Post-Collisional Ductile Extensional Tectonic Framework in the UHP and HP Metamorphic Belts in the Dabie-Sulu Region, China

The present-day observable tectonic framework of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts in the Dabie-Sulu region was dominantly formed by an extensional process, mostly between 200 and 170 Ma, following the Triassic collision between the Sino-Korean and Yangtze cratons. The framework that controls the present spatial distribution of UHP and HP metamorphic rocks in particular displays the typical features of a Cordilleran-type metamorphic core complex, in which at least four regional-scale, shallow-dipping detachment zones are recognized. Each of these detachment zones corresponds to a pressure gap of 0.5 to 2.0 GPa. The detachment zones separate the rocks exposed in the region into several petrotectonic units with different P-T conditions. The geometry and kinematics of both the detachment zones and the petrotectonic units show that the exhumation of UHP and HP metamorphic rocks in the Dabie-Sulu region was achieved, at least in part, by non-coaxial ductile flow in the multi-layered detachment zones, and by coaxial vertical shortening and horizontal stretching in the metamorphic units, under amphibolite- to greenschist-facies conditions, and in an extensional regime. All ductile extensional deformations occurred at depths below 10 to 15 km, i.e. below the brittle/ductile deformation transition.

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane,China: A Clear Record of Ultra-deep Subduction and Fast Exhumation

Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure （UHP） terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited （detrital） and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence （CL） imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones （domains） to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure （HP） metamorphic conditions of T = 571-668℃ and P =1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃ and P 〉 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600- 710℃ and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from - 55 km to 〉 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths 〉160 km to the base of the crust at -30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.

Helium Isotope Geochemistry of Ultrahigh-Pressure Metamorphic Eclogites From the Dabie-Sulu Terrane in East China

: The 3He/4He ratios of most eclogites from the Dabie-Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the 3He-4He diagram. The 3He/4He ratios of eclogites are obviously correlated with the types of their surrounding rocks. The helium isotope composition of the eclogites from the Bixiling complex possesses characters of mantle-derived rocks with the 3He/4He ratio being 5.6 Ra. The 4He concentration of the eclogites exhibits visible inverse correlation with the δ18O value of the quartz in the eclogites from the Sulu area. The δ18O values of the eclogites change synchronously with those of the country rocks. Those results suggest that protoliths of the eclogites were basic-ultrabasic rock bodies or veins intruding into the continental crust in the early stage; strong exchange and hybridization between the basic-ultrabasic rocks and continental rocks and the atmospheric water during the intrusion led to abrupt increase of the 3He/4He ratios, δ18O values and Nd(0) values of the intrusive bodies or veins, which show characters of continental rocks. This indicates that the eclogites are autochthonous.

Location of Triassic Tectonic Suture Between Collided Sino-Korean and Yangtze Cratons in Dabie-Sulu Region, China

The regional extent and spatial distribution of ultrahigh pressure metamorphic(UHPM) and high pressure metamorphic (HPM) rocks, and the geometrical relationships of various petrotectonic units in the Dabie-Sulu region indicate that the Triassic collisional suture line between the Sino-Korean and Yangtze cratons is situated at the northern margin of the Dabie massif, that is,along the Balifan-Mozitan-Xiaotian fault in the Dabie region, and possibly is linked to the Wulian-Yantai fault in the Sulu region to tbe east. The suture line has been strongly modified duriug and subsequent to UHPM aud HPM events.

Early Cooling History of Eclogites from the Dabie-Sulu Orogen: Constraints from Diffusion Kinetics of Garnet

For the first time, we apply different geospeedometric models to garnet zoning patterns that were obtained in this study from detailed EMP analyses for garnets from eclogites and granulite in the Dabie-Sulu orogen. Various zonings of cation diffusion were preserved in the garnets, enabling the acquirement of average cooling rates for the high-to ultrahigh-pressure rocks without using geochronological approaches. The coesite-bearing hot eclogites yield fast cooling rates of about 20 to 30℃/Ma subsequent to peak metamorphic temperatures, whereas the cold eclogite gives a relatively slow cooling rate of 8℃/Ma at its initial exhumation. A very slow cooling rate of <0.3℃/Ma is obtained for the granulite at Huangtuling, suggesting that the granulite may not be involved in the continental deep subduction.

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

Multi-Layered Low-Angle Ductile Detachment System in Dabie-Sulu UHPM and HPM Belts, China

The present-day observed crustal-scale tectonic style of ultrahigh-pressure metamorphic (UH-PM) and high-pressure metamorphic (HPM ) belts in the Dabie-Sulu region was dominantly formedby extensional processes, postdating the Triassic collision between the Sino-Korean and Yangtze cratons. The extensional structures overprinting the previous structures related to contraction that produced the thickened continental crust of the UHPM and HPM belts, in particular display the typical features of a Cordilleran-type metamorphic core complex, in which at least four regionalscale, low-angle ductile shear zones that constitute a detachment system, are recognized in the Dabie region. In the Sulu region, the extensionaI structures show in the form of small-scale domes or a regional-scale SE-dipping pseudo-monocline. The geometry and the kinematics of tbe detachment zones are briefly described and their significance for the exhumation of UHPM and HPM rocks is discussed. It is iniliated that the subhorizontal crustal-scale extensional flow in the middle-lower crust, under amphibolite- to greenschist-facies conditions, was an important tectonothermal process at 200- 170 Ma and the exhumation or the UHPM and HPM rocks was achieved at least in part along multi-layered detachment zones. The regional detachment system has been the main factor enabling UHPM and HPM rocks to be brought from middle-lower crustal levels to middle-upper crustal levels.

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.

Correlation between Rimjingang belt and Dabie-Sulu orogenic belt

The Dabie-Sulu orogenic belt in central-eastern China is considered as a high-pressure and ultrahighpressure metamorphic belt that demensions are comparatively large,and formed as a result of the collision of Sino-Korean and Yangtze cratons in eastern China. After continuous discoveries of high-pressure and ultra-high pressure metamorphic assemblages in the Dabie-Sulu area,the issue of the Dabie-Sulu orogenic belt extending eastward to the Korean Peninsula has been paid attentions widely. The discoveries of eclogites in the Hongsoeng area,the middle-western Korean Peninsula gives rise to the debate on the tectonic affiliations of the southern massifs. Although the Rimjingang belt in the Korean peninsula has been well investigated,the relation and comparative study to the Dabie-Sulu orogenic belt are lacking of detail work. In this paper,on the basis of informations and results of our previous works,some new contrastive considerations on the correlation between the Dabie-Sulu orogenic belt in central-eastern China and Rimjingang belt in the Korean Peninsula have are provided.

Remelting of subducted continental lithosphere: Petrogenesis of Mesozoic magmatic rocks in the Dabie-Sulu orogenic belt

The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermedi- ate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low εNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.

Post-collisional lithosphere delamination of the Dabie-Sulu orogen

The consistence between the first rapid cooling time (226-219 Ma) of the untrahigh pressure metamorphic (UHPM) rocks in the Dabie Mountains and the formation time (205-220 Ma) of the syncollisional granites in the Qinling and Sulu areas suggests that the first rapid cooling and uplift of the UHPM rocks may be related to breakoff of subducted plate. Therefore the second rapid cooling and uplift (180-170 Ma) of the UHPM racks needs a post-colli-sional lithosphere delamination which resulted in the granitic magmatism with an age of about 170 Ma. In addition, the rapid rising of the Dabie dome in the early Cretaceous (130-110 Ma) and the corresponding large-scale magmatism in the Dabie Mountains need another litho-sphere delamination. The geochronology of the post-collis-ional mafic-ultramafic intrusions and geological relationship between the mafic-ultramafic intrusions and granites suggest that partial melting was initiated in the mantle, and then progressively developed in the crust, suggesting a mantle

Ce anomaly in minerals of eclogite and garnet pyroxenite from Dabie-Sulu ultrahigh pressure metamorphic belt:Tacking subducted sediment formed under oxidizing conditions

In-situ excimer laser ICP-MS analysis of minerals of eclogites and garnet pyrox- enites from type localities (Shuanghe, Maowu, Bixiling, and Yangkou) in the Dabie-Sulu ultra- high-pressure metamorphic belt reveals highly variable Ce anomalies from negative to positive in garnet. Similar Ce anomalies are also present in omphacite or clinopyroxene but to a much lesser extent. Such mixed negative and positive Ce anomalies mimic those found in severe weathering profiles developed under oxidizing conditions. They suggest the presence of sub- ducted sediment components in the eclogites and garnet pyroxenites, which in turn points to the potential importance of the recycled sediments in modification of the mantle composition during the deep subduction of the continental crust.

Seismic tomography showing subduction and slab breakoff of the Yangtze block beneath the Dabie-Sulu orogenic belt

Seismic tomography reveals that a subducted ancient block has been preserved beneath the Moho of the Dabie-Sulu orogenic belt. Taking into account of geological and geochronological data, we inferred from the tomographic images that the Yangtze block was subducted northward beneath the Sino-Korean block and broken off at the depth 【200 km during 200-190 Ma. The slab breakoff of the Yangtze block is the most important dynamic mechanism to control the exhumation of UHP rocks.

Two Fresh Types of Eclogites in the Dabie-Sulu UHP Metamorphic Belt,China:Implications for the Deep Subduction and Earliest Stages of Exhumation of the Continental Crust

Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are dis- cernible in large eclogite bodies surrounded by country rock gneisses from the Dabie Sulu UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and grano- blastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions （~3.1-4.0 GPa, 800~50 ） within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic （-250-230 Ma）. The diagnostic UHP minerals, mineral assemblages and absence of notable macro- scopic deformation indicate the peak metamorphic ＇forbidden-zone＇ P-T conditions, an extremely low geothermal gradient （〈7 ＂C＇kma） and low differential stress. The foliated eclogite is composed of garnet＋omphacite＋rutile＋phengite＋kyanite＋zoisite＋talc＋nybtite^coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliatcd eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites de- fine UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three- dimensional relationship between the massive and foliated eclogites is well displayed a typical ＇block-in-matrix＇ rheological fabric pattern in- dicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted con- cept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic over- presure on UHP metamorphism is rather limited.

Petrology of the Non-mafic UHP Metamorphic Rocks from a Drillhole in the Southern Sulu Orogenic Belt,Eastern-Central China

The Drillhole ZK703 with a depth of 558 m is located in the Donghai area of the southern Sulu ultrahigh-pressure (UHP) metamorphic belt, eastern China, and penetrates typical UHP eclogites and various non-mafic rocks, including peridotite, gneiss, schist and quartzite. Their protoliths include ultramafic, mafic, intermediate, intermediate-acidic, acidic igneous rocks and sediments. These rocks are intimately interlayered, which are meters to millimeters thick with sharp and nontectonic contacts, suggesting in-situ metamorphism under UHP eclogite facies conditions. The following petrologic features indicate that the non-mafic rocks have experienced early-stage UHP metamorphism together with the eclogites: (1) phengite relics in gneisses and schists contain a high content of Si, up to 3.52 p.f.u. (per formula unit), while amphibolite-facies phengites have considerably low Si content (<3.26 p.f.u.); (2) jadeite relics are found in quartzite and jadeitite; (3) various types of symplectitic coronas and pseud