| SW Margin of the Yangtze Block, belonged to South part of the South China Plate, spans two geotectonic units of Yangtze Landmass and South China active zone. Its geographic scope includes whole Guizhou province, Eastern Yunnan, Northern Guangxi, Western Hunan, Southern Sichuan, and Southern Chongqing. In this area, the main characteristics is not only the large-scale distribution of sedimentary rocks, many kinds of sedimentary facies, relative integrity of sedimentary framework, development of event deposits,a mass of paleobiology, but also the enrichment of low temperature deposits. So it is the excellent area for studing sedimentary basin and elemental background, and this study has more significance in theory and practice.The dissertation discusses the filled characteristics and evolution of sedimentary basin and temporal and spatial changes of elemental contents in studied area. Based on the research the relationships between sedimentary basin and elemental background have been summarized in detail.The overhangs of innovation in the paper are : (1)Through the particular research on the Triassic strata with abundant fossils, the sedimentary sequence and the boundary age of systems tracts are determined . So the temporal and spatial framework with high resolution for the Triassic strata has been established. The characteristics of the Triassic sequences is closely related to the tectonic subsidence of the basin and changes of sea level. When the tectonic subsidence accelerated and sea level rose the retrograding sequences occurred and prograding sequence when the tectonic subsidence slows and sea level drops. (2) The volcanic event in early Mid-Triassic, caused by the collision of Indo-China plate and South China plate.The age of lower/mid Triassic boundary is about 247 Ma based on the volcanic tuff.(3) Through the quantitative analysis of subsident histories to the Youjiang basin in Triassic and sedimentary characteristics, it can be put forword that the South margin of South Chian plate showed the conversion from the divergent margin to convergent margin in latest Late Permian and from the rift basin to foreland basin about Youjiang basin. In the mid-stage evolution of foreland basin (middle to late Triassic) there is the basin margin uplift. Subsidence center of the basin moved from the south to north and basin margin uplift moves from north to south from early stage to late stage.Through detailed study, this paper drawed such main conclusions as follows:1. Sedimentary characteristics: two different depositional systems are divided including the terrigenous siliciclastic and carbonate systems. In Triassic, the terrigenous siliciclastic sediments is characteristic of turbidite with large-sized thickness and observed packing times by four. The turbidite overlaped gradually toward the carbonate platform. Their source derived from the Yunkai and Yuebai ancient lands. The carbonate deposits can be classified to two types of shallow water and deep water. Sedimentary facies models of shallow water carbonate are divided into carbonate ramp and rimmed carbonate platform. The reef and beach facies of platform margin are in coexistence. Platform and basin facies can be found in the interal part. Deep water carbonates include two types of gravity current deposit and suspension deposit which took place on the sloping orientations and interluded eachother.2. Event sedimentary characteristics: The black rock series, enriched nichel, molybdenum and vanadium etc, had been devoloped in Early Cambrian representing for an anoxic event. It is possiblely responsible for the Pangea break-up, sea level rise, hydrothermal exhalation, volcanic activity, upwelling ocean current and so on. The anoxic event in Wufengian is characterized by black color, carbon-rich and graptolite, which is caused by the glacier melting and sea level rise. The disaster event in Permian/Triassic resulted in a large scale extinction of biology and formed clay layer. This event is caused by volcanic activity, impact of out earth and changes of sea level. The volcanic event in early Mid-Triassic, caused by the collision of Indo-China plate and South China plate, formed the special deposits-thin and stable volcanic tuff.3. Lithostratigraphic framework: 31 sedimentary sequences of the South-Chinaic to Triassic strata with the characteristic of obvious cycle can be identified, then the lithostratigraphic framework has been established.4. Evolution of the sedimentary basin: The sedimentary basins of basement in studid area, in the whole, are formed in the tensional background evolving from the intercontinential rift basin in Mid-Proterozoic to continental margin rift basin in New-Proterozoic. The evolution of caprock includes two cycles. The first cycle is from South-Chinaian to Silurian and the second cycle from Devonian to Triassic. In each cycle the tectonic background changed from tension in early period to compression in later period. From South-Chinaian to Silurian the studied area undergone the evolution of rift basin to passive margin basin, and to foreland basin. From Devonian to Triassic the evolution is from rift basin to forelandbasin.5. Geochernical characteristics:(l)Major elements. On the whole, there are more differences of major-element contents between basement and caprock. For example, in the basement, the contents of MgO and CaO are lower and TiC>2 and P2O5 are close, whereas another oxide contents are higher than those in the caprock. It is possible that more carbonates occurred in the caprocks where the contents of SiO2,TiO2,Al2O3, E FeO,Na2O and K2O are relatively higher in South-Chinaian and Silurian, which is responsible for preponderant detrital rocks. Limestones occurred in Carboniferous and Permian characterized by higher CaO content. The lower parts of Datangpo Formation, of South-Chinaian and Wayao Form, of Triassic display MnO contents up 15% , but the four strata as South-Chinaian, Sinian, Cambrian and Silurian are characteristic of higher P2O5 contents. In the basement, The measured samples have similar major-element compositions but there are slight differences in the Qingbaikouian and Jixianian: SiO2, TiO2 and Fe2O3 contents are close, while A12O3, Na2O and K2O contents are higher in the Qingbaikouian with increased trend and FeO> MnO, MgO, CaO and P2O5 contents are of declined trend.(2) Trace elements. The trace elements showing the certain laws in the basement, caprock and crust are as follows: in the basement, the Ga and Nb contents are slightly higher, Mo, Ba, Hf, Sr and Zr contents are significantly higher in Qingbaikouian and other elements contents are lower than those in Jixianian. The Sr contents of Carboniferous disply the highest values of 496.6 lppm, but the contents ofSc, Cr, Co, Zn, Ga, Rb, Cs, W, Au, Tl, Pb and Th of Silurian are the highest with the lowest Hg content. In Sinian, Mo, Ag, Sb and Ba show the highest contents, while the lowest concentrations are Co, Ni and Sr with contents of 4.06,19.14 and 43.73 in ppm. The highest V, Cu, Nb and Ta contents occurred in Perimian when the Sb conten was the lowest value by 0.18ppm. The deep-water caprock are characterized by lower contents of Sr, Mo and Ag than the shallower-water one. In gernal, the trace-element contents of crust are higher than those of the sedimentary strata, but their enrichment degreens are not high. Comparatively, such these elements as Mo, Ag, Sb and Ba have higher concentrations and enrichment coefficient is up 2, however, Co, Sr,Ta and Hg are deficient. On the base of the above-mentioned, the lowest values of trace-element contents occurred in Carboniferous and the highest in Jixianian. In comparision with the basement, the caprock has slightly higher contents of Ni, Mo, Ag, Au, Hg and U and obviously higher contents of Sr.(3)Rare earth elements. The basement has similar REE distribution patterns showing negative Eu anomaly, but the total REE content by 225ppm in Qingbaikouian is higher than one by 196ppm in Jixianian. There are higher total REE contents relative to the North Margin of Yangtze Platform. The caprock, however, has REE enrichment of 217ppm in South-Chinaian and the sub-enrichment of 216ppm in Silurian. The total REE display lowest contents with value of 45ppm in Carboniferous. Except for Sinian, there are obviously negative Eu anomaly and slightly negative Ce anomaly.6. The relationships between sedimentary basin and geochemistry (l)Depositional systems and geochemistry. ?In the terrigenous siliciclastic systems, trace-element contents are commonly higher in the tidal-flate and shelf facies and are the lowest in the beach facies, but REE has silimary characteristics, are higher in the tidal-flate and shelf facies. In the tidal-flate, shelf and basin facies , there are higher Th/U ratios and lower Th/Sc and La/Sc ratios, however, the facts are contrary in the beach facies. (2)In the carbonate systems, geochemical characteristics of the trace elements, REE and the ratios of Th/U, Th/Sc and La/Sc are as follows: From slope to basin to restricted platform to open platform to semi- restricted platform facies, these values display the high to lowe changes. Therefore, different contents of the trace-element and REE occurred in the different facies, that is to say, the contents of terrigenous siliciclastic deposits are higher than that of carbonates.(2)Events and geochemistry. (Din Early Cambrian representing for an oceanic anoxic events, the REE is characterized by markedly high contents the and positive Eu anomaly. Anoxic stratum has higher V, Mo, Ba, Pb and U contents and Th/Sc values as well as lower Th/U values than the overlying and overlain strata.(g)In the anoxic sediments of Wufengian in Late Ordovician, the contents of major elements such as SiO2, A12O3 and TiO2 are higher, while trace elements of Li, Co, Sr, Zr, Hf and Th have lower enrichment, in especial, the V, Mo, Ag, Sb, Au and U contents are significantly higher than the overlying and overlain strata. REE characteristics indicate the anoxic sediments have higher total REE contents than the overlain stratum and lower than the overlying stratum.? In the P/T boundary, REE contents under the boundary are higher in common than up the boundary. Most trace elementsindicate higher contents under the boundary of shallow water or up the boundary of deep water. ?In the volcanic events in Middle Triassic, SiO2, A12O3, MgO and K2O display higher contents representing the origin of acid volcanic ash compared with the compositions of acid magmatite. The total REE contents range from 161ppm to 415ppm with the LREE concentrations, extensively negative Eu anomaly. With respect to trace elements, Rh, Th and U of higher contents occurred and Th content is higher than Ta. Above-mentioned characteristics imply the deposits in the volcanic events result from the calc-alkaline volcanic ash.(3)Sedimentary sequence and geochemisty. The REE, Th and Sc contents and Th/U, Th/Sc and La/Sc values in the systems tract have a negative correlation with the sea level changes. In the whole sequence by the transition from lowstand systems tract(LST) to transgressive systems tract(TST) ,and to highstand systems tract(HST), the values fluctuate from high to low, and to high, which indicates that they are declined during rise of the relative sea level, but the contrary phenomenon occurred during sea level drop.(4)Evolution of sedimentary basin and geochemistry. In course of basin evolution, various elements contents will vary in every stage. As example, major trace-element contents are highest in the intercontinental rift basin of Middle Proterozic, the highest REE contents display in the marginal rift basin of Early New-Proterozoic. The REE contents are gradually increased with the transition from rift basin to passive marginal basin in tensional backgrounds, and to forland basin in extrude backgrouds, and the major trace elements have the silimar changes.
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