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A new 140-km-long seismic reflection profile provides a high-resolution crustal-scale image of the southern Dabieshan high-pressure (HP) metamorphic belt and the Yangtze foreland fold-and-thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north-dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid-Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red-bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (∼30–33 km) are seismically prominent and segmented by a number of south-verging thrusts that were probably developed by foreland-directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic.

Two kinds of jadeite occur in the jadeite–quartzite from the Dabie Mountains, Eastern China: associated either with weak or strong deformation. The former shows a uniform composition while the latter shows both uniform and zoning composition. These jadeites were examined with infrared (IR) spectroscopy. All jadeite displays hydroxyl (OH) stretching bands, implying that hydrous components commonly exist in jadeite. The concentration of the hydrous components in the jadeite with weak deformation is homogenous, whereas the concentration of the hydrous components in strongly deformed samples is variable. The correlation between the intensity of OH-band absorbance and compositional zoning of jadeite with strong deformation indicates that Na-rich jadeite can accommodate more OH than Na-poor jadeite. Its interpretation is that there is more Na associated with greater OH availability in the initial crystallization environment. Our data of the comparatively high OH concentrations recorded in the core of the jadeite which contains relative high vacancies in M2 site imply that both OH content and the vacancies in M2 site could decrease during exhumation of the jadeite–quartzite. The decrease of OH solubility in jadeite would result in the formation of H2O fluid during the early exhumation of UHP metamorphic rocks. The H2O fluid transformed from OH during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep.

This study presents zircon ages and geochemical and Nd-isotopic data for metagranites and quartzites from the Susong high-pressure metamorphic zone in the Dabie–Sulu ultrahigh-pressure collisional belt. This belt belongs to the eastern part of the Qinling–Dabie orogenic belt that formed during Early Mesozoic collision of the North and South China Blocks. Two metagranites give zircon U–Pb model ages of 785±7 and 205±12 Ma, likely representing a Late Proterozoic magmatic event and an Early Mesozoic overprint. They have low initial eNd-values (−12.4 and −11.1 at 780 Ma), favouring a crustal origin. Flat heavy rare earth elements (REE) patterns (GdN/YbN∼1.2) probably reflect that melting took place at a shallow crustal section where heavy-REE-bearing mineral phases are unstable. All zircons of three quartzites yield young discordant U–Pb ages and define a discordia with U–Pb model ages of 784±6 and 213±3 Ma, identical to those of the metagranites. We assume that all detrital zircons had lost radiogenic Pb prior to the Early Mesozoic overprint, probably facilitated by fluid participation during a metamorphic event contemporaneous with the intrusion of the metagranites. This simultaneous metamorphic and magmatic event was probably related to a rift setting along the periphery of the Yangtze (South China) Block during Late Proterozoic.

Hualiangting whiteschist in the Dabie Shan ultrahigh-pressure eclogite complex is a specific Mg-Al- rich metamorphic rock. Its protolith was a felsic dyke, which suffered replacement by high-pressure fluids during eclogite-facies metamorphism. Based on fluid inclusion results in whiteschist, primary high-pressure fluids are CO2-bearing aqueous solutions. Those prove that there exists CO2-H2O fluid during the ultrahigh-pressure metamorphism. Pseudosecondary H2O inclusions were trapped during ultrahigh-pressure exhumation, whereas secondary H2O inclusions were related to the latest evolutionary stage of the ultrahigh-pressure exhumation.

More than 120 coesite and few polycrystalline quartz (coesite pseudomorph) inclusions were identified by Raman microspectroscopy in zircons of various kinds from gneissic rocks located in the Dabieshan and Sulu ultrahigh-pressure (UHP) metamorphic terranes in eastern China. The coesite inclusions have undergone, to various degrees, the coesite-quartz transition. Raman spectra of coesite and subsidiary quartz inclusions show various shifts, which are closely correlated to the extent of the transition. The coesite-pseudomorph inclusions in zircon show the highest Raman shift. Calibrations based on the experimental work of Hemley (1987) yielded inconsistent present-day overpressures (0–24 kbar) in coesite inclusions inside zircons. The availability of fluids, which resulted in the pervasive regional retrogression of the UHP gneisses during later exhumation stages, is regarded as the main factor controlling the extent of the coesite-quartz transition. This study underscores the need for the elastic model applied by previous researchers to explain the preservation of the coesite inclusions in rigid host minerals.

Abundant garnet-bearing granulite lenses are widely distributed in the northern part of the Sulu region and adjacent areas. They are possibly re-metamorphosed high-pressure metamorphic rocks. On the basis of detailed petrographic study, samples WD01, WD04 and ML06 from Laixi and Wendeng were identified as high-pressure granulites, and WH1 from Weihai as an original coesite-bearing eclogite. Three high-pressure granulite samples give mineral-WR isochron ages of 1846±76, 1743±79 and 1752±30 Ma. TDM ages are 3.3, 3.0 and 2.8 Ga. The Sm–Nd mineral-WR isochron ages are interpreted to date as the metamorphic resetting within the medium-pressure granulite facies, representing an isotopic re-homogeneity during uplifting of the high-pressure granulites from deep continent crust. It is important that Sm–Nd chronological characteristics are the same as Archaean high-pressure granulites in the North China craton. However, sample WH1 from Weihai demonstrates abnormal Sm–Nd characteristics. Its whole rock eNd (0) value is +129. TDM age is 1.3 Ga, and constrains the minimum age of re-metamorphosed eclogite protolith formation to the mid-Proterozoic. This result is identical to those reported by Jahn (1994) , showing complicated processes of metamorphism and metasomatism. The data in this paper provide further evidence to define the boundary between the North China craton and UHPM belt in eastern Shandong and to understand the geotectonic nature of the boundary.

Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.

Some geologists reported their discovery of sandwiched low-grade metamorphic slabs within UHP metamorphic Complexes in Changpu, Yuexi County, Anhui Province. They also suggested that some coesite-bearing eclogites are igneous veins, which intruded low-grade metamorphic slabs and other related rocks. Moreover they further called in question to UHP metamorphic process and continental collision tectonism in the Dabieshan terrane. Based on our recent study, so-called low-grade metamorphic slabs are strongly deformed fabric, fine-grained and tectonic recrystallized mylonites and tectonites. Their protolith rocks are garnet-bearing orthogneiss, eclogite and marble, as well as a few amounts of strongly deformed acid and basic veins. Their metamorphic, geochemical and geochronological characteristics are also identical with UHPM rocks and regional country orthogneisses. Therefore we conclude that there are not low-grade metamorphic slabs sand wiched with UHP metamorphic rocks in the Dabieshan terrane.

Could ultrahigh-pressure metamorphic rocks be caused, at depth of 32 km, by tectonic overpressure resulting from differential stress? The differential stress is limited by rock strength, which depends on strain rate and temperature. Therefore, tectonic overpressure could not go beyond 1 GPa, and could not cause the uitrahigh-pressure metamorphism. The stress-strain in plastic deformation, or in plastic stage of elasto-plastic deformation, has no linear relation, and should not be described by using linear equation of elastic deformation.

Coesite eclogite is associated with metagranitoid in a 50×100 m 2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

The SuLu ultrahigh-pressure (UHP) metamorphic terrane is part of the Qinling-Dabie suture zone formed by the collision between the Sino-Korean and Yangtze cratonic blocks. Geochemical and isotopic investigation of the UHP metamorphic rocks has revealed a spectacular history for some eclogitic rocks. Retrograded coesite-bearing eclogites from the Weihai area in the eastern end of the SuLu terrane have recorded an extraordinary Nd isotopic growth with a world-record high ϵNd-value of + 264 (range = + 170 to + 264). Such isotopic characteristics are consistent with the observed extreme LREE fractionation and very high Sm/Nd ratios. The SmNd isotopic data allow us to calculate the minimum age of protolith formation at ∼ 1.7 Ga, assuming their evolution from a depleted or a chondritic mantle. Geochemical data indicate that the eclogite protoliths were of basaltic compositions originally, but underwent metasomatism (water-rock interaction) at high pressures and temperatures leading to the present apparent andesitic composition and extreme LREE depletion. This process took place in the Proterozoic (∼ 1.7 Ga) and was not related to the Mesozoic continental collision and ultrahigh-pressure metamorphism at ∼ 210−220 Ma ago. Because of a protracted retrograde metamorphism, garnet SmNd systems were not closed until ∼ 160 Ma ago, ∼ 50 Ma after the peak eclogite metamorphism. This implies that the exhumation history is significantly different in different parts of the Dabie and SuLu UHP metamorphic terranes. From geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late Proterozoic can be identified prior to the last Mesozoic continental collision and accompanying UHP metamorphism in the SuLu and Dabie terrane.

Petrological studies of a serpentinized garnet lherzolite body in Rongcheng of the Su-Lu region of eastern China revealed unusually high pressure. Spinel lherzolite probably in a subducting slab was transformed to garnet lherzolite at mantle depth. During exhumation, they were subsequently subjected to the granulite and then amphibolite overprinting and a phase of serpentinization. The peak P–T conditions of the garnet lherzolite estimated after detailed analysis of the metamorphic texture are 4–5 GPa and 820°C or 5–6 GPa and 780°C, depending on the chosen geothermobarometers. The lower dP/dT of the garnet lherzolite can be interpreted as the results of subduction of an old (say 100 Ma older than the time of collision) and cold, slab underneath the margin of the Sino–Korean craton.

In the Su-Lu ultrahigh-P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyboite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith. The Su-Lu ultrahigh-P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh-P peak metamorphism took place at 700–890° C and P>28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh-P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.

Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al2O3 (28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.

Recent finds of coesite eclogites, Proterozoic blueschists, and 3.7-Ga granulites have stimulated world-wide attention for the studies of metamorphic rocks in China, which is now generally regarded as one of a few suitable areas to study deep structures of continental collision. The publication of Metamorphic Map of China in 1986 provided a sound basis for the recent successful researches. Recent introduction of quantitative phase petrology and the increase of radiometric age measurements enabled Chinese geologists to depict detailed pressuretemperature-time (PT-t) paths of a metamorphism. The presence or absence of Archean greenstone belts, igneous or sedimentary origin of some felsic gneisses, extent of “migmatization”, and nature of Archean regional granulite-facies metamorphism are unfixed debates, which will be solved in the next decade.