| Globally, the South China block is believed to retain the best-preserved middle Neoproterozoic (830-720 Ma) magmatism and sedimentary records related to the breakup of Rodinia. It is an important carrier for geological study in that period, the importance manifested by a succession of laterally discontinuous "wedge-shaped strata", whose basal boundary is diachronous, and the coeval intrusive or eruptive magmatic rocks. In-depth understanding of formation and evolution of the "wedge-shaped strata" and the characteristics of the contemporaneous magmatic rocks (especially the basaltic rocks which can represent magma composition and have specific meaning of tectonics and geodynamics), have a huge significance for the nature and dynamic mechanism research about the Neoproterozoic Southern China basin and the relationship with Rodinia supercontinent breakup. Until now the main research progress about Southern China nature in Neoproterozoic concentrated in the sedimentary, magmatic rock geochemistry and isotopic geochronology field which related to the basin evolution in southeastern and western margin of the Yangtze block, but for that of northern margin is relatively weak. It should be pointed out that, the northern margin of Yangtze is adjacent to the Qinling Mountains orogenic belt, and it is a tie to connect the interior of the Yangtze craton with the Qinling Mountains orogenic belt, it occupies an important position in the regional structure of china. Therefore, to conduct the Neoproterozoic sedimentary, isotopic geochronology and geochemistry research in the northern margin of the Yangtze block, is the key to investigate and solve the above-mentioned problems.Based on detailed field geological survey, after systematic study of sedimentology, isotopic geochronology, major and trace elements geochemistry and zircon Hf isotopic analysis, we proposed some new points in this paper:(1) The zircons LA-ICP-MS U-Pb dating analyzed for the basalt from Huashan Group and the tuff from the Dashigou and Sanlangpu Formation of the Xixiang Group in the northern margin of the Yangtze block. The results give weighted mean ages of (824±9) Ma for the basalt from Liufangzui Formation of the Huashan Group, moreover, (789.0±4.4) Ma and (760.4±4.5) Ma for the tuff from the Dashigou and Sanlangpu Formation respectively. According to existing data, the basal boundary age of the Xixiang Group should be ～820 Ma. Together with the underlying Sanhuashi Group, they represent two distinctive tectonic-sedimentary sequences respectively.(2) Recent field geological investigations and sedimentary facies analysis have disclosed that the previously reported "Macaoyuan conglomerates" actually should be assigned to the dolomitized intraclastic lime grainstones rather than the basal conglomerates, and display sedimentary-erosional conformable contacts with their underlying stratigraphic sequences. Therefore, there is no so-called "angular unconformity" and associated "Shennong movement" between the Macaoyuan Group and Shennongjia Group. The Macaoyuan Group dominantly consists of the mixed carbonate platform facies association and carbonate shelf facies association while the "Macaoyuan conglomerates" are mainly made up of the carbonate platform slope-channel gravity flow deposits. The sedimentary structures such as basal scouring structures, slump-convolute structures, imbricate structures, inverse grading bedding and lenticular filling deposits are well developed in the carbonate platform slope-channel gravity flow sedimentary sequences, whereas cross bedding, wavy bedding, parallel bedding, ripple bedding, normal grading bedding are common in the mixed inner shelf and platform-margin shoal sedimentary sequences. The SHRIMP and LA-ICP-MS zircon age dating techniques are employed for the synsedimentary tuff sample collected from the upper part and sedimentary detrital zircons from the topmost part of the Baliya Formation in the lower part of the Macaoyuan Group. The synsedimentary tuff sample gives a SHRIMP zircon U-Pb age of 1157±19 Ma, which agrees well with the threshold age of 1180 Ma for the sedimentary detrital zircons. It follows that the age of the deposition of the Macaoyuan Group should be traced back to the Mesoproterozoic rather than the Neoproterozoic. The ages of the detrital zircons are mainly concentrated to two time intervals:1.16 to 1.26 Ga and 2.60 to 2.95 Ga. The ca.2.60 Ga as a marked peak age may represents an important period for the crustal accretion and evolution of the Yangtze block.(3) Combined with the existed research result, we reconstructed the mid-Neoproterozoic sedimentary framework in northern margin of Yangtze block. Our study demonstrated that the Tiechuanshan Formation, Huashan Group, as well as Sunjiahe Formation of the Lower Xixiang Group from northern margin of Yangtze block can contrast with the Lower Banxi Group and the corresponding strata (e.g. Lower Xiajiang Group in southeastern Guizhou province, Lower Danzhou Group in northern Guizhou province, Lower Heshangzhen Group in northern Zhejiang province, etc.) from southeast margin of Yangtze block, as well as Lower Luliang Formation from western Yangtze block, all of them represent the bottom sediments of South China Neoproterozoic basin upsetting. Meanwhile, the Liantuo Formation, Dashigou Formation and Sanlangpu Formation of the Lower Xixiang Group from northern margin of Yangtze block can contrast with Upper Banxi Group and coeval strata from southeastern margin of Yangtze block, the Chengjiang Formation, Suxiong Formation and Kaijianqiao Formation from western margin of Yangtze block, their sedimentary age limits are 800-725 Ma.(4) Based on the reconstruction of the sedimentary framework, we compiled the lithofacies paleogeographic map for first time during mid-Neoproterozoic in northern margin of Yangtze block after paleocurrent direction, genetic facies and environmental facies analysis. It is reviews that the whole northern margin of Yangtze block presented as post-orogeinc uplift denudation zone during early Panchi stage, and mainly developed small size Hannan-Beiba volcanic basin and Huashan fault basin. During late Panchi stage, the ancient uplift denudation zone in the south of Xiangfan-Guangji fault provided the provenance to north and south directions, the southern Nanhua rift basin mainly developed the allochthonous coastal deposition of Liantuo Formation which overlapping in the north, however, the main rift system in the northern margin of Yangtze block should develop in the north of Xiangfan-Guangji fault, it is Wudang-Suixian rift basin.(5) We present geochemical and Hf isotopic evidence for the basalts from the Liufangzui Formation of the Huashan Group in the Dahongshan region of northern margin of Yangtze block. Geochemical analyze reviews that these basalts belong to the subalkaline tholeiite series and display slight enrichments in light rare earth elements (LREE) and varying degrees of deficiency of high field strength elements (HFSE) such as Nb, Ta and Ti. This pattern is very similar to that of continental flood basalts from the Bikou Group and Tiechuanshan Formation in the NW Yangtze block in both China and Siberia. According to the litho-genesis analysis proposed that the basaltic magmas underwent partially-fractional crystallization during ascent, but were not intensely influenced by crustal contamination. The characteristic element ratios and negative Hf isotopic analyses (εHf(t)=-6.6-2.6) in zircons indicate that the parental magmas of the basalts might have been derived from an enriched lithospheric mantle rather than from the depleted mantle such as normal mid-ocean ridge basalts (N-MORBs). The geochemical signatures and regional geological characteristics show that these basalts were formed along intraplate continental rifts rather than in island arcs or ocean basins. Considering the coeval basic volcanic rocks in South China, we propose that these Huashan Group basalts represent the remnants of plume-generated continental flood basalts.(6) Our 126 age data statistical analysis in South China about the mid-Neoproterozoic magmatic rocks manifested that there are three episodic magmatic activities during the Neoproterozoic epoch, whose peak ages are ～825Ma,～795Ma, and ～760Ma, respectively. The former two episodes are more obvious than the latter one, respectively representing the commence age and fully extensional age of the rift basin. The tuffs from the Dashigou and Sanlangpu Formation reported in this paper, as well as the basalts from Huashan Group respectively represent the above-mentioned three episodic magmatic activities, they are parts of the Neoproterozoic episodic magmatic activities, also closely related to the formation of the Neoproterozoic rift basin in the South China.(7) Considering our study results have similarity with Australia in fields of sedimentary basin records and rock geochemistry etc. Our results support the supercontinent Rodinia reconstruction model in which the South China block located in the SE of the Australia, and under the environment of supercontinent Rodinia, proposed the evolution characteristic of the ancient South China block aggregation and breakup:during 1000～900 Ma, Australia continent collided with northern margin of Yangtze block, meanwhile, the SE margin of Yangtze block is still an active continental subduction margin. At ～830 Ma, aggregation happened between Yangtze and Cathaysia (just subduction accretion, no collision orogenesis), resulted in South China block. And than the super mantle plume impacted the bottom (～825 Ma) of South China lithosphere, made the lithosphere breakup along the existing weak belt and finally formed the Neoproterozoic rift basin in South China block. |
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