辽吉古元古代活动带中变质富铝沉积岩系地球化学特征及其对物源的制约

Geochemistry of the metamorphic Al-rich sedimentary series and its constraint on provenance in the Paleoproterozoic Liao-Ji active belt

辽吉活动带分布巨量沉积岩系,不仅记录了自身物源信息,也对该带古元古代构造演化过程具有重要指示意义。选择活动带沉积岩系中最具代表性、成分成熟度较低的变质富铝碎屑岩样品进行主量、微量和稀土元素分析,结果表明,变质富铝碎屑岩的砂岩类型判别结果为硬砂岩、页岩,较低的CIA指数、中低等的风化程度及一致的稀土与微量元素配分形式,指示其物源单一,具有相对活动背景下快速堆积的特点,不存在明显的沉积再旋回。物源分析、构造环境判别及微量元素比值,反映富铝碎屑岩物质源为一套形成于大陆岛弧或活动陆缘构造背景下的长英质岩石。结合前人对活动带中变质沉积岩系碎屑锆石的研究成果,认为辽吉活动带中富铝沉积岩系主要物源的形成时代为古元古代,这意味着作为富铝碎屑岩物源区的古元古代地质体具有大陆岛弧或活动大陆边缘的构造属性,而辽吉活动带最终的闭合可能与活动陆缘背景下弧-陆碰撞有关。

Liao-Ji active belt contains various types of rocks and is characterized by widely distributed sedimentary rocks which can record material source information and provide significant evidence of Paleoproterozoic tectonic evolution of the Liao-Ji active belt.In this study,representative low compositional maturity sedimentary rock samples were collected from the active belt and a detailed research on geochemistry was carried out.It is shown by discriminant diagram that the sandstone types of he metamorphic Al-rich clastic rocks are greywacke and shale.Low CIA index,low to mid-degree weathering condition and the consistent rare earth and trace element distribution indicate that all the samples were from the same source which underwent rapid accumulation in an active background with no obvious sedimentary cycle.The geochemical features of the Al-rich clastic rock reveal that the material source was felsic rock formed in a continental island arc or an active continental margin tectonic setting.In combination with previous zircon age obtained in the study area,it is concluded that material source of the Al-rich sedimentary series was formed in Paleoproterozoic,implying that the Paleoproterozoic terrane which served as the material source of the Al-rich clastic rock had tectonic setting features of continental island arc or active continental margin,and that the closing process of the Liao-Ji active belt was probably associated with arc continental collision in an active continental margin tectonic setting.