滇西高黎贡变质岩带南段淡色花岗岩脉年代学特征及构造意义

Geochronological characterization and regional tectonic implication of the leucogranites in the southern segment of Gaoligong metamorphic zone,western Yunnan

高黎贡构造带作为藏东三江地区大地构造上的一条重要构造带,在印度-欧亚板块碰撞过程中起着重要的调节作用,并被认为是该地区发生"大陆逃逸"与地块旋转的西边界。本文以高黎贡构造带中的深变质岩为研究对象,通过对高黎贡变质岩带南段进行详细地构造解析,发现其内部广泛发育一系列或顺岩石面理、或切割面理分布的淡色花岗岩脉;并选取了5个淡色花岗岩脉样品进行锆石U-Pb测年,这5个样品的锆石^(206)Pb/^(238)U年龄值主要分布于二个区间:68~40Ma和24~21Ma,并在45Ma左右形成一个巨大的峰值。基于锆石的晶形、晶内结构和微量元素组成特征等方面进行锆石成因学分析,认为本文所测试的5个淡色花岗岩脉样品的锆石为深熔作用过程中形成的锆石;从而推测高黎贡变质岩带南段发生深熔作用并形成淡色花岗岩脉的时代主要集中于68~40Ma和24~21Ma。

As one of the significant tectonic belts in the southeastern Tibet and Three River-Indochina area, the Gaoligong belt plays important roles in accommodating the Indian-Eurasian plate collision and the subsequent post-collisional evolution, and is regarded as the western boundary of the southeastward extruded and rotated block in the ＂extrusion model＂. Based on detailed macro- and micro-structural observations in the high-grade metamorphic rocks of Gaoligong belt, it can be found that leucogranites are widely distributed within the metamorphic rocks and are parallel to, or cross-cut foliations in the host rocks at different scales. In this paper, five samples of leucogranites were chosen for LA-ICP-MS zircon U-Pb dating, giving two ranges of 206Pb/238U ages：68～40Ma and 24～21Ma respectively, and forming a peak at about 45Ma. According to analyses of shape, internal structure and in-situ trace element compositions of zircons, we interpret those zircons of leucogranites are anatectic in origin and grew during partial melting of the Gaoligong metamorphic rocks. Therefore, it can be inferred that the formation of the leucogranites related to anatexis mainly occurred during 68～40Ma and 24～21Ma.