大兴安岭中段索伦地区满克头鄂博组火山岩年龄、地球化学特征及其构造环境

摘要

对大兴安岭中段索伦地区满克头鄂博组火山岩进行了岩石地球化学、锆石U-Pb及Hf同位素研究,讨论该地区火山岩的形成时代和构造背景.用LA-ICP-MS测得索伦地区满克头鄂博组2个流纹岩样品的锆石206pb/238U年龄分别为138.0±1.5Ma和139.9±2.5Ma,表明火山岩的形成时代是早白垩世早期,而非晚侏罗世;地球化学数据显示其具有高硅、富碱、弱过铝质-过铝质(A/CNK=0.99～1.34)、分异程度高的特点,属于高钾钙碱性系列,相对富集轻稀土元素和部分大离子亲石元素(Rb、Th、U、K),亏损高场强元素(Nb、Ta、Ti、P).锆石Hf同位素组成表明,该区满克头鄂博组火山岩具有较高的εHf(t)值(+4.06～+8.24),二阶段模式年龄为936～664Ma,暗示其可能起源于从亏损地幔增生的年轻地壳物质的部分熔融.结合区域地质资料分析,满克头部博组火山岩形成于造山后伸展环境,可能与蒙古-鄂霍茨克洋闭合后造山带的垮塌密切相关.%In this study,the authors conducted detailed geochronological,geochemical and Hf isotopic researches on the volcanic rocks of the Manketouebo Formation from the Suolun area in the central Da Hinggan Mountains.LA-ICP-MS zircon U-Pb dating reveals that the Manketouebo volcanic rocks from the Suolun area were formed in 140～138Ma,i.e.,the early Early Cretaceous rather than Late Jurassic as previously thought.Geochemical data show that the volcanic rocks are characterized by high silicon,rich alkali,weakly peraluminous nature (A/CNK=0.99～1.34) and high degree of differentiation,thus belonging to high-k calc-alkaline series.They are enriched in LILEs (Rb,Th,U,K) and LtREEs,and depleted in HFSEs (Nb,Ta,Ti,P).Their zircon Hfisotopic compositions show that the Manketouebo volcanic rocks of the study area have relatively high εHf(t) values (+4.06～+8.24) with the two-stage model ages ranging from 936Ma to 664Ma,implying that they were probably derived from partial melting of juvenile crustal materials that accreted from the depleted mantle.According to the regional geologic data,it is concluded that the volcanic rocks of the Manketouebo Formation formed in a post-orogenic extensional setting,which was probably closely related to the collapse of orogenic belt after the closure of Mongol-Okhotsk Ocean.