Ultra-high-pressure metamorphism of carbonate rocks in the Dabie Mountains, central China

Abstract Widespread ultra-high-P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra-high-P metamorphic event occurred on a regional scale during Triassic collision between the Sino-Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite-bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra-high-P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine-grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite-bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra-high-P metamorphic terrane. Retrograde minerals are fine grained and replace coarse-grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An₁₈) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively. The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra-high-P conditions are consistent with the coexistence of grossular-rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low X_CO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.     

关于大别山榴辉岩相变质作用

大别山产出的榴辉岩相岩石包括石榴橄榄岩、榴辉岩、榴云片岩、榴辉片麻岩、榴玉英岩和榴辉大理岩等不同系列,它们均分布于花岗质片麻岩中。矿物共生序列研究表明,榴辉岩相岩石经历了从绿帘角闪岩相、柯石英榴辉岩相、角闪榴辉岩相、绿帘角闪岩相到绿片岩相的演化过程。花岗质片麻岩及变质火山—沉积岩系并未经历超高压变质作用,但却与榴辉岩相岩石经历了同一期绿帘角闪岩相变质事件,证明二者在地壳范围内发生了构造合并     

Fabric and Deformation of Omphacite in Dabie Ultra-high-pressure Ecologites

The rheological characters of omphacites in Dabie ultra-high-pressure eclogite have been studied interms of fabric, dislocation and micro-structures. 1. The eclogite has undergone high-temperature deformation, thus forming omphacite lattice preferred orientation. In addition to creep dislocation, the omphacite ductile deformation may have other mechanisms, such as diffusion creep and grain boundary migration. 2. The main-phase deformation of eclogite is coaxial, but asymmetry strain also exists due to strain partitioning in the Dabie erogenic belt. 3.The twin measured by the universal stage is (100), indicating that omphacite high-T deformation was superimposed by low-T deformation. 4. Subgrain structure is common in omphacite, but the deformation features of the omphacites in the Shuanghe area and Bixiling area are different, the latter being dominated by dynamic recrystallization. 5. The Flinn plots show that the strain of omphacite belongs to the constriction ellipsoid and stretching strain, which is     

大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

对大别地区角闪岩相－麻粒岩相和榴辉岩相岩石进行较系统的地球化学研究,探讨了超高压俯冲变质及折返过程中元素的再分配行为,并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明,大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用,并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩Ｎｂ／Ｔａ比值升高,但后期的退变质作用使榴辉岩与其寄主围岩间的Ｎｂ／Ｔａ比值相对     

Thermobarometry of phengite-bearing eclogites in the Dabie Mountains of central China

Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite-bearing eclogites from Dabieshan have been assessed through a consideration of Fe²⁺–Mg²⁺ partitioning between garnet–omphacite and garnet–phengite pairs and of the reaction equilibrium celadonite+pyrope+grossular=muscovite+diopside, which incorporates an evaluation of the extent of the strongly pressure-dependent inverse Tschermak's molecule substitution in the phengites. For the latter equilibrium, the calibration and recommended activity–composition models indicated by 45 ) have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others. Confirmation that in some phengite-eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite (and in some cases even diamond) rather negates previous suggestions that coesite may have been stabilized in only very localized, possibly just intracrystalline, domains. Inherent difficulties in the evaluation of peak metamorphic temperatures from Fe²⁺–Mg²⁺ partitioning between mineral phases, due to uncertainties over Fe³⁺/Fe²⁺ ratios in the minerals (especially omphacites), and to re-equilibration during extensive retrograde overprinting in some samples, are also assessed and discussed. Our results indicate the existence in south-central Dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions that south-central Dabieshan comprises a structurally coherent continental-crust terrane with a regional P–T gradient signalling previous deepest-level subduction in the north. Instead, we recognize the Central Dabie ultra-high-pressure (coesite eclogite-bearing) terrane to be structurally overlain by a Southern Dabie high-pressure (quartz eclogite-bearing) terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section.     

Unusual Hf contents in metamorphic zircon from coesite-bearing eclogites of the Dabie Mountains, east-central China: implications for the dating of ultrahigh-pressure metamorphism

Metamorphic zircon from coesite‐bearing eclogites in the Dabie Mountains encloses high‐P phases, and may have formed at the peak of ultrahigh‐pressure (UHP) metamorphism. Morphologically, the metamorphic zircon typically occurs as small, multi‐faceted, near‐spherical grains with homogeneous internal structure and weak backscattered electron (BSE) luminescence. Geochemically, it is characterized by extremely high and relatively constant contents of hafnium (Hf) and very low contents of Y, U and Th, reflecting the contraction of the zircon lattice under the UHP conditions. High contents of Hf may be characteristic of zircon formed during UHP metamorphism, which has important consequences for interpretation of geochronological results. We propose that the metamorphic zircon extremely enriched in Hf may be used to date the peak of UHP metamorphism that produced the coesite‐bearing eclogites in the Dabie Mountains, and potentially in other UHP terranes.     

大别山太湖—马庙断裂带两侧变质地体的增温变质作用

大别山榴辉岩地体以太湖 马庙断裂带为南界。该断裂带近东西走向, 向南倾斜。通过对石榴石中 Ｍｇ 含量由核部向边缘增高的生长环带构造的分析和详细的 Ｐ Ｔ 轨迹的计算表明, 断裂带北侧的榴辉岩和南侧的角闪岩及角闪片麻岩均经历了降压增温变质过程。热榴辉岩由地幔深处的折返和地温梯度的升高为可能的热源。     

Tectonic Framework and Evolution of the Dabie Mountains in Anhui, Eastern China

The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtze continental plate, the coesite- and diamond-bearing ultra-high pressure metamorphic zone and themeta-ophiolitic melange zone in the subducting basement, the fore-arc flysch nappe and the back thrust zoneoccurring respectively on the southern and northern margins of the Sino-Korean continental plate and the in-herited basin with molassic deposits on the northern margin. When the palaeo-Dabie oceanic plate subductednorthward in the Early Palaeozoic, volcanic arc and back arc basin probably formed on the southern margin ofthe Sino-Korean continental plate. The Sm / Nd isotopic dating of the strata and eclogite which were drawn in-to the foreland fold-thrust zone indicates that the intense collision of the two continental plates took place inthe Early Mesozoic.     

Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Ultra-high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9  mm to analytical point 18) has decreasing X_Sps and increasing X_Pyr, while Segment II (from analytical point 18, 1  mm outward to the rim) has increasing X_Sps and X_Alm and decreasing X_Pyr and X_Grs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P – T  conditions are estimated to be 1.35  GPa and 850  °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6  GPa and 700  °C. The well-preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra-high-pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.     

Microstructural Features of Garnets in Jadeite Quartzite from Dabie Mountains, China

The research into the high- pressure(HP) and ultrahigh-pressure (UHP) metam orphism has remained on the cuttingedge as well as a hot issue in the contemporary geosciencessince the coesite and diamond were discovered in the metamor-phic rocks.The jadeite quartzite from Anhui Province occur-ring as lenses within gneisses contains a m ineral assem blage ofjadeite,garnet and quartz.The discovery of coesite and itspseudomorphs in jadeite and garnet proves that the jadeitequartzite is an im portan…     

中国东部大别山超高压变质杂岩中的石英硬玉岩带

大别山的石英硬玉岩是大别山超高压变质杂岩中的重要成员,与大理岩和榴辉岩紧密共生,呈大小不等的构造透镜体产出在云母斜长片麻岩和含硬玉片麻岩中,分布在长约４０ｋｍ,宽约１ｋｍ的带内。透镜体中心常为花岗变晶结构,边部有不同程度退变并面理化,向外围逐渐变为含硬玉片麻岩。岩石的主要矿物组成为硬玉、石英、石榴石、金红石。退变的石英硬玉岩中还有钠长石、霓石、霓辉石、榍石等。硬玉和石榴石中都有柯石英包体。硬玉的Ｊｄ端元组分为８１．２５％～９０．２７％。恢复的石英硬玉岩的原岩为硬砂岩,与大理岩伴生的榴辉岩的原岩为泥灰岩。因此,石英硬玉岩与共生的大理岩和榴辉岩都属于榴辉岩相变质的表壳岩系,它的成带分布、其中有柯石英的产出,进一步证明大陆地壳能够俯冲到１００ｋｍ左右深度并迅速折返地壳后使其中的高压标志保存完好。     

Eclogites preserved as pebbles in Jurassic conglomerate, Dabie Mountains, China

Two types of eclogite pebbles were discovered in Middle to Upper Jurassic conglomerates from the Hefei Basin north of the Dabie ultrahigh-pressure (UHP) terrain, China. Type A eclogite pebbles are characterized by idioblastic garnet with well preserved chemical zonation. Si content in phengite is lower than 3.5 per formula unit (pfu). The maximum metamorphic pressure is lower than 2.5 GPa, and the temperature is below 600 °C. Type B eclogite pebbles contain coesite pseudomorphs in xenoblastic garnet. Si content in phengite is higher than 3.5 pfu. The maximum metamorphic pressure is 2.8–4.0 GPa at 700 °C indicating UHP metamorphism.

Types A and B eclogite pebbles are comparable with eclogites occurring in the southern portion of the Dabie UHP terrain. Based on the petrologic similarities and northeastwards directed paleocurrents, we infer that the eclogite pebbles were eroded from the Dabie UHP terrain. Sandstones containing detrital phengite with Si content higher than 3.5 pfu are also derived from UHP rocks. These petrologic and stratigraphic data place time constraints on exhumation and erosion history of the Dabie UHP terrain.     

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

Hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern Dabie Mountains, China. The δ¹⁸O 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 δ¹⁸O 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 δ¹⁸O 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.     

Isotopic Geochronological Evidence for the Caledonian Xiongdian Eclogite in the Northwestern Dabie Mountains,China

A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crystallization age of 399.5±1.6 Ma. 40Ar-39Ar dating on amphibole gives a plateau age.of 399.2 ± 4 Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high-pressure metamorphism is older than 399.5 ± 1.6 Ma, suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301± 2 Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm-Nd mineral isochron of garnet+omphacite gives     

大别山地壳结构的Pb同位素地球化学示踪

研究了大别山东部北大别变质杂岩，南大别变质杂岩和白垩纪花岗岩的全岩Pb同位素组成，结果表明，北大别变质杂岩与南大别变质杂岩相比，前者以相对低放射成因Pb同位素组成为特征，按照Pb同位素组成在地壳垂向剖面上的变化模型，指出在大别山地壳垂向结构上，北大别变质杂岩应位于南大别变质杂岩之下，这一认识得到大别山不同构造岩石单元中产出的白垩纪花岗岩Pb同位素对岩浆源区示踪的有力支持，因此南大别超高压变质带是发育在北大别杂岩之上的一个构造岩片，这对进一步确定扬子克拉通向华北克拉通俯冲－碰撞的缝合线位置具有重要意义。     

中国中部高铝质超高压变质岩

我国中部在超高压条件下形成的高铝质变质岩石, 以蓝晶黄玉石英岩为特征, 具有以蓝晶石、富水黄玉、富镁硬绿泥石、柯石英、硬水铝石等为代表的高压－超高压变质矿物, 矿物组合主要为蓝晶石＋ 黄玉＋ 石英 （柯石英）、蓝晶石＋ 蓝刚玉±富镁硬绿泥石等。化学成分富铝而贫镁, 其原岩为陆缘高铝质泥质沉积岩。实验岩石学的研究表明, 蓝晶石＋ 黄玉在１０００℃～１２００℃,３．１～３．７７ＧＰａ 条件下仍可稳定存在,而石英已部分转变为柯石英。蓝晶石＋ 黄玉与柯石英一起稳定于超高压条件下。这表明以高铝质岩石为代表的地壳物质已进入到地幔深度, 达到这样的深度只能由会聚板块的俯冲作用而引起。因此,高铝质岩石的存在可以作为Ａ型俯冲作用的指示剂。超高压变质的高铝质岩石将成为陆－陆碰撞事件的重要研究内容。     

U—Pb Dating of Marble Assoicated With Eclogite from the Dabie Mountains,East China

Marble is associated with ultrahigh pressure eclogite in the Dabie Mountains, East China. U-Pb isotope data for the marble define a²³⁸U-²⁰⁶Pb isochron age of 435 ± 45 Ma with an initial²⁰⁶Pb/²⁰⁴Pb value of 18.075 ± 0.006. The age of ∼435 Ma is interpreted to represent the time of limestone diagenesis rather than the time of metamorphic resetting accompanied by the formation of the ultrahigh pressure eclogites at Triassic. The paleontologic and stratigraphic studies also favor a local excursion in the carbon isotopic composition of the latest Ordovician water within a sedimentary basin between the Sino-Korean and Yangtze cratons. Because the latest Ordovician limestone was involved in the eclogite formation, the continental collision between the Yangtze and Sino-Korean plates would postdate Ordovician. This project was financially supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

大别山超高压变质带层析地震调查

１９９７年３月中德合作进行了大别山科学钻探选址区层析地震调查,研究结果揭示出大别山超高压变质带（ＵＨＰ）地壳精细结构。郯庐断裂带近垂直延伸至Ｍｏｈｏ界面（可能更深）,在Ｍｏｈｏ界面以上郯庐断裂带西侧大别山地壳物质没有迁移进入其东侧。在主测线（横中至潜山镇）２０ｋｍ深度发现一个由西向东倾斜的强反射层,推测可能是一个大的滑脱层。超高压变质岩石（或地幔物质）可能由地幔深处通过郯庐断裂带再沿此滑脱层析返到