ABSTRACT
Eclogites are high-pressure metamorphosed basalts, characteristically composed of about half red Ca-rich garnet and half green omphacite, an Na-Ca clinopyroxene intermediate between augite and jadeite. They form at temperatures and usually pressures higher than blueschists, but typically lower than those expected along a typical continental geotherm. Because of the unusual combination of relatively low temperature and high pressure, eclogites are typically found in fossil subduction zones where cold basaltic rocks were brought to great depths, then returned to the surface fast enough for the eclogite mineralogy to have been preserved. Eclogites can also be brought to the surface from the deep continental lithosphere as xenoliths in some alkali basalts and kimberlites. Typically, orogenic eclogites are found as boudins ranging from fist-size to hundreds of meters across. Eclogites can also occur as enormous masses or tectonic slabs that are kilometers across.
