The Basin And Tectonic Evolution Of The Neoproterozoic Banxi Period(Banxi Group) In Western Hunan

The Neoproterozoic strata were widely developed in the southeastern margin of Yangtze,which has recorded the whole process of the transition from orogenic environment to non-orogenic environment in the southeastern margin of the Yangtze block.The banxi strata in western hunan are completely exposed.There are a large number of related volcanic rock records in the early stage,and in the late stage,the strata of Wuqiangxi formation,Duoyitang formation,Baihelong formation and Niuguping formation were completely exposed.It is an ideal area to study the pattern and tectonic evolution of basin in Banxi period.For the research of the Neoproterozoic Banxi period,the research focus has been on Guangxi and Guizhou.The main reason is that the collision contact zone between Yangtze and Cathaysia block is mainly located in Guangxi,and the subsidence center of the basin is also located in Guangxi and Guizhou at the extensional stage.So little attention has been paid for the Xiangxi area.But the western Hunan area is also an indispensable part of the tectonic-sedimentary evolution in the Neoproterozoic period.In this paper,the Anhua section of " Red Banxi" ?the Anjiang section of "Black Banxi" at the late Banxi period and the Guzhang-Anjiang mafic-ultramafic rocks at the early Banxi period in Southeastern margin of Yangtze block have been studied.This paper mainly takes structural geology,sedimentology,magmatic petrology and stratigraphic sequence as theoretical guidance.Based on the analysis of the magmatic events of the multi-episode basic rocks,the geodynamic evolution process of the early Banxi period in the study area was determined,and the time of tectonic background transformation was constrained,so as to explore the dynamic principle of basin evolution.Meanwhile,the U-Pb chronology of detral zircons and Hf isotope analysis were used to determine the time of provenance transformation in different strata in the late Banxi period in the study area.By studying the peak of characteristic ages,the provenance source areas in and around the Yangtze were compared and analyzed to constrain the paleogeography of the sedimentary period.Based on the analysis of sedimentary facies,the basin evolution history of "Red Banxi" and "Black Banxi" in western Hunan was determined.Mainly achieved the following understanding:1.The sample results tested in this paper can determine the maximum sedimentary age of all strata in late Banxi Group and the Datangpo formation and Gucheng formation of Nanhua system.The maximum sedimentary age of Datangpo formation is 668 Ma,Gucheng formation is 679 Ma,Niuguping formation is 725 Ma,Baihelong formation is 732 Ma,Duoyitang formation is 748 Ma and Wuqiangxi formation is 772 Ma.Datangpo formation in Hunan can correspond to Datangpo formation in Guizhou,Gucheng formation of "Red Banxi" can correspond to the top of Fulu formation of "Black Banxi" in Hunan,and Gucheng formation can correspond to the Tiesiao formation in Guizhou.The Niuguping formation of "Red Banxi" in Hunan can correspond to the top strata of Longli formation in Guizhou and Gongdong formation in Guangxi,as well as the third section of Yanmenzhai formation of "Black Banxi" in Hunan.Baihelong formation of "Red Banxi" in Hunan can correspond to the Pinglue formation in Guizhou and the second section of Yanmenzhai formation of "Black Banxi" in Hunan.The Duoyitang formation can correspond to the top of the Qingshuijiang formation in Guizhou and the first section of Yanmenzhai formation.Wuqiangxi formation of "Red Banxi" corresponds to the Jiajiantian formation of "Black Banxi" and middle and lower strata of Qingshuijiang formation.2.There are two periods of source changes in Anhua section.The one is between in Baihelong formation and Niuguping formation: The ancient source?2.0 Ga and 2.5 Ga?input has been greatly weakened in Niuguping formation;The second is between in Niuguping formation and Gucheng formation?the boundary of strata is "Xuefeng movement" unconformity?: the input of the old provenance?2.0 Ga and 2.5 Ga?had began to increase and the new provenance of 680 Ma had been added in Gucheng formation.The variation of provenance in Anjiang section has three times.The one is between in Wuqiangxi formation and Duoyitang formation?between in the Jiajiantian formation and the first section of Yanmenzhai formation?,which shows the increase of provenance input of 2.0 Ga and 2.5 Ga in Duoyitang formation.The second is between in Baihelong formation and Niuguping formation?between the second section and the third section of Yanmenzhai formation?,which shows the near disappearance of 2.0 Ga and 2.5 Ga in the third section of Yanmenzhai formation.The third is between Chang 'an formation and Fulu formation,which shows that the input of 2.0 Ga and 2.5 Ga materols had increased again in Chang 'an formation.By studying the magmatic event of characteristic peak ca.680 Ma between the top of Banxi group and Nanhua System,based on the comparative analysis of the age spectrum of detrital zircons around the Yangtze and on the pre-cambrian basement inside the Yangtze and the Hf isotopes of zircons,it was found that during the deposition from the top of Banxi Group to the Nanhua System,the source area was the Southern Qinling area on the Northern margin of the Yangtze block.By comparing the age spectrum of the geological section along the western Exi strait,whether ca.680 Ma characteristic peak was found or not,the migration channel of its provenances was determined to be the Exi strait.3.Based on the comprehensive study,we believe that in Neoproterozoic,at least in ca.680 Ma,the Yangtze block was mainly composed of the upper Yangtze old land,the Ezhong old land,the Jiangnan orogenic belt,and the Southern Qinling area in the northern margin of the Yangtze.The uplift zone of the South Qinling exists in the north margin of the Yangtze block.It can be used as a provenance supply area.There may be uplift zone or trench near to Changyang and between in North Guangxi and Anhua.4.In the longitudinal direction,"Red Banxi" and "Black Banxi" show consistent changing rules: two sedimentary cycles with deepening water depth.On this basis,four stages of basin evolution are identified:?1?relatively stable stage;?2?relative activity stage;?3?tectonic violent change stage;?4?structural stability,transgressive expansion stage;Laterally,"Red Banxi" and "Black Banxi" show obvious differences in lithofacies and lithology:?1?In the early Banxi period,the Banxi Group deposited the epicontinental shore facies and the Gaojian Group deposited the shelf slope facies,and formed the terrigenous clastic rocks of the river-littoral facies and deltaic facies deposition in Hengluchong/Shiqiaopu formation,and the mixed deposition of calcarenite and carbonate rocks in the tidal-flat facies and continental shelf of Madiyi /Huangshidong formation;?2?The Zhuanqiangwan formation of "Black Banxi" forms black carbonaceous rocks in the undercompensated and trapped environment and low-density turbidity current sedimentary rocks in the retention environment.The Tongtawan formation of "Red Banxi" developed into a sandy and argillaceous deposit.Further north,the Shimen-Cili area?Red Banxi?remains continental facies.It shows the paleogeographic difference from north to south;?3?At the end of the Tongtawan /Zhuanqiangwan formation,due to the influence of the "Xihuangsahn movement",the tectonic changes gradually intensified,which directly led to large-scale regression and shallow water depth;?4?When the structure is stable,the sea level rises again.The Wuqiangxi formation of "Red Banxi" is mainly fluvial and littoral facies deposit,while the Jiajiangtian formation of "Black Banxi" is mainly littoral and shallow sea deposit.During the period of the Duoyitang-Niuguping /yanmenzhai formation,the Duoyitang-Niuguping formation of "Red Banxi" formed a series of terrigenous sandy and argillaceous--pyroclastic sedimentary rocks,while the Yanmenzhai formation of "Black Banxi" developed obvious turbidity current deposits.On the whole,the water depth of "Red Banxi" is shallow and "Black Banxi" is deep,and the paleogeographic pattern is high in the "Red Banxi" and low in the "Black Banxi".5.The U-Pb dating results of Guzhang-Anjiang basic rocks show that the formation time is ca.780 Ma.The basic rocks have widely distributed contents of SiO2?42.38-54.11 wt.%?,MgO?2.24-15.0 wt.%?,K₂O+Na₂O?1.96-11.19 wt.%?,Nb?17.55-49.62 ppm?and Ta?1.15-3.26 ppm?.It shows the geochemical characteristics of oceanic island basalt?OIB?,which is rich in rare-earth elements?LREEs?and large ion lithophile elements but poor in heavy rare-earth elements?HREEs?.The sample has the positive value of zircon ?_Hf?t??+1.86-+5.14?and the positive value of whole rock geochemical ?_Nd?t??+0.31-+1.80?.These characteristics indicate that the asthenosphere is the mantle of asthenosphere,which underwent the transformation of subduction oceanic crust in the early stage,and 2-5% partial melting occurred.Based on the study of the basic rocks in the western section of the Jiangnan orogenic belt in the Neoproterozoic,we believe that three tectonic stages occurred in the study area:?1?860-830 Ma,subduction of the oceanic crust;?2?830-805 Ma,the collision between the Yangtze block and the Cathaysia block;?3?780-750 Ma,from the orogenic stage to the post-orogenic stage,the Nanhua rift began to form.This paper constrains the time nodes of two construction transformations,ca.830 Ma and ca.780 Ma,respectively.The source area of these basic rocks changed spatially and evolved with time.The ca.780 Ma basic rock identified in this paper indicates a large scale asthenosphere upweling,representing that the tectonic background of the western part of the Jiangnan Orogenic belt shifted from collisional orogenesis to post-collisional stage.