Geochemistry And Implication To Paleoclimate Of The Ancient Weathering Crust Between The Tieling Formation And The Xiamaling Formation Of The Mesoproterozoic Erathem In The North China Craton

The iron-rich ancient weathering crust is widely identified at the boundary between the Mesoproterozoic Tieling Formation and the Xiamaling Formation in the north of the North China Craton.According to the recent dated zircon U-Pb ages of the potassium-bentonites in the Tieling Formation and the Xiamaling Formation,the ancient weathering crust can be calibrated at?1400Ma.Because the ancient weathering crust is well preserved at the Zishikou section and the Jixian section,this article focuses mainly on these two sections.Based on the analyses of petrology and mineralogy,it can be refered that the ancident weathering crust is mainly composed of limonite-bearing ferric mudstone,siltstone and illite claystone.Analyses of geochemistry show that the major element oxides Al2O3,TiO2,Fe2O3,K2O enrich in different degrees compared to the Upper Continental Crust,while SiO2 leaches slightly and CaO,MgO,Na2O show a serious leaching.The enrichment of Fe2O3 indicates an oxidizing environment,and the leaching of CaO indicates a high temperature and humid climate condition.In the trace elements,Zr,Hf,Sc,Cu,Ga,In,Nb,Ta,W,Th,Pb,U,Rb,Cs and Bi are unchanged or moderately enriched.Mo,Cr,Ni,Sr,and Cd,are loss.The variation of Ba indicates the weathering and leaching degree of the Jixian weathering crust is greater than the Zishikou weathering crust.The variation of Co indicates the Jixian weathering crust is much more alkaline than the Zishikou weathering crust.Covariance graphs of stable trace elements indicate the weathering crusts are both multi-source,probably related with detrital rocks.In general,the total amount of REE enriches.The LREE enrich relatively.Ce shows positive anomaly at the Zishikou section because of the absorption of clay.Negative anomalies of Eu at Zishikou section and Ce and Eu at the Jixian section are all inherited from the original rock.Weathering parameter U/Th indicates a gradual increase of oxidative or weathering intensity from the bottom up;Fe3+/Fe2+ ratio indicates the ancient weathering crust is formed in the strong oxidation environment;Fractionation of REE indicates that the ancient weathering crust is formed under a high temperature and humid climatic condition;CIA index indicates the weathering crust suffered a moderate to intense weathering degree,and formed in hot and humid weather condition.Comparatively speaking,however,it seems that the Jixian section experienced more intense weathering degree than the Zishikou section.In conclusion,the ancient weathering crust at the top of the Zishikou section and the Jixian section is high clay content,moderate to strong leaching,red soil weathering crust with the removal of silicon and enrichment of aluminum and iron.It formed in a hot and humid tropical-subtropical paleoclimate.Hence it could be expected that,more comprehensive research on the current ancient weathering crust at the top of Tieding Formation,more valuable information of the interaction of the earth's perispheres would be obtained,including that of the lithosphere,the atmosphere and the hydrosphere,at about 1400Ma of the Mesoproterozoic Era.