Ophiolite
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 is found in the name of ophiolites, because of the superficial texture of some of them. Serpentinite especially evokes a snakeskin. 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.
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 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
Emplacement by irregular continental margin
A hypothesis based on research conducted on the Bay of Islands complex in Newfoundland as well as the East Vardar complex in the Apuseni Mountains of Romania suggest that an irregular continental margin colliding with an island arc complex causes ophiolite generation in a back-arc basin and obduction due to compression. The continental margin, promontories and reentrants along its length, is attached to the subducting oceanic crust, which dips away from it underneath the island arc complex. As subduction takes place, the buoyant continent and island arc complex converge, initially colliding with the promontories. However, oceanic crust is still at the surface between the promontories, not having been subducted beneath the island arc yet. The subducting oceanic crust is thought to split from the continental margin to aid subduction. In the event that the rate of trench retreat is greater than that of the island arc complex's progression, trench rollback will take place, and by consequence, extension of the overriding plate will occur to allow the island arc complex to match the trench retreat's speed. The extension, a back-arc basin, generates oceanic crust: ophiolites. Finally, when the oceanic lithosphere is entirely subducted, the island arc complex's extensional regime becomes compressional. The hot, positively buoyant ocean crust from the extension will not subduct, instead obducting onto the island arc as an ophiolite. As compression persists, the ophiolite is emplaced onto the continental margin. Based on Sr and Nd isotope analyses, ophiolites have a similar composition to mid-ocean-ridge basalts, but typically have slightly elevated large ion lithophile elements and a Nb depletion. These chemical signatures support the ophiolites having formed in a back-arc basin of a subduction zone.As trapped forearc
Ophiolite generation and subduction may also be explained, as suggested from evidence from the Coast Range ophiolite of California and Baja California, by a change in subduction location and polarity. Oceanic crust attached to a continental margin subducts beneath an island arc. Pre-ophiolitic ocean crust is generated by a back-arc basin. The collision of the continent and island arc initiates a new subduction zone at the back-arc basin, dipping in the opposite direction as the first. The created ophiolite becomes the tip of the new subduction's forearc and is uplifted by detachment and compression. Verification of the two above hypotheses requires further research, as do the other hypotheses available in current literature on the subject.Research
Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so scientific understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust generally has a layered velocity structure that implies a layered rock series similar to that listed above. But in detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating to oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. However, seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, and these are quite thin. A few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro.The circulation of hydrothermal fluids through young oceanic crust causes serpentinization, alteration of the peridotites and alteration of minerals in the gabbros and basalts to lower temperature assemblages. For example, plagioclase, pyroxenes, and olivine in the sheeted dikes and lavas will alter to albite, chlorite, and serpentine, respectively. Often, ore bodies such as iron-rich sulfide deposits are found above highly altered epidosites that are evidence of 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. Beyond issues of layer thicknesses mentioned above, a problem arises concerning compositional differences of silica and titania. Ophiolite basalt contents place them in the domain of subduction zones, whereas mid-ocean ridge basalts typically have ~50% silica and 1.5–2.5% TiO2. These chemical differences extend to a range of trace elements as well. In particular, trace elements associated with subduction zone volcanics tend to be high in ophiolites, whereas trace elements that are high in ocean ridge basalts but low in subduction zone volcanics are also low in ophiolites.
Additionally, the crystallization order of feldspar and pyroxene in the gabbros is 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. This led to introduction of the term "supra-subduction zone" ophiolite in the 1980s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Consequently, some of the classic ophiolite occurrences thought of as being related to seafloor spreading were found to be "SSZ" ophiolites, formed by rapid extension of fore-arc crust during subduction initiation.
A fore-arc setting for most ophiolites also solves the otherwise-perplexing problem of how oceanic lithosphere can be emplaced on top of continental crust. It appears that continental accretion sediments, if carried by the downgoing plate into a subduction zone, will jam it up and cause subduction to cease, resulting in the rebound of the accretionary prism with fore-arc lithosphere on top of it. Ophiolites with compositions comparable with hotspot-type eruptive settings or normal mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.