Transformation between the Dabie Orogenic Belt and the Tan-Lu Fault zone: Insights from ENE-NE-trending gneiss belts at the tectonic node

Abstract

How the Dabie Orogenic Belt (DOB) and the Tan-Lu Fault Zone (TLFZ) transformed during the early Indosinian period is the key to reveal the convergence process between the North China Block (NCB) and the South China Block (SCB). Tongcheng area in the eastern margin of northern DOB is the tectonic node that connects the WNW-trending DOB and the NE-trending TLFZ. The typical ENE-NE-trending gneiss belts penetrably developed in Tongcheng area provide an ideal natural lab to decipher the transformation between the DOB and the TLFZ. Here we conduct an integrated structural and geochronological research on the ENE-NE-trending gneiss. Zircon U–Pb dating on the ENE-NE-trending gneiss yielded metamorphic ages ranging from 255 ± 9 Ma to 203 ± 10 Ma. The weakly deformed veins which intruded the surrounding gneiss yielded two groups of ages, 825 ± 29 Ma to 713 ± 7 Ma and 125 ± 2.6 Ma (weighted mean age), which indicate the protolith age of surrounding rocks and the intrusive timing of the vein, respectively. Integrated structural, microstructural and kinematic analysis indicate that no lateral structural superposed on the gneiss or veins. Therefore, it can be speculated that the deformation and metamorphism of the gneiss should simultaneously formed during the Early Triassic, as a result of continuous tearing from the DOB to the northeastern Sulu Orogenic Belt (SOB) during subduction of the Yangtze block beneath the NCB. Formation of the tearing belts, i.e., the ENE-NE-trending gneiss belts, accomplished the tectonic transformation between the DOB and the TLFZ. They could be regarded as an embryonic form of the TLFZ, which are also apparently different from the TLFZ by characteristics of non-strike slipping.