Ophiolite

Ordovician ophiolite in Gros Morne National Park, Newfoundland.

An ophiolite /ˈɒfiəlaɪt/ is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophio is Greek for snake (ὄφις), and lite means stone from the Greek lithos (λίθος), after the often-green-color rocks (spilites and serpentinites) that make up many ophiolites.

The term ophiolite was originally used by Alexandre Brongniart [1] for an assemblage of green rocks (serpentine, diabase) in the Alps; Gustav Steinmann[2] later modified its use to include serpentine, pillow lava, and chert ("Steinmann's trinity"), again based on occurrences in the Alps. The term was little-used in other areas until the late 1950s to early 1960s, with the recognition that this assemblage provided an analog for oceanic crust and the process of seafloor spreading. This recognition was tied to two events: (1) the observation of magnetic anomaly stripes on the seafloor, parallel to oceanic ridge systems, interpreted by Frederick Vine and Drummond Matthews [3] to represent the formation of new crust at the oceanic ridge and its subsequent symmetric spreading away from that ridge, and (2) the observation of a sheeted dike complex within the Troodos ophiolite (Cyprus) by Ian Graham Gass and co-workers,[4] which must have formed by repetitive extension of crust and intrusion of magma, resulting in a formation consisting of 100% dikes with no older wall rocks preserved within the complex. Moores and Vine[5] concluded that the sheeted dike complex at Troodos could form only by a process similar to the seafloor spreading proposed by Vine and Matthews.[3] Thus, it became widely accepted that ophiolites represent oceanic crust that had been emplaced on land.

Their great significance relates to their occurrence within mountain belts such as the Alps or the Himalayas, where they document the existence of former 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.

A simplified structure of an ophiolite suite:
1. axial magma chamber
2. pelagic sediments
3. pillow basalts
4. sheeted basaltic dykes
5. intrusive, layered gabbro
6. dunite/peridotite cumulates
Idealized stratigraphic sequence of an ophiolite.

Stratigraphy and definition

The stratigraphic sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges:

The Penrose field conference on ophiolites in 1972 redefined the term ophiolite to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on.[6] 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, and that in places volcanic rocks sit directly on peridotite tectonite, with no intervening gabbros.

Research

Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so their understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust has a layered velocity structure that implies a layered rock series similar to that listed above. In detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. 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.

The circulation of hydrothermal fluids through young oceanic crust causes serpentinization, alteration of the peridotites and alteration of minerals in the gabbros and basalts to lower temperature assemblages. For example, plagioclase, pyroxenes, and olivine in the sheeted dikes and lavas will alter to albite, chlorite, and serpentine, respectively. Often, ore bodies such as iron-rich sulfide deposits are found above highly altered epidosites (epidote-quartz rocks) that are evidence of (the now-relict) black smokers, which continue to operate within the seafloor spreading centers of ocean ridges today.

Thus there is reason to believe that ophiolites are indeed oceanic mantle and crust; however, certain problems arise when looking closer. Compositional differences regarding silica (SiO2) and titania (TiO2) contents, for example, place ophiolite basalts 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.[7]

The crystallization order of feldspar and pyroxene (clino- and orthopyroxene) in the gabbros is reversed, and ophiolites also appear to have a multi-phase magmatic complexity on par with subduction zones. Indeed, there is increasing evidence that most ophiolites are generated when subduction begins and thus represent fragments of 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.[8]

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 accretionary prism with fore-arc lithosphere (ophiolite) on top of it. Ophiolites with compositions comparable with hotspot-type eruptive settings or normal mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.

Ophiolite groups and assemblages

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

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 continental margin more or less intact (Tethys is the name given to the ancient sea that once separated Europe and Africa). Cordilleran ophiolites are characteristic of those that occur in the mountain belts of western North America (the "Cordillera" or backbone of the continent). These ophiolites sit on subduction zone accretionary complexes (subduction complexes) and have no association with a passive continental margin. They include the 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 SSZ in origin.[9]

Ophiolite assemblages in the Alps and some other collisional mountain belts are not formed during subduction, but rather represent the thinned margin of the continent that forms during rifting and continental drift. This incipient ocean crust remains locked to the continental margin when the ocean basin closes, emplacing the incipient ocean crust into the collision zone.

It is interesting to note that the age of ophiolite formation is often close to the age of their emplacement into the continental crust. Ophiolites are found in all the major mountain belts of the world whether collisional (e.g., Himalayas) or not (e.g., Andes). The subduction-related chemistry of ophiolites and their association with mountain belts suggests that their formation and emplacement are related to oceanic closure and continental collision (final stages of the Wilson Cycle) rather than oceanic opening and seafloor spreading as was first thought.

Furthermore, the occurrence of ophiolites throughout Earth history is not constant but rather formed and emplaced at specific intervals. These intervals correspond closely to times of super-continent break-up and dispersal — not because they form at the ridges that separate the drifting continents but because the large ocean basin coexisting with any super-continent must subduct along new subduction zones as rifting progresses.

Ophiolite formation and emplacement

Ophiolites have been identified in most of the world’s orogenic belts.[10] 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.[11]

Origin as ocean crust

Several studies support the conclusion that ophiolites formed as oceanic lithosphere. Seismic velocity structure studies have provided most of the current knowledge of the oceanic crust’s composition. For this reason, a seismic study was carried out on an ophiolite complex (Bay of Islands, Newfoundland) in order to establish a comparison. The study concluded that oceanic and ophiolitic velocity structures were identical, pointing to the origin of ophiolite complexes as oceanic crust.[12] The observations that follow support this conclusion. Rocks originating on the seafloor are of comparable chemical composition to unaltered ophiolite layers, from primary composition elements such as silicon and titanium to trace elements. Seafloor and ophiolitic rocks share a low occurrence of silica-rich rocks; those present have a high sodium and low potassium content.[13] The temperature gradients of the metamorphosis of ophiolitic pillow lavas and dykes are similar to those found beneath ocean ridges today.[13] Evidence from the 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 can be observed near oceanic ridges in a formation known as hydrothermal vents.[13] 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.[13] 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, ophiolites were found to have undergone emplacement when young and hot by radiometric and stratigraphic dating:[13] most are less than 50 million years old.[14] Ophiolites therefore can’t 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.[15]

Hypotheses for ophiolite generation and emplacement

Emplacement by irregular continental margin

A hypothesis based on research conducted on the Bay of Islands complex in Newfoundland suggests that an irregular continental margin colliding with an island arc complex causes ophiolite generation in a back-arc basin and obduction due to compression.[16] The continental margin, promontories and 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 won’t subduct, instead obducting onto the island arc as an ophiolite. As compression persists, the ophiolite is emplaced onto the continental margin.[16]

Ophiolites 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.[17] 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.[17] Verification of the two above hypotheses requires further research, as do the other hypotheses available in current literature on the subject.

Notable ophiolites

A pillow lava from an ophiolite sequence, Northern Apennines, Italy

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

Notes

  1. ↑ Brogniart, A. (1813)
  2. ↑ Steinmann, G (1927)
  3. 1 2 Vine F.J. and Matthews D.H. (1963)
  4. ↑ Gass, I.G. (1968)
  5. ↑ Moores E.M. and Vine, F.J. (1971)
  6. ↑ Dilek 2003, p. 5
  7. ↑ Metcalf, R.V. and Shervais, J.W., (2008)
  8. ↑ Shervais, J.W., (2001), Metcalf, R.V. and Shervais, J.W., (2008)
  9. ↑ e.g. Shervais, J.W., (2001)
  10. ↑ Ben-Avraham, Z., (1982)
  11. ↑ Kearey, P., et al., (2009)
  12. ↑ Salisbury, M.H., and Christensen, N.I., (1978)
  13. 1 2 3 4 5 Mason, R., (1985)
  14. ↑ Moores, E.M., (1982)
  15. ↑ Wakabayashi, J. and Dilek, Y., (2003)
  16. 1 2 Cawood, P.A. and Suhr, G., (1992)
  17. 1 2 Wakabayashi, J. and Dilek, Y., (2000)
  18. ↑ "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.
  19. ↑ Johnston, M. R.; Nineteenth-century observations of the Dun Mountain Ophiolite Belt, Nelson, New Zealand and trans-Tasman correlations, Geological Society, London, Special Publications 2007, v. 287, p. 375-387
  20. ↑ Rossman, D. L.; G. C. Castañada and G. C. Bacuta; Geology of the Zambales ophiolite, Luzon, Philippines, Tectonophysics, Volume 168, Issues 1–3, 20 October 1989, Pages 1–3, 7–22
  21. ↑ Acharyya, S.K.; K.K. Ray and Subhasis Sengupta; The Naga Hills and Andaman ophiolite belt, their setting, nature and collisional emplacement history, Physics and Chemistry of the Earth, Volume 18, Part 1, 1991, Pages 293–315

References

External links

The Wikibook Historical Geology has a page on the topic of: Ophiolites
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