DEFINITION OF ECLOGITE

A dense, coarse-grained granular green rock consisting

of the minerals garnet (almandine-pyrope varieties)

and sodic pyroxene (omphacite). Minor amounts of quartz,

rutile, orthopyroxene, white mica, and kyanite may also be

present. It has the chemical composition of basalt and gabbro,

and many examples of eclogite may have formed by the

progressive high-grade metamorphism of mafic rocks.

Eclogite facies refers to a group of metamorphic mineral

assemblages where mafic rocks are represented by the assemblage

omphacite plus garnet. Mineral and phase equilibrium

studies show that this assemblage forms at high pressures

(more than 10 kilobars, or greater than about 30 kilometers),

and some eclogites are known to have formed at more than

30 kilobars pressure, equivalent to more than 62 miles (100

km) depth.

Eclogites are uncommon rocks at the surface because

they must be brought to the surface by some unusual process.

They are found as inclusion in kimberlites or basalts, bands

and lenses in high-grade gneiss terrains, as blocks associated

with blueschists, and along some fault zones.

See also METAMORPHISM.

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