内蒙古迪彦钦阿木地区260Ma玄武岩地球化学特征及其地质意义

Geochemical Characteristics and Geological Significance of 260 Ma Basalt from Inner Mongolia, China

本文对中亚造山带东段内蒙古迪彦钦阿木地区玄武岩（~260 Ma）开展了岩石学和地球化学的系统研究。元素和Sr-Nd同位素地球化学研究表明,该区玄武岩具有富集LREE和亏损HREE的特征,呈现出明显的Eu正异常,而几乎无Ce异常。微量元素组成表现出富集LILE（如Rb、Th和K）和亏损HFSE（如Nb、Ta、Zr和Hf）的特征。同时,该区玄武岩具有较高的Nb/Ta比值（〉17）,较低的初始87Sr/86Sr比值（0.7026~0.7033）,以及正的Nd（t）值（3.6~4.6）。综合分析表明,该区玄武岩具有俯冲带火山岩的地球化学特征,其源区主要为亏损的岩石圈地幔,并局部有软流圈物质的加入,但岩浆演化过程中壳源物质混染程度不高。通过元素判别图解及区域地质资料分析,我们推测该区玄武岩的形成与弧后伸展环境下软流圈物质上涌而导致岩石圈地幔的部分熔融有关。因此,古亚洲洋的闭合时限应该不早于该玄武岩的喷发时间（~260 Ma）,并且同期存在古亚洲洋板块向北方的西伯利亚或者南蒙古微陆块的俯冲。

Diyanqinamu basalt emplaced at ~260 Ma and located at the eastern part of Central Asian Orogenic Belt（CAOB）. Geochemical results show that these basalts are enriched in LREE and depleted in HREE with distinct positive Eu anomaly and minor/no Ce anomaly. In the view of trace elements composition normalized to primitive mantle, they are also enriched in LILES（such as Rb, Ba and Th） and depleted in HFSES（such as Nb, Ta, Zr and Hf）. In addition, they are consistent with relatively high Nb/Ta ratios（〉17）, low initial 87Sr/86 Sr ratios（0.7026―0.7032） and positive Nd（t）（3.6―4.5）. These geochemical signatures indicate that the basalts from Diyanqinamu district are similar to the volcanics distributed along the subduction belt of the world. Compared with regional geological information, however, the basalts most probably inherited the geochemical information from history subduction events and sourced from the depleted lithospheric mantle with some contribution of asthenosphere material while the crustal contamination is feeble. Element discrimination diagram and regional geological analysis indicate that the formation of basalts from Diyanqinamu are connected with the lithospheric mantle partial melting triggered by the upwelling of asthenosphere material under back-arc extension environment. Then, the closure of Paleo-Asian Ocean should be later than the eruption age of the basalts（~260 Ma）. Meanwhile, the paleo-Asian Ocean plate is subducting underneath the Siberian Plate or South Mongolia Microplate.