火山岩中囊状体的特征及其鉴别

Characteristics and Identification of Saccules in Volcanic Rocks

熔岩和火山碎屑岩是两类最主要的火山岩,分别代表了两种不同的火山喷发方式和成岩过程,是火山岩分类命名中首先要注意区分的。囊状体具有与浆屑很相似的形态和结构,但对其研究还相当不足,易导致富含囊状体的流纹质—英安质熔岩与富含浆屑的熔结凝灰岩相混淆,产生熔岩与火山碎屑岩分类上的错误。本文首次介绍囊状体的基本特征并分析其与浆屑之区别。囊状体由矿物集合体(结晶内带)和外围脱玻体(脱玻外带)构成,因形态总体似囊状而得名,可呈条带状、透镜状,两端呈撕裂状或须状等,有时也呈斑块状消光和脱玻褪色体;它们是火山岩中微小的高温气相或液相长英质组份结晶、并释热诱发周围玻璃质脱玻形成的一种原生岩石结构。正确区分囊状体和浆屑对火山岩的分类命名及成因研究有重要作用,并避免火山碎屑岩的人为扩大化。

Lava and pyroclastic rock are main two sorts of volcanic rocks and represent two different eruptive types and petrogenic processes, so that they should be distinguished firstly for rock classification. Saccules have similar shapes and textures with fiammes, but are short of enough knowledge so far. Thus, Saccules-bearing rhyolitic--dacitic lavas are easily to be mistaken for fiamme-bearing welded tufts, which results in wrong classifications between lava and pyroclastic rock. This paper first introduces distinctive characters of saccules and their differences from fiammes. Saccules, named after their overall shape, are composed of mineral aggregate （ inner crystalline zone ） and surrounding devitrified body （ outer devitrified zone ）, usually stripped or lentiform with lacerated or whiskerlike ends, and also appear occasionally as patched extinction or devitrified fading body. They are formed by both fine crystallization of felsic high-temperature gas/liquid facies and devitrification of surrounding vitric components caused by the former~ heat releasing, which make up a primary texture in volcanic rocks. Identification between saccule and fiamme is helpful for accurate nomenclature and diagenesis of volcanic rocks, which can avoid broadening artificially the scope of pyroclastic rocks.