Ophiolites

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 Oman Ophiolite in Oman and the United Arab Emirates, Lizard Point in Cornwall, England, and the Klamath Mountains of northern California .

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: Gabbros, indicative of fractionated magma chamber.

Cumulate sequence: Dunite-rich layers of unfractionated magma.

Mantle sequence: Harzburgite/lherzolite-rich mantle rock.

The circulation of hydrothermal fluid through young oceanic crust causes 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.