Eclogite from the South Carolina, USA. (5.7 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 eclogite specimen shown above is from the Newberry Eclogite of South Carolina, USA. The original oceanic crustal rock was metamorphosed sometime during the Neoproterozoic to Ordovician. The Newberry Eclogite is part of the Silverstreet Subterrane of the Charlotte Belt (western Carolina Terrane) in the Southern Appalacian Piedmont. Newberry eclogite rocks occur as boudins in gneiss. They have been altered from the ideal eclogite composition of pyrope garnet + omphacite pyroxene. This rock consists of reddish garnet, greenish diopside clinopyroxene (replacing omphacite), and whitish albite plagioclase feldspar.

Geologic unit & age: Newberry Eclogite; metamorphic age - Neoproterozoic to Ordovician