On the survival of intergranular coesite in UHP eclogite
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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.Keywords:
Coesite
Phengite
Omphacite
Coesite
Omphacite
Phengite
Geothermobarometry
Pseudomorph
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Coesite- and kyanite-bearing eclogites are abundant in the southern part of the Dabie Mountains (southern Dabie terrane-SDT). Two types of eclogites from the SDT (Types III and IV) were selected for detailed paragenetic study. Type III eclogites, most abundant in the northern part of the SDT, occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite + garnet + phengite + epidote + coesite + kyanite + carbonate + rutile + ilmenite. These minerals exhibit weak compositional zoning and contain few mineral inclusions. Type IV eclogites, mostly in the southern part of the SDT, occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite + garnet + glaucophane + kyanite + epidote + phengite + quartz + rutile + ilmenite. Garnets of Type IV eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite, phengite, epidote, quartz, and rutile in the core and omphacite, barroisite, and Mg-katophorite in the rim. Prograde blueschist facies (~400°C) assemblages were partially preserved in Type IV eclogites. The eclogitic assemblages of both types of eclogite have been partially or completely retrograded to amphibolite and greenschist facies assemblages. Parageneses and compositions of minerals from eclogites indicate that these rocks have undergone a clockwise P-T evolution path. Within the SDT, the temperatures, estimated according to $$K_{D(Cpx-Gt)}$$ and $$K_{D(Gt-Phen)}$$ for eclogites, indicate a systematic decrease from about 770°C in the north to 580°C in the south. Such variation is also evident for pressure estimates, as coesite occurs only in eclogite in the north, whereas the assemblage omphacite + kyanite + quartz (without coesite) is found in the south. This study, together with ultrahigh-pressure mineral assemblages identified in the gneiss-marble country rock, suggests that the continental crust of the SDT has been subjected to a regional ultrahigh-pressure metamorphism as part of a north-dipping subduction zone formed between the Sino-Korean and Yangtze cratons in Triassic time.
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Abstract Major and trace elements in omphacite, including hydrogen, were determined in eclogites from two Variscan basement complexes in Germany: Erzgebirge (EG) and Fichtelgebirge (FG). Erzgebirge eclogite is derived from three units, showing different peak pressure (P) and temperature (T) conditions (Unit 1: 840–920°C/≥30 kbar, Unit 2: 670–730°C/24–26 kbar, Unit 3: 600–650°C/20–22 kbar). The peak conditions of FG eclogite (690–750°C/25–28 kbar) resemble those of EG Unit 2. Coesite eclogite occurs in EG Unit 1, and quartz eclogite in all other units. Omphacite from all samples shows four infrared (IR) absorption bands. Two prominent, sharp bands occur at 3,455 ± 10 cm −1 (band II) and 3,522 ± 10 cm −1 (band III). Band II is usually more prominent than band III, except for few samples with low jadeite content. A further, broad band is centred between 3,270 and 3,370 cm −1 (band I) and a fourth, minor band at 3,611–3,635 cm −1 (band IV). Bands II and III are due to hydrogen bound as structural OH − ions in omphacite. In most cases, this also applies to band IV. However, some spectra with extremely large type IV bands reflect phengite inclusions. The ambiguous band I may be due to different H 2 O species (molecular water, structural OH, and water in phengite). Omphacite of quartz eclogite has lower contents of TiO 2 , Zr, Hf, and REE, compared with that from coesite eclogite. By contrast, omphacite in quartz eclogite from both EG (H 2 O sample averages: 465–852 ppm) and FG (546–1,089 ppm) contains the same amount of structural OH (concentrations given in wt.‐ppm H 2 O) as omphacite in coesite eclogite (492–1,140 ppm). The obtained difference in the garnet‐omphacite H 2 O partition coefficient between quartz (0.01–0.03) and coesite eclogite (0.08–0.11) results from different H 2 O contents in garnet (coesite eclogite: 50–150 ppm; quartz eclogite: <2–50 ppm; Gose & Schmädicke, 2018). The total content of structural OH in omphacite is unrelated to its major and trace element composition. However, treating the individual IR bands separately, a relation between OH and mineral composition is observed. The OH amount defined by band II is positively correlated to Ti and tetrahedral Al, and that of band III shows a positive correlation with Ca and a negative one with Na (and jadeite). Both the total OH content of omphacite and the partial contents deduced from individual IR bands are unrelated to PT conditions. This implies that omphacite incorporated its structural H 2 O mainly in the quartz stability field, presumably during initial omphacite growth. Conversely, most OH in garnet was derived from the final breakdown of the last remaining calcic amphibole close to or within the coesite stability field. Our data suggest that coesite eclogite is able to transport a significant amount of H 2 O (average 550 ppm, maximum 730 ppm), corresponding to that in 3–4 vol.% calcic amphibole, via subduction to depths beyond 100 km. However, the majority of water liberated by dehydration reactions during subduction, including the breakdown of 5–10 vol.% eclogite facies and >10 vol.% pre‐eclogitic hydrous minerals, is not preserved in eclogite but liberated to the mantle wedge.
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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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Omphacite
Phengite
Geothermobarometry
Pseudomorph
Dalradian
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Coesite
Omphacite
Phengite
Glaucophane
Amphibole
Geothermobarometry
Titanite
Blueschist
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Coesite-bearing eclogite samples from the Chinese Continental Scientific Drilling main hole (100-2000 m) were examined. Three major evolutionary stages are distinguished from mineral assemblages and textural relations. (1) The pre-peak stage is indicated by inclusion assemblages in rutile eclogite such as amphibole and paragonite, with pre-peak P-T conditions of ~13 kbar and ~910°C. (2) The peak ultrahigh-pressure (UHP) metamorphic stage is characterized by the mineral assemblage garnet-omphacite-phengite-rutile-apatite-coesite/quartz pseudomorph ± kyanite; P-T conditions reached ~910°C and ~37 kbar for rutile eclogite, and ~850°C and ~35 kbar for phengite eclogite. (3) The retrograde stage produced decompression textures via back reactions, such as thin coronas of amphibole or zoisite on garnet, fine-grained amphibole-plagioclase symplectites on omphacite and biotite-plagioclase, or K-feldspar-albite on phengite, ilmenite or titanite on rutile, at P-T conditions of 530-560°C and 7-9 kbar. The presence of K-feldspar-albite symplectite on phengite and minor interstitial K-feldspar implies that post-peak decompressional partial melting occurred locally during rapid exhumation of the subducted UHP slab. Two alternative P-T paths may be constructed: (1) the first possesses a different pre-peak stage but similar peak UHP to post-peak decompression cooling for rutile and phengite eclogite; (2) the second shows a consistent evolution from a pre-peak stage via peak UHP stage to a post-peak stage of near-isothermal decompression, followed by near-isobaric cooling for both rutile and phengite eclogite. We prefer the first P-T path, because the inferred paths show a clear increase in pressure and temperature from near-peak to peak UHP stage, and this may be associated with subductionrelated tectonism as a result of continental collision between the North China and Yangtze blocks. Rapid subduction and fast retrograde exhumation is likely responsible for preservation of some pre-peak inclusion assemblages and prograde mineral growth zoning.
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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.
Coesite
Phengite
Omphacite
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Citations (27)