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 patterns are steeply right-inclined. The Pb, Nd and Sr isotopic compositions are (206Pb/204Pb)T=18.531 to 18.838, (207Pb/204Pb)T=15.749 to 15.859, (208Pb/204Pb)T=37.138 to 37.917; (87Sr/86Sr)i=0.70852 to 0.71086; (143Nd/144Nd)i=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 conclusion that the north Qiangtang block belong the Yangtze block, i.e. there exists the Proterozoic crystalline basement below the north Qiangtang basin.