Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China
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Abstract Nyböite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyböite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite. Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 ± 60°C and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyböite can occur in the Na-Al-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in Na B , variable in Si content, and richer in Na A variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.Keywords:
Amphibole
Coesite
Aegirine
Omphacite
Pyroxene
Grossular
Nepheline
Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite‐bearing eclogites from Dabieshan have been assessed through a consideration of Fe 2+ –Mg 2+ partitioning between garnet–omphacite and garnet–phengite pairs and of the reaction equilibrium celadonite+pyrope+grossular=muscovite+diopside, which incorporates an evaluation of the extent of the strongly pressure‐dependent inverse Tschermak's molecule substitution in the phengites. For the latter equilibrium, the calibration and recommended activity–composition models indicated by Waters & Martin (1993 ) have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others. Confirmation that in some phengite‐eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite (and in some cases even diamond) rather negates previous suggestions that coesite may have been stabilized in only very localized, possibly just intracrystalline, domains. Inherent difficulties in the evaluation of peak metamorphic temperatures from Fe 2+ –Mg 2+ partitioning between mineral phases, due to uncertainties over Fe 3+ /Fe 2+ ratios in the minerals (especially omphacites), and to re‐equilibration during extensive retrograde overprinting in some samples, are also assessed and discussed. Our results indicate the existence in south‐central Dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions that south‐central Dabieshan comprises a structurally coherent continental‐crust terrane with a regional P–T gradient signalling previous deepest‐level subduction in the north. Instead, we recognize the Central Dabie ultra‐high‐pressure (coesite eclogite‐bearing) terrane to be structurally overlain by a Southern Dabie high‐pressure (quartz eclogite‐bearing) terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section.
Phengite
Coesite
Grossular
Omphacite
Muscovite
Pyrope
Dalradian
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Phase relations of basic rocks under high pressure (HP) and ultrahigh pressure (UHP) metamorphic conditions are modelled on the basis of a MORB composition. The calculated pseudosections predict that basic rocks will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc under HP-lawsonite eclogite subfacies conditions (1.8-2.8 GPa, 500-600°C). In these assemblages, the pyrope content (Xpy) in garnet mainly increases with temperature rising, the grossular content (Xgr) chiefly decreases with pressure rising, and the silica content (Si-) in phengite increases linearly with increasing pressure; their contents are subtly affected by variations in bulk-rock composition. Thus, the isopleths of garnet and phengite compositions in P-T pseudosections potentially present a robust geothermobarometric method for natural glaucophane-bearing HP eclogites. Under low-T UHP conditions (>2.8 GPa, 550-650°C), a common assemblage for basic rocks is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. In this assemblage, the Xpy steadily increases as temperature rises and the Si-phengite increases with pressure rising, whereas the Xgr is very sensitive as pressure changes. The peak P-T conditions for low-T UHP eclogites can be determined using the isopleths of maximum Xpy and Si-phengite in P-T pseudosections. Under medium-T UHP conditions (>2.8 GPa and >650°C), basic rocks are predicted commonly to contain garnet + omphacite + lawsonite + phengite + coesite. In this assemblage, the Xpy mostly depends on bulk-rock compositions, whereas the Xgr and Si-phengite regularly increase, respectively, as temperature and as pressure rises, and thus, can provide good thermobarometric constraints for medium-T UHP eclogites. The decompression of these HP and UHP assemblages are modelled to be dominated by lawsonite dehydration reactions, which will result in disappearance of lawsonite and for- mation of glaucophane, epidote and/or kyanite with releasing a large amount of bound fluid.
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Omphacite
Lawsonite
Coesite
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Glaucophane
Pyrope
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Abstract Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P – T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P – T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ∼27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P – T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ∼70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.
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Glaucophane
Grossular
Coesite
Phengite
Pyrope
Lawsonite
Almandine
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Phengite
Omphacite
Coesite
Pyrope
Lawsonite
Grossular
Amphibole
Glaucophane
Almandine
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Abstract Phase relations of basic rocks under high pressure ( HP ) and ultrahigh pressure ( UHP ) metamorphic conditions are modelled on the basis of a MORB composition. The calculated pseudosections predict that basic rocks will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc under HP ‐lawsonite eclogite subfacies conditions (1.8–2.8 GPa , 500–600°C). In these assemblages, the pyrope content ( X py ) in garnet mainly increases with temperature rising, the grossular content ( X gr ) chiefly decreases with pressure rising, and the silica content ( S i‐) in phengite increases linearly with increasing pressure; their contents are subtly affected by variations in bulk‐rock composition. Thus, the isopleths of garnet and phengite compositions in P – T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing HP eclogites. Under low‐ T UHP conditions (>2.8 GPa , 550 – 650°C), a common assemblage for basic rocks is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. In this assemblage, the X py steadily increases as temperature rises and the S i‐phengite increases with pressure rising, whereas the X gr is very sensitive as pressure changes. The peak P – T conditions for low‐ T UHP eclogites can be determined using the isopleths of maximum X py and S i‐phengite in P – T pseudosections. Under medium‐ T UHP conditions (>2.8 GPa and >650°C), basic rocks are predicted commonly to contain garnet + omphacite + lawsonite + phengite + coesite. In this assemblage, the X py mostly depends on bulk‐rock compositions, whereas the X gr and S i‐phengite regularly increase, respectively, as temperature and as pressure rises, and thus, can provide good thermobarometric constraints for medium‐ T UHP eclogites. The decompression of these HP and UHP assemblages are modelled to be dominated by lawsonite dehydration reactions, which will result in disappearance of lawsonite and formation of glaucophane, epidote and/or kyanite with releasing a large amount of bound fluid.
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Lawsonite
Coesite
Grossular
Pyrope
Glaucophane
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Coesite
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Abstract Medium‐temperature ultrahigh pressure ( MT ‐ UHP ) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core–mantle increasing grossular ( X gr ) and decreasing pyrope ( X py ) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the X py and X gr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core–rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post‐peak stages. The maximum Si contents are mostly 3.60–3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT ‐ UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the X py in garnet mostly depends on bulk compositions, whereas the X gr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the X gr and Si isopleths in pseudosections, the peak P–T conditions were estimated to be 40 kbar/730 °C for the Jinheqiao, 41 kbar/726 °C for the Shuanghe, and 37–52 kbar and 700–830 °C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite ± phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early‐stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite‐bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid‐present conditions, it is difficult to recover real peak P–T conditions for UHP eclogites with lawsonite. This may be a reason why the P–T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late‐stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite‐absent MT ‐ UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting.
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Pyrope
Coesite
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Amphibole
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Laser Raman spectroscopy is used to identify mineral inclusions in situ within six Copeton diamonds and to determine the remnant internal pressure on them, namely 31-34 kbar for coesite, 13.6 and 23 kbar for diopsidic omphacite, and 8 kbar for grossular garnet. Using a published linear model of the host-inclusion volume system, each remnant pressure value generates a PT locus of diamond formation. The intersections of these define a range of diamond formation conditions from 250 ΰC, 43 kbar to 950 ΰC, 58 kbar with estimated intersection errors of α 70 ΰC and α 4 kbar. This range encompasses at least two sets of ultrahigh pressure (UHP) formation conditions derived from different protoliths involved with the termination of subduction, namely subducted oceanic slab (lower temperatures: eclogitic diamonds) and subducted continental rocks (higher temperatures: calcsilicate diamonds). Coesite has been found as a compound inclusion with omphacite (lower temperature set) and with grossular (higher temperature set). The measurements indicate that the Copeton stones are UHP diamonds (not cratonic), and their Carboniferous argon age dates on pyroxene inclusions should be interpreted as ages of crystallisation. The results and implications are consistent with a local source for Copeton diamonds, and imply there is a buried Carboniferous UHP terrane within eastern Australia.
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Omphacite
Grossular
Phengite
Lawsonite
Almandine
Peridotite
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Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh‐pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c . 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh‐pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh‐pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh‐ and high‐pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H 2 O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite‐bearing paragonite‐eclogite has re‐equilibrated at lower pressures. On the other hand, garnets from other coesite‐bearing but paragonite‐free kyanite‐zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.
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Omphacite
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Pyrope
Pseudomorph
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