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.