Zircon U-Pb ages, Hf isotopes and significance of the late Archean-Paleoproterozoic granitoicls from the Wutai-Luliang terrain, North China
吕梁-五台地区晚太古宙—古元古代花岗质岩石锆石U-Pb年代学和Hf同位素性质及其地质意义

We obtained a zircon U-Pb age of 2509 7.4Ma for a gneissic K-rich granitic pluton from eastern Wutai terrain,North China.The newly obtained age data,along with previously published data,define a better age framework for the late Archean subduction-related magmatism in the Wuta-Luliang terrain.We report in this paper zircon U-Pb ages and zircon in situ Hf isotopic analytical data in order to trace the source of the late Archean-Paleoproterozoic granitoids and,particularly,to study the nature and mechanism of continental growth at that time.Our data suggest that the late Archean gneissic K-rich granitoids likely represent products of arc magmatism that was developed upon the North China craton.The post-collisional granitoids(ca.1.8 Ga)were genetically related to a process of extensional collapse of the thickened orogen,which triggered partial melting of lithospheric mantle and subsequently underplating of resultant basaltic magmas in the lower crust.Underplating of the basaltic magmas,in turn,induced partial melting of lower crustal material,generating granitic melts.The granitic melts subsequently mixed in varied proportions with the mantle-derived basaltic magma to produce the compositionally complicated post-collisional magmas(mainly granitic in composition)in the Wutai-Luliang terrain.Zircon Hf isotopic data reveal that crustal component of older than 2.7Ga(2.9Ga?)may exist in the Wutai- Luliang terrain,although zircon debris of this age have not yet been found.