滇西南晚古生代火山岩与裂谷作用及区域构造演化

特提斯构造东南带的滇西南地区发育三个系列的晚古生代火山岩:碱性橄榄玄武岩系列,大陆拉斑玄武岩系列和类似MORB拉斑玄武岩系列。地质、地球化学特征反映它们可能是保山—掸邦地块东缘昌宁—孟连晚古生代裂谷(局部向初始洋盆转化),而不大可能是宽阔洋底和洋岛的火山作用产物。逐渐增强的前进式裂谷作用伴随陆壳的减薄(局部分离,洋壳诞生)和软流圈顶面的抬升,可能导致不同深度地幔产生不同程度熔融作用,形成本区三个系列岩浆。地幔对流可能引导陆缘裂谷、洋壳扩张、俯冲、微陆块碰撞以及岩石圈深部剪切作用,制约区域晚古生代至中生代早期的构造岩浆演化。

Late Paleozoic Rifting-related Volcanic Rocks and Tectonic Evolution in Southwestern Yunnan

Three different series of Carboniferous--Permlan volcanic rocks are reco-gnized in southwestern Yunnan of the eastern Tethyan orogenic belt, i. e.plateau tholeiitic, alkaline basaltic and MORB--like rocks. Geochemical studiessupport the understanding that the volcanic rocks are associated with thedevelopment of the Changning--Menglian rift basin dn the eastern side of theBaoshan--Shan State terrane, and are unlikely to represent rocks produced inocean floor and oceanic island environments. It is inferred that, with thethinning of the continental crust and the local emplacement of the oceaniccrust, this region experienced progressive rifting. As a result of different de-grees of melting at different depths of the mantle, three different sorts ofmagma were formed and then evolved into three series of volcanic rocks. Theclosure of the rift basin was related to the westward subduction of the Lancangjiang oc?an beneath the Baoshan--Shan State terrane, as is evidenced by the arcvolcanic suite of the Lincang--Jinghong belt east of the Changning--Menglianbelt. The convective mantle and the change in convection patterns are pre--sumed to have been responsible for the rifting, spreading of the initial coeaniccrust and collision of terrance.