柴达木北缘德令哈地区基性麻粒岩的发现及其形成时代

Discovery of basic granulite and its formation age in Delingha area, North Qaidam Monutains.

德令哈地区的基性麻粒岩呈透镜状或脉状分布于长英质片麻岩和泥质片麻岩中。基性麻粒岩的主要矿物组成为紫苏辉石、单斜辉石、斜长石、角闪石 ,含有少量的钛铁矿、锆石等副矿物。单斜辉石中具有针状的斜方辉石和钛铁矿的出熔片晶 ,代表了高温条件下的出熔结构。采用不同方法的二辉石温度计 ,估算出其麻粒岩相的温度为 72 0～ 92 5℃。基性麻粒岩的微量及稀土元素分析显示其原岩的形成环境可能为大陆边缘环境。全岩及矿物的 Sm- Nd等时线年龄为 1791± 37Ma,代表了麻粒岩相的变质时代。基性麻粒岩及其围岩的 Nd模式年龄显示出这一地区可能有太古宙的地壳物质存在。

The basic granulite from the Delingha area, Northern Qaidam Mts., occurs as lens or vein within quratz feldspathic gneisses. It consists of hypersthene, clinopyroxene, amphibole and plagioclase with minor ilmenite and zircon. Exsolution lamellaes of orthopyroxene and ilmenite within clinopyroxene, which represent exsolution texture under high temperature, can be observed. Using two pyroxene geothermometers of different methods, the temperature of 720～925℃ were estimated. The trace element, rare earth element and Sm Nd isotopic data suggest that the basic granulite have protolith feature drived from continental margin or continental rift. The Sm Nd isotopic data yield a whole rock pyroxene amphibole plagioclase isochron of 1791±37Ma age which is interpreted as the granulitic facies metamorphic age of the basic granulite. Nd medol ages of the basic granulite and gneiss indicate that there might be some Archean crustal materials. within basement in this area.