The Peak P-T Pattern for Eclogites Across the Southern Dabieshan Terrane, China, and Tectonic Significance

The architecture of the Southern Dabieshan Terrane (SDT) in China and the tectonic nature of its boundary are hotly argued topics. Wang et al. (1992) considered the SDT as a coherent terrane with a regional P-T gradient, Okay (1993) and Carswell et al. (1997) divided the SDT into high-pressure (HP) and ultrahigh-pressure (UHP) terranes, which was separated by Taihu-Mamiao fault. Across the Taihu region where the boundary between HP and UHP terranes was formed, hundreds of eclogites have been investigated, and occur mainly as lens or blocks in gneisses. The region can be divided into HP, `mixed' HP-UHP and UHP zones. Meanwhile, the peak P-T conditions for 61 samples of eclogite across the area are estimated using the Grt-Cpx Fe2 + -Mg thermometer (Krogh-Ravna, 2000) and the Grt-Cpx-phe barometers (Waters & Martin, 1996), and three different P-T regions can be identified: 1.38-2.36GPa and 535-768oC in HP zone, 2.00-3.54GPa and 641-839oC in `mixed' HP-UHP zone and 2.82-4.09GPa and 759-942oC in UHP zone. P-T values in the `mixed' HP-UHP bestride coestie-quartz inversion line, the average P-T (2.58GPa and 709oC) is close to the inversion line. Between HP and the `mixed' HP/UHP zones, P-T conditions show a line array and no break. It indicates that the HP terrane of Okay (1993) and Carswell et al. (1997) consists of HP and the `mixed' HP/UHP zones or includes at least some UHP components. Combined with the spatial distribution of the eclogites across the area, the peak P-T conditions decrease in order from north to south and define a reference ``geotherm'' of 5oCkm - 1, this suggest that SDT should be a coherent terrane. However, a P gap of 1.0GPa exists between UHP and the `mixed' HP-UHP zones, and suggests that at least 30km thick crust had been lost. It implies that the once deeply subducted terrane was exhumated as several slices rather than as a coherent terrane. The UHP terrane was juxtaposed against the `mixed' HP-UHP slice during exhumation.