内蒙古希仁布敦钾玄质火山岩锆石U-Pb年龄及构造环境

Zircon U-Pb age and tectonic setting of the shoshonitic volcanic rocks in the Xirenbudun area,Inner Mongolia

内蒙古希仁布敦地区钾玄质火山岩出露于贺根山缝合带梅劳特乌拉-迪彦庙蛇绿混杂岩带内,岩性为玄武粗安岩和粗安岩,研究其形成时代、岩石成因和构造环境,对探讨贺根山缝合带构造演化具有重要启示。粗安岩锆石LA-ICP-MS U-Pb同位素测年获得^(206)Pb/^(238)U年龄加权平均值为176.3±0.8 Ma,形成时代为早侏罗世。岩石相对富碱Na_(2)O+K_(2)O(5.28%~7.19%,>5%)、高Al2O3(16.13%~17.42%)、高K_(2)O/Na_(2)O值(0.66~1.14,>0.5)、高Fe_(2)O_(3)/FeO值(0.82~1.50,>0.5)、低TiO_(2)(0.91%~1.16%,<1.3%),富集Th、U、Sr等大离子亲石元素和轻稀土元素,亏损Nb、Ta、Ti等高场强元素。稀土元素含量为168.53×10^(-6)~233.03×10^(-6),稀土元素配分曲线为右倾式分布。岩石学和岩石地球化学特征表明,希仁布敦地区玄武粗安岩-粗安岩为后造山钾玄质岩石,与古亚洲洋俯冲作用有关,形成于古亚洲洋俯冲板片后撤-断离-后造山伸展构造环境。晚古生代末古亚洲洋俯冲洋壳+俯冲深积物析出流体交代上覆地幔楔,形成贺根山缝合带富集地幔,中生代早期古亚洲洋俯冲板片断离-后造山伸展作用诱发富集地幔部分熔融,产生该区钾玄质岩浆。

The shoshonitic volcanic rocks in the Xirenbudun area of Inner Mongolia occur in the Meilaotewula-Diyanmiao ophiolitic melange belt of the Hegenshan suture zone.The volcanic rocks include basaltic trachyandesite and trachyandesite.The study of the formation age,petrogenesis,and tectonic setting in the volcanic rocks has important implications for the tectonic evolution of the Hegenshan suture zone.The zircon LA-ICP-MS U-Pb dating for the trachyandesite yielded a weighted mean ^(206)Pb/^(238)U age of 176.3±0.8 Ma,suggesting that its formation age is Early Jurassic.The volcanic rocks are rich in Na_(2)O+K_(2)O(5.28%~7.19%,>5%),Al2O3(16.13%~17.42%)and high K_(2)O/Na_(2)O ratio(0.66~1.14,>0.5),Fe_(2)O_(3)/FeO ratio(0.82~1.50,>0.5)but low in TiO_(2)(0.91%~1.16%,<1.3%).The rocks are obviously enriched in Th,U,Sr and LREEs,and depleted in Nb,Ta and Ti,with remarkably negative Nb,Ta,P and Ti anomalies on primitive mantle-normalized incompatible element patterns.The contents of rare earth elements range from 168.53×10^(-6) to 233.03×10^(-6).The chondrite normalized REE distribution patterns are of right-inclined shape.The basaltic trachyandesite-trachyandesite belong to post orogenic shoshonitic rocks,were related to the subduction of the Palaeo-Asian Ocean(PAO),and formed in the subsequent subducted slab roll back-break off-post orogenic extension tectonic setting.In the end of Late Paleozoic,the fluids dehydrated from the subducted oceanic crust+sediments of the PAO metasomatized the overlying mantle wedge to form the enriched mantle of the Hegenshan suture.In the Early Mesozoic,the PAO subducted slab break off-the post orogenic extension triggered partial melting of the enriched mantle to produce the shoshonitic magma in this area.