Detrital Zircon And Apatite Fission Track Analysis Of The Mesozoic-Cenozoic Tectonic Evolution In Middle-north Part Of Lüliang Mountain

The Lüliang Mountain,one of the important mountains in the NCC,is an important mountain range in Shanxi block.Based on fission track analyses of granitoid intrusives from the Luyashan,Yunzhongshan and Yundingshan,we studied the tectono-geomorphic evolution process of the middle-north part of the Lüliang Mountain,and arrived at the following conclusions:1.Combined with coal vitrinite reflectance(Ro)and fission track data of sedimentary strata from Taiyuan Formation,Carboniferous in Ningwu-Jingle basin,we discussed geothermal gradient of the Lüliang Mountain.Nowadays,geothermal gradient in range from 1.92 to2.50?/100 m,the average is 2.21?/100 m,which belongs to the normal geothermal gradient.According to maximum burial depth and vitrinite reflectance(Ro),the paleo-geothermal gradient in range from 3.88 to4.34 ?/100 m,which is higher than the present data.Thus,a thermal event occurred in the Late Mesozoic.2.The apatite fission track data of samples from Yundingshan,Luyashan,Yunzhongshan and Ningwu-Jingle syncline recorded the multi-stage uplifting cooling events since the Late Cretaceous,especially the Cenozoic.Intensive tectono-geomorphic events occurred at 88~80Ma,75~70Ma,65~60Ma,56~53Ma and 23 Ma.3.Since the Late Cretaceous,the uplift and cooling/exhumation history recorded by bedrocks in the middle-north part of the Lüliang Mountain represented they underwent a different evolution stage.The results of apatite fission track indicated that there was a slowly uplifting and cooling stage in 130~80Ma and a rapidly stage in 70 Ma.In spatially,zircon fission track data show that Yunzhongshan began to uplift in the Late Proterozoic(885~691Ma),Guandishan in the Late Ordovician(447Ma).Apatite fission track data also indicate that uplifting of Yunzhongshan(154~122Ma)occurred earlier than that of Yundingshan(88~ 80Ma)and Luyashan(85.1 Ma).Erosion rate of YunZhongShan,Luyashan and Guandishan are all decreasing from middle to both sides.The present geomorphology has not formed until 23 Ma,or even later.4.Most of the fission track data in research area did not follow the "age-depth" law under the system of crustal isostatic uplift,which indicates the complexity of the Lüliang mountain uplifting mechanism.Combined with the difference of strata denudation extent and apatite fission track data,we proposed that there are some relationships betweenthe rapid uplift of the Lüliang mountain during the Cenozoic and thrust fault structural and the settlement of adjacent sag in the east.The uplifting and evolution of the Lüliang mountain during the Meso-Cenozoic,the uplifting and dying out of the Ordos basin and the development of the adjacent grabens controlled by the uniform regional dynamic setting,Apatite fission track data from the southeast margin of Ordos Basin,the Jinxi flexure belt,Qinshui Basin and Datong Basin all indicate that the uplifting and evolution of the Lüliang mountain mainly related with the compressional tectonic of the Tibetan plateau and the movement and evolution of the Pacific tectonic domain,while affected by wider regional geodynamic setting simultaneously.