碱性长石微结构特征与岩体冷却速率的关系  

RELATIONSHIP BETWEEN MICROSTRUCTURES OF ALKALI FELDSPAR AND THE COOLING RATE OF ROCKS

作　　者：陈小明[1] 陈克荣[1] 林承毅[2] 

机构地区：[1]南京大学地球科学系 [2]南京大学现代分析中心

出　　处：《地质论评》1996年第5期391-396,共6页Geological Review

基　　金：国家自然科学基金(编号49302025)

摘　　要：前人的研究(主要为实验研究)表明:岩体冷却速率越慢,隐纹长石出溶叶片周期越大。但笔者在研究中发现:如把不同地区的不同岩石按冷却速率的变化梯度排列起来,其隐纹长石出溶叶片周期变化为:熔结凝灰岩中300nm±,熔岩流中150nm±,较窄岩脉中100nm±,较宽岩脉中50nm±,火山颈相次火山岩中12nm±。可见随着冷却速率的变慢,碱性长石出溶叶片周期反而变小,这明显与前人的实验结果相矛盾。该结果可能由两个因素造成:一是以往的实验结果只是对由拐点分解形成的出溶叶片周期的研究,不适用于解释由非均匀成核及均匀成核形成的叶片周期;二是由于各个岩体各自的影响因素改变了它们的冷却速率。Previous experimental studies showed that the cooling rate decreases with increasing exsolu-tion lamellar periodicity of cryptoperthite. However, these experiments were all performed on alkali feldspars from the same rocks (i. e. with the same cooling rate). The paper focuses on the microstructures of alkali feldspars from different rocks with different cooling rates. Five types of rocks were chosen for the study. They were, in the order of the decreasing cooling rate, welded tuff, lava flow, narrow dike, wide dike and subvolcanic rock in a volcanic neck. TEM study shows that the lamellar periodicities of cryptoperthites in these rocks are on the average 300 nm ± , 150 nm± , 100 nm± , 50 nm± and 12 nm± respectively. This indicates that the lamellar periodicity of alkali feldspar decreases with the cooling rate of rocks. It is abvious that this result contradicts the previous experimental results. It may possibly be caused by two factors : first, the influence factors of these rocks themselves changed their cooling rates; second, the previous experiments just studied the lamellae formed by inflection point decomposition, so the results are not suitable for the lamellae formed by homogeneous nucleation and heterogeneous nucleation. But in nature it is very possible that cryptoperthites form at different cooling rates with different exsolution mechanisms. In fact the exsolution mechanisms are very likely to influence the lamellar periodicity.

关 键 词：碱性长石 岩体 冷却速率 火山岩 微结构 

分 类 号：P578.968[天文地球—矿物学]