Chronostratigraphic Correlation Of The Late Precambrian In The Northern Margin Of North China And Its Geological Significance

The study area is mainly located in the Bayan Obo and Shetai areas in central Inner Mongolia.The geotectonic position is located in the middle of the northern margin of North China and the southern margin of the Central Asian orogenic belt.Meso Neoproterozoic strata are widely developed in this area.The Bayan Obo Group,the Zhaertai group,the xilinhudong group and the Shinagan group are exposed from north to south.But for a long time,due to the lack of accurate isotopic age data,there have been different views on the stratigraphic division and correlation scheme of these groups.Recently,we found a granite vein in the basement of the lower part of the Shinagan group in the Semagou section.The granite vein cuts through the basement gneiss and is unconformably covered by the Pigan colored sandstone in the lower part of the Shinagan group.Its formation age should be younger than the basement and older than the caprock.The degree of zircon automorphism is euhedral crystal,which is basically long columnar,with grain size of 120 ? 150 ? m.The vibration band is obvious.Shrimp zircon dating data show that the zircon age of granite magma is 1800 ± 5m A(95% confidence,mswd = 1.2),which can limit the lower age of the Shinagan group to 1800 Ma.It is considered that the main sedimentary age of the Shinagan group is probably Jixian period;Based on the U-Pb isotopic dating data in recent years,the meso Neoproterozoic chronostratigraphic framework in the northern margin of the area is determined.It is suggested that the dulahala Jianshan formation of the Bayan Obo Group,the xilinhudong group and the shuligou zenglongchang formation of the Zhaertai group be divided into the Changcheng period;The main bodies of halahoete Birute formation of Bayan Obo Group,agulugou formation of Zhaertai group and Shinagan group are divided into Jixian period;Baiyinbaolage hujiertu formation of Bayan Obo Group and liuhongwan formation of Zhaertai group are divided into Qingbaikou period.After the Luliang movement in the late Paleoproterozoic,the North China Craton formed a stable and unified basement,which has basically become a consensus.However,there are different views on the understanding of the Luliang movement,regardless of its tectonic evolution.A large number of geological records indicate that the North China Craton entered into a fully extensional tectonic mechanism at about 1.8 GA.However,it is still unclear when the tectonic system of the North China Craton changed from collisional orogeny to extension in the late Paleoproterozoic.Early studies suggested that the transformation time was about 1800 ma.Recent studies on the eastern and central North China structural belts indicate that the transformation time of this structural system may have been earlier.Recently,we found a diabase vein in the heinaobao area of Bayan Obo in Yinshan block.The CL image shows that the zircon is small in size,long columnar and acicular.Shrimp zircon U-Pb data show that the age of the diabase magma is 1845 ± 6 Ma(95% confidence,mswd = 0.95).Combined with other data,it is considered that the Yinshan block has entered the extensional mechanism at least 1850 Ma,which is the same as that of other parts of the North China Craton.