Eclogitization

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Fig. 1 Metamorphic facies (bodies of rock with specific characteristics). Note the eclogite facies, which forms at the highest pressures. Metamorphic facies EN.svg
Fig. 1 Metamorphic facies (bodies of rock with specific characteristics). Note the eclogite facies, which forms at the highest pressures.

Eclogitization is the tectonic process in which the high-pressure, metamorphic facies, eclogite (a very dense rock), is formed. This leads to an increase in the density of regions of Earth's crust, which leads to changes in plate motion at convergent boundaries (where rock sinks beneath other rock).

Contents

Relationship to slab pull

There is the argument that collision between two continents should slow down because of continental buoyancy, and that for convergence to continue, it should do so at a new subduction zone where oceanic crust can be consumed. [1] Certain areas such as the Alps, Zagros, and Himalayas (where continental collisions have continued for tens of millions of years, in the middle of land, creating mountain ranges) contradict this argument, and have led geologists to propose a continental undertow that continues subduction. This continental undertow is explained by the slab pull concept. Slab pull is the concept that plate motion is driven by the weight of cool, dense plates and that heavier plates will begin to subduct. [2] Once a descending slab is disconnected there must be a force that continues subduction. Eclogitization is the mechanism for continuing subduction after slab detachment in a subduction zone. [1]

Geologic setting and effect of eclogitization

Fig 2. Eclogitization Schematic showing slab detachment within mantle and area of eclogitization and densification of subducting crust, which is a possible explanation for continental "undertow" Eclogitization schematic 1.png
Fig 2. Eclogitization Schematic showing slab detachment within mantle and area of eclogitization and densification of subducting crust, which is a possible explanation for continental "undertow"

Eclogitization typically occurs at two locations in a collisional fold mountain (fig 2): in the subduction of crust and at the base of the crustal root of the overriding crust. [3] At these zones high pressures are reached, as well as medium to high temperatures, and eclogitization commences. Metamorphic re-crystallization during burial can lead to a significant density increase (up to 10% in the case of eclogitization), [4] meaning approximately 300–600 kg/m3 of crustal rocks and continental lower crust and oceanic crust reach higher density than the mantle. [5]

This density increase acts as the main driver in the convection of Earth's mantle. It also explains the disconnection of a tectonic unit from the descending lithosphere, subsequent continuation of subduction, and the exhumation following subduction. [1]

Localities

Eclogitization is difficult to study because the rocks are rare: eclogites constitute only a very minor volume of continental basement exposed today at Earth's surface. [6] The few areas that are available to study eclogitization and view eclogites include garnet peridotites in Greenland and in other ophiolite complexes. Examples are also known in Saxony, Bavaria, Carinthia, Norway and Newfoundland. A few eclogites also occur in the northwest highlands of Scotland and the Massif Central of France. Glaucophane-eclogites occur in Italy and the Pennine Alps. Occurrences exist in western North America, including the southwest [7] and the Franciscan Formation of the California Coast Ranges. [8] Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Recently, coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya. Although limited localities are available to study, these areas provide the crucial samples to understand exhumation as well as continued subduction by continental "undertow."

Fluid influence on eclogitization

Fluids, rather than pressure and temperature conditions, are the key thing that makes the process of eclogitization, and the delamination (falling away) of crustal roots, in collisional orogens (fold mountains), possible. Partially eclogitized amphibolites, gabbros, and granulites from the Western Gneiss Region of Norway, the Marun-Keu Complex in the polar Ural Mountains, and the Dabie-Sulu belt in China demonstrate that fluid is required for complete eclogitization. [3] In these locations, eclogite occurs alongside unreacted rocks subjected to the same temperatures and pressures, with the eclogite forming where fluid can reach, for example along fractures.

An influx of fluids into the subduction zone or from the underlying mantle is vital for these metamorphic reactions to continue – fluids play a much more significant role in eclogite metamorphism than either temperature or pressure. [9] Without H2O, reactions will not proceed to completion, leaving metamorphic rocks metastable (stuck in an incomplete state) at unexpectedly high temperatures and pressures. Without the metamorphosis of less dense rocks to eclogite, which is eclogitization, continental "undertow" may be hindered, and subduction may be slowed down, or even eventually stop.

Field studies and simulations

Fig. 3 Cartoon Cross Section depicting tectonic evolution of eclogite terrain i.e. Laurentia and Baltic collision A)Early collisional phase with initial eclogitization of transitional margin between Laurentia and Baltica B)Continental Subduction C)Extension and exhumation where eclogites become exposed. Green eclogite symbols represent areas of active eclogitization and white symbols represent eclogites passing through retrograde conditions. Cartoon Cross Section depicting tectonic evolution of eclogite terrain.jpg
Fig. 3 Cartoon Cross Section depicting tectonic evolution of eclogite terrain i.e. Laurentia and Baltic collision A)Early collisional phase with initial eclogitization of transitional margin between Laurentia and Baltica B)Continental Subduction C)Extension and exhumation where eclogites become exposed. Green eclogite symbols represent areas of active eclogitization and white symbols represent eclogites passing through retrograde conditions.
  1. The force required for convergence at a constant velocity is significantly reduced when eclogitization has taken place, compared to models without eclogitization. [11]
  2. Models have shown that eclogitization does not impact subduction initiation, but eclogitized oceanic crust contributes to the slab negative buoyancy and could help the subduction of young oceanic lithosphere. [11]
  3. The consequences of eclogitization depend heavily on the temperature within the Mohorovičić discontinuity (MOHO) and decoupling in the crust.

Related Research Articles

Metamorphic rock Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock, in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Orogeny The formation of mountain ranges

Orogeny is the primary mechanism by which mountains are formed on continents. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. This leads to both structural deformation and compositional differentiation of Earth's lithosphere. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis. A synorogenic process or event is one that occurs during an orogeny.

Subduction A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.

Metamorphism Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).

Convergent boundary Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Eclogite A dense metamorphic rock formed under high pressure

Eclogite is a metamorphic rock formed when mafic igneous rock is subjected to high pressure. Eclogite forms at pressures greater than those typical of Earth's crust. An unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

Forearc The region between an oceanic trench and the associated volcanic arc

A forearc is the region between an oceanic trench and the associated volcanic arc. Forearc regions are found at convergent margins, and include any accretionary wedge and forearc basin that may be present. Due to tectonic stresses as one tectonic plate rides over another, forearc regions are sources for great thrust earthquakes.

Blueschist Metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition

Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

Continental collision Phenomenon in which mountains are produced on the boundaries of converging tectonic plates

In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. Continental collision is only known to occur on Earth.

Rock cycle Transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

Grenville orogeny

The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador to Mexico, as well as to Scotland.

In geology ultrahigh-temperature metamorphism (UHT) is extreme crustal metamorphism with metamorphic temperatures exceeding 900 °C. Granulite-facies rocks metamorphosed at very high temperatures were identified in the early 1980s, although it took another decade for the geoscience community to recognize UHT metamorphism as a common regional phenomenon. Petrological evidence based on characteristic mineral assemblages backed by experimental and thermodynamic relations demonstrated that Earth's crust can attain and withstand very high temperatures (900–1000 °C) with or without partial melting.

Ultra-high-pressure metamorphism refers to metamorphic processes at pressures high enough to stabilize coesite, the high-pressure polymorph of SiO2. It is important because the processes that form and exhume ultra-high-pressure (UHP) metamorphic rocks may strongly affect plate tectonics, the composition and evolution of Earth's crust. The discovery of UHP metamorphic rocks in 1984 revolutionized our understanding of plate tectonics. Prior to 1984 there was little suspicion that continental rocks could reach such high pressures.

High pressure metamorphic terranes along the Bangong-Nujiang Suture Zone

High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.

Archean subduction is a contentious topic involving the possible existence and nature of subduction in the Archean, a geologic eon extending from 4.0-2.5 billion years ago. Until recently there was little evidence unequivocally supporting one side over the other, and in the past many scientists either believed in shallow subduction or its complete non-existence. However, the past two decades have witnessed the potential beginning of a change in geologic understanding as new evidence is increasingly indicative of episodic, non-shallow subduction.

Paired metamorphic belts are sets of parallel linear rock units that display contrasting metamorphic mineral assemblages. These paired belts develop along convergent plate boundaries where subduction is active. Each pair consists of one belt with a low-temperature, high-pressure metamorphic mineral assemblage, and another characterized by high-temperature, low-pressure metamorphic minerals.

Subduction zone metamorphism Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

In geology, the term exhumation refers to the process by which a parcel of rock, approaches Earth's surface.

Sveconorwegian orogeny

The Sveconorwegian orogeny was an orogenic system active 1140 to 960 million years ago and currently exposed as the Sveconorwegian orogenic belt in southwestern Sweden and southern Norway. In Norway the orogenic belt is exposed southeast of the front of the Caledonian nappe system and in nappe windows. The Sveconorwegian orogen is commonly grouped within the Grenvillian Mesoproterozoic orogens. Contrary to many other known orogenic belts the Sveconorwegian orogens eastern border does not have any known suture zone with ophiolites.

South China Craton

The South China Craton or South China Block is one of the Precambrian continental blocks in China. It is traditionally divided into the Yangtze Block in the NW and the Cathaysia Block in the SE. The Jiangshan–Shaoxing Fault represents the suture boundary between the two sub-blocks. Recent study suggests that the South China Block possibly has one more sub-block which is named the Tolo Terrane. The oldest rocks in the South China Block occur within the Kongling Complex, which yields zircon U–Pb ages of 3.3–2.9 Ga.

References

  1. 1 2 3 Alvarez, Walter (May 22, 2010). "Protracted continental collisions argue for continental plates driven by basal traction". Earth and Planetary Science Letters. 296 (3–4): 434–442. Bibcode:2010E&PSL.296..434A. doi:10.1016/j.epsl.2010.05.030.
  2. Schellart, W. P.; Stegman, D. R.; Farrington, R. J.; Freeman, J.; Moresi, L. (16 July 2010). "Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab". Science. 329 (5989): 316–319. Bibcode:2010Sci...329..316S. doi:10.1126/science.1190366. PMID   20647465.
  3. 1 2 Leech, Mary L. (February 15, 2001). "Arrested orogenic development: eclogitization, delamination, and tectonic collapse". Earth and Planetary Science Letters. 185 (1–2): 149–159. Bibcode:2001E&PSL.185..149L. doi:10.1016/S0012-821X(00)00374-5.
  4. Jolivet, L; et al. (June 6, 2005). "Softening Triggered by Eclogitization, the first step towards exhumation during continental subduction" (PDF). Earth and Planetary Science Letters. 237 (3–4): 533–545. Bibcode:2005E&PSL.237..532J. doi:10.1016/j.epsl.2005.06.047 . Retrieved October 11, 2012.
  5. Doin, Marie- Pierre; et al. (December 2001). "Subduction initiation and continental crust recycling: the roles of rheology and eclogitization". Tectonophysics. 342 (1–2): 163–191. Bibcode:2001Tectp.342..163D. doi:10.1016/S0040-1951(01)00161-5.
  6. 1 2 3 Steltonphol, Mark; et al. (September 15, 2010). "Eclogitization and exhumationof Caledonian continental basementin Lofoten North Norway". Geologic Society of America. pp. 202–218. Retrieved October 12, 2012.
  7. William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages ISBN   1-897799-85-3
  8. C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham
  9. Austrheim, H. (1998). "Influence of fluid and deformation on metamorphism of the deep crust and consequences for the geodynamics of collision zones". In Bradley R. Hacker; Juhn G. Liou (eds.). When Continents Collide: Geodynamics and Geochemistry of Yltrahigh-pressure Rocks. Petrology and Structural Geology. 10. Kluwer Academic Publishers. pp. 297–323. doi:10.1007/978-94-015-9050-1_12. ISBN   978-90-481-4028-2.
  10. 1 2 Austrheim, H.; Griffin, W.L. (1985). "Shear deformation and eclogite formation with the granulite-facies anorthosites of the Bergen, Western Norway". Chem. Geol. 50 (1–3): 267–281. doi:10.1016/0009-2541(85)90124-x.
  11. 1 2 Doin, Marie-Pierre; et al. (December 2001). "Subduction initiation and continental crust recycling: the roles of rheology and eclogitization". Tectonophysics. 342 (1–2): 163–191. Bibcode:2001Tectp.342..163D. doi:10.1016/S0040-1951(01)00161-5.