湖南宁乡钾镁煌斑岩中火山微球粒的初步研究

A Preliminary Study of Volcanic Microspherolites from Lamproite, Ningxiang County,Hunan Province

本次研究从钾镁煌斑岩中挑选出上万个微球粒进行了系统研究。微球粒有三种：无磁性无色透明—半透明浅黄色、棕褐色至不透明黑色；弱磁性—强磁性黑色；强磁性钢灰色铁质。除微球粒外，还有岩浆溅射碎片和炉渣伏溅射碎片等火山尘。微球粒具有火山喷发条件下可出现的多种多样的表面结构和内部结构。其化学成分主要为：（１）贫铁型，富Ｓｉ、Ｍｇ、Ｃａ、Ｐ，贫Ｔｉ、Ｍｎ、Ｋ；（２）富铁型，富Ｓｉ、Ｔｉ、Ｍｎ、Ｋ，贫Ｃａ、Ｍｇ、Ｐ；（３）高铁型，以Ｆｅ为主。单个微球粒中还出现高Ｓｉ相和高Ｓｉ、Ｍｇ相。据微球粒化学成分的多样性推测，在钾镁煌斑岩浆侵位至地壳深部环境下，经历过液态熔离作用，当火山喷发时，在快速冷凝条件下形成火山微球粒。

A large number of microspherolites selected from Ningxiang lamproite,Hunan Pro-vince,South China,were examined and analysed by several approaches.In terms oftheir physical properties and chemical compositions,three groups of microspherolites canbe distinguished. They are considered as volcanic microspherolites although tbose fromsedimentary rocks and ocean floor are traditionally attributed to cosmic dusts.The microspherolites are described as of the following three groups:(1)nonmag-netic group, which is colorless and transparent,pale-yellow and semitransparent to darkyellow and opaque,(2)magnetic group, which is dark in color,(3)strong magneticgroup, which is steel grey and dominated by iron,In addition,some volcanic dusts,such as magmatic splashed fragments and blocks,can also be observed.The features ofmicrospherolites are varied,and their facies and textures show evidently that they areproducts of volcanism.Chemical compositions of these microsplierolites vary in a wide range,from irondeficiency to iron dominance:(1) Fe-poor type, which is rich in Si,Mg,Ca and P,but poor in Ti,Mn and K;(2)Fe-rich type,which is bigh in Si,Ti,Mn and K,but poor in Ca,Mg and P(3)Fe-high type,which is dominated by Fe.Lamproite microspherolites are characterized by diversity both in chemical composi-tion and in physical properties.It is concluded that liquation took place in the magmachamber after the emplacement of lamproite magma in the deep crust, and afterwardsmicrospherolites were formed as a result of rapid cooling and quenching during volcaniceruption.