Study On The Provenance And Sedimentary Environment Of Taiyuan Formation In Jiaozuo Area

This paper,which was guided by basic geological theories such as sedimentology,mineralogy,paleontology and stratigraphy,and zircon chronology,focused on the exposed Taiyuan Formation in Jiaozuo area.Reference was made to the latest research on limestone and clastic rock at home and abroad.This paper carried out geology tests like fossil identification,optical microscope observation of thin sections,zircon U-Pb dating and zircon Hf isotopic,ect.,after doing field outcrop observation and description,and systematicly sampling vertically,to conduct researches on sedimentary characteristics,depositional sequences,sedimentary environments and provenance of Taiyuan Formation in research area.Furthermore,the tectonic background in Taiyuan Formation period has been discussed.In this paper,we used the LA-ICP-MS detrital zircon U-Pb age dating and the in-situ Hf isotopic test and analysis,combined with the original rocks recovery and the ways of fragments transportation,to identify the provenance and its charactericts and sedimentary environment.The results show that the main detrital zircons of Taiyuan Formation in Jiaozuo area are mainly composed of the Hercynian magmatic zircons?280³59Ma?with a peak of²95Ma,the Caledonian magmatic zircons?377⁵23Ma?with a peak of⁴46Ma and the Precambrian magmatic zircons and metamorphic magmatic zircons?546³373Ma?.The Hercynian zircons and Caledonian zircons have sharp crystal edges,clearer concussion belts,and have not undergone obvious abrasion,which shows the nearer characteristics to the provenance area;while the Precambrian zircons are mostly round,with no concussion belts clearer concussion belts except that a few zircons have clear concussion belts.The ¹⁷⁶Hf/¹⁷⁷Hf values of the Hercynian zircons of Taiyuan Formation in Jiaozuo area are 0.282203⁰.282935,with an average value of0.282379;the values of?_Hf?t?are-13.3¹1.6,with an average value of-7.5 and the values of T_DM2 ages are 562²354Ma.The ¹⁷⁶Hf/¹⁷⁷Hf values of the Caledonian zircons are0.282131⁰.282932,with an average value of 0.282471;the values of?_Hf?t?are-12.7¹4.5,with an average value of-1.1 and the values of T_DM2 ages are 480²444Ma.The ¹⁷⁶Hf/¹⁷⁷Hf values of the Precambrian zircons are 0.281064⁰.282457,with an average value of0.281781;the values of?_Hf?t?are-15.9¹0.7,with an average value of-1.2 and the values of T_DM2 ages are 1199⁴843M.The limestones in the first cycle and second cycle in the bottom of Taiyuan Formation both contain Caledonian and Hercynian zircons;while the marlite in the first cycle and the lamelleted siliceous rock in the second cycle contain only Caledonian zircons.The limestones and clastic rocks in fifth and sixth cycles all transform into mainly containing zircons from the Hercynian period,with only a small amount of Caledonian zircons.It shows that with the tectonic movement and the transgression,the Qinling Orogenic Belt gradually decreased,and the proportion of sediment provided to the South China North China Basin reduced gradually.The way to carry out the zircons should be the magmatic rock fragements with low maturity,which is rapidly transported and deposited by strong hydrodynamic force caused by the rapid transgression.The transgression is mainly from the eastern part of the North China plate.Taiyuan group limestones deposition was caused by strong hydrodynamic force that transported hermatypic organisms,reef attached organisms and magmatic rock fragements within Hercynian zircons?with a peak value of²95Ma?located in the eastern part of the basin to the basin to and deposited them on the tidal flat of the land surface sea in a disorderly and fractured way,and caused scour to the lower rock layer.The Caledonian magmatic zircons?with a peak value of⁴46Ma?and Precambrian zircons in limestones are mainly derived from Caledonian intermediate acid granitic intrusive rocks in North Qinling Orogenic Belt in the south of North China plate,and the main metamorphic strata in the area that were invaded-the Kuanping group,the Erlangping group and the Qinling Mountains group.It was transported to the study area by sea water reflux of strong transgression that reached the pre-mountain area of North Qinling Orogenic Belt.The bioclastic in Taiyuan formation limestones is obvious in graded bedding and wavy bedding,combined with the zircons brought by strong hydrodynamic force caused by strong transgression,confirme that the hydrodynamic force was strong during sedimentary period of Taiyuan Formation.The limestones of Taiyuan Formation deposited under the intense transgression.Each layer of limestone represents a strong process of transgression.The multilayered limestone is the transition from the bottom thick stratiform limestone to the stratiform limestone,indicating that the intensity of the transgression gradually weakened as each time the limestone deposits were deposited.From the bottom to the top of the whole Taiyuan Formation,the number of thin layers of limestone in Taiyuan Formation was more and more,and the clastic rock intercalation was thicker,which indicates that the overall hydrodynamic condition of Taiyuan Formation was weaker and weaker.That is to say,the intensity of transgression in Taiyuan formation was weaker and weaker.