Characteristics And Significances Of Structural Deformation Of Western Part Of The North China Craton In The Late Paleoproterozoic

The North China Craton (NCC) is the largest craton in the China continent, and is one of the oldest cratonic blocks in the world. The NCC experienced multiple tectonic events and complex Precambrian tectonic evolution and record most of the significant tectonic events happened in the early history of the world, including the enormous crustal growth, the great oxygen event and the tectonic regime inversion from pre-plate tectonics to plate tectonics.The definition of Paleoproterozoic continent-continent collisional and orogenic events and the Mesoproterozoic extension rifting events initiated at about1.80Ga have been accepted by most of the researchers although various models have been postulated about the Precambrian tectonic evolution of the North China Craton (NCC). It is significant to study the characteristics of structural deformation and tectonic evolution of the NCC in the late for the following reasons. Firstly, the structural deformation and tectonic evolution between the orogeny at about1.85Ga and the rifting initiated at about1.80Ga been scarely studied, which have limited the understanding of the tectonic framework, evolution and mechanism of the NCC in the Paleoproterozoic. Secondly, the tectonic evolution in the late Paleoproterozoic controlled the tectonic framework of Meso-to Neo-proterozoic. Thirdly, the most important is the Paleoproterozoic might be the period of the tectonic regime inversion from pre-plate tectonics to plate tectonics, the study of the characteristics of structural deformation in the late Paleoproterozoic is helpful for confirming whether the NCC was controlled by the plate tectonic regim.The western part of the NCC is composed by several blocks which amalgamated along the collision orogenic belts, the combination of stable rigidity blocks and active orogenic belts could provide significant informations for understanding the characteristics of structural deformation and tectonic regim.A series of large scale ductile shear zones developed in the Paleoproterozoic orogenic belts in the NCC. Previous studies have shown that these ductile shear zones formed in the late Paleoproterozoic. The similar deformation ages indicate that the ductile shear zones might be genetically correlated, and are significant for studing the post orogeny structural deformation of the NCC.The characteristics and deformation ages of the ductile shear zones developed in the west, north and east margin of the Ordos block have been studied systematically through field investigation, mircro-structural analysis, zircon U-Pb dating and deformed mineral Ar-Ar dating. On the north margin of the Ordos block, the E-W trending Wulashan-Daqingshan ductile shear zone is characterized by dextral strike-slip shearing kinematics. On the west margin, the NE-NEE trending Zongbieli ductile shear zone is characterized by top-to-the-NW thrusting kinematics. On the east margin, the NE trending Xueling ductile shear zone is characterized by top-to-the-SE thrusting accompanied by sinistral strike-slip shearing. The40Ar-39Ar ages of deformed minerals from mylonites and LA-ICP-MS U-Pb dating results of zircons from syn-tectonic anatectic granites reflected that the ductile shear zones surrounding the Ordos block deformed between1.85and1.81Ga. The geometry, kinematics and geochronology characteristics of these ductile shear zones might defined the possible southwestward extrusion of the Ordos block in the late Paleoproterozoic (about1.85-1.80Ga).The extruded Ordos block was bounded on the northwest by the Zongbieli ductile shear zone which extended to the NE and connected with the Wulashan-Daqingshan ductile shear zone, and on the east by the Xueling ductile shear zone which extended to the NE and connected with the sinistral ductile shearing Zhujiafang shear zone.Under the modern palte tectonic mechanism, intracontinental tectonic movements including the intracontinental orogeny, intracontinental basining and the lateral and vertical extrusion of micro blocks would be initiated after the collision orogenic event. Whether the NCC was controlled by the plate tectonic mechanism was depended on the definition of intracontinent movement of the Ordos block in the late Paleoproterozoic, which was depended on the relationships of the Ordos block with its sorrunding blocks. The Yinshan block and Eastern block were connected to the Ordos block on the north and east, respectively. However, the relationship between the NCC and the Alax block in the late Paleoproterozoic was controversial and analysed by this work. Analysis of the zricon U-Pb dating results of the Langshan Group located at the neighbouring of the northeast margin of the Alax block and north margin of the NCC, and comprision of the detrital zircon U-Pb age compositions of the Precambrian sedimentary rocks developed in the Alax block and the NCC indicated that both of which have experienced similar tectonothermal events, which means that the Alax block was connected to the west margin of the NCC in the late Paleoproterozoic, and the west margin of the Ordos block was an intracontinental environment. And if there was space for the extrusion of the Ordos block depending on whether the NCC was connected with other blocks on the southwest margin. The Mesoproterozoic Changcheng-Jixian group developed in the southwestern part of the Ordos block is composed mainly by sandstones, and no rift related volcanic rocks similar to the Yanliao rift system and the Xiong’er rift system have been founded. Detrital zircon U-Pb age composition of the Changcheng-Jixian group in the southwestern part of the Ordos block was similar to which of the Meso-to Neo-proterozoic sedimentary rocks in the NCC, which indicated that the detritus were from the interior of the NCC. Both of this evidences indicated that no Mesoproterozoic rift system has developed in the southwest margin of the NCC. The conclusion of southwest margin of the NCC was not joined with other continents in the late Paleoproterozoic could be inferred, which offered the space of the extrusion.The west, north and east margin of the Ordos block in the western part of the NCC were in the intracontinental environment in the late Paleoproterozoic, whereas it was not connected with other blocks on the southwest margin. The western part of the NCC experienced intracontinental deformation represented by the SW extrusion of the Ordos block after the orogenic events at about1.85Ga. The results of this work defined the structural deformation and tectonic evolution of the NCC after the Paleoproterozoic orogeny at about1.85and before the Mesoproterozoic rifting at about1.80Ga. The combination of the orogenic events in the Paleoproterozoic and the post-orogenic intracontinent deformation indicated that the NCC has been controlled by the plate tectonic mechanism in the Paleoproterozoic. The structural deformation and tectonic evolution of the NCC in the Paleoproterozoic was correlated with the amalgamation of the Columbia supercontinent, which indicated that lateral and vertical extrusion of blocks might have happened after the orogeny in the Columbia super continent.