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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.

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.

A transitional eclogite- to high-pressure granulite-facies paragenesis (Omp+Pl+Qtz±Grt) after peak coesite–eclogite facies metamorphism and predating the later amphibolite-facies overprint is identified in coesite–eclogite from the Taohang area of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. These minerals were equilibrated at 17 kbar and 820°C. This reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the UHP rocks. The tectono-metamorphic significance of the unusually high pressure overprint is also discussed.

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.

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.

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.

Various types of eclogite, including coesite-bearing varieties formed under high-P-high-T conditions (up to 27 kbar, 700–800°C), and glaucophane schist occur in the Dabie Mountains and the SuLu terrane, eastern China. Dating of these high-pressure rocks by the SmNd mineral isochron and 40 Ar 39 Ar method suggests that the occurrence of the high-pressure metamorphism was during the early Triassic. LREE-enriched chondrite-normalized patterns for type-II eclogites and low initial ϵNd of − 14 to − 3 and the 87 Sr 86 Sr-values (0.706–0.710) for various types of eclogites suggest that their protoliths were mainly derived from Precambrian island arc or intraplate basalts in the basement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mountains suggests that the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabie Mountains in the late Jurassic and Cretaceous (at 134 Ma). It is proposed that the collision between the North China Block and Yangtse Block began in the late Permian or early Triassic with a north-dipping subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Triassic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histories for a coexisting gneiss and eclogite pair from Shima area suggest that the eclogite and their country rocks are not always coherent to each other. Some of them may have been juxtaposed through tectonic processes from different levels.

The Lancang metamorphic terrane consists of an eastern low-P/T belt and a western high-P/T belt divided by a N–S-trending fault. Protoliths of both units are mid–late Proterozoic basement and its cover. The low-P/T belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo-Triassic volcanic and volcaniclastic rocks. Most whole-rock Rb–Sr isochron and U–Pb zircon ages of the Lincang batholith are in the range 290–279 and 254–212 Ma, respectively. Metamorphism of the low-P/T belt reaches upper amphibolite with local granulite facies (735°C at 5 kbar), subsequently retrogressed at 450–500°C during post-Triassic time. The high-P/T rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P–T conditions follow the high-P intermediate facies series up to about 550–600°C, at which oligoclase is stable. The 40Ar/39Ar plateau age of sodic amphibole in blueschist is 279 Ma. The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south-east Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P–T conditions, perhaps during collision of the Baoshan block with south-east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in south-east Asia represents a suture zone between two contrasting microcontinents.