Yuanbao Wu

U-Pb age, trace-element, and Hf-isotope compositions of zircon in a quartz vein from eclogite in the western Dabie Mountains: Constraints on fluid flow during early exhumation of ultrahigh-pressure rocks

Abstract Quartz veins in high-pressure (HP) to ultrahigh-pressure (UHP) rocks are the products of fluid-rock interaction, and thus provide insight into fluid processes in subduction zones. In this paper, we report an integrated study of mineral inclusion, trace-element, U-Pb age, and Lu-Hf isotope compositions of hydrothermal zircon grains from a quartz vein within an UHP eclogite outcrop from the Hong’an area, western Dabie Mountains. These data are used to decipher the age, conditions of formation, and source of fluid for zircon formation during the exhumation of UHP rocks. Zircon grains from the vein have perfect euhedral shape, and show sector zoning or weak zoning, indicating that they precipitated from the aqueous fluid responsible for the vein formation. Raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet, omphacite, rutile, quartz, and H2O, implying that they crystallized from aqueous fluid under HP eclogite-facies conditions. The zircon grains show low Th/U and Lu/Hf ratios, nearly flat HREE patterns, absent Eu anomalies and low LREE contents. These characteristics are consistent with their precipitation in the presence of garnet and epidote, and absence of feldspar, and thus suggest that trace-element concentrations in hydrothermal zircon are controlled by co-precipitation of mineral assemblages. Crystallization temperatures of 670 to 712 °C, which were calculated using the Ti content of zircon, are consistent with their formation under eclogitefacies conditions and may correspond to the temperature of the infiltrating fluid. The weighted mean 206Pb/238U age of 224.7 ± 1.3 Ma is taken as the best estimate for the age of quartz-vein formation and records aqueous fluid flow during the early exhumation stage of UHP rocks. The zircon grains in the quartz-vein have Hf compositions similar to those in the host eclogite, which demonstrates isotopic equilibrium between fluid and rocks and that the fluid-rock ratio was likely low.

First record and timing of UHP metamorphism from zircon in the Xitieshan terrane: Implications for the evolution of the entire North Qaidam metamorphic belt

Abstract The Xitieshan terrane is one of four metamorphic terranes in the North Qaidam metamorphic belt, which is an early Paleozoic high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt in NW China. However, conclusive evidence and precise timing of UHP metamorphism in the Xitieshan terrane have not been well documented. In this study, we report an integrated study of zircon grains from an amphibolite in the Xitieshan terrane in terms of mineral inclusions, trace elements, and U-Pb age systematics. The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. The metamorphic zircon domains have weak or no zoning, very low Th/U ratios, insignificant Eu anomalies, and flat HREE patterns. Zircon grains contain inclusions of garnet, omphacite and rutile, in addition to the coesite inclusion. These inclusions indicate that the metamorphic zircon grains formed under UHP metamorphic conditions. The metamorphic zircon grains were dated by the SIMS and LA-ICPMS methods and yield weighted mean ages of 432 ± 14 and 441 ± 9 Ma, respectively. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted 460-440 Ma with the peak UHP metamorphism at 441 ± 9 Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam metamorphic belt might have experienced coeval UHP metamorphism during the early Paleozoic (420-450 Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle.

Tracing crustal evolution by U-Th-Pb, Sm-Nd, and Lu-Hf isotopes in detrital monazite and zircon from modern rivers

Detrital zircon U-Pb age and Hf isotope studies are useful for identifying the chemical evolution of the continental crust. Zircon, however, is typically a magmatic mineral and thus often fails to document the timing of low-grade metamorphism, and its survival through multiple sedimentary cycles potentially biases the crustal evolution record toward older events. In contrast, monazite typically records metamorphic events and is less likely to survive sedimentary recycling processes, thus providing information not available by zircon. Here, we demonstrate that monazite apparently faithfully records the Sm-Nd isotope composition of the bulk rock and can therefore track the record of crustal evolution and growth, similar to Hf isotopes in zircon. We examine the utility of detrital zircon and monazite for studies of crustal evolution through a comparison of age and tracer isotope information using sediments from two large rivers draining the South China block (SCB). Monazite and zircon grains yield mostly Mesozoic and Paleozoic U-Pb ages and depleted mantle model age peaks at ca. 1900–1300 Ma, indicating that both minerals preserve similar, yet critical, information on the crustal evolution of the catchment area. In contrast, zircon yields abundant Neoproterozoic and older U-Pb ages with a very large spread of model ages, preserving a history strongly skewed to older ages. Based on the lack of known rocks of this age in the catchments, ancient zircon was likely sourced from sedimentary rocks within the catchment area. This combined data set presents a more complete history of crustal evolution and growth in the SCB and demonstrates the advantages of an integrated approach that includes both detrital monazite and zircon.

Two-Stage Exhumation of Ultrahigh-Pressure Metamorphic Rocks from the Western Dabie Orogen, Central China

Zircon trace element, U-Pb age, and Lu-Hf isotope composition were determined for two eclogite samples from the Xinxian ultrahigh-pressure (UHP) unit in the western Dabie orogen to constrain its exhumation processes. Cathodoluminescence imaging reveals that most zircons are metamorphic, while some have inherited magmatic cores in one sample. The magmatic cores have high Th/U and Lu/Hf ratios, high trace element contents, and a clear negative Eu anomaly, consistent with their igneous genesis. They yield an upper intercept age of Ma and a positive ϵHf(t) value of 8.0, arguing for reworking of early Mesoproterozoic crust during the middle Neoproterozoic in the western Dabie orogen. The metamorphic zircons contain mineral inclusions of garnet, omphacite, rutile, and quartz. They are characterized by low Nb, Ta, Y, and heavy rare earth elements (HREE) contents, a nearly flat HREE pattern, and an insignificant negative Eu anomaly. These indicate that the metamorphic zircons formed under high-pressure (HP) quartz eclogite-facies metamorphic conditions. The metamorphic zircons in the two samples have Ti-in-zircon temperatures of and C, respectively, suggesting that they formed in different HP eclogite-facies metamorphic stages. The metamorphic zircons in the two samples yield weighted mean U-Pb ages of and Ma, which may date the initial exhumation to ∼90 km and ∼700°C and a subsequent HP quartz eclogite-facies retrogression at ∼50 km and ∼600°C for ultrahigh-pressure (UHP) rocks in the western Dabie orogen. This yields a maximum exhumation and average cooling rates of ∼0.33 cm/yr and 8°C/Ma, respectively. Based on the published Rb-Sr and Ar-Ar ages of ∼212 Ma for the UHP rocks in the area, the subsequent stage of exhumation of UHP rocks has a maximum exhumation and average cooling rates of 0.67 cm/yr and 65°C/Ma, respectively. This two-stage cooling process may be common for HP-UHP metamorphic terranes in continental collision zones.

Zircon Record of Ocean-Continent Subduction Transition Process of Dulan UHPM Belt, North Qaidam

The North Qaidam UHPM (ultra-high pressure metamorphism) belt is a typical continental subduction-collision belt containing continental crust deep subduction metamorphic products and oceanic crust relics, And it is an ideal region to study the ocean-continent transition and exhumation mechanism of oceanic UHP rocks during continental deep subduction process. In this paper, we report integrated in situ U-Pb, Lu-Hf and O isotope analyses of zircons from a serpentinized harzburgite as well as U-Pb dating for zircons from a kyanite eclogite from the North Qaidam Dulan UHPM terrane, and use these data to discuss the ocean-continent transition and exhumation mechanisms of oceanic UHP rocks during continental deep subduction. The serpentinized harzburgite was dated at 448±9 Ma, consistent with 455±5 Ma age for the kyanite eclogite within analytical errors. Zircons from the serpentinized harzburgite have uniform 176Hf/177Hf values ranging from 0.282 842 to 0.282 883 and εHf(t) values from 11.6 to 13.3. Zircon δ18O values of the serpentinized harzburgite vary from 4.47‰ to 5.29‰, slightly lower than the value of 5.3‰±0.6‰ for the normal mantle zircon. These Hf-O isotopic features indicate that the protolith of the serpentinized harzburgite was derived from depleted-mantle source, and might have experienced high-temperature rock-water interaction. Therefore, the serpentinized harzburgite was possibly located in the lower part of an oceanic section. The serpentinized harzburgite and kyanite eclogite were both formed due to the subduction of oceanic crust. The UHP metamorphism occurred successively from the oceanic crust to continental crust rocks of the North Qaidam UHP terrane. Low-density serpentinized peridotite and continental rocks possibly have negative buoyancy and play a key effect on preservation and exhumation of high-density oceanic eclogite.

Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and 40 Ar/ 39 Ar dating of eclogites

Publisher Summary The Central China Orogenic Belt (CCOB) is the largest known ultrahigh-pressure metamorphic (UHPM) belt. It extends from Sulu in Eastern China, via the Dabie, Qinling, and North Qaidam mountains to Altyn Tagh some 3000 km to the west. Since the discovery of coesite and diamond-bearing metamorphic rocks in Dabie Shan, the belt has been studied intensively. Some of the results have led to controversy regarding the number of (U)HP metamorphic events and their absolute and relative ages. The prominence of Dabie Shan as part of the largest UHPM belt is reflected by a wealth of published work since the discovery of coesite over 20 years ago. Despite two decades of efforts to decipher its geologic secrets, interpretation of the many reported isotopic ages of the Dabie Shan terrane that aimed to address its metamorphic history is still hampered by the absence of robust thermobarometric data for the same samples. Pre-Triassic metamorphism cannot be traced by Lu–Hf Grt–Cpx geochronology in the main volume of Dabie Shan, which was subjected to Triassic (U)HP metamorphism. The Triassic event erased previous signatures of some, but not all isotopic systems, as is evident from zircon U-Pb ages.

Oriented inclusions of pyroxene, amphibole and rutile in garnet from the Lüliangshan garnet peridotite massif, North Qaidam UHPM belt, NW China: an electron backscatter diffraction study

Oriented inclusions of clinopyroxene, orthopyroxene, sodic amphibole and rutile have been identified in garnet from the L€ uliangshan garnet peridotite massif in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, northern Tibetan Plateau, NW China. Electron backscatter diffraction (EBSD) analyses demonstrate that nearly half of the measured intracrystalline clinopyroxene (8 out of 17) have topotactic crystallographic relationships with host garnet, that is, (100)Cpx//{112}Grt, (010)Cpx//{110}Grt and [001]Cpx//<111>Grt. One-fifth of the oriented sodic amphibole (23 out of 110) inclusions of have topotactic crystallographic relationships with host garnet, that is, (010)Amp// {112}Grt, (100)Amp//{110}Grt and [001]Amp//<111>Grt. Over a third of rutile (36 out of 99) inclusions also show a close crystallographic orientation relationship with host garnet in that one <103>Rt and one <110>Rt parallel to two <111>Grt while the axes of [001]Rt exhibit small girdles centred the axes of <111>Grt. But, no ‘well-fit’ crystallographic relationship was observed between orthopyroxene inclusions and host garnet. Considering a very long and complex history for the L€ uliangshan garnet peridotite, we suggest that the low fit rates for these oriented minerals may result from several possible assumptions including different generations or multi-stage formation mechanisms, heterogeneous nucleation and growth under non-equilibrium conditions, and partial changes of initial crystallographic orientations of some inclusions. However, the residual quantitative ‘well-fit’ crystallographic information is sufficient to indicate that the nucleation and growth of many pyroxene, amphibole and rutile are controlled by the lattice of the host garnet. The revealed close topotactic relationships accompanied by clear shape orientations provide quantitative microstructural evidence demonstrating a most likely exsolution/precipitate origin for at least some of the oriented phases of pyroxene, sodic amphibole and rutile from former majoritic garnet and support an ultra-deep (>180 km depth) origin of the L€ uliangshan garnet massif.