisite Zoisite, first known as saualpite, after its type locality, is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2 Al3( Si O4)(Si2O7)O(O H). Zoisite occurs as prismatic, orthorhomb ...
'' * Pelites: ''Fe-Mg-carpholite +/-
chloritoid Chloritoid is a silicate mineral of metamorphic origin. It is an iron magnesium manganese alumino-silicate hydroxide with formula . It occurs as greenish grey to black platy micaceous crystals and foliated masses. Its Mohs hardness is 6.5, unusua ...
+/-
kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generall ...
+ zoisite +/-
pargasite Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa2(Mg4Al)(Si6Al2)O22(OH)2. It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality. It occurs in high temperature regional ...
or
phengite Phengite is a series name for dioctahedral micas of composition K(AlMg)2(OH)2(SiAl)4O10 it is common for Mg or Fe2+ to substitute for Al on the Y site and a corresponding increase in Si on the Z site in the mica group formula ''X''2''Y''4–6''Z' ...
+/- albite +/- quartz +/- talc +/- garnet'' * Granites: ''
kyanite Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generall ...
+/-
paragonite Paragonite is a mineral, related to muscovite. Its empirical formula is . A wide solvus separates muscovite from paragonite, such that there is little solid solution along the vector Na+ K+ and apparent micas of intermediate composition is most c ...
+/- chlorite +/- albite +/- quartz +/- pargasite or phengite'' * Calc-silicates: Various * Limestones and marble:
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
transforms to
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pr ...
at high pressure, but typically reverts to calcite when exhumed Blueschist facies generally is considered to form under pressures of >0.6
GPa Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
, equivalent to depth of burial in excess of 15–18 km, and at temperatures of between 200 and 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, New Zealand and New Caledonia. Continued
subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
of blueschist facies
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...
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 15–18 km 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 wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the ...
s 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). It has been held that the absence of blueschist dating to before the
Neoproterozoic Era The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is p ...
indicates that currently exhumed rocks never reached blueschist facies at
subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
zones before 1,000 million years ago. This assertion is arguably wrong because the earliest
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...
would have contained more magnesium than today's crust and, therefore, would have formed greenschist-like rocks at blueschist facies.


History and etymology

In Minoan Crete blueschist and greenschist were used to pave streets and
courtyard A courtyard or court is a circumscribed area, often surrounded by a building or complex, that is open to the sky. Courtyards are common elements in both Western and Eastern building patterns and have been used by both ancient and contemporar ...
s between 1650 and 1600 BC. These rocks were likely quarried in
Agia Pelagia Agia Pelagia ( el, Αγία Πελαγία, " Saint Pelagia") is a popular seaside resort 23 km northwest of Crete's capital city of Heraklion. In former times, Agia Pelagia was a tiny fishing village and a place for the residents of nearby Ach ...
on the north coast of central Crete. In 1962, Edgar Bailey of the
U.S. Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...
, introduced the concept of "blueschist" into the subject 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


References


External links


Blueschist facies - Rock Library Glossary, Imperial College London
{{Metamorphic Facies Schist Metamorphic petrology Volcanic rocks he:צפחה#צפחות כחולות