What is the stratigraphic sequence of an ophiolite?
Stratigraphic sequence of an ophiolite. The stratigraphic -like sequence observed in ophiolites corresponds to the lithosphere -forming processes at mid-oceanic ridges : Sediments: muds (black shale) and cherts deposited since the crust formed. Extrusive sequence: basaltic pillow lavas show magma /seawater contact.
What is ophiolite?
Ophiolite literally means snakestone. In modern usage it refers to an intact section of oceanic lithosphere that has been thrust onto a continental margin. A typical section of oceanic lithosphere consists of ultramafic rock and serpentinite near its base, overlain by gabbro and basalt, with bedded chert or turbidite at the top.
How thick is the ophiolite layer?
These layers, from the base to the top, include tectonized and cumulate peridotites, layered and massive gabbro, dikes, basaltic lavas, and volcanic rocks. The entire sequence of ophiolite layers is about 15 km thick.
Why are ophiolites so similar to peridotite?
And when hot peridotite reacts with seawater, the product is the soft and slippery serpentinite that is so common in ophiolites. This detailed resemblance led geologists in the 1960s to a working hypothesis: ophiolites are tectonic fossils of the ancient deep seafloor.
Where do ophiolite sequences originate?
Many geologists believe that ophiolites formed at oceanic ridges were emplaced by tectonic forces at convergent plate boundaries and then became exposed in highly deformed orogenic (mountain) belts. In fact, the same sequences of rocks were first reported in the Alps and were considered…
What does ophiolite consist of?
An ophiolite sequence consists of variably altered oceanic rocks, including marine sediments, ocean crust, and part of the mantle. The name ophiolite means “snakestone” from “ophio” (snake) and “lithos” (stone) in Greek.
What are the 3 main layers in an ophiolite?
Sediments (layer 1) Volcanic Crust (layer 2) Plutonic Crust (layer 3) Mantle Harzburgite (layer 4) 4 layers distinguished from Geophysics: Typical Ophiolite: 1.
What type of rock is ophiolite?
Origin and evolution of the concept In the first, he used ophiolite for serpentinite rocks found in large-scale breccias called mélanges. In the second publication, he expanded the definition to encompass a variety of igneous rocks as well such as gabbro, diabase, ultramafic and volcanic rocks.
What is an ophiolite quizlet?
An ophiolite is a section of oceanic crust and upper mantle (oceanic lithosphere) that has been transported along thrust faults and uplifted and emplaced on the margins of a continent or other land mass.
Where is ophiolite located in the Philippines?
Abstract. The Zambales Ophiolite Complex (ZOC) in the western portion of Luzon Island, Philippines represents a typical exposure of an emplaced crust-upper mantle section of an ancient lithosphere.
What are ophiolite suites?
[ ŏf′ē-ə-līt′, ō′fē- ] A sequence of rocks consisting of deep-sea marine sediments overlying (from top to bottom) pillow basalts, sheeted dikes, gabbro, dunite, and peridotite.
What rocks are included in an ophiolite sequence list them stratigraphically from bottom to top?
The stratigraphic sequence observed in an ophiolite corresponds to layered oceanic crust: from top to bottom, an upper layer of oceanic sediment (siliceous or carbonate ooze); a layer of pillow basalt; a layer of sheeted near-vertical dykes; layers of gabbro and plagioclase and mafic cumulates; and a basal layer of “ ...
How does an ophiolite form at the subduction zone?
Supra-subduction zone (SSZ) ophiolites have the geochemical characteristics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere.
Why is it rare to see an entire ophiolite sequence in an accreted terrane?
It is rare to find ophiolites on land because oceanic crust is usually dragged down into the mantle at subduction zones.
What is the best preserved and largest ophiolite?
Jormua ophioliteThe Jormua ophiolite is the best preserved one along a larger chain of ophiolites that occur within the Kainuu Schist Belt. The crustal rocks of the ophiolite include pillow lava, hyaloclastite, sheeted dykes, gabbro, plagiogranite and ultramafic cumulate rock.
What do xenoliths mean?
A xenolith is a piece of rock trapped in another type of rock. Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. Magma is the molten rock beneath the Earths crust that emerges as lava during a volcanic eruption.
What is an ophiolite?
The Penrose field 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 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 tectonite, with no intervening gabbros .
What are the two components of ophiolite formation?
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.
How is ophiolite subduction explained?
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 (over the accretionary wedge) 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.
Why do ophiolites have fore arcs?
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 (ophiolite) 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.
Why did researchers conduct a seismic study on an ophiolite complex?
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.
What is the upper mantle of the Earth?
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
Where did the term "ophiolite" come from?
The term ophiolite originated from publications of Alexandre Brongniart in 1813 and 1821. In the first, he used ophiolite for serpentinite rocks found in large-scale breccias called mélanges. In the second publication, he expanded the definition to encompass a variety of igneous rocks as well such as gabbro, diabase, ultramafic and volcanic rocks. Ophiolites thus became a name for a well-known association of rocks occurring in the Alps and Apennines of Italy. Following work in these two mountains systems, Gustav Steinmann defined what later became known as the "Steinmann Trinity": the mixture of serpentine, diabase - spilite and chert. The recognition of the Steinmann Trinity served years later to build up the theory around seafloor spreading and plate tectonics. A key observation by Steinmann was that ophiolites were associated to sedimentary rocks reflecting former deep sea environments. Steinmann himself interpreted ophiolites (the Trinity) using the geosyncline concept. He held that Alpine ophiolites were "submarine effusions issuing along thrust faults into the active flank of an asymmetrically shortening geosyncline". The apparent lack of ophiolites in the Peruvian Andes, Steinmann theorized, was either due to the Andes being preceded by a shallow geosyncline or representing just the margin of a geosyncline. Thus, Cordilleran-type and Alpine-type mountains were to be different in this regard. In Hans Stille 's models a type of geosyncline called eugeosynclines were characterized by producing an "initial magmatism" that in some cases corresponded to ophiolitic magmatism.
What is an ophiolite?
An ophiolite is a part of the Earth’s oceanic crust that is uplifted and exposed above sea level. Geologists first observed ophiolites in the European Alps. In 1821, Alexandre Brongniart used the name “ophiolite” to refer to an assemblage of green rocks that had serpentinite features on it. 150 years later, during the advent of plate tectonics, researchers adopted the name ophiolites to refer to the pieces of oceanic crusts attached to the continents. Ophiolites are important to geologists since they occur within mountain belts like the Himalayas and Alps, and help in identifying former ocean basins and are the center of the plate tectonic theory. The formation of the ophiolites did not occur all at once, but rather in intervals.
How thick is the ophiolite?
The entire sequence of ophiolite layers is about 15 km thick.
Where are the Cordilleran ophiolites found?
Examples of the Cordilleran ophiolites include the Josephine ophiolite found in the Klamath Mountains in Oregon and California, ...
Why are ophiolites important?
Ophiolites are important to geologists since they occur within mountain belts like the Himalayas and Alps, and help in identifying former ocean basins and are the center of the plate tectonic theory. The formation of the ophiolites did not occur all at once, but rather in intervals.
Where did ophiolites originate?
One theory suggests they are formed as ocean lithosphere. Researchers proved this theory by studying a an ophiolite complex in the Bay of Islands, Newfoundland.
What are the materials that make peridotite?
The materials that make the peridotite found at the base of the ophiolites is harzburgite rock. The dikes and volcanic rocks that form the cap of ophiolites result from the eruptions that occur on the ocean floor.
Where is the ophiolite sequence located?
Ophiolite sequences occupy a considerable part of eastern Sabah . The chromitite concentrations in the peridotite and serpentinized peridotite are too small for commercial mining (Bailey, 1963; Hutchison and Dhonau, 1971 ). The reason for the absence of large deposits from Sabah, as compared with the Zambales of nearby Philippines, is that only the upper layers of the Sabah ophiolite are exposed, and the chromitite layers are thin and podiform. They represent the lower ultrabasic cumulate part of the gabbro layer. By contrast the rich Zambales deposits occur well into the mantle sequence.
What is ophiolite in the ocean?
Ophiolite literally means snakestone. In modern usage it refers to an intact section of oceanic lithosphere that has been thrust onto a continental margin. A typical section of oceanic lithosphere consists of ultramafic rock and serpentinite near its base, overlain by gabbro and basalt, with bedded chert or turbidite at the top. Turbidite is a fine-grained sediment (or sedimentary rock) that gradually changes from coarse- to fine-grained and that was deposited by turbidity currents. It is rare for a complete, intact section of oceanic lithosphere to be thrust (obducted) onto a continent. The only complete section in the Appalachians is the Bay of Islands Ophiolite, which is spectacularly exposed in Gros Morne National Park in the Long Range Mountains of western Newfoundland. Fairly complete sections in the Cordillera include the Trinity ophiolite in the Klamath Mountains and the Coast Range ophiolite in the Coast Range of California. The obduction of intact oceanic lithosphere onto a continental margin implies that a subduction zone once existed at that location, but it does not imply collision between two terranes. Obduction can occur during the subduction process, and following obduction the subduction process could continue. Complications arise if collision with another terrane occurs long after obduction as depicted in Figure 20.11. In this case, a geologist must differentiate between the obduction event and the later collision event. This can be an especially difficult task if rocks representing the two events are intermingled.
What is the name of the rock that is uplifted and exposed above sea level?
Ophiolite is thrust sheets of ancient oceanic crust and upper part of mantle rocks that have been uplifted and exposed above sea level and often emplaced on top of the continental lithosphere.
What is the granular texture of mantle peridotite?
Figure 4.12. The photomicrograph of mantle peridotite showing crushed to fine-grained matrix with a granular texture of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) and resembled harzburgite. Serpentinization has taken place mostly around uncrushed olivine grains. The ophiolite environment is for chromium and associated mineral deposits.
Where is the only complete section of the oceanic lithosphere?
The only complete section in the Appalachians is the Bay of Islands Ophiolite, which is spectacularly exposed in Gros Morne National Park in the Long Range Mountains of western Newfoundland.
Who made the study of the Sabah chromite?
Hutchison (1972) made a systematic study of the Sabah chromite. It shows a range of Al/Cr substitution and the unit cell (a,Å) edge varies systematically thus:
Can geochemical studies be supported by structural information?
In such ophiolites, one might question the results of geochemical studies that cannot be supported by any structural information. By contrast, in an ophiolite whose internal structure has been preserved and decrypted, the solution of further problems depends largely on dedicated geochemical studies.
What type of crust is a Franciscan ophiolite?
Caribbean-type ophiolites represent the subduction of oceanic plateaus or Large Igneous Provinces. Franciscan-type ophiolites are accreted pieces of oceanic crust scraped off the subducted plate onto the upper plate, as in Japan today.
How do ophiolites differ from seafloor crust?
Ophiolites differ from intact seafloor crust in some important ways, most notably in that they aren't intact. Ophiolites are almost always broken apart, so the peridotite, gabbro, sheeted dikes and lava layers don't stack up nicely for the geologist. Instead, they are usually strewn along mountain ranges in isolated bodies. As a result, very few ophiolites have all the parts of the typical oceanic crust. Sheeted dikes are usually what is missing.
What is the name of the assemblage of serpentinite?
In 1821 Alexandre Brongniart named this assemblage ophiolite ("snake stone" in scientific Greek) after its distinctive exposures of serpentinite ("snake stone" in scientific Latin). Fractured, altered and faulted, with almost no fossil evidence to date them, ophiolites were a stubborn mystery until plate tectonics revealed their important role.
What is the product of hot peridotite?
And when hot peridotite reacts with seawater, the product is the soft and slippery serpentinite that is so common in ophiolites. This detailed resemblance led geologists in the 1960s to a working hypothesis: ophiolites are tectonic fossils of the ancient deep seafloor.
Which type of ophiolites formed during the interaction of two oceanic plates like today's Izu-?
Mediterranean-type ophiolites formed during the interaction of two oceanic plates like today's Izu-Bonin forearc.
Do ophiolites have all the crust?
As a result, very few ophiolites have all the parts of the typical oceanic crust. Sheeted dikes are usually what is missing. The pieces must be painstakingly correlated with each other using radiometric dates and rare exposures of the contacts between rock types.
What is the study of ophiolites?
oceanic crust. In oceanic crust: Study of ophiolites. Great strides in understanding the oceanic crust were made by the study of ophiolites. These are slices of the ocean floor that have been thrust above sea level by the action of plate tectonics.
What are ultramafic rocks?
…ultramafic rocks are known as ophiolites. Many geologists believe that ophiolites formed at oceanic ridges were emplaced by tectonic forces at convergent plate boundaries and then became exposed in highly deformed orogenic (mountain) belts. In fact, the same sequences of rocks were first reported in the Alps and were considered…
What is the name of the crust that is preserved on land?
In plate tectonics: Mountains by terrane accretion. …of ocean crust, known as ophiolites, are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land.
What is an ophiolite?
Wikimedia list article. Ophiolites are sequences of mafic to ultramafic rock generally believed to represent ancient oceanic lithosphere. They are distributed all across the world being all of them located at present or past orogenic belts, sites of mountain building processes.
Where are ophiolites found?
Ophiolites are common in orogenic belts of Mesozoic age, like those formed by the closure of the Tethys Ocean. Ophiolites in Archean and Paleoproterozoic domains are rare. A pillow lava from an ophiolite sequence, Northern Apennines, Italy.
Overview
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.
Several studies support the conclusion that ophiolites formed as oceanic lithosphere. Seismic vel…
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: muds (black shale) and cherts deposited since the crust formed.
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 c…
Groups and assemblages
Ophiolites are common in orogenic belts of Mesozoic age, like those formed by the closure of the Tethys Ocean. Ophiolites in Archean and Paleoproterozoic domains are rare.
Most ophiolites can be divided into one of two groups: Tethyan and Cordilleran. Tethyan ophiolites are characteristic of those that occur in the eastern Mediter…
Origin and evolution of the concept
The term ophiolite originated from publications of Alexandre Brongniart in 1813 and 1821. In the first, he used ophiolite for serpentinite rocks found in large-scale breccias called mélanges. In the second publication, he expanded the definition to encompass a variety of igneous rocks as well such as gabbro, diabase, ultramafic and volcanic rocks. Ophiolites thus became a name for a well-known association of rocks occurring in the Alps and Apennines of Italy. Following work in these t…
Notable ophiolites
Examples of ophiolites that have been influential in the study of these rocks bodies are:
• Coast Range Ophiolite in the California Coast Ranges, from Santa Barbara through San Francisco Counties, California.
• Semail Ophiolite in Oman and the United Arab Emirates, widely considered one of the best exposed ophiolite seq…
External links
• Ishiwatari, A. (2001). "Introduction to opholites". Kanazawa University. Retrieved 26 July 2016.
• Shervais, J. W. (2001). "Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites" (PDF). Geochemistry, Geophysics, Geosystems. 2 (1): n/a. Bibcode:2001GGG.....2.1010S. CiteSeerX 10.1.1.538.2375. doi:10.1029/2000gc000080. Retrieved 26 July 2016.
What Is An Ophiolite?
Components of An Ophiolite
- Ophiolites contain several layers of rocks. These layers, from the base to the top, include tectonized and cumulate peridotites, layered and massive gabbro, dikes, basaltic lavas, and volcanic rocks. The entire sequence of ophiolite layers is about 15 km thick. The materials that make the peridotite found at the base of the ophiolites is harzburgite rock. The dikes and volcani…
Ophiolite Groups and Assemblages
- Two broad categories of ophiolites exist: Cordilleran and Tethyan. Cordilleran ophiolites form in the mountain belts of the Cordillera (which is the name given to western North America). Their locations include subduction zones and have no relation to the passive continental margins. Examples of the Cordilleran ophiolites include the Josephine ophiolite found in the Klamath Mou…
Origin of Ophiolites
- Several theories attempt to explain the origin of ophiolites. One theory suggests they are formed as ocean lithosphere. Researchers proved this theory by studying a an ophiolite complex in the Bay of Islands, Newfoundland. The study revealed that ophiolitic and oceanic velocity structures were indeed identical. Hence, this proved that ophiolites originated as ocean crusts. The secon…
Typical Ophiolites and Where to Find them?
- Examples of areas that have ophiolites include Oman, California, New Guinea, Cyprus, and Newfoundland. The Samail ophiolite in southeastern Oman has been studied the most by researchers. Furthermore, some popular ophiolites include: Macquarie Island and Tasmania (Australia); Yakuno, Poroshiri, and Horokanai (Japan); and the Zambales ophiolite complex (Phili…