桂北新寨角闪花岗岩角闪石矿物化学特征及其指示意义
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P578.955;P588.12+1

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国家自然科学基金项目(41572191,41703054);广西自然科学基金项目(2016GXNSFCA380022,2017GXNSFBA198164,2019GXNSFDA24009)


The composition of amphibole in northern Guangxi and its implications for the magma evolution of the Xinzhai amphibole granite
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    摘要:

    对桂北新寨侵入岩体中的角闪花岗岩进行了详细的偏光显微镜观察和系统的矿物化学研究,并在此基础上,基于电子探针分析结果选取共生的角闪石和斜长石,估算了该岩体侵位时的温压条件、氧逸度和含水量。岩相特征观察显示,新寨角闪花岗岩中主要发育有自形、未蚀变半自形/他形和强交代半自形/他形3种主要类型的角闪石,是岩浆侵位过程中在不同深度的结晶产物或交代蚀变产物。电子探针研究结果显示,新寨花岗岩中角闪石成分变化较大,且在岩浆侵位过程中呈现出Al2O3、FeOT、Na2O、TiO2、K2O含量降低但MgO、SiO2含量升高的趋势。矿物温压计估算结果显示自形和未蚀变半自形/他形角闪石的结晶压力分别为0.28~0.30 GPa和0.19~0.26 GPa,对应的结晶温度分别为767~783℃和740~764℃。温压计算结果表明新寨岩体初始侵位深度应大于11.3 km,且侵位过程是一个近乎等温降压的过程,变压结晶作用为新寨侵入体持续侵位过程中的主要结晶方式。角闪石结晶时具有较高的氧逸度,变化范围在ΔNNO+0.1(log fO2=-13.5)到ΔNNO+0.5(log fO2=-12.9)之间。角闪石结晶时熔体含水量为4.9%~6.4%,但在侵位到中地壳7.0~9.5 km时经历了由于降压引起的流体出溶。角闪石化学组成指示其母岩浆属于钙碱性的中酸性岩浆,来源于壳幔混合,且在侵位过程中不断有壳源物质加入。

    Abstract:

    Detailed petrographic observation and systematic mineralogical chemistry studies of the Xinzhai granite from the north Guangxi were performed in this paper. Based on electron microprobe analysis (EMPA) data, the authors calculated pressure, temperature, oxygen fugacities (log fO2) and H2O in co-existent melts conditions of the crystallization of these granites. According to the petrographic features, there are three types of amphiboles in Xinzhai amphibole granite, i.e., euhedral amphibole, unaltered subhedral/anhedral amphiboles and strongly metasomatized subhedral/anhedral amphiboles, which are products of crystallization or metasomatic alteration of the Xinzhai intrusion formed at different depths and stages in the process of emplacement. The EMPA results show that the major element compositions of amphibole in Xinzhai granites are varied depending on crystal morphology, characterized by the decreasing of Al2O3, FeOT, Na2O, TiO2, K2O but increasing of MgO and SiO2 during magmatic emplacement. The values of crystallization pressures estimated from aluminium-in-amphibole barometry of the euhedral amphibole and unaltered subhedral/anhedral amphiboles are 0.28~0.30 and 0.19~0.26 GPa respectively, indicating that the emplacement depth of the Xinzhai pluton should be greater than 11.3 km. Furthermore, the corresponding crystallization temperatures of emplacement calculated with the amphibole-plagioclase thermometry and amphibole thermometry of the euhedral amphibole and unaltered subhedral/anhedral amphiboles are 767~783℃ and 740~764℃ respectively. Based on the p-T conditions of the pluton crystallization, it is suggested that the Xinzhai pluton experienced isothermal decompression and polybaric crystallization during the emplacement. The calculated oxygen fugacities of the unaltered amphiboles are in the range of ΔNNO+0.1 (log fO2=-13.5)~ΔNNO+0.5 (log fO2=-12.9), indicating a relatively high oxygen fugacity. Moreover, the fractional crystallization enriched the water content of the residual melt (4.9%~6.4%) and experienced exolved aqueous fluids when the evolved magma intruded into the shallower depth (7.0~9.5 km) mainly due to depressurization. Genetic mineralogy studies show that amphibole in the Xinzhai granite belongs to calc-alkaline intermediate and acid magma, which was likely derived from a crust-mantle mixed origin and continuously contaminated by crust materials in the process of emplacement.

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胡荣国,刘楚晴,冯佐海,赵义来,蔡永丰,刘奕志,夏志鹏,2020,桂北新寨角闪花岗岩角闪石矿物化学特征及其指示意义[J].岩石矿物学杂志,39(6):703~717.

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  • 收稿日期:2020-04-14
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  • 在线发布日期: 2020-11-13
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