New U-Pb Geochronological Constraints on Formation and Evolution of the Susong Complex Zone in the Dabie Orogen

Objective The Susong complex zone（SCZ）is a relatively lowgrade metamorphic unit mostly with an epidoteamphibolite facies,located in the southernmost part of the Dabie orogen.However,its rock compositions,ages,

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.

A Uniform Orogen-parallel Extension System of the Shear Zones in the Tongbai-Dabie Orogenic Belt, Central China

Large-scale magmatism affected the Tongbai-Dabie orogenic belt during post-collisional lateral tectonic extension in the Cretaceous, which was suggested to account for the widespread deformation and migmatization in the Tongbai-Dabie complexes. However, it cannot explain the most deformations in the shear zones. The northwest-southeast shear zones are developed around or wrapped the Tongbai-Dabie complexes. They play an important role for the interpretation of the tectonic evolution of the Tongbai-Dabie orogenic belt. By a systematically observation and description of the geometry and kinematics of these shear zones, we found that the shear zones to the north dip NE and have a uniform sinistral shear sense, the shear zone to the south dips SW and has a uniform dextral shear sense, and the shear zones at the core are sub-horizontal and have a uniform top-to-NW sense of shear. Combining with the comparison of previous and our geochronological studies, we interpret these associations as indicating that these shear zones were originally a single, more flat-lying and sub- horizontal shear zone with a uniform top-to-NW shear sense before the folding-doming of the Tongbai- Dabie complexes and suggest that the Tongbai-Dabie orogenic belt experienced a uniform top-to-NW orogen-paraUel extension in the ductile lithosphere before the widespread magmatism in the Cretaceous.

A Comparison about Metamorphism among the Oldest-Rock Units from Orogenic Belts of Dabie,Eastern Qinling and Eastern Kunlun of Central Mountain Ranges,China

The Dabie orogen, Eastern Qinling orogen and Eastern Kunlun orogen are the major components of the Central Mountain Ranges of China and each has distinctively metamorphic processes in their oldest rock units. The Dabie orogen oldest rock units had experienced an intermediate-higher pressure, upper amphibolite to lower granulite facies metamorphism in the Indosinian intracontinental subduction collision event. The clockwise pt path, synchronous attainment of t max and p max and a segment of high slope retrograde path suggest a tectono driven rapid exhumation of the oldest rock units to upper middle crustal level following the end of the subduction collision process. The oldest rock units, also called Qinling Group, of Eastern Qinling Mountains suffered intensively collisional metamorphism at an immature plate tectonic framework during Jinningian movement about 1 000 Ma ago. The clockwise pt path with reach of t max following several hundreds of MPa decreasing from p max suggests that the denudation of the deeply burial rock units was due essentially to isostatic relaxation. During the Caledonian stage, the highly metamorphosed oldest rock units suffered from a high t thermal event in arc environment and superimposed by contact metamorphism. After thermal peaks, the rock units were exhumed in a short distance and cooled down isobarically. No regional metamorphism higher than upper greenschist facies condition since 300 Ma has been traced within exposed rocks in the eastern portion of Northern Qinling orogen. The oldest rock units in the Eastern Kunlun orogen were metamorphosed to upper amphibolite facies to lower granulite facies rocks early or during the Luliang movement (some 1 800 Ma ago). The high grade metamorphic rock units had a long resident time at the deep crustal level, and were exhumed to middle upper crustal level during the Caledonian to Hercynian tectonometamorphic events. Shallow erosion of the orogenic belt led to good preservation of the low pressure metamorphic belt.

The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt(East-Central China)

The Xiaotian–Mozitan Shear Zone(XMSZ)is the boundary of the Dabie High-grade Metamorphic Complex(DHMC)and the North Huaiyang Tectonic Belt.It was deformed in ductile conditions with a top-to-NW/WNW movement.Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation,respectively,which indicates a retrograde process of evolution.A few groups of zircon U-Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone.Zircons from the felsic ultramylonites are all magmatic,producing a weighted mean 206Pb/238U age of 754±8.1 Ma,which indicates the time of magmatic activities caused by rifting in the Neoproterozoic.Zircons from the granitic veins,cutting into the mylonites,are also of magmatic origin,producing a weighted mean 206 Pb/238 U age of 130±2.5 Ma,which represents the time of regional magmatic activity in the Cretaceous.Zircons from the mylonitic gneisses are of anatectic-metamorphic origins and are characterized by a core-mantle interior texture,which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206 Pb/238 U age of 762±18 Ma,similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206Pb/238U age of 143±1.8 Ma,indicating the anatectic metamorphism in the Dabie Orogenic Belt(DOB).Based on integrated analysis of the structure,thermal conditions of ductile deformation and the contact relations of the dated rocks,the activation time of the Xiaotian–Mozitan Shear Zone is constrained between~143 Ma and 130 Ma,during which the DOB was undergoing a transition in tectonic regime from compression to extension.Therefore,the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process.This research also inclines us to the interpretation of it as an extensional detachment,with regard to the tectonic properties of the shear zone.It may also be part of a continental scale extension in the background of the North China Block’s cratonic destruction,dominated by the subduction and roll-back of the Paleo-Pacific plate,but more detailed work is needed in order to unravel its complicated development.

Architecture and Kinematics of the Dabie Orogen,Central Eastern China

The geometry of the Dabie Mountains is manifested in terms of the distribution of petro-tectonic units in three dimensions. It is identified into three segments from east to west, four horizons in vertical profiles and eight petro- tectonic units from north to south. Three segments are the east, middle and west segments. Four horizons, from top to bottom, are two different meta-tectonic mélange in the uppermost part, underthrust basement and cover below them, and mantle at the bottom of the profiles. Eight petro-tectonic units from north to south are: (1) the hinterland basin, (2) the meta -flysch, (3) the ultramafic rock belt (UM) + Sujiahe eclogite belt (SH), (4) eclogite belt 2 (Ec2) with most eclogites of continental affinity, (5) eclogite belt 1 (Ecl1) with some eclogite of oceanic affinity, (6) the Dabie complex or underthrust basement of the Yangtze continent, (7) the Susong and Zhangbaling Groups or underthrust cover of the Yangtze continent and (8) the foreland belt. The (3), (4) and (5) units belong to meta-tectonic mélange. Some ultrahigh pressure metamorphic minerals such as coesite and micro-diamonds have been found in (3) and (4) units; a possible ultrahigh pressure mineral, clinozoisite aggregate pseudomorph after lawsonite, was found in unit (5). The three tectonic units are speculated to be coherent initially; the UM and SH units are suggested to be the root belt in the east, middle and west segments respectively. The kinematics of the Dabie orogen is divided into three stages: top-to-south thrusting during the eclogite-granulite facies metamorphism, top-to-north extension during the amphibolite metamorphic stage, and faults or shear bands of brittle deformation and greenschist facies metamorphism were formed in the post-orogenic stage since the Late Jurassic and the movement pictures of these faults is different from each other.

Pb Isotope Mapping in the Tongbai-Dabie Orogenic Belt, Central China

Tongbai-Dabie orogenic belt in Central China is a part of the collisional belt between the Yangtze and North China cratons. It represents one of the most extensive ultrahigh-pressure （UHP） and high-pressure （HP） metamorphic rocks in the world. The Pb isotope mapping in this area is a significant method to constrain the crustal structure and tectonic evolution and to identify the tectonic boundaries within the vertical tectonic stack. Based on the Pb isotope compositions of the Dabie complex （DBC）, the Tongbai complex （TBC）, UHP and HP metamorphic rocks and associated foliated granites, the lower metamorphosed rocks from North Huaiyang （NHY） tectonic belt, and Cretaceous granites in the Tongbai-Dabie orogenic belt, we determined the Pb isotope geochemical map of the Tongbai-Dabie orogenic belt. The Pb isotope map shows that the Pb isotope compositions are similar within each geological body or lithotectonic unit, but the Pb isotope compositions of different lithotectonic units show systematic variations in the Tongbai-Dabie orogenic belt. The NHY tectonic belt contrasts strongly with the Tongbai-Dabie UHP.HP metamorphic belt in Pb isotope compositions. It is suggested that the line along the Xiaotian-Mozitan fault, the north limit of the Tongbai-Dabie UHP and HP metamorphic rocks, represents an important tectonic boundary. Within the Tongbai-Dabie HP -UHP metamorphic belt, to the south of Xiaotian-Mozitan fault, the vertical variations of Pb isotope compositions in different lithotectonic units and the spatial relationship among different major lithotectonic units have been constrained.

Structural Evidence for the in-situ Origin of the HP and UHP Eclogites in the Dabie-Sulu Orogenic Belt

Whether the HP and UHP metamorphic rocks of the Dabie-Sulu orogenic belt are of an "in-situ" or "foreign" origin is a long-standing dispute among geologists. Eclogites preserved today in the HP and UHP units constitute merely 5-10%, which are not isolated exotic bodies tectonically intruding into amphibolite facies gneiss, but remnants of once pervasive or widespread eclogite-facies terranes or slabs. The present spatial distribution and forms of the eclogites have resulted from polyphase and progressive deformation and strain partitioning of the HP and UHP slabs. From their formation in deep mantle to their exhumation to the surface, the eclogites have experienced long-term deformation with different strain regimes. The dominant regime responsible for the present spatial distribution and forms of the eclogites is the shear process. The deformation patterns of the eclogites and gneiss matrix also clearly show that the eclogites were metamorphosed in situ. The original distribution area of the eclogites

Geochemistry of Gneisses from Dabie Complex and Tongbai Complex in Qinling-Tongbai-Dabie Orogenic Belt:Implications for Location of Yangtze-Sino-Korean Suture

The Dabie complex (DC) and the Tongbai complex (TBC) are separately distributed in the middle and eastern parts of the Qinling-Tongbai-Dabie orogenic belt. In this study, the Dabie complex can be divided into two units: one is the complex with no high pressure and ultrahigh pressure metamorphic rocks (DC1), and the other is the complex containing coesite-bearing eclogite lenses or boudins (DC2). Gneisses are predominant in the TBC, DC1 and DC2. Major and trace element data of gneisses in the TBC, DC1 and DC2 show them to be the orthogneisses. The gneisses in the DC1 have higher incompatible element contents and higher ratios of w(K 2O)/w(Na 2O) and w(La) n/w(Yb) n than those in the DC2. However, no obvious differences arise in other element contents and the ratios of w(La)/ w(Nb), w(Nb)/w(Th), w(Nb)/w(Hf), w(Ba)/w(La), w(Sm)/w(Nd) and w(Th)/w(U) between the gneisses in the DC2 and those in the DC1. These observations suggest that the protoliths of the gneisses in the DC2 have affinities to those in the DC1. The difference between the DC1 and DC2 gneisses in incompatible element contents could reflect the difference in their partial melting extent. The TBC gneisses are geochemically similar to the DC1 gneisses, suggesting that the TBC and DC1 gneisses are the same lithologic unit in the Qinling-Tongbai-Dabie orogenic belt and that they have experienced similar formations and evolution histories. In the Qinling-Tongbai area, the TBC is part of the northern blocks of the Yangtze craton. Given the similarity of geochemical characteristics, the rock assemblage and the ages between the TBC and DC1 gneisses, we can infer that the Dabie complex also belongs to the northern blocks of the Yangtze craton. In terms of the distribution of eclogites and metamorphic facies, we propose that the collisional suture in the Dabie area is distributed along the Xiaotian-Mozitan fault, at the contact with the Shang-Dan-Tongbai fault to the west.

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.

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.

Dynamic Settings and Interactions between Basin Subsidence and Orogeny in Zhoukou Depression and Dabie Orogenic Belt

This paper presents a study of the geo dynamic setting and the relation between orogenic uplift and basin subsidence in the inland Zhoukou depression and Dabie orogenic belt. Since the Mesozoic the evolution of Zhoukou depression can be divided into three stages: (1) foreland basin, (2) transitional stage, (3) fault depression. Formation and variations of basin were not only related to the orogenesis, but also consistent with the orogenic uplift.

A Comparison of Nd Isotopes of Granitoids from the Central Asian Orogenic Belt and Qinling-Dabie Orogen,and Implications for Understanding of Crustal Growth from Accretion to Collision

Orogens can generally be divided into two types:accretionary and collisional.The fundamental differences in deep-crustal compositions and architecture from accretion to collision and how to identify them is not well understood.This is one of the major aims of the IGCP 662 project(www.igcp662.org.cn).

TECTONICS OF THE DABIE OROGENIC BELT, CENTRAL CHINA

Tectonically the Dabie orogenic belt consists mainly of the Dabieshan Yanshanian uplifted zone and the Beihuaiyang Variscan Indosinian folding zone. In the north boundary adjoining the North China Block, there are an Early Palaeozoic ophiolitic mixtite belt and the Hefei Mesozoic Cenozoic faulted basin which overlaps on the suture belt. In the south of Dabie orogen, there is a secondary tectonic unit called Foreland thrust faulted structural zone which was mainly formed by the intracontinental subductions during Mesozoic era. The study shows that the Dabie Block is a part of mid late Proterozoic palaeo island arc at the north margin of Yangtze Block. During Caledonian period,as a submerged uplift at the northen continental margin of Yangtze Block, the Dabie Block collided with the early Palaeozoic palaeo island arc at the south margin of North China Block, resulting in the convergence of the North and South China Blocks and the disappearance of oceanic crust. Since then, large scale intracontinental subductions were followed. Dabie Orogenic Belt is the product of overlapping of Yangtze Block, Dabie Block and North China Block under the mechanism of intracontinental subduction. Indosinian period is the period of chief deformation and high pressure dynamic metamorphism for Dabie Block, and Yanshan period is the main orogenic period in which the remelting of crust caused by basement shearing resulted in large scale thermometamorphism. The present tectonic framework of the orogen was finally formed by the rapid uplifting of the Dabieshan mountains and gliding southwards, which result in the developing of thrust belt on south side and the extensional tectonic movement on north side.

Tectonic Subdivision of Dabie Orogenic Belt, Central China: Evidence from Pb Isotope Geochemistry of Late Mesozoic Basalts

It has long been debated that the Dabie orogenic belt belongs to the North China or Yangtze craton. In recent years, eastern China has been suggested, based on the Pb isotopic compositions of Phanerozoic ore and Mesozoic granitoid K-feldspar (revealing the crust Pb) in combination with Meso-Cenozoic basalts (revealing the mantle Pb), being divided into the North China and Yangtze Pb isotopic provinces, where the crust and mantle of the Yangtze craton are characterized by more radiogenic Pb. In this sense, previous researchers suggested that the pro-EW-trending Dabie orogenic belt with less radiogenic Pb in the crust was part of the North China craton. In this paper, however, the Late Cretaceous basalts in the central and southern parts of the Dabie orogenic belt are characterized by some more radiogenic Pb (\{\{\}\+\{206\}Pb\}/\{\{\}\+\{204\}Pb\}=\{17.936\}-\{18.349\}, \{\{\}\+\{207\}Pb\}/\{\{\}\+\{204\}Pb\}=\{15.500\}-\{15.688\}, \{\{\}\+\{208\}Pb\}/\{\{\}\+\{204\}Pb\}=\{38.399\}-\{38.775\}) and a unique U-Th-Pb trace element system similar to those of the Yangtze craton, showing that the Mesozoic mantle is of the Yangtze type. In addition, the decoupled Pb isotopic compositions between crust and mantle were considerably derived from their rheological inhomogeneity, implying a complicated evolution of the Dabie orogenic belt.

Zircon U-Pb age evidence of the Mesoarchean(2.9–3.2 Ga)crustal remnant in the Southern Dabie Orogen,South China

1.Objective Unlike the North China Plate where Archean and Paleoproterozoic crustal rocks are widely distributed,Early Precambrian basement rocks in the Yangtze Block of South China are locally exposed(Fig.1a),such as in the Kongling Complex,the Zhongxiang Complex,and the Douling Complex(Zhao GC and Cawood PA,2012).The Dabie Orogen is the eastward extension of the Qinling Orogen.

Transpressive Deformation across Tongbo-Dabie Orogenic Belt

Deformation characteristics of the dextral Dashankou ductile shear zone , parallel to the Tongbo-Dabie Orogenic Belt and regional stretching lineation patterns within the blueschist belt in Northern Hubei , China , are examined at various scales , respectively . The new data , combined with those obtained in the Wudangshan area , indicate a transpressive deformation involving a WNW directed nearly horizontal shearing accompanied by shortening across the orogenic belt . A kinematic model is proposed for development of the deformation within the Tongboshan segment . Based on these findings it can be seen that the convergence and collision between the North China Craton and the Yangtze Craton may locally be oblique during the Proterozoic Jinningian Orogeny . This oblique collision is probably related to a certain extent to the shapes of the older craton margins .

Ray equation migration of wide-angle reflections in Dabie orogenic zone

ay equation migration of wide-angle reflections in 2-D medium is one kind of Kirchhoff prestack depth migration method. Based on ray theory, this method can be used extensively in 2-D inhomogeneous medium, and shows its advantage in wide-angle reflection study. After calculating ray fields, we can get the wave fields of sources and receivers by interpolation, and the intensity (or amplitude) AP corresponding to the depth point P at travel-time tP=tSP+tPR can be found in seismograms. Ray equation migration is completed by transforming AP according to the specific relation, migrating it to the depth point, then calculating the ray fields of lots of sources and receivers in the same way and finally stacking the outcomes. Numerical calculation has yielded satisfactory results. Data processing of the Zhuangmu-Yuexi-Huangmei-Zhanggongdu profile passing through the Dabie orogenic zone provides a structural form of M-discontinuity along the profile. The result shows that the high pressure metamorphic rock zone of South Dabie rock mass is related to the uplifting of M-discontinuity here. 'Mountain root' exists under North Dabie rock mass, which conforms with gravity isostasy theory. The Xiaotian-Mozitan fault is a suture zone as a result of the collision of North China plate with Yangtze plate, and extends through M-discontinuity. The abyssal fault near Liu'an is the extended eastern section of the Luonan-Minggang fault, which is also confirmed here.

Petrogeochemistry, geochronology and its geological significance of the granite from Baishipo Ag deposit, Dabie orogen

1 Introduction Granitoid magmas have been widely used as a natural probe for tracing the crustal evolution process(Martin et al.,2005).Mesozoic granitoids are widespread in the Dabie

Petrogenesis of granite from Xiaofan Mo deposit, Dabie Orogen

The Mesozoic granitoids in the Dabie Orogen are of particular geological interest as indicators for Mesozoic lithospheric evolution because of their close association with porphyry Mo mineralization. Here, we present a study using petrogeochemistry data to constrain the petrogenesis of the Xiaofan granites in the Dabie Mo mineralization belt （DMB）, Henan Province, China. Field investigations show that the Xiaofan pluton mainly consists of porphyritic granite. The Xiaofan granites have high SiO2 contents of 74.29 wt%-76.07 wt% （average： 75.18 wt%）, A1203 contents of 11.66 wt%-12.83 wt% （average： 12.13 wt%）, and K20 contents of 5.37 wt%-7.90 wt% （average： 6.86 wt%） and low MgO （0.06 wt%-0.16 wt%）, TiO2 （0.09 wt%-0.10 wt%）, and P205 （0.047 wt%-0.103 wt%） contents. They are enriched in Rb, U, K and Hf but depleted in Ba, Nb, Ta, Sr and Ti. By geochemical and mineralogical features, we propose that the Xiaofan granites belong to A-type type granite and dominantly sourced from the crust. The granites from the Xiaofan Mo deposit may have formed in a post-collision extensional setting.