The tectonic setting of early Permian bimodal volcanism in central Inner Mongolia: continental rift, post-collisional extension, or active continental margin?

ABSTRACTThe petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.

摘要：本文针对中国内蒙古中部西乌珠穆沁旗南部后头庙地区的早二叠世火山岩开展岩石学、年代学与地球化学研究，旨在阐明该区域早二叠世的构造背景。该套火山岩与砂岩互层，兼具镁铁质与长英质组分，表现出双峰式特征。锆石U-Pb定年结果显示，火山岩形成于274~278 Ma，与邻区双峰式岩浆作用的年龄范围一致。镁铁质岩石由拉斑玄武岩、玄武安山岩、玄武粗安岩及粗安岩组成，富集Th、U及大离子亲石元素（large ion lithophile elements, LILEs），亏损高场强元素（high field strength elements, HFSEs）Nb、Ta与Ti，ε_Nd(t)值为+3.6~+7.9，均为正值。地球化学分析表明，镁铁质岩石起源于受交代作用改造的岩石圈地幔。长英质火山岩以流纹岩为主，辅以少量粗面岩与英安岩；其主量元素演化趋势、球粒陨石标准化稀土元素配分模式及同位素组成均与镁铁质火山岩存在显著差异，排除了镁铁质熔体分离结晶形成长英质岩石的成因机制。长英质岩石的ε_Nd(t)值与区域内石炭纪宝力达多弧岩相近，据此认为二叠纪长英质熔体起源于石炭纪新生弧相关岩石的部分熔融。通过与裂谷、碰撞后构造背景、弧后盆地以及美国盆地与山脉省等典型构造环境下的新生代双峰式火山岩进行对比，发现内蒙古中部的双峰式火山岩与弧后盆地及美国盆地与山脉省的火山岩具有相似的岩石学与地球化学特征。综合区域地质资料分析可知，研究区早二叠世双峰式火山岩形成于晚石炭世俯冲-增生作用之后的西伯利亚古板块伸展大陆边缘环境。