The U-Pb Geochronology Of Detrital Zircon And Its Geological Significance Of The Upper Paleozoic In The Northeastern Ordos Basin

The Ordos Basin forms part of the Western Block of the North China Craton(NCC).The sedimentary evolution in the northern part of the basin is always controlled by the tectonic evolution of the Paleo-Asian Ocean(PAO)in the northern margin of the NCC.The chronology of detrastic zircons in the same orogenic basin can not only determine the source of the material,but also better reflect the evolution of adjacent orogenic belts.The Ordos Basin is located in the southern edge o f the Xing-Meng Orogenic Belt(XMOB),and the detrital materials in the basin can clearly record the process of subduction orogenesis of the PAO.We carried out LA-ICP-MS zircon U-Pb geochronology analysis of the Upper Paleozoic drill core in the Yulin-Shenmu area of northeastern Ordos Basin,and combined with the outcropping situation and tectonic evolution characteristics of the surrounding basin,to determine the source of materials in the study area and to invert the geological and historical events in the northern and surrounding areas of the Ordos Basin.Detrital zircon ages provide for three principal episodes of deposition to this basin: Neoarchean(2550?2400Ma),Paleoproterozoic(2000?1700Ma),and late Early Carboniferous-Early Permian(330?284 Ma),showing three peak ages of 2500 Ma,1850 Ma,300 Ma and two weak peak ages of 200 Ma and 400 Ma.The populations were dominated by Paleoproterozoic zircon(77%?93%),followed by zircon of Early Carboniferous-Early Permian(8%?20%).Moreover,Mesoproterozoic and 3 billion year old detrital zircons have been found.The distribution of U-Pb ages in zircon is extensive,and records from the Archeozoic and Paleozoic have been recorded,which can better reflect important events in geological history.A comparative study of the age of detrital zircons in the study area and the age of zircons in the surrounding orogenic belt shows that the upper Paleozoic sediments in the northern Ordos Basin are mainly derived from the Yinshan Blocks and the Khondalite Belt in the northern margin of the NCC.In addition,the young age component to the detrital zircon populations indicated that the Inner Mongolia Uplift started large-scale uplift in the late Early Carboniferous(?330 Ma)affected by the PAO last dive since Carboniferous period.This also provides a timeframe for closure of the PAO suggesting its completion by the time of deposition of the Early Permian Shanxi Formation.The ordos basin was transformed from an epicontinental sea basin to an intracontinental depression basin.Following ocean closure the Inner Mongolia Uplift entered a phase of stable uplift and denudation,during which time the XMOB and the Inner Mongolia Uplift began to jointly provide stable clastic sediments into the Ordos Basin.