Exhumation tectonics of the HP-UHP orogenic belt in Eastern China: New structural–petrological insights from the Tongcheng massif, Eastern Dabieshan

The Tongcheng massif, a structure lying to the east of the central Dabieshan domain, previously considered a unique unit metamorphosed under Ultrahigh-Pressure (UHP) conditions, similar to those involved in the South Dabieshan and wrenched by the Tan–Lu fault during the Mesozoic. Detailed field survey and laboratory work in the areas of structural geology and petrology indicates that the Tongcheng massif is a tectonic stack of allochthonous sheets. From the bottom to the top, this massif is divided into three units: Lower, Middle and Upper Unit. The Lower Unit suffered an UHP metamorphism with a P-T condition of about 3.21–3.31 GPa and 590–651 °C. Eclogites in the Middle Unit underwent high pressure (HP) and medium temperature metamorphism at 2.46–2.64 GPa and 500–565 °C. In the Upper Unit, the peak metamorphic P-T conditions for eclogites were estimated as 1.54–2.36 GPa and 389–510 °C. These petrological results correspond well to the geometry of the field observation. Structural analysis indicates that the Tongcheng massif has experienced polyphase deformation. The earliest event (D1) corresponds to top-to-north (northeast) shearing associated with the early exhumation stages of HP-UHP rocks during the late Triassic. The D2 deformation overprints D1 and coincides with a domal structure similar to a metamorphic core complex of early Cretaceous age. The D3 deformation is expressed by a pervasive ENE-WSW trending, subvertical foliation and a subhorizontal lineation or penetrative slickenline with a sinistral sense of shear, probably related to the NE-SW contraction during the NW-SE subhorizontal extension, the strike slip deformation raised by the N-S compression during the early Cretaceous. The D4 event is represented by a brittle NE striking, moderately to steeply dipping to the SE fault with down-dip slickenlines. Kinematic indicators show a normal displacement with the east side moving downward. Half-grabens of late Cretaceous to early Cenozoic age are widespread throughout Eastern China. Based on structural analysis, mineral chemistry data and, in particular, the results of P-T calculations for eclogites in different units, a geodynamic scenario for the evolution of the HP-UHP orogenic belt of the Dabieshan is proposed, and the significance of the Tan–Lu fault is also discussed.