Retrograde eclogite from the Eocene of Switzerland. (field of view 5.0 centimeters across)

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 Swiss eclogite shown here is part of the Zermatt-Saas Ophiolite, exposed in a collisional nappe in the Piedmont Zone of the Swiss-Italian Alps. Eclogite-grade metamorphism occurred during the Eocene, at about 40 to 50 million years. The original greenish omphacite pyroxene has been retrograde metamorphosed into a bluish-gray mineral, possibly glaucophane.