Related Research Articles
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.
Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.
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 an igneous, sedimentary, or existing metamorphic rock.
Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 to 200 °C, and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.
Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
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.
Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are classed as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral. The Anthophyllite crystal is characterized by its perfect cleavage along directions 126 degrees and 54 degrees.
Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.
Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness gives the rock schistosity Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).
Talc carbonates are a suite of rock and mineral compositions found in metamorphosed ultramafic rocks.
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.
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.
In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.
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.
The Zambezi Belt is an area of orogenic deformation in southern Zambia and northern Zimbabwe. It is a segment of a broader belt lying between the Congo Craton and the Kalahari Craton, which also includes the Lufilian Arc and the Damaran Belt. The eastern margin of the belt interacts with the north-south Eastern African orogen.
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.
The Pressure-Temperature-time path is a record of the pressure and temperature (P-T) conditions that a rock experienced in a metamorphic cycle from burial and heating to uplift and exhumation to the surface. Metamorphism is a dynamic process which involves the changes in minerals and textures of the pre-existing rocks (protoliths) under different P-T conditions in solid state. The changes in pressures and temperatures with time experienced by the metamorphic rocks are often investigated by petrological methods, radiometric dating techniques and thermodynamic modeling.
A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are plotted as metamorphic reaction boundaries in a pressure–temperature cartesian coordinate system to produce a petrogenetic grid for a particular rock composition. The regions of overlap of the stability fields of minerals form equilibrium mineral assemblages used to determine the pressure–temperature conditions of metamorphism. This is particularly useful in geothermobarometry.
Blue Ridge Ophiolite is an ultramafic series of pods found in the Blue Ridge Mountains of the Appalachian mountain chain. The pods formed before the Taconic orogeny. Throughout the middle and late Ordovician era, the rocks were affected by regional metamorphism leading to resulting in altered mineralogy for some pods.
References
- ↑ Schreyer, W. (1973). "Whiteschist: a high-pressure rock and its geologic significance". The Journal of Geology. 81 (6): 735–739. Bibcode:1973JG.....81..735S. doi:10.1086/627926. JSTOR 30059001. S2CID 128944616.
- ↑ Schreyer, W.; Abraham K. (1976). "Three-Stage Metamorphic History of a Whiteschist from Sar e Sang, Afghanistan, as Part of a Former Evaporite Deposit". Contributions to Mineralogy and Petrology. Springer-Verlag. 59 (2): 111–130. Bibcode:1976CoMP...59..111S. doi:10.1007/BF00371302. S2CID 129384880.
- 1 2 3 4 5 6 Johnson, J. "Preliminary investigation of in-situ, high-pressure, high-ƒO2 metasomatism and metamorphism of meta-basalt to whiteschist" (PDF). Frontier Research on Earth Evolution. 1. Retrieved 21 December 2011.
- ↑ Rolfo, F.; Compagnoni R.; Xu S.; Jiang L. (2000). "First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China". European Journal of Mineralogy. 12 (4): 883–898. doi:10.1127/0935-1221/2000/0012-0883 . Retrieved 21 December 2011.
- ↑ Wyllie, P.J. (1992). "Experimental petrology: Earth materials science". In Brown G., Hawkesworth C. & Wilson C. (ed.). Understanding the Earth (2 ed.). Cambridge University Press. pp. 82–84. ISBN 978-0-521-42740-1 . Retrieved 21 December 2011.
- ↑ Schreyer, W. (1977). "Whiteschists: Their compositions and pressure-temperature regimes based on experimental, field, and petrographic evidence". Tectonophysics. Elsevier. 43 (1–2): 127–144. Bibcode:1977Tectp..43..127S. doi:10.1016/0040-1951(77)90009-9.
- ↑ John, T.; Schenk V.; Mezger K.; Tembo F. (2004). "Timing and PT evolution of whiteschist metamorphism in the Lufilian Arc-Zambezi Belt orogen (Zambia): Implications for the assembly of Gondwana". The Journal of Geology. 112 (1): 71–90. Bibcode:2004JG....112...71J. doi:10.1086/379693. S2CID 73549945 . Retrieved 21 December 2011.