内蒙古北山地区黑红山一带斑状花岗闪长岩地球化学特征、LA-ICP-MS锆石U-Pb年龄及其地质意义

Geochemical characteristics and LA-ICP-MS zircon U-Pb age of porphyritic granodiorite in Heihongshan area of Beishan region, Inner Mongolia, and their geological significance

北山地区黑红山一带出露的晚石炭世斑状花岗闪长岩大地构造位置位于北山-天山造山系、额济纳-北山弧盆系,至今未见可靠的同位素年龄报道。在运用阴极发光技术对岩石中的锆石进行内部结构分析的基础上,利用LA-ICP-MS锆石UPb同位素原位定年方法获得206Pb/238U年龄加权平均值为312.1±3.2Ma。岩石化学研究显示,斑状花岗闪长岩属中钾钙碱性、准铝质-弱过铝质系列,具I型花岗岩特征;轻稀土元素富集,轻、重稀土元素分馏明显,稀土元素配分曲线右倾;微量元素不同程度地富集Rb、Th、K、Hf,亏损Ta、Nb、Ti、P等元素,显示其形成于板块碰撞前的火山弧环境。结合区域地质背景分析,红石山-百合山-蓬勃山蛇绿岩带代表的洋盆在晚石炭世尚未消亡。

The tectonic location of Late Carboniferous porphyritic granodiorite of Heihongshan area in Beishan region belongs to Beishan-Tianshan orogenic system and Ejina-Beishan arc basin system,but without reliable zircon U-Pb dating report.On the basis of the detailed internal structure analysis of zircons from the porphyritic granodiorite using the technology of cathodoluminescence,the authors conducted isotope age analysis using LA-ICP-MS zircon U-Pb dating method.The result shows that porphyritic grano-diorite formed at 312.1±3.2Ma.Geochemical study shows that porphyritic granodiorite belongs to medium potash-calc alkaline,qua-si-aluminous granite system,being I type granite.It has characteristics of light REE enrichment and characteristics of enrichment of Rb,Th,K,Hf and depletion of Ta,Nb,Ti,P.It formed in a pre-collision tectonic environment.Combined with regional geologi-cal background,It is considered that ocean basin,represented by Hongshishan-Baiheshan-Pengboshanophiolite,should not have been closed in Late Carboniferous period.