Time-event Series Of Formation And Evolution Of Xishan Coal Bearing System,Taiyuan And Its Geological Factors

Carboniferous-Permian standard stratotype section is located in Xishan coalfield of central North China.In order to discuss Time-event Series of Upper Paleozioc and LEarly Mesozoic Formation and Evolution of Xishan coalfield,Taiyuan,three sections(Kuangchanggou,Qiligou and sigou)were as research area.Eleven sandstones from the marked layers were sampled,which were prefonned on zircon U-Pb and Lu-Hf isotopic analyses,as well as zircon/apatite fission track dating.Detrital zircon U-Pb dating showed that the maximum sedimentary age of Taiyuan formation is not earlier than 298Ma,which suggested the Taiyuan formation is in early Permian instead of Late Carboniferous.The maximum sedimentary ages of Shangshihezi and Shiqianfeng formation were 266Ma and 250Ma,respectively.The Permian-Triassic boundary in Xishan coalfield is located at the bottom of Shiqianfeng formation.Combined lithological feature,zircon U-Pb dating with Lu-Hf isotopic analyses and compared with the potential source areas,provenances of upper Paleozioc-lower Mesozoic strata were confirmed.Precambrian zircons of all sandstone samples were from the ancient basement of north China Craton.Howere,the phanerozoic zircons have different provenances for different formations.Provenance for Taiyuan formation was from Inner Mongolia uplift of northern margin of North China Cratoa However,Phanerozoic zircons in Shanxi and Xiashihezi formations were both from Inner Mongolia uplift and Central Asian Orogenic Belt.The provenances for Shangshihezi and Shiqianfeng formation were mainly from the ancient basement of north China Craton,and little Phanerozoic zircons were from the Inner Mongolia uplift.What's more,sources for Liujiagou,Heshanggou and Ermaying formations were from the Inner Mongolia uplift as well as Central Asian Orogenic Belt.It is suggested that the North China craton stairted to collage with the central Asian orogenic belt in the Shanxi period,and finished before Liujiagou period.Phanerozoic zircons showed three periods of magmatic activity in the source area,including 230?340Ma with an apparent peak at 291 Ma,365?41OMa and 415?500Ma.These magmatic activities indicated that the passive continental margin changed into active continental margin in northern margin of NCC during the palezoic.Furthermore,basin-moutain model were identified by the periods of magmatic activity.After Ermaying formation deposition,the coal metamorphism in the Xishan coalfield began.Zircon fission track data indicated a thermal tectonic events during 181?216Ma.The coalfield showed a thermal evolution difference in time and space.The coalification finished during 181?216Ma and 130Ma in the eastern area and western area,respectively.The results of apatite fission track revealed that the samples had an apatite fission-track age between 48±4 and 52±6 Ma.By thermal historical modeling,it was found that the Taiyuan Xishan coalfield experienced a three stages5 tectonic up lift-cooling process during the Cenozoic:(1)the fast uplifting stage before 50 Ma,(2)the slow uplifting stage between 50 and 5 Ma,and(3)the intense uplifting stage after 5 Ma.Compared with the apatite fission-track data of the surrounding area,we found that the Luliang Mountains showed differential uplifting characteristics.The uplifting of the northern region occurred earlier than that of the central region.