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Anophiolite is a section of Earth'soceanic crust and the underlyingupper mantle that has been uplifted and exposed, and often emplaced ontocontinental 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 Greeklithos, meaning "stone".) Some ophiolites have a green color. The origin of these rocks, present in many mountainousmassifs, remained uncertain until the advent ofplate tectonic theory.
Their great significance relates to their occurrence withinmountain belts such as theAlps and theHimalayas, where they document the existence of formerocean basins that have now been consumed bysubduction. This insight was one of the founding pillars ofplate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.
Thestratigraphic-like sequence observed in ophiolites corresponds to thelithosphere-forming processes atmid-oceanic ridges. From top to bottom, the layers in the sequence are:
AGeological 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.[1] This definition has been challenged recently because new studies of oceanic crust by theIntegrated Ocean Drilling Program and other research cruises have shown thatin situ ocean crust can be quite variable in thickness and composition, and that in places sheeted dikes sit directly onperidotitetectonite, with no interveninggabbros.
Ophiolites have been identified in most of the world'sorogenic belts.[2] 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 densitycontinental crust.[3]
Several studies support the conclusion that ophiolites formed as oceaniclithosphere.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.[4] 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.[5] The temperature gradients of the metamorphosis of ophioliticpillow lavas anddykes are similar to those found beneath ocean ridges today.[5] Evidence from themetal-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 ashydrothermal vents.[5] 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.[5]Despite the above observations, there are inconsistencies in the theory of ophiolites as oceanic crust, which suggests that newly generated ocean crust follows the fullWilson 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 andstratigraphic dating has found ophiolites to have undergone emplacement when young and hot:[5] most are less than 50 million years old.[6]Ophiolites therefore cannot have followed the full Wilson cycle and are considered atypical ocean crust.
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 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 contact with air.[7]
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[8] suggest that an irregular continental margin colliding with anisland arc complex causes ophiolite generation in aback-arc basin andobduction due to compression.[9] The continental margin,promontories andreentrants 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.[9] 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.
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.[10] 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 (over theaccretionary wedge) bydetachment and compression.[10] Verification of the two above hypotheses requires further research, as do the other hypotheses available in current literature on the subject.
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 ofin 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.[citation needed]
The circulation ofhydrothermal fluids through young oceanic crust causesserpentinization,alteration of the peridotites and alteration of minerals in the gabbros and basalts to lower temperature assemblages. For example,plagioclase,pyroxenes, andolivine in the sheeted dikes and lavas will alter toalbite,chlorite, andserpentine, respectively. Often,ore bodies such asiron-richsulfide deposits are found above highly alteredepidosites (epidote-quartz rocks) that are evidence of relictblack smokers, which continue to operate within the seafloor spreading centers of ocean ridges today.[citation needed]
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 ofsilica (SiO2) andtitania (TiO2). Ophiolite basalt contents place them 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 oftrace elements as well (that is, chemical elements occurring in 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.[11]
Additionally, thecrystallization order offeldspar andpyroxene (clino- and orthopyroxene) 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 offore-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. Consequently, some of the classic ophiolite occurrences thought of as being related to seafloor spreading (Troodos inCyprus, Semail inOman) were found to be "SSZ" ophiolites, formed by rapid extension of fore-arc crust during subduction initiation.[12]
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 theaccretionary prism with fore-arc lithosphere (ophiolite) on top of it. Ophiolites with compositions comparable withhotspot-type eruptive settings or normalmid-oceanic ridgebasalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.[citation needed]
Ophiolites are common inorogenic belts ofMesozoic age, like those formed by theclosure of theTethys Ocean. Ophiolites inArchean andPaleoproterozoic domains are rare.[13]
Most ophiolites can be divided into one of two groups: Tethyan and Cordilleran. Tethyan ophiolites are characteristic of those that occur in the eastern Mediterranean sea area, e.g. Troodos in Cyprus, and in the Middle East, such as Semail in Oman, which consist of relatively complete rock series corresponding to the classic ophiolite assemblage and which have been emplaced onto a passivecontinental margin 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. They include theCoast Range ophiolite of California, the Josephine ophiolite of theKlamath Mountains (California, Oregon), and ophiolites in the southernAndes of South America. Despite their differences in mode of emplacement, both types of ophiolite are exclusively supra-subduction zone (SSZ) in origin.[14]
Based on mode of occurrences, theNeoproterozoic ophiolites appear to show characteristics of both mid-oceanic ridge basalt (MORB)-type and SSZ-type ophiolites and are classified from oldest to youngest into: (1) MORB intact ophiolites (MIO); (2) dismembered ophiolites (DO); and (3) arc-associated ophiolites (AAO) (El Bahariya, 2018). Collectively, the investigated ophiolites of theCentral Eastern Desert (CED) fall into both MORB/back-arc basin basalt (BABB) ophiolites and SSZ ophiolites. They are spatially and temporally unrelated, and thus, it seems likely that the two types are notpetrogenetically related. Ophiolites occur in different geological settings, and they represent change of the tectonic setting of the ophiolites from MORB to SSZ with time.
The termophiolite originated from publications ofAlexandre Brongniart in 1813 and 1821. In the first, he usedophiolite forserpentinite rocks found in large-scalebreccias calledmélanges.[15][16] In the second publication, he expanded the definition to encompass a variety ofigneous rocks as well such asgabbro,diabase,ultramafic andvolcanic rocks.[16] Ophiolites thus became a name for a well-known association of rocks occurring in theAlps andApennines of Italy.[16] Following work in these two mountains systems,Gustav Steinmann defined what later became known as the "Steinmann Trinity": the mixture ofserpentine,diabase-spilite andchert.[16] The recognition of the Steinmann Trinity served years later to build up the theory aroundseafloor spreading andplate tectonics.[17] A key observation by Steinmann was that ophiolites were associated tosedimentary rocks reflecting former deep sea environments.[16] Steinmann himself interpreted ophiolites (the Trinity) using thegeosyncline concept.[18] He held that Alpine ophiolites were "submarine effusions issuing along thrust faults into the active flank of an asymmetrically shortening geosyncline".[19] The apparent lack of ophiolites in the PeruvianAndes, Steinmann theorized, was either due to the Andes being preceded by a shallow geosyncline or representing just the margin of a geosyncline.[18] Thus, Cordilleran-type and Alpine-type mountains were to be different in this regard.[18] InHans Stille's models a type of geosyncline called eugeosynclines were characterized by producing an "initial magmatism" that in some cases corresponded to ophiolitic magmatism.[18]
Asplate tectonic theory prevailed in geology[1] and geosyncline theory became outdated[20] ophiolites were interpreted in the new framework.[1] They were recognized as fragments ofoceanic lithosphere, and dykes were viewed as the result ofextensional tectonics atmid-ocean ridges.[1][21] Theplutonic rocks found in ophiolites were understood as remnants of former magma chambers.[1]
In 1973,Akiho Miyashiro revolutionized common conceptions of ophiolites and proposed anisland arc origin for the famousTroodos Ophiolite inCyprus, arguing that numerous lavas and dykes in the ophiolite hadcalc-alkaline chemistries.[22]
Examples of ophiolites that have been influential in the study of these rocks bodies are:
