黑龙江镜泊湖地区蛤蟆塘火山空落堆积物形成机制研究

火山碎屑物的粒度、粒形和分布特征蕴含着其形成机制和喷发的环境信息。基于镜泊湖地区蛤蟆塘火山的一个空落堆积剖面的野外地质和岩相学，以粒度分析和分形理论定量研究了火山碎屑物的粒度分布、粒形几何及其分形特征。蛤蟆塘火山空落碎屑粒度分布均为单峰式，由岩浆爆炸形成的空落浮岩粒度峰值较小，而由射气岩浆喷发形成的含细花岗岩碎屑夹层的碎屑粒度峰值较大。空落浮岩颗粒的类球度、长宽比和凸度都小于含细花岗岩碎屑夹层的数值，表明空落浮岩颗粒相对不规则的特点。利用多段幂律方法拟合了蛤蟆塘火山空落碎屑颗粒分布规律，发现空落浮岩颗粒存在四个幂律分布段（即对应四个分形维数），这是由于岩浆初始破碎、火山通道内的二次破碎以及风力筛选作用等造成的；含细花岗岩碎屑夹层的碎屑分布有两个幂律分布段（对应两个明显不同的分形维数），即浮岩和花岗岩碎屑的形成是因不同破碎机制造成的。

Study on formation mechanism of fallout deposits in Hamatang volcano, Jingpo Lake area, Heilongjiang

The grain size, shape and distribution characteristics of pyroclasts contains their formation mechanism and environmental information during eruption. Based on the field geology and petrography of a fallout deposit section of Hamatang volcano in Jingpo Lake area, the distribution of particle size, the geometry of particle shape and their fractal characteristics of pyroclasts are quantitatively studied by particle size analysis theory and fractal theory. The particle size distribution of pyroclasts in Hamatang volcano is unimodal, and the particle size peak of fallout scoria formed by magmatic explosion is small, while the particle size peak of pyroclasts in the interlayer with fine- grained granite clasts formed by phreatomagmatic eruption is large. The sphericity, aspect ratio and convexity of the fallout scoria particles is less than the one of pyroclasts in the interlayer with fine- grained granite clasts, which indicates the relatively irregular characteristics of the fallout scoria particles. The multi- segment power- law method is used to fit the distribution law of pyroclastic particles in Hamatang volcano. It is found that there are four power- law distribution segments of fallout scoria particles (i.e. corresponding to four fractal dimension values), which are caused by the initial fragmentation of magma, the secondary fragmentation in volcanic channel and the sorting effect of wind. The distribution of pyroclasts in the interlayer with fine- grained granite clasts has two power- law distribution segments (i.e. corresponding to two obviously different fractal dimension values), and the formations of scoria and granite clasts are caused by different fragmentation mechanisms.