Structural development of the Tso Morari ultra-high pressure nappe of the Ladakh Himalaya
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Accretionary wedge
Coesite
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Omphacite
Phengite
Geothermobarometry
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Abstract Coesite has been found as an inclusion in garnet from the Mengzhong eclogite in Donghai county, northeastern Jiangsu province, eastern China. The host eclogite occurs closely associated with regionally developed gneiss and was formed under P-T conditions of about 810°C and more than 28 kbar.
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A large eclogite belt developed along the Dabie Mountains and Subei-Jiaonan Rise in central China is a result of collision between the North China and Yangtze blocks. Recently, coesite and coesite pseudomorphs are recognized as an inclusion in both garnet and
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Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a pali-sade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7―3.25 GPa, 670―730℃). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.
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The water content of garnet was determined for eclogite from two Variscan complexes in Germany: the Erzgebirge (EG), Saxony, and the Fichtelgebirge (FG), Bavaria. Erzgebirge eclogites occur in three units, each of which experienced specific peak conditions (unit 1: 840–920°C/≥30 kbar, unit 2: 670–730°C/24–26 kbar, unit 3: 600–650°C/20–22 kbar). Peak conditions of the FG eclogite (690–750°C/25–28 kbar) are close to those of eclogite from EG unit 2. Coesite eclogite is restricted to the EG ultra-high pressure (UHP) unit 1. Garnet shows infrared absorption bands at ca. 3650, 3580–3630, and 3570 cm-1, ascribed to structural water. Many garnets also contain molecular water (in sub-microscopic fluid inclusions), which is irregularly distributed on the grain scale and of secondary origin. Grain volumes with molecular water invariably reveal a band at 3580–3630 cm-1 attributed to a hydrogarnet substitution. Because structural water due to this substitution positively correlates with molecular water, the primary content of structural water can only be deduced from grain volumes that are free of molecular water as demonstrated by Schmädicke & Gose (2017; American Mineralogist 102, 975–986). This primary content is typically low in garnet from quartz eclogite (<2–50 ppm); averages for most samples fall in the range of 8–28 ppm. Garnet from coesite eclogite hosts more water (50–180 ppm) except for garnet from an unusual, phlogopite-bearing coesite eclogite that contains only 19–55 ppm. Structural water in garnet is unrelated to metamorphic peak pressure but governed by the presence (or absence) of eclogite-facies hydrous minerals such as calcic amphibole, zoisite, and, or, phlogopite. In the case that hydrous minerals were stable at peak metamorphism—as in quartz eclogite—garnet hosts little or no water. If hydrous minerals are not part of the peak assemblage— as in common coesite eclogite—garnet contains distinctly more water. The latter was apparently derived from eclogite-facies hydrous minerals, which decomposed and liberated their H2O due to overstepping their stability field during UHP metamorphism. Moreover, garnet in coesite eclogite is more Ca-rich than garnet in quartz eclogite. This is ascribed to the breakdown of prograde zoisite, liberating Ca and facilitating a higher grossular content, which, in turn, enhances the garnet's capacity for water storage. This study further suggests: (1) post-peak metamorphic introduction of secondary fluid; (2) relatively dry conditions prior to fluid influx, because only water-deficient garnet is able to incorporate additional structural water; (3) The determined primary contents of structural water were probably not modified by decompressional water loss, because the latter should only occur if the water content at peak pressure is ≥75 % of the maximum storable amount; (4) Since garnet from both eclogite types was water-deficient at the metamorphic peak it is unlikely that the different water contents are related to pressure; (5) The mineral assemblage and the dehydration of hydrous minerals is definitely more important in this context; (6) Garnet and, by implication, omphacite from both eclogite types was able to incorporate only part of the water liberated by hydrous minerals, a great part must have been released to hanging-wall rocks; and (7) The study points to a moderate amount of water (several hundred ppm) that is transported by subducting coesite eclogite to depths of >100 km, an amount equivalent to that in ca. 1–2 % calcic amphibole.
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In recent years, coesite-bearing eclogites have been discovered at many sites in the eastern part of the Qinling-Dabie Orogene. However, these ultrahigh-pressure metamorphic rocks were found only in the southern Qinling Orogene before. In 1993, they were reported in the Qinling Group. Recently, coesites have been found in eclogites.
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Abstract Coesite is typically found as inclusions in rock‐forming or accessory minerals in ultrahigh‐pressure ( UHP ) metamorphic rocks. Thus, the survival of intergranular coesite in UHP eclogite at Yangkou Bay (Sulu belt, eastern China) is surprising and implies locally “dry” conditions throughout exhumation. The dominant structures in the eclogites at Yangkou are a strong D 2 foliation associated with tight‐to‐isoclinal F 2 folds that are overprinted by close‐to‐tight F 3 folds. The coesite‐bearing eclogites occur as rootless intrafolial isoclinal F 1 fold noses wrapped by a composite S 1 –S 2 foliation in interlayered phengite‐bearing quartz‐rich schists. To evaluate controls on the survival of intergranular coesite, we determined the number density of intergranular coesite grains per cm 2 in thin section in two samples of coesite eclogite (phengite absent) and three samples of phengite‐bearing coesite eclogite (2–3 vol.% phengite), and measured the amount of water in garnet and omphacite in these samples, and also in two samples of phengite‐bearing quartz eclogite (6–7 vol.% phengite, coesite absent). As coesite decreases in the mode, the amount of primary structural water stored in the whole rock, based on the nominally anhydrous minerals ( NAM s), increases from 107/197 ppm H 2 O in the coesite eclogite to 157–253 ppm H 2 O in the phengite‐bearing coesite eclogite to 391/444 ppm H 2 O in the quartz eclogite. In addition, there is molecular water in the NAM s and modal water in phengite. If the primary concentrations reflect differences in water sequestered during the late prograde evolution, the amount of fluid stored in the NAM s at the metamorphic peak was higher outside of the F 1 fold noses. During exhumation from UHP conditions, where NAM s became H 2 O saturated, dehydroxylation would have generated a free fluid phase. Interstitial fluid in a garnet–clinopyroxene matrix at UHP conditions has dihedral angles >60°, so at equilibrium fluid will be trapped in isolated pores. However, outside the F 1 fold noses strong D 2 deformation likely promoted interconnection of fluid and migration along the developing S 2 foliation, enabling conversion of some or all of the intergranular coesite into quartz. By contrast, the eclogite forming the F 1 fold noses behaved as independent rigid bodies within the composite S 1 –S 2 foliation of the surrounding phengite‐bearing quartz‐rich schists. Primary structural water concentrations in the coesite eclogite are so low that H 2 O saturation of the NAM s is unlikely to have occurred. This inherited drier environment in the F 1 fold noses was maintained during exhumation by deformation partitioning and strain localization in the schists, and the fold noses remained immune to grain‐scale fluid infiltration from outside allowing coesite to survive. The amount of inherited primary structural water and the effects of strain partitioning are important variables in the survival of coesite during exhumation of deeply subducted continental crust. Evidence of UHP metamorphism may be preserved in similar isolated structural settings in other collisional orogens.
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