Study on peak metamorphism conditions and zircons U-Pb chronology in the "low-grade metamorphism" belt: Foziling Group, Beihuaiyang metamorphic unit

The Foziling Group (FZG) is an important component of the Beihuaiyang metamorphic unit in the northernmost margin of the Dabie collisional orogenic belt, and is located in the crucial position between the Yangtze and North China plates. For a long time, the FZG has been regarded as the sedimentary product of the northern margin of the Yangtze Plate as an accretionary wedge. However, the disputes on the tectonic attribute and dynamic formation mechanism of the FZG are incessancy due to the different viewpoints from studies of its geochronology and structural geology. The reason for this situation lies on the lack of the accurate appraise for the metamorphic attribution of the FZG. In this study, detailed fieldworks and regional metamorphic investigations on the FZG, together with abundant analysis of its metamorphic petrology, thermodynamic characteristics and geochronological dating have been carried out. The result indicates that the FZG is mainly lithologically composed of fine-grained mica-quartz schists, quartz-mica schists and garnet mica schists. The estimates of the main metamorphic P-T conditions of the FZG show that it has metamorphosed in a middle-high pressure amphibolite facies with a P-T range of 610 similar to 620 degrees C and 0. 9 similar to 1. 0GPa, respectively, and it should not be the lower'-grade metamorphic unit. which demonstrates that the FZG might be involved into the process of subduction and exhumation in the deeper level. On the basis of zircon U-Pb dating, a large age range of 3101 +/- 48Ma similar to 413 +/- 9Ma has been gained for the FZG, which can be divided into 5 groups with the peak age values of 2537Ma, 1567Ma, 940Ma, 749Ma and 440Ma, respectively. These peak ages imply that the FZG has a mixture source from North China and Yangtze plates, and the youngest peak age suggests that the FZG has been formed in late Early Paleozoic.