About 2.5 Ga Tectono-Metamorphic Event in Southern Margin of North China Craton and Its Significance

Cao Zhengqi; Zhai Wenjian; Jiang Xingfu; Hu Zhengxiang; Zeng Zuoxun; Cai Yitao; Zhang Xiong; Xu Shaopeng; Guo Jungong

doi:10.3799/dqkx.2016.047

Abstract：Linshan Group,located in the southern margin of the central orogenic belt of the North China Craton,is a Precam-brian basement.It is important to study the collision era of the central orogenic belt and the distribution of the southern margin of the North China Craton.A study was carried out on the metabasite (plagioclase-amphibole schist)of the Linshan Group for the purpose of constraining the collisional time of the central orogenic belt and its distribution through the zircon U-Pb dating and rock geochemical methods.The results obtained show that most of zircon grains from plagioclase-amphibole schist are long columnar,euhedral to subhedral,and show typical core-rim structure with irregular shape,weak zoning,taxitic zoning and sector zoning structure.Some zircons preserve the inherited cores of the early magma.Zircon rims display characteristics of dis-solution texture with low Th/U ratios (most of them less than 0.09),however,zircons??cores show typical oscillatory zoning structure and relatively high Th/U ratios ranging from 0.10 to 9.24.The evidence suggests that the zircons of plagioclase-amphibole schist could be metamorphic origin.Relict cores and rims of the zircons from the plagioclase-amphibole schist yield ages of 2 528±43 Ma (MSWD=1.5,n =10)and 2 474±22 Ma (MSWD=1.3,n =21),respectively.The former data repre-sent residual age of protolith,and the latter indicate a circa 2.5 Ga metamorphic event in the central orogenic belt of the North China Craton.Whole-rock geochemical results show that metabasite samples from the Linshan Group have relatively low con-tents of SiO 2 (47.70%-50.77%,average=48.90%)and K2 O (0.56%- 1.78%,average= 1.13%),and are depleted in high field strength elements (eg.,Nb,Th,Ti and Zr)with flat REE patterns.Moreover,the geochemical samples have low content of Zr (less than 130×10 -6 )and ratios of Zr/Y (less than 4),indicating that the Linshan Group could have been formed in the subducted-related island arc environment.Therefore,the above evidence show that the central orogenic belt that formed by the collision orogenesis between eastern block and western block of the North China Craton experienced late Neoarchean to early Paleoproterozoic subduction-collisional event.