The Dabie Mountains is a collisional orogen between the North China and Yantze Continental plates. It is composed, from south to north, of the foreland fold and thrust belt alternated with molasse basin, the subducted cover and basement of the Yangtze continental plate, the meta-ophiolitic melange belt, the forearc meta-flysch nappe (bounded by southward and northward thrust belts) in which there may be a buried volcanic arc and a relict back-arc basin (Fig. 1A) (Xu et al., 1992a, 1994).

Marbles and interlayered coesite-bearing eclogites near the village ofSanqingge in the Sulu ultrahighpressure (UHP) terrane ofeastern China were studied to estimate their P-T evolution. Using garnet, omphacite and phengite as geothermobarometers, the coesite eclogites are calculated to have experienced P-T conditions of3.4 –3.7 GPa and ~600ºC (stage I), followed by decompression and a slight temperature decrease to 2.7–3.2 GPa and 520–560ºC respectively (stage II) and later to 2.6–2.8 GPa and ~500ºC (stage III). No water influx affected the eclogites until reaching amphibolite facies conditions of 0.5–1.3 Gpa and 595–685ºC (stage IV). As we interpret the occasional appearance ofcalcite with magnesite relics in the core as a reaction ofUHP dolomite and magnesite with Ca-rich fluids at stage IV to form CaCO3, the calculated pressure for stage I could be the maximum pressure experienced by these rocks. Thus, the crustal material ofthe Sanqingge quarry, originally sedimentary carbonates (now marbles) and interstratified basic tuffs (now eclogites), has been buried to a depth of ≥ 120 km at ~600ºC. This burial occurred in a subduction setting along a very low geotherm of 5–6ºC/km. The exhumation possibly occurred in the environment ofa subduction channel.

Nyböite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyböite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite.

Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 ± 60°C and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyböite can occur in the Na-Al-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in NaB, variable in Si content, and richer in NaA variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.

Diamond and coesite occur in granulites of the Internal Zone of the Rif belt in northwest Africa. Diamond, identified by optical microscopy, electron microprobe analysis, Raman spectroscopy, cathodoluminescence and microstructural electron backscattered diffraction, is present as inclusionsup to 20 μm across in garnet, K-feldspar, coesite relics and quartz. Thermobarometric estimates yield P >4.3 GPa and T >1100°C, which corresponds to a depth of formation >150 km. The estimates suggest that the diamond-bearing peridotites and adjacent crustal rocksexperienced similar P–T conditions. If this is correct, there is an old (undated) core in the Betic–Rif cordillera and the current models of the tectonic evolution of the area, which are based on 'full Alpine' evolution, must be revised. This discovery provides furthervaluable information about the complex geotectonic environment of the southeast Spain and north Moroccan collisional orogen.

Coesite has been found as an inclusion in garnet from the Mengzhong eclogite in Donghai county, northeastern Jiangsu province, eastern China. The host eclogite occurs closely associated with regionally developed gneiss and was formed under P-T conditions of about 810°C and more than 28 kbar.

Fluid inclusions trapped in coesite-bearing rocks provide important information on the fluid phases present during ultrahigh-pressure metamorphism. The subduction-related coesite-bearing eclogites of the Tso Morari Complex, Himalaya, contain five major types of fluids identified by microthermometry and Raman spectroscopy. These are: (1) high-salinity brine, (2) N2, (3) CH4, (4) CO2 and (5) low-salinity aqueous fluids. These fluids were trapped during both deep subduction and exhumation processes. The coesite-bearing rocks are inferred to have been buried to a depth of >120 km, where they experienced ultrahigh-pressure metamorphism. The fluid–rock interaction provides direct evidence for fluid derivation during a deep subduction process as demonstrated by silica–carbonate assemblages in eclogite. High salinity brine, N2 and CH4 inclusions are remnants of prograde and peak metamorphic fluids, whereas CO2 and low-salinity aqueous fluids appear to have been trapped late, during uplift. The high-salinity brine was possibly derived from subducted ancient metasedimentary rocks, whereas the N2 and CH4 fluids were likely generated through chemical breakdown of NH3-bearing K minerals and graphite. Alternatively, CH4 might have been formed by a mixed fluid that was released from calcareous sediments during subduction or supplied through subducted oceanic metabasic rocks. High density CO2 is associated with matrix minerals formed during granulite-facies overprinting of the ultrahigh-pressure eclogite. During retrogression to amphibolite-facies conditions, low-salinity fluids were introduced from external sources, probably the enclosing gneisses. This source enhances salinity differences as compared to primary saline inclusions. The subducting Indian lithosphere produced brines prior to achieving maximal depths of >120 km, where fluids were instead dominated by gaseous phases. Subsequently, the Indian lithosphere released CO2-rich fluids during fast exhumation and was then infiltrated by the low-salinity aqueous fluids near the surface through external sources. Elemental modelling may improve quantitative understanding of the complexity of fluids and their reactions.