Eclogite facies is determined by the temperatures and pressures required to metamorphose basaltic rocks to an eclogite assemblage. The typical eclogite mineral assemblage is garnet plus clinopyroxene. Eclogites record pressures over at about and usually over. This is high-pressure, medium- to high-temperature metamorphism. Diamond and coesite occur as trace constituents in some eclogites and record particularly high pressures. Such ultrahigh-pressure metamorphism has been defined as metamorphism within the eclogite facies but at pressures more than the quartz-coesite transition. Some UHP rocks appear to record burial at depths greater than if diamond occurs in these rocks. Eclogites containing lawsonite are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form during normal subduction of oceanic crust at depths between about. The rarity of lawsonite eclogites therefore does not reflect unusual formation conditions but unusual exhumation processes. Lawsonite eclogite is known from the U.S. ; Guatemala, Corsica, Australia, the Dominican Republic, Canada, and Turkey. Eclogite is the highest pressure metamorphic facies and is usually 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 crust. Eclogites are helpful in elucidating patterns and processes of plate tectonics because many represent the crustal rocks that were 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 or garnet, 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, 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. Xenoliths of eclogite occur in kimberlite pipes 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 greater than 45 km and rapid exhumation via thrust faults 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 directly 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 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. Some andesite-like rocks could be produced from partial melting of eclogite; for instance, an unusual rock type called adakite has been proposed to be a product of partial melting of eclogite in subducting oceanic crust. Likewise, partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts. Basalt is generally created as a partial melt of peridotite at 20–120 km 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 carried down into the mantle. If that subducted eclogite is subsequently carried upward with peridotite, as in a mantle plume, it may melt by decompression melting at lower temperature than the accompanying peridotite. Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted. The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.
Eclogite diamonds
Many diamonds from eclogite xenoliths have a 13C:12C 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 chromian diopside inclusions, minerals which are not typically found in eclogites.
Distribution
Eclogites occur with garnet peridotites in Greenland and in other ophiolite complexes. Examples are known in Saxony, Bavaria, Carinthia, Norway and Newfoundland. A few eclogites also occur in the northwest highlands of Scotland and the Massif Central of France. Glaucophane-eclogites occur in Italy and the Pennine Alps. Occurrences exist in western North America, including the southwest and the Franciscan Formation of the California Coast Ranges. Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Recently, coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya. The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.
