内蒙古解放营子花岗闪长岩岩浆混合机制——来自斜长石和角闪石的矿物化学证据

Magma mixing mechanism of the Jiefangyingzi granodiorite in Inner Mongolia: Evidence from mineral chemistry of plagioclase and hornblende

为揭示华北克拉通北缘中晚三叠世解放营子花岗闪长岩的岩浆混合机制,对寄主岩石和镁铁质包体中斜长石和角闪石开展了电子探针分析。分析结果显示,多斑和少斑包体边部的斜长石斑晶发育An值增加的突变环带,环带的An值为32~46,明显高于核部和边部斜长石的An值(18~31),而核部的An值与寄主岩石中斜长石的An值一致,该特征指示突变环带结构是镁铁质岩浆捕获并改造寄主岩石中斜长石的结果。少斑包体中心部位的斜长石斑晶核部具有熔蚀结构,残余部位的An值(40~47)高于熔蚀部位的An值(32~36),但总体大于边部和基质中细粒斜长石的An值(17~35),反映包体的中心部位受到了长英质成分的改造。另一方面,寄主岩石中角闪石斑晶TiO_(2)含量(0.50%~1.10%)明显低于包体角闪石TiO_(2)含量(1.17%~2.47%),而寄主岩石基质中细粒角闪石TiO_(2)含量(1.58%~2.26%)与包体角闪石TiO_(2)含量相近,指示寄主岩石演化晚期有富TiO_(2)的镁铁质成分混入。综合分析表明,解放营子岩体形成过程中,镁铁质和长英质岩浆之间既存在宏观尺度上的机械混合,也存在微观尺度上的以矿物间元素迁移为特征的化学混合。

In order to reveal the magma mixing mechanism of the Mid-late Triassic Jiefangyingzi Pluton on the northern margin of the North China Craton, this paper carried out electron microprobe analyses of plagioclase and hornblende in host rock and mafic enclaves. The results show that the plagioclase phenocrysts in rich-phenocrysts enclaves and the edges of poor-phenocrysts enclaves develop the spike zone of increased An value. The spike zone of An value(32~46) is higher than those of the core and rim in same plagioclase grain, and the An value of the core is consistent with that of plagioclase in host rock, indicating the spike zone was the result of the mafic magma capturing and modifying the plagioclase from host rock. The core of plagioclase at the central part of poor-phenocrysts enclaves has an erosion sieve structure,and the An value of the residual part( 40 ~ 47) is higher than that of the erosion part( 32 ~ 36) in the core of the plagioclase grain,but generally higher than that of rim in the phenocryst and fine-grained plagioclase in the matrix( 17 ~ 35),indicating the central part of the enclaves has been modified by felsic composition. On the other hand,the TiO_(2)content of hornblende phenocrysts in host rocks( 0. 50% ~ 1.10%) is significantly lower than that of hornblende in enclaves( 1.17% ~ 2.47%),while the TiO_(2)content of finegrained hornblende in the matrix( 1.58% ~ 2.26%) is similar to that of hornblende in enclave,indicating that there was a mixture of mafic components at the late stage of host rocks evolution. The above studies show that magma mingling on a macroscopic scale and the migration of minerals and elements of magma mixing on a microscopic scale occurred between mafic and felsic magma during the formation of Jiefangyingzi Pluton.