UHP Metamorphism Research Articles

Excess and Radiogenic Argon in High‐Pressure Mylonites of the Tan‐Lu Fault Zone, Eastern China

Abstract: Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan‐Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the Tan‐Lu sinistral faulting, and provide geochronological evidence for the syn‐orogenic faulting of the Tan‐Lu fault zone. The results show that the phengite formed during the retrograde eclogite‐facies mylonitization was not contaminated with excess argon and can be used for dating the deformation. Argon closure in previous K‐bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward‐convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre‐existing phengite after the Tan‐Lu eclogite‐facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.

Triassic continental subduction in central Tibet and Mediterranean-style closure of the Paleo-Tethys Ocean

The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QMB are 244–223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a Paleo-Tethys arc terrane. This collision, and the coeval South China–North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm.

Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates

Three distinct groups of eclogites (low-Mg–Ti eclogites, high-Ti eclogites and Mg-rich eclogites) and ultramafic rocks from the depth interval of 100–680 m of the Chinese Continental Scientific Drill Hole were studied. The low Mg#s (= 100 ⁎molar Mg/(Mg + Fe)) (81–84%) and low Ni (1150–1220 ppm) and high Fe 2O 3 total (13–15 wt.%) contents of ultramafic rocks suggest a cumulate origin. Mg-rich eclogites show middle and heavy REE enrichments, which could not be produced by metamorphic growth of garnet. Instead, if the rocks formed from a light REE enriched magma, there may be an igneous precursor for some garnets in their protolith. Alternatively, perhaps they formed from a light REE depleted magma without garnet. The high-Ti eclogites are characterized by unusually high Fe 2O 3 total contents (up to 24.5 wt.%) and decoupling of high TiO 2 from low Nb and Ta contents. These features cannot be produced by concentration of rutile during UHP metamorphism (even for samples with TiO 2 > 4 wt.%) of high-Ti basalts, but could be attributed to crystal fractionation of titanomagnetite (for those with TiO 2 < ∼ 4 wt.%) or titanomagnetite + ilmenite (for those with TiO 2 > ∼ 4 wt.%). Thus, we suggest that protoliths of the high-Ti eclogites were titanomagnetite/ilmenite-rich gabbroic cumulates. As a whole, the low-Mg–Ti eclogites are geochemically complementary to the high-Ti eclogites, Mg-rich eclogites and ultramafic rocks, and could be metamorphic products of gabbroic/dioritic cumulates formed by high degree crystal fractionation. All these observations suggest that parental materials of the ultramafic rock-eclogite assemblage could represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas at intermediate to high pressure, which were later carried to ultrahigh-pressure conditions during a continental collision event.

Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition

In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks. Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones.

Comment on ‘Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40Ar/ 39Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans’

In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/ 39Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta 70, 2354–2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions. However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar. If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210–240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40Ar ∗ component with mean 40Ar/ 36Ar value of ∼5000. This value is similar to the composition of extraneous 40Ar ∗ in other eclogite facies terrane.

Response to the comment by Z.F. Zhao and T.S. Gao (2007) on “Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in dabieshan, China: New insights from 40Ar/ 39Ar dating by stepwise crushing”

In our paper we supposed that the Paleozoic ages of the Bixiling eclogitic garnets by 40Ar/ 39Ar crushing in vacuo were related to the UHP metamorphism. Zhao and Gao [Zhao Z.-F. and Gao T.-S. (2007) Comment on “Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China:Newinsights from 40Ar/ 39Ar dating by stepwise crushing” by Qiu and Wijbrans (2006). Geochim. Cosmochim. Acta 71(24), 6046–6050] dispute our conclusions and re-interpret them as “the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic” without presenting any new petrological evidence. Based on our present understanding of 40Ar/ 39Ar crushing technique and petrological observations, we do not agree with their re-interpretation and still prefer our original interpretation.

Petrology and U-Pb SHRIMP Geochronology of a Garnet Peridotite, Sulu UHP Terrane, East-Central China

Garnet clinopyroxenite at Rizhao occurs as a lens 100 x 225 m2 within a serpentinized peridotite body, in fault contact with felsic gneiss. Minor dunite also occurs within the same peridotite. The clinopyroxenite consists of porphyroblastic Grt-bearing (type 1), megacrystic Grt-bearing (type 2), porphyroblastic Cpx-bearing (type 3), and equigranular (type 4) garnet clinopyroxenite. Lamellaerich, coarse-grained Cpx occurs as inclusions in megacrystic Grt from type 2 clinopyroxenite, and as porphyroblasts in a matrix of Cpx + Grt + Ilm from type 3 clinopyroxenite; these clinopyroxenes contain abundant exsolution lamellae of Grt (Prp16-19Grs62Alm19-21Sps1) + Ilm ± Mag ± Amp (Mg#: 0.88-0.98; Na2O: 0.5-3.3 wt%; K2O: 0.7-1.0 wt%; TiO2: 0.1-0.2 wt%). Megacrystic and porphyroblastic garnet (Prp35-42Grs34-45Alm19-24Sps1) also occurs in a matrix of fine-grained Cpx + Grt + Ilm; spinel (Mg#: 0.60-0.62) is present as inclusions in both porphyroblastic Grt and lamellae-rich Cpx. Petrochemical data support a mantle origin involving crystallization of majoritic garnet. We propose that the protolith of the Rizhao clinopyroxenite initially was a spinel clinopyroxenite, and was convected to great depths (>450 km) to form a majoritic garnet that contains high CaO and considerable amounts of FeO and TiO2. During later decompression, the majoritic garnet broke down to form intergrowths of Cpx + Grt + Ilm ± Mag ± Amp at T = 700-800°C, P ≤ 3 GPa. It was then emplaced into a subduction zone and experienced UHP metamorphism at 620-880°C, 3 GPa during Triassic continental collision. Megacrystic/porphyroblastic garnet formed during this stage, and lamellaerich Cpx recrystallized to the matrix assemblage of Cpx + Grt + Ilm subsequently. The clinopyroxenite also experienced later retrogression during exhumation. Zircon U-Pb geochronology yields 215 ± 2 Ma for the Rizhao garnet clinopyroxenite; occurrence of garnet and clinopyroxene inclusions in zircon suggests that this age reflects the UHP metamorphism.

Timing of UHP metamorphism in the Hong’an area, western Dabie Mountains, China: evidence from zircon U–Pb age, trace element and Hf isotope composition

The Hong’an area (western Dabie Mountains) is the westernmost terrane in the Qinling-Dabie-Sulu orogen that preserves UHP eclogites. The ages of the UHP metamorphism have not been well constrained, and thus hinder our understanding of the tectonic evolution of this area. LA-ICPMS U–Pb age, trace element and Hf isotope compositions of zircons of a granitic gneiss and an eclogite from the Xinxian UHP unit in the Hong’an area were analyzed to constrain the age of the UHP metamorphism. Most zircons are unzoned or show sector zoning. They have low trace element concentrations, without significant negative Eu anomalies. These metamorphic zircons can be further subdivided into two groups according to their U–Pb ages, and trace element and Lu–Hf isotope compositions. One group with an average age of 239 ± 2 Ma show relatively high and variable HREE contents (527 ≥ LuN ≥ 14) and 176Lu/177Hf ratios (0.00008–0.000931), indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction. The other group yields an average age of 227 ± 2 Ma, and shows consistent low HREE contents and 176Lu/177Hf ratios, suggesting their growth with concurrent garnet crystallization and/or recrystallization. These two groups of age are taken as recording the time of prograde HP to UHP and retrograde UHP–HP stages, respectively. A few cores have high Th/U ratios, high trace element contents, and a clear negative Eu anomaly. These features support a magmatic origin of these zircon cores. The upper intercept ages of 771 ± 86 and 752 ± 70 Ma for the granitic gneiss and eclogite, respectively, indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the Yangtze Block. Young Hf model ages (TDM1) of magmatic cores indicate juvenile (mantle-derived) materials were involved in their protolith formation.

Reducing role of sulfides and diamond formation in the Earth's mantle

Sulfides are abundant inclusions in diamond, but their role in the diamond genesis is still debatable. To address this issue, experimental modeling of natural diamond-forming processes with the participation of sulfides has been performed with the MgCO 3–SiO 2–Al 2O 3–FeS system at 6.3 GPa in the temperature range of 1250–1800 °C, using a multi-anvil high pressure apparatus of the “split-sphere” type. As a result of redox reactions involving carbonate, oxides and sulfide, diamond and/or graphite are produced in association with garnet, orthopyroxene, coesite and sulfides (pyrite, pyrrhotite). Diamond crystals, formed from the carbon of initial carbonate, are found to contain nitrogen impurity with total concentration of approximately 1500 ppm, and defects related to hydrogen impurity. Based on the experimental data and thermodynamic calculations, the processes of the carbonate–oxide–sulfide interaction are reconstructed, revealing the role of sulfides as a reducing agent for CO 2-fluid. It is established that pyrrhotite acts as the reducing agent irrespective of its aggregate state (solid or melt). At temperatures below melting, pyrrhotite is enriched with sulfur and depleted with iron from FeS to Fe 0.85S. At higher temperatures, sulfide melt is enriched with sulfur and crystallizes as pyrite and pyrrhotite during quenching. The medium in which diamond and/or graphite crystallize is a CO 2-dominated fluid containing dissolved carbon, silicates, oxides and sulfides. The investigated processes and mechanisms of diamond crystallization can be considered as a possible model of the diamond formation in sulfide-bearing paragenesis in mantle metasomatism or UHP metamorphism of crustal material.

Coupled Lu–Hf and Sm–Nd geochronology constrains prograde and exhumation histories of high- and ultrahigh-pressure eclogites from western Norway

Eclogites from the Western Gneiss Region (WGR), Norway, that formed over a wide range in temperature (∼ 600 °C to > 850 °C) and pressure (high- and ultrahigh-pressure conditions; UHP) have a diverse range of Lu–Hf and Sm–Nd ages that correlate with their P–T histories. Three eclogites (∼ 676–850 °C and ∼ 2.0–2.85 GPa) yield Lu–Hf ages of 419.5 ± 4.3 Ma, 416.3 ± 3.7 and 412.0 ± 4.7 Ma, and two Sm–Nd ages of 402.7 ± 4.6 and 398.3 ± 5.5 Ma. The former are interpreted to date early garnet growth and the latter are interpreted to date near-peak metamorphism. These results support previous proposals that, for eclogites which recrystallized below Lu–Hf and Sm–Nd closure temperatures, the differences in ages for the two isotopic systems constrain the period of prograde garnet growth. In the case of the WGR, garnet growth is estimated to have occurred over a period of at least 20 m.y., beginning at ∼ 425–420 Ma. In contrast, two UHP eclogites (> 800 °C and > 3 GPa) yield significantly younger and overlapping Lu–Hf and Sm–Nd ages of 390–370 Ma; these ages are significantly younger than ages that are generally accepted for peak eclogite-facies conditions in the WGR, although they overlap 40Ar/ 39Ar ages that have been previously determined on white mica from the same samples or localities, indicating that the Lu–Hf and Sm–Nd ages may reflect rapid exhumation and cooling during the latest stages of HP and UHP metamorphism in the WGR.

H 2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Garnet and omphacite from ultrahigh-pressure eclogites from the Dabie orogen, eastern China were investigated by Micro-FTIR. The results show that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH), with H 2O contents varying from 14 to 1915 ppm (H 2O wt) and from 105 to 695 ppm, respectively. Within the same sample, the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim. Low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation. The δD values of omphacites are between −108.4‰ and −114.2‰, and independent of water contents. Heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during UHP metamorphism was very limited. The estimated whole-rock water content based on mineral H 2O contents is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle (at least) several hundreds ppm water to mantle depths. The preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after subduction. The water released during decompression might represent the early-stage retrograde fluid.

Dabieshan UHP Metamorphic Terrane: Sr-Nd-Pb Isotopic Constraint to Pre-metamorphic Subduction Polarity

The Qinling-Dabie orogen in east-central China was produced by Triassic collision between two Precambrian cratons. The traditional view about the pre-collisional plate movement is that the South China Block (SCB = Yangtze craton) underwent northerly subduction beneath the North China Block (NCB = Sino-Korean craton). In this paper, we use the Sr-Nd-Pb isotopic tracer technique to test the validity of this view. In all published models, ultrahigh-pressure and high-pressure metamorphic rocks and subjacent "basement" gneisses of Dabieshan are considered to be the deeply subducted, then exhumed crustal slices of the South China Block. Cretaceous igneous rocks, including granitic batholiths generated by partial melting of the Yangtze middle to lower crust and mafic rocks derived from the subjacent upper mantle, should carry the isotopic signatures of the SCB lithosphere. However, Sr-Nd-Pb isotopic tracer analyses indicate that the Cretaceous granitoids and mafic-ultramafic rocks of Dabieshan have an NCB isotopic signature; hence the subducted lithosphere has little resemblance to the Yangtze craton. We conclude that an opposite polarity of the Triassic subduction is more likely, namely that the NCB plate was subducted southward beneath the SCB plate. A new tectonic model is presented, and we suggest that the plate boundary is located in the southern margin of the Dabieshan.

The onset of the Tan–Lu fault movement in eastern China: constraints from zircon (SHRIMP) and 40Ar/39Ar dating

AbstractIn eastern China, the Dabie Shan–Su–Lu orogenic belt has been separated by the Tan–Lu sinistral strike–slip fault. Mylonites are exposed along the strike–slip fault system in the southern segment, and along the eastern margin of the Dabie Shan orogenic belt. The country rocks of the mylonites are retrograde UHP eclogites, gneissic granites, muscovite granites and gneisses. The ductile strike–slip shear zone trends 30–40°N (NE30–40°‐trending) and exhibits stretching lineations and nearly vertical, SE‐dipping foliations. Most of the zircon grains separated from mylonites have a weighted average radiometric age of 233 ± 6–225 ± 6 Myr. These data constrain the onset of the Tan–Lu sinistral strike–slip movement and imply that the Tan–Lu sinistral strike–slip motion developed after retrograde UHP metamorphism. The related phengite within the eclogite rocks on the western side of the Tan–Lu fault, with 40Ar/39Ar plateau ages of c. 182–190 Myr, is also deformed and aligned parallel to the almost NE trending stretching lineations. Non‐metamorphosed granites exhibit sinistral strike–slip shearing and indicate that the Tan–Lu fault initially developed after 182–190 Myr. Muscovite collected from the mylonite yields 40Ar/39Ar plateau ages of 162 ± 1–156 ± 2 Myr. The zircon SHRIMP age data, the muscovite 40Ar/39Ar plateau ages, together with structural and petrological field information support the interpretation that the Tan–Lu strike–slip fault was not related to the Yangtze–north China plates collision, but corresponded to the formation of a NE‐trending tectonic framework in eastern China starting c. 165–160 Ma.

Rb/Sr and Sm/Nd isotope analyses of CCSD eclogites (Sulu, China): A test for the closure temperature concept

The age of UHP metamorphism in the Sulu belt is now well established between 228 and 221 Ma ((Liu et al., 2004) and references therein). However, precise interpretation of the whole set of geochronological data in terms of P–T path that allows to deduce exhumation rates remains uncertain. Here we may pose two key questions: (1) Do the zircon U/Pb ages date the prograde, peak or retrograde stage of the P–T path? (2) Should the zircon ages be interpreted as the crystallization or cooling ages? We choose to address the second question here. Eclogite samples from the Chinese Continental Scientific Drilling (CCSD) borehole in Donghai (Sulu Belt, eastern China), provide a unique opportunity to test the closure temperature concept (Dodson, 1973). Our sample set is composed of ten eclogites selected along a 2000 m core transect. All the eclogites recorded the same P–T path (peak at 850–910 C, >35 kbar followed by nearly adiabatic decompression, (Tong et al., accepted)), however, they are characterized by a wide variety of modal and chemical compositions (i.e., pyrope content in garnet from 10 to 50%), as well as granulometry (grain size between 2 mm and 200 lm). The closure temperature theory expects that grain size, modal composition and chemical composition of the mineral phases control the temperature at which a radiochronometer is set to function during cooling. Therefore, the comparison of geochronological data obtained for a given isotopic system applied to the same mineral species in the 10 samples should provide an excellent test for this concept since all the parameters given before are variable. The results of Sm/Nd dating on garnets and Rb/Sr dating on phengites of different size fractions of the same sample will be discussed at the conference.

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P– T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism. Abundant H 2O–CO 2, H 2O- or CO 2-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low- P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H 2O- and/or CO 2-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.

Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust?

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO 2 (2–5 wt%), medium TiO 2 (1–2 wt%) and low TiO 2 (<1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative εNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction. Geochronological data indicate that the UHPM occurred about 500–440 Ma ago, and that there were two eclogite protoliths with ages of 800–750 and ∼1000 Ma, respectively. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, north Qaidam mountains, which have Sr and Nd ages of 768±39 and 780±22 Ma. The age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites. A later series of calc-alkaline, island arc volcanic rocks was formed at about 500 Ma ago, essentially contemporaneous with the UHP metamorphism. We propose the following tectonic model for the evolution of the north Qaidam UHP belt. At about 1000 Ma ago, a number of continents were amalgamated to form the Rodinian continent in this general area, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800–750 Ma ago to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the north Qaidam mountains. In the early Paleozoic another Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma ago and the subduction-related granites (470–450 Ma). Once the oceanic lithosphere was consumed, the subsequent continent–continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites, to cause the ultrahigh pressure metamorphism.

The age of deep, steep continental subduction in the NW Himalaya: Relating zircon growth to metamorphic history. Comment on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005)

Leech et al. [Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83–97], present 3 clusters of ages for growth stages in zircon from quartzo-feldspathic gneisses hosting coesite-bearing eclogite from the Tso Morari Complex, NW India. These age clusters, from oldest to youngest, are interpreted to represent the age of ultrahigh-pressure metamorphism, a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of Indian crust to have started earlier than previously accepted. However, no petrographic evidence, such as inclusions in the zircons relating to particular metamorphic events, is presented to substantiate the proposed sequence of metamorphic stages. Previously published data from eclogites of the same area indicate that coesite–eclogite is not the first but at least the second eclogite facies stage. In addition, the newly proposed time interval between coesite–eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet–garnet couples in eclogite. Neither the age of ultrahigh-pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data.

Reply to comment by P.J. O'Brien on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth Planetary Science Letters 234 (2005) 83–97

Reply to comment by P.J. O'Brien on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth Planetary Science Letters 234 (2005) 83–97

U–Pb isotopic studies of eclogites and their host gneisses in the Xitieshan area of the North Qaidam mountains, western China: New evidence for an early Paleozoic HP–UHP metamorphic belt

Using both TIMS and SHRIMP methods, U–Pb isotopic dating was carried out on zircons from eclogites and their host gneisses in the Xitieshan area of the north Qaidam mountains, western China. Zircons from both eclogites and retrograded eclogites with metamorphic characteristics gave ages of 485–500 Ma, representing the age of eclogite facies metamorphism. Euhedral zircons from the eclogites, which show a magmatic character (Th/U>1), yielded ages of 750–800 Ma, which are believed to represent the magmatic crystallization age of the protlith. TIMS dating of zircons from granitic gneiss enclosing retrograded eclogite gave an upper intercept age of 952±13 Ma and a lower intercept age of 478±44 Ma. The former probably represents the magmatic crystallization age of the granitic protolith, and the latter is in agreement with the age of metamorphic zircons from the eclogite within the margin of error. This implies that the eclogites and their enclosing gneisses in Xitieshan area as a whole underwent HP–UHP metamorphism during an early Paleozoic orogeny to form one part of the north Qaidam HP–UHP metamorphic belt. These results also suggest an ‘in situ’ rather than ‘tectonic'relationship between the eclogites and their host gneisses.

Dating of subduction and differential exhumation of UHP rocks from the Central Dabie Complex (E-China): Constraints from microfabrics, Rb–Sr and U–Pb isotope systems

The correlation of deformation fabrics and metamorphic reactions with geochronologic data of UHP metamorphic rocks demonstrate that the multistage subduction and exhumation history of the Central Dabie Complex requires rapid subduction and rapid initial exhumation. Moreover, these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems. Thus, our age data do not simply reflect a thermal/cooling history. In the investigated section, the maximum age for UHP is given by the 244 ± 3 Ma (2 σ) U–Pb age of a pre-UHP titanite phenocryst that survived UHP metamorphism and subsequent tectonometamorphic events. A minimum age for UHP is set by the 238 ± 1 Ma (2 σ) 238U– 206Pb mineral isochron age of titanite and cogenetic epidote. These minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust. In the period of ca. 238–218 Ma, the UHP terrain records HT metamorphism, local partial melting, and extensive pervasive strain below the eclogite (jd + grt) stability field. Exhumation was polyphase with a first phase of fast exhumation, succeeded by episodes of HT metamorphism and concomitant deformation at deep/mid crustal level between 238 and 218 Ma. Slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca. 209–204 Ma. The resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep, which was the dominant deformation mechanism in quartz–feldspar rocks, in combination with fluid influx.