Geochemical characteristics and geological significance of chert nodules in Qixia formation,Pingding Mountain, lower Yangtze Plate

摘要

The Permian is an important period for the deposition of siliceous rocks on a global scale,but the genesis of chert is still controversial.To better understand the mechanism of chert nodules deposition from Qixia Formation(P1q)(Lower Yangtze Plate,China),we analyzed the major,trace and rare earth elements of these chert nodule samples(CN)and surrounding rock samples(SR)using Inductively Coupled Plasma Mass Spectrometry(ICPMS)and X-ray fluorescence(XRF)technique.The redox state,siliceous sources and depositional environment analysis show that CN have higher contents of SiO2,Fe2O3 and MnO comparing with SR,and their geochemical properties are different.Trace and rare earth elements characteristics reveal that nodules were deposited under the action of biochemistry in the basin,no terrigenous materials contamination was observed and the contribution of upwelling was emphasized.The Al/(Al+Fe+Mn),Fe/Ti and Al-Fe-Mn discrimination diagram indicated the chert nodules are influenced by hydrothermal action.Evidence from redox-sensitive elements such as Th,U,V and Sc suggests that nodules were deposited in a reducing environment.In the convective mixing of hydrothermal action and normal seawater,excessive dissolved silicon and nutrients are absorbed and precipitated by biological action,and then transported by upwelling to shallow water for deposition.All the evidence comes from the trace elements enrichment characters,Rare earth elements and Y(REY)distribution patterns and La/La^(*),Ce/Ce^(*),Eu/Eu^(*),LREE/HREE,Y/Ho and∑REE characteristics.