酸性岩浆不混溶演化路径探索——来自福建省政和县一个“超级实验室”的野外证据

Exploration of immiscible evolution path of acidic magma——field evidence from a “super laboratory” in Zhenghe County,Fujian Province

一个多世纪以来经历过几代人的不懈努力,尤其是近年来岩浆不混溶作用在业界得到重视,使其理论研究和实践运用都达到了前所未有的高度。笔者在中国东南沿海火山岩中首次发现大规模酸性岩浆不混溶现象,通过详细的岩相学研究揭示,大约温度在900~650℃之间、剧烈的过冷度变化条件下岩浆发生不混溶两液相,一相为富硅的球粒相,另一相为富铁的基体相。不混溶球粒相发展,最初可能由大黏度富桥氧的硅氧四面体-络阴离子团聚集成核,成核后形成珠滴状液滴,液滴聚集成塑性小球,小球碰撞合并奠定球粒个体,球粒个体先后经历过三个世代的反复交叠生长而定型。在浮力和岩浆上侵的综合作用下,无数个球粒个体与熔浆分离、上浮、云集成规模宏大的富硅球粒相,即球粒流纹斑岩地质体,富铁基体相下沉形成英安岩地质体。地球化学特征表明,球粒相富集主元素SiO_(2)、K_(2)O、Na_(2)O和微量元素Sr、Ba;基体相富集主元素Al_(2)O_(3)、TiO_(2)、MgO、TFe_(2)O_(3)和微量元素Rb、Cs、Zr、Hf、Th、Ta、Nb、La、Ce、Ni、Co、ΣREE。随着两相间的物质交换迁移,球粒相所占比例增大,基体相不断减少,明显表现基体残余岩浆朝着基性方向演化,这是本文揭示的酸性岩浆不混溶成分演化的主要路径。

After more than a century of unremitting efforts over several generations,particularly in recent years,magmatic immiscibility has been received attention from industry,reaching unprecedented heights in theory and practice.Large-scale immiscibility of acidic magma was first discovered by the authors in volcanic rocks along the southeastern coast of China.Through a detailed petrographic study,it was revealed that the magma has two liquid phases immiscible separations under the condition of temperature between 900℃ and 650℃ and the change of supercooling degree.One phase is the silicon-rich spherule phase and the other is the iron-rich matrix phase.The spherules may be formed by the anionic agglomeration of silica tetrahedron with high viscosity and rich bridged oxygen phase.After nucleation,emulsion droplets,that will be integrated into plastic spherules will be formed.The ball collision will be combined to establish spherical individuals.The spherical individuals have experienced three generations of collision and repeated overlapping growth.Under the comprehensive action of buoyancy and magma upwelling,numerous spherical particles are separated from molten slurry,floating up and cloud integrated with large-scale silicon-rich spherical facies,namely spherule rhyolite porphyry geological body,and the iron rich matrix facies sinks to form the dacite geological body.Geochemical characteristics show that the main elements SiO_(2),K_(2)O,Na_(2)O and trace elements Ba and Sr are enriched in the spherule phase,while the main elements Al_(2)O_(3),TiO_(2),MgO,TFe_(2)O_(3)and trace elements Rb,Cs,Zr,Hf,Th,Ta,Nb,La,Ce,Ni,Co and ΣREE are enriched in the matrix phase.With the increase in the proportion of spherule phase,that is,the spherule growth,the matrix phase is decreasing,and the material exchange and migration between the two phases obviously show that the residual magma is evolving towards the basic direction,which may be the main path of the immiscibility evolution of acid magma.