- 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 of theEarth . Second, being unusually dense rock, eclogite can play an important role in drivingconvection within the solid Earth.The fresh rock can be striking in appearance, with red to pink
garnet (almandine-pyrope) in a green matrix ofsodium -richpyroxene (omphacite ). Accessory minerals includekyanite ,rutile ,quartz ,lawsonite ,coesite ,amphibole ,phengite ,paragonite ,zoisite ,dolomite ,corundum , and, rarely,diamond .Plagioclase is not stable in eclogites.Glaucophane andtitanite (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 (typicallybasalt orgabbro ) as it plunges into the mantle in asubduction zone . Such eclogites are generally formed from precursor mineral assemblages typical ofblueschist metamorphism. Eclogite can also form frommagma 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. 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 andcoesite 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 ofoceanic 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.Importance of eclogite
Eclogite is a rare and important rock because it is formed only by conditions typically found in the mantle or the lowermost part of thickened continental crust.
Eclogites are helpful in elucidating patterns and processes of
plate tectonics because many representoceanic 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 andplagioclase 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 thekimberlite pipes of thediamond mines of Africa, Russia, Canada, and elsewhere. Eclogites in granulite terranes are known from the Musgrave Block of centralAustralia 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 viathrust fault s prevented significant melting. Felsic rocks in these terranes containsillimanite ,kyanite ,coesite ,orthoclase andpyroxene , 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 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 asharzburgite andlherzolite . 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 producegranodiorite -like granitic melts.Basalt is generally created as a partial melt of peridotite at between 20-120km depth. Eclogite is denser 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 duringmantle convection together with peridotite, then it would melt by decompression melting (see discussion inigneous rock ) at lower temperature than the accompanying peridotite. Eclogite-derived melts may therefore be part of the melt contribution derived frommantle 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 13C:12C
isotope ratio different from that typical of diamonds fromperidotite 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 , chromianspinel and Cr-diopside inclusions, minerals which are not typically found in eclogites.Distribution
Eclogites occur with garnet
peridotite s inGreenland and in otherophiolite complexes. Examples are known inSaxony ,Bavaria , Carinthia,Norway and Newfoundland. A few eclogites also occur in the north-west highlands ofScotland and the Massif Central ofFrance . Glaucophane-eclogites occur inItaly and thePennine Alps .Transitional Granulite-Eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of thePetermann 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]
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