Sedimentary Records And Tectonic Environmental Response Of Shangxi Group Of Neoproterozoic In Shexian-Jixi Area, South Anhui Province

Shangxi Group of Neoproterozoic in Shexian-Jixi area is mainly a set of epimetamorphic rock which is primarily composed of mudstone, slate, argillaceous siltstone, silty sandstone, fine sandstone, conglomeratic sandstone and sandstone. Shangxi Group in Zhaixi-Qiashe profile, South Anhui,13244.9 meters, can be divided into three segments. The upper segment of this profile,1330.01 meters in thickness, is composed of thin to medium bedded argillaceous siltstone, siltstone, fine sandstone with straight lamination. The single bedding of the strata is 3 to 20 centimeters. Sedimentary structures like horizontal bedding, graded bedding, lapped bedding are developed in the strata and Td and Te of Bouma sequence can be recognized. Locally, banded and lenticular sandstone body, ranging from1 to 6 centimeters in width, can be found in mudstone.The whole upper segement of Shangxi group shows a trend of coarsening and thickening upward that indicates that this segment should be deposited in outer channel-lobe transition zone. The middle segment of the Shangxi group,6422.30 meters in thickness, is composed by fine sandstone and thin bedded silty mudstone, slate. Bedding is curved in this section and the continuity of bedding is extremely poor. Thickness of single bedding varies from 7 to 42 centimeters. Horizontal bedding, graded bedding, lapped bedding, slump structure and deformation structure are widely distributed in this section. Tc and Td of Bouma sequence can be found. Similar with the upper segment, the upper part of middle segment also shows tendency of coarsening and thickening upward, which reveals that the upper part of middle segment was formed in channel-lobe transition zone of out fan. While the beddings of middle and lower part of middle segment are much more stable. Thickness of single bedding varies from 18 to 63centimeters. Sandstone belt can be observed in the strata. The middle and lower part of middle segment, showing different trend from the upper segment that the size of grain is getting smaller and the thickness is becoming thinner upward, was regarded to form in channel of middle fan.The lower segment,5492.49 meters in thicness, is made up by shallow grey thick and medium bedded argillaceous siltstone and fine sandstone, ash black thin and medium bedded silty mudstone, mudstone. The thickness of single bedding is between 2 and 49 centimeters, with 21 centimetres in average.Compared with the upper and middle segment in the profile, the grain size is smaller and the lateral extensibility of bedding in much better in the lower segment. Horizontal bedding, graded bedding are in wide distribution in this segment. Also, Tde, Tce of Bouma sequence can be recognized in the lower segment which was considered to be depositd in the lobe of outer fan.Shangxi group in Yangxi profile is also primary a set of epimetamorphic rocks of Neoproterozoic which can be divided into three parts. The upper part,254.8 meters in thickness, is mainly composed of siltstone, argillaceous siltstone and silty mudstone. The beddings in upper part, ranging from 2 to 9 centimeters in thickness, extend laterally well. Horizontal laminations are developed in the beddings with 0.2 tol centimeter in width. Horizontal bedding and parallel bedding are extensively distributed in the strata of upper part. In general, grain size is coarsening and thickness of bedding is thickening downward, which predicts that the upper part was formed in lobe of outer fan. The middle part,192.5 meters in thickness, is made up of ash black thin to medium bedded fine sandstone and siltstone. The thickness of single bedding is between 1 to 15 centimeters. Sand belts are also common in the strata of middle part, whose thickness change from 7 to 9 centimeters. By comparing the strata of upper and middle part, we found that the content of sandstone in middle part is higher than that in upper part. Therefore, grain size in this part is larger than that in the upper part generally. However lateral continuity of strata is poorer in this part. All the features referred above prove that the middle part was deposited in lobe of middle-outer fan zone. The lower part,192.4 meters in thickness, is mainly a set of conglomerate. Gravels,with poor sorting and roundness, were in extremely disorder in the strata of this part.According to the features of this part, we think that was deposited in channel of upper fan. Meanwhile silty mudstone was interlaid between gravels. Tde of Bouma sequences can be observed in these mudstones. They are supposed to be channel overflow deposits.The compositional maturity and structural maturity in the study area are relatively low. In Zhaixi-Qiashe profile, the contents of quartz, feldspar and lithic debris in the clastic rocks are 21.9%?40.1%, 297.7%?56.4%,19.1%?31.4% respectively. Lithic debris are mainly composed of andesitic and granitic debris. While the percentage of quartz, feldspar and lithic debris in the clastic rocks are 42.7%?56.5%, 19.9%?40.0%,13.0%?24.1% respectively in Yangxi profile. The component characteristics indicate that the clastic rocks are close to their source area and the source rocks are closely related to volcanic rocks. In QFL and related discrimination diagrams, samples fall into arc tectonic zone and active continent margin area, predicting that the tectonic setting of source area is a combination of island and active continent margin.The geochemical analysis shows that the content of SiO2?Al2O3 & Fe2O3 are 69.3%?14.58%?5.54% respectively. The discrimination diagrams of tectonic setting according to major elements imply that the feature of both island arc and active continent margin, which is similar to that in QFL diagrams. In the plots of La/Th-Hf and Co/Th-La/Sc based on trace elements, the results suggest that the provenances are primarily felsic rocks. Meanwhile tectonic setting discrimination diagrams of Th-Co-Zr/10?La-Th-Sc?Th-Sc-Zr/10 demonstrate that the source area would be continent island arc.The average amount of rare earth element in the study area is 153.55×10-6. The average ratio of (La/Yb) N is 7.51, which implying that enrichment of light rare earth element. Also, obvious negative Eu anomalies (? Eu=0.61) suggest that the source rock would be granites.Hence, according to the combination of petrological features and geochemical features on the two profiles, we infer those provenances of Shangxi group are mainly acidic volcanic rocks. The tectonic setting should be both island arc and active continent margin. In the Neoproterozoic, Cathaysian plate subducted northward the Yangtze plate. On the southeast edge of Yangtze plate a large amount of magma arc and the back-arc basin were formed. These magmatic rocks experienced weathering and formed initial crustal basement. After the collision of Yangtze and Cathaysian plates, the tectonic setting turned into extensional accretionary orogenic belt. Meanwhile because of the thickening and remelting of the crust in the southeast in the Yangtze plate, a series of magmatic tectonic belt was constructed along South Anhui. Magmatic rocks also provided large amount of source to the back-arc basin. The tectonic setting in the study varied from restricted sea basin to back-arc basin. Along with the transformation of tectonic setting from tension into extrusion back, the sea level in the study area also saw a process of decreasing.