摘要
罗波岗日地区大地构造位置处于冈底斯花岗岩带南亚带,该区出露较大规模的二云二长花岗岩体。LA-ICP-MS U-Pb年龄结果表明,锆石^(206)Pb/^(238)U加权平均年龄为16.15±0.19 Ma,表明上述岩体形成于新近纪中新世布尔迪尔期。二云二长花岗岩的w(SiO_2)为71.77%~73.05%,组合指数δ为1.76~2.23,A/CNK的比值为1.114~1.213,A/NK的比值为1.36~1.54,属于高钾钙碱性强过铝质S型花岗岩系列。在ORG标准化蛛网图中,二云二长花岗岩微量元素总体呈右倾富集特征,相对富集Rb,Ce,Hf,Sm,而亏损Nb,Zr。球粒陨石标准化的稀土元素配分曲线呈右倾富集型,稀土元素总量变化较大,∑REE=98.92×10^(-6)~184.17×10^(-6),LREE/HREE=11.24~25.16,(La/Yb)N=14.85~56.98,轻重稀土分馏显著,Eu的负异常不明显。样品的Rb/Sr比值为0.34~0.60,大于0.32,TFeO+MgO+TiO_2含量为2.53%~3.05%,CaO/Na_2O比值为0.32~0.41,说明罗波岗日二云二长花岗岩的源岩主要为来自于上地壳的砂质岩。与该区域的浅色花岗岩对比研究表明,罗波岗日地区二云二长花岗岩的成因,是在水等流体在伸展体制背景下沿着构造薄弱带迁移、聚集,在上地壳物质重新调整过程中,使得以砂质岩为主的源岩发生"湿"深熔作用,生成过铝质岩浆,最终结晶形成二云二长花岗岩。
Luobogangri monzonitic granite located in the Gangdese magma belt, on the north of Yaluzangbu suture zone. The LA-ICP-MS zircon U-Pb dating indicate the intrusion was generated in 16.15 ± 0.19 Ma. The monzonitic granite show high SiO_2 content（71.77% ~ 73.05%）, high A/CNK values（1.114 ~ 1.213） and high A/CNK values（1.36 ~ 1.54）, which suggest that the monzonitic granite belongs to peraluminous S type granite with high-K calc alkaline nature. The intrusion show the light rare earth element（REE） enrichment pattern in chondrite-normalized andORG-normalized diagram. The fractional degree of HREE and LREE is striking, with the values of LREE/HREE vary from 11.24 to 25.16, and（La/Yb）Nvary from 14.85 to 56.98. The slight high Rb/Sr values, slight high CaO/Na_2O values, and medium content of TFeO＋MgO ＋TiO_2 indicated that the source rock of monzonitic granite is mainly sandy rock from upper crustal. Comparative study show the Miocene granite fromed in the initial extensional stage after the crustal tectonic regime had changed from compression to extension, and closely related to decompression of base, represents the ＂wet＂ crustal anatexis.
出处
《甘肃地质》
2017年第3期1-10,共10页
Gansu Geology
基金
中国地质调查局地调项目(1212011221058)资助
