Ophiolites are pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates. They provide models for processes at mid-ocean ridges.
Ophiolites are an assemblage of mafic and ultramafic lavas and hypabyssal rocks found in association with sedimentary rocks like greywackes and cherts. They are found in areas that have complex structure. Cross-sections simplified from R.C. Coleman, 1981, Journal of Geophysical Research, v. 86, p. 2497-2508.
Ophiolites have been found in Cyprus, New Guinea, Newfoundland, California, and Oman. The Samail ophiolite in southeastern Oman has probably been studied in the greatest detail. The rocks probably formed in the Cretaceous not far from the what is now the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle) westward onto the Arabian shield.
Ophiolites are characterized by a classic sequence of rocks. This sequence is well exposed at the Samail ophiolite. The base of the sequence is sedimentary rocks of the Arabian shield, not part of the ophiolite, on which the oceanic plate was pushed. From base to top the ophiolite is made of: peridotite, layered gabbro, massive gabbro, dikes, and volcanic rocks. At Samail this entire sequence is 15 km thick. The basal peridotite is made of a rock called harzburgite (made mostly of the minerals olivine and enstatite). Within the peridotite are many dikes of gabbro and dunite. The peridotite is deformed. The peridotite is overlain by dunite (an intrusive igneous rock made mostly of the mineral olivine) that grades upward to gabbro (an intrusive igneous rock made mostly of plagioclase and clinopyroxene - augite). The sequence is capped by dikes and volcanic rocks (pillow basalts that erupted on the ocean floor). Sequence of rocks simplified from R.C. Coleman (1981).
From a tectonic perspective, the peridotite is depleted mantle that was under the magma chamber at the mid-ocean ridge crest. The gabbro layer is related, in some way, to the crystallization of the magma chamber (probably with repeated injections of magma). The dikes and volcanic rocks are formed by magma in transit to or at the surface. Cross-section simplified from Pallister, J.S., and Hopson, C.A., 1981, Journal of Geophysical Research, v. 86, p. 2593-2644.
To learn more about gabbro and other igneous rocks review the Geotour Igneous Rocks Module or visit the petrography homepage.
To see a classic bit of oceanic crust that has been thrusted up on a
continent (an ophiolite ) visit the Oman Virtual Field Trip.
McBirney, A.R., 1985, Igneous petrology: Freeman Cooper & Co.