Eclogite from the Middle Paleozoic of Norway. (~6.05 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 seen here represents Late Silurian to Early Devonian (~400-423 million years old) very high-grade burial metamorphism (mantle depths) of a Proterozoic protolith. Metamorphism took place during the Scandian Orogeny (oceanic lithosphere adjacent to ancient Baltica was subducting beneath Laurentia - the ancient North American-Greenland continent). The eclogite was brought back to upper crustal levels during the Devonian phase of the Caledonian Orogeny (~383-404 Ma).