The Geochronology Of The Meso-Neoproterozoic Strata In The Southern Margin Of North China And Its Geological Significance

In rencent years, the geological studies of the North China Craton more and more focused by the researchers. Many papers published over the past decade, one of the focued subjucts is material composition, deposition times and times division of the Neoproterozoic strata. Neoproterozoic sedimentary strata is covered in a large ara on the crystalline basement of the North China Craton Archean-Proterozoic, including the Great Wall, Jixian and Qingbaikou in the north and the east, the Langshan- Zhaertaishan Group, Baiyunebo Group and the Huade Group in the northern margin, the sedimentary rocks of the southern of Liaoning, Shandong and Xuhuai area, as well as the Xiong’er Group volcanic-sedimentary rocks and the Neoproterozoic strata covered in the southern margin.As we all know, the Great Wall, Jixian and Qingbaikou strata of Jixian area in the northern of Yanliao rift basin has been used as the Neoproterozoic standard sequence profiles, but since 2007, there was made significant progress of its formation geochronology, Middle Proterozoic era of Xiamaling Group was re-calibrated between 1.65-1.32Ga, and need reestablished the Jixian which original recording 1.2～1.OGa profile. It is important that looking for the standard profile of this period, some scholars hope the results about the Neoproterozoic strata of the southern margin of North China to fill this gap.In addition, the era of the lower boundary of Middle Proterozoic is same wih the era of lower boundary of the Great Wall, the era of the lower boundary of the Great Wall in China (1.8Ga) is different with the international arena (1.6Ga), which is mainly based on the era of the lower boundary of the Yanliao rift of the northern margin of North China or the Xionger rift of the southern margin of North China. Because the era of the lower boundary of the Yanliao-Great Wall Department is limited to about 1.65 Ga, indicating Yanliao rifting time is later than the Xiong’er rifting time (1.80～1.75Ga), and the Xiong’er Group is volcanic rock. Therefore, some scholars advocated the starting time of the Mesoproterozoic sedimentary in China is 1.65Ga, consistent with the international standard. However, this establishment of the proposition mainly depends on the Mesoproterozoic sedimentary stratigraphic era on the Xiong’er Group of the southern margin of North China. That is, whether the era of Yunmengshan Group or the Gaoshanhe Group is older than the Changzhougou Group of Yanliao area.For a long time, there is some controversy about the era of the Neoproterozoic strata over Xiong’er Group in the southern margin of the North China, there is three opinions that attributed to Changchengian Period, Jixianian Period or Qingbaikouian Period, if the deposition occured mainly during Changchengin Period, it is likely that is continuous deposition after Xiong’er Group, it means that the "Changcheng system" more complete in the western region of Henan province, then the bottom boundary of the Changchng system should be the bottom line of Xiong’er Group; if it is deposited during the original "Jixianian Period", it is possible to fill the gap of Proterozoic stratigraphic column? if it is deposited during the Qingbaikou Period, there is a long temporal discontinuiry,how to explanation? These are closely related to the major basic geological problems about the stratigraphic division and the tectonic evolution of Proterozoic in China.The Neoproterozoic in the sourthern margin of North China is mainly distributed in the border area of Henan-Shaanxi-Shanxia, which is used to call the "Henan-Shaanxi rift," which mainly is among the southwest of Henan province. The Neoproterozoic strata in tihis area is complete, continuous outcrop, it is one of the best areas that the Mesoproterozoic strata is exposed in North China Craton. Volcanic rocks of Xiong’er Group is below Neoproterozoic strata, Luoqun Group is over Neoproterozoic strata, so this area is one of Neoproterozoic-related research hot areas in North China; 1990s, this area has became an important research window for th early evolution of life due to some macro-algae, special micro ancient plants and suspected fossils have been found;In recent years, some scholars proposed that the southern margin of the North China is as the main south of "the orogenic belt in the central of North China", in the Discussion of the evolution relations beween the Neoproterozoic in North China Craton and the Colombia, Rodini Ya, the region has aroused more attention.In the traditional scheme, the Neoproterozoic strata in the southwest of Henan province is included Xiong’er Group of Mesoproterozoic Changcheng System, Ruyang Group of Jixian System, Guandaokou Group (with Gaoshanhe Formation) of Jixian System, Luoyu Group and Luanchuan Group of Neoproterozoic Qingbalkou system, on which was unconformably covered by the Huanglianduo Formation, Dongjia Formation, Luoquan Formation of Sinian system or Xinji Formation of Cambrian System. But all the researchers are troubled that:there are some uncertainty abot the era of the Neoproterozoic strata in the Southwest of Henan province, it has been the lack of reliable zircon U-Pb isotopic age to qualify, it has been argued about the deposition Times endlessly over the years, resulted in many uncertainties to the geological workers and geological researchers. Therefore, it is very urgent to determine the Neoproterozoic era in the southern margin of the North China.In recent years, with the rapid development of isotope dating techniques and stratigraphy fine research, it is becming possible to precisely define the era of Neoproterozoic in the southern margin of North China. Geplogical Scholars have come to realize that a suitable dating carrier is needed to precisely define stratigraphic formations, especially some volcanic rocks sandwich or acidic intrusive rocks, but lack of such substances in the Neoproterozoic strata of the southwest of Henan, although the andesitic volcanics sandwich were found in Yunmengshan Group of Ruyang Formation and Gaoshanhe Fomation of Guandaokou Group, but so far no reported publishcations about zircon U-Pb isotopic age. So, is the set of volcanic is laminated formations? Whether Its exact age can determine? In addition, there are other volcanic sandwich in the Neoproterozoic strata in the Southwest of Henan? Can we obtain accurate isotopic ages? Whether there is intrusive rocks that can be closely related with the formation and can define stratigraphic age? Is there another way to constrain the set of stratigraphic age?Since 2010, The author is engaged in geological survey work in the Southwest of Henan, and carried out a lot of work about data analysis, field survey and sample test analysis. Rocks samples about volcanic rocks of Gaoshanhe Formation, volcanic rocks of Yunmengshan Formation, sandstones of Gaoshanhe Formation, sandstones of Yunmengshan Formation and tuff rocks of Luoyukou Formation for each typical profiles were collected. On the basis of petrology, rock chemical analysis, zircon U-Pb isotopic dating is finished using LA-ICP-MS method, and a Group of reliable, high-quality zircon U-Pb isotopic data is obtained. Based on extensively reviewed and then analiyzed the regional intrusive rocks chronological data, the stratigraphic deposition time of Guandaokou Group, Ruyang Group and Luoyu Group was demonstrated by different ways. The results is the important evidence to further define the era of the Neoproterozoic strata framework in the southern margin of North China, and also provided the basis for the establishment of the Neoproterozoic stratotype section and stratigraphic framework in the southern margin of North China, and also provide basis for exploring Cambrian geological evolution in the southern margin of North China, also for Proterozoic strata divided in North China Craton, for the establishment and contrast of stratigraphic framework, as well as provided an important geological basis for the comparison of the global Proterozoic boundary layer type profile.The main conclusion as follows:(1) Based on isotopic dating of zircon U-Pb, we measured the zircon U-Pb isotopic age values (161 ± 1Ma) of the original "volcanic rocks of Gaoshanhe Formation" and "volcanic rocks of Yunmengshan Formation", it presented the volcanic events during the same period, and ruled out the possibility of defining Proterozoic stratigraphic age. Then combined with the field information and the geochemical characteristics of the inner plate backgrounds, we knew that the volcanic rocks of Ruyang Group and Guandaokou Group should be attribute in the super-shallow sub-volcanic rocks of the Mid-Late Jurassic. In addition, the volcanic events are very rare in the southwestern region of Henan province during the Mid-Late Jurassic, it is also important for the tectonic-magmatic event during the early Yanshan Period.(2) We confirmed that the middle layer of tuff zircon of the Luoyukou Formation of the Ruyang of Henan profile is magmatic zircons, and got the precision age (1640 ± 16Ma), and demarcated the age of the Luoyu Group-Ruyang Group which belongs to the Statherian (1800 ～1600 Ma).(3) we acquired some important information about the chronology of rock through the information analysis about the geochronology of the intrusive rocks of Guandaokou Group, the zircon age value(1598 ± 9Ma) of the Maping alkali syenite of the invasive Longjiayuan Formation of the Luonan country in Shanxi province, the point age of the Xiaohe rock which intrusive the Fengjiawan Formation (1463Ma), it represents the approximate upper age limit of the Group, and the Guandaokou Group can be roughly calibrated to Mesoproterozoic Statherian. evolution of life and the research of the supercontinent Columbia.(4) Through the zircon U-Pb isotopic dating of the clastic rocks of the bottom of the Gaoshanhe Formation and the bottom of the Yunmengshan Formation, we accessed its youngest age data (1706 ± 61Ma,1711 ± 37Ma),and may be initially limited the boundaries of its bottom(about 1710 Ma), it means that the ages of whole Guandaokou Formation, Ruyang Formation and Luoyu Formation of the southern margin of the North China generally limited 1710～1600 Ma, further defined the starting time limit of the Proterozoic sedimentary strata. Above all the age data, and combined with other information, we established the stratigraphic framework of the Proterozoic age of the southern margin of the North China. It is very important for the establishment of the stratigraphic framework of Proterozoic of the southern margin of the North China, the stratigraphic correlation between the northern margin and the southern margin of the North China, and the establishment of the standard profile and the definition of the lower boundary of Changchengian Period of Mesoproterozoic age, and also has important significance for the early(5) Based on the five samples of detrital zircons 207Pb/206Pb age range(3.48～1.70Ga) of the Gaoshanhe Formation and Yunmengshan Formation, it shows that its geological provenance mainly belongs to the late Archean-Proterozoic, and there are obvious age peak (1.83Ga,1.95 Ga,2.15Ga,2.53 Ga,2.73 Ga), two age peak of which(1.95Ga and 1.83 Ga) represent the peak metamorphism of two commonly occurring in the North China, The above information provide the important basis for the limit the era framework of the Neoproterozoic and discussing the pre-Cambrian geological evolution of the southern margin of the North China.