Sedimentary Source-to-Sink Regionalization And Paleogeography For The Nanhuan–Middle Ordovician South China

Global contemporaneous orogenic belts are crucial factors for connecting the blocks within a supercontinent.Information of the orogenic belts can be preserved in the terrigenous sediment archives of these blocks through the denudation-accumulation system.In the perspective of the source-to-sink relationship,a series of studies on the earliest sedimentary records since the formation of South China was conducted.The results not only enriched current geological data,but also were used to clarify the disputes on the paleogeographic understanding and shed new light on the paleoposition of South China within the global tectonic system.Based on the extensive field investigations,a study on the new materials from the Nanhuan-middle Ordovician strata in the central Yangtze Block,the southeastern margin of the Yangtze Block(SEMYB)and the east Cathaysia Block was carried out.The following results are obtained.(1)4480 zircons obtained from 42 samples have U-Pb ages of 3731–487 Ma(except for the ages resulted from the metamorphism of samples),¹⁷⁶Hf/¹⁷⁷Hf values of 0.280347–0.282764,?_Hf(t)values ranging from-41.4 to+13.8,and Hf-T_DM2 of 4.3–0.8 Ga.Those results show significant differences between two regions,in the southern SEMYB(i.e.,northeast Jiangxi and northwest Zhejiang)as well as the northeastern Cathaysia Block(i.e.,central-east Jiangxi and northwest Fujian),detrital zircon samples exhibit unimodal age peaks at 833–738 Ma(except for one sample with an age peak at 880 Ma),Hf-T_DM2generally within 3.0–1.9 Ga,and most of the zircons from the Nanhuan-Sinian and Cambrian-Lower Ordovician systems are angular-subangular and rounded-subrounded,respectively.However,in the Cathaysia Block that includes the central-west and south parts of Jiangxi and central-southeast Fujian,detrital zircon samples display age spectra with major peaks at 984–940 Ma and Hf-T_DM2 clustering at2.2–1.7 Ga,and the majority of zircons are rounded-subrounded.In addition,three samples from the south Anhui,central Fujian and northwestern central Jiangxi have major age peaks at 789–753 Ma,and one sample from the central Fujian has a major age peak at 518 Ma,whose 0.65–0.5 Ga zircon grains are angular-subangular,while others are generally rounded-subrounded.(2)Directional significances for 98 datasets within 5 categories of directional structures show that the directions of sedimentary transport and layer slump were SW-SSW-SSE-SE in the places including the central Yangtze Block,northern SEMYB,and Pingxiang area and northwest Fujian of the Cathaysia Block;while the NW-NNW-NNE-NE directions were suggested in the southern SEMYB and the Cathaysia Block,including the central-west and south parts of Jiangxi and central-southeast Fujian.Additionally,the transport direction of siliciclasts in the central-east Jiangxi of the Cathaysia Block was SWW-NWW,and the orientation of the deep-water facies was the NW-direction of the southernmost SEMYB.The sedimentary slump direction was SWW-NWW or SEE-NEE indicated by one peculiar syndepositional fold in the southernmost SEMYB.(3)Analytical results of siliciclastic components(e.g.,quartz,feldspar and lithic fragment)in 163thin-sections from 61 samples show that the siliciclasts probably were from the stable craton for the northern SEMYB,the recycled orogen concerning the southern SEMYB and the northeastern Cathaysia Block,and the sources characterized by recycled orogen and stable craton for the Cathaysia Block including the central-west and south parts of Jiangxi and central-southeast Fujian,respectively.According to these results above and the available data with particular reference to 29400 U-Pb ages and 8500 Hf-T_DM2 ages of detrital zircons,the source-to-sink regionalization and its paleogeographic implication were further carried out.The results can be concluded as follows.(A)It is reasonable to divide the Nanhuan-middle Ordovician South China into three sedimentary source-to-sink regions(S2SRs).(a)The S2SR of Eastern Jiangnan-Northern Wuyi area,which includes the southern SEMYB and the northeastern Cathaysia Block(including northeast Jiangxi,northwest Zhejiang,central-east Jiangxi,northwest Fujian and central Zhejiang provinces)and is characterized by the detrital zircons with age peak at 0.9–0.7 Ga and Hf-T_DM2 peaks at 2.6–2.0 Ga,1.6–1.0 Ga,and 3.2–2.7Ga.(b)The S2SR of Nanling-Southern Wuyi area,which consists of the Cathaysia Block(except for its northeastern part)and is characterized by the detrital zircons with age peak at 1.2–0.9 Ga and Hf-T_DM2peak at 2.4–1.6 Ga.(c)The S2SR of Pan-Yangtze area,which is composed of the Yangtze Block(except for the southern SEMYB)and is featured by the detrital zircons with age peak at 0.9–0.7 Ga and Hf-T_DM2peak at 3.5–3.0 Ga and/or 2.0–1.1 Ga;in addition,the detrital zircons from the middle and eastern regions have distinctively minor age peaks of 2.0 Ga and/or 2.5 Ga.The boundaries of the three S2SRs probably are the Xiajiang-Shicheng-Taining-Shunchang-Zhenghe-Longquan area,the provincial border of Zhejiang and Anhui,and the zone between the Maguan-Chengbu-Xinning area and the Daxin-Yongfu-Pingxiang area.(B)During the Nanhuan-middle Ordovician period,the landscape of central-eastern South China was composed of four paleogeographic domains as well as an unknown land adjacent to the southeast of the Cathaysia Block.From north to south,these domains and their features are listed below:(i)the Yangtze Platform;(ii)the Zhe-Gan-Wan and Xiang-Gan-Gui-Yue basins,which had opposite dips on their north and south sides,and were located separately in the SEMYB and in the central-southern Yangtze Block as well as the central-western Cathaysia Block;(iii)the Zhe-Min Land,that was located in the junction region of Zhejiang-Fujian-Jiangxi and central Zhejiang provinces and was the only long-existing land in the Cathaysia Block;(iv)the Gan-Min-Zhe Basin,that was located in the southeastern Cathaysia Block with the NW-N-NE dip of its south flank;and(v)the unknown land,which was the main source terrane of siliciclastic sediments in the Cathaysia Block and the southern margin of the Yangtze Block,and underwent the Grenvillian and Pan-African tectonic-magmatism.(C)In the early Neoproterozoic,the Cathaysia Block was probably a peripheral component of Rodinia,close to the northwestern margin of Australia-East Antarctica.During the Pan-African stage,the South China Block was likely located in the margin of East Gondwana and the Cathaysia Block was close to the northwestern Australia.