扬子陆块西缘陆良地区牛头山组火山岩的厘定及对Rodinia超大陆演化规律的指示

滇中地区位于扬子陆块西缘, 发育有新元古代火山岩, 是开展Rodinia超大陆演化研究的重要窗口。其中牛头山组是滇中地区重要的新元古代地层, 由于缺乏精确的年代学依据, 严重影响了区域地层格架的划分, 并制约了对Rodinia超大陆构造演化时限的认识。本文对牛头山组的流纹质玻屑凝灰岩开展了岩相学、锆石U-Pb年代学、锆石原位Hf同位素、锆石微量元素等方面的研究, 结果显示凝灰岩均由火山物质组成, 锆石具明显岩浆震荡环带和自形程度高等特征; 凝灰岩的锆石U-Pb年龄为817.9±7.7Ma、818.3±8.3Ma和822±13Ma, 锆石ε_Hf(t)值兼具正值和负值, 岩浆源区伴有地幔物质的混入, 及主要与古元古代(1523.9~2107Ma)陆壳物质的熔融有关; 锆石研究显示凝灰岩形成于岩浆弧, 属陆壳锆石类。岩石地球化学表明, 凝灰岩属硅质火山岩类, 具弧火山岩性质。综合研究认为, 牛头山组是华夏陆块向扬子陆块俯冲末期的沉积-岩浆响应, 形成于扬子陆块西缘的弧后盆地环境, 该弧后盆地可能是华南新元古代康滇裂谷的初期阶段。牛头山组沉积作用的结束, 指示了扬子陆块与华夏陆块已完成汇聚, Rodinia超大陆可能形成了统一板块, 同时Rodinia超大陆开始向裂解转换, 及转换时间约为820Ma。

Determination of volcanic rocks of Niutoushan Formation in Luliang area,western margin of Yangtze block and its indication to the evolution law of Rodinia supercontinent

Located at the western margin of the Yangtze Block, the central Yunnan region has Neoproterozoic volcanic rocks, which is an important window to study the evolution of the Rodinia supercontinent. The Niutoushan Formation is an important Neoproterozoic stratum in the central Yunnan region. Due to the lack of accurate geochronology, the division of the regional stratigraphic framework and the tectonic evolution of the Rodinia supercontinent had been seriously restricted. In this paper, the petrography, as well as zircon U-Pb geochronology, in-situ Hf isotope, and trace elements of the rhyolite tuff from the Niutoushan Formation have been studied. The results show that the tuff is composed of volcanic materials, and the zircons without grinding have obvious magmatic oscillating zones. The zircon U-Pb ages of tuff are 817.9±7.7Ma, 818.3±8.3Ma, and 822±13Ma, and εHf(t) values are both positive and negative. The magma source is associated with the mixing of mantle material, and mainly related to the melting of Paleoproterozoic continental crust material (1523.9~2107Ma). Zircon studies show that tuff was formed in a magmatic arc and belongs to the continental crust. The petrogeochemistry shows that the tuff belongs to siliceous volcanic rocks and has arc volcanic properties. It is concluded that the Niutoushan Formation is a sedimental-magmatic response to the subduction process of the Cathaysia block to the Yangtze block. It was formed in the back-arc basin environment of the western margin of the Yangtze block, and this back-arc basin might be the early stage of the Kangdian rift in Neoproterozoic South China. The end of the Niutoushan Formation sedimentation indicated that the Yangtze and Cathaysia blocks had converged, and the Rodinia supercontinent might have formed a unified plate. At the same time, the Rodinia supercontinent began to break up and the transition time was about 820Ma.