Western Dabie Research Articles

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.

S-wave velocity structure of the lithosphere beneath the western Dabie Mountain, China

S-wave velocity structure of the lithosphere beneath the western Dabie Mountain, China

Exhumation P– T path of UHP eclogites in the Hong'an area, western Dabie Mountains, China

Four generations of mineral assemblages have been identified in ultrahigh-pressure (UHP) eclogites from the Hong'an area, recording four metamorphic stages of the exhumation trajectory, i.e., garnet + omphacite + kyanite+ zoisite/epidote + rutile + coesite formed at the UHP stage, garnet + omphacite + paragonite + phengite + amphibole + epidote + quartz at the high-pressure (HP) eclogitic stage, amphibole + epidote + albite–oligoclase + paragonite + quartz at the epidote-amphibolite facies (EA) stage, and actinolite+ albite and pumpellyite + albite + K-feldspar + muscovite + chlorite + margarite + epidote + quartz at the greenschist facies (GS) stage. Garnet formed at the UHP stage was overgrown by atoll garnet at the HP stage. X-ray images, mineral compositions and compositional profiles indicate that the UHP garnet experienced a variable Fe–Mg modification in response to Fe–Mg exchange between garnet and omphacite prior to the overgrowth of atoll HP garnet. A volume diffusion zone was also observed between the UHP and HP garnets. The following P– T conditions were estimated for these mineral assemblages: 480 to 560 °C and 2.5 to 2.9 GPa with median values of 520 °C and 2.8 GPa at UHP stage, 575 to 685 °C and 1.6 to 1.9 GPa with the average results of 634 ± 37 °C and 1.8 ± 0.06 GPa at the HP stage, 500 to 640 °C and 0.4 to 1 GPa at the EA stage, and 160 to 320 °C and 0.2 to 0.8 GPa at the GS stage. The Hong’an UHP eclogites experienced a temperature increase of over 100 °C from the UHP to HP stages and continued to exhume from the HP to EA stages through a process of approximately isothermal depression, which was followed by a significantly cooling from the EA to GS stages. In the process of exhumation, UHP eclogites witnessed a series of events involving fluid influx, resulting in the formation of hydrous minerals at different stages. The nature of fluids changed from K-rich at the HP stage to K-poor at the EA stage and then K-rich again at the GS stage.

Triassic paragonite- and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan

Paragonite- and garnet-bearing high-grade epidote-amphibolite (PGEA) in the Ise area of the Hida Mountains, Japan is characterized by the high-pressure (HP) epidote-amphibolite facies parageneses ( M 1), garnet + hornblende + clinozoisite + paragonite + quartz + rutile. Paragonite and garnet of the peak M 1 stage are locally replaced by retrograde albite (+ oligoclase) and chlorite ( M 2), respectively. Phase equilibria constrain peak metamorphic conditions of P = 1.1–1.4 GPa and T = 530–570 °C, and a decompressional P– T path for this rock. Mineral parageneses of prograde epidote-amphibolite facies are comparable to some HP rocks from the Hongan region of western Dabie, but differ from other HP mafic schists with cooling ages of c. 330 Ma in the Hida Mountains. New paragonite K–Ar dating for the PGEA yields a Triassic cooling event at 210 Ma that is coeval with regional cooling and exhumation of the Sulu–Dabie–Qinling (SDQ) belt. Both petrological and geochronological data of the Triassic HP epidote-amphibolite in Hida Mountains support our earlier hypothesis that the SDQ belt extends across the Korean Peninsula to SW Japan.

Provenance of Jurassic sediments in the Hefei Basin, east-central China and the contribution of high-pressure and ultrahigh-pressure metamorphic rocks from the Dabie Shan

The provenance of the Jurassic sediments in the Hefei Basin is constrained by compositions of the detrital K-white micas and garnets, and SHRIMP dating of the detrital zircons, which can help to understand the evolution and to reconstruct the paleogeographic distribution of HP–UHP rocks in the Jurassic Dabie Shan. (1) For the oldest Mesozoic sediments at the bottom of the Fanghushan Formation ( J 1), the predominance of the early Paleozoic and Luliang (1700–1900 Ma) zircons indicates a major source from the North China Block. However, Neoproterozoic zircons as the major component in other Jurassic sediments indicate that the source rocks were mainly derived from the exhumed Yangtze Block in the Dabie Shan. (2) The co-occurrence of high-Si phengites and Triassic zircons provides stratigraphic evidence that the first exposure of the UHP rocks at the Earth's surface in the Dabie Shan occurred in the Early Jurassic during deposition of the Fanghushan Formation. (3) From the east to the west of the Hefei Basin, there is a spatial variation in the compositions for detrital micas and garnets, and in the U–Pb ages of detrital zircons. Evidently, HP–UHP rocks were widely distributed at outcrop in the eastern Dabie Shan. In contrast, they were less important in the western Dabie Shan during the Jurassic.

Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

AbstractOmphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H2O, and Amp + Ab = Omp ± Czo + Qtz + H2O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H2O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.

Carboniferous and Triassic eclogites in the western Dabie Mountains, east‐central China: evidence for protracted convergence of the North and South China Blocks

AbstractSHRIMP U–Pb dating and laser ablation ICP‐MS trace element analyses of zircon from four eclogite samples from the north‐western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events. Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma. One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra‐high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt. REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions. Zircon also grew during later retrogression. Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy‐REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks. Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block. In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone. The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone. The first collision produced local eclogite facies metamorphism in the Huwan shear zone. The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone.

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

SHRIMP dating of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains, China

This note reports the SHRIMP U-Pb data of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains. Zircons from the rock occur mainly in garnet and other metamorphic minerals with sharp boundaries and exhibit textures growing under metamorphic conditions. Analyses of 7 grains give206Pb/238U ages ranging from 335 to 424 Ma, showing a certain degree of radiogenic lead loss. This suggests a minimum age of (424± 5) Ma for the metamorphic zircons, as well as the high-pressure metamorphic event. The outer peripheral zone of a zircon gives206Pb/238U age of about 300 Ma. Combined with Sm-Nd,40Ar-39Ar, U-Pb and207Pb/206Pb ages, the peak metamorphism of the Xiongdian eclogite is documented between 424–480 Ma.

Petrographic and SHRIMP Studies of Zircons from the Caledonian Xiongdian Eclogite, Northwestern Dabie Mountains

AbstractThe Xiongdian eclogite occurring in the Sujiahe tectonic mélange zone at Luoshan County, Henan Province, in the western Dabie Mountains, is typical high‐pressure (HP)‐ultrahigh‐pressure (UHP) and medium‐temperature eclogite. The occurrence, internal texture and surface characteristics of zircons in eclogite were studied rather systematically petrographically combined with the cathodoluminescence (CL) and scanning electron microscope (SEM) methods. Zircons are mainly hosted in garnet and other metamorphic minerals with sharp boundaries, have a multifaceted morphology and are homogeneous or exhibit a metamorphic growth texture in the interior, thus indicating that they are the product of metamorphism. SHRIMP analyses give zircon 206Pb/238U ages of 335 to 424 Ma and show a certain degree of radiogenic Pb loss; therefore it may be inferred that the age of 424±5 Ma represents the minimum age of a HP‐UHP metamorphic age. From the above analyses coupled with previous Sm‐Nd, 40Ar‐39Ar, U‐Pb and 207Pb/206Pb age data, it is suggested that the peak metamorphic age of the Xiongdian eclogite should be between 424 and 480 Ma. This study further validate the view of the existence of a Caledonian HP‐UHP metamorphic event in the western Dabie Mountains.

Stable Isotope Geochemistry of High to Ultrahigh Pressure Metamorphic Rocks in the Dabie Mountains

The identification of coesite and micro-diamond in the metamorphic rocks of the Dabie and S U ~ U terranes implies that the crustal materials canbe subducted to mantle depths of a t least 100 km (Okay et al., 1989; Wang et al., 1989; Xu et al., 1992). Stable isotope analyses of high to ultrahigh pressure (UHP) metamorphic rocks can be used to determine whether isotopic compositions are largely inherited from pre-metamorphic protoliths or, alternatively, if compositions have been modified by the infiltration of external fluids. Previous investigations show that minerals in eclogites from Qinglongshan in the western S U ~ U have extreme negative 6I8O values of -10 to -8%, whereas eclogites from Shuanghe in the eastern Dabie have 6180 values of -3 to +7%0 (Yui et al., 1995; Zheng et al., 1996, 1998). Preservation of oxygen isotope equilibrium between the eclogite minerals suggests that the eclogite protoliths underwent hydrothermal alteration by fluids of meteoric origin prior to subduction. Limited crust-mantle interaction has been inferred between the '*O-depleted eclogites and the mantle when the slab containing the eclogite protoliths was subducted to mantle depths, implying a short residence time of the eclogite-containing slab in the mantle (Zheng et al., 1998). Concordant oxygen isotope temperatures are obtained from different mineral pairs, suggesting great cooling rates of the eclogite slab at the initial stage of exhumation. In this study, the carbon, oxygen and hydrogen isotope compositions of mineral separates have been determined for marble, eclogites, and two types of gneiss (biotite gneiss and granitic gneiss) from the Dabie Mountains. The marble, eclogites and biotite gneiss from the eastern Dabie contain coesite and thus belong to the UHP slab, whereas there is no mineralogical evidence for the marble and eclogite from the western Dabie and the granitic gneiss from the eastern Dabie for having experienced the UHP metamorphism. The results place constraints on the sedimentary environment of marble protolith, the scale of fluid-rock interaction during plate subduction, and tectonic relationship between UHP and non-UHP units.

Proterozoic Metamorphic Rock-Hosted ZR, Y, and HREE Mineralization in the Dabie Mountain Area, Central China

Zr, Y, and heavy rare earth element (HREE) mineralization hosted by late Proterozoic metamorphic rocks from the western Dabie Mountains, northern Hubei Province, central China was discovered in 1978 by the Hubei Regional Geological Party (Li et al., 1981) and is examined further in this paper. The area is located in the EW-trending fold belt between the Sino-Korean and Yangtze cratons. Highly metamorphosed hornblende-plagioclase gneiss, granitic gneiss, and migmatites of the late Archean Dabie Group constitute the basement, which is unconformably covered by the late Proterozoic Hongan Group—a sequence of phosphorus-bearing layer(s), dolomitic marbles, muscovite-albite schist, felsic schists, and a variety of greenschists. The stratiform Zr, Y, and HREE mineralization is hosted by felsic and albite-muscovite schists of the lower part of the Hongan Group, which was highly stressed and deformed. The mineralized layer, about 10 m in thickness, occurs concordantly in the strata and extends discontinuously more...