内蒙古狼山地区晚志留世石英闪长岩的发现及其地质意义

The discovery of Late Silurian quartz diorite in Langshan area of Inner Mongolia and its geological implications

内蒙古狼山山脉西侧分布零星的早古生代岩浆岩,晚志留世石英闪长岩主要出露于哈日音熬瑞一带。该岩体岩性为石英闪长岩,块状构造,LA-ICP-MS锆石U-Pb年龄显示,其206Pb/238U年龄加权平均值为419.0±4.0~413.3±1.8Ma。岩石暗色矿物为黑云母,富铁(4.27%~4.65%)、富钠(4.17%~4.45%),富集大离子亲石元素Sr、K、Pb,不同程度亏损高场强元素P、Ti、Nb,轻稀土元素富集,重稀土元素亏损,弱负Eu异常,这些特征与弧岩浆岩的地球化学特征相似;La/Ba-La/Nb图解显示俯冲交代地幔的源区特点;岩石负的εHf(t)值(-12.1^-8.8)及较大的TDM2值(1981~2362Ma)反映了古元古代古老地壳熔融的特点。花岗岩类构造判别图解及区域地质资料显示岩体形成于弧陆拼贴的构造背景。对比白乃庙地区新发现的晚志留世弧陆拼贴的石英闪长岩,两者的岩石地球化学特征及同位素特征基本一致,因此,华北地块北缘晚志留世弧陆拼贴作用向西延伸至狼山地区。

The Early Paleozoic magmatites are sporadically distributed in the west of Langshan Mountain of Inner Mongolia, whereas the Late Silurian quartz diorite is mainly exposed in the zone of Hariyinaorui. LA-ICP-MS zircon U-Pb dating shows that the weighted average 206Pb/238U age of quartz diorite is 419.0±4.0~413.3±1.8Ma. The characteristics of quartz diorite mainly exhibit that the biotite is the main dark mineral, which is characterized by enrichment of TFeO (4.27%~4.65%) and Na2O (4.17%~4.45%), enrichment of LILE (Sr, K, Pb), depletion of P, Ti and Nb, enrichment of LREE, and depletion of HREE with weak negative Eu anomaly, similar to the characteristics of arc magmatite. La/Ba-La/Nb determinant diagram shows the characteristics of metasomatic mantle, and isotopic characteristics reflect the characteristics of melting of ancient continental crust which has an obvious negative εHf(t) value (-12.1^-8.8) and older TDM2 value (1981~2362Ma). Tectonic discrimination diagrams and regional geological data show the pluton probably formed the tectonic setting of arc-continent collision. A comparative study of recently published relevant data of Late Silurian quartz diorite in the Bainaimiao area shows that the geochemical characteristics are basically the same, and, therefore, arc-continent collision of Late Silurian extended westward to Langshan area on the northern margin of North China Craton.