Eclogite

Eclogites are metamorphic rocks with basaltic chemistry ( metabasites ), which were formed through a metamorphosis at high (HP) or even ultra high ( UHP) pressure conditions while the temperature was relatively low. Eclogites along the suture line ( Geosutur ) between two formerly separate continents are evidence of the former presence of an ocean or sea basin with oceanic crust between two continental plates.

Eklogit has a density of 3.2-3.6 g / cm ³. He is therefore the densest of all silicate rocks that is open at the surface, very resistant and weather resistant. Because of its limited distribution it is still rarely used.

Chemical composition

Eclogites with basaltic chemical composition made ​​of green clinopyroxene ( omphazitreich: (Ca, Na) (Mg, Al) Si2O6 ) and red garnet ( pyropreich ). In addition, often quartz, kyanite, rutile, titanite and pyrite contain. Eclogites with granitic chemistry often contain aluminum-rich white mica ( phengite ). Characteristic for all types of eclogites is the lack of plagioclase ( feldspar ), which is reduced when the pressure increase after the reaction albite = Jadeit quartz. At extremely high pressures from about 27kbar ( ultrahigh -pressure metamorphism ) is also coesite containing high-pressure modification of quartz. From 27kbar/500 ° C rising to can 35kbar/700 ° C with corresponding chemical composition also diamond in eclogite be included. Coesite or diamond are usually found in the stable mineral phases such as garnet, kyanite or omphacite and after exhumation at the surface often only indirectly detectable.

Formation

Eclogites occur at high pressures from about 10 kbar (corresponding to about 35 km depth) and medium to high temperature (500 to 1000 degrees Celsius ), but this is only the case in regions with low geothermal gradient. Thus eclogites are often viewed as an indicator of Paläosubduktionszonen. Eklogitfazielle rocks can also occur at the base of a highly thickened continental crust, but such discoveries are rare. The uplift to the surface the eclogites can be overprinted by retrograde metamorphism, with minerals such as Plagioclase, amphibole, epidote and biotite form, but which may not be attributed to the actual eklogitfaziellen mineral paragenesis. Due to its high density, the oldest preserved eclogite findings apply in Fennoscandian Shield ( Kola Peninsula ) as evidence for the existence of subduction and plate tectonics in the modern sense before 2.87 billion years.

Occurrence

The oldest found so far Eklogitfragmente have a maximum age of 3.2 billion years that eclogites previously rare or has not occurred probably due to the then much hotter mantle. The largest Eklogitvorkommen in Central Europe are in the range of " Münchberger gneiss mass," the largest single occurrence, the white stone with regular stream. The parent rocks of the eclogites of Münchberger gneiss mass were undersea volcanic rocks, which in the Precambrian before about 570 formed Ma ( million years).

The Alpine eclogites in the central part of the Alps are represented with about 100 Ma as relatively young. For some parts of an older age is represented: eclogites of the central Ötztalkristallins and the Bohemian Massif are a Subduktionsvorgang before about 360 Ma ( latest Devonian) and thus dated from the beginning of the Variscan orogeny. The latter is with multiple stages of metamorphosis for the eclogites of the Kor in the Styrian mountain range represented ( Middle East Alpine ceiling floor, Koralm crystalline).

The youngest eclogites with an age of only about 4.3 Ma ( uncertainty ± 0.4) were discovered in 2004 in the eastern Papua New Guinea.

The vast majority of known eclogites are of terrestrial origin as in meteorite impacts, but indeed the correct pressures prevail too much heat, arise if at all only very small amounts of eclogite. Only since 2013 are known samples of extraterrestrial eclogite, which originate from a crashed meteorite must have been about as big as the moon or even larger.

Significance for the geology

The study of eclogites is useful for paleogeographic reconstructions. Eclogites, which have emerged from a mid-ocean - ridge basalt ( MORB ), which represent a piece of former oceanic crust, which was swallowed up in a Paläosubduktionszone and transported to great depths before it passed through exhumation again to the surface, here are usually only Eklogitlinsen in material (eg blue slate ) before having a lower density.

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