青藏高原东北缘祁连山三叠系砂岩碎屑锆石U-Pb定年及其物源分析

Detrital Zircon U-Pb Dating and Provenance Analysis for the Triassic Sandstone in Qilianshan Orogen,NE Margin of Tibetan Plateau

三叠系沉积物广泛覆盖青藏高原东北缘,其中松潘—甘孜地区三叠系的沉积物得到了较系统的研究,但是青藏高原北缘的祁连山三叠系盆地的研究却较为缺乏。为了丰富相关研究和揭示区域构造演化的特点,通过古水流方向统计、砂岩中碎屑矿物统计和碎屑锆石U-Pb测年等方法对祁连山三叠纪盆地物源进行系统研究。结果表明,祁连山三叠系盆地的古流向主要有南东向、正南向、南西向,物源来自岩浆弧和大规模褶皱造山作用的混合区。祁连山三叠系砂岩中的碎屑锆石的年龄谱主要峰值集中在250~290 Ma、360~460 Ma、1600~2000 Ma和2200~2600 Ma这4个年龄段。通过对比分析华北板块、华南板块中和秦祁昆中央造山带中岩浆锆石年龄谱特征可知:1600~2000 Ma和2200~2600 Ma年龄段的锆石来自华北板块,360~460 Ma年龄段的锆石来自北祁连造山带,250~290 Ma年龄段的锆石来自东昆仑的火山岛弧。此外,600~1000 Ma年龄段锆石很少,这些锆石来自扬子板块,表明在三叠纪扬子克拉通和华北克拉通发生碰撞形成了秦岭造山带,阻断了来自扬子克拉通的物源。

Triassic detrital sediments occurring in the northern margin of the Tibetan Plateau,which has been well studied in the Songpan-Ganzi terrane,yet the Triassic basin in the Qilianshan remain poorly understood.In this contribution,we integrated stratigraphy,sedimentology,zircon U-Pb dating on the Triassic sandstones in the Qilianshan to decipher their complex regional tectonic evolution.The results show that the ages of detrital zircons concentrate at four populations in 250-290 Ma,360-460 Ma,1,600-2,000 Ma,and 2,200-2,600 Ma.By comparing with the zircon age spectra in the North China Craton,South China Craton,and the Qinling-Qilian-Kunlun Central Orogenic belt,we suggest that the 1,600-2,000 Ma and 2,200-2,600 Ma zircon age populations were probably sourced from North China Craton,whereas a zircon age population of 360-460 Ma was derived from the North Qilian Orogenic Belt.The 250-290 Ma zircon population may have sourced from the volcanic arc of Eastern Kunlun Orogen in the south.The rare 600-1,000 Ma zircons were probably sourced from the Yangtze Block,since the Triassic North China-Yangtze collision along the Qinling Orogen may have acted as a barrier for the sediments from the Yangtze Block.