(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037)

Through the previous research work, this topic has successively investigated several South China profiles, such as Wangfenggang in Yichang, Hubei, Yangjiaping in Shimen, Hunan, and Fulu in Sanjiang, Guangxi. After study and comparison, Yangjiaping section in Shimen, Hunan Province is recommended as the candidate stratotype section of Nanhua system. At the same time, in order to solve the problem that the study of isotopic chronology of Sinian-Nanhua system is relatively weak, the search for volcanic tuff interlayer of Sinian and Nanhua system in South China and the study of isotopic chronology have been strengthened. South China is found in the west wing of Huangling anticline in Xia Dong, Hubei, the Sinian section of Jiuqunao in Zigui, the Nanhua section of Heishuixi manganese mine in Songtao, Guizhou, the Sinian-Nanhua section of Yangjiaping in Shimen, Hunan, the Sinian-Nanhua section of Xiayabu in Jiande, Zhejiang, the Sinian-Nanhua section of Shilonggang in Jiangshan, Zhejiang, the Sinian-Nanhua section of Xishankou, Korla, Xinjiang, the Sinian section of Doushan Phosphate Mine in Wengbei and Jinzhong Town, Kaiyang County. Through the study of SHRIMP zircon U-Pb dating, a large number of SHRIMP zircon U-Pb dating data of South China and Sinian are obtained, which basically solves the division and correlation of South China in stratigraphic regionalization, Yangtze River (National Stratigraphic Committee, 2008). However, how to compare it with the stratigraphic division in Jiangnan and the ice age division in South China has become a top priority.

The lower strata of Nanhua system in South China can be roughly divided into three different sedimentary assemblage zones. (1) The northern belt is represented by the periphery of Huangling anticline, and the lower part is mainly composed of braided river gravel deposits in the coastal zone. The upward transition is siltstone and mudstone mixed with tuff and sandstone and tuff thin layer. (2) The middle belt is wide, represented by Changyang in Hubei, Shimen in Hunan and Dayong-Yuanling. It is characterized in that the lower part is mainly braided river-estuary bar gravel, and a small amount of debris (several meters thick) flows upward to the shallow sea to deposit manganese-containing siltstone, shale and siliceous rocks, which shows the sedimentary combination characteristics of continental facies-slope break-continental slope. (3) The southern belt is represented by the area south of Hunan-Huaihua-Sanjiang Anhua Line in Guangxi, and the lower part is mainly continental slope debris flow deposits, and the upper part is manganese-containing siltstone, shale, siliceous mudstone and manganese-containing dolomite lens (Fulu Formation). Manganese-bearing siltstone and mudstone represent the sediments in the maximum flooding period of South China system. At present, the focus of the research group is to solve the stratigraphic correlation problem between the southern belt and the northern belt and the central belt.

1 main research overview and research content

The research team collected paleontological, petrological and geochemical samples of related strata, focusing on the South China profile of Sanjiang Guangdong in Guangxi and Zhaoxing in Liping, Guizhou. At the same time, fine detrital samples from Liantuo Formation to Nantuo Formation in eastern Xia Dong were collected systematically, and chemical alteration indexes were studied. On the basis of previous research work, a multidisciplinary comprehensive research method combining field geological survey and indoor research is adopted. Concentrate on the multidisciplinary comprehensive study of the typical Nanhua stratigraphic profile in the south of the Yangtze River, focusing on the sedimentary characteristics, stratigraphic framework and isotopic age determination of key horizons in the Lower Glaciation and Interglacial Period of Nanhua. Further improve the division and comparison of the Nanhua system in South China. The research goal is to put forward the division and comparison scheme of South China System in Jiangnan and Yangzi regions through systematic field investigation and comprehensive indoor research, further improve the chronostratigraphic system of South China System in China and lay the foundation for gradually connecting with the international community.

2. Main research progress and achievements

2. 1 SHRIMP zircon U-Pb age of Fulu Formation in South China.

The tuff samples of Fulu Formation were collected from the lower part of Fulu Formation of Nanhua System in Longshuicha section of Zhaoxing Town, Liping County, Guizhou Province. This section is located in the stratigraphic division of the south of the Yangtze River, and represents the slope-basin facies sedimentary type of Nanhua-Sinian system in South China. This type is mainly distributed in the south of Anhua, Hunan-Tongren, Guizhou, and the Nanhua system is divided into Chang 'an Formation, Fulu Formation and Nantuo Formation from bottom to top. Chang 'an Formation is a set of glacial migmatite deposits, including glacial turbidite deposits and typical glacial conglomerate deposits (Figure 2a). It is generally believed that the Fulu Formation consists of iron-bearing and manganese-bearing clastic rocks and dolomite under warm and humid climate conditions, with iron-bearing sandstone at the bottom. Nantuo Formation, like other areas, is a set of stable moraine deposits. Sample T06 1 17- 1 is taken from the bottom of the middle and lower member of Nanhuafulu Formation in Longshengcha, Zhaoxing. The lower part of Fulu Formation in this profile is grayish brown silty feldspar timely sandstone, and there is a yellow-green tuff interlayer near the bottom with a thickness of 7 ~ 10 cm (Figure 1, Figure 2b).

Figure 1 Location of the middle and lower members and sampling points of Nanhua Fulu Formation in Longshengcha, Zhaoxing, Liping, Guizhou.

Fig. 2 Section of Nanhua System in Longshengcha, Zhaoxing, Liping, Guizhou

2. 1. 1 zircon characteristics and SHRIMP dating results

The sample T06 1 17- 1 is yellow-green tuff with a thickness of about 7 ~ 10 cm. Crushing by conventional method, directly washing by hand, and finally picking out 100% pure zircon under the microscope. Under the microscope, zircon is obviously colorless or white transparent, equiaxed or short columnar authigenic crystal with well-preserved and smooth crystal face, and no traces of corrosion and wear are found, which belongs to magmatic zircon. The long axis of zircon is mostly 100 ~ 150μ m, and clear banded structure can be seen in all the cathodoluminescence images of zircon (Figure 3). In addition, the cathodoluminescence images of some zircons show obvious nuclear and marginal proliferation zones (Figure 3, F), and some zircons also show the phenomenon of marginal proliferation zones after two nuclei are spliced (Figure 3, G).

Fig. 3 Cathodoluminescence images of some zircons from the Fulu Formation in Zhaoxing show obvious nuclear and marginal zones.

The ages of 58 zircon grains in this sample were determined at 58 measuring points. The analysis results show that, except for the oldest point, which is slightly inconsistent with the consistent curve due to the loss of Pb, the results of other measuring points all fall on the consistent harmonious curve. The results of 57 measuring points are obviously divided into two groups (Figure 4, table 1), and the results of young group 5 1 measuring point are mainly concentrated between 660 and 820 Ma, with three peaks (Figure 4b). The weighted average age of 206Pb/238U of the four measuring points with the smallest peak value is (669 13) Ma (MSWD = 0.95), which should represent the formation age of tuff at the bottom of Fulu Formation (Figure 5).

Fig. 4 T061KLOC-0/7-1zircon U-Pb diagram of tuff samples at the bottom of the Fulu Formation in Zhaoxing.

Fig. 3 is a partial zircon cathodoluminescence image of sample T06 1 17- 1 and the position map of the measuring point. Photos A-D show the cathodoluminescence images of four zircon particles with the smallest peak and the positions of the measuring points. It can be seen that all of them are zircon particles with good crystal form, without obvious crystal nucleus, and there is an obvious annular zone at the edge, and the measuring points are all located in the annular zone at the edge. The measured result is 206Pb/238U age (1), and the measuring point l061107-3.1is (659.19.8) ma; (2) The measuring point L261107-3.1is (689.015.6) ma; (3) The measuring point L261107-1.1is (668.5 19.5) mA; (4) The measuring point L26 1107-13.1is (675.0 14.8)Ma (table1). Fig. 3. e-h has a well-developed nucleus with a proliferation band outward. In Figure G, two clear nuclei are spliced together and there is a proliferation band outward. Their results are as follows: (5) The measuring point 061107-12.1is (718.71.9) mA; (6) The measuring point L061107-5.1is (710.1.3) mA; (7) The measuring point L061107-14.1is (834. 1 8.8) Ma, which shows that they are obviously larger than zircon without obvious nuclei. Figure H shows the morphology of a group of zircon in recent 2 billion years. (8) The measuring point 0023-22. 1 is (2060.9 24.9) mA.

Fig. 5 T061KLOC-0/7-1zircon U-Pb diagram of tuff samples at the bottom of the Fulu Formation in Zhaoxing.

Table 1 sample T06 1 17- 1 zircon SHRIMP U-Pb dating results

sequential

Note: the error is 1σ, and Pbc and Pb* are ordinary Pb and radioactive Pb respectively; Calibrate 204P with ordinary lead.

2. 1.2 Geological significance

In Sanjiang and Qiandongnan areas in northern Guangxi, the lower part of Fulu Formation often contains banded iron-bearing sandstone, the middle part occasionally sees thin suspected moraine conglomerate with gravel, and the upper part is manganese-bearing shale and siltstone. According to lithologic comparison, some researchers think that the manganese-bearing shale and siltstone in the upper part of Fulu Formation can be compared with Tangpo Formation in northern Guizhou, the gravelly moraine in the middle part can be compared with Dongshanfeng Formation in western Hunan or Tiexian 'ao Formation in northern Guizhou, and the banded iron-bearing sandstone in the lower part separated from the moraine in the lower Chang 'an Formation during the interglacial period. It is considered that the Nanhua system in South China experienced three glacial periods from bottom to top: Chang 'an Glaciation, Gucheng Glaciation and Nantuo Glaciation (Xue et al., 2001; Zhou et al, 2004). In recent years, zircon U-Pb ages are (662.9 4.3) Ma (=1.24, Zhou et al, 2004) and (667 9.9) Ma (=1.24), respectively. The age value of (669 13) Ma (MSWD = 0.95) obtained from the bottom of South China Fulu Formation in Longxingcha section of Zhaoxing, Liping County, southeastern Guizhou is basically consistent with the lower limit age of the above-mentioned Datang Slope interglacial period, which proves that although the lithology and thickness of South China Fulu Formation in southeastern Guizhou are different from those of Tangpo Formation in northern Guizhou, the lower limit age is the same, which is the product of the temperature and humidity environment during the interglacial period, and the lower limit age of Fulu Formation and Datang Slope Formation is basically consistent.

Fig. 6 Comparison between Zhao Xing and Yangjiaping Nanhua profiles in Shimen, Hunan and Yichang Nanhua profiles in Hubei.

At the same time, the zircon U-Pb age at the bottom of the Fulu Formation in the Longshuicha section of Zhaoxing, Liping County, southeastern Guizhou Province can further determine the lower limit age of the interglacial period in Datangpo, South China, which proves that the records of glacial cold events below this interglacial period are different in different paleogeographic locations. The most complete and largest sedimentary thickness in the Lower Glaciation developed in the basin facies areas divided by Jiangnan strata, such as Chang 'an Formation in southeastern Guizhou and northern Guangxi. The development degree of the Lower Glaciation changed from the southeast to the northwest of stratigraphic regionalization in the south of the Yangtze River (Figure 6), until the Datangpo Formation and the Lower Glaciation in Hubei Province were completely lost and replaced by Liantuo Formation (Jiang et al., 2003; Zhou et al., 2004). According to the zircon U-Pb age data obtained in recent years and the new isotopic age results obtained in this project, it is further proved that the Nantuo Ice Age of South China is equivalent to the Neoproterozoic Marinoan Ice Age, which is widely developed in the world, and the time limit is roughly between 660 and 635 Ma. The lower glaciation of the Nanhua system (Chang 'an or Gucheng Glaciation) is similar to that of Studt, and the time limit is roughly between 750 and 670 Ma.

2.2 Changes of chemical alteration indexes of Liantuo Formation of Nanhua system in eastern Xia Dong and their environmental significance.

2.2. 1 sample collection and analysis processing

In the east of Xia Dong, only the Lower Liantuo Formation (50 ~ 246m thick) and the Upper Nantuo Formation (35 ~180m thick) developed in the Nanhua System. Nantuo Formation is composed of glacial sediments, including typical moraine conglomerate and glacial water sediments. The bottom of Liantuo Formation is unconformity with Huangling granite, and it is purplish red conglomerate and glutenite with medium and small oblique bedding and trough bedding. The middle part is grayish purple and grayish green medium-fine grained feldspar timely sandstone, sandy siltstone, siltstone and a small amount of shale; The upper part is composed of grayish green sandy siltstone mixed with purple gray, siltstone mixed with shale and a small amount of purple medium-fine grained sandstone (Tzi-chiang Chao et al.,1985; Xing Yusheng et al.,1996; Liu Hongyun, 199 1). The chemical alteration index samples of Liantuo Formation are taken from the cutting section of Shibanxi Bridge east to Huajipo Highway in the south of Sandouping (sample number R 041016-17), and the samples of Nantuo Formation (including the top samples of Liantuo Formation) are taken from the measured section of Jiulongwan-Gaotoushi in the southeast of Sandouping (sample

All samples are fresh fine clastic rocks. First, cut the sample into fresh surface, and then take 200 mg rock powder for chemical analysis within the range of 3 ~ 5 mm by hand drill (Chemical Analysis Laboratory of China Geo University). The analysis accuracy of the main elements is better than 6%, and H2O+ and H2O- have been obtained in the analysis, so the loss on ignition in Table 2 can basically represent the content of CO2, and participate in the calculation to obtain n(CaO*). See Table 2 for sample lithology and analysis results.

2.2.2 Analysis of sedimentary environment of sediments

The analysis results show that the CIA value of Nantuo Formation of Upper South China System is mainly between 60 and 65 (except for the first two samples). The distribution of CIA values in Liantuo Formation of Lower Series is complex, and the CIA values of the first two samples (R 041016-1~ 2) near Nantuo Formation are between 55 and 60. The CIA values of 8 samples (r04 10 16-3 ~ 5, 7 ~ 8, 10 ~ 12) in the upper part of Xialiantuo Formation are between 65 and 75. The CIA values of the bottom five samples (r041016-13 ~17) fell back to the range of 50 ~ 65. From the A-CN-K triangle diagram, it can be seen that the samples from different horizons of the South China system are obviously concentrated in three compact and stable areas. The research shows that the CIA value of Nantuo Formation samples in Sandouping area is roughly equivalent to that of Pleistocene glacial clay, which is caused by glacial sediments. The CIA values of the first two samples (r041104-1,r04110) are 70, indicating that the cold climate of Nantuo Formation is steep.

Liantuo Formation is composed of fluvial glutenite deposits with a thickness of tens of meters at the bottom, which gradually developed into meandering river-reticulated river sandstone-siltstone deposits in the offshore. The CIA values of the top two samples of Liantuo Formation (r04116-1~ 2) are 6 1 and 58, respectively, indicating that low-grade chemical weathering deposits with dry and cold climate have appeared at the top of the Formation. The ClA value of the upper samples of liantuo formation is between 65 and 75, which shows moderate chemical weathering deposition under warm and humid climate conditions; The CIA value of the lower samples of this group decreased to 55 ~ 65 again, indicating that it was deposited with low chemical weathering degree under cold and dry climate conditions. These changes show that the Liantuo Formation deposits started in the environment with low chemical weathering degree in cold and dry climate, and then transferred to the environment with moderate chemical weathering degree in warm and humid climate, and the environment with low chemical weathering degree reappeared at the top in cold and dry climate until the stable glacial deposits appeared in Nantuo Formation.

Figure 7 clearly shows the above changes. Figs. 7a, 7b and 7c show three relatively dense areas, respectively. Fig. 7(a) shows the distribution of sampling points in Nantuo Formation, which can be further divided into two sub-regions. Plot I * * * has five points, and their CIA values are in the range of 60 ~ 65. The X-ray diagram contains two sample points representing the top of Nantuo Formation, which may indicate that the climate is warming. Fig. 7b shows the area with relatively dense sampling points at the top and bottom of Liantuo Formation. All sample points are located in the low value area of CIA value 50 ~ 65. Fig. 7c shows the sample points in the upper part of Liantuo Formation, which all fall within the CIA value range of 65 ~ 75 in the upper part of the triangle. The above three relatively dense areas represent three different relatively stable stages in the sedimentary evolution of the Nanhua period. However, as far as the A-CN-K triangle of the whole South China system is concerned, the distribution of sample points is quite scattered, and the CIA value fluctuates greatly at different stages. This feature reflects that the South China era (Cryogenian in the Ice Age) was in the period of disintegration of the ancient continent. Before the land mass was put in place, the tectonic environment and climatic conditions of the Yangtze ancient continent were in an overall unstable state compared with the Paleozoic era, whether it was sediment characteristics or lithofacies changes (Wang Ziqiang et al., 2006b).

2.3 Changes of chemical alteration indexes and environmental analysis of Nanhua system in southern Guizhou and northern Guangxi.

Nanhua Formation in southern Guizhou and northern Guangxi is divided into three groups from bottom to top, namely Chang 'an Formation in the lower part, Fulu Formation in the middle part and Nantuo Formation in the upper part. The Chang 'an Formation is pseudo-integrated or unconformity on the Neoproterozoic Danzhou Group. Although there are different understandings of its composition, most researchers tend to think that this stratum is glacial marine sediments. Fulu Formation above Chang 'an Formation is a set of clastic rock deposits containing iron and manganese, which is the product of interglacial period. The Shangnantuo Formation represents another glacial deposit. Through the study of chemical stratigraphy in this area, our research group attempts to further explore the comparison between the Nanhua system in Guizhou, Guangxi and the Nanhua system in Yangtze area. The chemical alteration index samples of Nanhua system in this area are mainly taken from Fulu Formation in Guandong-Mu Feng section of Sanjiang in Guangxi, the top of Danzhou Formation in east of Shuikou in Liping, Guizhou, and the lower part of Chang 'an Formation and Nantuo Formation in Zhaoxing section (Figure 8). Because the lithology of Fulu Formation in Zhaoxing area is too thick for CIA to study, it was not adopted. However, there are porphyritic interlayers in the middle and lower part of Fulu Formation in this area, and the zircon U-Pb age is (669 13)Ma, which provides a strong basis for the division and correlation of South China series.

Table 2 chemical analysis of whole rock and conjugated linoleic acid and ICV values of Nanhua series samples in Sandouping, Yichang

Fig. 7 decomposition A-CN-K triangle diagram of Nanhua system in Sandouping, Yichang

Figure 8 Measured profiles of Guandong Cave in Sanjiang, Guangxi, Shuikou in Liping, Guizhou and Zhaoxing.

The study samples of chemical alteration index of the above sections are all taken from the cements of siltstone, sandy siltstone and glacial conglomerate. The results of whole rock chemical analysis, ICV and CIA are shown in Table 3.

Table 3 Chemical analysis, ICV and CIA values of whole rocks of Nanhua series samples from Sanjiang, Guangxi and Liping, Guizhou.

Note: 1 106- 1 ~ 5 is the sample of Guangdong group, 1 106 ~ 6 ~ 24 is the sample of Chang 'an group,1104-9 ~/kl.

Because the lithology of Fulu Formation in Zhaoxing section of Liping, Guizhou Province is coarse, it is not suitable as the research object of chemical alteration index. Only 1 107-5 and 1 107-7 samples are taken from the top of Fulu Formation in this section. The remaining samples of Fulu Formation (1 104-9 ~ 12) were taken from the lower part of Fulu Formation in the Guandong section of Sanjiang, Guangxi. The samples in the lower part of Fulu Formation in Guandong section are extremely thin shales and mudstones, and their CIA values are all between 85 and 95, which reflects that they are deposits with strong chemical weathering in hot and humid climate, especially the ICV values of the above samples are all greater than 1, indicating that these samples may be the first deposits with relatively active structures on the continental margin. The CIA values of samples 1 107-5 and107-7 were 69 and 70, respectively, indicating that the climate had cooled before the Nantuo Ice Age.

The sample 1 107-8 ~ 13 is taken from the lower part of Zhaoxing Nantuo Formation in Liping. The CIA values of the six samples were all between 60 and 65, which was 66.3 113. It is basically consistent with the CIA value obtained from Nantuo Formation in the east of Yichang Gorge. It shows that it is transformed into deposition in low chemical weathering environment with cold and dry climate again.

Division and regional comparison of Neoproterozoic in southern China

The division and comparison of Neoproterozoic in South China have long been different, but two aspects are basically recognized. First, the Neoproterozoic is bounded by the Jinning movement interface, and the lowest stratigraphic unit is Qingbaikou System (that is, Tononi period, equivalent to the international stratigraphic table), which should include Canglangpu Group and Banxi Group and their equivalent strata (National Stratigraphic Committee, 200 1, 2002; Lu Songnian, 2002; Zheng, 2003). Second, the Sinian system in China has been clearly defined (Ma Guogan et al.,1984; Yin et al., 2005). At present, the focus is on the understanding of Liantuo Formation of Nanhua System and its position in Nanhua System.

The Nanhua system in South China can be roughly divided into three types: Type I is distributed in the border areas of Hunan, Guizhou and Guangxi provinces, represented by Sanjiang section of Guangxi. The Nanhua Formation consists of three formations: Shangnantuo Formation (Glacier Rock); Middle section of Fulu Formation (siltstone containing iron and manganese, dolomite lenticular shale containing manganese); Lower Chang 'an Formation (conglomerate deposited in glacial sea). Type ⅱ is mainly distributed in northeast Guizhou-northwest Hunan, represented by Yangjiaping section in Shimen, Hunan, and the Nanhua system consists of 4 formations. The top is the glacier deposit of Nantuo Formation; The Upper Datangpo Formation (called Hu Meng Formation in Hunan) is a black and purple-brown manganese-bearing fine clastic rock, which forms a recoverable manganese deposit in northeastern Guizhou and parts of Hunan. Dongshanfeng Formation in the middle part is glacial sedimentary conglomerate with small thickness. Xiaqinshuihe Formation is composed of purple-gray and gray-green glutenite, sandstone and siltstone, with shale and a small amount of thin bentonite. The bottom boundary of Type I and Type II Nanhua System is unconformity on Banxi Group and its horizon. Type ⅲ South China system is mainly distributed in western Hubei and the middle and northern parts of the lower reaches of the Yangtze River. The eastern part of Yichang Gorge consists of two strata. The upper Nantuo Formation is glacial sedimentary conglomerate, and the lower Liantuo Formation is composed of purplish red glutenite, sandstone, grayish green sandstone and siltstone mixed with shale. The difference is that the deposits in this section are not integrated on Huangling granite. According to the principle of lithostratigraphy, it can be clearly considered that the manganese-bearing strata and lower glacial migmatite under Nantuo Formation are missing in the east of Xiaxia (including Tiexian 'ao Formation in Guizhou, Dongshanfeng Formation in Hunan and so-called "Xiao Bing" in other areas, and Chang 'an Formation in the border area of Hunan, Guizhou and Guangxi). At the same time, it can be compared with Lianshuihe Formation and Liantuo Formation in Shimen, Hunan, and it can be considered that Liantuo Formation under Chang 'an Formation is missing even Banxi Group and Liantuo Formation (Liu Hongyun,/kloc-

Fig. 9 Profile of different types of sedimentary facies belts of Neoproterozoic Nanhua System in South China.

Feng Lianjun et al. (200 1) put forward that the CIA values of Upper Qinshuihe Formation and Dongshanfeng Formation in Yangjiaping, Shimen are 60 ~ 65 and 60 ~ 70, respectively, which are basically consistent with Pleistocene glacial clay. It is considered that Qinshuihe Formation in this area is the product of cold and dry climate and low degree of chemical weathering. The CIA of the upper part of Laoshanya Formation of Banxi Group underlying Qinshuihe Formation is 70 ~ 75, which should be the product of warm and humid climate and moderate chemical weathering. At present, the author's study on the chemical alteration index of Nanhua System in Sandouping, Yichang, Hubei Province shows that Liantuo Formation under Nantuo Formation also experienced a sedimentary process controlled by cold-dry-warm-wet-cold-dry climate change, so the author puts forward a scheme for dividing and comparing different facies areas of Nanhua System (Figure 9).

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