Analysis Of Zircon Chronology For The Mesozoic Sediments In The Hefei Basin And Their Tectonic Implications

Evolution of the Hefei Basin,located to the north of the Dabie Orogen and west of the Tan-Lu Fault Zone is closely related to the two major tectonic belts.Our LA-ICP-MS zircon dating work on Mesozoic sandstones and volcanic rocks from the southern and eastern Hefei Basin gives constraints on depositional time of the different formations,yields eruption time of the dated volcanic rocks and indicates provenance for the Mesozoic sediments.The dating results show that the Mesozoic clastic rocks in the southern Hefei Basin include the Lower Jurassic Fanghushan Formation,the Middle Jurassic Yuantongshan or Sanjianpu formation,the Lower Cretaceous Fenghuangtai or Zhougongshan Formation and Upper Cretaceous Qijiaqiao Formation from the bottom to the top,with the absence of Upper Jurassic strata.The exposed Cretaceous clastic rocks in the eastern Hefei Basin are the Lower Cretaceous Zhuxiang or Xiangdaopu Formatiom and Upper Cretaceous Zhangqiao Formation from the bottom to the top.The zircon data indicate eruption time of Early Cretaceous(130-120 Ma)for the exposed Maotanchang or Baidafeng volcanic rocks in the basin.The detrital zircon demonstrate that the Lower to Middle Jurassic and Cretaceous clastic rocks in the southern basin were derived from the Dabie Orogen as a result of the post-orogenic exhumation and late-orogenic extensional uplifting respectively.The provenance of the Cretaceous clastic rocks in the eastern Hefei Basin is the Zhangbaling uplifting belt which is a rising wall of the Tan-Lu Fault Zone during its normal faulting.Continent–continent collision between the North China and South China plates place along the Qinling-Tongbai–Hong'an–Dabie orogens during the Triassic.A micro-continent with Paleozoic arcmagmatism has been recognized in the northern Qinling–Tongbai orogens;however,it remains unclear whether the micro-continent extended to the Dabie Orogen to form a ribbon-shaped micro-continent,due to later burialby the Hefei Basin in the north.To solve this problem,we conducted LA-ICP-MS U/Pb dating of zircons from Silurian to Cretaceous sandstones and volcanic rocks from the southern margin of the basin.The age spectra of detrital zircons suggest that the Dabie Orogen and later basin cover were the sources of the analyzed sandstones.The detrital and inherited zircons indicate Neoproterozoic,early and late Paleozoic magmatism in the Beihuaiyang unit in the north of the Dabie Orogen.The zircon and previous geophysical data show that a micro-continent bounded by the Feizhong Suture in the north and the Xiaotian–Mozitang Suture in the southexisted between the North China Plate and the Triassic Dabie Orogen,and its northern half is buried by the Jurassic-Paleogene Hefei Basin.The Beihuaiyang micro-continent experienced early Paleozoic arc magmatism caused by southwardsubduction of the Erlangping oceanic crust and late Paleozoic magmatism related to northward subduction of the Paleotethyan oceanic crust.The micro-continent was accreted to the southern edge of the North China Plate at the end of the Early Devonian via arc–continent collision.Similarly to the Qinling-Tongbai orogens,the Dabie Orogen contains a Paleozoic accretionary system in the north and a Triassic collisional system in the south;thus,it is suggested that a ribbon-shaped micro-continent,>900 km long and 50-100 km wide,was present along the entire Qinling-Tongbai-Hong'an-Dabie orogens prior to the middle Paleozoic.This micro-continent might have originated as a result of middle Neoproterozoic rifting along the margin of the Yangtze Plate.