Eclogite

Eclogite is a coarse-grained, mafic-to-ultramafic grouping of metamorphic rocks of special interest due to the variety of minerals they contain and their microscopic structures and geological relationships. The fresh rock can be very striking in appearance, with red to pink garnets, almandine-pyrope in a green matrix of sodium-rich pyroxene, and pale green or nearly colorless augite (omphacite). Accessory minerals are kyanite, rutile, occasional quartz, hornblende, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamonds. Feldspar is rare; if present, belongs to basic varieties rich in lime. Other minerals which have been found in eclogites are bronzite, olivine and glaucophane.

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Eclogite typically results from medium temperature and high-pressure metamorphism of mafic igneous rocks (typically basalt) as it plunges into the mantle in a subduction zone. Under mantle conditions generally greater than 45 km depth at 400–1000 °C and > 12 kbar, the basaltic mineralogy is transformed into the eclogite facies assemblage. High pressure metamorphism of lower temperature (less than 400 °C) is in the blueschist facies.

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Some consider eclogite to be representative of the upper mantle and see tholeiitic basalt volcanism typical of eclogite melts. Eclogite that is brought to shallow conditions is quite unstable, and retrograde metamorphism often occurs with secondary amphibole and plagioclase forming reaction rims on the primary pyroxene minerals. Xenoliths of eclogite reportedly occur in the kimberlite pipes of the diamond mines of South Africa. Diamonds from eclogite xenoliths have a 13C:12C isotope ratio different from peridotite associated diamonds and indicate a possible biogenic component. This is compatible with a subduction zone eclogite origin. Eclogites also occur with garnet peridotites in Greenland and in other ophiolite complexes.

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The eclogites in their chemical composition show close affinities to gabbros. Examples are known in Saxony, Bavaria, Carinthia and Norway. A few eclogites also occur in the north-west highlands of Scotland. Glaucophane-eciogites occur in Italy and the Pennine Alps.

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Mafic: In geology, mafic minerals and rocks are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. The term is a combination of "magnesium" and ferrum, the Latin word for iron http://dictionary.reference.com/search?q=mafic....

Metamorphic rock: Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form" (from the Greek words meta, "change", and morphe, "form"). The protolith is subjected to extreme heat (greater than 150 degrees Celsius) and...

Mineral: This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band)....