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.
