青藏高原北缘上新世巨厚砾岩的同位素沉积年代及成因研究

Prodigious quantities of conglomerates were deposited into the surrounding areas of the Tibetan Plateau during the transition between Tertiary and Quaternary: to the northwest the Xiyu conglomerates; to the north the Yumen conglomerates; to the northeast the Jishi conglomerates; and to the east the Dayi conglomerates. While some scientists propose that the presence of these conglomerates are crucial evidence of uplift of the Tibetan Plateau starting from the end of Tertiary, some believe that formation of these conglomerates relates to global climate cooling. Up to date magnetostratigraphy is the only technique used to date the northern conglomerate strata, although the strata are either incompletely preserved or contain million-year-scale sedimentation hiatus. This project will pursue detailed field investigations of the Xiyu conglomerates, the Yumen conglomerates and the Jishi conglomerates. Further lab analyses will reveal the deposition environments, the provenance and deposition ages of these conglomerates together with their underlying deposits via sporo-pollen assemblage, detrital zircon U-Pb age distribution, Hf isotopic composition and conglomerate minerology, and cosmogenic nuclide burial dating, respectively. Through such systematic studies, we aim to decipher the origin of these conglomerates and understand the uplift history of the Tibetan Plateau and its influence on the global climate pattern.

在第三纪末与第四纪初交替之际，青藏高原周缘都发育有巨厚的砾岩，如：西域砾岩、玉门砾岩、积石砾岩、大邑砾岩等。一些学者把这些砾岩的存在看成是青藏高原从第三纪末开始强烈抬升的关键证据，还有一些学者认为这些砾岩的产生是全球气候变冷的结果。尽管青藏高原北缘相关的沉积地层要么存在沉积间断,要么剖面保存不完整，但迄今为止对西域砾岩、玉门砾岩和积石砾岩的定年还仅限于磁性地层学的研究工作。本项目将采用宇宙成因核素埋藏年龄法对青藏高原北缘的西域砾岩、玉门砾岩和积石砾岩及其下伏地层的顶部进行精确的同位素定年，利用碎屑锆石U-Pb年龄分布和Hf同位素组成以及砾岩岩性特征进行物源示踪，通过野外沉积学调查和沉积相分析以及沉积地层中孢粉的种属组合来解析砾岩的沉积环境，从而系统地认识这些砾岩的形成机制，为全面认识青藏高原的隆升历史及其对全球气候格局产生的影响提供新的证据。