Eclogite

 

Eclogite, Nordfjord. Red-brown garnet, green omphacite, bluish kyanite.

Kyanitic eclogite (Late Cretaceous, 90-95 Ma , near Fost , Slovenska Bristica Municipality , Slovenia)

~8.0 centimeters across at its widest

Eclogite is an attractive, scarce, crystalline-textured, very high-grade metamorphic rock. It is dominated by green and red minerals. The red is pyrope or almandine garnet. The green is omphacite pyroxene. Eclogite appears to be moderately common in portions of the upper mantle, but it occurs in very few places at the Earth’s surface. They have the same chemistry as basalts and gabbros (= oceanic crustal rocks), but have different mineralogies. Eclogites form by very high grade metamorphism of oceanic crust at mantle depths along subduction zones. Uplift of eclogites back to the surface often involves some retrograde metamorphism and the formation of new minerals, resulting in retrograde eclogites.

The eclogite seen here is from the Eastern Alps in Slovenia. It has a kyanite component. Kyanite is a bluish to bluish-gray aluminum silicate mineral - Al2SiO5.

Eclogite boudin within gneiss.

 Eclogite , Thin section . High pressure rock exhibiting large garnet phenocrysts and the pyroxene omphacite (green in plane polarized view). Some low relief quartz are also present.

Eclogite is a metamorphic rock; is formed by metamorphism of gabbro or basalt. This sample comes from Cabo Ortegal, La Coruna, Galicia, Spain.

Besides its aesthetic beauty and awesome vibrant colors, eclogite is a very rare and important rock. Eclogites are mafic metamorphic rocks that are formed in subduction zone settings. They experience metamorphism at extremely high pressures (1.2 GPa), greater than those typical of the Earth’s crust, as well as medium to high temperatures.

They are responsible for slab pull, the pulling down and subduction of a tectonic plate. The reddish/brown colored minerals are garnet and the greenish minerals are omphacite, a Na-rich pyroxene.

Subduction and accumulation of lawsonite eclogite and garnet blueschist in eastern Australia.

Shallowly-inclined surface showing leucosome dike with enclaves of retrograded UHP eclogite in lower half and retrograded migmatitic eclogite with quartz veins in upper half. Note quartz along the top edge of the leucogranite dike. These features suggest evolution from supercritical fluid to hydrous melt and aqueous fluid during exhumation across the critical curve in the granite-H₂O system.