Retrograde eclogite from the Middle Paleozoic of France. (6.0 centimeters across at its widest)

Eclogite is an attractive, uncommon, crystalline-textured, very high-grade metamorphic rock. It is dominated by green & 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, but different mineralogy, as basalts & gabbros (= oceanic crustal rocks). Eclogites form by very high grade metamorphism of oceanic crust (basalts & gabbros) 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 French eclogite shown here has been significantly altered by retrograde metamorphism during uplift from mantle depths. Note that the garnets have reactions rims and the omphacite pyroxene matrix around the garnets has been altered to other minerals. Eclogite-facies metamorphism occurred in the Late Silurian to Early Devonian, at about 400 to 420 million years. Published research indicates that uplift, exhumation, and retrograde metamorphism occurred during the Late Devonian, at about 360 to 380 million years ago.