Eclogite
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[[Image:100 5860.JPG|thumb|350px|right|Eclogite piece with a [[garnet]] (red) and [[omphacite]] (greyish-green) [[groundmass]]. The sky-blue crystals are [[kyanite]]. Some white [[quartz]] is seen too, it was probably once [[coesite]]. A few gold-white [[phengite]] [[mica]] minerals can be seen at the top. [[1 euro coins|Coin of 1 euro]] (2,3 [[centimetre|cm]]) for scale.]]
[[Image:Eclogite.jpg|200px|thumb|Eclogite]]
'''Eclogite''' ({{pronEng|ˈɛklədʒaɪt}}) is a coarse-grained [[mafic]] ([[basalt]]ic in composition) [[metamorphic rock]]. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the [[Crust (geology)|crust]] of the [[Earth]]. Second, being unusually dense rock, eclogite can play an important role in driving [[convection]] within the solid Earth.
The fresh [[Rock (geology)|rock]] can be striking in appearance, with red to pink [[garnet]] (almandine-pyrope) in a green matrix of [[sodium]]-rich [[pyroxene]] ([[omphacite]]). Accessory minerals include [[kyanite]], [[rutile]], [[quartz]], [[lawsonite]], [[coesite]], [[amphibole]], [[phengite]], [[paragonite]], [[zoisite]], [[dolomite]], [[corundum]], and, rarely, [[diamond]]. [[Plagioclase]] is not stable in eclogites. [[Glaucophane]] and [[titanite]] (sphene) form in eclogite as pressures decrease during exhumation of the rocks, or may be earlier formed minerals that did not entirely react away.
===Origins===
Eclogite typically results from high-pressure metamorphism of mafic [[igneous rock]] (typically [[basalt]] or [[gabbro]]) as it plunges into the [[Mantle (geology)|mantle]] in a [[subduction zone]]. Such eclogites are generally formed from precursor mineral assemblages typical of [[blueschist]] metamorphism. Eclogite can also form from [[magma]]s that crystallize and cool within the mantle or lower crust of continents.
==Eclogite facies==
Eclogite [[facies]] is determined by the temperature and pressure conditions required to metamorphose basaltic rocks to an eclogite assemblage.
<!-- Unsourced image removed: [[Image:Metamorphic Facies.JPG||left|thumb|300px|Eclogite facies rocks form at higher pressures than typically found in the crust.]] -->
The typical eclogite mineral assemblage is garnet (pyrope to almandine) plus clinopyroxene (omphacite).
Eclogites record pressures in excess of 1.2 GPa (45 km depth) at >400–1000 °C and usually in excess of 600-650 °C. This is extremely high pressure, medium to high temperature metamorphism. [[Diamond]] and [[coesite]] occur as trace constituents in some eclogites and record particularly high pressures. In fact, ultrahigh-pressure (UHP) metamorphism has been defined as metamorphism within the eclogite facies but at pressures greater than those of the quartz-coesite transition (the two minerals have the same composition -- silica). Some UHP rocks appear to record burial at depths greater than 150 km.
Eclogites containing [[lawsonite]] (a hydrous calcium-aluminium silicate) are very rarely exposed at the Earth's surface, although they are predicted from experiments to form during normal subduction of [[oceanic crust]] at depths between ~ 45-300 kilometers. The rarity of lawsonite eclogites therefore does not reflect unusual formation conditions but unusual exhumation processes. Examples of lawsonite eclogite are known from the U.S. (Franciscan Complex of California; xenoliths in Arizona); Guatemala (Motagua fault zone), Corsica, Australia, the Dominican Republic, Canada (British Columbia), and Turkey.
Eclogite is the highest pressure metamorphic facies and is usually only the result of advancement from [[blueschist]] metamorphic conditions.
{{Metamorphic Facies}}
==Importance of eclogite==
[[Image:Eclogite dlw.jpg|thumb|right|Photomicrograph of a thin section of eclogite from Turkey. Green omphacite (+ late chlorite) + pink garnet + blue glaucophane + colorless phengite.]]
Eclogite is a rare and important rock because it is formed only by conditions typically found in the [[mantle (geology)|mantle]] or the lowermost part of thickened continental crust.
Eclogites are helpful in elucidating patterns and processes of [[plate tectonics]] because many represent [[oceanic crust]] that has been subducted to depths in excess of 35 km and then returned to the surface.
Eclogite that is brought to shallow conditions is unstable, and retrograde metamorphism often occurs: secondary [[amphibole]] and [[plagioclase]] may form reaction rims on the primary pyroxene, and titanite may form rims about rutile. Eclogite may completely retrogress to amphibolite or granulite during exhumation. In some retrogressed eclogites and accompanying more silica-rich rocks, UHP (ultrahigh-pressure) metamorphism has been recognized only because of the preservation of coesite and/or diamond inclusions within trace minerals such as zircon and titanite.
[[Xenolith]]s of eclogite occur in the [[kimberlite]] pipes of the [[diamond]] mines of Africa, Russia, Canada, and elsewhere. Eclogites in granulite terranes are known from the Musgrave Block of central [[Australia]] where a continental collision took place at 550-530 Ma, resulting in burial of rocks to >45km (15 kilobars) and rapid (in less than 10 million years!) exhumation via [[thrust fault]]s prevented significant melting. Felsic rocks in these terranes contain [[sillimanite]], [[kyanite]], [[coesite]], [[orthoclase]] and [[pyroxene]], and are rare, peculiar rocks formed by an unusual tectonic event.
===Eclogite and basalt petrogenesis===
Peridotite is the dominant rock type of the upper mantle, not eclogite, as established by seismic and petrologic evidence. Likewise, peridotite is a much more important source rock of common magmas.
Melting of eclogite to produce basalt is generally not supported in modern petrology. Unreasonably high degrees of [[magma|partial melting]] are required to attain basaltic compositions. To get a basalt from melting an eclogite (ie; a rock with basalt composition) it has to undergo 100% partial melting. Instead, basalts can be modelled as having been produced by 1 to 25% partial melting of [[peridotite]], such as [[harzburgite]] and [[lherzolite]]. However, some andesite-like rocks could be produced from partial melting of eclogite; for instance, an unusual rock type called adakite (first described from Adak Island in the Aleutians) has been proposed to be a product of partial melting of eclogite. Likewise, partial melting of eclogite has been modeled to produce [[granodiorite]]-like granitic melts.
<!-- these are facts, 70.113.x.x, this is NOT in dispute. If you remove this, I will put it back. I do not get bored with this. I am more pedantic than you. For instance;
"The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.
Robert P. Rapp, Nobumichi Shimizu and Marc D. Norman, see link
There you go. You are wrong, I am right. Cheers, Rolinator -->
Basalt is generally created as a partial melt of peridotite at between 20-120km depth. Eclogite is more dense than the surrounding asthenosphere. Unless the eclogite is created in very young oceanic crust, it is cool at the time of initial subduction and so is usually carried down to great depths without melting. If that subducted eclogite is subsequently carried upwards during [[mantle convection]] together with peridotite, then it would melt by decompression melting (see discussion in [[igneous rock]]) at lower temperature than the accompanying peridotite. Eclogite-derived melts may therefore be part of the melt contribution derived from [[mantle plume]]s.
[http://www.nature.com/nature/journal/v425/n6958/abs/nature02031.html Eclogite melting creates granite; Nature '''425''', 605-609 (9 October 2003)]
===Eclogite diamonds===
Many diamonds from eclogite xenoliths have a <sup>13</sup>C:<sup>12</sup>C [[isotope]] ratio different from that typical of diamonds from [[peridotite]] xenoliths. The carbon isotopic differences between harzburgitic and eclogitic diamonds supports the hypothesis that those eclogite xenoliths formed from basalt carried down within subduction zones.
Eclogite diamonds are also typically higher in nitrogen, and will have a different suite of mineral inclusions than harzburgitic diamonds. Harzburgitic diamonds typically have titaniferous [[pyrope]], chromian [[spinel]] and Cr-[[diopside]] inclusions, minerals which are not typically found in eclogites.
==Distribution==
Eclogites occur with garnet [[peridotite]]s in [[Greenland]] and in other [[ophiolite]] complexes. Examples are known in [[Saxony]], [[Bavaria]], [[Carinthia (state)|Carinthia]], [[Norway]] and [[Newfoundland (island)|Newfoundland]]. A few eclogites also occur in the north-west highlands of [[Scotland]] and the Massif Central of [[France]]. Glaucophane-eclogites occur in [[Italy]] and the [[Pennine Alps]].
Transitional Granulite-Eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the [[Petermann Orogeny]], central Australia.
==References==
* Blatt, Harvey and Robert Tracy, 1995, ''Petrology: igneous, sedimentary, and metamorphic'', Freeman, ISBN 0-7167-2438-3
* Camacho, A., Hensen, B.J., Armstrong, R., ''Isotopic test of a thermally driven intraplate orogenic model, Australia', Geology, '''30''', pp. 887-890
* [http://www.ees.adelaide.edu.au/research/geology/cerg/projects/intracratonic_deformation/petermann.html The Petermann Orogeny, Central Australia]
* [http://www.nature.com/nature/journal/v425/n6958/abs/nature02031.html Rapp, Robert P., Shimizu Nobumichi, and Marc D. Norman. Growth of early continental crust by partial melting of eclogite. ''Nature'' '''425''', 605-609 (9 October 2003)]
==External links==
*[http://www.mantleplumes.org/Eclogite.html Mantle eclogites]
*[http://csmres.jmu.edu/geollab/Fichter/MetaRx/Rocks/Eclogite1.html Eclogite sample]
*[http://www.venuewest.com/8IKC/s2oral.htm Eclogite melting in production of Archaean TTG granitoids]
[[Category:Metamorphic rocks]]
[[Category:Petrology]]
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