The Permian-Triassic Tectonic Evolution Of The Ningwu And Longyan Sedimentary Basins In South China

The Late Paleozoic-Mesozoic continental margin of the eastern and northern South China witnessed the subduction of the Paleo-Pacific Plate and the collision of South China with North China,which basically established the current tectonic framework of eastern China.Constraining the initial uplift and exhumation of orogens through sedimentary basins is of great significance to the study of the history of the South China and paleogeographic reconstruction.Focusing on the initial time and the tectonic background of the subduction of the Paleo-Pacific Plate to the southeastern South China and the initial time of Dabie Orogenic Belt becoming a source area in the Lower Yangtze region basins,this work conducted a detailed provenance study of the Late-Paleozoic to Mesozoic Ningwu Basin and Longyan Basin.The source area of the Lower Silurian-Middle Permian sandstone in the Ningwu Basin has changed.Sandstones of the Lower Silurian Fentou Formation are feldspatho-quartzose with a few igneous and low-grade metamorphic rock fragments.Sandstones of the Devonian Wutong Formation are quartzose with a few quartz-rich metamorphic rock fragments.The Fentou and Wutong formations share similar detrital-zircon age spectra,with peaks mainly at 465-420 Ma and 838-812 Ma,indicating a source from the Wuyi terrane.The lithology of Permian Longtan Formation are litho-quartzose and feldspatho-quartzose,with dominant felsic and intermediate volcanic lithic fragments,zircon age spectra from the Longtan Formation in the Chaohu area display a diagnostic peak at ?1866 Ma,indicating provenance from the Paleoproterozoic basement of the Wuyi terrane.Instead,detrital zircons in the Longtan Formation in the Nanjing area display peaks at ?445 Ma and ?970Ma,similar to those in the underlying Paleozoic sandstones.The Wuyi terrane in the Cathaysian block thus remained as a major source area for long.The Longtan Formation also contain detrital zircons with ages distributed continuously from 341 to 254 Ma,reflecting renewed intermediate and felsic magmatism in the source area.The source area of the Ningwu Basin in the Middle Triassic changed again from the Paleozoic.The lithology of Huangmaqing Formation in Ningwu Basin is dominated by lithoquartzose sandstone and feldspathic-litho-quartzose sandstone,the content range of quartz is76%-84%,lithic fragment is 10%-14%,and feldspar is 6%-11%.Among the lithic fragments,quartz-rich metamorphic rock fragments or metapelite grains rock debris are dominant.The heavy minerals of the samples are mainly zircon,rutile,apatite,tourmaline,hornblende,garnet and muscovite,which further indicate the existence of special sources of metamorphic rocks.The detrital zircon age spectrum shows the main mode at 2000-1600 Ma?350-250 Ma?420-400 Ma?900-700 Ma?2500-2100 Ma.This paper believes that materials of Huangmaqing Formation mainly comes from the middle-low degree metamorphic belts of the early exhumation of the Dabie orogen,such as the current Bei Huaiyang belt.The materials of the early uplift may include the sedimentary cover of the southern margin of the North China and the northern margin of the South China,and also the Paleozoic arc magmatic rocks of the Qinling-Dabie Mountains.Through comparison with the sedimentary records of other early Mesozoic basins around the Dabie orogen,it is found that the basins have similar provenance in the Middle-Late Triassic.Combining the previous study of the South China-North China collision occurred "early in the east and late in the west" and the exhumation mode of the subducted South China crust,this paper believed the Huangmaqing Formation,which is deposited during Middle Triassic Lardian(242-237 Ma),represents the earliest record of orogenic denudation.The depositional age can be used to limit the latest uplift and exhumation time of the Dabie orogen.The lithology of the Upper Devonian Tianwadong Formation,Taozikeng Formation,and Lower Carboniferous Lindi Formation in Longyan Basin are mainly quartzose,and sharing the age mode of detrital zircons of 480-380 Ma,inside of which are 3 minor age peaks as 440 Ma,420 Ma and 390 Ma,indicate that provenance areas are within 150 km from Longyan.Combining the single age peak distribution of detrital zircons U-Pb ages and the low structural maturity of the sandstones,it is believed that the Longyan Basin before Permian had a nearway provenance,with the source area in the east of the Nanling terrane and the west and middle of the Wuyi terrane;the Permian Tongziyan formation,Cuipingshan Formation,and Luokeng Formation share the same age mode,meanwhile,and similar to Ningwu Basin,detrital zircons with age of 350-250 Ma occur and indicate the magmatic events in the southeast coast of South China have joined the provenance supply system of Permian sandstone both in the Longyan Basin and Ningwu Basin.This study discusses the nature and initial time of magmatic activity from the Late Carboniferous to Early Permian along the southeast coast of South China.Combining the characteristics of intermediate-acid igneous rocks,long-lasting magmatic activity,wide range of provenance systems,and positive Hf isotopic trend,this paper believes that the nature of magmatic activity that occurred at about 290 Ma is most likely to be a continental arc,formed in the early subduction stage of the paleo-Pacific plate;the original magmatism record may be pulled away from the mainland and some slices are preserved in Hainan Island China,the Kurosegawa belt in southwest Japan,and Mindoro Island in the western Philippines.The regional Permian sandstone detrital zircon Hf isotope data set shows that the intensified melting of the newly-born crust of the continental arc occurred at 310 Ma,marking the initial time of the continental arc.In addition,the uplift of the continental arc caused the diachronous retreat(with timespan of 40 Ma)of the Late Carboniferous-Early Permian carbonate platform that once existed extensively in the inland of the Cathaysia plate.The retreat direction was from the coast to the inland.