Continental Subduction-zone Metamorphism Research Articles

Zircon geochemical constraints on the protolith nature and metasomatic process of the Mg-rich whiteschist from the Western Alps

The Mg-rich silicic crustal rocks characterized by high SiO2 and MgO contents are widespread in the Alps, providing rare targets to investigate the origin of Mg-rich fluids in the continental subduction zone. However, it is a prerequisite to elucidate the protolith nature of such Mg-rich rocks, including the possible alteration process experienced before the Alpine metamorphism. For this purpose, we carried out a detailed study of trace elements, O-Hf isotopes and U-Pb ages for zircons from the coesite-bearing whiteschist and its country rock metagranite at Dora-Maria in Western Alps. The whiteschist is characterized by much higher MgO contents than the adjacent metagranite. Relict magmatic zircons from the two types of rocks give essentially the same U-Pb ages of ~262Ma, similar REE contents and patterns, the same range of εHf(t1) values from −6.4 to 2.8 and δ18O values from mostly 9 to 11‰. These features provide compelling evidence that the whiteschist and metagranite share the same protolith, i.e., the Permian granite. All the above geochemical compositions are consistent with the argument that the high MgO content of the whiteschist was acquired from high Mg fluids from dehydration of the serpentinized peridotite at the slab-mantle interface in the continental subduction channel. The hypothesis of a pre-Alpine hydrothermal alteration of the granitic protolith to cause the MgO enrichment is not favorable. Zircon in the metagranite does not show any metamorphic overgrowth during the Alpine orogeny, in contrast to the extensive overgrowth of metamorphic rims at ~35Ma in the whiteschist. The metamorphic rims show relatively low REE contents with variably flattened HREE patterns, different from the relict magmatic zircons that exhibit relative high REE contents with steep HREE patterns. This difference suggests that the breakdown of hydrous minerals plays a crucial role in dictating mineral reactions for zircon growth during the continental subduction-zone metamorphism. This interpretation is also consistent with the local occurrence of garnet coronae along the grain margins of biotite in the metagranite. Therefore, this study provides not only the constraints on the protolith nature and metasomatic process of Mg-rich whiteschist from the Western Alps, but also insights into the role of metamorphic fluids in controlling mineral reactions for the metamorphic growth of both zircon and garnet in the continental subduction zone.

High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U–Pb dating for granitic gneiss from the northeastern Sulu orogen, east-central China. Zircon U–Pb dating yields protolith ages of 753±15Ma to 780±13Ma and metamorphic ages of 209±3 to 244±7Ma. The Neoproterozoic cores with concordant U–Pb ages exhibit a wide δ18O range from −11.0‰ to 5.8‰, which is nearly the same as those for cores with discordant U–Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around −10‰ whereas the rims of the other samples show a wider range from −9.8‰ to 5.0‰. The δ18O values as negative as −11.0‰ for zircons with concordant Neoproterozoic U–Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith zircon cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than −9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water–rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen

A combined study of mineral inclusions, U–Pb ages, trace elements and Hf-O isotopes was carried out for zircons from a coesite-bearing jadeite quartzite in the Dabie orogen. The results provide insights into the action of ultrahigh-pressure (UHP) metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone. The zircons mostly show core-rim structures in cathodoluminescence images. The overgrown rims contain rare mineral inclusions, and exhibit concordant U–Pb ages of 225 to 246Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the relict cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U–Pb ages ranging from 983 to 2045Ma and steep REE patterns with significant negative Eu anomalies. The U–Pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000±43Ma and 234±18Ma, respectively. We interpret the rims with Triassic ages as the new growth of metamorphic origin and the cores with Precambrian ages as the protolith relics of magmatic origin. The relict magmatic zircons underwent two subtypes of metamorphic recrystallization, i.e., solid-state transformation and metasomatic alteration. The solid-state recrystallized zircons exhibit slightly discordant U–Pb ages close to the protolith age, steep MREE–HREE patterns, and almost unchanged Hf isotope ratios. These observations point to the lowest degree of resetting to the geochemical composition of protolith zircons. In contrast, the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon REE composition and U–Pb and Lu–Hf isotopic systems. All of the zircon domains, regardless of the rims and cores, show relatively consistent δ18O values of 4.0±0.2‰. Such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the UHP metamorphic fluids of Triassic age. The metasomatic recrystallization of protolith zircons is indicated by the occurrence of UHP inclusion minerals such as coesite, rutile and jadeite in sealed microcracks. In this regard, the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. As such, the metasomatic recrystallization has heterogeneously reset the U–Pb and Lu–Hf isotope systems of protolith zircons. The composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in Si, Ti, Na and Al. These elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss. Therefore, the jadeite quartzite would be precipitated from the UHP metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction-zone metamorphism.

Garnet geochemistry records the action of metamorphic fluids in ultrahigh-pressure dioritic gneiss from the Sulu orogen

A combined study of major and trace elements, water content and oxygen isotope composition was carried out for newly grown garnets in ultrahigh-pressure dioritic gneiss from the Sulu orogen. The results indicate the occurrence of a core–rim structure of garnet grains with growth of rims over cores via the mechanism of coupled dissolution–reprecipitation. This provides new insights into the action of geofluids on garnet growth during continental subduction-zone metamorphism. The garnet cores and rims show distinct geochemical compositions. The cores have high spessartine contents (4.2–19.4mol%), relatively low grossular contents (26.5–29.8mol%) and low δ18O values (5.0±0.2‰). The rims have relatively low spessartine contents (1.0–3.4mol%), high grossular contents (28.7–36.1mol%) and high δ18O values (6.1±0.3‰). Furthermore, the rims show much lower contents of Li and HREE, and considerably higher contents of Sc, V and Ga than the cores. In addition, the cores exhibit very low contents of structural hydroxyl in 32±10 to 48±20ppm H2O, whereas the rims show much higher contents of structural hydroxyl in 112±16ppm to 171±34ppm H2O. Although the boundary between core and rim is highly irregular in the garnet grains, the zoning in water contents, element and oxygen isotope compositions are all spatially correlated with each other. Taken all the observations together, it is suggested that the geochemical compositions of garnet cores and rims would be primarily dictated by two types of geofluids with distinctive compositions. Some garnet fragments, bounded by fractures, show the two types of domains with distinct water contents. This difference indicates insignificant exchange of structural hydroxyl by diffusion during growth of the rims over the cores. Mineral-pair O isotope thermometry yields consistent temperatures of ~550°C for the quartz-garnet rim and quartz-titanite, and titanite U–Pb dating yields a metamorphic age of 226±6Ma. Thus, the growth of garnet rims is linked to the action of geofluids (probably anatectic melts) during the exhumation of deeply subducted continental crust. Therefore, the compositions of garnet cores and rims are primarily controlled by the composition of attending fluids. As such, garnet provides a mineralogical record of the fluid action during subduction-zone metamorphism.

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

AbstractComposite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.

Protolith control on fluid availability for zircon growth during continental subduction-zone metamorphism in the Dabie orogen

Different episodes of zircon growth are recognized by a combined study of CL images, mineral inclusions, U–Pb ages, trace elements and Lu–Hf isotopes for ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Dabie orogen. The results provide insights into the effect of protolith property on fluid liberation during continental collision. Fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth. Zircon U–Pb dating for UHP metabasalt and metasediment (eclogite and its host paragneiss) yields two groups of ages at 244±3 and 225±2Ma, respectively. Mineral inclusion, trace element and Lu–Hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation, respectively. In contrast, zircon U–Pb dating for UHP metaintrusive rocks (granitic orthogneiss and its hosted eclogite) gave only one group of age at 222±2Ma. Mineral inclusion, trace element and Lu–Hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation. The difference between the two groups of zircon U–Pb dates is attributed to the difference in their protolith origin. Volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water (pore fluid), so that considerable amounts of aqueous fluid can be released from them during subduction. This episode of fluid action was recorded by the growth of anatectic zircon at ∼244Ma. In contrast, intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals, so that little fluid can be released from them during subduction. Nevertheless, large amounts of retrograde fluid were released from UHP metamorphic rocks regardless of their protolith origin during decompression exhumation. This episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220–225Ma. Therefore, the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction, which has great bearing on partial melting, element transport and mineral growth during continental subduction-zone metamorphism.

Formation of metamorphic and metamorphosed garnets in the low-T/UHP metagranite during continental collision in the Dabie orogen

A combined study of major and trace elements in garnet was carried out for low-T/UHP metagranite in the Dabie orogen. The results show different fashions of element zoning in the garnet, suggesting occurrence of both metamorphic and metamorphosed garnets. Three generations of garnet growth are distinguished on the basis of the assumption that Ca contents, Mn contents and Fe/Mg ratios of garnet in metagranites are a function of pressure and temperature. The first generation of garnet (Grt-I) occurs in the core of a skeletal garnet, showing homogeneously low XGrs values and Fe/Mg ratios but high XSps values. This implies that the core garnet grew at the highest temperature but the lowest pressure, representing a metamorphosed residue of magmatic garnet in protolith granite. The second generation of garnet (Grt-II) occurs in the mantles and cores of many garnet grains, exhibiting increased XGrs values but decreased Fe/Mg ratios. This is ascribed to a continuous increase in temperature and pressure till the peak pressure, corresponding to metamorphic growth (or overgrowth) during the prograde subduction. The third generation of garnet (Grt-III) occurs in the rims of all garnet grains, displaying decreased XGrs values and Fe/Mg ratios in response to a pressure decrease but a temperature increase till the peak temperature. These rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation. Grt-I shows steep MREE-HREE patterns and profoundly negative Eu anoalies, consistent with growth from granitic melt. This kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements (such as REE, Rb, Ba, Sr, Pb, Th, U, Nb and Ta) despite the low-T/UHP metamorphism. Grt-II and Grt-III in one sample exhibit steep MREE-HREE patterns, with a continuous decrease in REE contents. This suggests their growth from the almost same matrix of mineral assemblages (plagioclase+K-feldspar+muscovite) during metamorphism. However, Grt-II in the other sample displays flat to steep MREE-HREE patterns, with an increase of REE contents from core to mantle. This implies that the matrix of mineral assemblages for Grt-II changes from common rock-forming minerals (e.g., feldspar, muscovite and biotite) to REE-rich minerals (e.g., epidote, allanite, zircon, amphibole, apatite and titanite). Grt-III in the two samples all displays significantly lowered REE contents compared to those of Grt-I and Grt-II. This may be due to a decrease in pressure and an increase in temperature during the initial exhumation. Therefore, the two-stage growth of metamorphic garnet is evident in the low-T/UHP metagranite during the continental subduction-zone metamorphism. In addition, the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims. This suggests that the peak pressure (Pmax) did not occur contemporarily at the peak temperature (Tmax), corresponding to the “hot” exhumation.

Multistage growth of garnet in ultrahigh-pressure eclogite during continental collision in the Dabie orogen: Constrained by trace elements and U–Pb ages

Major and trace elements for garnet and other minerals, and U–Pb ages and trace elements for zircon (as inclusions in garnet or as matrix) were determined on thin sections of UHP eclogite from the western part of the Dabie orogen. The results provide temporal constraints on growth of garnet during continental subduction-zone metamorphism. Major elements show homogeneous distribution across garnet grains, with weak or no zonation of Ca and almost flat distribution patterns of Mg and Fe. However, their REE compositions display two groups of patterns relative to chondrite. Group I garnet is characterized by steep patterns from LREE to HREE, whereas Group II garnet shows enrichment of MREE relative to LREE and HREE. These two groups of garnet also have different parageneses of mineral inclusions. Group I garnet always contains abundant inclusions of amphibole+epidote/allanite+rutile+plagioclase+quartz±zircon, whereas Group II garnet occasionally contains inclusions of omphacite±rutile±apatite±zircon. LA-ICPMS U–Pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet. Four types of zircon were identified based on their CL images, REE distributions and U–Pb ages. Types I and II zircons are metamorphosed after protolith magmatic zircon, with Type I being modified by solid-state recrystallization and Type II by dissolution recrystallization. Types III and IV zircons are metamorphic and grew in the stability field of garnet. They gave consistent 206Pb/238U ages of 239±6Ma and 222±4Ma, respectively, in association with Groups I and II garnets. This provides a temporal constraint on the two-stage growth of garnet, respectively, predating and postdating the peak UHP metamorphic event. Therefore, we conclude that there are two stages of zircon and garnet growth during continental collision, which occurred during the final subduction from HP to UHP phase and the early exhumation from UHP to HP eclogite facies, respectively.

Fluid action on zircon growth and recrystallization during quartz veining within UHP eclogite: Insights from U–Pb ages, O–Hf isotopes and trace elements

In-situ SIMS analyses of O and U–Pb isotopes as well as LA-(MC)-ICPMS analyses of trace elements, U–Pb, and Lu–Hf isotopes were carried out for zircon from a quartz vein and its host UHP eclogite in the Dabie orogen. The results are integrated to decipher the effect of aqueous fluid on the new growth of metamorphic zircon and on the recrystallization of protolith zircon during continental subduction-zone metamorphism. Most zircons show distinct core–(mantle)–rim structures in CL images. The rims are unzoned and have low trace element contents, low Th/U ratios of 0.003 to 0.06 and shallow MREE–HREE patterns, and insignificant negative Eu anomalies, indicating their growth from metamorphic fluid. The rims from the vein have concordant U–Pb ages of 222±6 to 212±4Ma, suggesting protracted growth during exhumation of the deeply subducted continental crust. In contrast, the rims from the eclogite gave two groups of concordant U–Pb ages at 221±3 and 208±3Ma, respectively, corresponding to two episodes of fluid action. The all rims have relatively uniformly negative δ18O values of −2.59 to −1.23‰ for the vein and −3.37 to −0.33‰ for the eclogite, suggesting their growth from negative δ18O metamorphic fluids of the same origin. The cores exhibit clearly oscillatory or patchy zonation, high trace element contents, high Th/U ratios of 0.13 to 2.12, steep MREE–HREE patterns, and significant negative Eu anomalies. They gave discordia Neoproterozoic upper-intercept U–Pb ages for the both vein and eclogite. Most cores have discordant U–Pb ages and variably positive δ18O values, with no correlation between the two variables. This suggests non-linear isotope alteration of the protolith zircon by metamorphic recrystallization in the presence of aqueous fluid. The highest δ18O value of 4.76‰ with the concordant Neoproterozoic U–Pb age is considered as a proxy for the primary δ18O value for the protolith zircon. In this regard, the cores would crystallize from slightly low δ18O magmas in the Neoproterozoic, and interacted with the negative δ18O fluids of metamorphic origin in the Triassic. In particular, the cores in the vein are the xenocrystal that was trapped by the metamorphic fluids from the eclogite. Therefore, the occurrence of the both magmatic cores and metamorphic growth in the vein suggests that sinking of the metamorphic fluids into eclogite factures brought about both the physical transport of protolith zircon and the chemical transport of element Zr during the continental collision.

Metamorphic growth and recrystallization of zircons in extremely 18O-depleted rocks during eclogite-facies metamorphism: Evidence from U–Pb ages, trace elements, and O–Hf isotopes

A combined in situ SIMS and LA-(MC)-ICPMS study of U–Pb ages, trace elements, O and Lu–Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U–Pb ages of 769 ± 9 Ma have positive δ 18O values of 0.1–10.1‰, high Th/U and 176Lu/ 177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ 18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U–Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ 18O values of −10.0‰ to −2.2‰, low Th/U and 176Lu/ 177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ 18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ 18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ 18O magmatic core and the negative δ 18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ 18O fluids. The zircons recrystallized in solid-state maintained positive δ 18O values, and REE and Lu–Hf isotopes of protolith zircon, but their U–Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ 18O values similar to the metamorphic growths, almost completely reset U–Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ 18O values, and partially reset REE, and U–Pb and Lu–Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.

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.

Mineral hydrogen isotopes and water contents in ultrahigh-pressure metabasite and metagranite: Constraints on fluid flow during continental subduction-zone metamorphism

Systematic measurements of hydrogen isotopes and total water contents were carried out for metamorphic minerals from ultrahigh-pressure (UHP) eclogite, amphibolite, schist and granitic gneiss in continuous core segments from the main hole of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. Together with previous studies of both outcrop and drilling samples, the results yield a dataset that is the most extensive for hydrous and nominally anhydrous minerals from UHP metamorphic rocks (especially for UHP metagranite). This provides insights into the property and scale of fluid flow during continental subduction-zone metamorphism. Hydrous and nominally anhydrous minerals show large variations in δD values from −155 to −20‰, consistent with the incorporation of meteoric water into protoliths of UHP metaigneous rocks by high-T hydrothermal alteration. Both equilibrium and disequilibrium H isotope fractionations occur between coexisting hydrous and nominally anhydrous minerals, indicating the differential effects of retrograde metamorphism on the H isotope systems of UHP minerals during the exhumation of deeply subducted continental crust. Besides the structural OH, nominally anhydrous minerals (NAMs) also contain large amounts of water in the form of molecular H2O that is depleted in D relative to the structural OH. There are negative correlations between total water contents and δD values for garnet, omphacite and rutile in the eclogite, indicating a preferential loss of the D-poor molecular water from the NAMs by diffusion relative to the structural OH during the decompression exhumation. As a consequence, symplectites after the mafic minerals are depleted in D relative to their precursors. On the other hand, amphibole and plagioclase in the amphibolite have δD values similar to, or greater than, garnet and omphacite from the adjacent eclogite. This suggests that the retrograde fluid is enriched in D relative to the NAMs of eclogite and thus derived from the decomposition of D-rich hydrous minerals. In either case, the retrograde fluid is internally buffered in stable isotope compositions. Nevertheless, the exsolution of molecular water and structural hydroxyl from the NAMs is responsible for D-poor fluid, whereas the decomposition of hydrous minerals is responsible for the D-rich fluid. The systematic measurements of water contents for various minerals from the different UHP lithologies demonstrate significant differences in total water contents between eclogite and gneiss. This may be a basic cause for their different behaviors of fluid activity during the exhumation.

Zr-in-rutile thermometry of eclogite in the Dabie orogen: Constraints on rutile growth during continental subduction-zone metamorphism

The Zr content of rutile was analysed by both EMP and LA-ICPMS for low-T/UHP eclogite and enclosed kyanite–quartz veins in the Dabie orogen, China. Zr-in-rutile temperatures were calculated at different pressures and then compared with temperatures derived from Ti-in-zircon, mineral-reaction, quartz–mineral O-isotope and garnet–clinopyroxene Fe–Mg exchange thermometers. All thermometric data are interpreted within the framework of petrologically and geochronologically constrained P– T– t path. As a consequence, variable Zr-in-rutile temperatures for different occurrences of rutiles are used to indicate their growth in different stages during continental subduction-zone metamorphism. Thus, some rutiles would grow at the peak pressure, whereas other rutiles would either grow before the peak pressure during the subduction or grow at or after the peak temperature during exhumation. The mineral O-isotope and Fe–Mg exchange thermometers also yield variably lower temperatures for the eclogite. The similar results were obtained for mid-T/UHP eclogites in the Dabie-Sulu orogenic belt. This is ascribed to the characteristic P– T– t path of UHP metamorphic rocks, in which metamorphic temperatures at peak pressures are lower than the peak temperatures at decreased pressures during exhumation. Thus, there is a contest between thermodynamics and kinetics during metamorphic reactions in response to P– T changes. Therefore, the reasonable interpretation of thermometric data requires comprehensive understanding of thermometric methodology from physicochemical principles to geological applicabilities.

Trace elements in zircon and coexisting minerals from low-T/UHP metagranite in the Dabie orogen: Implications for action of supercritical fluid during continental subduction-zone metamorphism

Simultaneously in-situ analyses of U–Pb isotopes and trace elements were carried out for zircons, in combination with the in-situ analyses of trace elements in coexisting minerals, from low-T/UHP metagranite in the Dabie orogen. The results provide geochemical evidence for the existence of supercritical fluid during continental subduction-zone metamorphism. The zircons are categorized into three types based on their patterns of REE distribution. Type I zircons show increasing enrichment from La to Lu, with prominent positive Ce anomalies and negative Eu anomalies, which are typical of magmatic zircon. Some of them display regular or blurred oscillatory-zoned texture and apparent 206Pb/ 238U ages of 341 to 780 Ma, suggesting metamorphic modification by solid-state recrystallization with no significant involvement of metamorphic fluid. Type II zircons share similar Th, U and HFSE contents and REE patterns to Type I zircons. However, they exhibit blurred oscillatory-zoned texture or are unzoned, have apparent 206Pb/ 238U ages of 348 to 709 Ma, and are LREE-enriched relative to Type I zircons. This suggests that they underwent metamorphic reworking by replacement recrystallization in the presence of metamorphic fluid. The LREE enrichment is due to the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. Type III zircons, representing the majority of the present analyses, are characterized by spongy texture and consistent enrichment of LREE, HREE, Th, U and HFSE relative to Type I zircons. They yield nearly concordant U–Pb ages close to the discordia lower-intercept. The consistent enrichment of trace elements relative to the magmatic zircon indicates involvement of a special UHP metamorphic fluid that has a strong capacity to extract significant amounts of LREE, HREE, Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite–water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Thus, a supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. The mechanism of dissolution recrystallization is responsible for the spongy texture and the very high concentration of trace elements in this type of metamorphic zircons. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions.

Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen, a series of combined studies were carried out for UHP metamorphic rocks from the main hole (MH) at continuous depths of 100 to 5000 m. The results provide new insights into the chemical geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as follows. (1) An O isotope profile of 100 to 5000 m is established for the UHP metamorphic minerals, with finding of 18O depletion as deep as 3300 m. Along with areal 18O depletion of over 30,000 km 2 along the Dabie–Sulu orogenic belt, three-dimensional 18O depletion of over 100,000 km 3 occurs along the northern margin of the South China Block. (2) Changes in mineral O isotope, H isotope and water content occur in eclogite-gneiss transitions, concordant with petrographic changes. The contact between different lithologies is thus the most favorable place for fluid action; fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance, petrography and δ 18O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. (3) Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE. Some eclogites have andesitic compositions with high SiO 2, alkalis, LREE and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks, and thus the possible presence of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO 2, LREE and LILE occur at the contact between eclogite and gneiss. This indicates their mobility between different slab components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the widespread retrogression, retrograde fluid was internally buffered in stable isotope compositions, and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals the episode of HP eclogite-facies recrystallization at 216 ± 3 Ma, with timescale of 1.9 to 9.3 Myr or less. Collectively, the Dabie–Sulu UHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780 Ma. The first type of bimodal magmatism was formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In conclusion, the geochemical studies of CCSD-MH core samples have placed important constraints on the nature and scale of fluid action and element mobility during the continental subduction and UHP metamorphism.

U–Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U–Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of 206Pb/ 238U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ 18O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low 176Lu/ 177Hf ratios of 0.000001–0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high 176Lu/ 177Hf ratios of 0.000192–0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε Hf ( t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between 206Pb/ 238U ages and Lu–Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu–Hf and U–Th–Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.