太行山—大兴安岭东麓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 East Foot of the Taihang-Da Hinggan Mountains: Lithogeochemical Characteristics and Their Implications

太行山—大兴安岭东麓晚中生代碱性侵入岩带由20余个碱性岩体组成,南北纵跨华北克拉通和兴蒙海西造山带两个大地构造单元,具有十分独特的地质意义。本文通过分布于不同大地构造单元的7个典型碱性岩体中各类碱性岩的主量、稀土和微量元素成分分析和综合研究,首次系统地讨论了该碱性岩带岩石地球化学特征及其时空分布规律,并在此基础上对该带碱性岩形成的大地构造环境、岩浆物质来源性质和深度的变化规律等重大地质问题开展了较全面的新探讨。分布于不同的大地构造单元的碱性岩岩石地球化学特征具有非常明显的差别,但呈现一定的空间变化规律:均属于铝过饱和的酸性岩类,以富碱高钾,贫钙、铁、镁为特征。空间上由南至北,岩石酸度呈增高的趋势,碱度随酸度增高而增高,而Al、Ca、Fe和Mg含量则呈降低的变化趋势。华北克拉通区碱性岩轻、重稀土分馏作用明显,但太行山南段岩石基本上无铕异常,而北段则以明显的铕负异常为特征,高Sr、Ba,贫Rb,普遍亏损高场强元素Nb、Ta、Zr、Hf;而兴蒙海西造山带内岩石稀土配分曲线呈特征的V字型,显示出较弱的轻、重稀土分馏作用和显著铕负异常;以贫Sr、Ba,高Rb,富集高场强元素为特征。由此推断,华北克拉通碱性岩浆来源可能与富集地幔物质关系密切,源区较深,而兴蒙造山带内岩浆来源则与富集地幔物质关系不大,源区相对较浅,该带碱性岩岩浆源区深度由南至北逐渐由深而浅。此外,认为该带碱性岩均形成于拉张环境。可能属于A1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 intrusive belt at the east foot of the Taihang-Da Hinggan Mountains, which consists of more than twenty alkaline intrusives, passes through two tectonic units, namely, the North China Craton (NCC) and the Inner Mongolian Orogenic Zone (IMOZ). The authors made for the first time a systematic lithogeochemical study of seven typical complexes along the belt. The results indicate that the alkaline intrusives in different tectonic units have obviously different characteristics, which can be used to reveal their tectonic settings, different source region features and depths. According to the bulk compositions, these rocks belong to acid igneous protoliths with over-saturated aluminum, characterized by the enrichment of alkaline, high K, and low Ca, Fe and Mg content, which reflects the rising acidity and alkalinity and the decreasing Al, Ca, Fe and Mg content of rocks from south to north. The tectonic environment of the late Mesozoic rocks in this belt is mainly of the extensional type. According to REE characteristics, the rocks in the NCC are all characterized by strongly depleted heavy rare elements (HREE), those in south Taihang area are noted for the absence of Eu anomalies, representing a deep magma source, and those in north Taihang area have clear negative Eu anomalies. The rocks in the IMOZ are characterized by weakly fractionated REE and distinct negative Eu anomalies. The δEu values in the belt gradually decrease from south to north, which shows that the source region in the north section is shallower than that in the south one. As for the trace element features, the alkaline intrusives in the NCC are characterized by high Sr, high Ba, low Rb, and relative depletion of the high field strength elements such as Nb, Ta, Zr and Hf, but things are just the opposite for those in the IMOZ, suggesting that the alkaline magma source in the NCC is related to the enriched mantle.