File:Eclogite (late Neoproterozoic metamorphism; Borborema Province, Pernambuco State, eastern Brazil) 1 (15098058265).jpg

Eclogite from the Neoproterozoic of Brazil. (wet, cut surface; 6.5 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 shown above has pyrope garnet (brownish-red) and jadeitic omphacite pyroxene (greenish). It formed by ultrahigh-pressure metamorphism during the Pan-African Orogeny (late Neoproterozoic), during which the ancient continent of Gondwana formed. Gondwana was a small supercontinent that principally consisted of the modern landmasses of South America, Africa, Arabia, Madagascar, India, Antarctica, and Australia.