The isotopic compositions of basement in the Qilian fold belt and their geological implication

摘要

1 IntroductionThe Qilian fold belt is the important segment of Central Orogenic System (COS) of China. In past decades , previous studies of the Qilian orogenic belt have focussed mainly on Phanerozoic rocks, absence of studies Precambrian basements(Xu et al., 1994; Xia et al., 1996; Feng et al., 1996). Precambrian metamorphic rocks are widespread in the central-Qilian blocks of the Qilian fold belt, eg "Huangyuan Group; Hualong Group; Maxianshan Group" et, these metamorphic rocks are important science significance of the formation of the Qilianshan fold belt, whereas the debate on Precambrian basements in the Qilian fold belt, as to: ① the Neoarchean or Paleoproterozoic Huangyuan Group (BGMRQH, 1991), but Guo et al (2000) meterage zircon U-Pb ages are 900 Ma of the Huangyuan Group. ② Wan et al (2003) meterage the zircon U-Pb ages are (750±30) Ma of the potassium feldspar granite in the Hualong Group consider it formed from 800 Ma to 600Ma, nevertheless, Xu et al (2007) studies of zircon U-Pb ages of clastic rocks and the granitic gneiss in the Hualong Group, the data is 891-875Ma.