长江流域现代沉积物碎屑锆石U-Pb年龄物源探讨

PROVENANCE STUDY BY U-Pb DATING OF THE DETRITAL ZORCONS IN THE YANGTZE RIVER

长江流域上游到入海口共10个现代沉积物样品碎屑锆石U-Pb年龄分析表明,200～300、400～500、750～850和1800～2000Ma这4组年龄是长江流域沉积物U-Pb年龄的主要组成,出现在大多数样品中,其中又以200～300、750～850Ma占主体。10个样品中大多数锆石颗粒Th/U>0.3,说明这些锆石多是岩浆成因的锆石。所有样品锆石年龄的重叠度和相似度都在0.6以上,说明长江流域现代沉积物在不同河段,其锆石U-Pb年龄相近。通过与长江流域可能物源区羌塘地块、义敦岛弧、松潘甘孜复理石盆地、扬子克拉通、秦岭造山带、大别造山带几个构造单元锆石U-Pb年龄对比发现,除了羌塘地块和义顿岛弧之外,其他几个构造单元几乎都包括200～300、400～500、750～850和1800～2000Ma这4组年龄,因此,由锆石U-Pb年龄判别不出具体的物源供给。在青藏高原东部的红河和湄公河沉积物中反映出与长江流域相似的U-Pb年龄谱,从而进一步证实由于长江流域可能物源区锆石U-Pb年龄的相似性,单一碎屑锆石U-Pb年龄方法不能把各个物源区区分开来,有必要与其他物源分析方法相结合。

Four major U-Pb age groups of detrital zircons are recognized in the modern Yangtze River sediments. They are 200-300 Ma, 400-500 Ma, 750-850 Ma and 1 800-2 000 Ma respectively with 200-300 Ma and 750-850 Ma dominating. Most grains studied are granitic zircons with the Th/U over 0.3. The overlap and similarity analysis of all the samples are in an excess of 0.6. It means that all the samples have the similar U-Pb ages. By comparison of the U-Pb age patterns with the potential source areas of Qiangtang Block, Yidun Volcanic Arc, Songpan-Ganze basin, Yangtze Craton, Qinling orogen and Dabie orogen, it is found that the four age groups mentioned above appeared in almost all the regions except the Qiangtang Block and Yidun Volcanic Arc, hence it is hard to distinguish the final source by U-Pb dating. Meanwhile, the U-Pb age patterns of the Red River and Mekong River sediments are close to those of the Yangtze River, suggesting that the U-Pb dating of detrital zircons can not work well in the Yangtze River as there are no exclusive age indicators. Other methods should be used for seeking conclusions.