Main Metamorphic Conditions And Chronology Of The Foziling Group In The Dabie Collision Orogenic Belt

Beihuaiyang metamorphic unit is located in the northern margin of the Dabie orogenic belt and is an important component of the Dabie orogenic belt.The Foziling Group is the main unit of the Beihuaiyang metamorphic unit.So far,the research on the Foziling Group is relatively weak,compared with the rich research achievements of other rock units in the Dabie orogenic belt.Firstly,there is still no precise limit on the P-T conditions and time limits of its main metamorphosis and it is usually considered that the Foziling Group is low-grade metamorphic rock unit in the Dabie orogenic belt;Secondly,there is no consistent statement about its structural attribution.It restricts the comprehensive understanding and interpretation of the structural framework of the Dabie orogenic belt and its formation mechanism.Therefore,our studying conduct a detailed and systematic research in aspects of field geological investigation,regional metamorphic petrology,microphysiography,thermodynamics and zircon U-Pb geochronology.In order to further enrich and confirm the formation environment of the Foziling Group,and to improve the metamorphic evolution history and structural framework of the Dabie orogenic belt.The main contents and results of the research are:(1)Evaluation of the P-T condition of the Foziling Group: The petrographic study shows that the garnet mica schist contains a complete mineral composition with sufficient garnet + plagioclase + biotite + muscovite + quartz.Relatively speaking,as the main rock of the region,quartz mica schist and mica quartz schist,the mineral combination is relatively simple,the main minerals are quartz + plagioclase + muscovite + biotite.The former is the key research object of this research.On the basis of accurate mineral chemical analysis,a reasonable combination of GB & GBPQ geothermobarometer is selected for joint solution,and the temperature range is calculated as P=0.93-1.12 GPa and T=613.-669 °C;for the latter,the pressure was calibrated to 1.0 GPa using a Bt-Ti thermometer and the result was 543-654 °C.Based on the calculation results of GB & GBPQ thermometer,the estimated temperature of Bt-Ti thermometer and the study of regional metamorphism petrology,when the pressure is 1.0 GPa,the main metamorphic temperature of the Foziling Group is in the range of 543-669 °C and has generally experienced high-pressure amphibolite facies metamorphism.It shows that the Foziling Group is involved in a deep subduction during the orogenic process,and the subduction depth can reach 33 km.(2)Zircon U-Pb chronological analysis: We analyzed four samples of zircon samples from the Foziling Group.The CL images and dating results indicate that they are detrital zircons.Combined with previous research data,the statistical results show that the 147 detrital zircon harmonic age range of the Foziling Group is 413±9 Ma-3101±48 Ma,and the peak ages are 435 Ma?848 Ma?957 Ma?1713 Ma and 2531 Ma.The smallest clastic zircon age is 413 Ma,which limits the Lower limit of sedimentation of the Foziling Group to the Early Devonian.Next,there is a lack of 580-510 Ma magmatism in the Foziling Group.The two peak ages of 848 Ma and 957 Ma indicate that the provenance of the Foziling Group is related to Yangtze plate.The two peak ages of 1713 Ma and 2531 Ma are closely related to the Wutaian cycle and Lüliangian-Zhongyuean cycle of the North China Plate.The Foziling Group is characterized by a mixed source.