藏东南地区早白垩世的安第斯型弧火

Early Cretaceous Andean type arc volcanics in SE Xizang, China

首次在藏东南发现一套早白垩世的陆相火山岩， 称雅则火山岩， 全岩 Ｒｂ Ｓｒ 等时线年龄１０７４ Ｍａ。其主体是高钾的安山岩系， 以强亏损高场强元素、富集大离子亲石元素和轻稀土、 Ｌａ／ Ｎｂ 比值 ＞ ２ 为特征， 属活动大陆边缘火山岩； 微量元素等指示它产出在安第斯型陆缘环境。其南延即滇西南的腾冲铁窑山大松坡消减型花岗岩带 （１００～１１２ Ｍａ）， 反映新特提斯洋在早白垩世有过一次重要的消减过程；晚白垩世的花岗岩具不同的岩石组合和成矿特征，记录了消减后的陆弧碰撞事件。

The Early Cretaceous andesitic volcanic rocks are firstly discovered in Yaze to the south of Ranwu, Basu County, SE Xizang (Tibet), with a whole rock Rb Sr isochron age of 107 4±5 2 Ma The field observations reveal that the volcanic lava erupted in a continental environment, and the synchronous deposits east of it named as the Duoni Formation, are red beds of continental facies The Yaze volcanic rocks, consisting mainly of high Potassium andesitic series, are characterized with high depleted HFSE, rich in LILE and LREE, and La/Nb ratios > 2, which indicate the tectonic setting being a convergent plate boundary or an active continental margin The diagrams of SiO 2 vs K 2O, K +/(K ++Na +)vs (K ++Fe +3 )/(K ++Na ++Fe +3 +Fe +2 ), La vs K 2O and La vs P 2O 5 confirm further that the Yaze volcanic rocks represented an arc volcanic belt of Andean type It is well known that the Neotethyan oceans subducted in Late Cretaceous,evidenced by the Gandise arc,were closed in Early Tertiary The recent researches in SE Xizang and SW Yunnan demonstrate that the Gandise magmatic belt must be a composite one, consisting of granitoids in ages of Early Cretaceous, Late Cretaceous and Tertiary The Yaze volcanic rocks and related granitoids in SE Xizang can be correlaed to the Tengcong Tieyaoshan Dasongpo granitic belt in SW Yunnan (in age of 100～112Ma), standing for a giant active continental margin of Andean type The Late Cretaceous granitoids should be a geological record of continent arc collision followed by the subduction with rock association, geochemical features and mineralization being different from those of Early Cretaceous The Early Cretaceous magmatic arc comprehends that the Neotethyan evolution must have been a long and complicated process with multi generations of subduction, and the Yanshanian movement in the area of subduction related orogeny