Yong-Fei Zheng

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 δ18O 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 δ13C values of −28‰ to −21‰ for apatite as well as host-eclogites and gneisses; (4) positive δ13C 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 δ18O from −11‰ to +9‰, with U–Pb ages of 700 to 800 Ma for the timing of low-δ18O magma crystallization. It appears that the UHP metamorphic rocks exhibit ranges of δ18O 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 δ18O values by meteoric-hydrothermal alteration before the UHP metamorphism. Thus, the UHP metamorphic rocks largely reflect the δ18O 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 earths surface, and subjected to varying degrees of water–rock interaction at some time before plate subduction. The low-δ18O 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 earths 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-δ18O 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 δ18O 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.

Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates

Abstract The modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl-bearing silicate minerals. The order of 18O enrichment obtained in common rock-forming minerals is: pyrophyllite > kaolinite > tourmaline ⩾ talc > prehnite ⩾ topaz > illite > phengite > lepidolite ⩾ muscovite ⩾ staurolite > epidote > glaucophane > serpentine ⩾ chlorite > tremolite > hornblende > phlogopite ⩾ biotite > humite > norbergite > ilvaite. Hydroxyl-bearing silicates are enriched in 18O relative to hydroxyl groups but depleted in 18O relative to anhydrous counterparts. Three sets of self-consistent fractionation factors: between quartz and the hydroxyl-bearing silicate minerals, between calcite and the silicate minerals, and between the silicate minerals and water, have been calculated for a temperature range of 0–1200°C. The fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous, metamorphic and sedimentary petrology. They can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest. The difference in oxygen isotope composition between the hydroxyl-bearing mineral and the OH group is quantitatively demonstrated to be temperature dependent and, therefore, can be used as a single-mineral geothermometer.

Genesis of zircon and its constraints on interpretation of U-Pb age

Zircon U-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of U-Pb age. Three methods have been proposed to link zircon U-Pb age to metamorphic conditions: (i) internal structure; (ii) trace element feature; (iii) mineral inclusion composition. Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned, sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances, with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks. Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore, interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.

Oxygen and hydrogen isotope geochemistry of ultrahigh-pressure eclogites from the Dabie Mountains and the Sulu terrane

The oxygen and hydrogen isotope compositions of mineral separates have been determined for ultrahigh-pressure (UHP) eclogites from Shuanghe in the eastern Dabie Mountains and from Donghai in the western Sulu terrane, East China. The results show a large variation in δ18O values of garnet and omphacite (−2.6 to +7.0‰ for Shuanghe and −10.4 to +4.8‰ for Donghai) but a small range in phengite δD value (−104 to −73‰). Oxygen isotope equilibrium has been preserved between the eclogite minerals and thus records the metamorphic temperatures of 550–730°C for the Shuanghe eclogites and 650–750°C for the Donghai eclogites. These not only demonstrate that the UHP rocks acquired the unusual δ18O values prior to eclogite-facies metamorphism by interaction with 18O-depleted fluids, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O depletion in the eclogites. Ancient meteoric water is assumed to exchange oxygen isotopes with the eclogite precursors on the continental crust prior to plate subduction. The extremely low δ18O values (−10 to −9‰) and δD values (−104 to −100‰) for the Qinglongshan eclogite may represent the oxygen and hydrogen isotope compositions of ancient meteoric water at some earlier time than subduction. The survival of the oxygen and hydrogen isotopic signature of meteoric water in the UHP eclogites indicates that the eclogites resided at mantle depths only for a short time, otherwise the extremely 18O-depleted eclogites would be re-equilibrated isotopically with the mantle due to diffusion and recrystallization. This suggests restricted fluid mobility and limited crust–mantle interaction during the UHP metamorphism. The consistency of oxygen isotope temperatures between different mineral pairs in this study suggests relatively rapid cooling and ascent for the UHP eclogites in the early stage of their exhumation. However, there are differential exchanges of oxygen and hydrogen isotopes in hydroxyl-bearing minerals (and rutile) with retrograde fluid during exhumation, which has not only resulted in lower oxygen isotope temperatures for mineral pairs containing zoisite and rutile, but also disequilibrium and reversed hydrogen isotope fractionations between phengite, amphibole and zoisite.

Low-Grade Metamorphic Rocks in the Dabie-Sulu Orogenic Belt: A Passive-Margin Accretionary Wedge Deformed during Continent Subduction

Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic rocks of the subducting accretionary wedge consist of two parts: (1) large masses of metasedimentary rocks (including slates, schists, phyllites, metasandstones, and marble) along the northern margin of the Dabie-Sulu orogenic belt and deformed igneous rocks of Middle Neoproterozoic age; (2) sporadic outcrops in the interior of the belt of metavolcanics, metaclastics, phyllite, and marbles. During Triassic subduction of the Yangtze plate, the sedimentary cover and its underlying basement were partly scraped off by the overthrusted North China plate. The scraped-off materials accumulated in front of the overriding plate, forming an accretionary wedge that underwent deformation and metamorphism under greenschist-facies conditions. The present study also provides a constraint on the location of the Triassic suture zone between the North China and Yangtze plates. It is located below, or north of, the accretionary wedge (i.e., the Beihuaiyang zone in the Dabie region and the Wulian-Penglai zone in the Sulu region) rather than along the northern margin of the ultrahigh-pressure metamorphic zones.

Calculation of oxygen isotope fractionation in magmatic rocks

The increment method is applied to calculation of oxygen isotope fractionation factors for common magmatic rocks. The 18 O-enrichment degree of the different compositions of magmatic rocks is evaluated by the oxygen isotope indices of both CIPW normative minerals and normalized chemical composition. The consistent results are obtained from the two approaches, pointing to negligible oxygen isotope fractionation between rock and melt of the same compositions. The present calculations verify the following sequence of 18 O-enrichment in the magmatic rocks: felsic rocks>intermediate rocks>mafic rocks> ultramafic rocks. Two sets of internally consistent fractionation factors are acquired for phenocryst–lava systems at the temperatures above 1000 K and rock–water systems in the temperatures range of 0–1200 jC, respectively. The present calculations are consistent with existing data from experiments and/or empirical calibrations. The obtained results can be used to quantitatively determine the history of water–rock interaction and to serve geological thermometry for various types of magmatic rocks (especially extrusive rocks). D 2002 Elsevier Science B.V. All rights reserved.

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Abstract Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.

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.

Hydrogen and oxygen isotope evidence for fluid-rock interactions in the stages of pre- and post-UHP metamorphism in the Dabie Mountains

Abstract Hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern Dabie Mountains, China. The δ 18 O values of eclogites cover a wide range of −4.2 to +8.8‰, but the δD values of micas from the eclogites fall within a narrow range of −87 to −71‰. Both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some eclogite samples. The δ 18 O values of −4 to −1‰ for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water–rock interaction before the high to ultrahigh pressure metamorphism. Heterogeneous δ 18 O values for the eclogite protoliths implies not only the varying degrees of the water–rock interaction before the metamorphism at different localities, but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction. Retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals.

Geochemistry of continental subduction-zone fluids

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.