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 crust. Eclogites are helpful in elucidating patterns and processes of plate tectonics because many represent the crustal rocks that were 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, ultrahigh-pressure (UHP) 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 greater than 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 typically result from high to ultrahigh pressure metamorphism of mafic rocks at low thermal gradients of <10 ℃/km as they were subducted to the lower crust to upper mantle depths in a subduction zone. They are generally formed from precursor mineral assemblages typical of blueschist-facies metamorphism.

Eclogite is commonly bright red and green, which make it a popular rock option in construction, particularly for countertops and other decorative elements. Unsurprisingly, it is a popular choice for jewelry, especially because eclogite often contains large garnets. It has also been used in cosmetics.

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 and the Franciscan Formation of the California Coast Ranges. 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. The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.

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 over 1.2 GPa (170,000 psi) (45 km (28 mi) depth) at about 400 to 1,000 °C (752 to 1,832 °F) and usually over 600–650 °C (1,112–1,202 °F). This is high-pressure, medium- to high-temperature metamorphism. Diamond and coesite occur as trace constituents in some eclogites and record particularly high pressures. Such ultrahigh-pressure (UHP) metamorphism has been defined as metamorphism within the eclogite facies but at pressures more than the quartz-coesite transition (the two minerals have the same composition—silica). Some UHP rocks appear to record burial at depths greater than 120 km (75 mi) if diamond occurs in these rocks. 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 about 45–300 km (28–186 mi). 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.

Eclogite is believed to support spiritual growth, clearing the heart and solar plexus chakras. It is said to balance the emotions, relieving the user from negative feelings of guilt, fear and anger and replacing these with positive energy. Some say meditating with this stone will help to increase self-esteem and restore confidence and motivation.