UHT represents extreme crustal met with T exceeding 900 1100C P 713 Kb Granulite facies rocks metamorphosed at very high temperatures were identified in the early 1980s ID: 1022191
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1. UHT/ UHP Met
2. UHTUltrahigh-temperature metamorphism (UHT) represents extreme crustal met with T exceeding 900 -1100°C, P = 7-13 Kb Granulite facies rocks metamorphosed at very high temperatures were identified in the early 1980sPetrological 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.
3. UHTMet of crustal rocks in which peak T exceeds 900 °C, recognized either by robust thermobarometry or by the presence of a diagnostic mineral assemblage in an appropriate bulk composition and oxidation state, such as assemblages with opx + sill + qtz, sapphirine + quartz or spinel + quartz, generally at pressure conditions of sillimanite stability in metapelite
4. UHT : evidence, identificationPetrological indicators are usually preserved in extremely Mg-Al-rich rocks which are usually dry and restitic in nature. Mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartzTypical Min ass: (i) Sapphrine+ Qtz(ii) Spinel+Qtz(iii) Opx+sill+QtzDirect contact b/w sapphirine and Qtz is regarded as the most robust evidence
5. OccurrenceUHT terranes are : Archaean UHT rocks are distributed in East-Antarctica, South Africa, Russia and CanadaPaleoproterzozoic UHT granulites were reported from the North China craton (during accretion Columbia supercon), northwestern Canada and Scotland.UHT rocks from the Neoproterozoic Grenville orogeny are distributed in the Eastern Ghats ProvinceNeoproterozoic-Cambrian (Pan-African): East Antarctica, southern Madagascar, Sri Lanka and southern India
6. HypothesisCorrelation proposed between the episodic formation of UHT granulites and the episodic assembly and disruption of supercontinents or the plume activity during various periods in Earth historyUHT granulites characterized by dry mineral assemblages, require low water activities and involvement of CO2-rich fluids in generating diagnostic UHT assemblagesUHT rocks might represent windows for the transfer of CO2 from the mantle into the mid crust and ultimately to the atmosphere
7. Ultra-high-pressure metamorphismUHP met refers to processes at pressures high enough to stabilize coesite, high-P polymorph of SiO2 ≥27kbar (2.7GPa) = 100km28 kb, 700-800 C, 100-200km 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 formation of many UHP terrains has been attributed to the subduction of microcontinents or continental margins and the exhumation of all UHP terrains has been ascribed principally to buoyancy caused by the low density of continental crust—even at UHP—relative to Earth's mantle.
8. UHP indicatorspresence of metamorphic coesite, diamond, or majoritic garnet are diagnosticIn UHP eclogite the best geobarometers involve garnet + clinopyroxene + kyanite + coesite/quartz
9. DistributionFirst evidence of UHP from Greenland Caledonides, from kyanite eclogite and associated host gneisses Phanerozoic continental orogenic belts : Most occur in Eurasia Coesite is relatively widespread, diamond less so, and majoritic garnet is known from only rare localities. The oldest UHP terrain is 620 Ma and is exposed in Mali the youngest is 8 Ma and exposed in Papua New Guinea.Discovery of coesite from Indian Himalaya = UHP met in Indian subcntinental crust (P, T = 28Kb, 700-800 C) = indicating Indian plate subducted at steeper angle, depth 100-200km
10. MechanismSubduction of continental crust is the preferred mechanism for UHP, bcz it can account for rocks being transported to depths in excess of 100km to form eclogite. Continental subduction also provides a low geother that accounts for T below 750C common to UHP terraneAlternate mechanism is crustal thickening via imbrication by thrust stacking
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14. THE FATE OF SUBDUCTED CONTINENTAL CRUST: EVIDENCE FROM RECYCLED UHP –UHT MINERALS Elements 2013