{"id":10413,"date":"2023-04-24T12:10:17","date_gmt":"2023-04-24T10:10:17","guid":{"rendered":"https:\/\/ccgm.org\/?page_id=10413"},"modified":"2023-06-26T12:49:06","modified_gmt":"2023-06-26T10:49:06","slug":"eclogites-roches-de-choix","status":"publish","type":"page","link":"https:\/\/ccgm.org\/en\/eclogites-roches-de-choix\/","title":{"rendered":"Eclogites - rocks of choice"},"content":{"rendered":"
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A temporary exhibition at the Geological Society of France prepared by : Camille FRAN\u00c7OIS, Solen LE GARDIEN, Violaine SAUTTER & David C. SMITH.<\/p>

With the support and assistance of Mary Inglis, A. Rosu & S. Chaimbault<\/p>

Download the brochure here<\/a>.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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ABOUT THE EXHIBITION<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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In collaboration with the Mus\u00e9um National d'Histoire Naturelle and the Commission de la Carte G\u00e9ologique du Monde, this exhibition allows visitors to discover eclogites, rocks formed in the depths of the Earth, through works from the SGF's documentary collection and, above all, samples from the COSEM: the Commission for the Geological Map of the World. Co<\/strong>llection S<\/strong>mith des E<\/strong>clogites M<\/strong>wave.<\/p>

Why the term \"eclogite\" and why Norway? <\/strong><\/p>

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Abb\u00e9 Ren\u00e9-Just Ha\u00fcy coined the term \"eclogite\" from the Greek \"\u1f10\u03ba\u03bb\u03bf\u03b3\u03ae\" (eklog\u00ea) meaning \"choice\". He believed that these rocks \"chose\" to be different from others, and described them in his Trait\u00e9 de min\u00e9ralogie of1822.<\/p>

In 1920, Pentti Eskola (1883-1964) published a classification of metamorphic rocks based on the innovative concept of \"facies\", including a description of what he defined as an \"eclogitic facies\". The following year, he published a work on the eclogites of Norway in \"Le Gneiss de l'Ouest\", an enormous unit of more than 200 km of nappes that have been metamorphosed.<\/p>

At the same time, Yvonne Bri\u00e8re (1891-1981), one of the first female geologists in France, proposed a metamorphic origin for eclogites in her thesis \"Les \u00e9clogites fran\u00e7aises: leur composition min\u00e9ralogique et chimique; leur origine\", defended at the Facult\u00e9 des Sciences de Paris and published in 1920. This idea, which was strongly criticised by his thesis jury, is now unanimously accepted.<\/p>

Since the 1960s, with the advent of new analytical methods in petrology (electron microprobe, etc.), many other researchers have taken an interest in eclogites to understand how they can outcrop on the Earth's surface after having been created at such great depths.<\/p>

A paradigm shift in geodynamics: subduction of continental crust<\/strong><\/p>

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In 1984, the discovery of the mineral coesite in rocks of the continental crust in Italy (Chopin, 1984) and in eclogites in Norway (Smith, 1984) revolutionised geodynamics. The presence of coesite in eclogites proves burial at depths two to three times greater than previously assumed (around 100 km). It also shows that the continental crust can subduct to great depths.<\/p>

A new discipline was born, called UHPM (Ultra-High Pressure Metamorphism).<\/p>

This exhibition begins with a definition of eclogites and metamorphism (showcases 1 to 3) and the geodynamic processes involved in the formation of these rocks (showcases 4 to 7). It continues with the highly diverse mineralogy of Norwegian eclogites (display cases 8 to 14). The study of COSEM rocks has enabled David and his colleagues to describe new species: nyb\u00f6ite, Mg-Al & Fe-Al-taramite, lisetite and davidsmithite, all recognised by the International Mineralogical Association (showcases 15 & 16). The exhibition concludes with the geodynamic implications of the discovery of these rocks throughout the history of our Earth (showcase 17).<\/p>

This exhibition will take you on a journey from the infinitely large to the infinitely small. The study of eclogites is a vast field, ranging from regional geodynamics to crystallochemistry.<\/p>

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1. DEFINITION OF ECLOGITE & HISTORY<\/a><\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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Eclogite is a grey rock composed of red garnet and green pyroxene (omphacite, Greek for green grapes). In 1822, Abb\u00e9 Ren\u00e9-Just Ha\u00fcy was fascinated by this rock (sample 505<\/b> showcase) with bright colours (no feldspar), made it a rock of choice by naming it \"eclogite\" (from the Greek \u1f10\u03ba\u03bb\u03bf\u03b3\u03ae meaning \"choice\", referring to the selection of minerals that make up this rock). For Ha\u00fcy, diallage (the old name for clinopyroxene) was \"\u00a0considered to be a basic function and, with garnet, formed a binary combination to which kyanite, quartz, epidote and laminar amphibole were supposed to be accidentally added\". <\/em>Before Ha\u00fcy, these rocks had been spotted 40 years earlier in Bavaria by Horace B\u00e9n\u00e9dict de Saussure.<\/p>

The great German geognosist Abraham Gottlob Werner was also familiar with some eclogites from the Austrian Alps and southern Germany, in particular the well-known Silberbach deposit (sample 25 Ec<\/b>). He described them as consisting of garnet, \"omphazit\" and, occasionally, \"cyanit\" (Werner, 1817).<\/p>

The name eclogite was used by European geologists to designate rocks from Germany, Austria, the Alps and Norway. In France, the first discovery of eclogite was made by Auguste Rivi\u00e8re in the Vend\u00e9e region (sample P1567<\/b>), where he discovered \"the beautiful rock that bears the name of eclogite\" (Rivi\u00e8re, 1835). The first study of these rocks was carried out by Alfred Lacroix in 1891, then by his student Yvonne Bri\u00e8re in 1920 (see<\/i> window 2).<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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This thin blade on display is a kimberlite eclogite <\/strong>composed mainly of red garnet and green pyroxene (Roberts Victor mine, South Africa). A kimberlite (from the town of Kimberley in South Africa) is a very brecciated ultrabasic magmatic rock deeply rooted in the mantle and rising to the surface at Mac 2 speeds, with nodules of eclogite, peridotite and diamonds.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t

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Books on display:<\/u><\/strong><\/p>

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Hommage \u00e0 Ha\u00fcy: Soci\u00e9t\u00e9 fran\u00e7aise de min\u00e9ralogie (1945). - Ren\u00e9-Just Ha\u00fcy, 1743-1822, Paris, Masson, 348 p.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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\n\t\t\t\t\t\t\t\t\t\t\t\t\t\"Ren\u00e9-Just\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t
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Ren\u00e9-Just Ha\u00fcy<\/strong> (1743-1822) was a French mineralogist and the founder of crystallography (with Jean-Baptiste Rom\u00e9 de Lisle). Destined for the priesthood, he discovered botany and then mineralogy, and in 1781 published an article on the structure of garnet crystals. Admitted to the Acad\u00e9mie des Sciences in 1783, then to the Mus\u00e9um National d'Histoire Naturelle, he described and named a large number of mineral species in his A treatise on mineralogy<\/em>published in 5 volumes (including one with plates) in 1801. It was in the second edition of this Treaty<\/em>published in 1822, that he described a rock he called \"eclogite\", in a chapter on diallage (clinopyroxene): \"I gave this rock the name of eclogite, which means choice, election, because its components, not being of those which commonly exist several together in the primitive rocks, like feldspar, mica, amphibole, seem to have been chosen to form a separate band\". (Trait\u00e9 de Min\u00e9ralogie, 2nd edition, Vol. II, p. 456). The sample H505<\/strong> on display is a Bavarian eclogite studied by Ha\u00fcy.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Saussure, H.-B. de (1779-96). - Voyages dans les Alpes, pr\u00e9c\u00e9d\u00e9s d'un essai sur l'histoire naturelle des environs de Gen\u00e8ve, Neuch\u00e2tel, S. Fauche, 4 vols, Vol. I, pp. 155-156 :<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Horace-B\u00e9n\u00e9dict de Saussure<\/strong> (1740-1799) was a Genevan scholar interested in physics, geology, mineralogy, meteorology, botany and philosophy. Fascinated by the Alps from an early age, in 1760 he offered a reward for the ascent of Mont Blanc, which was successfully completed in 1786 by Jacques Balmat and Michel Paccard. Saussure reached the summit the following year and recounted his experiences of the Alps in his Trips in the Alps<\/em>published in 4 volumes between 1779 and 1796.<\/p>

In the first volume of his Travel,<\/em>\u00a0Saussure describes a rock \"described nowhere: it is a mixture of jade, green spathic schorl and garnet in mass. This stone, of considerable hardness and density, takes on a beautiful polish, and the large red, green and yellow spots form a very beautiful effect\". (Trips in the Alps<\/em>Vol. I, pp. 155-156). This would appear to be the first description of eclogite, more than forty years before Ha\u00fcy described and named it.<\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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The copy presented bears the inscription \"Ex-libris Ellenberger\". Before being donated to the SGF library, the book was part of the collection of Fran\u00e7ois Ellenberger (1915-2000), a French geologist whose work included the structure and metamorphism of the Alps. He was also the founder of the French Committee for the History of Geology (COFRHIGEO) in 1976, and took an interest in the figure of Horace-B\u00e9n\u00e9dict de Saussure in his work History of Geology. <\/em><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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