The Xilingele complex from the eastern part of the Central Asian-Mongolia Orogenic Belt, China: Products of Early Variscan orogeny other than ancient block: Evidence from zircon SHRIMP U-Pb ages
中亚-蒙古造山带东段的锡林郭勒杂岩：早华力西期造山作用的产物而非古老陆块 —— 锆石SHRIMP U-Pb年代学证据

The Xilingele complex is one of the maximal series of metamorphic rocks outcropped in the Paleozoic folded region along the northern margin of the North China Craton, has been ever considered as a Precambrian block before. In this work, we acquired zircon SHRIMP U-Pb data of the complex in order to gain the forming ages of their protoliths and metamorphism ages to constrain the orogenic processes of the eastern part of the Central Asian-Mongolia Orogenic Belt. The results are as follows: Most zircon grains from the paragneiss were formed in magma, their weighted mean 206Pb/238U age is 406±7Ma, taken to represent the emplacement age of the source rock (arc-related granite?) and the lower limited age of the sedimentary rock. The magmagene zircon grains from the granitic gneiss yield a single, coherent population with a weighted mean 206Pb/238U age of 382±2Ma, taken to represent the emplacement age of its protolith. The metamorphic age of the paragneiss and the granitic gneiss, dating from the metamorphic overgrowth rims of zircon grains, is 337±6Ma. This metamorphism might be related with an major collission in the Hougenshan suture. Taken together, it is suggested that the Xilingele complex is the products of Early Variscan magmatism, sedimentation and metamorphism other than an ancient block. The duration of the Xilingele complex formed is only about 70Ma, which may imply that the Xilingele complex developed at fore-arc environment. The tectonic pattern before collision in the eastern part of the Central Asian-Mongolia Orogenic Belt might be not a typical ocean with poly-islands. Owing to the absence of ancient block, the orogenic process is more possibly a continentized ocean, i.e., arc islands formed by subduction within ocean, then convergence of arc islands along the passive continental margin and developed a new continent.