Sedimentary Framework Of The Neoproterozoic In The Western Part Of North China Craton And Its Tectonic Significance

The tectonic evolution and attribution of the Alxa Block in the western part of NCC have been under debate for a long time.Recently,a lot of Neoproterozoic granitic igneous rocks?⁰.92 Ga,⁰.8 Ga?and mafic rocks?⁰.82 Ga?were discovered in the Precambrian metamorphic basement of the Alxa Block.Based on these discoveries,more disputes were raised about the tectonic background of the Alxa Block and the relationship between the Alxa Block and North China Craton:?1?Some researchers believe that Alxa Block is one of the micro Precambrian blocks which constitute the North China Craton or the western part of the Yinshan Block;?2?Some researchers think that the Alxa Block is an independent micro block before amalgamating with North China Craton in the Late Paleozoic or Late Mesozoic;?3?Some researchers consider that the Alxa Block has close paleogeographic relationship with the Yangtze Craton or Cathysia Craton in the Neoproterozoic;?4?Some researchers deem that the Alxa Block had affinity with Tarim Craton from the Precambrian to the Early Paleozoic.In this study,we intend to re-tease the Neoproterozoic sedimentary framework of North China Craton,trace the provenance of the Neoproterozoic,re-analysis the sedimentary framework of the western part of the North China Craton,discuss the tectonic relationship between the Alxa Block and North China Craton,and re-establish the sedimentary and tectonic evolutionary model for the Neoproterozoic in the western part of the North China Craton.According to the paleocurrents of the Sinian System and Early Cambrian System in the Helanshan area,western margin of the Ordos Block,the provenance mainly comes from the NE-NW of the Helanshan area.The lithology content of the gravels indicates that the gravels mainly come from the Mesoproterozoic Changcheng System Huangqikou Group and Jixian System Wangquankou Group.In the Helanshan area,a lot of Neoproterozoic detrital zircons i are first found n the Sinian and Early Cambrian strata with age peaks at 818Ma?n=88?and 905Ma?n=20?.However,there is a lot of Neoproterozoic igneous rocks developed at the northern margin of the Alxa Block and the western part of the Ordos Block.Moreover,a part of the Neoproterozoic zircons in the Sinian system Zhengmuguan Formation have similar Hf isotopic characteristics(n=10,?_Hf?t?:-7.812³.274,T_D^C _M:2211Ma¹578Ma)with the Neoproterozoic igneous rocks in the Langshan area(n=23,?_Hf?t?:-1.105⁵.928,T_D^C _M:1.75Ga¹.38Ga)and the Bayanxibie area.Therefore,the Helanshan area received clastics from the northern margin of the Alxa Block.Combined with the similarity of the Sinian sedimentary framework at the southern margin of the Ordos Block and the Alxa Block,it can be suggested that the NCC have collided with the Alxa Block before the Late Neoproterozoic.According to the detrital zircon U-Pb geochronology research,the Sinian System and the Early Cambrian Formaiton at the southern margin of the Ordos Block has similar zircon U-Pb age pattern with the Precambrian basement and Mesoproterozoic strata.And the Sinian System and the Early Cambrian Formation have similar zircon U-Pb age pattern with the basement and Mesoproterozoic in the southern part of the Alxa Block.Therefore,it can be inferred that the southern margin of the Ordos Block and Alxa Block received clastics from the interior of the Ordos Block and the Alxa Block,respectively.In the Tonian period,clastics provenance from the Neoarchean to Paleoproterozoic basement of the Alxa Block and the northern part of North China Craton,Khondalite Belt in the western part of North China Craton,the Mesoproterozoic strata in the Bayan Obo-Zhaertai-Langshan Rift and Ordos Block,and the Mesoproterozoic igneous rocks in the Laurentian-Baltica Craton sedimented at the northern margin of the Alxa Block.The Late Mesoproterozoic detrital zircons?1.3-1.0 Ga?have the same zircon U-Pb age pattern with the Mesoproterozoic igneous rocks in the Laurentia-Baltica Craton.Therefore,the Laurentia-Baltica Craton might provide clastics from the Mesoproterozoic igneous rocks to the northern margin of North China Craton in the Tonian period.It can be concluded that North China Craton had paleographic affinity with Laurentia-Baltica Craton in the Early Tonian period.According to the zircon LA-ICP-MS U-Pb dating research about the Sinian system at the northern margin of the Alxa Block,a lot of Late Mesoproterozoic?1.3-1.0 Ga?detrital zircons are discovered in the Tonian system Zhulazhagamaodao Formation in the Bayanxibie area at the northern margin of the Alxa Block,and the Tonian system Langshan Formation in the Langshan area at the northern margin of the western part of the Ordos Block.Besides that,there is also a small quantity of Late Mesoproterozoic detrital zircons in the Tonian system Baiyinbaolage Formation and Huerjiertu Formation in the Bayan Obo area,and the Tonian system Sanxiatian Formation and Hujiertu Formation in the Huade area at the northern margin of the NCC.The Late Mesoproterozoic is the most important stage of the convergence of the Rodinia supercontinent.Meanwhile,the Grenville Orogenic Belt developed in the Laurentia and Baltica Craton accompanied by the development of the Late Mesoproterozoic igneous rocks during the convergence of the Rodinia Supercontinent.What's more,the zircon U-Pb age patterns of the Late Mesoproterozoic igneous rocks highly consistents with the U-Pb age patterns of the Late Mesoproterozoic detrital zircons in the Tonian system at the northern margin of the western part of the NCC.Therefore,the Laurentia Craton and Baltica Craton have paleogeographic affinity with the NCC,and provide clastics for the northern margin of the NCC in the Tonian period.The western part of North China Craton and the Alxa Block share the same sedimentary framework in the Neoproterozoic:?1?The Tonian System at the northern margin of the Alxa Block and in the Bayan Obo-Zhaertai-Langshan rift at the northern margin of North China Craton have the same lithology combination,sedimentary facies and sequences evolution process.According to the provenance analysis,the Precambrian basement of the Alxa Block and the northern margin of North China Craton,theKhondalite,andtheMesoproterozoicintheBayan Obo-Zhaertai-Langshan rift and Ordos Block provided clastics for the Tonian System at the northern margin of the Alxa Block.During the transgressive sequence evolution from the northern margin to the interior of North China Craton,there sedimented fluvial to shore facies coarse clastics.Companied by the transgressive sequence evolved from shore to neritic facies,the lithology combination mainly consists of shale and carbonate rocks.After that,weak regressive sequence developed and the sandstone and sandy shale in the shore facies developed.In the upper part of the Tonian System,transgressive sequence developed and the lithology combination is comprised of shale and limestone in the shore-neritic facies;?2?From the Nanhua to the Middle Sinian period,there is no contemporary sedimentary records and igneous events which might indicate that the tectonic background remains stable uplift;?3?At the end of the Sinian period,there existed glacial deposits from the southern margin,western margin of the Ordos Block to the southern margin of the Alxa Block.After the glacial period,there developed transgressive sequence from the southwestern margin to the interior part of North China Craton.Combining the Tonian System in the northern margin of the Alxa Block provenancing from the northern part of North China Craton,and the Sinian System in the Helanshan area provenancing from the Alxa Block,the Alxa Block and the western part of North China Craton might share the same sedimentary framework in the Neoproterozoic.Based on the current sedimentary and geochronology framework for the Meso-Neoproterozoic of the NCC,the northern margin of the NCC and the Alxa Block share same sedimentary framework.Under the control of the Rodinia Supercontinent's break-up,the northern margin of the NCC lied in a regional extensional tectonic setting at the beginning of the Tonian period?0.92-0.82 Ga?;After that,the northern margin of NCC might start to rift from the Laurentia-Baltica Craton at the middle to late Tonian period?⁰.82-0.72 Ga?;From the Nanhua period to the middle Sinian period?⁰.72-0.58 Ga?,contemporary sedimentary record and igneous events haven't been found in the NCC,and the southern margin of NCC rifted away from the Qilian Block;In the Late Sinian?⁰.58-0.54 Ga?,there developed broad tillites at the south-western margin of the NCC and transgression sequence after the glacial period.