辽宁赛马霓霞正长岩黑云母地球化学特征、40Ar-39Ar年龄及其地质意义

黑云母不仅是理想的40Ar-39Ar年代学定年矿物，其化学组成还可用来指示母岩浆物理化学条件、岩浆源区及分异演化程度、成矿潜力和成岩构造环境.本文对辽宁赛马碱性杂岩体中分布最为广泛的霓霞正长岩中的黑云母开展系统电子探针分析和40Ar-39Ar定年，结合pMELTS软件对前人全岩主量元素分析数据开展了平衡结晶计算，探讨了母岩浆物理化学性质、演化过程和成岩构造背景.电子探针分析结果表明赛马霓霞正长岩黑云母具有高铁高钛特征，属铁质黑云母.根据黑云母主量元素含量及比值估算黑云母结晶温度为770~800℃，lgf_O2介于-16~-14，而pMELTS平衡结晶计算结果表明整个霓霞正长岩岩浆体系于1 300℃以上便已开始结晶，且随温度降低氧逸度呈现不断降低的趋势，这种较高温度和持续降低的氧逸度环境不利于岩浆热液的分异，使得碱金属（Na）、挥发分及铀钍稀有元素保留在岩浆房内并在后期钠质岩浆（异霞正长岩）结晶分异过程中富集成矿.部分黑云母发育完好的振荡环带，且核部较幔部具有更高的TiO₂、Na₂O含量和更低的SiO₂含量和Fe3+/（Fe3++Fe2+）比值，也进一步证实黑云母结晶后残余岩浆具有更低氧逸度和更高Na含量.赛马碱性岩是华北克拉通北缘近东西向碱性岩带的一部分，本文获得其黑云母40Ar-39Ar年龄为222 Ma左右，形成于古亚洲洋闭合之后的后碰撞伸展构造背景. 

Geochemistry Characteristics and 40Ar-39Ar Age of Biotite from the Saima Aegirine-Nepheline Syenite and Its Geological Significance

Biotite is not only a robust 40Ar-39Ar geochronometer, but also commonly used to constrain the physico-chemical conditions, magma source and evolution process, mineralization potential and tectonic settings of the parental magma. In this study, we have obtained the 40Ar-39Ar age and major element compositions of biotite from the most widely outcropped aegirine-nepheline syenite of the Saima alkaline complex. We further discuss the physicochemical property, the evolution process and tectonic settings of the rock by the combination of equilibrium calculation using pMELTS software on previously reported data. The biotite crystals from the aegirine-nepheline syenite have high Fe and Ti contents and belong to annite in both the APSE classification diagram and Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn) diagram for biotite. On the basis of the major element compositions of the biotite, the crystallization temperatures and oxygen fugacity (lgf_O2) are estimated to range from 770 to 800℃ and -16 to -14, respectively, while the equilibrium calculation by pMELTS indicates that the magmatic system crystallized at >1 300℃ and the oxygen fugacity have decreased since the crystallization begun. The high temperature and decreasing oxygen fugacity prevent the early exsolution of a fluid phase, and the alkalis and volatiles are therefore retained in the melt and finally enriched in the hydrothermal fluids related to the late sodic lujavrite. Some of the biotite plates show oscillatory zoning, high TiO₂, Na₂O but low SiO₂ contents and low Fe3+/(Fe3++Fe2+) ratio from the core to the rim, further indicating that the residual magmas after biotite crystallization would have lower temperature, lower oxygen fugacity but higher alkalis contents. The Saima alkaline complex is part of the E-W-trending alkaline rock belt at the northern margin of the NCC, and it is formed at~222 Ma as indicated by biotite 40Ar-39Ar age in post-collisional extension tectonic settings after the closure of the palaeo-Asian Ocean.