RETROGRADE ECLOGITE
Eclogites are very high-grade, garnet-omphacite metamorphic rocks. They form at mantle depths by burial metamorphism of oceanic crustal rocks (basalts & gabbros). Uplift of eclogites back to the surface often involves retrograde metamorphism and the formation of new minerals. Shown below are retrograde eclogites from various localities.
Retrograde eclogite (above & below; above: 8.6 cm across; below: ~4.2 cm across) from Jenner, California. This rock 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).
The rock is dominated by four minerals: pyrope garnet (reddish brown rounded crystals), omphacite pyroxene (greenish), glaucophane (bluish), and muscovite mica (whitish patches). The original eclogite was dominated by the garnet & omphacite. After uplift from mantle depths, the eclogite underwent retrograde metamorphism. Some of the omphacite pyroxene was altered into glaucophane & muscovite. But the original eclogite’s mineralogy & overall texture is fairly well preserved.
Locality: 100 meter-sized eclogite body (“knocker”) in blueschist included in mid-Cretaceous Franciscan Complex serpentinitic melange; outcrop just north of Jenner, western margin of Sonoma County, western California, USA.
Retrograde eclogite (7.8 cm across at its widest) from the Belomorian Complex (Belomorian Eclogite Province) near Polarnie Zory, Shirokaya Salma District, Kola Peninsula, Baltic Shield/Fennoscandian Shield, far-northwestern Russia. The protolith rock was subjected to eclogite-facies metamorphism at 2.7 to 2.8 billion years ago, during the early Neoarchean. Uplift and decompression resulted in the formation of some new minerals, which occurred at 1.89 billion years (late Paleoproterozoic). Specimen owned by James Cheshire.
Retrograde eclogite (above & below; above: 6.0 cm across at its widest; below: 2.15 cm across) from near Tulle, Massif Central, Correze Department, France. This eclogite has been significantly altered by retrograde metamorphism during uplift from mantle depths. Note that the garnets have reactions rims (see photo below) and the omphacite pyroxene matrix around the garnets has been altered to other minerals. Eclogite-facies metamorphism occurred in the Late Silurian to Early Devonian, at about 400 to 420 million years. Published research indicates that uplift, exhumation, and retrograde metamorphism occurred during the Late Devonian, at about 360 to 380 million years.
Specimen owned by James Cheshire.
Retrograde eclogite (above & below; above: 9.6 cm across at its widest; below: 5.0 cm across) from near Zermatt, southern Valais Canton, southwestern Switzerland. This eclogite 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.
