Peak Ultrahigh-pressure Metamorphism Research Articles

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

Isua (Greenland) ~3700 Ma meta-serpentinite olivine Mg# and δ18O signatures show connection between the early mantle and hydrosphere: Geodynamic implications

The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ18OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo96-98 olivine has δ18OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals.We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism.In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.

Melting of subducted continental crust during collision and exhumation: Insights from granitic rocks from the North Qaidam UHP metamorphic belt, NW China

The formation and reworking of continental crust have close connections with orogeny. Syn- and post-collisional magmatism is key to understanding crustal growth and reworking during continental collision and post-collisional extension. Here we report a series of syn-collisional felsic magmatic rocks intruded into migmatized gneiss and amphibolite/eclogite in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, to provide insights into melting/reworking of UHP metamorphosed continental crust during continental collision and exhumation. The syn-collisional magmas are mainly tonalite-trondhjemite and mica granite that occur as plutons and dykes, and high potassium granite veins in migmatites. They were formed by decompression melting of subducted oceanic and continental crust at different levels during exhumation, without any discernible mantle contribution. The tonalite-trondhjemites show adakitic signatures of both oceanic crust and mafic continental crust with varying Nd isotopic compositions. The mica granite plutons have high silica and alkali contents, are weakly to strongly peraluminous, and have crustal SrNd isotope compositions. These mica granites were formed by partial melting of previously subducted continental crust, and formed mostly in high-pressure conditions, in the garnet stability field. The protolith is predominantly granitic gneiss, with minor meta-sediment and metabasite. A compilation of published ages and our results show that these syn-collisional magmatic rocks formed between 437 and 397 Ma, overlapping with UHP metamorphic ages (440–420 Ma), and suggests that exhumation occurred synchronously with continental subduction. Individual zircon grains show metamorphic cores and magmatic rims with a ≤ 20 Myr time interval, interpreted as the time between peak UHP metamorphism and melting during exhumation. Such long-lived and episodic melting of UHP metamorphic continental crust should result from periodic processes, involving melt weakening and decompression by upward movement within the subduction channel, rather than coherent motion of the entire UHP belt. This process represents crustal reworking during collision and exhumation, without input of mantle melts.

Origin of atoll garnets in ultra-high-pressure eclogites and implications for infiltration of external fluids

Atoll-shaped and normal garnets in a low-temperature (LT) and ultra-high-pressure (UHP) metamorphic eclogite of the Dabie orogen were studied by detailed petrographic observation and pseudosection modelling. The normal and atoll garnets exhibit pronounced chemical variations, characterized by the overall increase of pyrope and decrease of grossular content from core to rim. A compositional discontinuity between the inclusion-rich core and the inclusion-poor mantle was defined by a sharp decrease in grossular content and an increase in pyrope. Pseudosection modelling using different effective bulk compositions (EBCs) obtained using XRF and the point-counting method shows that the eclogite experienced two stages of eclogite-facies metamorphism from a prograde amphibole-eclogite-facies (∼500–700 °C, <2.2 GPa) to the peak UHP lawsonite-eclogite-facies (∼650 °C, ∼3.3 GPa), followed by an isothermal decompression (ITD) exhumation. Garnet essentially did not grow owing to the parallels between the prograde P–T path and the garnet mode isopleths after the formation of the garnet core. The stable field and the compositions of the peak mineral assemblage at LT-UHP conditions insignificantly depend on the scale of equilibrium. Partial lawsonite and omphacite should be subtracted from the EBCs to obtain reasonable P–T estimates/pseudosections that are consistent with the genesis of atoll-garnet-bearing eclogite. Pseudosection modelling indicates that the infiltration of external fluid into the “dry” eclogite weakened the rigid garnet and produced micro-cracks. These cracks served as permeable pathways, leading ultimately to the consumption of previously formed garnet core and the formation of atoll texture. Previous P–T estimates using conventional garnet-omphacite-phengite thermobarometry are less reliable because omphacite was highly altered owing to net transfer reactions involving omphacite. The atoll texture was developed from prograde to peak UHP metamorphism, and the atoll-garnet forming event in the Dabie eclogite predates the final stage of the UHP metamorphism. The previous Lu–Hf date of c. 221 Ma is thus a maximum estimate of the termination of UHP metamorphism.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

In-situ U–Pb dating and Nd isotopic analysis of perovskite from a rodingite blackwall associated with UHP serpentinite from southwestern Tianshan, China

Metamorphic perovskite associated with diopside+chlorite occurs in blackwalls surrounding rodingitized mafic dikes that are included in ultrahigh-pressure (UHP) serpentinized ultramafic rocks from Changawuzi, Chinese southwestern Tianshan. Perovskite forms discrete crystals in the chlorite-rich matrix or appears at the rim of ilmenite. The metamorphic perovskites from the blackwalls are characterized by high concentrations of REE, Nb, Ta, Th and U. The primitive mantle normalized patterns of perovskite are two to three orders of magnitude higher but parallel to the bulk rock patterns, demonstrating that perovskite is a major host of these incompatible trace elements in the ultramafic rocks. The bulk-rock major and trace element compositions of rodingite dikes and associated blackwalls display variable patterns, consistent with different mafic protoliths of the rodingites.Perovskites from three samples were analysed for U–Pb by in-situ secondary ion mass spectrometry. Two samples contained essentially common Pb whereas one sample had enough radiogenic Pb to define a discordia in the Tera–Wasserburg diagram with an age intercept of 317±11Ma. If all analyses are pooled together, a 238U–206Pb age of 308±5 Ma is obtained. These ages are in agreement with the age of peak UHP metamorphism in Chinese southwestern Tianshan. In-situ Nd isotopes of perovskite from the three samples were obtained by laser ablation multi-collector inductively coupled plasma mass spectrometry. The ɛNd(t)310 of two samples are 8.37±0.14 and 7.88±0.25, respectively, which suggests that they have similar protoliths. These samples plot close to a 310Ma Sm–Nd isochron. In contrast, the third sample has a different ɛNd(t)310 of 0.90±0.51, indicating a completely different protolith. Thus, in-situ analyses of perovskite can provide important information on the protolith as well as the metamorphic evolution of reaction zones between mafic and ultramafic rocks.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717±38Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221±2Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ18O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded εNd (t)=−12 to −10 and TDM=2.2–2.8Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (206Pb/204Pb)i=15.34–16.50, (207Pb/204Pb)i=15.23–15.32, and (208Pb/204Pb)i=35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.

U-Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P-T-t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh-pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction-zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti-in-zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction-zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti-in-zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr-in-titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr-in-titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction-zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid-T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.

Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: Evidence from zircon U–Pb age, trace element and mineral inclusion

The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complex zone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South China Block and display a multiple metamorphic evolution. However, the exact timing of the UHP metamorphism in the NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. In order to constrain the ages of peak UHP metamorphism and subsequent retrogression during continental subduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combined petrological, trace element and U–Pb isotopic study. In combination with petrological data, the present zircon SHRIMP and LA-ICPMS U–Pb dating provides precise constraints on the timing of multistage metamorphic events on the eclogites in the region. The U–Pb isotope in zircon from the eclogites records the times of multiple discrete events in the history of the rock such as Neoproterozoic, 238±2Ma, 222±4–227±2Ma, 210±4–215±2Ma, 199±2Ma and 176±2–188±2Ma. Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. By U–Pb age, trace element and mineral inclusion of zircon, at least two episodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226±3 and 214±3Ma, respectively. The younger ages of 199±2 and 176–188Ma most likely record the granulite-facies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of 238±2Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus, Neoproterozoic mafic lower continental crust was subducted to depths greater than 120km (corresponding to the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226±3Ma. Then these UHP metamorphic rocks were exhumed to about 60km depth at 214±3Ma and experienced high-pressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustal levels and overprinted by granulite- and amphibolite-facies metamorphism, respectively.

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 � C. These are considered as minima because of the potential for a diffusion-related modification and re- equilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at � 3 GPa as associated kyanite eclogites. Decompression and cooling to � 0.5 GPa and 550 � C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar � sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 � 10, 258 � 7 Ma) and Triassic (238 � 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 � 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1-4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 � 0.5 Ma age obtained in this study agrees with that (93-91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a melange-like assemblage of rocks, which followed different P-T-t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

Microgeochemistry of rutile and zircon in eclogites from the CCSD main hole: Implications for the fluid activity and thermo-history of the UHP metamorphism

Rutile and zircon in eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD-MH) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe (EMP). Both single spot and profile analyses of large matrix rutiles (> 60 μm) generally demonstrate core–rim variability of Pb, Sr, Zr and U. Pb and Sr show enrichment at the rims, while Zr and U decrease in concentration from the cores to the rims. Correspondingly, temperatures estimated by Zr-in-rutile geothermometers are higher within the cores (mainly in the range of 680–740 °C) than at the rims by 10–30 °C. We note that estimated temperatures within the cores of matrix rutiles correlate positively with grain diameter. This suggests that the Zr content in rutiles is not only dependent on temperature and pressure, but is also affected by fluid activity and Zr diffusion. Estimated temperatures within the cores of matrix rutiles merely represent lower limits of the peak ultra-high pressure (UHP) metamorphic temperatures. Combined with Th/U and Ce/Lu ratios, rare earth element (REE) patterns and 206Pb/ 238U ages, Ti-in-zircon geothermometers can be used to distinguish the thermal evolution of eclogites during the UHP metamorphism. Zircons of 745 ± 8.0 Ma (2 σ, n = 3) show evidence of the temperatures at which they crystallized from the original magma. Zircons of 240 ± 7.0 Ma (2 σ, n = 11) with flat HREE patterns suggest a definite temperature range (689 ± 18 °C (2 σ, n = 11)) for the prograde metamorphism. Although 226 Ma is generally considered the time of the peak UHP metamorphism for the Dabie–Sulu orogen, except for a few grains with a maximum temperature of 744 °C, most of the 226 ± 2.0 Ma (2 σ, n = 12) zircons indicate lower temperatures (average = 698 ± 12 °C (2 σ, n = 12)). These data imply that the 226 Ma zircons were probably not products of the peak UHP metamorphism, but might instead be associated with the pressure decrease in the post-peak metamorphism. The youngest zircons (< 220 Ma) provide evidence for temperatures (682 ± 14 °C (2 σ, n = 11)) associated with the retrograde metamorphism. Geothermometers that incorporate both rutile and zircon indicate that there may have been experienced a locally short-lived heating event for eclogites from the CCSD-MH.

Fluid regime in continental subduction zones: petrological insights from ultrahigh-pressure metamorphic rocks

Abstract: Intensive studies of ultrahigh-pressure (UHP) metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation. Recent studies in the Dabie–Sulu orogen of China have made important contributions to understanding fluid action in such systems, hence these rocks are the main focus of this review. Sources of metamorphic fluid can be deduced from study of hydrous minerals, structural hydroxyl and molecular water in nominally anhydrous minerals. Fluid and solid inclusions of UHP metamorphic minerals are also a record of fluid action in deep subduction zones. The presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization, making available materials for U–Pb dating. Isotopic dating demonstrates that after peak UHP metamorphism, retrograde fluid with deuteric origin became active. The decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite-facies retrogression within UHP slabs. Aqueous fluid released by the exsolution of molecular water and structural hydroxyl from UHP nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions. Therefore, one part of the UHP slab is a source of aqueous fluid, whereas another part is a sink. In both cases, the fluid action during subduction and exhumation is episodic rather than continuous. Supplementary material: A description of the Dabie–Sulu UHP metamorphic rocks, analytical methods, LA-ICPMS zircon U–Pb isotope data and rare earth element compositions are available at http://www.geolsoc.org.uk/SUP18355 .

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ∼ 2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature ( PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism (∼ 2.7 Gpa) began at 242.2 ± 0.4 Ma ( n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (> ∼ 4 Gpa) at 227.3 ± 0.7 Ma ( n = 72), before exhumation (< ∼ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.

Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.

Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya

U-Pb ages of zircon and allanite from the coesite-bearing ultrahigh-pressure (UHP) units in the Kaghan Valley, northern Pakistan, demonstrate that peak UHP metamorphism along the northern margin of the Indian plate occurred at 46.4 ± 0.1 Ma at peak pressure-temperature conditions of >27.5 kbar (>100 km) and 720–770 °C. Much lower pressure retrogressive growth of titanite took place between 46.4 and ca. 44 Ma, indicating that the eclogites were exhumed to 35 km depth at or before 44 Ma, implying very rapid exhumation rates within the mantle of ∼30–80 mm/yr or more, comparable to rapid plate velocities. Once entrained in the base of the crust, the eclogites followed a slower cooling history from 45 Ma, similar to the amphibolite facies gneisses of the Pakistan Himalaya.

SHRIMP zircon U-Pb dating in Jingshan “migmatitic granite”, Bengbu and its geological significance

The petrographic characteristics of Jingshan “migmatitic granite” and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433—722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by the UHPM. The UHPM could be the reason for the major Pb loss at ±220 Ma.

Low-T eclogite in the Dabie terrane of China: petrological and isotopic constraints on fluid activity and radiometric dating

While extensive studies have demonstrated fluid release during subduction of oceanic crust, little attention has been paid to fluid activity during subduction and exhumation of continental crust. Abundant occurrence of quartz veins within eclogites in the Dabie-Sulu orogenic belt of China provides us with an opportunity to study the origin and role of vein-forming fluids with respect to heat and mass transfer during ultrahigh pressure (UHP) metamorphism and its relevant processes. This study focuses on kyanite-quartz vein that occurs as polycrystalline aggregates within the low-T eclogite in the Dabie terrane, which are interpreted as pseudomorphs after former porphyroblasts of lawsonite. Coesite pseudomorphs were found for the first time in eclogite garnet, resulting in a revised estimate of peak P–T conditions at 670°C and 3.3 GPa for the eclogite and thus upgrading the high-P unit to an UHP unit. On the basis of the relationship between calculated P–T path and metamorphic reactions as well as the absence of foliation texture, and undulose extinction of quartzes in the vein, we conclude that lawsonite breakdown into kyanite–quartz–zoisite assemblage took place at the onset of exhumation subsequent to peak pressure. Retrograde metamorphism caused O and H isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in stable isotope compositions. Zircon U–Pb dating and whole-rock Nd–Sr isotope analyses indicate that eclogite protolith is the paleoceanic basalt that was derived from the depleted mantle by magmatism at about 1.8 to 1.9 Ga but experienced hydrothermal alteration by surface waters. The altered basalt underwent UHP metamorphism in the Triassic that caused fluid release for zircon growth/overgrowth not only at about 242±3 Ma prior to the onset of peak pressure but also at about 222±4 Ma during decompression dehydration by lawsonite breakdown and hydroxyl exsolution in the low-T/UHP eclogite. Consistent ages of 236.1±4.2 Ma and 230±7 Ma were obtained from mineral Sm–Nd and Rb–Sr isochron dating, respectively, indicating attainment and preservation of Nd and Sr isotope equilibria during the Triassic UHP eclogite-facies metamorphism. Ar–Ar dating on paragonite from the eclogite gave consistent plateau and isochron ages of 241.3±3.1 Ma and 245.5±9.8 Ma, respectively, which are interpreted to date paragonite crystallization during the prograde eclogite-facies metamorphism. The timing of peak UHP metamorphism for the low-T eclogite is constrained at sometime prior to 236.1±4.2 Ma. Thus the termination age of peak UHP metamorphism may be different in different slices of deep-subducted slab.

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

Discovery of coesite, diamond, and extreme 18O-depletion in eclogites from the Dabie–Sulu orogen in central-east China has contributed much to our understanding of subduction of continental crust to mantle depths and its subsequent exhumation. Hydrogen, oxygen, and carbon isotope distributions were systematically investigated in the past 8 years for ultrahigh pressure (UHP) eclogites, gneisses, granulites, marbles, and peridotites from this exciting region. The available data show the following characteristic features: (1) variable δ 18O values of −11‰ to +10‰ for the eclogites and gneisses, with both equilibrium and disequilibrium fractionations of oxygen isotopes among minerals; (2) disequilibrium fractionation of hydrogen isotopes between mica and epidote from both eclogites and gneisses, with low δD values up to −127‰ to −100‰ for phengite; (3) negative δ 13C values of −28‰ to −21‰ for apatite as well as host-eclogites and gneisses; (4) positive δ 13C values of +1‰ to +6‰ for coesite-bearing marble associated with eclogites; (5) zircons from metamorphic rocks of different grades show a large variation in δ 18O from −11‰ to +9‰, with U–Pb ages of 700 to 800 Ma for the timing of low- δ 18O magma crystallization. It appears that the UHP metamorphic rocks exhibit ranges of δ 18O values that are typical of potential precursor protolith rocks. Preservation of the oxygen isotope equilibrium fractionations among the minerals of the UHP eclogites and gneisses suggests that these rocks acquired the low δ 18O values by meteoric-hydrothermal alteration before the UHP metamorphism. Thus, the UHP metamorphic rocks largely reflect the δ 18O values of their premetamorphic igneous or sedimentary precursors. The stable isotope data demonstrate that basaltic, granitic, and sedimentary protoliths of the eclogites, orthogneiss, and paragneiss in the orogen were at or near the earth's surface, and subjected to varying degrees of water–rock interaction at some time before plate subduction. The low- δ 18O rocks were isolated from water–rock interactions during their descent to and return from mantle depths. It appears that the oxygen, hydrogen, and carbon on the earth's surface were recycled into the mantle at depths of >200 km by the continental subduction. A fried ice cream model is advanced as an analogy to the rapid processes of both plate subduction and exhumation, with a short residence time of the UHP slab at mantle depths. The entire cycle of subduction, UHP metamorphism, and exhumation is estimated to take place in about 10 to 20 Ma. The 18O-depleted zircons and other minerals acquired their oxygen isotope compositions from low- δ 18O magmas that incorporated the isotopic signatures of meteoric water in rifting tectonic zones prior to solidification. The U–Pb discordia dating for the 18O-depleted zircons revealed that the meteoric water–rock interaction occurred at Neoproterozoic, a time being much earlier than the UHP metamorphism at Triassic, but correlated with the Rodinian breakup, positive carbon isotope anomaly in carbonates, and the snowball earth event. The unusually low δ 18O values can be acquired from either the meteoric water of cold paleoclimates or the melt water of glacial ice or snow. Neoproterozoic rift magmatism along the northern margin of the Yangtze craton may have provided sufficient heat source to trigger the meteoric-hydrothermal circulation. It is possible that the unusual 18O-depletion in the meta-igneous rocks occurs at some time prior to the snowball earth event, when there is a transition from a very cold earth with continental glaciers to a widely glaciated earth where bulk of the earth is covered by sea ice as defined for the snowball earth. The heterogeneity of oxygen isotope compositions at outcrop scales demonstrates the absence of pervasive fluid infiltration during prograde, peak UHP, and retrograde metamorphism; most rocks appear to have recrystallized under virtually closed system conditions characterized by widespread lack of an aqueous fluid phase. Volatiles may not escape from the rock series during the rapid subduction of the continental crust, resulting in a general lack of syn-collisional arc-magmatism in this orogen. Big differences in pressure and time from the peak UHP stage to the retrograde HP eclogite-facies stage cause significant release of aqueous fluid by dehydration from decompressing slabs during exhumation. As a result, fluid flow occurred in a channellized way on small scales subsequent to the UHP metamorphism, with very limited mobility of fluid at peak UHP conditions. The fluid for retrograde reactions was internally buffered in stable isotope compositions. While some fluids were locally derived from the surrounding gneisses, more fluid was probably derived from internal dehydration of the rock units in question. In addition to the breakdown of hydroxyl-bearing minerals, exsolution of structural hydroxyl dissolved in nominally anhydrous minerals due to abrupt decrease in pressure may have been an important source for the retrograde fluid.

Diamond growth during ultrahigh‐pressure metamorphism of the Kokchetav Massif, northern Kazakhstan

Abstract Characteristic features of in situ diamonds can be used to retrace diamond formation during ultrahigh pressure (UHP) metamorphism of the Kokchetav Massif, Kazakhstan. These features include the nitrogen aggregation state in diamond, dissolution features observed on diamond surfaces, and the carbon and nitrogen isotopic composition of the diamonds. The minerals in which the diamonds are included provide additional information and support the view that at least some of the Kokchetav microdiamonds are the products of prograde or peak UHP metamorphism. The coexistence of diamond and graphite is evaluated within this framework.