阿尔泰造山带喀纳斯群变质岩地球化学特征及形成环境

Geochemical characteristics and formation environments of metamorphic rocks in the Kanas Group of Altay tectonic belt

阿尔泰造山带喀纳斯群变质岩的地球化学研究表明,该套变质岩的原岩主要为泥质沉积岩,个别为砂质沉积岩,源区物质主要为中—酸性大陆岛弧火山岩,并夹杂少量其他碎屑物。尽管16个样品间岩性有差异,但其化学蚀变指数(CIA)几乎全部小于75,成分变异指数(ICV)接近或大于1,指示源区物质绝大多数经历了温暖、湿润条件下的中等化学风化作用,且成熟度较差,为活动大陆边缘首次沉积的产物。该套变质岩中的La、Ce、∑REE、δEu、LREE/HREE的值显示其原岩主体形成于大陆岛弧背景。其地球化学特征表明,喀纳斯群变质岩很可能沉积于活动大陆边缘的大陆岛弧相关环境,推测喀纳斯群发生沉积作用与变质作用的时间在(550±18)Ma至430 Ma之间;并根据相关对应时间,推测喀纳斯群变质期次为加里东期。

The geochemical study of metamorphic rocks of the Kanas Group in Altay tectonic belt shows that the metamorphic rocks are mainly argillaceous sedimentary rocks,with small amounts of sandy sedimentary rocks,Source material is mainly of acidic continental arc volcanic rocks mixed with a small amount of other debris.Although there are differences in lithology between16samples,the chemical etching of the variable indexes(CIA)is almost all less than75,indexes of compositional variation(ICV)are close to or higher than1,indicating that the source region must have experienced a warm,humid conditions of moderate chemical weathering and the maturity was poor,being the first deposition product of active continental margin.The La,Ce,ΣREE,LREE/HREE,sigma delta Eu values of the metamorphic rocks indicate that the protolith formed in a continental arc setting.These geochemical characteristics suggest that the Kanas Group metamorphic rocks probably deposited on an active continental margin of continental island arc environment.It is inferred that Kanas Group sedimentation and metamorphism occurred between550±18ma to430Ma and,according to the corresponding time,the metamorphism of the Kanas Group took place in Caledonian.