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Ophiolite

A distinctive assemblage of mafic plus ultramafic rocks generally considered to be fragments of the oceanic lithosphere that have been tectonically emplaced onto continental margins and island arcs. Ophiolite was named by A. Brongniart, a nineteenth-century French naturalist, who considered its scaly appearance and the greenish color of its main constituent rock, serpentinite. An ophiolite is a formation made up of an association of typical rocks in a clearly defined sequence. As shown in Fig. 1, a complete idealized ophiolite sequence from bottom to top includes (1) an ultramafic tectonite complex composed mostly of multilayered, deformed harzburgite, dunite, and minor chromitite; (2) a plutonic complex of layered mafic-ultramafic cumulates at the base, grading upward to massive gabbro, diorite, and possibly plagiogranite; (3) a mafic sheeted-dike complex; (4) an extrusive section of massive and pillow lavas, pillow breccias, and intercalated pelagic sediments; and (5) a top layer of abyssal or bathyal sediments, which may include ribbon chert, red pelagic limestone, metalliferous sediments, volcanic breccias, or pyroclastic deposits. Most ophiolites lack complete sections, and are dismembered and fragmented. Their estimated original thickness is variable, ranging from about 2 km (1.2 mi) to more than 8 km (5 mi).  See also: Earth crust; Lithosphere

Fig. 1  Idealized cross section of (a) an oceanic ridge showing stratigraphy of the intrusive and extrusive sequences, and circulation of heated seawater at the top of the oceanic crust, and (b) an idealized ophiolite succession compared with various exposed ophiolites.

Occurrence

Ophiolites typically occur in collisional mountain belts or island arcs and define a suture zone marking the boundary where two plates have welded together. The ophiolite complex is interpreted as evidence for a closed marginal ocean or back-arc basin. Typical examples of such sutures are found along the northern flank of the Himalayas (the Indus suture) and in the central Urals; both extend for more than 1000 km (600 mi). The occurrence of a suite of deep-sea sediments, pillow basalts, gabbros, and serpentinized ultramafic rocks within these sutures suggests that they constituted oceanic lithosphere that was subsequently thrust onto the continental margins by a process known as obduction.

According to plate tectonics, the destiny of the ocean floor is subduction into the mantle. In contrast, geologists have proposed the term “obduction” to describe the particular destiny of ophiolites that, during the advance of oceanic lithosphere against a continent, end up stranded on the edge of a continent instead of disappearing into the subduction zone below it.  See also: Plate tectonics; Subduction zones

Throughout the world, ophiolites occur as long narrow belts, up to 10 km (6 mi) wides, that can extend more than 1000 km (600 mi) in length, in two distinct geographic settings. (1) Those in the Alpine-Mediterranean region, Tethyan ophiolite, were formed in small ocean basins that were surrounded by older, attenuated continental crust. Many of the classical ophiolites found in Cyprus and Oman belong to this group, which have a nearly complete sequence and were brought above sea level during the collapse of small ocean basins by the convergence of neighboring plates. (2) Those in western North America and the Circum-Pacific (Cordilleran) region seem to have formed in inter-arc basins. The Cordilleran ophiolites, such as the Trinity ophiolite and the Coast Range ophiolite of California, are generally incomplete, metamorphosed, or dismembered, but they commonly form the basement rocks for many North American continental margin terranes. Tethyan ophiolites are characterized by the occurrence of harzburgitic ultramafic rocks and pronounced thick layers of gabbroic rock, whereas Cordilleran ophiolite, such as the Trinity, have undepleted lherzolites and thin layers of recrystallized and deformed gabbros. Such differences have been attributed to the rate of spreading, with fast spreading accounting for more partial melting of the primary mantle rocks, and hence the thicker mantle residue as harzburgite and greater differentiation of gabbroic magma for the mafic-ultramafic rocks of the Oman ophiolite.  See also: Basin; Continental margin; Geodynamics; Structural geology

These two types of ophiolites strongly resemble the oceanic lithosphere insofar as the latter is known from dredging, shallow drilling, and geophysical studies. However, the crustal section of most ophiolites is significantly different from the abyssal oceanic crust of the Atlantic and Pacific oceans. Many ophiolitic basalts possess chemical affinities of back-arc basin basalts or island-arc basalts, rather than those of mid-oceanic ridge basalt (MORB). For example, the potassium oxide (K2O) content in MORB is usually less than 0.2 wt %, whereas it is higher than 0.2 wt % in back-arc basin basalts and the extrusive rocks of the ophiolite sequence. Many ophiolites carry a weak-to-strong subduction zone geochemical imprint with relative depletions in tantalum and niobium in normalized incompatible-element distribution diagrams compared to MORB. The sedimentary sequence overlying most ophiolites grades from thin layers of pelagic or cherty sediments upward to turbidite or calc-alkaline volcaniclastics, suggesting that the oceanic crust of most ophiolites formed in back-arc basins. This is also supported by the fact that the crustal sequence of most ophiolites is thin (<5 km; 3 mi) in comparison with average oceanic crust developed in large ocean basins (7–10 km; 4–6 mi).  See also: Basalt; Mid-Oceanic Ridge; Oceanic islands

Origins

Many ophiolites are thought to have formed in submarine extensional tectonic settings with extremely high heat flow, such as the present-day East Pacific Rise and Mid-Atlantic Ridge, where new oceanic crust is being generated. The analogy of the ophiolite sequence with the oceanic lithosphere (Fig. 1) is supported by the gross similarity in chemistry, metamorphic grades corresponding to temperature gradients existing under spreading centers, the presence of similar ore minerals, and the occurrence of deep-sea sediments. However, in recent years this simple analogy has been challenged. It has been suggested that ophiolites do not represent the typical oceanic lithosphere and do not belong to a unique species; instead they have formed in other extensional regimes, including island-arc or marginal-basin settings, above a subduction zone. Ophiolites are derived from a variety of oceanic sites on which new lithosphere is formed. Despite this ongoing controversy, the general mechanism by which a complete ophiolite succession forms is reasonably well understood and agreed upon.

In extensional oceanic environments, heat flow is high and the mantle asthenosphere is rising. As the pressure decreases in the rising asthenosphere beneath ocean ridges or back-arc basins, undepleted mantle lherzolites partially melt to form basaltic magma according to the general reaction, lherzolite harzburgite (75%) + basaltic magma (25%). These magmas collect in a chamber (or chambers) at depths about 4–6 km (2.5–4 mi), and 25 km (16 mi) long, and undergo fractional crystallization. Some magma rises as dikes, forming a sheeted-dike complex, and are extruded as pillow lavas in the axial rift on the sea floor. The remaining magma within the chamber fractionates upon cooling to form the layered and massive rocks of the plutonic sequence. Fractional crystallization of the magma gives rise locally to diorite and plagiogranite. The petrologic and chemical data indicate that the lavas, dikes, gabbros, and underlying tectonized harzburgite are all cogenetic; the harzburgite represents a crystalline residue from partial melting in the mantle that produced the overlying igneous rocks.  See also: Asthenosphere; Igneous rocks; Magma; Pluton

Ages of formation and emplacement

At the base of most ophiolites, a thin metamorphic sole occurs and records the travel history of the obduction of the oceanic lithosphere on land. The sole ranges in thickness from 10 to 50 m (33 to 165 ft) and can extend laterally for more than 100 km (60 mi). It is composed of highly deformed amphibolites and metasedimentary greenschists, and shows a sharp decrease in metamorphic grade from top to bottom. When young and hot oceanic lithosphere thrusts upon the oceanic crust near a continent margin, it heats the underlying crustal rocks like a flat iron. These rocks start to recrystallize while being deformed under the moving load of the overlying lithosphere, and are subjected to intense metamorphism. During its oceanic travels at a rate of several centimeters per year, the ophiolite nappe loses heat; the temperature at its base will have dropped and the underlying crust will transform into low-temperature metamorphic rocks, such as greenschist. The metamorphic soles formed, apparently, by successive underplating and welding onto the base of an ophiolite as the hot, young oceanic slab migrated toward the continent. Dating the metamorphic soles' thermally recrystallized minerals provides ages for ophiolite emplacement onto the continental margins.  See also: Metamorphic rocks

Ages of ophiolite formation can be obtained by direct radioactive lead (U/Pb) dating of zircon from diorite or plagiogranites of the plutonic sequence of an ophiolite. They can also be constrained by determining the ages of radiolarian fossils in pelagic chert overlying these ophiolites. Ophiolites are rather abundant in Phanerozoic orogenic belts: however, several Precambrian ophiolites with ages ranging from 600 million years to about 2 billion years have also been described. Age gaps between the deposition of pelagic sediments and ophiolite emplacement are probably less than 25 million years. Such a short duration is consistent with the life span of a back-arc basin (less than 20 million years), and indicates that obduction of many ophiolites occurred soon after their creation. Ophiolites consequently represent young oceanic lithosphere that was detached while still hot.  See also: Lead isotopes (geochemistry); Rock age determination; Zircon

Hydrothermal alteration and formation of massive sulfide deposits

Immediately after new oceanic crust forms, seawater percolates downward through fractures and faults from the flanks of the rifted valley. It easily penetrates the crust down to 2–3 km (1–2 mi) at the base of the dike complex. Heated to 400–450°C (752–842°F), the water circulates toward the ridge and starts to ascend, becoming progressively channeled, and finally discharges along the rift axes at temperatures up to 380°C (716°F). During this high-temperature circulation, the hydrothermal solution alters and corrodes the crustal rocks, dissolving metals. When this discharged hydrothermal solution, consisting of metal sulfides leached from the oceanic crust, meets the cold seawater on the ocean floor, a black cloud of metallic sulfides results. Such hot springs are known as black smokers. Subsequent precipitation at, around, and beneath these chimneys can produce substantial amounts of massive sulfide ores in the upper parts of the extrusive sequence (Fig. 1). Such processes for precipitation of massive sulfide deposits were observed on the present-day ocean floor at locations along Pacific, Atlantic, and Indian ocean ridges.  See also: Hydrothermal vent

Figure 2 shows a black smoker spewing dark, mineral-rich fluids as observed by scientists during a dive of the deep-sea submersible Alvin on the East Pacific Rise (latitude 21°N) in 1979. The dissolved minerals and the heat given off by the black smoker favor the prolific growth of chemoautotrophic bacteria which, in turn, provide nourishment for the surrounding colonies. Around the hot springs, abundant and unusual sea life—giant tubeworms, huge clams, and mussels—has been discovered.  See also: Marine microbiology

Fig. 2  View of the first high-temperature geothermal vent (380°C or 716°F) ever seen by scientists during a dive of the deep-sea submersible Alvin on the East Pacific Rise in 1979. (Photograph by Dudley Foster, RISE Expedition, courtesy of W. R. Normark, U.S. Geological Survey)

In the upper parts of the extrusive sequences of many ophiolites, thin lenses (up to 50 m or 165 ft thick and ∼500 m or 1650 ft across) of sulfide ore occur in some fault-controlled depressions. The ore lenses consist of massive sulfide minerals, including pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite (ZnS), pyrrhotite (FeS), and minor molybdenite (MoS2), and have been mined as major copper deposits. Such copper-zinc deposits are presently exploited in the basalts of ophiolite sequences. The best-known copper deposit is located on Cyprus and is associated with the genesis of the Troodos ophiolites, and it was known to the ancient Greeks, who named the island after the metal (cupros, or copper). These Cyprus deposits contain a total reserve of 1.5 megatons of copper. A similar, contemporaneous mineralization of the Kuroko-type black-ore deposits in Japan was generated along embryonic oceanic ridges, which developed in a back-arc setting.  See also: Copper

The circulation of heated seawater through cooling, fractured crustal sections also causes significant alteration of the primary minerals. Formation of secondary minerals, including clay minerals and zeolites in basaltic rocks, is selective and incomplete and increases the volatile content of oceanic metabasalts. Hydrothermal metamorphism at depths results in the formation of epidote, chlorite, and amphiboles in the gabbroic sequence, and minor serpentinization of the ultramafic rocks.  See also: Serpentinite

Summary

Ophiolites represent new oceanic crust formed in a variety of spreading environments, including oceanic ridge, back-arc basin, and island arcs above a subduction zone, and subsequently emplaced onto the continents. Their occurrence along plate sutures marks the sites of ancient tectonic interaction between oceanic and continental crust. Ophiolites can form in multitude of tectonic settings, and the process of new ocean crust generation in spreading centers can produce different magma types. Depending on the rate of influx of ascending asthenosphere and the spreading rate of the oceanic lithosphere, ophiolites may not consist of plutonic and extrusive sequences resulting from a single magmatic pulse, but several, evidenced by the many intrusive relations. For example, the plagiogranites of the best-preserved Oman ophiolite are the product of fractional crystallization of the second magmatic event. Similarly, many Tethyan ophiolites, including the one in Oman, are characterized by thick harzburgites and well-layered gabbros resulting from fast spreading and large magma chambers. Many Circum-Pacific ophiolites, such as the Trinity ophiolite of California, have relatively thin lherzolitic mantle sequences, which are dismembered and recrystallized as a result of slow spreading at their ridges. In spite of extensive research efforts, it is still difficult to establish what kind of oceanic crust some ophiolites represent, what the actual processes were that formed ophiolites in spreading systems, how particular ophiolites formed in various tectonic settings compared with the oceanic lithosphere, and how they were emplaced onto the continental margins. Nevertheless, ophiolites provide the best opportunity for geologists to walk across the ocean floor on land; they also offer vertical sections in addition to horizontal distributions. Moreover, ophiolite formations record the ages of oceanic fragments that escaped disappearing into subduction zones.