内蒙古北山地区蒜井子一带晚奥陶世花岗岩地球化学特征及构造环境分析

Geochemical characteristics and tectonic environment of Late Ordovician intrusive rocks in Suanjingzi,Beishan area,Inner Mongolia

本文报道了北山造山带蒜井子晚奥陶世花岗岩的LA-ICP-MS锆石U-Pb年龄与全岩地球化学,以探讨其形成时代、岩石成因及构造环境。蒜井子二长花岗岩LA-ICP-MS锆石U-Pb定年结果显示其形成于456.3±3.5 Ma。英云闪长岩具有低的SiO_(2)(61.99%~66.62%)、ALK(4.66%~5.16%)、K_(2)O/Na_(2)O(0.52~0.64)和A/CNK为0.89~1.05,而二长花岗岩具有高的SiO_(2)(71.93%~73.20%)、ALK(6.44%~6.89%)、K_(2)O(K_(2)O/Na_(2)O为1.32~1.94)和A/CNK(1.04~1.05),低的A/NK(1.49~1.64)。该岩体富集轻稀土(LREEs)和大离子亲石元素(LILE)Rb、Th、K,亏损重稀土元素(HREEs)和高场强元素Nb、Ta、Ti,具有弱的负Eu异常。上述特征表明,研究区晚奥陶世花岗岩具有准铝质-弱过铝质钙碱性岩石的特征,属于高分异Ⅰ型花岗岩,岩浆源区以壳源物质为主,有少量幔源物质的加入,形成的构造环境为岛弧。结合区域地质背景,认为北山洋在456.3±3.5Ma之前已开始向北俯冲。

In this paper,LA-ICP-MS zircon U-Pb age and whole rock geochemistry of Suanjingzi late Ordovician granite in Beishan orogenic belt are reported to discuss its formation age,petrogenesis and tectonic setting.LAICP-MS zircon U-Pb data from the monzogranite show that it was formed at 456.3±3.5Ma.The tonalite have low SiO_(2),ALK,K_(2)O/Na_(2)O and A/CNK,ranging from 61.99% to 66.62%,4.66% to 5.16%,0.52 to 0.64,and 0.89 to 1.05,respectively,while the monzogranite have higher SiO_(2)(71.93%~73.20%),ALK(6.44%~6.89%),K_(2)O(K_(2)O/Na_(2)O 1.32~1.94)and A/CNK(1.04~1.05).These rocks are enriched in LREEs and LILE(e.g.,Rb,Th and K),while depleted in HREEs and HFSEs(e.g.,Nb,Ta and Ti),with weak negative Eu anomaly.These above characteristics show that the rocks belong to metaluminium-weak peraluminous calc-alkaline high differentiation I-type granite,and their primary magma is dominated by crust-derived materials with a small amount of mantlederived materials,formed under the tectonic setting of island arc in late Ordovician.Combining with regional geologic background,we propose that the Beishan Ocean have began to subduct northward before 456.3±3.5 Ma.