南秦岭元古宙西乡群大陆溢流玄武岩的确定及其地质意义

THE CONFIRMATION OF CONTINENTAL FLOOD BASALT OF THE PROTEROZOIC XIXIANG GROUP IN THE SOUTH QINLING MOUNTAINS, AND ITS GEOLOGICAL IMPLICATIONS

中元古代西乡群火山岩是南秦岭元古宙最老的火山岩系,以其大面积分布、具浅海-陆相火山岩组合、属大陆拉斑玄武岩系列,具大陆板内火山岩岩石地球化学特征及与大的基性侵入体共生等特点,可确认其属大陆溢流玄武岩系。西乡群火山岩具有较高的值及低-中等的(^(87)Sr/^(87)Sr)_i值,可判定其源区为与洋岛玄武岩相似的软流圈地幔柱源,在岩石圈较大程度的拉张作用下,上隆的地幔柱减压熔融,产生岩浆。扬子克拉通北缘南秦岭广泛发育中-晚元古代大陆裂谷火山岩系,显示在中-晚元古代该区有大规模大陆张性拉伸作用,晚元古代晚期至早古生代,这种张性拉伸作用发展为陆壳碎裂直至形成洋盆。西乡群大陆溢流玄武岩与扬子克拉通北缘的破裂有密切关系。

The volcanic rock series of the Xixiang Group on the northern margin of the Yangtze craton dating 1451 - 987 Ma is probably the oldest one of Proterozoic volcanic series. This volcanic rock series belongs with continental flood basalts, with the following five characteristics. (1) the volcanic rock series consists of shallow-sea facies and continental facies volcanic rocks; (2) the 'source' magma series belongs probably to continental tholeiitic magma series; (3) the volcanic rock series possesses petrogeochemical features of continental intraplate volcanic rocks; (4) acid volcanic rocks are located in the upper part of the volcanic rock series; (5) the volcanic rocks appears to be associated with large and high-level basic layered intrusions. The basalts of the Xixiang Group characterized by low-middle 'Sr/86Sr ratios and high εNd values indicate that the basalts would be derived from asthenospheric plume, similar to the source of oceanic island basalts. Under the extension of the lithosphere, the upwelling upper mantle plume due to release of pressure melted thus producing magma. The emergence of the continental flood basalts indicates that in the Middle Proterozoic the tensional fractures began to take place in the South Qinling Mountains. Not only the Xixiang Group but also the Middle-Late Proterozoic Yunxi Group, Yaolinghe Group and Bikou Group volcanic rock series in the South Qinling Mountains belong to continental rift volcanic rock series. Therefore the authors deduce that during the Middle-Late Proterozoic the tensional fault occurred widely in the South Qinling Mountains. With the time progression the extension intensity increased, and at the end of the Late Proterozoic-Early Palaeozoic,break-up of the continental crust occurred, thus generating an oceanic basin. The authors suggest there is closely relationship between the Xixiang continental flood basalt and the continental-margin break-up on the northern margin of the Yangtze craton.