江西路迳金伯利质煌斑岩中金云母斑晶的蚀变机制研究

A study on the alteration mechanism of phlogopite phenocrysts in the Lujing Kimberlitic lamprophyre in the Anyuan area,Jiangxi Province

层状硅酸盐矿物的蚀变演化过程可记录岩矿成因及地质环境变迁。三八面体云母是超基性岩中最常见的层状硅酸盐矿物,其矿物学特征已被证明能够有效地反映深部的结晶条件,但超基性岩云母的次生演化过程却尚未得到系统研究。为探明超基性岩中原生云母的后期演化规律及其内在机理,本研究采用微区X射线衍射、电子探针、X射线光电子能谱、聚焦离子束扫描电镜和高分辨率透射电镜等多种微区微束技术,对江西路迳金伯利质煌斑岩中的金云母及其演化产物进行了分析。结果显示,在蚀变过程中,大部分Si离子保留在金云母中,而Fe、Ti、Mn、K和Na离子则大量流入热液、次生钛铁矿和碳酸盐(而非硅酸盐产物)中。金云母的蚀变反应从缺陷位点向外扩散,首先通过溶解再结晶在缺陷处形成绿泥石、高岭石、蛇纹石等蚀变序列产物,而后通过外延生长在裂隙中充填伊利石,并在蚀变较弱的部位通过固相转变形成层间亏损金云母;蚀变产物的形成位点由相变机制控制;随着蚀变程度的降低,产物与母矿物金云母的结构相似性逐渐增加。

The alteration evolution process of phyllosilicate minerals can record genetic informations of diagenesis and metallogenesis and changes of geological environmental conditions.Trioctahedral mica is a common phyllosilicate mineral in the ultra basic rocks.Its mineralogical characteristics have been proved to be effective in reflecting the crystallization conditions at depth/and reflects alterations in host rocks.However,the secondary evolution process of mica in ultrabasic rocks have not been systematically investigated.In oreder to explore the late-stage evolutionary regulation and internal mechanism of original mical in ultrabasic rocks,by using multiple microzone microbeam techniques,including the microXRD spectrometer,electron probe microanalyzer,X-ray photoelectron spectroscopy,focused ion beam scanning electron microscopy,and high-resolution transmission electron microscope(TEM),we here have analyzed phlogopite and its evolutionary products in the Lujing kimberlitic lamprophyres in Jiangxi Province,China.Our study reveals that phlogopite could be altered to various secondary phyllosilicates simultaneously at different crystallographic sites.In the alteration process,most Si ions were remained within phlogopite,while Fe,Ti,Mn,K,and Na ions were largely migrated into hydrothermal fluids,secondary titanium-iron oxides,and carbonates(rather than silicate products).The alteration reaction of phlogopite spreads outward from the defect site.The alteration sequence products such as chlorite,kaolinite,serpentinite were formed at the defect sites through dissolution and recrystallization,then illite was filled in the fissure through epitaxial growth,and interlayer-deficit phlogopite was formed at the weakly altered site through solid phase transformation.The formation sites of alteration products are controlled by the phase transformation mechanism.With the decrease of alteration degree,the structural similarity between products and the parent mineral phlogopite is gradually increased.