Two generations of crustal anatexis in association with two-stage exhumation of ultrahigh-pressure metamorphic rocks in the Dabie orogen

The Dabie-Sulu orogenic belt was originally generated by collisional thickening of the South China Block with the North China Block and subsequent exhumation of ultrahigh-pressure (UHP) metamorphic rocks in the Triassic. However, its present tomography is primarily by post-collisional reworking through anatectic meta-morphism and felsic magmatism in the Early Cretaceous. Because of the tectonic superimposition, much confusion has arisen in the partial melting of UHP rocks between exhumation of the deeply subducted conti-nental crust in the terminal stage of continental collision and collapse of the orogenic lithosphere in the post-collisional stage. A resolution to this issue is to determine the timing, P-T conditions, and mechanisms of crustal anatexis in the collisional orogen. For this purpose, we have performed a combined study of petrological and geochronological analyses, as well as phase equilibrium modelling, for tonalitic migmatites from the Dabie orogen. We distinguished three types of leucosomes in the migmatites with variable modal amounts of quartz, K-feldspar and plagioclase, respectively. Two generations of partial melting were identified to be associated with two stages of crustal exhumation in the collisional orogen. The first generation of crustal anatexis took place during the early exhumation of UHP metamorphic slices in the Late Triassic. The occurrence of rimmed garnet with negative Eu anomalies, single mineral inclusion of rutile, and multiphase inclusions of Grt +/- Ab +/- Kfs +/- Qz in zircon with U-Pb ages of 220-200 Ma indicate the partial melting during high-pressure (HP) granulite facies metamorphism in the Late Triassic. Melt-reintegrated phase equilibrium calculations suggest the anatectic conditions of 850-960 degrees C and 1.9-2.1 GPa. The isothermal decompression from the assumed UHP eclogite facies (M1) to the HP granulite faces (M2) was accompanied by a phengite dehydration melting reaction of Phg + Cpx + Coe/Qz = Grt-II + Kfs + Pl + Melt. An increase of geothermal gradient during the early exhumation from <10 C/km (M1) to-12-15 degrees C/km (M2) is probably associated with slice thrusting along a continental sub-duction channel. The second generation of crustal anatexis occurred during the post-collisional stage in the Early Cretaceous. Phase equilibrium calculations suggest the operation of not only a biotite dehydration melting re-action of Bt + Qz = Amp-II + melt at 670-830 degrees C at 0.5-0.7 GPa but also a fluid-present melting reaction of Bt + Pl + Qz + H2O = Amp-I + melt at-670 degrees C/0.5-0.7 GPa. The two types of partial melting could be coupled in both space and time to occur at high geothermal gradients of 30-50 degrees C/km (M3). The resulted rocks were exhumed through the mechanism of domical extrusion subsequent to orogenic collapse. There was a significant supply of external heat to the second-stage melting at low-pressure amphibolite to granulite facies in the Early Cretaceous. This high heat flow is attributable to continental rifting due to upwelling of the asthenospheric mantle consequential to thinning of the lithospheric mantle in the collisional orogen. Therefore, the two gen-erations of crustal anatexis in the Dabie orogen are temporally associated with the two-stage exhumation of UHP metamorphic rocks in the different mechanisms. This dichotomy in the mechanism of crustal exhumation and its associated anatexis may be common in collisional orogens. It has great bearing on changes in the dynamic regime and thermal state of collisional orogens after plate convergence.