内蒙古北山石板井地区晚古生代中酸性侵入杂岩LA-ICP-MS年龄和地球化学特征及其地质意义

The LA-ICP-MS Zircon U-Pb ages and geochemical characteristics of Late Paleozoic intermediate—acidic intrusive complexes in Shibanjing area, Beishan Mountains, Inner Mongolia, and their geological significance

花岗岩的时代和地球化学特征可以揭示岩浆源、形成环境和构造演化。近期我们在从事内蒙古黑鹰山地区矿产地质调查和相关科研工作过程中,在石板井地区发现一套中泥盆世中酸性侵入杂岩,为了揭示它们的岩浆源、形成环境和构造演化,我们在野外地质调查研究的基础上,开展了锆石U-Pb定年、岩石元素地球化学的系统测定。研究表明:细粒辉石闪长岩-中粒花岗闪长岩-中细粒花岗闪长岩-中粒黑云母二长花岗岩形成时代为中泥盆世(385~380 Ma),具有同源岩浆演化特征。其中,细粒辉石闪长岩具有钙碱性、钠质、准铝质岩石属性;中粒黑云母二长花岗岩-中粒花岗闪长岩-中细粒花岗闪长岩具有高钾钙碱性-钙碱性、钾质-钠质、过铝质-准铝质岩石属性;初始岩浆可能是俯冲板片流体交代的地幔楔,地幔楔部分熔融形成玄武质岩浆,玄武质岩浆底侵而导致上覆年轻地壳的部分熔融的产物,在此过程中伴随着分离结晶作用;中泥盆世中酸性侵入杂岩具有I型花岗岩的特征,其形成于大洋岛弧背景下的火山弧环境,可能是小黄山洋向南俯冲的产物。

Objectives: The magmatic source,forming environment and tectonic setting were revealed by the age and geochemical characteristics of granite. Recently,during the geological survey and related scientific research work in the Shibanjing area of Inner Mongolia,we identified a set of Middle Devonian intermediate-acidic intrusive complexes in the Shibanjing area. In this study,we discuss the petrogenesis,forming environment and tectonic setting of intemediate-acidic intrusive complexes.Methods: Based on field work,through the microscope,zircon U-Pb dating,whole-rock major and trace element geochemistry were carried out.Results: The results show that the fine-grained augite diorite,medium-grained granodiorite,medium to finegrained granodiorite and medium-grained biotite monzogranite formed in the Middle Devonian( 385 ~ 380 Ma),and they have homologous magma evolution characteristics. The fine-grained pyroxenite diorite belongs to the calcalkaline,sodic,peraluminous series. The medium-grained granodiorite,medium to fine-grained granodiorite and medium-grained biotite monzogranite characterized by the high-K calc-alkaline to calc-alkaline, sodic,metaluminous-peraluminous series.Conclusions: The parent magma has been considered to be connected with the mantle wedge was metasomatized by the subducted plate fluid,and the partial melting of the mantle wedge forms basalt magma. The basalt magma underplating during the partial melting of lower crust. In the process,it is accompanied by the crystallization and differentiation of magma. The Middle Devonian intermediate-acidic intrusive complexes have the characteristics of type I granite,and it is volcanic arc environment formed in the background of the Oceanic island arc,the diagenetic environment may belongs to the southward subduction under the background of the Xiaohuangshan Ocean.