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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。
In this paper, we systematically studied the petrology and geochemistry of basalts (~ 260 Ma) in the Chinamu area, Diyan, Inner Mongolia, in the eastern part of the Central Asia orogenic belt. Elemental and Sr-Nd isotopic geochemical studies show that the basalts in this area are characterized by enrichment of LREE and depleted HREE, showing obvious positive Eu anomalies with almost no Ce anomaly. The trace element composition shows the enrichment of LILEs (such as Rb, Th and K) and depleted HFSE (such as Nb, Ta, Zr and Hf). Meanwhile, the basalts in this area have higher Nb / Ta ratio (> 17), lower initial 87Sr / 86Sr ratios (0.7026-0.7033) and positive Nd (t) values (3.6-4.6). The comprehensive analysis shows that the basalts in the area have the geochemical characteristics of volcanic rocks in subduction zone, the source area is mainly depleted lithospheric mantle, and asthenospheric material is added locally. However, the degree of contamination of shell source material during the magmatic evolution is not high . By means of elemental discrimination diagrams and regional geological data analysis, we speculate that the formation of basalts in this area is related to the partial melting of asthenospheric mantle due to upwelling of asthenosphere under the post-arc extension environment. Therefore, the closure time of the Paleo-Asian Ocean should not be earlier than that of the basalt (~ 260 Ma), and the subduction of the paleo-Asia Ocean plate to the north of Siberia or southern Mongolia micro-mass occurs in the same period.