P-T evolution of elusive UHP eclogites from the Luotian dome (North Dabie Zone, China): How far can the thermodynamic modeling lead us?

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.