Zircon U-Pb ages of the Xilinhot metamorphic complex and blueschist, and implications for tectonic evolution of the Solonker suture
锡林浩特杂岩和蓝片岩的锆石U-Pb年代学及其对索仑缝合带演化的意义

We report the zircon U-Pb age data for the Xlinhot metamorphic complex and the blueschist block from a mélange belt (south Sonidzuoqi) along the Solonker suture zone, Inner Mongolia. Detrital zircons from the Xilinhot complex have complicated origins, with U-Pb ages ranging from Late Archean to Late Paleozoic times. A few zircon grains show U-Pb age in the range 280~340Ma, identical to those of the youngest arc magmas from Sonidzuoqi and the northern north China craton. This suggests that formation of the protolith of the Xilinhot complex was finally completed in the Late Paleozoic, rather than in Precambrian times. The protolith of the Xilinhot complex probably represented forearc sediments.Zircons from the blueschist show U-Pb ages in the range 338~280Ma, with a mean 206Pb/238U age of 318±5Ma.These zircons show oscillatory and sector-zoning, typical of magmatic origin; thus the age probably represents the formation age of the protolith of bluschist, and the metamorphism could occur later than 280Ma. Therefore, the Xilinhot complex and blueschist formed likely in Late Permian times, related to the collision between arc series and continental margin, implying that the suturing of the Slonker suture could have happened in the Late Permian, rather than in Devonian as previously suggested by many others.