内蒙古扎鲁特旗地区晚侏罗世火山岩岩石学及地球化学特征

Petrological and Geochemical Characteristics of Late Jurassic Volcanic Rocks in Zhalute Area,Eastern Inner Mongolia

为了探讨内蒙古扎鲁特旗地区晚侏罗世火山岩成因及其构造背景,在详细的野外地质调查基础上,从岩石学、岩石地球化学、稀土元素和微量元素特征方面对其进行了研究。结果表明,研究区晚侏罗世火山岩为中性—酸性的火山熔岩和火山碎屑岩组合。化学成分显示其以安山岩、粗面安山岩、粗面(英安)岩和流纹岩为主,属于高钾钙-碱性系列岩石,并富碱、高铝。稀土元素总量变化大,轻重稀土元素分馏明显,配分曲线右倾,具Eu负异常;富集大离子亲石元素Rb、Ba、K、Th和LREE,亏损高场强元素Ta、Nb、Ti、P,基性相容组分Cr、Co、Ni质量分数和Mg#较低,显示出岩浆来源于下地壳,并混染了上地壳物质;w(SiO2)与大多数常量元素(Na2O、K2O、TiO2、TFeO、MgO、CaO、Al2O3和P2O5)质量分数具有很好的相关性,反映同源岩浆的演化特征和岩浆结晶分异演化的趋势。综合研究表明,火山岩形成于张性构造环境,与蒙古—鄂霍茨克洋闭合造山之后因重力坍塌岩石圈伸展减薄作用有关。

In order to discuss the genesis and tectonic setting of Late Jurassic volcanic rock in Zhalute area,the authors studied it from petrology,rock geochemistry,rare earth elements and trace element characteristics on the basis of detailed field of geological investigation.The results show that the Late Jurassic volcanic rocks in Zhalute area are a combination of neutral to acid volcanic lava and volcaniclastic rock.Chemical composition shows Late Jurassic volcanic rocks is mainly andesite,trachyandensite,trachyte and rhyolite that belong to high-K calc-alkaline series.The negative Eu anomalies and right-wing distribution curve owned by all REE demonstrate the characteristics of LREE enrichment and HREE loss.In trace elements they are enriched in lithophile elements Rb,Ba,K,Th and LREE,depleted in high field strength elements Ta,Nb,Ti,and P,and have less basic compatible components（Cr,Co,Ni）and Mg#,which show the magma derived from the lower crust and was blended in the upper crust.While the good correlation of SiO2 and most macroelement（Na2O,K2 O,TiO2,TFeO,MgO,CaO,Al2O3 and P2O5）reflects the evolution characteristics of comagma and evolution trend of fractional crystallization of magma.Comprehensive analysis shows that the Mesozoic volcanic rocks formed in an extensional environment,which should be the result of lithospheric extension after the closure of Mongol-Okhotsk Ocean.