File:Eclogite (Franciscan Complex, Jurassic; Valley Ford, California, USA) (28009322418).jpg

Eclogite from the Jurassic of California, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

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 California eclogite shown here represents Middle Jurassic-aged oceanic crust (basalt/gabbro) that was subducted & cooked underneath North American continental lithosphere during the Late Jurassic (~155-158 m.y. metamorphic date). Many California eclogites have been retrograde metamorphosed and have a significant component of bluish-colored glaucophane.

Geologic unit & age: Franciscan Complex, Middle Jurassic oceanic crust metamorphosed in the Late Jurassic, 155-158 Ma