湖南锡田岩体的岩浆混合成因:岩相学、岩石地球化学和U-Pb年龄证据  

Magma mixing in the Xitian pluton of Hunan Province: Evidence from petrography, geochemistry and zircon U-Pb age

作　　者：陈迪 陈焰明 马爱军 刘伟 刘耀荣 倪艳军 

机构地区：[1]湖南省地质调查院,湖南长沙410116

出　　处：《中国地质》2014年第1期61-78,共18页Geology in China

基　　金：中国地质调查局区域地质调查项目(基[2011]02-14-04)资助

摘　　要：南岭中段的锡田岩体燕山期岩石以斑状黑云母二长花岗岩为主,岩体中广泛发育暗色微粒包体.暗色微粒包体为岩浆结构,大多数具有塑性外形,发育淬冷边、反向脉,存在多种不平衡结构和矿物组合,如钾长石环斑、石英眼斑、针状磷灰石等,显示岩浆混合特征.岩石地球化学方面,暗色微粒包体具有比寄主岩贫硅、贫碱,富K、Fe、Mg、Ca、Ti特征;暗色微粒包体及寄主岩富集轻稀土元素（LREE/HREE=2.6～8.8）,具有中-强的铕负异常（δEu=0.09～0.74）以及具有相似的稀土元素配分曲线和微量元素蛛网图;在主要氧化物含量Harker图解中投点多呈直线变异趋势,反映了两者具有密切的亲缘关系,利用共分母和不共分母图解进行判别,暗色微粒包体与寄主岩具岩浆混合特征,在MgO-TFeO判别图解中也指示包体为岩浆混合成因.上述岩相学和元素地球化学特征表明暗色微粒包体是基性岩浆侵入到酸性岩浆中淬冷形成的,指示锡田岩体存在两种岩浆的混合作用.通过LA-ICP-MS锆石U-Pb定年,得到寄主岩形成年龄（150.04±0.52）Ma,暗色包体形成年龄（145.09±0.63）Ma,二者年龄在测试误差范围内一致,显示岩浆混合作用发生的时间大致为晚侏罗世.The Xitian pluton area is located in the middle segment of the Nanling Mountains, and the Yanshanian granites mainly consist of porphyritic biotite monzogranites with many dark microgranular enclaves. These dark microgranular enclaves have plastic shape and magmatic structure, quench boundary, reverse veins and unbalanced textures, such as acicular apatite, quartz ocellar texture and K-feldspar zoning. These characteristics indicate magma mixing. Geochemically, these enclaves are poor in SiO2 and ALK, and the values of K, Fe, Mg, Ca and Ti in the enclave samples are higher than those in their host granites. The enclaves and their host rocks are rich in LREE, with LREE/HREE ratios ranging from 2.6 to 8.8, and display moderate to strong negative europium anomalies （δEu=0.09-0.74）. The selected major oxides of enclaves and porphyritic biotite granite show a linear correlation in Harker diagrams. The enclaves and their host rocks display obvious features of magma mixing in the Covariant diagrams. The TFeO-MgO discrimination diagrams also suggest the magma mixing origin of the enclaves. The petrography and geochemistry also indicate magma mixing. The enclaves from the Xitian pluton were probably derived from quenching of marie magma during its intrusion into felsie magma. The LA-ICP-MS single-zircons U-Pb dating demonstrate that the magmatic age is （150.04±0.52） Ma for the biotite monzogranites and （145.09±0.63） Ma for the enclave. The two ages are basically the same, suggesting that the mixing event took place during the period of Late Jurassic.

关 键 词：锡田岩体 黑云母二长花岗岩 暗色微粒包体 锆石U-PB LA-ICP-MS 岩浆混合 南岭 

分 类 号：P597[天文地球—地球化学] P588.121[天文地球—地质学]