Eclogite

Source: Wikipedia, the free encyclopedia.
groundmass. The sky-blue crystals are kyanite. Minor white quartz is present, presumably from the recrystallization of coesite. A few gold-white phengite
patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale.

Eclogite (

subducted, altered oceanic crust
.

Origins

Eclogites typically result from high to ultrahigh pressure

subduction zone.[1]

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.[2][3] The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet.[3] 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. [4]

The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface.

glaucophane schist, primarily exemplified by the New Caledonia tectonic block off the coast of California. [5]

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. [6] [7][8]

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to

cumulate model, where garnet and clinopyroxene bulk compositions derive from residues of partial melting within the mantle. [16] Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range. [17]

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 45–300 km (28–186 mi).[18]

Importance

Photomicrograph of a thin section
of eclogite from Turkey. Green omphacite (+ late chlorite) + pink garnet + blue glaucophane + colorless phengite.

Formation of igneous rocks from eclogite

Eclogite

Partial melting of eclogite has been modeled to produce

tonalite-trondhjemite-granodiorite melts.[19] Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted.[20] The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.[21]

Distribution

Eclogite from Almenning, Norway. The red-brown mineral is garnet, green omphacite and white quartz.

Occurrences exist in western North America, including the southwest

Himalaya.[24] The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.[25][26]

References

  1. ISSN 1367-9120
    .
  2. ^
    ISSN 0024-4937
    .
  3. ^
    ISSN 0016-7606
    .
  4. ISSN 0016-7606. Archived
    from the original on 2022-02-12. Retrieved 2021-11-30.
  5. ISSN 0016-7606. Archived
    from the original on 2022-02-12. Retrieved 2021-11-30.
  6. ISSN 0024-4937. Archived
    from the original on 2022-02-12. Retrieved 2021-11-30.
  7. ISSN 0012-8252
    .
  8. ISSN 0040-1951
    .
  9. S2CID 123640184. Archived
    from the original on 2021-11-04. Retrieved 2021-11-30.
  10. ISSN 0024-4937
    .
  11. ISSN 2156-2202
    .
  12. S2CID 46321182
    .
  13. ISSN 2156-2202
    .
  14. ISSN 0009-2541
    .
  15. ISSN 0012-821X
    .
  16. ISSN 0012-8252
    .
  17. ISSN 0024-4937
    .
  18. S2CID 135327696. Archived
    (PDF) from the original on 2010-06-17. Retrieved 2019-09-24.
  19. S2CID 4333290
    .
  20. ISBN 978-1-4051-6148-0. Archived
    from the original on 2017-11-25. Retrieved 2011-03-16.
  21. S2CID 1565886
    .
  22. ISBN 1-897799-85-3
  23. ^ "C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham". Archived from the original on 2011-06-10. Retrieved 2009-01-14.
  24. doi:10.1016/j.lithos.2009.07.015
    .
  25. doi:10.1016/S0009-2541(01)00264-9
    .
  26. doi:10.1016/j.gr.2014.09.013
    .

External links
Wikimedia Commons has media related to Eclogite.
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