Eclogite
A eclogite (of the Greek eklogê , choice) is a metamorphic Roche which was formed in the facies eclogitic, it is with-to say having undergone the conditions pressure, temperature of this field.
Historically, Escola defined the metamorphic facies on rocks resulting from protolithes basic ( rock before being metamorphized contained less than silica 53% and was rich in ferro-magnesian). Initially, the eclogite name was reserved has a basic rock of nature.
But, by abuse language, one employs the term of eclogite for the facies eclogitic and thus an eclogite can be acid (more than silica 53%) or basic.
Composition
A basic eclogite is made up of Omphacite and Grenat (mixture of Almandin, Grossulaire and Pyrope). Omphacite is a solid solution between the 2 following poles: the Jadeite, sodic orthopyroxene and it Diopside, clinopyroxene.
A acid eclogite is made up of Grenat, rather frequent Mica white (Phengite, Paragonite). It can have also Talc.
Chemin P (ression), T (empérature), T (emps)
Omphacite and garnet for basic eclogite, garnet, phengite and talc constitute stable mineral associations in conditions P, T given which one calls paragenesis .
However, one finds another mineral present on the basic eclogite which is the amphibole and which seemed to thus develop with the garnet-red interface omphacite/at the expense of these 2 phases. The presence of amphibole shows that once formed eclogite underwent a change of conditions P, T and was found in the field of amphibolites.
A metamorphic rock thus passes under various conditions P, T during its formation. During the hiding or way prograde, the conditions P, T increase. Then it reaches its metamorphic maximum (here, stage eclogite). Lastly, at the time of the exhumation, it follows a retrograde way where the conditions P, T decrease (here, presence of amphibole).
For this eclogite (see photo eclogites of the Vendée, opposite), garnet WP and the Cpx clinopyroxene are not more in balance, as the black crown around garnet testifies some. This black edge consists of hornblende and plagioclase, consequence of the destabilization of the Gt assembly + Cpx: reaction WP + Cpx + Q = Lorraine Area + Pl. Cpx consists of Cpx + Pl. This last results from the destabilization of omphacite, according to: Omphacite = Cpx + White feldspar.
These rocks can contain disthene in the form of small beaches dew, and of the rutile in the form of small grains bright red if it is thin or brown if it is thicker. With the contact (possible!) between magnesian amphibole and disthene, it can be formed a very rare mica: the Préisweirkite, of which there exist only 8 layers in the world.
The rocks are bad conductive of heat. Of this fact the geothermic and metamorphic gradient are not the same ones.
In general, the retrograde way is preserved better than the way prograde.
Time (for example the maximum of hiding) is estimated by radiochronology. One can use the couple / or the method Ar-Ar.
Geological significance
Eclogites are characteristic of a metamorphic gradient high pressure, low temperature. Their presence indicates a zone of subduction or a paléo-subduction.
Layers
One finds eclogites in the Italian Alps (Dora Maira), with the Japan, in China, of the eclogite relics in the Massif Central and the Armorican solid mass…
Eclogites meet in small masses or lenses inside other metamorphic rocks or in increase of Lave S.
An important eclogite career is located at “Gerbaudière”, Saint-Philbert-of-Large-Place, France.
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