Eclogite

    Eclogite (pronounced /ˈɛklədʒaɪt/) is a coarse-grained mafic (basaltic 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 the Earth. Second, being unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

The fresh 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.

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Origins

Eclogite typically results from high-pressure metamorphism of mafic igneous rock (typically basalt or gabbro) as it plunges into the mantle in a subduction zone. Such eclogites are generally formed from precursor mineral assemblages typical of blueschist metamorphism. Eclogite can also form from magmas 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 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.

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 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.

Xenoliths 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 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 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 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.

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 plumes.

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 from peridotite xenoliths. The carbon isotopic differences between harzburgitic and eclogitic diaomonds 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 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 north-west highlands of Scotland. 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
• The Petermann Orogeny, Central Australia
• 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)