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Medium-temperature ultrahigh pressure (MT-UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core-mantle increasing grossular (X gr ) and decreasing pyrope (X py ) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the X py and X gr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core-rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post-peak stages. The maximum Si contents are mostly 3.60-3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT-UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the X py in garnet mostly depends on bulk compositions, whereas the X gr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the X gr and Si isopleths in pseudosections, the peak P-T conditions were estimated to be 40 kbar/730°C for the Jinheqiao, 41 kbar/726°C for the Shuanghe, and 37-52 kbar and 700-830°C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite AE phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early-stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite-bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid-present conditions, it is difficult to recover real peak P-T conditions for UHP eclogites with lawsonite. This may be a reason why the P-T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late-stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite-absent MT-UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting.Discoveries of coesite-and diamond-bearing eclogites from the south Dabie eclogite belt (Okay et al., 1989;Wan...

Garnet amphibolites can provide valuable insights into geological processes of orogenic belts, but their metamorphic evolution is still poorly constrained. Garnet amphibolites from the Wutai-Hengshan area of the North China Craton mainly consist of garnet, hornblende, plagioclase, quartz, rutile and ilmenite, with or without titanite and epidote. Four samples selected in a south-north profile were studied by the pseudosection approach in order to elucidate the characteristics of their metamorphic evolution, and to better reveal the northwards prograde change in P-T conditions as established previously. For the sample from the lower Wutai Subgroup, garnet exhibits obvious two-substage growth zoning characteristic of pyrope (X py ) increasing but grossular (X gr ) decreasing outwards in the core, and both X py and X gr increasing outwards in the rim. Phase modelling using THERMOCALC suggests that the garnet cores were formed by chlorite breakdown over 7-9 kbar at 530-600°C, and rims grew from hornblende and epidote breakdown over 9.5-11.5 kbar at 600-670°C. The isopleths of the minimum An in plagioclase and maximum X py in garnet were used to constrain the peak P-T conditions of~11.5 kbar/670°C. The modelled peak assemblage garnet + hornblende + epidote+ plagioclase + rutile + quartz matches well the observed one. Plagioclase-hornblende coronae around garnet indicate post-peak decompression and fluid ingress. For the samples from the south Hengshan Complex, the garnet zoning weaken gradually, reflecting modifications during decompression of the rocks. Using the same approach, the rocks are inferred to have suprasolidus peak conditions, increasing northwards from 11.5 kbar/745°C, 12.5 kbar/780°C to 13 kbar/800°C. Their modelled peak assemblages involve diopside, garnet, hornblende, plagioclase, rutile and quartz, yet diopside is not observed petrographically. The post-peak decompression is characterized by diopside + garnet + quartz + melt = hornblende + plagioclase, causing the diopside consumption and garnet compositions to be largely modified. Thus, the pesudosection approach is expected to provide better pressure results than conventional thermobarometry, because the later approach cannot be applied with confidence to rocks with multi-generation assemblages. U-Pb dating of zircon in the Wutai sample records a protolith age of c. 2.50 Ga, and a metamorphic age of c. 1.95 Ga, while zircon in the Hengshan samples records metamorphic ages of c. 1.92 Ga. The c. 1.95 Ga is interpreted to represent the prepeak or peak metamorphic stages, and the ages of c. 1.92 Ga are assigned to represent the cooling stages. All rocks in the Wutai-Hengshan area share similar clockwise P-T morphologies. They may represent metamorphic products at different crustal depths in one orogenic event, which included a main thickening stage at c. 1.95 Ga followed by a prolonged uplift and cooling after 1.92 Ga.