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In those ultrahigh pressure (UHP) terranes that experienced protracted high/ultrahigh temperature (HT/UHT) exhumation histories, slow exhumation rates and a widespread anatexis, the UHP metamorphism is often elusive and difficult to be constrained. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution. Different peak P-T conditions, varying between non-eclogitic to UHP conditions, have been proposed so far for various high-grade metamorphic rocks from the NDZ. Few attempts were also made to reconstruct its P-T evolution. Most of the proposed P-T paths, based on conventional thermobarometry, follow a clockwise trajectory at relatively HT (> 750 °C) and almost none of them infer the prograde portion. In this paper we present a detailed petrologic study on two eclogites from the Luotian dome of the NDZ: sample 11-7c2 shows a well preserved eclogitic assemblage (Grt + Cpx + Rt), whereas sample 11-9c1 is a Qtz-Ky-bearing eclogite pervasively retrogressed under granulite-facies conditions. The results of the thermodynamic forward modelling allowed to constrain, for the first time, the prograde portion of the NDZ P-T evolution; the decompression evolution at granulite-facies conditions has been also precisely constrained. However, due to the HT overprinting and to poorly reactive bulk compositions, this method alone is not sufficient to reconstruct the whole P-T trajectory of the NDZ: more specifically, unambiguous evidence of the attainment of UHP conditions have not been found. Different “unconventional” thermobarometric methods (such as those based on trace element and textural characterization of zircons) might be more suitable to decipher the HP/UHP history of this “really hot and slow” UHP terrane. [ABSTRACT FROM AUTHOR]

As a contribution to the discussion about the “in situ” vs. “exotic” origin of the ultrahigh-pressure metamorphic (UHPM) rocks, and to the real size of the exhumed tectonic units, the Wuhe–Pailou Unit (WPU) in eastern Dabie Shan was studied. The WPU typically consists of two micas—epidote gneiss, marble with minor eclogite, and jadeite-bearing granofels with rare garnetite and eclogite nodules; these lithologies are enclosed within leucocratic granitic gneiss.Jadeite-bearing granofels is the most peculiar lithology of the WPU, being the only felsic rock in Dabie Shan still preserving extensive relics of the UHPM mineral assemblage. The UHP peak assemblage is coesite+jadeite+garnet±epidote±phengite±kyanite, with accessory rutile, apatite, and zircon. All eclogites consist of garnet, omphacite, quartz/coesite, rutile or high-Al titanite±phengite±clinozoisite±kyanite. Two micas—epidote gneiss associated with the jadeite-bearing granofels typically consists of epidote, muscovite, biotite, plagioclase, K-feldspar, rare garnet, and accessory rutile, titanite, apatite, and zircon. Locally, the WPU paragneiss contains garnet with inclusions of phengite, kyanite, rutile, and amphibole+Na plagioclase symplectite after omphacite.A clockwise P–T path was inferred for the WPU, which consists of an early prograde part up to coesite–eclogite facies conditions (P∼3.3 GPa and T∼720 °C), an early decompression stage and a late part characterised by a near-adiabatic decompression down to medium pressure (P∼1.1 GPa, T∼670 °C) and to low-pressure amphibolite-facies conditions, followed by a final cooling to greenschist-facies conditions.Both field and laboratory data consistently suggest that the WPU, characterised by the close association of paragneiss, marble, eclogite, and granofels, is a supracrustal metasedimentary sequence, which underwent subduction, UHPM, and exhumation as a coherent tectonic unit. Therefore, within collisional orogens, large coherent slices of continental crust may be subducted at depths greater than 100 km. [Copyright &y& Elsevier]

Deciphering the P-T-t path of ultrahigh-pressure (UHP) rocks is crucial for reconstructing the evolution of continental crustal terranes involved in very deep subduction followed by exhumation. Achievieng this goal is especially challenging for UHP terranes which experienced protracted high/ultrahigh temperature (high-T/UHT) exhumation histories, because the widespread anatexis associated to retrograde re-equilibration often obliterate the evidence of UHP metamorphism. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) is the archetype of such UHP terranes experiencing a "slow and hot" exhumation. The NDZ stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution. This paper presents the results of a multidisciplinary study integrating P–T pseudosection of phase equilibrium modeling, conventional geothermometry and zircon U–Pb dating with trace element analysis applied on the high–T UHP eclogites from the NDZ. The studied eclogites underwent a multistage metamorphic evolution including post-peak granulite-facies overprinting and subsequent amphibolite-facies retrogression. P–T conditions of the granulite-facies overprinting and the amphibolite-facie retrogression have been estimated at >830 °C, 7–12 kbar and 560–880 °C, 4.3–8.9 kbar, respectively. New zircon U–Pb dating, mineral inclusion and trace element analysis, combined with modeled plagioclase modal variation by pseudosection approach, allowed constraining the ages of prograde metamorphism, eclogite-facies metamorphism, granulite-facies overprinting and amphibolite-facies retrogression of the eclogites at 247–230 Ma, 226–219 Ma, 217–212 Ma and 214–198 Ma, respectively. These results provide new P–T-t estimates and age data for the multiple metamorphic stages during continental collision, contributing to precisely reconstruct the whole P–T–t path of the NDZ. Unlabelled Image • Better constrained P-T estimates than previously obtained using conventional thermobarometry. • X(H 2 O)-T pseudosection to discuss the influence of water content on the P-T pseudosection. • Clarified the influence of the later granulite-facies overprinting on earlier granulite-facies and amphibolite-facies retrograde metamorphisms. • Provides detailed REE data of the metamorphic zircons formed at different prograde and retrograde stages. [ABSTRACT FROM AUTHOR]