内蒙古苏尼特左旗南两类花岗岩同位素年代学及其构造意义

Subduction- and Collision-related Granitoids from Southern Sonidzuoqi, Inner Mongolia: Isotopic Ages and Tectonic Implications

内蒙古北部索伦缝合带(索伦-苏尼特左旗-锡林浩特)被多数中外学者认为是西伯利亚板块南缘和华北板块北缘的最终缝合带。本文选择该缝合带上苏尼特左旗南两类花岗岩——与俯冲有关和与碰撞有关的岩浆岩(分别叫弧岩浆岩和碰撞花岗岩)进行同位素年代学研究。结果表明:①弧岩浆活动有两期,分别约为490Ma和310Ma(锆石U-Pb,SHRIMP);碰撞花岗岩的侵位年代在230～250Ma(Rb-Sr全岩和锆石U-Pb);②根据本文新的年代学数据,索伦缝合带的最终缝合时间可能是在230～310Ma,这显然不同于国内多数学者坚持的“晚泥盆世碰撞”模式;而Sengor等推测的“晚二叠世碰撞”模式与本文数据一致。

The Solonker zone (Solonker-Sonidzuoqi-Xilinhot) of northern Inner Mongolia is thought by most authors to represent the suture between the southern margin of the Siberian plate and the northern margin of the North China plate. The subduction- and collision-related granitoids that occur along the suture zone in southern Sonidzuoqi are selected for isotopic dating. Two conclusions are achieved: (1) Two phases of arc magmatism happened in the area, at about 490 Ma and 310 Ma, revealed by zircon U-Pb SHRIMP method; the collisional granites were emplaced at the time interval of 230 - 250 Ma (Rb-Sr whole-rock and zircon U-Pb); (2) Final collision and suturing between the two plates is supposed to happen during the period of 230~310 Ma based on our new data. This does not support the 'late Devonian collision' model advocated by most local geologists, rather, is consistent with a 'late Permian collision' model proposed by Sengor et al. (1993).