Eclogite

Origins

Eclogite typically results from high-pressure metamorphism of mafic 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. 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 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 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 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 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 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 diopside inclusions, minerals which are not typically found in eclogites.

Distribution

Eclogites occur with garnet 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)