新藏公路奇台达坂晚中新世火山岩的发现及^40Ar-^39Ar定年

在新藏公路奇台达坂东约10km、海拔5600m的晚三叠世花岗岩之上发现厚约10m的玄武岩和粗面英安岩。地球化学数据显示，该火山岩高碱，富集大离子亲石元素和轻稀土元素，亏损重稀土元素，Eu中等负异常，高Sr，低Nd，属钾玄岩系列，可能源于壳幔混合层。测定全岩^40Ar-^39Ar坪年龄为8．27Ma±0．32Ma（900-1400℃，^39Ar累积释放量为66％），与受火山岩烘烤后花岗岩的磷灰石裂变径迹年龄（7．9Ma±1．0Ma）在误差范围内一致．表明该火山岩喷发于约8Ma的晚中新世，与相邻的大红柳滩火山岩的时代（7．97Ma±0．14Ma）~近。新藏公路奇台达坂晚中新世火山岩的发现丰富了青藏高原西北缘晚新生代岩浆活动的资料，表明在晚中新世-上新世康西瓦-泉水沟一带火山活动非常频繁。并显示火山活动与大型断裂带运动的关系非常密切。

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-mande mixed layer. Determination of whole-rock 4^40Ar-^39Ar plateau age is 8.27Ma± 0.32Ma （900-1400℃, 39Ar cumulant 66%）. The apatite fission track age of the granite atier 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-Quanshnigou during Late Miocene-Pliocene, and indicate that volcanic activity and the movement of large-scale fault zone have very close relationship.