Ophiolites

Ophiolites are sections of oceanic crust (~7km in thickness) that have been uplifted or emplaced to be exposed at the land surface. Examples include the Troodos Ophiolite in Cyprus, the Oman Ophiolite in the UAE, and Lizard Point in Cornwall, UK.

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

SEDIMENTS: Muds and cherts deposited since the crust formed

EXTRUSIVE SEQUENCE: Pillow lavas show magma/seawater contact

SHEETED DYKES: Vertical, parallel features which fed pillows 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, for example, in the sheeted dykes. Often, ore bodies such as epidosites (Fe-rich deposits) are found - evidence of (the now relict) black smokers that continue to dominate the seafloor setting today.

Thus there is reason to believe that ophiolites are indeed oceanic 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 - MORBs - 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.