Anatexis (via Latin from Greek roots meaning "to melt down") is the partial melting of rocks. [1] Traditionally, anatexis is used specifically to discuss the partial melting of crustal rocks, while the generic term "partial melting" refers to the partial melting of all rocks, in both the crust and mantle.[ citation needed ]
Anatexis can occur in a variety of different settings, from zones of continental collision to mid-ocean ridges. [2] It is believed that anatexis is the process largely responsible for the formation of migmatites. [1] Furthermore, scientists have recently discovered that partial melting plays an increasingly important role in active crustal processes, including the advancement of active deformation and the emplacement of crustal granites. [3] As a result, active feedback between crustal shearing, melting, and granite emplacement [3] has become largely accepted in the place of large scale, unreasonable models involving fractional melting of the mantle into granitic batholiths and plutons. [4] Evidence for this can be seen in the physical, mineralogical, and isotopic signatures of countless granites. [5]
Crustal anatexis is not restricted to a single tectonic setting, but rather is controlled by four primary parameters: temperature, pressure, volatile content, and rock type/composition. [2] These parameters are highly variable and depend on depth, crustal thickness, and local variations of the Earth's geotherm. [2] [6] The amount and composition of partial melts likely varies locally, reflecting the heterogeneity of the Earth's crust. [6]
In order to induce crustal melting, the temperature must be increased past the normal geotherm. [2] [7] Possible sources of heat include primordial heat originating from the core of the Earth as well as the decay of radioactive elements. [7] This heat is distributed throughout the Earth's crust by a number of different processes, including radiation, conduction, convection, and advection. [7]
The emplacement of magmatic intrusions is also commonly associated with local increases in temperature. [2] [7] If the increase in temperature is sufficient, this can lead to partial melting of adjacent country rocks. [7] If partial melting does occur, then the degree of melting is controlled by the amount of available heat in the magmatic body. [7]
Beneath the Earth's surface, pressure increases with depth due to the accumulation of overlying rock. [7] At a given temperature, a decrease in pressure can result in localized melting. [7] Melting that is caused by a drop in pressure is referred to as decompression melting. [8] Decompression melting can occur in thickened portions of the Earth's crust and may be the result of a variety of processes, including erosion, tectonic denudation, and lithospheric thinning. [8]
The amount of water available in the system plays a major role in controlling the degree of melting at a given temperature. [2] [7] Low water availability will suppress melting. [1] Furthermore, the degree of water saturation of a system will affect the composition of any melt generated. [1] Water can be derived from a variety of sources, including from surrounding country rocks (pore water) or from the decomposition of hydrous minerals (e.g. micas, amphiboles). [2] Melting reactions involving water liberated from hydrous minerals are often referred to as dehydration melting reactions or vapour-absent reactions. [1] [2] Over time, dehydration melting reactions will consume all of the hydrous phases in a rock, meaning that the amount of melt generated through these reactions is controlled by the abundance and stability of specific hydrous phases. [2] Depending on the tectonic setting, water can also be introduced to the system through the dehydration of a subducting hydrated oceanic plate or magmatic underplating. [2]
The composition of a parent rock has a direct effect on the composition of the resulting melt. [2] Granitic melts are commonly classified based on the nature of their source rock. [2] One of the more popular classification schemes for granites was first introduced by White and Chappell in 1974. [2] This classification scheme categorizes granites based on whether they are the result of the melting of sedimentary rocks (S-type granites) or the melting of igneous rocks (I-type granites). [9] This genetic difference is reflected in the geochemical signature of the melts themselves. [2]
Where partial melting is associated with regional tectonics and differential stresses, the production of melt creates instabilities in pore spaces and eventually along grain-boundaries that localize strain into crustal-scale shear zones. [3] These zones promote melt flow out of the anatectic system as a mechanism to accommodate strain which in turn promotes more partial melting. The feedback loop that develops between the advancement of deformation and partial melting is referred to as syntectonic crustal anatexis. Syntectonic anatectic migmatites at Hafafit region, Eastern Desert, Egypt as a part of the Nubian Shield are a good example of such crustal melts. [10] [11]
Segregation of granitic melts from their residual solids begins with the onset of partial melting along the grain boundaries of reactant minerals, namely the ferromagnesian phases of micas and amphiboles. [3] Such reactions produce large positive volume changes within the metamorphic system causing melt enhanced embrittlement. [12] [4] This, coupled with an increasing melt fraction, alters the deformation mechanisms acting among grains and decreases the strength of the rock significantly. [3] Melt filled pores eventually coalesce and melt flow parallel to the elongation lineation of grains (or along planes of foliation) is promoted. [13] [3]
As a rock partially melts and begins to flow, its rheology changes significantly. Such changes will localize the strain created by regional tectonics and as per Le Chatelier's Principle, the system responds by pumping melt towards zones of dilatancy (lower pressure) thereby segregating the melt from its anatectic source on a local scale. [3] Where this has occurred and been preserved in the rock record, one can expect to see macroscopic melt-rich layers (leucosomes), and macroscopic residual solid layers (melanosomes). These layers will commonly be oriented parallel to the fabric of the host rock. As the amount of accumulated melt in the surrounding rock increases, melt will travel further from its source towards growing transverse structures such as the aforementioned embrittlement fractures. Eventually, this leads to the formation and development of an interconnected accumulation network. [13]
When the transport of melt occurs on larger scales, anatexis can lead to the ascent and emplacement of large granitic bodies in the upper crust. This transition is generally marked by the change from shear-driven melt migration to buoyancy-driven melt migration. This final step in the extraction process requires an optimal balance between melt fraction and melt distribution in the local rock. [13]
The ascent of this magma, while previously thought to have occurred as large, slow-rising and buoyant bodies, is now largely attributed to fast-moving narrow conduits and self-propagating dykes. [4] These faster moving models have overcome major thermal and mechanical problems embedded in older theories as well as the granite problem and near surface felsic volcanism. As the flow of rising magma then changes from vertical back to horizontal, emplacement is initiated. [4] This process is episodic and accommodated by both ongoing regional tectonics and emplacement-generated wall rock structures allowing the pluton to spread laterally and thicken vertically. Syntectonic anatectic migmatites at Hafafit region, Eastern Desert, Egypt, Nubian Shield provide an example of the close relation between orogeny (tectonic), metamorphism and granite generation and emplacement. [10] [11]
Granite is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.
Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles.
Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("neosome"), while the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance ("paleosome"). Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains.
Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.
A volcanic arc is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentine minerals. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.
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.
Restite is the residual material left at the site of melting during the in place production of granite through intense metamorphism.
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of magmas produced by igneous differentiation is known as a magma series.
Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.
In geology, an igneous intrusion is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and compositions, illustrated by examples like the Palisades Sill of New York and New Jersey; the Henry Mountains of Utah; the Bushveld Igneous Complex of South Africa; Shiprock in New Mexico; the Ardnamurchan intrusion in Scotland; and the Sierra Nevada Batholith of California.
Adakites are volcanic rocks of intermediate to felsic composition that have geochemical characteristics of magma originally thought to have formed by partial melting of altered basalt that is subducted below volcanic arcs. Most magmas derived in subduction zones come from the mantle above the subducting plate when hydrous fluids are released from minerals that break down in the metamorphosed basalt, rise into the mantle, and initiate partial melting. However, Defant and Drummond recognized that when young oceanic crust is subducted, adakites are typically produced in the arc. They postulated that when young oceanic crust is subducted it is "warmer" than crust that is typically subducted. The warmer crust enables melting of the metamorphosed subducted basalt rather than the mantle above. Experimental work by several researchers has verified the geochemical characteristics of "slab melts" and the contention that melts can form from young and therefore warmer crust in subduction zones.
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.
Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. This process creates a magma that has a higher proportion of the minerals that have melted than the original rock. Partial melting is an important part of the formation of all igneous rocks and some metamorphic rocks, as evidenced by a multitude of geochemical, geophysical and petrological studies.
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The Huangling Anticline or Complex represents a group of rock units that appear in the middle of the Yangtze Block in South China, distributed across Yixingshan, Zigui, Huangling, and Yichang counties. The group of rock involves nonconformity that sedimentary rocks overlie the metamorphic basement. It is a 73-km long, asymmetrical dome-shaped anticline with axial plane orientating in the north-south direction. It has a steeper west flank and a gentler east flank. Basically, there are three tectonic units from the anticline core to the rim, including Archean to Paleoproterozoic metamorphic basement, Neoproterozoic to Jurassic sedimentary rocks, and Cretaceous fluvial deposit sedimentary cover. The northern part of the core is mainly tonalite-trondhjemite-gneiss (TTG) and Cretaceous sedimentary rock called the Archean Kongling Complex. The middle of the core is mainly the Neoproterozoic granitoid. The southern part of the core is the Neoproterozoic potassium granite. Two basins are situated on the western and eastern flanks of the core, respectively, including the Zigui basin and Dangyang basin. Both basins are synforms while Zigui basin has a larger extent of folding. Yuanan Graben and Jingmen Graben are found within the Dangyang Basin area. The Huangling Anticline is an important area that helps unravel the tectonic history of the South China Craton because it has well-exposed layers of rock units from Archean basement rock to Cretaceous sedimentary rock cover due to the erosion of the anticline.
Tonalite–trondhjemite–granodiorite (TTG) rocks are intrusive rocks with typical granitic composition but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean TTG rocks are present in arc-related batholiths, as well as in ophiolites, while Archean TTG rocks are major components of Archean cratons.
The geology of Peru includes ancient Proterozoic rocks, Paleozoic and Mesozoic volcanic and sedimentary rocks, and numerous basins and the Andes Mountains formed in the Cenozoic.
The Dharwar Craton is an Archean continental crust craton formed between 3.6-2.5 billion years ago (Ga), which is located in southern India and considered as the oldest part of the Indian peninsula.
Volcanic and igneous plumbing systems (VIPS) consist of interconnected magma channels and chambers which are responsible for the production, storage and transportation of magma in Earth's crust. Volcanic plumbing systems can be found in all active tectonic settings, such as mid-oceanic ridges, subduction zones, and mantle plumes, when magmas generated in continental lithosphere, oceanic lithosphere, and in the sub-lithospheric mantle are transported. Magma is first generated by partial melting, followed by segregation and extraction from the source rock to separate the melt from the solid. As magma propagates upwards, a self-organised network of magma channels develops, transporting the melt from lower crust to upper regions. Channelled ascent mechanisms include the formation of dykes and ductile fractures that transport the melt in conduits. For bulk transportation, diapirs carry a large volume of melt and ascent through the crust. When magma stops ascending, or when magma supply stops, magma emplacement occurs. Different mechanisms of emplacement result in different structures, including plutons, sills, laccoliths and lopoliths.
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