Retrogressive Microstructures: A Key to Post-Collisional Uplift and Extension Tectonics of Ultrahigh-Pressure Metamorphic Rocks from Dabie Orogenic Belt, China

A wealth of retrogressive microstructures have been discovered from the UHP metamorphic rocks in Dabie orogenic belt, namely, the ultrahigh-pressure (URP) eclogites, jadeite quartzites and kyanite-zoisite-quartz vein. The most important are pseudomorphic replacements of UHP minerals like coesite, the corona reaction textures iuduced by solid-solid reactions as well as the corona and symplectites induced by reactions involving fluid. According to the textural relationships the sequence of mineral Paragenesis and the metamorphic stages in the UHP eclogites can be delineated; the mineral geobarothermometry of the various stages of retrograde metamorphism is studied and a clockwise, nearly isothermal decompressive metamorphic PT-trajectory for the UHP eclogites can be reconstructed. In terms of the PT-trajectory the two stage post collision uplirt and exhumation processes are reflected. When the UHP metumorphic rocks extruded to the lower-middle crust partial melting happened which bad in turn caused tke crustal extension and the further exhumation or the UHP metomorphic rocks. Based on the field strain analysis combined with geochronological data a scenario or post collision uplift aud exhumation model is presented.

The Lanshantou Kyanite-bearing Eclogite with Coesite Inclusions in the Sulu Ultrahigh-Pressure Metamorphic Belt and Its PTt Path

Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhumation.

Geochemical Characteristics of Pb Isotope of High-Pressure Metamorphic Rocks and Foliated Granites from HP Unit of Tongbai-Dabie Orogenic Belt

Whole-rock Pb isotopic compositions of the high-pressure (HP) metamorphic rocks, consisting of two-mica albite gneisses and eclogites, and foliated granites from the HP metamorphic unit of the Tongbai-Dabie orogenic belt are firstly reported in this paper. The results show that the HP metamorphic rocks in different parts of this orogenic belt have similar Pb isotopic compositions. The two- mica albite gneisses have 206 Pb/ 204 Pb=17.657-18.168, 207 Pb/ 204 Pb=15.318-15.573, 208 Pb/ 204 Pb=38.315-38.990, and the eclogites have 206 Pb/ 204 Pb=17.599-18.310, 207 Pb/ 204 Pb=15.465- 15.615 , 208 Pb/ 204 Pb=37.968-39.143. The HP metamorphic rocks are characterized by upper crustal Pb isotopic composition. Although the Pb isotopic composition of the HP metamorphic rocks partly overlaps that of the ultrahigh-pressure (UHP) metamorphic rocks, as a whole, the former is higher than the latter. The high radiogenic Pb isotopic composition for the HP metamorphic rocks confirms that the subducted Yangtze continental crust in the Tongbai-Dabie orogenic belt has the chemical structure of increasing radiogenic Pb isotopic composition from lower crust to upper crust. The foliated granites, intruded in the HP metamorphic rocks post the HP/UHP metamorphism, have 206 Pb/ 204 Pb=17.128-17.434, 207 Pb/ 204 Pb=15.313-15.422 and 208 Pb/ 204 Pb=37.631-38.122, which are obviously different from the Pb isotopic compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP unit. This shows that the foliated granites from the HP and UHP units have common magma source. Combined with the foliated granites having the geochemical characteristics of A-type granites, it is suggested that the magma for the foliated granites in the UHP and HP unit would be derived from the partial melting of the retrometamorphosed UHP metamorphic rocks exhumed into middle to lower crust, and partial magmas were intruded into the HP unit.

Elastic and Seismic Properties of Dabie-Sulu Ultrahigh Pressure Metamorphic Rocks

Lame modulus （λ） and shear modulus （μ） are among the most important, intrinsic, elastic constants of rocks. Using 7. and μ could be much more advantageous than using P- and S-wave velocities （Vp and Vs）. Here we quantified these equivalent isotropic elastic moduli for 115 representative rocks from the ultrahigh pressure （UHP） metamorphic terrane of the Dabie-Sulu orogenic belt （China） and their variations with pressure （P）, temperature （T）, density （p）, Vp, Vs and mineralogical composition. Both moduli increase nonlinearly and linearly with increasing pressure at low （〈200-300 MPa） and high （〉200-300 MPa） pressures, respectively. In the regime of high pressures, 7. and IX decrease quasi-linearly with increasing temperature with temperature derivatives dλ/dT and dμ/dT generally in the range of -10×10-3 to -1×10-3 GPa/℃. Dehydration of water-bearing minerals such as serpentine in peridotites and chlorite in retrograde eciogites results in an abrupt drop in 7. while μ remains almost unchanged. In Z-p, μ-p and 7.-IX plots, the main categories of UHP rocks can be characterized. Serpentinization leads to significant decreases in μ and 7. as serpentine has extremely low values of Z, μ and p. Eclogites, common mafic rocks （mafic gneiss, metagabbro and amphibolite）, and felsic rocks （orthogneiss and paragneiss） have high, moderate and low μ and λ values, respectively. For pyroxenes and olivines, λ increases but μ decreases with increasing Fe/Mg ratios. For plagioclase feldspars, both Z and μ exhibit a significant positive correlation with anorthite content. SiO2-rich felsic rocks and quartzites are deviated remarkably from the general trend lines of the acid-intermediate-mafic rocks in Vs-p, μ-p, λ-Vp,λ-Vs and μ-λ diagrams because quartz has extremely low λ （-8.1 GPa） and p （2.65 g/cm3） but moderate μ （44.4 GPa） values. Increasing the contents of garnet, rutile, ilmenite and magnetite results in a significant increase in the λ and μ values of the UHP metamorphic rocks. However, either λ or μ is insensitive to the compositional variations for pyralspite （pyrope-almandine-spessartine） solution series. The results provide potentially improved constraints on characterization of crustal composition based on the elastic properties of rocks and in situ seismic data from deep continental roots.

The Dabieshan Coesite-bearing Eclogite Terrain-A Late Archaean Ultra - high- Pressure Metamorphic Belt

A U -Pb zircon age of 2774±24 Ma for eclogite from the Bixiling rock body of Anhui Province, central China, indicates that the Dabieshan coesite-bearing eclogite was probably formed in the Late Archaean. A phengite Ar-Ar isochron age of 662±13 Ma for the eclogite confines also an upper limit age of its subsequent retrograde metamorphism in the Precambrian. The results of isotopic dating for such type of eclogite coincide with the geological features of its restricted occurrence within the Archaean metamaorphic terrain composed of the Dabie Group. It is believed that the Dabieshan coesite-bearing eclogite terrain might be a Late Archaean ultra-high-pressure metamorphic belt. The Dabie Mountains area was the eastward extension of the southern Qinling structural belt during the Triassic. Both the Dabie Group and the coesite-bearing eclogite hosted therein underwent a late-stage dynamic metamorphic event. The present authors have obtained a muscovite Ar-Ar isochron age of 192.6±2.8 Ma from plagioclase gneiss and a hornblende Ar-Ar plateau age of 230.7±4.6 Ma for the low amphibolite in eclogite respectively, which represent the Indosinian reworking ages of the original metamorphic rocks of the Dabie Group gneiss and coesite-bearing eclogite.

Interpretation of Aeromagnetic Anomalies of the Sulu Ultrahigh-Pressure(UHP) Metamorphic Belt,Eastern China

The Sulu ultra-high pressure(UHP)metamorphic belt in Eastern China is well known as the eastern extension of the Qingling-Dabie orogenic belt formed by subduction and collision between the Sino-Korean and Yangtze cratons.The main hole of the Chinese Continental Scientific Drilling(CCSD)project is located at the southern segment of the Sulu UHP metamorphic belt(34°25′N/118°40′E),about 17 km southwest of Donghai County.Integrated geophysical investigations using gravity,magnetic,deep

Kanfenggou UHP Metamorphic Fragment in Eastern Qinling Orogen and Its Relationship to Dabie-Sulu UHP and HP Metamorphic Belts, Central China

In the Central Orogenic Belt, China, two UHP metamorphic belts are discriminated mainly based on a detailed structural analysis of the Kanfenggou UHP metamorphic fragment exposed in the eastern Qinling orogen, and together with previous regional structural, petrological and geochronological data at the scale of the orogenic domain. The first one corresponds to the South Altun North Qaidam North Qinling UHP metamorphic belt. The other is the Dabie Sulu UHP and HP metamorphic belts. The two UHP metamorphic belts are separated by a series of tectonic slices composed by the Qinling rock group, Danfeng rock group and Liuling or Foziling rock group etc. respectively, and are different in age of the peak UHP metamorphism and geodynamic implications for continental deep subduction and collision. Regional field and petrological relationships suggest that the Kanfenggou UHP metamorphic fragment that contains a large volume of the coesite and microdiamond bearing eclogite lenses is compatible with the structures recognized in the South Altun and North Qaidam UHP metamorphic fragments exposed in the western part of China, thereby forming a large UHP metamorphic belt up to 1 000 km long along the orogen strike. This UHP metamorphic belt represents an intercontinental deep subduction and collision belt between the Yangtze and Sino Korean cratons, occurred during the Paleozoic. On the other hand, the well constrained Dabie Sulu UHP and HP metamorphic belts occurred mainly during Triassic time (250-220 Ma), and were produced by the intracontinental deep subduction and collision within the Yangtze craton. The Kanfenggou UHP metamorphic fragment does not appear to link with the Dabie Sulu UHP and HP metamorphic belts along the orogen. There is no reason to assume the two UHP metamorphic belts as a single giant deep subduction and collision zone in the Central Orogenic Belt situated between the Yangtze and Sino Korean cratons. Therefore, any dynamic model for the orogen must account for the development of UHP metamorphic rocks belonging to the separate two tectonic belts of different age and tectono metamorphic history.

Fluid Activity and Tectonic Evolution in the Northern Qilian High-pressure Metamorphic Belt

The Northern Qilian high-pressure metamorphic belt has experienced multipledeformation-metamorphism, which consists of at least four stages. In 550.8-526 Ma, eclogites wereformed. High temperature and pressure caused the escape of a large quantity of gas-liquid fluidsfrom rocks while silicate melt was generated. In the late stage, small amounts of CO_2 and H_2Oinfiltrating along fractures were introduced. In the formation of glaucophane schist (447-362 Ma),devolatilization reactions were dominated during the subduction-uplift stage of the paleoplate. Inthe uplift-exhumation stage (400-380 Ma) the increase of internal space of fractures in the rocksfavoured fluid infiltration and concentration. These fluids participated in hydration reactions inthe retro-metamorphism. The fluids participating in the mineral reactions have the compositions ofCaCl_2-NaCl-H_2O. In subsequent thrusting (<380 Ma), the metamorphic terrain was uplifted to theshallower crust and ductile-shearing deformation took place, which caused mainly dehydrationreactions of minerals. In a near-surface environment the metamorphic terrain experienced brittledeformation, forming many accompanying fractures. Immiscible CO_2 and low-salinity aqueous fluidsoccurred in these secondary microfractures and were trapped and sealed. The thermodynamic conditionsof different deformation-metamorphic stages of the metamorphic terrain were calculated and thecorresponding P-T-t path was deduced, showing that the metamorphic terrain has experienced aclockwise path indicated by T- and P-rising, and T- and P-falling processes. This reveals that thesubduction zone has undergone multiple tectono-dynamic processes, i.e. initial deep burial,subsequent quick uplift and near-surface tectonism.

U-Pb Dating of Marble Associated 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 a 238U-206Pb isochron age of 435±45a with an initial 206Pb/204Pb value of 18.075±0.006,The age of -435 Ma is interpreted to represent the time of limestone diagensis rather than the time of metarmorphic 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 cratos.Becasuse the latest ordovician limestone was involved in the eclogite formation,the continental collision between the Yangtze and Sino-Korean Plates would postdate ordovican.

Crustal structure of the eastern Dabie orogenic belt from the seismic tomography and wide-angle reflection studies

The studies of seismic tomography and wide-angle reflection have been carried out to reveal the velocity structUrebeneath the eastern Dabie orogenic belt. The result from the seismic tomography shows the high velocity bodiesmight be positioned to a depth of only about 1 .5 km below sea level within the Dabie ultra-high pressure metamorphic (UHPM) belt; the fan-profile shows the Shuihou-Wuhe fault, the demarcation between the South Dabieand the North Dabie, slopes to the south-west at a dip angle of about 45° in the bottom of upper crust. The wideangle reflection shows the middle crustal boundaries and the complex features from the lower crust.

On Continent-Continent Point-Collision and Ultrahigh-Pressure Metamorphism

Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650-800℃ and 2.6-3.5 GPa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt-an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5-9 times higher than those in their surroundings). The tectonic stresses may account for 20-35% of the total UHP. So we may infer that the HP (nigh-pressure)-UHP rocks in the Dabie Mountains were formed at depths of 60-80 km. Thus the authors propose a new genetic model of UHP rocks-the point-collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do nol exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point-collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou （看丰沟 ） and Xiangfanggou （ 香坊沟 ） slices exposed in the eastern Qinling （秦岭） orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine-grained coesite and quartz pseudomorphs after coesite- and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South AItun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age （ -500 -400 Ma）. The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie （ 大别 ）-Sulu （苏鲁 ） ultrahigh/high pressure metamorphic belt of Triassic age （-250-220 Ma）. The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping （二郎坪） rock group. The East Qinling UHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These two ultrahigh metamorphic belts are separated by the suture-like Shangnan （ 商南 ）-Danfeng （丹凤 ） fault system or Guishan （ 龟山 ）-Meishan （ 梅山） fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultrahigh/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam- North Qinling ultrahigh metamorphic belt probably represents an intercontinental deep subduction/ collision belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/ collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of 4200--300 Ma, responsible for the formation of the two ultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.

The white schist assemblage in the kyanite quartzite from the ultrahigh-pressure metamorphic belt, eastern Dabie Mountains

The kyanite quartzite from the ultrahigh-pressure metamorphic belt in the eastern Dabie Mountains is composed mainly of quartz, kyanite, talc and epidote, which contains a typical mineral assemblage of white schist. It suffered an eclogite facies metamorphism of P =(2 4±0 3) GPa and T =630℃ together with coexisting eclogite. The protolith may be an aplite replaced by high-pressure fluid of eclogite facies.

U-Pb isotopic compositions of the ultrahigh pressure metamorphic (UHPM) rocks from Shuanghe and gneisses from Northern Dabie zone in the Dabie Mountains, central China: Constraint on the exhumation mechanism of UHPM rocks

The U-Pb isotopic study of the ultrahigh pressure metamorphic (UHPM) rocks and gneisses from the Dabie Mountains shows that the UHPM rocks exposed in the Southern Dabie zone have relatively low Pb contents (most < 4×10-6), high U/Pb ratios (most > 0.1), and a large variation of Pb isotopic ratios with relatively high radiogenic Pb (206Pb/204Pb = 17.026—20.781). Their low Pb contents could be the result of Pb loss caused by fluid expulsion during continental subduction, while their high radiogenic Pb values can be explained by the mixing of the upper crust Pb and the mantle Pb. In contrast, the gneisses exposed in the Northern Dabie zone have higher Pb contents (most > 4×10-6), lower U/Pb ratios (< 0.07), and lower Pb isotopic ratios (206Pb/204Pb = 15.799—17.204), which are similar to those of the Mesozoic granites developed in both Northern and Southern Dabie zones. It is suggested that the Northern Dabie zone may have experienced a smaller fluid expulsion and Pb loss during the continental subduction, and their Pb isotopic compo-sitions could be explained by the mixing of the lower crust Pb and the mantle Pb. Their initial Pb isotope ratios at 230 Ma suggest that the U/Pb ratios of the UHPM rocks in the Southern Dabie zone are higher than those of the gneisses in the Northern Dabie zone in a long period of time be-fore the continental subduction. The above observations suggest that the protoliths of the UHPM rocks in the Southern Dabie zone are upper crustal rocks in the subducted continental crust, while the gneisses in the Northern Dabie zone have the middle-lower crust features. Based on these observations, a model for the exhumation of UHPM rocks is proposed, i.e. the detachment be-tween the upper crust and lower crust in the subducted continental crust could have occurred dur-ing subduction, thus the subducted upper crust was uplifted by buoyancy and moved southward along the thrust.

The recognition of adakite-type gneisses in the North Dabie Mountain and its implication to ultrahigh pressure metamorphic geology

Adakite is an igneous rock generated by the partial melting of subducting ocean slab in an island arc setting. Meanwhile, residuals of the slab-melting are transformed to eclogite or garnet-amphibolite during a slab subduction process. This note reports the adakite-type grey gneisses discovered in the North Dabie Mountain. These adakitic rocks are characterized by high AI2O3(14.97%-17.56%), Sr (545-941 μg/g), Sr/Y (44.71-218.98), La/Yb (16.70-97.50), and low Y (3.32-12.19μg/g), Yb (0.31-1.34 μg/g), Sc (0.92-4.2μg/g), as well as positive Sr anomaly but absent or positive Eu anomaly. They are different from the low aluminum grey gneisses with low AI2O3 (13.73%-14.38%), Sr (262-409 μg/g), Sr/Y (6.46-15.97) and La/Yb (1.90-22.60), high Y(21.79-36.24 μg/g), Yb (2.00-5.59 μg/g) , Sc (8.30%-12.70%), and negative Sr, Eu anomaly in the North Dabie Mountain. The adakite-type grey gneisses in the North Dabie Mountain were most probably formed by the partial melting of subducting ocean slab before the

Deep structures of the east Dabie ultrahigh-pressure metamorphic belt, East China

The geophysical investigations with seismic and MT methods were carried out in the east Dabie Mountain area in 1997, producing detailed results about crustal structures with good compatibility between different geophysical methods. After integrated interpretation of both avail-able geophysical and geological data, the author compiles a crustal tectonic section across the east Dabie Mountain (Plate II), which provides much more structural details with improved reliabil-ity due to geophysical constraints applied to the deep structures. The east Dabie orogenic belt can be divided into 4 geologic units from north to south: the north Huaiyang, the north Dabie, the south Dabie and the Susong. The Mesozoic northward subduction of the Yangtze craton caused the Yangtze crust to insert into the middle and lower crust of the Susong high-pressure metamorphic zone, while the middle and lower crust below the north Huaiyang and the Hefei basin contains the basement of the Sino-Korean craton. The middle and lower crust of the south Dabie is rather dif-ferent from that of the north Dabie, showing that the north and the south Dabie had different evolu-tional trajectories and should not belong to a single tectonic unit. The current crustal pattern has resulted mainly from deformation caused by the post-collisional intracontinental subduction of both the Yangtze and Sino-Korean cratons before the late Jurassic, and deformation caused by later crustal extension including doming and unroofing around the north Dabie. It can be inferred that the suture zone of the Triassic collision between the Sino-Korea and the Yangtze was located along the Xiaotian-Mozitan fault zone, which contains a group of normal faults dipping north in the upper crust, but becomes steep thrusts of dipping south in the middle crust, accompanied by ex-tensive deformation between this fault zone and the Hefei basin. The middle crust below the north Huaiyang unit is connected to the basement of the Sino-Korean craton, showing the plate com-pressional convergence between the Yangtze and Sino-Korean cratons in the post-collisional stage. The influence of this convergent event reached as far as to Huainan, the northern boundary of the Hefei basin. As a clear reflection appears at 22s TWT in the stack reflection sections, the lithospheric thickness of the east Dabie is about 78 km. The newly obtained geophysical data in-dicate that the thickness of the east Dabie UHPM rock slices is no more than 8 km, therefore they do not present evidences to support the hypothesis involved in whole-plate exhumation of the UHPM terranes.

Detachment within subducted continental crust and multi-slice successive exhumation of ultrahigh-pressure metamorphic rocks: Evidence from the Dabie-Sulu orogenic belt

Although tectonic models were presented for exhumation of ultrahigh-pressure (UHP) metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various depths within deeply subducted continental crust. This lends support to the multi-slice successive exhumation model of the UHP metamorphic rocks in the Dabie-Sulu orogen. The available evidence is summarized as follows: (1) the low-grade metamorphic slices, which have geotectonic affinity to the South China Block and part of them records the Triassic metamorphism, occur in the northern margin of the Dabie-Sulu UHP metamorphic zone, suggesting decoupling of the upper crust from the underlying basement during the initial stages of continental subduction; (2) the Dabie and Sulu HP to UHP metamorphic zones comprise several HP to UHP slices, which have an increased trend of metamorphic grade from south to north but a decreased trend of peak metamorphic ages correspondingly; and (3) the Chinese Continental Science Drilling (CCSD) project at Donghai in the Sulu orogen reveals that the UHP metamorphic zone is composed of several stacked slices, which display distinctive high and low radiogenic Pb from upper to lower parts in the profile, suggesting that these UHP crustal slices were derived from the subducted upper and middle crusts, respectively. Detachment surfaces within the deeply subducted crust may occur either along an ancient fault as a channel of fluid flow, which resulted in weakening of mechanic strength of the rocks adjacent to the fault due to fluid-rock interaction, or along the low-viscosity zones which resulted from variations of geotherms and lithospheric compositions at different depths. The multi-slice successive exhumation model is different from the traditional exhumation model of the UHP metamorphic rocks in that the latter assumes the detachment of the entire subducted continental crust from the underlying mantle lithosphere and its subsequent exhumation as a whole. This also reveals the difference between the continental subduction and oceanic subduction. In addition, several important proposals concerning the multi-slice successive exhumation model are made for further studies.

大陆俯冲隧道中的应变不均一分布:来自大别山超高压变质岩的记录

俯冲隧道是俯冲板片与上覆板块之间的剪切带,也是高压—超高压变质岩折返和深部流/熔体活动的通道。大别山超高压变质岩分布广泛,变形程度差异很大,是研究大陆俯冲隧道中岩石变质-变形过程的理想地区。本文系统总结了前人对中大别双河地区超高压变质岩的岩石学和年代学研究成果,在双河地区开展了地质填图、应变分析和三维构造重建。通过将超高压变质岩的变形特征与P-T-t轨迹结合,识别出超高压变质岩折返过程中的三期韧性变形。在双河北部发现了一个上盘向NW剪切的千米尺度的榴辉岩相鞘褶皱,枢纽向SE倾伏,倾伏角约20°,与榴辉岩、片岩和长英质片麻岩的拉伸线理平行,表明超高压变质岩初始折返阶段的流体活动使榴辉岩的强度显著降低,榴辉岩与围岩一起发生韧性变形。该期变形被角闪岩相退变质阶段上盘向NW的剪切叠加,此时应变集中于片麻岩、片岩、大理岩等非能干层,强度较高的榴辉岩成为构造透镜体。而绿片岩相变质阶段上盘向SE方向的剪切与早白垩世北大别花岗片麻岩穹隆的形成有关。对双河南部弱变形花岗片麻岩的锆石U-Pb定年揭示了757±14 Ma的原岩年龄和240~216 Ma的变质年龄,与双河北部含柯石英强变形花岗片麻岩类似,暗示其也经历了三叠纪超高压变质作用及随后的角闪岩相退变质作用。通过计算长英质片麻岩的有效黏度,发现无水碱长花岗片麻岩的有效黏度高于黑云斜长片麻岩,折返阶段的流体活动使超高压变质岩的强度显著降低,当局部的流体活动不足以弱化碱长花岗岩体时,应变集中于黑云斜长片麻岩。因此,大陆俯冲隧道中的应变分布受矿物组成、流体活动和岩体规模的共同影响。

大别山晓天—磨子潭断裂附近退变榴辉岩的厘定及其时代和大地构造意义

研究区位于大别造山带晓天—磨子潭断裂附近(图1)。晓天—磨子潭断裂是北大别杂岩带和北淮阳带之间的边界断裂,部分研究者认为该断裂带为华北板块和扬子板块之间的缝合线以及大别山超高压变质岩折返的重要边界(Faure et al.,1999;徐树桐等,2002),但对此仍存在较大争议。

Extensional deformation of post ultrahigh-pressure metamorphism and exhumation process of ultrahigh-pressure metamorphic rocks in the Dabie massif,China

A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the post-Indosinian continent-continent oblique collision between the Sino-Korean and Yangtze cratons and ultrahigh-pressure metamorphism (UHPM). The configuration is characterized by a regional tectonic pattern similar to metamorphic core complexes and by the development of multi-layered detachment zones. On the basis of the identification of compressional and extensional fabrics, it is indicated that the exhumation and uplift of ultrahigh-pressure (UHP) metamorphic rocks from the mantle depth to the surface can be divided into at least three different decompression retrogressive metamorphism and tectonic deformation stages, in which the subhorizontal crustal-scale extensional flow in the middle-lower crust under amphibolite facies conditions is an important geodynamic process in the exhumation of UHP metamorphic rocks. Moreover, the extensional flow is probably driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.