Ultrahigh-temperature metamorphism
Encyclopedia
Ultra-high-temperature metamorphism (UHT) represents extreme crustal metamorphism
with metamorphic temperatures exceeding 900 °C. Granulite
facies
rocks metamorphosed at very high temperatures were identified in the early 1980s, although it took another decade for the geoscience community to recognize UHT metamorphism as a common regional phenomenon. Petrological
evidence based on characteristic mineral
assemblages backed by experimental and thermodynamic
relations demonstrated that the Earth’s crust
can attain and withstand very high temperatures (900-1000 °C) with or without partial melting.
+ quartz
, sapphirine
+ quartz or spinel
+ quartz, generally at pressure conditions of sillimanite stability in metapelite
s [after Brown (2007) following proposal by Harley (1998)].
in nature. Mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz, osumulite and spinel + quartz provide straight away evidence for such extreme conditions. Occasionally widespread assemblages like garnet
+ orthopyroxene, ternary feldspar
s, (F-Ti) pargasite
or metamorphic inverted pigeonite
are taken as typical indicators of UHT metamorphism.
s with diagnostic UHT indicators have been reported over the globe, related to both extensional and collisional tectonic environments; the two fundamental types of Earth orogenic
systems. The major Archean
UHT rocks are distributed in East-Antarctica, South Africa, Russia and Canada. The Paleoproterozoic
UHT granulites were reported from the North China Craton
(during the accretion
of the supercontinent
Columbia
), Taltson magmatic zone, northwestern Canada and South Harris, Lewisian complex
, Scotland. UHT rocks from the Neoproterozoic
Grenville orogeny
are distributed in the Eastern Ghats Province, India. Neoproterozoic-Cambrian (Pan-African) UHT occurrences are mainly distributed in Lutzow-Holm Bay, East Antarctica, Sri Lanka and southern India UHT rocks are also reported from younger terrenes like the Triassic
Kontum Massif, Vietnam, Cretaceous
Higo belt, Japan and Paleogene
Gruf Complex, central Alps
.
activity during various periods in Earth history. UHT granulites are generally characterized by dry mineral assemblages, the stability of which require low water activities. The direct evidence for the involvement of CO2
-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO2 fluid inclusions
in buffering
the water activity
and stabilizing the anhydrous
mineralogy of UHT rocks have come from the finding of abundant pure CO2 fluid inclusions in these rocks (Tsunogae et al., 2008;. UHT metamorphism has been recently evaluated in the plate tectonic context using modern analogues and it has been suggested that both post-collisional extension and rift
ing play a crucial role. The abundant CO2 liberated by subsolidus
decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect
thereby triggering the deglaciation of snowball Earth
. Based on an evaluation of the distribution of carbonated subcontinental mantle
in global reconstructions of the Proterozoic supercontinent assembly, and their link with crustal domains that have undergone CO2-aided dry metamorphism at extreme conditions, Santosh and Omori (2008b) speculated that the UHT rocks might represent windows for the transfer of CO2 from the mantle into the mid crust and ultimately to the atmosphere
.
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
with metamorphic temperatures exceeding 900 °C. Granulite
Granulite
Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure...
facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...
rocks metamorphosed at very high temperatures were identified in the early 1980s, although it took another decade for the geoscience community to recognize UHT metamorphism as a common regional phenomenon. Petrological
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
evidence based on characteristic mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
assemblages backed by experimental and thermodynamic
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
relations demonstrated that the Earth’s crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
can attain and withstand very high temperatures (900-1000 °C) with or without partial melting.
Definition
Metamorphism of crustal rocks in which peak temperature exceeds 900 °C, recognized either by robust thermobarometry or by the presence of a diagnostic mineral assemblage in an appropriate bulk composition and oxidation state, such as assemblages with orthopyroxene + sillimaniteSillimanite
Sillimanite is an alumino-silicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman . It was first described in 1824 for an occurrence in Chester, Middlesex County, Connecticut, USA....
+ quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, sapphirine
Sapphirine
Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula 86O20 . Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones...
+ quartz or spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
+ quartz, generally at pressure conditions of sillimanite stability in metapelite
Pelite
Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...
s [after Brown (2007) following proposal by Harley (1998)].
Identification
Petrological indicators of UHT metamorphism are usually preserved in extremely Mg-Al-rich rocks which are usually dry and restiticRestite
Restite is the residual material left at the site of melting during the in place production of granite through intense metamorphism.Generally, restite is composed of a predominance of mafic minerals because these are harder to melt . Typical minerals are amphibole, biotite, pyroxene, ilmenite or...
in nature. Mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz, osumulite and spinel + quartz provide straight away evidence for such extreme conditions. Occasionally widespread assemblages like garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
+ orthopyroxene, ternary feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
s, (F-Ti) pargasite
Pargasite
Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
or metamorphic inverted pigeonite
Pigeonite
Pigeonite is a mineral in the clinopyroxene group. It has a general formula of Si2O6. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations....
are taken as typical indicators of UHT metamorphism.
Global distribution
UHT rocks are now identified in all major continents and span different geological ages ranging from c. 3178 to 35 million years associated with major geological events. More than 46 localities/terraneTerrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s with diagnostic UHT indicators have been reported over the globe, related to both extensional and collisional tectonic environments; the two fundamental types of Earth orogenic
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
systems. The major Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
UHT rocks are distributed in East-Antarctica, South Africa, Russia and Canada. The Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
UHT granulites were reported from the North China Craton
North China craton
The North China Craton is one of the smaller continental cratons of the Earth. It covers a total area of around 1.7 million square kilometres in the northeast of China, most of Korea and the southern part of Mongolia, and has a shape quite akin to a funnel, with a long east-west axis in the...
(during the accretion
Accretion (geology)
Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features.-Description:...
of the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
Columbia
Columbia (supercontinent)
Columbia, also known as Nuna and Hudsonland, was one of Earth's oldest supercontinents. It was first proposed by J.J.W. Rogers and M. Santosh and is thought to have existed approximately 1.8 to 1.5 billion years ago in the Paleoproterozoic Era. Zhao et al...
), Taltson magmatic zone, northwestern Canada and South Harris, Lewisian complex
Lewisian complex
The Lewisian complex or Lewisian Gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the...
, Scotland. UHT rocks from the Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
Grenville orogeny
Grenville orogeny
The Grenville Orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador to Mexico, as well as to Scotland...
are distributed in the Eastern Ghats Province, India. Neoproterozoic-Cambrian (Pan-African) UHT occurrences are mainly distributed in Lutzow-Holm Bay, East Antarctica, Sri Lanka and southern India UHT rocks are also reported from younger terrenes like the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
Kontum Massif, Vietnam, Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
Higo belt, Japan and Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
Gruf Complex, central Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
.
Recent hypothesis
A correlation has been proposed between the episodic formation of UHT granulites and the episodic assembly and disruption of supercontinents or the plumeMantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
activity during various periods in Earth history. UHT granulites are generally characterized by dry mineral assemblages, the stability of which require low water activities. The direct evidence for the involvement of CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
-rich fluids in generating diagnostic UHT assemblages has been recorded from the common occurrence of pure CO2 fluid inclusions
Fluid inclusions
thumb|250px|Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more...
in buffering
Mineral redox buffer
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions at which a rock forms and evolves can be important for interpreting the rock history...
the water activity
Activity (chemistry)
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...
and stabilizing the anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
mineralogy of UHT rocks have come from the finding of abundant pure CO2 fluid inclusions in these rocks (Tsunogae et al., 2008;. UHT metamorphism has been recently evaluated in the plate tectonic context using modern analogues and it has been suggested that both post-collisional extension and rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing play a crucial role. The abundant CO2 liberated by subsolidus
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
decarbonation along consuming plate boundaries was probably one of the factors that contributed to the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
thereby triggering the deglaciation of snowball Earth
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
. Based on an evaluation of the distribution of carbonated subcontinental mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
in global reconstructions of the Proterozoic supercontinent assembly, and their link with crustal domains that have undergone CO2-aided dry metamorphism at extreme conditions, Santosh and Omori (2008b) speculated that the UHT rocks might represent windows for the transfer of CO2 from the mantle into the mid crust and ultimately to the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
.
Further reading
- Santosh, M., Osanai, Y. and Tsunogae, T. 2004, Ultrahigh temperature metamorphism and deep crustal processes Journal of Mineralogical and Petrological Sciences v. 99 (part 1 & 2) , n. 4-5, 137-365
- Maruyama, S and Santosh, M. 2007, Island arcs: Past and present Gondwana Research, v. 11, no 1-2, p. 1-262.
- Brown, M. and White, R.W. 2008, Processes in granulite metamorphism Journal of Metamorphic Geology, v. 26, p. 125-299.
- Maruyama, S and Santosh, M. 2008, Models on Snowball Earth and Cambrian explosion Gondwana Research, Volume 14 (in press; published online)
- Sajeev, K. and Santosh, M. 2006, Extreme crustal metamorphism and related crust-mantle processes. Lithos v. 92 n. 3-4, p. 321-624.