Tectonic Affinity, T-t Path and Uplift Trajectory of Eclogites from Northern Dabie Mountains, Central-Eastern China

INTRODUCTIONTheDabieMountainsaretheeasternsectionoftheE WtrendingQinling DabieorogenicbeltbetweentheNorthChinablockandYangtzeblock ,andistransectedatitseasternendbyTan Lufault.Ultrahigh pressuremetamorphic (UHPM)rocksincludingcoesites (Okayetal.,1989;Wangetal.,1989)andmicrodiamonds (Xuetal.,1992b)bearingeclogitesareexposedintheDabieMountainsandSu Luorogen ,whichisthelargestUHPMbeltontheearthsurface .TheUHPMrocksarecomposedofdifferentkindsofmetamorphicrockssuchascoesite anddiamo…     

Retrogressive Microstructures of the Eclogites from the Northern Dabie Mountains, Central China： Evidence for Rapid Exhumation

The ultrahigh-pressure eclogites from the northern Dabie Mountains in central China occurred as tectonic lens or blocks within granitic gneisses or meta-peridotites. Petrologic studies suggest that the studied eclogites experienced strongly retrogressive metamorphism and produced a series of characteristic retrogressive microstructures. The retrograde structures mainly include: (1) oriented needle mineral exsolution, e. g. , quartz needles in Na-clinopyroxene and rotile, clinopyroxene and apatite exsolution in garnet formed under decreasing pressure conditions during exhumation; (2) symplectite, especially, two generations of symplectites developed outside the garnet grains, which are called ““double symplectite““ here; (3) compositional zoning of minerals such as garnet and clinopyroxene; (4) minerals with a reaction rim or retrograde rim, e.g. , omphacite with a diopside rim, diopside with an amphibole rim and rutile with a rim of ilmenite. These retrograde textures, especially mineral zoning and symplectite, provide important petrologic evidence for the exhumation process and uplift of high-grade metamorphic rocks such as eclogite in the northern Dabie Mountains, indicating a rapid exhumation process.     

Eclogites of the Dabie Region: Retrograde Metamorphismand Fluid Evolution

Based upon fluid effects, retrograde metamorphism of eclogites in the Dabie region can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stage is marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and is thought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely to have occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyanite porphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals such as phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such as amphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with lower amphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. The product of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have     

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.     

Aragonite from Mulanshan Glaucophane Schist: Implications for Regional Evolution of Southwestern Dabie Mountains, Central China

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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.     

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

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The Key Influence of Fluid in the Preserves, Mineral Assemblages, Compositionsand Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains，Central China

正Pseudosection modeling for the garnet amphibolite samples from the Western Dabie Mountains show they have experienced similar HP metamorphic evolution with that of the adjected eclogites.The common assemblage of     

The Key Influence of Fluid in Metamorphic Rock Preserves, Mineral Assemblages, Compositions and Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains，Central China

正The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should     

Geochemical Characteristics and Genesis of the Granitic Gneisses from the Southeastern Dabie Mountains

The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.     

大别山北部榴辉岩的退变质特征及其地质意义

研究了大别山北部榴辉岩的变质岩岩石学。结果表明，该区榴辉岩相变质作用可分为早期(超高压)和晚期(高压)两个阶段，并在折返过程中形成了一系列特征性的退变质显微构造。其中，退变质结构主要包括：(1)由于压力降低而出溶形成的一些定向针状或叶片状矿物包裹体，如钠质单斜辉石中石英及石榴子石中的金红石、单斜辉石和磷灰石等；(2)冠状体或后成合晶，特别是石榴子石外围发育两期(“双层”)后成合晶；(3)反应边或退变边，如绿辉石的透辉石退变边、透辉石的角闪石退变边和金红石的钛铁矿退变边等。这些退变质结构为本区榴辉岩高级变质岩的快速折返过程和抬升历史提供了强有力的岩石学依据；石榴子石中针状矿物出溶体进一步证明研究区榴辉岩早期经历了超高压变质作用，峰期变质压力应大干4．0GPa，甚至可能达到5～7GPa或更高。     

TEM and HRTEM Study of Microstructures in Omphacite from UHP Eclogites at Shima, Dabie Mountains, China

Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra-high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocation substructures, including free dislocations, loops, tiltwalls, dislocation tangles and subboundaries. They were produced by high-temperature ductile deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470±6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APDs in ordered omphacites from Shima, coincident with ages given by single-zircon U-Pb dating (471±2 Ma). HRTEM reveals C2/c and P2ln space groups in different parts of one single omphacite crystal, and no exsolution is observed in the studied samples, which is attributed to rapid cooling. It is suggested that the UHP eclogites underwent a long     

大别高压和超高压榴辉岩中绿辉石的透射电镜研究

近年来有关大别山超高压变质岩的研究取得了重要进展。但对榴辉岩的显微和超微构造的研究仍然不足。榴辉岩主要由石榴石和绿辉石组成,其中石榴石基本上相当于刚性体,除非在很高温条件下石榴石才发生韧性变形［１～４］。因此,笔者对高压和超高压榴辉岩中的绿辉石进行了...     

大别山北部榴辉岩的大地构造属性及冷却史

岩石地球化学及同位素年代学研究表明，大别山北部榴辉岩的大部分属印支期扬子俯冲陆壳（下地壳）的一部分；大致沿磨子潭－晓天断裂附近分布的含榴辉岩、大理岩和变质橄榄岩的镁铁－超镁失质岩带可能代表扬子与华北2个大陆板块之间的变质构造混杂岩带，它应接近于扬子与华北2个大陆板块之间的缝合线（镁铁－超镁铁质岩带以北），而五河－水吼剪切带则可能代表扬子腐冲陆壳内部的1条拆离带或滑脱带。冷却史研究表明，大别山北部榴辉岩峰期变质后初期（230－210Ma)仅抬升到下地壳水平，因而未经历降温过程并具有比大别山南部榴辉岩较低的抬升速率，峰期变质后较慢的抬升速率及相应的近于等温或局部升温阶段也许是造成大别山北部榴辉岩与南部超高压带中榴辉岩差异的重要原因之一；同时，也许是研究区榴辉岩很少见有保留早期超高压变质证据的重要原因。     

安徽大别山北部榴辉岩的分布及主要特征

安徽大别山北部新发现的榴辉岩主要产于花岗质片麻岩及基性—超基性岩中 ,与大别山南部的榴辉岩在产出围岩、矿物组成及地球化学特征等方面均十分相似。现今榴辉岩南多北少的分布 ,主要是在中生代热隆构造体制下剧烈差异抬升剥蚀 ,中生代花岗岩大面积侵位所致。     

大别山榴辉岩氢氧同位素组成及其地球动力学意义

碧溪岭榴辉岩石至少经历了５个阶段的变质作用，矿物对氧同位素地质测温结果与通过岩石学方法计算的温度吻合得非常好，表明碧溪岭榴辉岩体中大部分同位素体系接近或达到平衡，且退变质作用未明显破坏这种平衡，榴辉岩全岩δ＾１８Ｏ为３‰～５‰指示其母岩玄武岩曾与地表水在变质作用之前发生过一定程度的交换作用。