羌塘中生代(T3-K1)盆地演化新模式

在羌塘盆地上三叠统那底岗日组陆相火山岩—沉火山碎屑岩及冲洪积相砂砾岩之下，作者发现了一个十分重要的古风化壳。该风化壳穿时超覆于肖茶卡组(T3上三叠统?)及其以下的二叠系及石炭系地层之上，沉积超覆于古风化壳之上的那底岗日组陆相火山岩—沉火山碎屑岩及冲洪积相砂砾岩代表了羌塘中生代(T3—K1，晚三叠世卡尼期至早白垩世时期)新一轮沉积作用的开启。采用SHRIMP锆石U-Pb同位素定年方法，作者在羌塘盆地胜利河地区和望湖岭地区分别获得了一组那底岗日组玻屑凝灰岩和晶屑凝灰岩的年龄，其值为216.8±2.1 Ma和217.3±2.5 Ma；这些同沉积年龄证据证实了羌塘中生代盆地的开启时间应该为晚三叠世卡尼—若利期。羌塘中生代盆地早期沉积作用经历了一个由陆相至海相的沉积超覆过程。伴随着岩浆侵入、火山爆发及火山碎屑沉积作用，沉积超覆作用是从冲洪积相开始。总体上，表现为一个向上由浅变深的海侵序列，显示为被动陆缘裂陷盆地特征。

A New Sedimentary Model for the Qiangtang Basin

The newly discovered paleoweathering crust sediments are developed beneath the continental volcanic rocks, sedimentary volcaniclastic rocks, and alluvial diluvial sandstones and conglomerates from the Upper Triassic Nadigangri Formation, and diachronously overlapped upon the Upper Triassic(?) Xiaochaka Formation and its underlying strata including the Carboniferous Permian strata in the Qiangtang Basin, indicating a sedimentary gap between the Nadigangri Formation and its underlying strata. These volcanic rocks, sedimentary volcaniclastic rocks and alluvial diluvial sandstones and conglomerates from the Nadigangri Formation deposited above the paleoweathering crust represent the onset of the Qiangtang Basin sediments during the Mesozoic. The SHRIMP zircon U Pb isotopic dating gives the ages of 219.5±2.1 Ma and 219±2 Ma for the basalt and granodiorite from central Qiangtang, and 216.8±2.1 Ma and 217.3±2.5 Ma for the vitric tuff and crystal tuff from the Nadigangri Formation in the Shenglihe and Wanghuling areas in the Qiangtang Basin. All these age determinations imply that the onset of the Mesozoic Qiangtang Basin should be traced back to the Late Triassic. The deposition went through the continental to marine sedimentary overlap during the early evolution of the Mesozoic Qiangtang Basin. The sedimentary overlap started from the alluvial diluvial facies and accompanied by magma intrusion, volcanic eruption and pyroclastic deposition. On the whole, the deposition displays a deepening upward transgressive sequence, characteristic of a passive marginal rift basin.