Eclogite

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. Coin of 1 euro (23 mm) for scale.
Eclogite

Eclogite /ˈɛklədʒaɪt/ is a mafic (basaltic in composition) metamorphic rock. Eclogite forms at pressures greater than those typical of the crust of the Earth. An 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 eclogite.

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-facies or amphibolite-facies metamorphism. Eclogite can also form from magmas that crystallize and cool within the mantle or lower crust.

Eclogite facies

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 (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 high-pressure, medium- to high-temperature metamorphism. Diamond and coesite occur as trace constituents in some eclogites and record particularly high pressures. In fact, such 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 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 ~ 45-300 kilometers.[1] The rarity of lawsonite eclogites therefore does not reflect unusual formation conditions but unusual exhumation processes. Lawsonite eclogite is 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 the result of advancement from blueschist metamorphic conditions.

Importance of eclogite

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 or the lowermost part of thickened 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 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, 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 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 >45 km (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 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 (i.e.; 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. 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 in subducting oceanic crust. Likewise, partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts.[2]

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 (see discussion in igneous rock) 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.[3]

The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.[4]

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 Cr-diopside inclusions, minerals which are not typically found in eclogites.

Distribution

Eclogite from Almenning, Norway. The red-brown mineral is garnet, green omphacite and white quartz.

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[5] and the Franciscan Formation of the California Coast Ranges.[6] 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.

References

  1. ↑ Hacker, B.R., 2008, H2O subduction beyond arcs, Geochemistry, Geophysics, Geosystems, v. 9, doi:10.1029/2007GC001707.
  2. ↑ Robert P. Rapp, Shimizu Nobumichi and Marc D. Norman. Growth of early continental crust by partial melting of eclogite. Nature 425, 605-609 (9 October 2003)
  3. ↑ Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0.
  4. ↑ Sobolev, A. V., Hofmann, A. W., Sobolev, S. V. & Nikogosian, I. K. (2005). An olivine free mantle source of Hawaiian shield basalts. Nature 434, 590-593
  5. ↑ William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages ISBN 1-897799-85-3
  6. ↑ C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham
Notes

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