Discovery of Late Miocene volcanic rocks at Qitaidaban of Xinjiang-Tibet Highway，Chnia and its 40Ar-39Ar dating.

The basalt and trachydacite with about 10m in thickness are found overlying on the Late Triassic granite at Qitaidaban, about 10km east to the Xinjiang-Tibet Highway and 5600m above sea level. Geochemical data showed that the high-alkaline volcanic rocks are enriched in LILE and LREE, loss of HREE, middle negative Eu anomaly and high Sr, low Nd. They belong to shoshonitic series and may arise from a crust-mantle mixed layer. Determination of whole-rock 40Ar-39Ar plateau age is 8.27Ma±0.32Ma(900~1400℃, 39Ar cumulant 66%). The apatite fission track age of the granite after baking by the volcanic rocks is 7.9Ma±1.0Ma. The similar ages of volcanic rocks and the apatite track of granites indicate that the eruption of volcanic rocks occurred at about 8Ma in the Late Miocene. In addition, they have similar age to Dahongliutan volcanic rocks(7.97Ma±0.14Ma). The discovery of Qitaidaban volcanic rocks make the data relating to Late Cenozoic magmatic activity in the northwestern margin of the Qinghai-Tibet Plateau more plentiful and the results showing that the volcanic acitivity is frequent along Kangxiwa-Quanshuigou during Late Miocene-Pliocene, and indicate that volcanic activity and the movement of large-scale fault zone have very close relationship.