山东西部中生代青山群火山岩的地球化学特征及其岩石成因

Geochemistry and Petrogenesis of the Mesozoic Volcanic Rocks from the Qingshan Group in Western Shandong Province

以山东西部西董地区中生代青山群火山岩为研究对象,在进行系统的元素-同位素分析的基础上,探讨其岩浆源区和岩石成因,同时丰富该区域岩石地层的地球化学特征。西董青山群火山岩Si O2含量变化于52.8%~59.8%之间,主要为钙碱性系列岩石,岩石组合为玄武质粗面安山岩-玄武质安山岩-粗面安山岩,Mg O、TFe2O3、Mn O、Ti O2、Ca O与Si O2具有明显的负相关关系,Al2O3和Na2O与Si O2具有正相关关系;微量元素富集Rb、Ba、Th、U和K等大离子亲石元素,亏损Nb、Ta和Ti等高场强元素,稀土元素总量∑REE变化范围为100×10-6~187×10-6,(La/Yb)N比值范围为8.2~11.1,无明显的Eu异常(δEu=0.9~1.02);显示富集且均一的Sr-Nd同位素组成((87Sr/86Sr)t=0.7047~0.7050,εNd(t)=-16.5^-14.2)。西董青山群火山岩的Nd、Sr同位素组成与Si O2缺乏明显负相关关系,Nb/Ta与La/Nb之间不具有负相关关系以及Nb与Th之间具有正相关关系,指示岩浆在上升和演化过程中地壳混染作用对青山群火山岩岩浆化学组成的影响并不显著;火山岩Mg O含量明显高于玄武岩部分熔融形成的熔体,说明青山群中基性火山岩岩浆的形成过程可能与大陆地壳和地幔岩系之间的相互作用有关。综合分析表明,西董中生代青山群火山岩是大陆下地壳物质经拆沉作用于地幔环境中发生部分熔融,与上地幔岩石发生交代作用后进入地表形成。

With volcanic rocks of the Mesozoic Qingshan Group in the Xidong area in western Shangdong province as study objective, we carried out systematical analysis of element isotope geochemical analysis on the volcanic rocks to understand source area of magmatic rocks and their petrogenesis. This study will expand the geochemical features of rock sequences in this area. SiO2 content of Qingshan Group volcanic rocks ranges from 52.8% to 59.8%, suggesting they belong to calc alkaline series. The rock assemblage consists of basaltic trachyandesite to trachyandesite. SiO2 shows negative relation to MgO, TFe2O3, MnO and TiO2, but positive to Al2O3 and Na2O. Large ion lithophile elements such as Rb, Ba, Th, U and K are enriched while high field strength elements such as Nb, Ta and Ti are depleted. The total REE ranges from 100×10^-6to 187×10^-6, with （La/Yb）N =8.2-11.1 and no significant Eu anomaly （Eu=0.9-1.02）. The volcanic rocks have homogeneous and enriched Sr Nd isotopic compositions with （^87Sr/^86Sr）t ratios of 0.7047-0.7050 and Nd（t） values of -16.5--14.2. Nd and Sr isotopic compositions of Qingshan Group volcanic rocks in the Xidong area show no distinct negative relation to SiO2, Nb/Ta and La/Nb has no negative relation, and Nb and Th shows positive relation. All these features indicate that crust contamination had much influence on chemical compositions of magma of Qingshan Group volcanic rocks during the uprising and evolution process of magma. The fact that the volcanic rocks have higher MgO content than melt resulted from partial melting of basalt indicates that forming process of intermediate mafic volcanic rocks of the Qingshan Group was probably related to interaction between continental crust and mantle rock series. Integrated analysis indicates that the volocanic rocks of Mesozoic Qingshan Group in the Xidong area were the emplacement of partially melting continental lower curst materials onto the Earth surface, which were firstly metasomatized with upper mantle rocks in the mantle environment due to delamination.