| The Xiajiang Group in the southeastern Guizhou Province, is located in the western part of the Jiangnan Orogen Belt, and consists of Precambrian low metamorphic terrigenous clastic sequence interbedded within volcaniclastic rocks. But its age and stratigraphic correlation with regional strata are still controversial due to the absence of lithological marker bed, fossils and insufficient geochronological data. Furthermore, there also are controversy on the sedimentary evolution, basin types and dynamic mechanisms about the Xiajiang Group. The thesis chiefly applies the theories and methods of the orogenic-sedimentology to study the sedimentary geology, bulk-rock geochemistry, sandstone clastic composition, detrital zircon trace element geochemistry and U-Pb geochronology of the Xiajiang Group in southeast Guizhou. The major conclusions are outlined as following:(1) The Neoproterozoic Xiajiang Group in the southeast of Guizhou was mainly composed of terrigenous clastic rocks interbedded with volcaniclastic rocks characterized by flysch sequence. It mainly consists of silty slate, argillaceous siltstone, argillaceous siltstone-sandstone, tuffaceous slate, and tuffaceous siltstone. Thehorizontal bedding, parallel bedding, graded bedding, cross-bedding, massive bedding, homogeneous bedding, flaser bedding, lenticular bedding etc.and event deposits like slump and turbidity current were well developed in these stratigraphic sequences. The Xiajiang Group was deposited in a coast-shallow ocean-bathyal marine environment.(2) On the basis of detailed analyses of LA-ICP MS U-Pb geochronology on zircons from tuff, tuffaceous and clastic rocks from the Hecun Formation, Sibao Group, and Wuye, Qingshuijiang, Pinglue formations, Xiajiang Group, we obtain high resolution geological data, including the 819.8±6.4 Ma,779.5±4.7 Ma,764.0±6.3 Ma and (756.8±7.6 Ma,756±13 Ma), 733.9±8.8 Ma,725±10 Ma, which help to constrain the deposition of the Xiajiang Group to between 815 Ma and 720 Ma. The Xiajiang Group could be divided into Wuye, Qingshuijiang, Pinglue and Longli formations from bottom to top. Based on these zircon age data, we suggested the depositional time of first member of the Jialu Formation was about 815～805 Ma, the second member was about 805～800 Ma, the first member of the Wuye Formation was about 800～780 Ma, the second member of the Wuye Formation and the Fanzhao Formation was about 780～770 Ma, the Qingshuijiang Formation was about 770～745 Ma, the Pinglue and Longli formations was about 745～720 Ma. Combined with the field observation that the Longli and Chang’an formations are continuous deposits and the global low latitude glaciation event of the Nanhuaian began at 717.4 Ma, the depositional time of the Xiajiang Group can be constrained from 815 Ma to 720 Ma, with the Neoproterozoic. The tectonic setting and characteristics of the basement of the Jiangnan orogen belt were not related with the Grenville orogeny.(3)The basin evolution during the Xiajiang period was constrained by the sedimentary environments and facies analyses of the Xiajiang Group: ① Stage of planation and rapid filling of back-arc basin during early Jialu period.② Stage of rapid basinal development during late Jialu period. ③ Stage of rapid basinal development during the Wuye and Fanshao period. ④ Compression and uplift stage in the Qingshuijiang period. ⑤The basin rapidly vanished during Pinglue period to Longli period.(4) The statistics of sandstone clastic compositions suggest that sandstones from the Xiajiang group are moderately sorted and rounded and contain many rock fragments and feldspars with tufaceous matrixes. Part of strata experienced low-grade metamorphism, silicification, sericitization or pelitization. There are some accessory minerals, such as mica, zircon, tourmaline, limonite, chlorite and so on. Feldspars are obviously weathered and altered and the volcanic rock detritus have altered microcrystalline structure. Statistics of component shows that quartz accounts for 7.7%～89.3%, single-crystal quartz accounts for 5.5%～78.9%, multi-crystal quartz accounts for 2.1%～38.6%, feldspar accounts for 2.1%～34.3%, plagioclase accounts for 2.1%～27.0%, alkali feldspar accounts for 1.1%～36.8%, rock fragments accounts for 4.9%～78.6%, sedimentary and metamorphic fragment accounts for 4.4%～73.1% and volcanic rock fragment accounts for 0.0%～65.9%. Q-F-L, Q-M-Lt and Qp-Lv-Ls triangular diagrams reflect that sandstones of the Xiajiang group in southeast of Guizhou had a complicated material provenance. The primary sources were from the recycling orogen belt and volcanic- magma arc. Therefore, the sources of sedimentary debris were primary from the recycling orogeny which related to the magma arc. The diagram tell us that the magma arc was exist in western Jiangnan Orogen Belt during the Xiajiang period. Combining with the regional geological background, this magma arc was considered to be located in Longsheng area, Guangxi province. The Xiajiang Group sediments were deposited in back-arc environment. (5) The whole-rock geochemical study, including the concentration and ratios of major element, trace and rare-earth elements, suggests that clastic sdiments of the Xiajiang Group were formed in an active continental margin environment. (6) The detrital zircons from clastic rock and tuff samples of the Xiajiang Group and Sibao Group were divided into six different age groups:723 Ma-745 Ma、745～775 Ma、775 Ma-815 Ma、815 Ma-825 M、825 Ma-875 Ma and>875 Ma. On Th/U-Nb/Hf and Th/Nb-Hf/Th diagrams, all zircons are ploted in the arc-related/anorogenic zone. On Hf-Th/Yb、Hf-U/Yb、 Y-U/Yb and Y-Th/Yb diagrams, they are ploted in the continental granite zone and more than half zircons are ploted in the mixed zone between the continental crust and oceanal crust. The result indicates that a subduction zone may exist in the Qingshuijiang area, which may continue until at least Xiajiang Group (760 Ma±).(7) Peak age spectrums of detrital zircons from the Shuangqiaoshan Group in the east are similar with those from the Fanjingshan Group, Lengjiaxi Group and Sibao Group in the west of the southeast Yangtze Block, and also, the Xiajiang, Danzhou and Banxi groups in the west are similar to the Xiajiang Group in the east, except two age-older peaks, these indicate that the provenances were approximately the same. When we contrast the Sibao Group with the Xiajiang Group, we can find that the 1600 Ma and 1950 Ma peak age spectrums of detrital zircons are more obvious in the Sibao Group, while the 2500 Ma and 2000 Ma peak age spectrums are more obvious in the Xiajiang Group. This indicates that material sources were changes and related to changes in denuded layers. are similar to those of magmatic rocks along the margin of the Yangtze Block, but are obviously different from those of the Cathaysia Block or interior Yangtze Block. This indicates that the provenances were located on the hyperplasia belts of the margin of the Yangtze Block. In addition, the younger age peaks reflect that there were obvious magmatic activities in the depositional period. Thus, the sedimentary setting was active tectonic setting but not steady environment.The absence of the Cathaysia provenance signal may reveal that the Yangtze Block may not collide with the Cathaysia Block during the Xiajiang period in the southeastern Guizhou Province, which may be separated by a depth ocean (trench). The west margin of the Jiangnan orogen belt belongs to the trench-arc-basin system during the Xiajiang period(8) The age spectras of detrital zircons from the Xiajiang group at the southeastern Guizhou Province reveal that the provenance has changed after the Qingshuijiang period. Before the Qingshuijiang Formation, the main source was the margin and basement of the Yangtze Block. However, the source during the late Xiajiang Group has changed to be the deposits of the early Xiajiang Group and few were derived from the deposits of Sibao period and the basement of the Yangtze Block. Zircon U-Pb data from tuff of the Qingshuijiang Formation yields ages of 770-745Ma, which is similar to the age (760 Ma) of the pillow basalts and basalt from Longsheng area, Guangxi province, and Guzhang, Huan province. These data indicate that the detritus of the Qingshuijiang Formation may be sourced from the Longsheng area, Guangxi province, and Guzhang, Huan province. Middle Neoproterozoic (770-740 Ma) experienced obvious volcanic activities in the northern and weastern of the Yangtze Block and Xuefeng area. Howevever, these areas could not be the possible sources due to the separation from the Yangtze Block. On the north and west margins of the Yangtze Block and the Xuefeng area, the Xiajiang Group and overlying Nanhua System changed from weak angular unconformity into disconformity and then conformity from the northwest to southeast. This change reveals that the core area of the Xuefeng movement was located in the northern part of the Yangtze Block. The southeastern Guizhou located in the southern part of the Yangtze Block is the margin of the Xuefeng movement. The Xuefeng movement resulted in the extinction of the volcanic arc.(9) This paper establishes an Arc-continental subduction dynamic model for the southeastern Guizhou. (a) 870～830 Ma:the western part of Jiannan orogenic belt belonged to trench-arc-basin system, where deposited a suit of flysch formation of Fanjingshan/Sibao group; (b) 830-815 Ma:the strong compression orogenic movement caused the extensive magma activity, the Jiangnan fold belt was formed; (c) 815～770 Ma:The continuous compression existed in the Jiangnan orogenic belt and arc-related environment was developed in the Longsheng area, Guangxi province. During this period, the Jialu and Fanzhao formations of the lower part of the Xiajiang Group were deposited in the southeast Guizhou province to north part of the Guangxi province, (d) 770～745Ma:the Cathaysian Block subducted under the Yangtze Block, meanwhile, the Yangtze Block moved to west (north), which counteracted the ability to converge of the two Block. Moreover, because the continuous converge, the angular unconformity occurred in Xuefeng area, the western and northern margins of the Yangtze Block. Although the Jiangnan orogenic belt experienced arc-continent converge, but it only caused differential uplift of basement and weak fold deformation. (e) 745～720 Ma:after the Xuefeng movement, the Yangtze and Cathaysia blocks amalgamated together. The Pinglue and Longli formations are formed in a passive continental margin basin environment. |
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