吉林省龙岗火山群南龙湾第四纪火山碎屑颗粒特征研究

Study on the characteristics of Quaternary volcanic pyroclasts from Nanlongwan volcano,Longgang volcanic cluster,Jilin Province.

爆炸性火山喷发形成的碎屑颗粒的粒度、分选性、表面结构和内部结构等特征与火山喷发的机制、岩浆与水作用的程度、搬运过程等有着重要的联系。本文以此为线索,研究了龙岗火山群南龙湾火山的一个剖面,以探讨其喷发类型和特征。在该剖面上采集了不同层位火山碎屑颗粒样品,然后进行显微形貌观测、粒度分析和扫描电镜形貌观测。显微镜下观测表明,射汽爆发、射汽岩浆爆发和岩浆爆发的碎屑颗粒具有不同的成分和形貌特征。粒度分析结果显示,粒度与喷发类型之间存在很好的对应关系,不同的爆发类型具有不同的分维值 D 范围。SEM 分析可以提供有关火山喷发特征对火山碎屑颗粒的影响。本文的研究结果表明,南龙湾火山喷发为爆炸式喷发,包括早期的射汽岩浆爆发,到岩浆爆发至晚期以射汽爆发为主的射汽岩浆爆发的不同阶段,该区火山喷发的不同时期,水参与喷发的程度不同。

The particle size, separation, surface texture, and internal structure of pyroclasts produced by explosive volcanic eruption bear a close relation to the mechanism of volcanic eruption, the reaction degree between the magma and water, the emplacement processes, and so on. In the light of this recognition, a profile of Nanlongwan volcano, Longgang volcanic cluster was studied in this paper to gain an insight into the eruptive type of the volcano. Samples of pyroclasts from each layer of this profile have been collected, and microscopic observation of pyroelastic morphology, analysis of particle size, and scanning electron microscopic （SEM） observation of pyroclastic surface textures have been carried out on all these samples. Microscopic observation makes clear that the pyroclasts from phreatic, phreatomagmatic and magmatie explosive eruption have different compositions and morphology. Particle size analysis indicates that particle size distribution feature bears a good relation to eruptive type, and different eruptive type has different fractal dimension scope. SEM analysis may provide information on the effect of the characteristics of the volcanic eruption on the features of pyroelasts. The results of this study show that the eruption of Nanlongwan volcano can be assigned to explosive volcanic eruption, which includes the early phreatomagmatic eruption advancing to magmatic eruption, and the late phreatomagmatic eruption, but dominated mainly by phreatic eruption. The reaction degree between the magma and water is different at different eruptive stages.