Eclogite Views
Eclogite (pronounced /ˈɛklədʒaɪt/) is a coarse-grained mafic (basaltic in composition) metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth. Second, being unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.
The fresh rock can be striking in appearance, with red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamond. Plagioclase is not stable in eclogites. Glaucophane and titanite (sphene) form in eclogite as pressures decrease during exhumation of the rocks, or may be earlier formed minerals that did not entirely react away.
Eclogite typically results from high-pressure metamorphism of mafic igneous rock (typically basalt or gabbro) as it plunges into the mantle in a subduction zone. Such eclogites are generally formed from precursor mineral assemblages typical of blueschist metamorphism. Eclogite can also form from magmas that crystallize and cool within the mantle or lower crust of continents.
Eclogites record pressures in excess of 1.2 GPa (45e km depth) at dgt;400–1000 °C and usually in excess of 600-650 °C. This is extremely high pressure, medium to high temperature metamorphism. Diamond and 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 150u km.
Eclogite Images
