eclogite

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eclogite Eclogite is a distinctive, dense, green rock, composed principally of pyroxene and garnet and with no plagioclase. It has the bulk chemical composition of basalt or gabbro, and in some instances textural evidence shows it to have formed from progressive metamorphism of such rocks. Eclogite can also be produced by the primary crystallization of basaltic magmas at upper mantle pressures, but it appears that most eclogites are a product of metamorphism at similarly high pressures. Under these conditions, plagioclase is unstable: the notional albite component is present in eclogite as jadeite in solid solution in the pyroxene (which is omphacite), and as minor quartz, while the notional anorthite component occurs primarily as grossular in garnet, but sometimes also as zoisite. Other common minor constituents of eclogite include kyanite, orthopyroxene, rutile, amphibole, pyrite, and white mica.

There are three main settings in which eclogites are found: as xenoliths in kimberlite or basalt (as at Oahu crater, Hawaii), as bands or lenses in high-grade gneiss terranes (as in west Norway or the Dabie Mountains of central China), and as bands or isolated blocks associated with blue-schists (for example in the Cyclades). Mineral chemistry indicates significant temperature differences between these modes of occurrence, with xenoliths representing the highest temperatures and bodies in blueschist terranes the lowest.

The origin of eclogites, especially those from gneiss terranes, has been controversial for many years. While some geologists have favoured a primary origin by crystallization of basalt magmas in the upper mantle, others have argued that these eclogites originate at sub-solidus temperatures by metamorphism of basalt or gabbro. Although eclogites in west Norway and other gneiss terranes were seen for many years as tectonically emplaced slices of mantle, modern work has largely moved away from this view. Some eclogite bodies retain in part the textures of precursors formed at lower pressure; for example, eclogite assemblages may develop only locally in shear zones, because it appears that deformation or ingress of water, or both, is needed to catalyse the reactions that convert dry granulite or gabbro to eclogite. Another argument for tectonic emplacement from the mantle was that the host gneisses themselves recorded no comparable history of high-pressure metamorphism. However, in 1982 C. A. Heinrich showed that, whereas retrogression of eclogite requires influx of water to proceed, high-pressure gneiss assemblages undergo spontaneous and pervasive dehydration as they are uplifted, and are thus much less likely to retain any high-pressure relicts by the time they reach the surface. Nevertheless, some evidence of high-pressure metamorphism has been reported from plagioclase-bearing gneisses associated with eclogists. For a long time, battle was joined over the issue of the pressure (and hence depth) required for eclogites to form, with the ‘crustal eclogite lobby’ consistently claiming lower formation pressures than the ‘mantle school’. This dispute was finally resolved by the discovery in 1984 by D. C. Smith of tiny inclusions of coesite in pyroxene from a Norwegian eclogite. Coesite has subsequently been reported from a number of eclogite terranes worldwide, although it is more normally hosted by garnet. As a result, a distinct group of ultra-high-pressure eclogites is now recognized, formed at pressures of around 30 kbar or higher, and corresponding to depths of burial of at least 100 km. Interestingly, many of these eclogites appear to have originated in the crust, rather than in the mantle.

Some occurrences of eclogite in blueschist terranes are closely associated with blueschists of broadly similar metabasite compositions, and several suggestions have been made as to the origin of the association. In the Cyclades, eclogite appears to correspond to relatively massive gabbro bodies, while metavolcanic rocks have blueschist assemblages. Restricted access of water to the gabbros may thus have favoured production of the more anhydrous eclogite assemblage, but there are chemical differences in Mg/Fe and silica content that may also control the assemblages.

Bruce W. D. Yardley

Bibliography

Carswell, D. A. (ed.) (1990) Eclogite facies rocks. Blackie, Glasgow.

eclogite Very rare, coarse-grained, igneous rock with a chemical composition similar to that of basalt, but noted for the presence of the rare, bright-green pyroxene omphacite, and red, almandine—pyrope garnets. Eclogites may be basalt metamorphosed by high temperatures and pressures.