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

报道了内蒙索仑缝合带附近的锡林浩特杂岩和苏左旗混杂带中蓝片岩块的锆石U-Pb年代学数据.锡林浩特杂岩中碎屑锆石来源复杂,从晚太古至晚古生代均有.最年轻的碎屑锆石U-Pb年龄为280～340Ma,与该区最年轻的弧岩浆岩年龄一致,表明该杂岩的原岩最终形成于晚古生代,而不是前寒武陆块残留.锡林浩特杂岩原岩的沉积源区主要是古生代的孤岩浆岩和部分裸露的前寒武陆壳,可能是弧前沉积建造.蓝片岩的锆石U-Pb年龄为318±5Ma(范围338～280Ma),这些锆石显示典型岩浆振荡环带结构,因此该年龄可能代表其原岩形成年龄,而其变质应该在280Ma之后.所以,锡林浩特杂岩和混杂带中蓝片岩块的变质变形作用可能发生在晚二叠,与索仑缝合带强烈的弧陆碰撞有关,暗示索仑缝合带的碰撞缝合时间在晚二叠,而不是许多人坚持的泥盆纪.

Zircon U-Pb ages of the Xilinhot metamorphic complex and blueschist, and implications for tectonic evolution of the Solonker suture

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.