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Ophiolites

From Academic Kids

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. Examples include the Troodos Ophiolite in the Troodos Mountains of Cyprus; the Semail Ophiolite in Oman and the United Arab Emirates; the Betts Cove, St. Anthony, Little Port, Advocate, Gander River and Annieopsquotch ophiolites in Newfoundland; the Lizard Point in Cornwall, England; the Coast Range, Smartville, and Klamath Mountains of northern California; and the Papuan ophiolite in Papua-New Guinea. The Yakuno, Horokanai, and Poroshiri are three full ophiolite sequences in Japan. The Bay of Islands Ophiolite in Gros Morne National Park, Newfoundland was named a UNESCO World Heritage Site in 1987 because of its superbly exposed complete ophiolite stratigraphic sequence.

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.
Missing image
Ophiolite2.jpg
Idealized vertical section through an ophiolite. Note the position of two different Mohorovicic discontinuity or Mohos, separating crust from mantle. Seismic Moho distinguishes mantle from crust on the basis of seismic velocities, whereas Petrologic Moho distinguishes the entire crust as resulting from magma (including cumulate peridotite at the base) from the residue it was derived from (tectonized peridotite).

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. One of the big problems 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.

The circulation of hydrothermal fluid through young oceanic crust causes serpentinization alteration of the minerals observed: chlorite and serpentine, for example, in the sheeted dikes. 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 content, for example, place ophiolite basalts in the domain of subduction zones (~55% silica), whereas mid-ocean ridge basalts typically have a value ~50%. 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. 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 up it up and cause subduction to cease, resulting in the rebound of the continental crust with forearc lithosphere (ophiolite) on top of it.

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