Clay Mineral Characteristics Of Permian Carbonate Rocks In Zhen'an Area,Shaanxi Province And Its Paleoclimate Significance

Permian is a key period in geological history,with the specific studies of Permian climate are scarce,so it's of great significance to further the study of paleoclimate.During Permian,the study area located in Zhen'an area in Shanxi Province today,where Permian stratum is continuously developed and huge-thickly accumulated,was an isolated platform in Eastern Paleo-Tethys.Clay minerals,as an important indicator of paleoclimate,were selected to recovery paleoclimate.In this paper,we analyze the compositions and relative contents of Permian clay minerals of Zhen'an area to explore the characteristics and evolution of Permian paleoclimate,we also compare the plaeoclimate with that in South China,Paleo-Tethys and the rest of the world to broaden its significance.We carried out specific field measurement and systemic sampling on the Permian Xikou section in Zhen'an,Shaanxi,with total length of 7300 m and total thickness of 3134 m.The corresponding lithostratigraphic units are Sanlichong Formation,Shimengya Formation and Yazi Formation at Lower Permian,Wulipo Formation and Shuixiakou Formation at Middle Permian,and Xikou Formation,Yundoutan Formation and Longdongchuan Formation at Upper Permian.272 samples were collected and 172 representative carbonate rock samples were picked out for the qualitative and semiquantitative analysis of clay mineral.The main steps of the experiment of analyzing clay minerals include crushing samples,removing carbonate and organic matter and clay mineral extraction.Each clay mineral sample was prepared into three oriented slices: air drying oriented slices(N),ethylene glycol treatment oriented slices(T)and heat treatment oriented slices(550?),and three X-ray diagrams were then measured by XRD for the analysis and semiquantitative calculations of the clay minerals.The analysis shows that clay minerals of the study area were comprised of Illite(70.9%),Kaolinite(7.9%),Montmorillonite(4.4%),Chlorite(8.6%)and Illite-smectite mixed layer(8.3%)during Permian.The values of Kübler are almost greater than 0.25??° before calibration and are all greater than 0.42??° after calibration,indicating the crystallinity of clay mineral is poor and the clay mineral has a low metamorphic degree and clay minerals can be used to restore the paleoclimate under which they formed.Based on the characteristics of clay mineral assemblages in the study area,the relative content of illite was chosen as the index to restore the paleoclimate.The high content of illite indicates that the climate was cold and dry,and contrarily the climate was warm and humid.According to the change of illite's relative content,Permian palaeoclimate fluctuations in the study area can be divided into 8 phases: P1,from Asselian to middle Kungurian,was a dry-cold period;During the middle of Kungurian to middle Capitanian,the climate fluctuated from warm-wet period(P2)to dry-cold period(P3),and to the alternation of dry-colding and warm-weting(P4);P5,from middle Capitanian to early Wuchiapingian,was a cooling period;a short warm-wet period happened in early Wuchiapingian(P6);an increasingly cooling period from early Wuchiapingian to the end of Wuchiapingian(P7);and an alternation of dry-cold and warm-wet climate in Changhsingian(P8).The contrastive studies of plaeoclimate among the study area,South China,Western Europe(Western Palaeo-Tethys)and Japan(Panthalassa)show that the change of Permian palaeoclimate in Zhen'an area were influenced by the Late Paleozoic Ice Age(LPIA),Kamura Event and G-LB biological extinction event.In addition,a short cold period(~2.4 Ma)during Roadian is identified by this study,which is different from previous views that the climate was gradually warming up from Kungurian onwards.This cold period is just located in third ice age(P3),suggesting the broader meaning of the contrast.