Ophiolites
Encyclopedia
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
al rocks. Ophio- is Greek for "snake" (ὄφις), -lite means "stone" from the Greek lithos (λίθος), after the often green-colored rocks (spilite
Spilite
Spilite is a fine-grained igneous rock, resulting particularly from alteration of oceanic basalt.The term was introduced into the geological literature by Alexandre Brongniart in 1827...
s and serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s) that make up many ophiolites.
The term ophiolite was originally used by Alexandre Brongniart
Alexandre Brongniart
Alexandre Brongniart was a French chemist, mineralogist, and zoologist, who collaborated with Georges Cuvier on a study of the geology of the region around Paris...
for an assemblage of green rocks (serpentine, diabase
Diabase
Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...
) in the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
; Steinmann later modified its use to include serpentine, pillow lava
Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
, and chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
("Steinmann's trinity"), again based on occurrences in the Alps. The term was little used in other areas until the late 1950s to early 1960s, with the recognition that this assemblage provided an analog for oceanic crust and the process of seafloor spreading
Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
. This recognition was tied to two events: (1) the observation of magnetic anomaly stripes on the seafloor, parallel to oceanic ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
systems, interpreted by Frederick Vine and Drummond Matthews
Drummond Matthews
Drummond Hoyle Matthews FRS was a British marine geologist and geophysicist and a key contributor to the theory of plate tectonics...
to represent the formation of new crust at the oceanic ridge and its subsequent symmetric spreading away from that ridge, and (2) the observation of a sheeted dike complex
Sheeted dyke complex
A sheeted dyke complex or sheeted dike complex is a normal component of an ophiolite, a piece of oceanic crust that has been emplaced within a sequence of continental rocks. The dykes act as feeders for the overlying sequence of extrusive rocks typically pillow lavas, together forming layer 2 of...
within the Troodos ophiolite
Troodos Mountains
Troodos is the biggest mountain range of Cyprus, located in the center of the island. Troodos' highest peak is Mount Olympus at 1,952 metres.The Troodos mountain range stretches across most of the western side of Cyprus...
(Cyprus
Cyprus
Cyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
) by Ian Graham Gass
Ian Graham Gass
Ian Graham Gass, FRS, geologist, was Professor of Earth Sciences and Head of Discipline at the Open University, Milton Keynes and he was President of the IAVCEI . He was married to Mary Pearce ....
and co-workers, which must have formed by repetitive extension of crust and intrusion of magma resulting in a formation consisting of 100% dikes with no older wall rocks preserved within the complex. Moores and Vine concluded that the sheeted dike complex at Troodos could only form by a process similar to the seafloor spreading proposed by Vine and Matthews. Thus, it became widely accepted that ophiolites represent oceanic crust that had been emplaced on land.
Their great significance relates to their occurrence within mountain belts such as the Alps or the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
. This insight was one of the founding pillars of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.
Stratigraphy and definition
The stratigraphicStratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
sequence observed in ophiolites corresponds to the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
-forming processes at mid-oceanic ridges:
- Sediments: muds (black shaleShaleShale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
) and cherts deposited since the crust formed. - Extrusive sequence: basaltBasaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic pillow lavaPillow lavaPillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...
s show magmaMagmaMagma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
/seawater contact. - Sheeted dikes: vertical, parallel dikesDike (geology)A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
which fed lavas above. - High level intrusives: isotropic gabbroGabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
, indicative of fractionated magma chamber. - Layered gabbro, resulting from settling out of minerals from a magma chamber.
- Cumulate peridotite: duniteDuniteDunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
-rich layers of minerals that settled out from a magma chamber. - Tectonized peridotite: harzburgite/lherzolitePeridotiteA peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
-rich mantle rock.
An international conference on ophiolites in 1972 redefined the term ophiolite to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on. This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program
Integrated Ocean Drilling Program
The Integrated Ocean Drilling Program is an international marine research program. The program uses heavy drilling equipment mounted aboard ships to monitor and sample sub-seafloor environments...
and other research cruises have shown that in situ ocean crust can be quite variable, and that in places volcanic rocks sit directly on peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
tectonite
Tectonite
Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid flow during formation. Planar foliation results from a parallel orientation of...
, with no intervening gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
s.
Research
Scientists have only drilled about 1.5 km into the 6–7 km thick oceanic crust, so their understanding of oceanic crust largely comes from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust has a layered velocity structure that implies a layered rock series similar that listed above. In detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. Seismic sounding of mid-ocean ridges has only revealed 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 fluid through young oceanic crust causes serpentinization
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
, alteration
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
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
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
and serpentine, respectively. Often, ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
bodies such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
-rich sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
deposits are found above highly altered epidosite
Epidosite
Epidosite is a highly altered epidote and quartz bearing rock. It is the result of slow hydrothermal fluid alteration of basalt that occurs below the black smokers within mid-oceanic ridge spreading centers. Similar altered rocks have been identified within ophiolite complexes and associated...
s (epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
-quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
rocks) that are evidence of (the now 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. Compositional differences regarding silica (SiO2) and titania (TiO2) contents, for example, place ophiolite basalts in the domain of subduction zones (~55% silica, <1% TiO2), 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 (that is, chemical elements occurring amounts of 1000 ppm or less). In particular, trace elements associated with subduction zone (island arc) 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 .
The crystallization order of feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
in the gabbros is unexpectedly 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" (SSZ) ophiolite in the 1980s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Ironically, some of the classic ophiolite occurrences used to relate ophiolites to seafloor spreading (Troodos in Cyprus, Semail in Oman) 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 crust, 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 continental crust with forearc lithosphere (ophiolite) on top of it. Ophiolites with compositions comparable with hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
-type eruptive settings or normal mid-oceanic ridge basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.
Ophiolite groups and assemblages
Continental margin
The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
more or less intact (Tethys is the name given to the ancient sea that once separated Europe and Africa). Cordilleran ophiolites are characteristic of those that occur in the mountain belts of western North America (the "Cordillera" or backbone of the continent). These ophiolites sit on subduction zone accretionary complexes (subduction complexes) and have no association with a passive continental margin. These include the Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America. Despite their differences in mode of emplacement, both types of ophiolite are exclusively SSZ in origin.
Ophiolite assemblages in the Alps and some other collisional mountain belts are not formed during subduction, but rather represent the thinned margin of the continent that forms during rifting and continental drift. This incipient ocean crust remains locked to the continental margin when the ocean basin closes, emplacing the incipient ocean crust into the collision zone.
Interestingly, the age of ophiolite formation is often surprisingly close to the age of their emplacement into the continental crust. Ophiolites are found in all the major mountain belts of the world whether collisional (e.g. Himalayas) or not (e.g. Andes). The subduction-related chemistry of ophiolites and their association with mountain belts suggests that their formation and emplacement are related to oceanic closure and continental collision (final stages of the Wilson Cycle
Supercontinent cycle
The supercontinent cycle describes the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being...
) rather than oceanic opening and seafloor spreading as was first thought.
Furthermore, the occurrence of ophiolites throughout Earth history is not constant but rather they were formed and emplaced at specific intervals. These intervals correspond closely to times of super-continent break-up and dispersal—not because they form at the ridges that separate the drifting continents, but because the large ocean basin that must coexist with any super-continent must subduct along new subduction zones as rifting progresses.
Notable ophiolites
Examples of ophiolites that have been influential in the study of these rocks bodies are:- Semail Ophiolite in OmanOmanOman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...
and the United Arab EmiratesUnited Arab EmiratesThe United Arab Emirates, abbreviated as the UAE, or shortened to "the Emirates", is a state situated in the southeast of the Arabian Peninsula in Western Asia on the Persian Gulf, bordering Oman, and Saudi Arabia, and sharing sea borders with Iraq, Kuwait, Bahrain, Qatar, and Iran.The UAE is a...
, widely considered one of the best exposed ophiolite sequences - Troodos Ophiolite in the Troodos MountainsTroodos MountainsTroodos is the biggest mountain range of Cyprus, located in the center of the island. Troodos' highest peak is Mount Olympus at 1,952 metres.The Troodos mountain range stretches across most of the western side of Cyprus...
of CyprusCyprusCyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
, in these case its study has partly been economically motivated as the copper deposits of Cyprus are part of the ophiolite. - Bay of Islands Ophiolite in Gros Morne National ParkGros Morne National ParkGros Morne National Park is a world heritage site located on the west coast of Newfoundland. At , it is the second largest national park in Atlantic Canada ....
, Newfoundland, named a UNESCOUNESCOThe United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations...
World Heritage SiteWorld Heritage SiteA UNESCO World Heritage Site is a place that is listed by the UNESCO as of special cultural or physical significance...
in 1987 because of its superbly exposed complete ophiolite stratigraphic sequence - Yakuno, Horokanai, and Poroshiri, three full ophiolite sequences in JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
- Dun Mountain Ophiolite Belt, South Island, New ZealandNew ZealandNew Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. The rocktype duniteDuniteDunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
is named after this locality.