Abstract: The pyroclastic deposits from tephra fall,pyroclastic-flow,base-surge,and lahar are well developed in Changbaishan Mountain area where volcanic activity is active in the Holocene.These different pyroclastic deposits are difficult to distinguish from each other in the field because of their similar physical and geological appearance.Through field investigation of pyroclastic deposit sections in the study area,we systematically collected different pyroclastic deposit samples at different sites,and then calculated the grain size parameters and plotted out the probability cumulative curves of all samples in the laboratory.Our results indicate that the pyroclastic samples from different origins can be distinguished by characteristic grain-size diagrams,e.g.median diameter(Md_Ф)versus sorting(σ_Ф),F1 versus F2 diagram.In terms of texture,most pyroclastic deposits show similar characters.However,with the increase of distance from the volcano crater,grain size decreases correspondingly.Finally,Laser Particle Size analyzer is utilized at first time to analyze the subpopulation of fine ash(≤62.5μm)of our samples.On the basis of the fine ash subpopulation histogram distribution and weight cumulative curves,we found that the fine ashes with grain-size from 0.02μm to 62.5μm in pyroclastic-flow deposits and pyroclastic-surge deposits are significantly different due to their different formation origin and different transportation processes.

Key words: pyroclastic deposit, fine ash, grain-size analyses, cumulative curve, Changbaishan area

摘要： 长白山地区全新世火山活动活跃,发育了良好的火山空降、火山碎屑流、火山涌流和火山泥石流堆积物。这些堆积物交错堆积,野外区分较为困难。在火山碎屑地层剖面调查基础上,系统采集了各种类型的火山碎屑堆积物样品。在实验室通过粒度参数和概率累积曲线分析,对堆积物成因类型进行了判别,讨论了火山空降堆积物和火山碎屑流堆积物随着与火口距离变化的规律。首次对研究区内粒度范围为62.5～0.02μm的细火山灰进行了粒度分析,对火山碎屑流和火山碎屑涌流中细火山灰端元分布特征和地质意义进行了分析和讨论。

关键词: 火山碎屑, 细火山灰, 粒度分析, 概率累积曲线, 长白山地区