Bin Fu

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. n nIt 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. n nThe 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. n nThe 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.

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.

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.

Transition of UHP eclogites to gneissic rocks of low-amphibolite facies during exhumation: evidence from the Dabie terrane, central China

The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of ∼20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has 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. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers.

Petrological, isotopic and fluid inclusion studies of eclogites from Sujiahe, NW Dabie Shan (China)

In addition to the Triassic Hongan low-T–high-P eclogite and the Xinxian coesite-bearing kyanite-glaucophane eclogite, Silurian coesite-free amphibole eclogites occur in the Sujiahe region, NW Dabie Shan of central China. A comprehensive study of petrology, Nd–Sr, O–H isotopes and fluid inclusions has been carried out for the Sujiahe eclogites. Geothermobarometry of mineral assemblages gave peak P–T conditions of 600–730 °C and 14–19 kbar, and retrograde metamorphism of 530–685 °C and 6 kbar. The eclogites have high Nd(t) values of −1.9 to 5.8, indicating that their protoliths were derived from oceanic basalts. Whole-rock δ18O values for eclogites at Xiongdian and at Hujiawan range from 8.8‰ to 11.2‰, whereas δ18O value of the Yangchong eclogite is as low as 4.3‰. Whole-rock δD values range from −55‰ to −75‰. The results exclude that the eclogite protoliths were isotopically exchanged with ancient meteoric water prior to plate subduction, thus distinguishing them from the other Dabie–Sulu eclogites. Fluid inclusions show the presence of early highly saline brine and late low-salinity aqueous fluids in the Sujiahe eclogites during the high-P metamorphism. The isotopic data suggest that the Sujiahe eclogite protolith was metamorphosed during subduction of an oceanic plate beneath the Sino-Korean Craton in the Middle Paleozoic, different from the other eclogites in the Dabie–Sulu orogen which were formed by the ultrahigh pressure metamorphism during the Triassic.

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope

δ13C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO2 despite a large variation in δ18O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO2 in the UHP metamorphic fluid. The 13C-poor CO2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. n nZircons from two samples of the granitic orthogneiss exhibit low δ18O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U–Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both 13C- and 18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.

Oxygen isotope composition of quartz-vein in ultrahigh-pressure eclogite from Dabieshan and implications for transport of high-pressure metamorphic fluid.

Abstract The oxygen isotope composition of minerals from quartz-veins and host-eclogites in Dabieshan, China was measured in order to place geochemical constraints on the origin and transport of high-pressure metamorphic fluids. The results, along with structural and petrological relationships between vein and wallrock, show that the quartz veins are the high-pressure metamorphic and thus formed prior to eclogite-facies recrystallization when they were exhumated from mantle depths to deep crustal levels. Not only is the oxygen isotope composition of the vein-quartz identical to that of the host-eclogite, but in addition the oxygen isotope geothermometry of mineral-pairs from the quartz-veins yield temperatures that are close to the eclogite-facies temperatures. Therefore, the vein-forming fluid was likely derived from the local host-eclogites by the exsolution of dissolved hydroxyls within eclogite minerals due to significant pressure decrease. Local advective transport of fluid is the predominant mechanism in the processes of vein precipitation. Fluid flow prior to the eclogite-facies recrystallization may occur mainly along pressure gradients. The loss of the UHP or HP fluid at the different depths during exhumation may be the potential cause for concordant and discordant isotope temperatures between different mineral-pairs in the eclogites.

Oxygen isotope composition of granulites from Dabieshan in eastern China and its implications for geodynamics of Yangtze plate subduction.

Abstract The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ18O values relative to the normal mantle δ18O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ18O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ18O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ18O values before the granulite-facies metamorphism by interaction with a certain 18O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

Carbon concentrations and isotopic ratios of eclogites from the Dabie and Sulu terranes in China

Abstract Both concentration and isotope composition of bulk carbon in apatite and host eclogites from the Dabie–Sulu ultrahigh pressure (UHP) terranes in China have been determined along with their oxygen isotope composition. The results show significant 13 C-depletion in the apatite (δ 13 C=−27.7‰ to −20.8‰) with the carbon concentrations of 0.59 to 1.65 wt.% CO 2 despite a large variation in δ 18 O (−6.5‰ to +9.5‰). The bulk carbon in 21 of the 24 eclogites has low δ 13 C values of −26.1‰ to −17.9‰ with low carbon content of 500 to 1000 ppm, whereas the other three samples show high δ 13 C values of −7.1‰ to −2.8‰ with high carbon contents of 2400 to 4300 ppm. Noncarbonate carbon was measured by treating the all eclogites with 5 N HCl solution, yielding uniformly low δ 13 C values of −27.9‰ to −24.2‰ and carbon contents of 200 to 300 ppm. Carbonate carbon is thus calculated by mass balance to have also low δ 13 C values of −25.6‰ to −15.1‰ for the 21 samples but high δ 13 C values of −4.3‰ to −1.2‰ for the remaining three samples. Secondary carbonate was identified in the three eclogites that are also depleted in 13 C primarily, but subjected to overprint of 13 C-rich CO 2 -bearing fluid subsequent to the UHP metamorphism. The isotopically light carbon in both eclogite and apatite is interpreted to represent the isotope composition of carbon in eclogite precursors before plate subduction, and thus has an origin of organic carbon from the Earths surface. The uniformly low δ 13 C values of apatite suggest that the CO 2 of metamorphic fluid in equilibrium with the host-eclogites would be derived from the oxidation of organic carbon rather than the decarbonation of underthrust carbonates during progressive metamorphism. Protoliths of the eclogites are inferred to be of igneous origin, which underwent more extensive interaction with organic matter than with meteoric-hydrothermal fluid on the subsurface of the continental crust. The break off of the subducted plate containing the isotopically light carbon and subsequent interactions with the surrounding mantle could produce the mafic and/or silicic magmas that are significantly depleted in 13 C relative to the primary mantle carbon. This may provide evidence for a linkage of the 13 C-depleted mantle carbon to a surficial source via plate subduction.

Mössbauer spectroscopy of omphacite and garnet pairs from eclogites: Application to geothermobarometry

Abstract Cation partition among coexisting minerals has been widely applied to eclogite thermometry, but an accurate estimation of Fe3+ content compared to total Fe is crucial in obtaining reasonable temperatures for petrologic studies. Room-temperature Mössbauer spectroscopy was measured for garnet-omphacite pairs in high-pressure (HP) and ultrahigh-pressure (UHP) eclogites from the Dabie terrane in east-central China. The results show very low Fe3+/ΣFe ratios of 0.026 to 0.082 in garnet but high Fe3+/ΣFe ratios of 0.240 to 0.689 in omphacite. The hyperfine parameters of minerals record the HP-UHP conditions that the eclogites experienced. Fe2+ in clinopyroxenes with low Na + Ca contents in their M2 sites shows pressure-induced occupation in M1 site. The quadrupole splitting of Fe2+ in HP-UHP garnets (3.61 to 3.77 mm/s) and omphacites (2.77 to 3.06 mm/s) are among the highest values ever reported, indicating effectively pressure-regulated polyhedral sites. After the Fe3+ was corrected, Fe2+-Mg partitioning not only significantly narrow the ranges relative to those without Fe3+ correction, but also yield temperatures about 8 to 370 ºC lower than the uncorrected temperatures for the same garnet-pyroxene pairs. The recalculated temperatures are constrained to narrow ranges of 477 to 647 °C for quartz-bearing eclogites and 624 to 843 °C for coesite-bearing eclogites. These maximum values provide close proxies to peak metamorphic temperatures provided that the retrograde exchange of Fe-Mg cations by diffusion between minerals during exhumation is taken into account.