青藏高原北羌塘盆地上三叠统那底岗日组火山岩的地球化学特征及其意义

北羌塘盆地南缘的晚三叠世那底岗日组火山岩主要由英安岩、流纹岩、凝灰岩等组成。其主量元素具有高SiO2（66．58%-80．90％）、低TiO2（0．12％-0．42％）的特征，属钙碱性系列（σ平均为1．245）；微量元素表现为大离子亲石元素K、Rb、Ba及不相容元素Th的高度富集和高场强元素Nb、Ta、Ti的亏损；稀土元素（La／Yb）N〉10，Eu亏损较明显（δEu=0．53-43．88），配分曲线右倾、较陡；Pb、Nd、Sr同位素组成为（^206Nd／^204Nb）T=18．531-18．838、（^207Pb／^204Pb）T=15．749-15．859、（^308Pb／。^204）T=37．138-37．917、（^87Sr／^86Sr）i=0．70852-0．71086、（^143Nd／^144Nd）。=0．511779-43．511896、εNa（t=208Ma）=-9．3--11．6。上述岩石地球化学特征表明，那底岗日和石水河的那底岗日组火山岩的岩浆源区为上地壳；同时利用同位素地球化学数据证明了北羌塘地块归属扬子陆块群的论断，即北羌塘盆地存在元古宙结晶基底的信息。

Geochemical characteristics of volcanic rocks of the Upper Triassic Nadi Kangri Formation in the north Qiangtang basin,Qinghai-Tibet Plateau

The volcanic rocks of the Late Triassic Nadi Kangri Formation on the southern margin of the north Qiangtang basin consist predominantly of dacite, rhyolite and tuff, with a zircon U-Pb age of 208 Ma. They have high SiO2 （66.58-80.90%） and low TiO2 （0.12-0.42%）, belonging to the calc-alkali series （σ=1.245）. In terms of the trace elements, they exhibit strong enrichment in the large ion lithophile elements K, Rb and Ba and the incompatible element Th and depletion in the high field strength elements Nb, Ta and Ti; in terms of REE, they show （La/Yb）N〉10 and relatively strong Eu depletion with δEu=0.53-0.88, and the REE distribution pat- terns are steeply right-inclined. The Pb, Nd and Sr isotopic compositions are .（^208Pb/^204Pb）v=18.531 to 18.838, （^207Pb/^204Pb）v=15.749 to 15.859, （^208Pb/^204Pb）v=37.138 to 37.917; ^87Sr/^86Sr）,=0.70852 to 0.71086; （^143Nd/^144Nd）,=0.511779 to 0.511896, with εNd （t=208 Ma）= -9.3 to -11.6. Therefore, the above-mentioned geochemical characteristics indicate that the magma source of the volcanic rocks of the Nadi Kangri Formation at Nadi Kangri and Shishuihe was upper crust. Additionally, the isotopic geochemical data verify the conclnsion that the north Qiangtang block belong the Yangtze block, i.e. there exists the Proterozoic crystalline basement below the north Qiangtang basin.