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| Eclogite from the Mariánské Lázně Complex in the west Czech Republic. Keele collection. Check out those gorgeous pink garnets! Photo courtesy of Ian Stimpson. |
A high-pressure, high-temperature, coarse-grained metamorphic rock consisting primarily of pink-red garnet (almadine-pyrope variety) and green pyroxene (omphacite, a sodium-rich variety). Eclogites may also contain small amounts of other high-pressure minerals such as kyanite, quartz, hornblende, and zoisite. Eclogites form when mafic rock (basalt or gabbro) descends deep within the Earth, generally at a subduction zone. Mafic rocks consist primarily of pyroxene and plagioclase (along with some amphibole and olivine). At high pressures and temperatures, the original minerals in mafic rock are squished into the more compact (denser) minerals garnet and omphacite, and the mafic rock becomes eclogite. Eclogites form when mafic rock encounters temperatures greater than ~400 degrees Celsius and pressures greater than ~12 kbar (or ~1.2 GPa). These temperatures and pressures mean that eclogites form at a minimum depth of ~40 km; some eclogites may form as deep as ~150 km. As a reference, ocean crust (which is comprised primarily of basalt and gabbro) is generally only 6-10 km thick. Because they are very dense and inclined to descend even deeper into Earth's mantle, eclogites are rarely brought to Earth's surface. Eclogites may be exposed in ophiolite sequences and other places where deep mantle rocks are brought to Earth's surface. Often, eclogites experience partial or full retrograde metamorphism as they are brought to Earth's surface. That is, if eclogites are brought to the surface slowly, their minerals may change back into minerals that are stable at lower temperatures and pressures. Sometimes, higher-pressure minerals will have rims of lower-pressure minerals around them.
