Ophiolite

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Ordovician ophiolite in Gros Morne National Park, Newfoundland
Chromitic serpentinite, Bay of Islands Ophiolite, Lewis Hills, Newfoundland

An ophiolite is a section of Earth's

upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal
rocks.

The Greek word ὄφις, ophis (snake) is found in the name of ophiolites, because of the superficial texture of some of them.

plate tectonic
theory.

Their great significance relates to their occurrence within

ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics
, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.

Pseudostratigraphy and definition

Stratigraphic sequence of an ophiolite.
pillow basalts
  • sheeted basaltic dykes
  • intrusive, layered gabbro
  • dunite/peridotite cumulates

    • The

    mid-oceanic ridges
    . From top to bottom, the layers in the sequence are:

    A Geological Society of America Penrose Conference on ophiolites in 1972 defined the term "ophiolite" to include all of the layers listed above, including the sediment layer formed independently of the rest of the ophiolite.[1] This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable in thickness and composition, and that in places sheeted dikes sit directly on peridotite tectonite, with no intervening gabbros.

    Formation and emplacement

    Ophiolites have been identified in most of the world's orogenic belts.[2] However, two components of ophiolite formation are under debate: the origin of the sequence and the mechanism for ophiolite emplacement. Emplacement is the process of the sequence's uplift over lower density continental crust.[3]

    Origin as ocean lithosphere

    Several studies support the conclusion that ophiolites formed as oceanic

    metal-ore deposits present in and near ophiolites and from oxygen and hydrogen isotopes suggests that the passage of seawater through hot basalt in the vicinity of ridges dissolved and carried elements that precipitated as sulfides when the heated seawater came into contact with cold seawater. The same phenomenon occurs near oceanic ridges in a formation known as hydrothermal vents.[5] The final line of evidence supporting the origin of ophiolites as seafloor is the region of formation of the sediments over the pillow lavas: they were deposited in water over 2 km deep, far removed from land-sourced sediments.[5]
    Despite the above observations, there are inconsistencies in the theory of ophiolites as oceanic crust, which suggests that newly generated ocean crust follows the full
    Wilson cycle before emplacement as an ophiolite. This requires ophiolites to be much older than the orogenies on which they lie, and therefore old and cold. However, radiometric and stratigraphic dating has found ophiolites to have undergone emplacement when young and hot:[5] most are less than 50 million years old.[6]
    Ophiolites therefore cannot have followed the full Wilson cycle and are considered atypical ocean crust.

    Ophiolite emplacement

    There is yet no consensus on the mechanics of emplacement, the process by which oceanic crust is uplifted onto continental margins despite the relatively low density of the latter. All emplacement procedures share the same steps nonetheless: subduction initiation, thrusting of the ophiolite over a continental margin or an overriding plate at a subduction zone, and contact with air.[7]

    Hypotheses

    Emplacement by irregular continental margin

    A hypothesis based on research conducted on the Bay of Islands complex in Newfoundland as well as the East Vardar complex in the Apuseni Mountains of Romania

    reentrants along its length, is attached to the subducting oceanic crust, which dips away from it underneath the island arc complex. As subduction takes place, the buoyant continent and island arc complex converge, initially colliding with the promontories. However, oceanic crust is still at the surface between the promontories, not having been subducted beneath the island arc yet. The subducting oceanic crust is thought to split from the continental margin to aid subduction. In the event that the rate of trench retreat is greater than that of the island arc complex's progression, trench rollback will take place, and by consequence, extension of the overriding plate will occur to allow the island arc complex to match the trench retreat's speed. The extension, a back-arc basin, generates oceanic crust: ophiolites. Finally, when the oceanic lithosphere is entirely subducted, the island arc complex's extensional regime becomes compressional. The hot, positively buoyant ocean crust from the extension will not subduct, instead obducting onto the island arc as an ophiolite. As compression persists, the ophiolite is emplaced onto the continental margin.[9]
    Based on Sr and Nd isotope analyses, ophiolites have a similar composition to mid-ocean-ridge basalts, but typically have slightly elevated large ion lithophile elements and a Nb depletion. These chemical signatures support the ophiolites having formed in a back-arc basin of a subduction zone.

    As trapped forearc

    Ophiolite generation and subduction may also be explained, as suggested from evidence from the Coast Range ophiolite of California and Baja California, by a change in subduction location and polarity.[10] Oceanic crust attached to a continental margin subducts beneath an island arc. Pre-ophiolitic ocean crust is generated by a back-arc basin. The collision of the continent and island arc initiates a new subduction zone at the back-arc basin, dipping in the opposite direction as the first. The created ophiolite becomes the tip of the new subduction's forearc and is uplifted (over the accretionary wedge) by detachment and compression.[10] Verification of the two above hypotheses requires further research, as do the other hypotheses available in current literature on the subject.

    Research

    Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so scientific understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust generally has a layered velocity structure that implies a layered rock series similar to that listed above. But in detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating to oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. However, seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, and these are quite thin. A few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro.[citation needed]

    The circulation of

    black smokers, which continue to operate within the seafloor spreading centers of ocean ridges today.[citation needed
    ]

    Thus, there is reason to believe that ophiolites are indeed oceanic mantle and crust; however, certain problems arise when looking closer. Beyond issues of layer thicknesses mentioned above, a problem arises concerning compositional differences of

    silica (SiO2) and titania (TiO2). Ophiolite basalt contents place them in the domain of subduction zones (~55% silica, <1% TiO2), whereas mid-ocean ridge basalts typically have ~50% silica and 1.5–2.5% TiO2. These chemical differences extend to a range of trace elements as well (that is, chemical elements occurring in amounts of 1000 ppm or less). In particular, trace elements associated with subduction zone (island arc) volcanics tend to be high in ophiolites, whereas trace elements that are high in ocean ridge basalts but low in subduction zone volcanics are also low in ophiolites.[11]

    Additionally, the

    fore-arc lithosphere. This led to introduction of the term "supra-subduction zone" (SSZ) ophiolite in the 1980s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Consequently, some of the classic ophiolite occurrences thought of as being related to seafloor spreading (Troodos in Cyprus, Semail in Oman) were found to be "SSZ" ophiolites, formed by rapid extension of fore-arc crust during subduction initiation.[12]

    A fore-arc setting for most ophiolites also solves the otherwise-perplexing problem of how oceanic lithosphere can be emplaced on top of continental crust. It appears that continental accretion sediments, if carried by the downgoing plate into a subduction zone, will jam it up and cause subduction to cease, resulting in the rebound of the

    mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.[citation needed
    ]

    Groups and assemblages

    Classic ophiolite assemblage in Cyprus showing sheeted lava intersected by a dyke with pillow lava on top.

    Ophiolites are common in orogenic belts of Mesozoic age, like those formed by the closure of the Tethys Ocean. Ophiolites in Archean and Paleoproterozoic domains are rare.[13]

    Most ophiolites can be divided into one of two groups: Tethyan and Cordilleran. Tethyan ophiolites are characteristic of those that occur in the eastern Mediterranean sea area, e.g. Troodos in Cyprus, and in the Middle East, such as Semail in Oman, which consist of relatively complete rock series corresponding to the classic ophiolite assemblage and which have been emplaced onto a passive

    Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America. Despite their differences in mode of emplacement, both types of ophiolite are exclusively supra-subduction zone (SSZ) in origin.[14]

    Based on mode of occurrences, the Neoproterozoic ophiolites appear to show characteristics of both mid-oceanic ridge basalt (MORB)-type and SSZ-type ophiolites and are classified from oldest to youngest into: (1) MORB intact ophiolites (MIO); (2) dismembered ophiolites (DO); and (3) arc-associated ophiolites (AAO) (El Bahariya, 2018). Collectively, the investigated ophiolites of the Central Eastern Desert (CED) fall into both MORB/back-arc basin basalt (BABB) ophiolites and SSZ ophiolites. They are spatially and temporally unrelated, and thus, it seems likely that the two types are not petrogenetically related. Ophiolites occur in different geological settings, and they represent change of the tectonic setting of the ophiolites from MORB to SSZ with time.

    Origin and evolution of the concept

    The term ophiolite originated from publications of

    serpentine, diabase-spilite and chert.[16] The recognition of the Steinmann Trinity served years later to build up the theory around seafloor spreading and plate tectonics.[17] A key observation by Steinmann was that ophiolites were associated to sedimentary rocks reflecting former deep sea environments.[16] Steinmann himself interpreted ophiolites (the Trinity) using the geosyncline concept.[18] He held that Alpine ophiolites were "submarine effusions issuing along thrust faults into the active flank of an asymmetrically shortening geosyncline".[19] The apparent lack of ophiolites in the Peruvian Andes, Steinmann theorized, was either due to the Andes being preceded by a shallow geosyncline or representing just the margin of a geosyncline.[18] Thus, Cordilleran-type and Alpine-type mountains were to be different in this regard.[18] In Hans Stille's models a type of geosyncline called eugeosynclines were characterized by producing an "initial magmatism" that in some cases corresponded to ophiolitic magmatism.[18]

    As

    plutonic rocks found in ophiolites were understood as remnants of former magma chambers.[1]

    In 1973, Akiho Miyashiro revolutionized common conceptions of ophiolites and proposed an island arc origin for the famous Troodos Ophiolite in Cyprus, arguing that numerous lavas and dykes in the ophiolite had calc-alkaline chemistries.[22]

    Notable examples

    See also: List of ophiolites
    Apennines, Italy

    Examples of ophiolites that have been influential in the study of these rocks bodies are:

    Notes

    1. ^ a b c d e Dilek 2003, p. 5
    2. ^ Ben-Avraham, Z., (1982)
    3. ^ Kearey, P., et al., (2009)
    4. ^ Salisbury, M.H., and Christensen, N.I., (1978)
    5. ^ a b c d e Mason, R., (1985)
    6. ^ Moores, E.M., (1982)
    7. ^ Wakabayashi, J. and Dilek, Y., (2003)
    8. OCLC 1188715024
      .
    9. ^ a b Cawood, P.A. and Suhr, G., (1992)
    10. ^ a b Wakabayashi, J. and Dilek, Y., (2000)
    11. ^ Metcalf, R.V. and Shervais, J.W., (2008)
    12. ^ Shervais, J.W., (2001), Metcalf, R.V. and Shervais, J.W., (2008)
    13. ISBN 9780080457598
      .
    14. ^ e.g. Shervais, J.W., (2001)
    15. ^ Brogniart, A. (1813)
    16. ^ a b c d e Dilek 2003, p. 1
    17. S2CID 128688781
    18. ^ a b c d Şengör & Natal'in (2004), p. 682
    19. ^ Şengör & Natal'in (2004), p. 681
    20. ^ Şengör (1982), p. 44
    21. ^ Dilek 2003, p. 4
    22. ^ Dilek 2003, p. 6
    23. ^ "Macquarie Island World Heritage values". World heritage places. Australian Government Department of the Environment. 24 April 2008. Archived from the original on 17 April 2012.
    24. S2CID 129776536
      .
    25. doi:10.1016/0040-1951(89)90366-1
      .
    26. ISSN 2156-2202
      .
    27. doi:10.1016/j.tecto.2004.04.010
      .
    28. doi:10.1016/0079-1946(91)90006-2
      .

    References

    External links

    The Wikibook Historical Geology has a page on the topic of: Ophiolites
    Wikimedia Commons has media related to Ophiolite.
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