吉林保安屯地区侏罗世花岗闪长岩及其闪长质包体成因研究

Study on the Cause of Jurassic Granodiorite and its Dioritic Enclaves in Baoantun Region of Jilin Province

本文对小兴安岭-张广才岭岩浆弧南段吉林保安屯出露的侏罗纪花岗闪长岩及其内部的闪长质包体开展了详细的地质调查及系统的岩石地球化学、锆石U-Pb年代学研究。研究表明寄主花岗闪长岩与闪长质包体的母岩浆不是同源的,寄主花岗闪长岩的源区壳源性质更明显,而闪长质包体的源区则更接近于幔源;此外,闪长质包体的形成温度和压力较寄主花岗闪长岩更高。二者均具有轻稀土富集、重稀土亏损、轻重稀土分馏明显的特征,在球粒陨石标准化分布型式图中显示,闪长质包体的Eu负异常更为明显。锆石U-Pb定年表明闪长质包体形成年龄为182.0±1.3Ma,为早侏罗世,而寄主岩体花岗闪长岩的U-Pb同位素年龄为168.1±1.6Ma~172.1±1.2Ma,属中侏罗世。上述地球化学特征及形成时代的差异表明闪长质包体可能代表晚期花岗闪长质岩浆上侵时捕获的围岩物质,其成因应属捕掳体,间接说明张广才岭南段侏罗世花岗岩类中广泛发育的暗色包体类型至少有两种,一是捕掳体成因,二是岩浆混合成因。

In this paper,detailed geological survey and systematic studies on petrogeochemistry and zircon U-Pb dating were carried out on the Baoantun granodiorite and its dioritic enclaves in the southern section of Xiaoxinganling-Zhangguangcailing magmatic arc.The studies show that the magmas of the granodiorite and its dioritic enclaves have different origins.the host granodiorite has an affinity to crustal origin,whereas the dioritic enclaves display some signatures of mantle.In addition,the formation temperature and pressure of the enclaves are higher than those of the host granodiorite.Both the lithologies are enriched in light rare earth elements and depleted in heavy rare earth elements.Compared with the granodiorite,the dioritic enclaves show obvious nagative Eu anomaly in the chondrite-normalized diagram.Zircon U-Pb dating reveal that the dioritic enclaves formed at 182.0±1.3Ma,whereas the host granodiorite formed at 168.1±1.6Ma~172.1±1.2Ma.The distinct differences in the geochemistry and formation ages suggest that the dioritic enclaves were captured from the wall-rock during the granodioritic magma ascending.Therefore,the dioritic enclaves are belonged to tiny xenoliths within the host granodiorite.Indirectly,there are at least two types of dark xenoliths widely developed in Jurassic granite in the Southern Zhangguangcai Range,one is xenoliths,the other is cause of magma mixing.