Abstract: By dating detrital zircon U-Pb ages of deposition sequence in foreland basins, we can analyze the provenance of these zircons and further infer the tectonic history of the mountain belts. This is a new direction of the zircon U-Pb chronology. The precondition of using this method is that we have to have all-around understanding to the U-Pb ages of the rocks of the orogenic belts, while the varied topography, high altitude of the zircon U-Pb ages of the orogenic belts are very rare and uneven. This restricts the application of this method. Modern river deposits contain abundant geologic information of their provenances, so we can probe the zircon U-Pb ages of the geological bodies in the provenances by dating the detrital zircon U-Pb ages of modern rivers' deposits. We collected modern river deposits of 14 main rivers draining from Pamir, South Tian Shan and their convergence zone and conducted detrital zircon U-Pb dating. Combining with the massive bed rock zircon U-Pb ages of the magmatic rocks and the detrital zircon U-Pb ages of the modern fluvial deposit of other authors, we obtained the distribution characteristics of zircon U-Pb ages of different tectonic blocks of Pamir and South Tian Shan. Overlaying on the regional geological map, we pointed out the specific provenance geological bodies of different U-Pb age populations and speculated the existence of some new geological bodies. The results show that different tectonic blocks have different age peaks. The main age peaks of South Tian Shan are 270~289Ma and 428~449Ma, that of North Pamir are 205~224Ma and 448~477Ma, Central Pamir 36~40Ma, and South Pamir 80~82Ma and 102~106Ma. The Pamir syntaxis locates at the west end of the India-Eurasia collision zone. The northern boundary of the Pamir is the Main Pamir Thrust(MPT)and the Pamir Front Thrust(PFT). In the Cenozoic, because of the squeezing action of the India Plate, the Pamir thrust a lot toward the north and the internal terranes of the Pamir strongly uplifted. For the far-field effect of the India-Eurasia collision, the Tian Shan on the north margin of the Tarim Basin also uplifted intensely during this period. Extensive exhumation went along with these upliftings. The material of the exhumation was transported to the foreland basin by rivers, which formed the very thick Cenozoic deposition sequence. These age peaks can be used as characteristic ages to recognize these tectonic blocks. These results lay a solid foundation for tracing the convergence process of Pamir and South Tian Shan in Cenozoic with the help of detrital zircon U-Pb ages of sediments in the foreland basin.

Key words: detrital zircon U-Pb dating, Pamir, South Tian Shan, convergence zone, modern river deposit

摘要： 现代河流沉积物携带了其物源区丰富的地质信息，通过对现代河流沉积物进行碎屑锆石U-Pb测年，可以了解其物源区地质体中锆石U-Pb年龄的组成。文中对帕米尔、南天山及其会聚带14条主干河流采集的现代河沙样品进行了碎屑锆石U-Pb测年，结合前人在该区域的锆石U-Pb测年结果，确定了帕米尔、南天山不同构造单元的碎屑锆石U-Pb年龄特征；结合区域地质图，揭示了不同年龄峰可能的物源地质体，并推测了一些新的地质体的存在。结果显示，该区域不同构造带具有明显不同的锆石U-Pb年龄峰：南天山构造带的主要年龄峰为270~289Ma和428~449Ma，北帕米尔构造带的主要年龄峰为205~224Ma和448~477Ma，中帕米尔构造带的主要年龄峰为36~40Ma，南帕米尔构造带的主要年龄峰为80~82Ma和102~106Ma。这些年龄峰可以作为识别上述构造单元的特征年龄峰，为利用前陆盆地沉积地层碎屑锆石U-Pb年龄来示踪帕米尔和南天山在新生代的会聚过程提供了基础数据。

关键词: 碎屑锆石U-Pb测年, 帕米尔, 南天山, 会聚带, 现代河流沉积物