Eclogite Explained

Eclogite is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum and, rarely, diamond. The chemistry of primary and accessory minerals is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.

The name eclogite is derived from the Ancient Greek word for 'choice', meaning 'chosen rock' on account of its perceived beauty. It was first named by René Just Haüy in 1822 in the second edition of his work Traité de mineralogie.[1]

Origins

Eclogites typically result from high to ultrahigh pressure metamorphism of mafic rock at low thermal gradients of < as it is subducted to the lower crust to upper mantle depths in a subduction zone.[2]

Classification

Eclogites are defined as bi-mineralic, broadly basaltic rocks which have been classified into Groups A, B and C based on the chemistry of their primary mineral phases, garnet and clinopyroxene.[3] [4] The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet. The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higher alkali contents.[5]

The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface. Group A derive from cratonic regions of Earth's crust, brought to the surface as xenoliths from depths greater than 150 km during kimberlite eruptions.[3] Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by peridotitic mantle material. Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the New Caledonia tectonic block off the coast of California.[6]

Surface versus mantle origin

The broad range in composition has led a longstanding debate on the origin of eclogite xenoliths as either mantle or surface derived, where the latter is associated with the gabbro to eclogite transition as a major driving force for subduction.[7] [8] [9]

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition.[10] [3] Models proposing a primary surface origin as seafloor protoliths strongly rely on the wide range in oxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of the Samail ophiolite.[11] [12] The variation found in some eclogite xenoliths at the Roberts Victor kimberlite pipe are a result of hydrothermal alteration of basalt on the seafloor.[11] This process is attributed to both low- and high-temperature seawater exchange, resulting in large fractionations in oxygen isotope space relative to the upper mantle value typical of mid ocean ridge basalt glasses.[13] [14] Other mechanisms proposed for the origin of Group A eclogite xenoliths rely on a cumulate model, where garnet and clinopyroxene bulk compositions derive from residues of partial melting within the mantle.[8] Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range.[15]

Eclogite facies

This facies reflects metamorphism at high pressure (at or over 12kbar) and moderately high to very high temperatures. The pressures exceed those of greenschist, blueschist, amphibolite or granulite facies.

Eclogites containing lawsonite (a hydrous calcium-aluminium silicate) are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form during normal subduction of oceanic crust at depths between about .[16]

Importance

Formation of igneous rocks from eclogite

Partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts.[17] Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted.[18] The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.[19]

Distribution

Occurrences exist in western North America, including the southwest[20] and the Franciscan Formation of the California Coast Ranges.[21] Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya.[22] The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.[23] [24]

External links

Notes and References

  1. Web site: Eclogite History. Internation Eclogite Conference. 25 May 2024.
  2. Zheng. Yong-Fei. Chen. Ren-Xu. September 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences. 145. 46–73. 10.1016/j.jseaes.2017.03.009. 1367-9120. 2017JAESc.145...46Z. free.
  3. Jacob. D. E.. 2004-09-01. Nature and origin of eclogite xenoliths from kimberlites. Lithos. Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume. en. 77. 1. 295–316. 10.1016/j.lithos.2004.03.038. 0024-4937.
  4. COLEMAN. R. G. LEE. D. E. BEATTY. L. B. BRANNOCK. W. W. 1965-05-01. https://doi.org/10.1130/0016-7606(1965)76[483:EAETDA2.0.CO;2 Eclogites and Eclogites: Their Differences and Similarities]. GSA Bulletin. 76. 5. 483–508. 10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. 0016-7606.
  5. COLEMAN. R. G. LEE. D. E. BEATTY. L. B. BRANNOCK. W. W. 1965-05-01. https://doi.org/10.1130/0016-7606(1965)76[483:EAETDA2.0.CO;2 Eclogites and Eclogites: Their Differences and Similarities]. GSA Bulletin. 76. 5. 483–508. 10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. 0016-7606. 2021-11-30. 2022-02-12. https://web.archive.org/web/20220212025752/https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/76/5/483/5909/Eclogites-and-Eclogites-Their-Differences-and?redirectedFrom=fulltext. live.
  6. COLEMAN. R. G. LEE. D. E. BEATTY. L. B. BRANNOCK. W. W. 1965-05-01. https://doi.org/10.1130/0016-7606(1965)76[483:EAETDA2.0.CO;2 Eclogites and Eclogites: Their Differences and Similarities]. GSA Bulletin. 76. 5. 483–508. 10.1130/0016-7606(1965)76[483:EAETDA]2.0.CO;2. 0016-7606. 2021-11-30. 2022-02-12. https://web.archive.org/web/20220212025750/https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/76/5/483/5909/Eclogites-and-Eclogites-Their-Differences-and?redirectedFrom=fulltext. live.
  7. Jacob. D. E.. 2004-09-01. Nature and origin of eclogite xenoliths from kimberlites. Lithos. Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume. en. 77. 1. 295–316. 10.1016/j.lithos.2004.03.038. 0024-4937. 2021-11-30. 2022-02-12. https://web.archive.org/web/20220212025749/https://www.sciencedirect.com/science/article/abs/pii/S0024493704001045. live.
  8. O'Hara. M. J.. 1968-01-01. The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth-Science Reviews. en. 4. 69–133. 10.1016/0012-8252(68)90147-5. 0012-8252.
  9. Ringwood. A. E.. Green. D. H.. 1966-10-01. An experimental investigation of the Gabbro-Eclogite transformation and some geophysical implications. Tectonophysics. en. 3. 5. 383–427. 10.1016/0040-1951(66)90009-6. 0040-1951.
  10. 1980-07-24. Chemical variations in upper mantle nodules from southern African kimberlites. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. en. 297. 1431. 273–293. 10.1098/rsta.1980.0215. 123640184 . 0080-4614. 2021-11-30. 2021-11-04. https://web.archive.org/web/20211104080455/https://royalsocietypublishing.org/doi/10.1098/rsta.1980.0215. live.
  11. MacGregor. Ian D.. Manton. William I.. 1986. Roberts victor eclogites: Ancient oceanic crust. Journal of Geophysical Research: Solid Earth. en. 91. B14. 14063–14079. 10.1029/JB091iB14p14063. 2156-2202.
  12. Gregory. Robert T.. Taylor. Hugh P.. 1981. An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges. Journal of Geophysical Research: Solid Earth. en. 86. B4. 2737–2755. 10.1029/JB086iB04p02737. 46321182 . 2156-2202.
  13. Muehlenbachs. Karlis. 1998-04-15. The oxygen isotopic composition of the oceans, sediments and the seafloor. Chemical Geology. en. 145. 3. 263–273. 10.1016/S0009-2541(97)00147-2. 0009-2541.
  14. Mattey. David. Lowry. David. Macpherson. Colin. 1994-12-01. Oxygen isotope composition of mantle peridotite. Earth and Planetary Science Letters. en. 128. 3. 231–241. 10.1016/0012-821X(94)90147-3. 0012-821X.
  15. Huang. Jin-Xiang. Gréau. Yoann. Griffin. William L.. O'Reilly. Suzanne Y.. Pearson. Norman J.. 2012-06-01. Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects. Lithos. en. 142-143. 161–181. 10.1016/j.lithos.2012.03.002. 0024-4937.
  16. 10.1029/2007GC001707. H2O subduction beyond arcs. Geochemistry, Geophysics, Geosystems. 9. 3. . 2008. Hacker. Bradley R.. 2008GGG.....9.3001H. 10.1.1.513.829. 135327696 . 2019-09-24. 2010-06-17. https://web.archive.org/web/20100617131553/http://www.geol.ucsb.edu/faculty/hacker/viz/Hacker08_H2O_subduction_beyond_arcs.pdf. live.
  17. 10.1038/nature02031. 14534583. Growth of early continental crust by partial melting of eclogite. Nature. 425. 6958. 605–609. 2003. Rapp. Robert P.. Shimizu. Nobumichi. Norman. Marc D.. 2003Natur.425..605R. 4333290 .
  18. Book: Plates vs. Plumes: A Geological Controversy . Foulger, G.R. . 2010 . 978-1-4051-6148-0 . Wiley-Blackwell . 2011-03-16 . 2017-11-25 . https://web.archive.org/web/20171125195240/http://www.wiley.com/WileyCDA/WileyTitle/productCd-1405161485.html . live .
  19. Sobolev. Alexander V.. Hofmann. Albrecht W.. Sobolev. Stephan V.. Nikogosian. Igor K.. March 2005. An olivine-free mantle source of Hawaiian shield basalts. Nature. 434. 7033. 590–597. 10.1038/nature03411. 15800614. 0028-0836. 2005Natur.434..590S. 1565886 .
  20. William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages
  21. Web site: C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham . 2009-01-14 . 2011-06-10 . https://web.archive.org/web/20110610100125/http://www.megalithic.co.uk/article.php?sid=19244 . live .
  22. Wilke . Franziska D.H. . O'Brien . Patrick J. . Altenberger . Uwe . Konrad-Schmolke . Matthias . Khan . M. Ahmed . Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes . Lithos . January 2010 . 114 . 1–2 . 70–85 . 10.1016/j.lithos.2009.07.015. 2010Litho.114...70W .
  23. Bor-ming Jahn. Jahn. Bor-ming. Caby. Renaud. Monie. Patrick. 2001. The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications. Chemical Geology. 178. 1–4. 143–158. 10.1016/S0009-2541(01)00264-9. 2001ChGeo.178..143J.
  24. Santos. Ticiano José Saraiva. Amaral. Wagner Silva. Ancelmi. Matheus Fernando. Pitarello. Michele Zorzetti. Fuck. Reinhardt Adolfo. Dantas. Elton Luiz. 2015. U–Pb age of the coesite-bearing eclogite from NW Borborema Province, NE Brazil: Implications for western Gondwana assembly. Gondwana Research. 28. 3. 1183–1196. 10.1016/j.gr.2014.09.013. 2015GondR..28.1183D.

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