An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ophis (snake) is found in the name of ophiolites, because of the superficial texture of some of them. Serpentinite especially evokes a snakeskin. (The suffix -lite is from the Greek lithos, meaning "stone".) Some ophiolites have a green color. The origin of these rocks, present in many mountainous massifs, remained uncertain until the advent of plate tectonic theory. Their great significance relates to their occurrence within mountain belts such as the Alps and 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 and the interpretation of ancient mountain belts.
Pseudostratigraphy and definition
The stratigraphic-like sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges. From top to bottom, the layers in the sequence are:
Pelagic sediments: mostly siliceous oozes, calcareous oozes and red clays deposited since the crust formed. Extrusive sequence: basaltic pillow lavas show magma/seawater contact. Sheeted dike complex: vertical, parallel dikes that fed lavas above. High level intrusives: isotropic gabbro, indicative of a 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. A Geological Society of America Penrose Conference on ophiolites in 1972 defined the term "ophiolite" to include all of the layers listed above, including the sediment layer formed independently of the rest of the ophiolite. 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 in thickness and composition, and that in places sheeted dikes sit directly on peridotite tectonite, with no intervening gabbros.
Formation and emplacement Ophiolites have been identified in most of the world's orogenic belts. However, two components of ophiolite formation are under debate: the origin of the sequence and the mechanism for ophiolite emplacement. Emplacement is the process of the sequence's uplift over lower density continental crust.
Origin Several studies support the conclusion that ophiolites formed as oceanic lithosphere. Seismic velocity structure studies have provided most of the current knowledge of the oceanic crust's composition. For this reason, researchers carried out a seismic study on an ophiolite complex (Bay of Islands, Newfoundland) in order to establish a comparison. The study concluded that oceanic and ophiolitic velocity structures were identical, pointing to the origin of ophiolite complexes as oceanic crust. The observations that follow support this conclusion. Rocks originating on the seafloor show chemical composition comparable to unaltered ophiolite layers, from primary composition elements such as silicon and titanium to trace elements. Seafloor and ophiolitic rocks share a low occurrence of silica-rich minerals; those present have a high sodium and low potassium content. The temperature gradients of the metamorphosis of ophiolitic pillow lavas and dykes are similar to those found beneath ocean ridges today. Evidence from the metal-ore deposits present in and near ophiolites and from oxygen and hydrogen isotopes suggests that the passage of seawater through hot basalt in the vicinity of ridges dissolved and carried elements that precipitated as sulfides when the heated seawater came into contact with cold seawater. The same phenomenon occurs near oceanic ridges in a formation known as hydrothermal vents. The final line of evidence supporting the origin of ophiolites as seafloor is the region of formation of the sediments over the pillow lavas: they were deposited in water over 2 km deep, far removed from land-sourced sediments. Despite the above observations, there are inconsistencies in the theory of ophiolites as oceanic crust, which suggests that newly generated ocean crust follows the full Wilson cycle before emplacement as an ophiolite. This requires ophiolites to be much older than the orogenies on which they lie, and therefore old and cold. However, radiometric and stratigraphic dating has found ophiolites to have undergone emplacement when young and hot: most are less than 50 million years old. Ophiolites therefore cannot have followed the full Wilson cycle and are considered atypical ocean crust.
Ophiolite emplacement There is yet no consensus on the mechanics of emplacement, the process by which oceanic crust is uplifted onto continental margins despite the relatively low density of the latter. All hypotheses regarding emplacement procedures share the same steps nonetheless: subduction initiation, thrusting of the ophiolite over a continental margin or an overriding plate at a subduction zone, and exposure at the surface.
Hypotheses
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