吉林龙岗火山群基浪搬运堆积机制研究

Longgang volcanic cluster is one of the important volcanic areas in Northeast China. It erupted frequently in Quaternary. And it is famous for its phreatomagmatic eruptions which produced a large number of base-surge deposits and maar lakes. Phreatomagmatic eruption is an important and special type of volcanic eruption. It represents the interaction mechanism between magma and water underground. The base-surge transportation and deposition process usually can not be observed directly because of the covering of ash-cloud which is accompanied with the base-surge. So some problems are still plaguing volcanic researchers, such as “Is the base-surge wet or dry?” , “What is the mechanism of base-surge transportation and deposition ?” ..In this application, the base-surge deposits strata of Longquanlongwan volcano, Nanlongwan volcano and Sanjiaolongwan volcano in Longgang volcanic cluster would be described and studied.The macro-deposition information would be got by spatial station scaning the strata.According to the different diffusion characteristics of air-fall fine ash between the moist and dry environments,through the analyses of microscopic morphology and distribution of fine ash in the ash-cloud layer, wet or dry of the base-surges would be discussed. Based on the filed investigation of base-surge strata and combined with the grain-size distribution and microscopic morphology analysis, characteristics of base-surge deposits facies would be established. And the mechanism of transportation and deposition of base-surge in longgang would be discussed..These research would have a positive significance for mechanism of phreatomagmatic eruption, volcanic hazard prediction and assessment tourism resources development in Longgang volcano cluster..

射汽岩浆爆破式喷发是一种特殊的火山喷发类型，是岩浆与水作用的体现。吉林龙岗火山群第四纪火山活动频繁，是中国近代单成因火山区之一，以射汽岩浆爆破式喷发为特色，形成众多玛珥湖和大量基浪堆积物。由于基浪流动时外围笼罩着湍流搬运的灰云浪，不能被直接观测。因此“如何判断基浪流体干湿状态”以及“基浪是如何搬运堆积”等问题仍然在困扰着研究者。本次项目以吉林龙岗龙泉龙湾火山、南龙湾火山以及三角龙湾等火山的基浪堆积地层为研究对象，使用空间测站仪扫描地层剖面，分析各种层理特征，获取基浪堆积物宏观动力学信息；借鉴空降细火山灰在潮湿环境喷发和干冷环境喷发下不同扩散机制，对基浪中灰云浪层细火山灰聚合状态以及显微结构进行分析，判断龙泉龙湾等火山基浪流体干湿状态；以地层研究为基础，通过成分分析、粒度分析以及显微形貌特征分析，建立龙泉龙湾火山基浪堆积物相模式，并对其搬运堆积机制进行探讨，为基浪堆积物灾害评估提供参考。

岩浆与水作用是龙岗火山群的一个显著特征，形成了一系列的玛珥火山湖、凝灰岩环和凝灰火山锥。这些火山具有多火口、堆积序列复杂等特征，地貌上呈现为葫芦状。本项目以西大甸子为代表的玛珥火山系统为研究对象，开展了火山地层重构及对比、C14测年、粒度分布、组分分析以及扫描电镜解译，建立了西大甸子玛珥火山系统，提出了西大甸子火山是由南北两个火口组成。西大甸子火山形成于约19000年前，由射汽岩浆喷发、岩浆爆破式喷发和干热的射汽岩浆喷发三个喷发阶段形成，北火口由最后一次喷发形成。研究区位于坚硬的花岗片麻岩峡谷地区、喷发具有多阶段以及火口向北迁移的特征，由此推测基岩以及峡谷的裂隙控制了火山的展布，在岩浆爆破式喷发后期，裂隙水的补给影响了火山喷发的类型。西大甸子火山作为龙岗火山群目前已知年龄最年轻的火山，沼泽化以及破坏程度远超本区其它火山。项目组认为金龙顶子熔岩流的形成破坏了西大甸子南火口的南侧火口垣，并导致西大甸子成为一个低洼地带，快速的沼泽化。西大甸子火山年代的确定，丰富了龙岗火山群晚更新世晚期的火山活动；展布稳定的火山渣及基浪堆积地层，可作为区分本区四海火山渣的标志层；西大甸子火山系统的重构可为相似坚硬基岩区射汽岩浆喷发提供借鉴。