内蒙古阿鲁科尔沁旗安乐屯岩体形成时代、成因及对古亚洲洋主缝合带位置的启示

Age and petrogenesis of the Anletun pluton in Ar Horqin Banner,Inner Mongolia and its implications for the major suture zone of the Paleo-Asian Ocean

对内蒙古阿鲁科尔沁旗南部安乐屯岩体进行了岩石学、锆石U-Pb年龄及元素-同位素地球化学分析研究,以揭示其形成时代及成因,并讨论其构造意义。LA-ICP-MS锆石U-Pb定年结果显示,该岩体形成于早二叠世(275±1 Ma),而非前人认为的早侏罗世。岩石学和元素地球化学研究表明,该岩体由石英二长闪长岩-花岗闪长岩-二长花岗岩组成,整体上富钠、低钾,具有中钾钙碱性系列特征,属于准铝质系列岩石;岩体整体上表现出Eu的正异常特征,并相对富集Rb、Ba、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素。这些地球化学特征与俯冲带成因的I-型花岗岩类似。锆石Hf同位素分析结果表明,该岩体的ε_(Hf)(t)值介于-7.0~-1.9之间,结合其Nb、Ta亏损而Th相对富集的壳源属性,推断其源区为古老下地壳物质。通过区域对比分析,认为安乐屯岩体形成于古亚洲洋板块向南俯冲于中朝古板块之下的活动大陆边缘环境,属于中朝古板块北缘古生代增生带的一部分。

This paper reports petrology,zircon U-Pb dating,and element and isotope geochemical analyzing results for the Anletun pluton in Ar Horqin Banner,Inner Mongolia with the aim of revealing the formation age and petrogenesis of this pluton,and discussing its tectonic significance.Zircon LA-ICP-MS U-Pb dating results indicate that the Anletun pluton formed at 275±1 Ma,i.e.,the Early Permian,not the early Jurassic as previously believed.Lithological and geochemical analysis show that the Anletun pluton are composed mainly of a suite of quartz monzodiorites,granodiorites,and monzogranites.They have high Na 2 O and low K 2 O contents,belong to the middle-K calc-alkaline and metaluminous series,display positive Eu anomalies,and are enriched in large ion lithophile elements(LILEs)such as Rb,Ba and K,and depleted in high field strength elements(HFSEs)such as Nb,Ta and Ti.Geochemical compositions of the pluton are similar to those of I-type granites formed in a subduction zone at active continental margin.The zirconε_(Hf)(t)values of the sample range from-7.0 to-1.9.Together with the crustal nature of enrichment in Nb,Ta,and depletion in Th,the Anletun pluton probably originated by partial melting of ancient lower crustal material.By contrast to the regional geology between the north margin of the Sino-Korean paleo-plate and the south accretionary margin of the Siberian paleo-plate,it is speculated that the Anletun pluton formed in an active continental margin setting related to the southwards subduction of the Paleo-Asian oceanic plate beneath the Sino-Korean paleo-plate,and was attached to north accretionary zone of the Sino-Korean paleo-plate in the Paleozoic.