ECLOGITE

 

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 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.  Shown below are eclogites from various localities.

 

Eclogite (9.2 cm across) with almandine garnet and omphacite pyroxene, from an unrecorded locality.

 


 

Eclogites (above & below) (above: 6.1 cm across) from the Nordfjord area (Sogn og Fjordane County) of southwestern Norway.  These rocks represents Late Silurian to Early Devonian (~400-423 million years) 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 eclogites were brought back to upper crustal levels during the Devonian portions of the Caledonian Orogeny (~383-404 million years).

 

Eclogite (6.0 cm across at its widest) from the Nordfjord area, southwestern Norway.

 

Eclogite (7.1 cm across at its widest) from the Nordfjord area, southwestern Norway.

 

Eclogite (4.8 cm across at its widest) from the Nordfjord area, southwestern Norway.

 


 

Eclogite (above & below; above: 7.4 cm across at its base; below: 9.0 cm across at its widest) from the Münchberg (Muenchberg) Gneiss Massif in Bavaria, Germany.  These rocks formed by deep burial metamorphism of near-earliest Ordovician mafic igneous rock during the Devonian Hercynian (Variscan) Orogeny.

Protolith age: early Early Ordovician, ~480 m.y.

Metamorphic age: late Late Devonian, ~365 m.y.

Locality: Weissenstein (Weißenstein), a hill just south of the town of Stammbach, Frankenwald Forest, Bavaria, southeast-central Germany (~50° 07’ 48” North, ~11° 41’ 27” East).  The host rocks for eclogite at this locality include amphibolite, gneiss, and marble.

Specimens owned by James Cheshire.

 

Eclogite (field of view ~3.5 cm across) - close-up of 1st Münchberg eclogite shown above.

 

Eclogite (field of view ~2.6 cm across) - close-up of 2nd Münchberg eclogite shown above.

 


 

Eclogite (5.7 cm across) from the Newberry Eclogite near Newberry, northwest-central South Carolina, USA.  The original oceanic crustal rock was metamorphosed sometime during the Neoproterozoic to Ordovician.  The Newberry Eclogite is part of the Silverstreet Subterrane of the Charlotte Belt (western Carolina Terrane) in the Southern Appalacian Piedmont.  Newberry eclogite rocks occur as boudins in gneiss.  They have been altered from the ideal eclogite composition of pyrope garnet + omphacite pyroxene.  This rock consists of reddish garnet, greenish diopside clinopyroxene (replacing omphacite), and whitish albite plagioclase feldspar.

 


 

Eclogite (above & below) (above: 6.5 cm across at its widest) from Borborema Province, Pernambuco State, northeastern margin of the São Francisco Craton, eastern Brazil.  These eclogites have pyrope garnet (brownish-red) and jadeitic omphacite pyroxene (greenish).  They formed by ultrahigh-pressure metamorphism during the Pan-African Orogeny (late Neoproterozoic), during which the ancient continent Gondwana formed.  Specimen owned by James Cheshire.

 

Eclogite (1.6 cm across; same specimen as above) from Pernambuco State, eastern Brazil.

 

Eclogite (2.6 cm across; same specimen as above) from Pernambuco State, eastern Brazil.

 

Eclogite (5.8 cm across the base) from Pernambuco State, eastern Brazil.

 


 

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