Abstract: The field investigation shows that the volcanism in Longgang,Jilin has three distinct eruption ways,i.e. phreatic eruption,phreatomagmatic eruption and magmatic eruption. The materials of phreatic eruption are rock fragments from basement or volcanic channel with a grain size less than 5mm and nearly isoaxisal shape. The products of phreatomagmatic eruption include basaltic pyroclasts and rock fragments of basement with greatly different grain sizes and isoaxial structure. The magmatic eruption generates blowhole-shaped pyroclasts which are of grain sizes 8～0.5mm,mostly nearly equi-axial structured and some being non-equiaxial. Based on the fractal theory,we analyze the fractal features of the grain size of pyroclasts from the Longgang volcanoes,Jilin Province. Our results show that the fractal dimension of the pyroclasts of phreatomagmatic eruption is smaller than that of the phreatic eruption,but greater than that of the magmatic eruption. The values of fractal dimension can be used as parameters to distinguish different types of volcanic eruption. For the Longgang volcanoes,the fractal dimension of pyroclasts at Jinglongdingzi on the upper profile over the water resisting layer is greater than 2,and those at Xiaojinglongdingzi on the lower profile is less than 2. Fractal dimension values can be regarded as an indicator to differentiate various eruptive sources and to classify stratum sequences associated with volcanoes. Our research demonstrates that the pyroclasts with fractal dimension less than 2 contain anisometric grains of various content,and their values of fractal dimension are negatively correlated with the grain content. This is the first time to study the fractal dimension of the grain sizes of pyroclasts for this area. It suggests a new approach to distinguishing eruptive types,strata sequence and eruption intensity of volcanoes in a quantitative manner.

Key words: pyroclasts, fractal dimension, Longgang Jinlin

摘要： 用分形理论分析了吉林龙岗火山碎屑物粒度的分形结构特征。结果显示,射气喷发碎屑物分维值>射气岩浆喷发碎屑物分维值>岩浆喷发碎屑物分维值,分维值可作为区分火山不同喷发类型的定量参数。而对于龙岗岩浆喷发碎屑物,不同火山喷发的碎屑物其分维值也有差别,晚期喷发的金龙顶子火山碎屑分维值>2,早期喷发的小金龙顶子碎屑分维值>2,火山碎屑物分维值可作为区分不同喷发源和划分火山喷发地层序列的一种指标。研究表明,分维值<2的火山碎屑中有不同含量的非等轴颗粒,且分维值与非等轴颗粒的含量呈负相关。

关键词: 火山碎屑, 分维数, 吉林龙岗