Geochemistry And Chronology Of Alkaline Rocks In Fengcheng,Liaodong And Their Geological Implications

The lithogeochemistry,isotope geochemistry and isotope chronology of Fengcheng alkaline rocks in the northern margin of the North China Craton has been systematically studied in this paper.Based on the above reserches,we discussed their magma sources,tectonic environment et al,providing some basic evidences for study on the nature of the early Mesozoic lithospheric mantle and tectonic evolution of the north of the NCC.The Fengcheng alkaline rocks are limited between 40°50?N and 41°10?N,which are distributed in Taizi River-Hun ancient hollow and Yingkou-Kuandian paleohighs junction part.Representative rocks include Saima,Gujia,Aiyang and Bolinchuan alkaline rocks.This paper mostly focuses on the Saima and Bolinchuan alkaline rocks.According to rock composition,Fengcheng alkaline rocks belong to intermediate rocks,with few acid rocks.The major rock types are aegirite nepheline syenite,alkaline syenite and nepheline syenite.The rocks are characterized by enrichment of alkalis,high K and poor Ca and Mg.Total REE concentration of the rocks is in the range of 29.8⁵083.4?g/g,with the mean value 858.77?g/g.All alkaline rocks in the Fengcheng are enriched in large ion lithophile elements?LILEs?and light rare earth elements?LREEs?,and depleted in high field strength elements?HFSEs?with significant negative Sr,Ti and Gd anomalies.The ¹⁷⁷Hf/¹⁷⁶Hf values of the studied Fengcheng alkaline rocks are 0.282260⁰.282520,and the ?Hf?t?values are-8.318^-16.851,with an average of-13.823.The magmatic crystallization temperature calculated by Ti zircon thermometry mainly lies between 649?and 877?,indicating the Fengcheng alkaline rocks diagenetic temperature was relatively high.A11 these major elements,rare earth elements,trace elements and isotopes suggest that Fengcheng alkaline rocks in the north margin of the NCC magma source are related to the enriched lithospheric mantle and suffered contamination of crustal components.LA-ICP-MS zircon U–Pb dating gives consistent ages of 221²26 Ma for these rocks,suggesting that they are coeval,which are consistent with the post-orogenic magmatism in Liaodong and the intraplate alkaline rocks in the northern NCC.Thus,the Fengchegn alkaline complex formed in a combined post-orogenic and intraplate extensional setting induced by the collapse of the Sulu Orogenic belt.It is also related to the post orogenic events in the North China craton.