Eclogite

An eclogite is a high-pressure metamorphic rock that commonly derives from basalt or gabbro under subduction-zone conditions. Its defining mineral assemblage is garnet and omphacite, a sodium-rich clinopyroxene. Accessory minerals such as rutile, ilmenite, quartz, and locally coesite or stishovite may occur, but plagioclase is typically absent or scarce in the rock.

In hand specimen, eclogites are dense and usually feature red to pink garnet grains intergrown with green to dark pyroxene (omphacite). The texture is often granuloblastic, with garnet and pyroxene forming a coarse intergrowth. The mineral composition can vary, with garnet commonly pyrope- and almandine-rich, and omphacite containing jadeite components.

Formation and pressure–temperature conditions: eclogite forms at high pressures and relatively high temperatures associated with subduction zones, representing the metamorphism of oceanic crust that has descended through the mantle. Typical P–T conditions are on the order of 1–2 GPa and several hundred to around 1000°C, though exact values depend on protolith and uplift history. Some eclogites preserve evidence of ultra-high-pressure metamorphism, indicated by coesite or other diagnostic minerals.

Occurrence and significance: eclogites occur in subduction-related belts, ophiolites, and high-pressure metamorphic terranes worldwide. They are important because they record the deep recycling of oceanic crust into the mantle and help constrain subduction-zone processes and plate tectonics. Subtypes include jadeite-bearing varieties and other assemblages that reflect variations in chemical composition and P–T pathways.