Ophiolites

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Ophiolites are sections of the oceanic crust and the subjacent upper mantle that have been uplifted or emplaced to be exposed within continental crustal rocks. Ophio is Greek for "snake", lite means "stone".

The term ophiolite was originally used by Alexandre Brongniart (1813) for an assemblage of green rocks (serpentine, diabase) in the Alps; Steinmann (1927) 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 1950's to early 1960's, 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 Vine and Matthews (1963) to represent the formation of new crust at the oceanic ridge and its subsequent spreading away from that ridge, and (2) the observation of a sheeted dike complex within the Troodos ophiolite (Cyprus) by Gass and co-workers, which must have formed by 100% intrusion of new magma, since no older wall rocks are preserved within the complex (Gass 1968). Moores and Vine (1971) concluded that the sheeted dike complex at Troodos could only form by a process similar to the sea-floor spreading proposed by Vine and Matthews (1963). 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.

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

Sediments: Muds (black shale) and cherts deposited since the crust formed.
Extrusive sequence: Basaltic pillow lavas show magma/seawater contact.
Sheeted dikes: Vertical, parallel dikes which fed the pillow lavas above.
High level intrusives: Isotropic Gabbro, indicative of fractionated magma chamber.
Layered Gabbro, resulting from settling out of minerals from a magma chamber.
Cumulate peridotite: Dunite-rich layers of minerals that settled out from a magma chamber.
Tectonized peridotite: Harzburgite/lherzolite-rich mantle rock.

An international 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. 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. Many ophiolites have similar variations in their stratigraphy, some of which are primary and others of which formed later (for example, during emplacement into a mountain belt).

Ordovician ophiolite in Gros Morne National Park, Newfoundland.
Ordovician ophiolite in Gros Morne National Park, Newfoundland.

Examples of ophiolites include:

We have only drilled about 1.5 km into th 6-7 km thick oceanic crust, so our understanding of oceanic crust largely comes 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 that listed above. In detail there are problems, with many ophiolites exhibiting thinner accumulations of igneoous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. Seismic sounding of mid-ocean ridges has only revealed 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 fluid 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 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.

The crystallization order of feldspar and pyroxene in the gabbros is unexpectedly 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 1980's to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Ironically, some of the classic ophiolite occurrences used to relate ophiolites 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.

A fore-arc setting for most ophiolites also solves the otherwise perplexing problem of how do you emplace oceanic lithosphere on top of continental crust? It appears that continental crust, 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 continental crust with forearc 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.

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 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. These 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 (e.g., Shervais, 2001).

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.

Interestingly, the age of ophiolite formation is often surprisingly 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 they were 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 that must coexist with any super-continent must subduct along new subduction zones as rifting progresses.

[edit] References

Brogniart, A. (1813) "Essai de classifacation mineralogique des roches melanges" Journal des Mines, v. XXXIV, 190-199.

Gass, I.G. (1968) "Is the Troodos massif of Cyprus a fragment of Mesozoic ocean floor?" Nature', 220, 39-42.

Moores E.M. and Vine, F.J. (1971) "The Troodos massif, Cyprus, and other ophiolites as oceanic crust: Evaluation and implications" Philosophical Transactions of the Royal Society of London, 268A, 443-466.

Moores, E.M. (2003) "A personal history of the ophiolite concept" in Dilek and Newcomb, editors, Ophiolite Concept and the Evolution of Geologic Thought Geological Society of America, Special Publication 373, 17-29.

Shervais, J.W., 2001, "Birth, Death, and Resurrection: The Life Cycle of Suprasubduction Zone Ophiolites," Geochemistry, Geophysics, Geosystems, vol. 2, (Paper number 2000GC000080), 20,925 words, 8 figures, 3 tables. [online journal]

Steinmann, G (1927) "Die ophiolitshen zonen in den mediterranen Kettengebirgen", translanted and reprinted by Bernoulli and Friedman, in Dilek and Newcomb, editors, Ophiolite Concept and the Evolution of Geologic thought (Geological Society of America, Special Publication 373), 77-91.

Vine F.J. and Matthews D.H. (1963) "Magnetic anomalies over ocean ridges", Nature 199, 947-949.


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