Eclogite

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Grinding of an eclogite

Eclogites are (according to the definition of the IUGS ) metamorphic rocks that are free of plagioclase and are composed of ≥75% omphacite and garnet . They originated from parent rocks (protoliths) with basaltic chemistry ( metabasites ), which were formed by metamorphosis under high (HP) or even ultra-high (UHP) pressure conditions at medium to high temperatures (over approx. 14 kbar and from approx. 500 ° C) (so-called eclogite facies ). Eclogites along the seam line ( geosutur ) between two formerly separate continents are evidence of the earlier existence of an ocean or sea basin with an oceanic crust between two continental plates. Eclogite has a density of 3.2-3.6 g / cm³. He is therefore the densest of all silicate rocks , which on the surface open-minded and is considered a main drive of plate tectonics since mesoarchean.

As a natural stone, despite its solid and weather-resistant properties, it is rarely used because it is very rarely exposed on the surface and some of the deposits are also under geotope protection.

Mineralogical composition

Eclogites consist of green clinopyroxene ( omphacite-rich : (Ca, Na) (Mg, Al) Si 2 O 6 ) and red garnet ( pyro-rich ). Quartz , thisthen , rutile , titanite , phengite and pyrite are also often included. Characteristic of all types of eclogites is - by definition - the lack of plagioclase ( feldspar ), which is broken down when the pressure increases after the reaction albite = jadeite + quartz. At extremely high pressures from approx. 27 kbar ( ultra- high pressure  metamorphosis ), Coesite , the high-pressure modification of quartz, is also contained. From 27 kbar / 500 ° C increasing to 35 kbar / 700 ° C, with the appropriate chemical composition, eclogite can also contain diamond . Coesite or diamond are mostly found in the stable mineral phases such as garnet, thistle or omphacite and are often only indirectly detectable after exhumation on the earth's surface.

Emergence

Eclogites arise at high pressures from approx. 10 kbar (corresponds to approx. 35 km depth) and medium to high temperatures (500 to 1000 degrees Celsius), but this is only the case in regions with low geothermal gradients . Thus, eclogites are often viewed as an indicator of paleosubduction zones . Eclogite facial rocks can also form at the base of a heavily thickened continental crust , but such finds are quite rare. When raised to the surface of the earth, the eclogites can be overprinted by retrograde metamorphosis, whereby minerals such as z. B. form plagioclase , amphibole , epidote and biotite , which, however, must not be assigned to the actual eclogite facial mineral paragenesis. Due to their high density, the oldest preserved eclogite finds in the Fennoscandian Shield ( Kola Peninsula ) are also considered evidence of the existence of subduction and plate tectonics in the current sense 2.87 billion years ago. Older eclogites are considered unlikely due to the higher geothermal gradient in the earlier Archean and Hadean times.

The oldest eclogite fragments found so far are no more than 3.2 billion years old. The fact that eclogites rarely or never formed before that is probably due to the then much hotter mantle of the earth.

Occurrence

The type of rock is located near Kupplerbrunn (municipality of Eberstein ) on the Carinthian Saualpe . The largest eclogite deposits in Central Europe are in the Münchberger Gneissasse area . The largest single occurrence is the Weißenstein near Stammbach . The starting rocks of the eclogites of the Münchberg gneiss mass were submarine volcanics that formed in the Precambrian about 570  Ma (million years).

The alpine eclogites in the central part of the Alps are represented as relatively young at around 100 Ma. A higher age is represented for some parts: eclogites of the central Ötztal crystal - and also of the Bohemian mass - are said to come from a subduction process before about 360 Ma (youngest Devonian ) and thus from the beginning of the Variscan orogeny . The latter is also represented with multiple stages of metamorphosis for the eclogites of the Koralpe in the Styrian foothills (Middle Eastern Alpine ceiling storey, Koralm crystalline).

In the Himalaya Mountains, very young eclogites were found in the northern part of the Kaghan Valley (Pakistan) and at Tso Morari in Ladakh ( India ). The dating showed an Eocene age around 47 million years before today. Since it can be assumed that the sinking of Indian crustal material under Eurasia, and the formation of eclogites, with the continent-continent collision took place, the approx. 47 million years are also to be regarded as the age of the northwestern Himalaya mountains. Mineral dating also allows the statement that the exhumation of the eclogites from a depth of approx. 140 km after their formation between 47 and 46 million years ago took place very quickly, down to crustal depths (approx. 40 km) and then much more slowly passed.

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

The vast majority of known eclogites are of terrestrial origin. Since the pressures are right but the heat is too great when a meteorite hits, only very small amounts of eclogite are produced here, if at all. Samples of an extraterrestrial eclogite have only been known since 2013, which come from a shattered meteorite that must have been about the size of the moon or even larger.

Retrograde metamorphosis

Eclogite in the microscope
Partially retrograde embossed eclogite from Förstenreuth (thin section, LPL): In the middle, relictically preserved omphacite, surrounded by symplectic adhesions of diopside and plagioclase. Garnet on the right at the edge of the picture (with high relief).
Eclogite partially retrograde embossed (thin section, XPL): The relics of Omphacit stand out due to their brightly colored interference colors. Next to it, quartz (gray), zoisite (blue), garnet (black).

Since the pressure and temperature conditions of the eclogite facies can only be reached at greater depths, longer periods of time may be required to bring the correspondingly metamorphosed rock back to the earth's surface. If the conditions of low-grade metamorphic facies areas are maintained for a longer period of time, mineral paragenesis can partly adapt to these conditions. Often the omphacite of the eclogite which was originally present breaks down into a mixture of pyroxene and plagioclase; as the retrograde metamorphosis progresses, the pyroxene can also be replaced by amphibole . The corresponding mixtures often form symplectic adhesions. Macroscopically, these changes are not necessarily particularly noticeable; In the microscopic picture, however, the conversion products immediately catch the eye.

Significance for geology

The examination of eclogites is helpful in paleogeographic reconstructions. Eclogites, which emerged from a mid-ocean ridge basalt (MORB), represent a piece of former oceanic crust that was swallowed in a paleosubduction zone and transported to great depths before it returned to the earth's surface through exhumation, mostly only eclogite lenses are found here in material (e.g. blue schist ) with a lower density.

literature

  • Werner Geigner, Brigitta Hella Keil: Geological-mineralogical hiking and excursion guide Eklogit. Conventus Musicus Verlag, Dettelbach 2002, OCLC 163309536 .

See also

Web links

Commons : Eklogit  - collection of images, videos and audio files

Individual evidence

  1. ↑ Douglas Fettes, Jacqueline Desmons (Ed.): Metamorphic Rocks. A Classification and Glossary of Terms . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-33618-5 , pp. 147 .
  2. ^ Gregor Markl: Minerals and Rocks . 2nd Edition. Spectrum, Heidelberg 2008, ISBN 978-3-8274-1804-3 , p. 104 .
  3. ^ Walter Schumann: The new BLV stone and mineral guide. Munich 1997, p. 320.
  4. ↑ lfu.bayern.de
  5. ↑ CS Kennedy, GC Kennedy: The equilibrium boundary between graphite and diamond. In: Journal of Geophysical Research. vol, 81, 1976, pp. 2467-2470.
  6. ↑ geology.gsapubs.org
  7. ↑ Harald Furnes, Minik Rosing et al: Isua supracrustal belt (Greenland) —A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology. In: Lithos. 113, 2009, pp. 115-132, doi: 10.1016 / j.lithos.2009.03.043 .
  8. ↑ Steven B. Shirey, Stephen H. Richardson: Start of the Wilson Cycle at 3 Ga Shown by Diamonds from Subcontinental Mantle. In: Science. 333 (6041), July 22, 2011, pp. 434-436. doi: 10.1126 / science.1206275
  9. ↑ Friedhelm Thiedig: Eklogit - An interesting Carinthian rock - history of its discovery, distribution and formation . In: Carinthia II . tape 200 , 2010, p. 7–48 ( PDF on ZOBODAT [accessed December 18, 2019]).
  10. ^ Hans Georg Krenmayr, Albert Daurer (editor): Rocky Austria. A colorful geological history of Austria. Federal Geological Institute, Vienna 1999, ISBN 3-85316-006-9 , p. 36.
  11. ↑ Helmut W. Flügel, F. Neubauer: Geology of the Austrian federal states in brief individual representations. Styria. Explanations of the geological map of Styria 1: 200,000. Federal Geological Institute, series of federal states . Vienna 1984, p. 70.
  12. ^ Franziska DH Wilke, Patrick J. O'Brien, Axel Gerdes, Martin J. Timmerman, Masafumi Sudo: The multistage exhumation history of the Kaghan Valley UH P series, NW Himalaya, Pakistan from U-Pb and 40 Ar / 39 Ar ages . In: European Journal of Mineralogy . tape 22 , no. 5 , October 1, 2010, ISSN  0935-1221 , p. 703–719 , doi : 10.1127 / 0935-1221 / 2010 / 0022-2051 ( ingentaconnect.com [accessed September 3, 2018]).
  13. ^ SL Baldwin, BD Monteleone, LE Webb, PG Fitzgerald, M. Grove, E. June Hill: Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. In: Nature. 431, 2004, pp. 263-267.
  14. ↑ M. Kimura, N. Sugiura, T. Mikouchi, T. Hirajima, H. Hiyagon, Y. Takehana: Eclogitic clasts with omphacite and pyrope-rich garnet in the NWA 801 CR2 chondrite. In: American Mineralogist. 98 (2-3), 2013, pp. 387-393. doi: 10.2138 / am.2013.4192
  15. ↑ Roland Vinx: Rock determination in the area . 3. Edition. Spectrum, Heidelberg 2011, ISBN 978-3-8274-2748-9 , p. 412-415 .