大兴安岭北段晚中生代碱性侵入岩岩石地球化学特征及其意义

The Late Mesozoic Alkaline Intrusive Rocks at the North of the Da Hinggan Mountains: Lithogeochemical Characteristics and Their Implications

大兴安岭北段晚中生代碱性侵入岩处于兴蒙海西造山带大地构造单元,有十分独特的地质意义。本文通过呼中区六个碱性岩体的主量、稀土和微量元素成分分析和综合研究,首次系统讨论了碱性侵入岩的岩石地球化学特征及其时空分布规律,并在此基础上较全面地探讨了碱性岩形成的大地构造环境、岩浆物质来源、性质和深度等地质问题。研究表明,不同时期的碱性岩岩石地球化学特征差异不明显:均属于过碱性的中酸性岩类,以相对贫Sr、Ba,高Rb,富集高场强元素为特征,稀土配分曲线呈特征的V字型,显示由弱变强的轻、重稀土元素分馏作用和逐渐变小的铕负异常。由此推断兴蒙造山带内岩浆来源较浅,而源区逐渐加深。碱性岩均形成于拉张环境;可能属于A型花岗岩,形成于非造山板内裂谷或热点地幔柱环境。

The Late Mesozoic alkaline intrusion at the north of the Da Hinggan Mountains consists of more than twenty alkaline intrusive rocks and passes through the Inner Mongolian Orogenic Zone （IMOZ）. This paper systematically studied the lithogeochemistry of the six typical complexes in this area for the first time. The results in dicated that these alkaline intrusive rocks have very similar characteristics. According to the bulk composition stud ies, these rocks belong to acid igneous protoliths with over saturated alkaline and are characterized by alkaline enrichment （i. e. high K content） , which reflects the rising acidity and alkalinity. The late Mesozoic rocks in this area were formed in a tectonic environment of extensional type. According to their REE characteristics, the rocks in the IMOZ are characterized by weakly fractionated REE distribution and significant negative Eu anomalies. The δEu values in the area are similar gradually indicated shallow source regions in the north section. The trace element studies showed that the alkaline intrusions are characterized by low content of Sr and ha, high content of RB, and relative depletion of the high field strength elements such as Nb, which suggests that the alkaline magma source is in the deep of the crust magma.