BIMODAL INTRUSIVE ROCKS IN XIAOHONGSHAN VANADIUM-TITANIUM MAGNETITE OREFIELD IN BEISHAN AREA OF INNER MONGOLIA: Petrology, Geochronology, Geochemistry and Geological Implication
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摘要:
对内蒙古北山地区小红山钒钛磁铁矿区内侵入岩的岩石学、锆石U-Pb年代学和全岩地球化学资研究显示，该岩体岩性为辉长岩和花岗岩，LA-ICP-MS锆石U-Pb测年结果表明成岩年龄分别为431.1±2.4 Ma和424.9±2.3 Ma，形成时代相近，属同一构造岩浆作用事件. 该套侵入岩的SiO2含量呈双峰式，稀土、微量元素特征表明它们具有岛弧岩浆岩地球化学特征，其源区可能受到了俯冲流体交代作用的影响. 地球化学特征指示小红山花岗岩源于下地壳物质在高温条件下的部分熔融，辉长岩为富集地幔部分熔融的产物. 结合区域地质背景及构造判别，认为小红山中晚志留世双峰式岩浆组合是北山洋南向俯冲诱导大陆边缘伸展环境下的产物.
Abstract:Study on the petrology, zircon U-Pb chronology and whole-rock geochemistry of intrusive rocks in Xiaohongshan vanadium-titanium magnetite orefield of Beishan area in Inner Mongolia shows that the lithology of the intrusion is gabbro and granite with the diagenetic age of 431.1±2.4 Ma and 424.9±2.3 Ma respectively. The two types of rocks were formed at a similar time of the same tectono-magmatism event. The SiO2 content in the intrusive rocks shows double peaks, and the characteristics of rare earth and trace elements indicate that they have the geochemical features of island-arc magmatic rocks, with the source area metasomatized by subduction-related fluid. The geochemical characteristics reflect Xiaohongshan granite is originated from the partial melting of lower crust materials at high temperature while the gabbros resulted from the partial melting of enriched mantle. Combined with the regional geological background and tectonic discrimination, it is concluded that the Middle and Late Silurian bimodal magmatic assemblage in Xiaohongshan area is the product of continental margin extension induced by the southward subduction of Beishan ocean.
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