云南腾冲大六冲火山机构及其火山碎屑岩研究

Tengchong volcanic cluster, located adjacent to the border of western Yunnan province and Myanmar, comprises famous Quaternary volcanic field in China. The special tectonic situation, on the northeast edge of the collision zone between Indian and Eurasian plates, gave rise to the active volcanism. Explosive eruptions of large scale once occurred pervasively in Tengchong in early Quaternary, which hasn't yet come to notice. Daliuchong volcano, situated in the middle of the volcanic field, has the highest peak in Tengchong area. A volcanic conduit with its diameter of the plug more than one hundred meters was found the first time in the south of the Daliuchong peak. It was believed to be the primary conduit in the region through which most of explosive volcanic products erupted and almost covered half of the volcanic field. Daliuchong volcanic apparatus characterized for its acidic explosive materials in early Quaternary was yet poorly known. The research target of the project is first to identify the different volcanic facies of the explosive eruption, lava flow and intrusive rock on the basis of systematic geologic field investigations and indoor identifications, second to identify different explosive eruption process based on the analysis of characteristic texture and distribution of crystal size and vesicle size, third to determine volcanic chronology as well as eruption episodes through combined research work of K-Ar dating for various eruption products and petrography work. The final target is to reconstruct the volcanic apparatus of Daliuchong as well as explosive eruption process, then to give an early warning for future secondary geological disaster by designating the dangerous area in the relevant geologic map.

腾冲火山区位于我国滇西和缅甸交界处，地处印度板块俯冲最前缘，特殊的构造位置导致岩浆活动频繁，是我国著名的活动火山区。在第四纪早期腾冲曾发生过大规模爆炸式喷发，却从未引起人们的注意。位于腾冲中部的大六冲是本区最高峰，在顶峰南侧发现一个直径超百米的火山通道，可能是区内早期火山喷发的主通道，其喷发物占据了腾冲盆地相当大的面积，是本区早第四纪酸性火山岩爆破式喷发的重要代表，但目前对此次喷发知之甚少。本课题以腾冲火山区第四纪早期爆破式喷发的大六冲火山为研究对象，通过详细调查大六冲的火山地质情况和喷发物的产状与分布，恢复古火山机构，确定喷发规模；通过分析大六冲火山碎屑岩的晶屑、气孔大小形态等特征，研究对爆炸喷发的指示意义；通过对大六冲不同层位喷发物测年，结合野外岩相及位置，确定大六冲火山活动的时代及期次；本课题还试图圈定可能发生滑坡和泥石流的危险区，为将来可能发生的次生地质灾害提供一定程度预警。

腾冲火山区位于青藏高原东构造结，构造活动强烈，第四纪以来地震、火山活动频发，大六冲火山是区内海拔最高、规模最大的火山，具有代表性。.通过解译卫片和野外考察，对大六冲火山机构及岩相分布进行了详细调查。首次发现顶峰以南存在着一个大致圆形、直径超百米的火山通道，由近直立产出英安质岩石组成，侵出相围绕火山通道产出，锥体上部及东北部部分区域被爆发相所覆盖，溢流相从锥体中部向外广泛分布。火山颈、熔岩穹丘、岩墙、火山角砾、爆发相与溢流相喷发堆积物构成了大六冲完整的火山机构。估算了大六冲火山喷发物面积约为90km2，（疏松）体积约为32km3。.镜下观察和电子探针测试发现大六冲火山岩和火山碎屑岩基质都发育玻晶交织结构，斜长石微晶大致定向分布于玻璃质中。火山碎屑岩熔结程度不一，晶屑丰富，有塑性浆屑，但镜下不见岩屑和玻屑。扫描电镜下两种岩石微观形貌有明显区别，前者基质均匀，孔洞极少，有少量雏晶生长；后者基质不均匀，有大量形态不一的微孔隙。.对大六冲爆发相和溢流相斜长石晶屑CSD统计表明，前者CSD曲线呈上凹形，未发生明显转折，说明岩浆房中未发生混合作用，但暗示有结晶分异作用。后者CSD曲线为微上凹形，与前者相似。研究还表明大六冲火山早期发生溢流式喷发，而后经历了较大规模的爆炸式喷发。.分别采集了火山岩、火山碎屑岩、炭化木及土壤烘烤层样品，通过14C、OSL、K-Ar和Ar-Ar等多种测年手段开展了包括大六冲火山在内的腾冲火山群年代学研究，对更新世以来的火山活动进行了年代学约束。研究结果表明，黑空山和城子楼火山在全新世有过喷发，距今3600年前可能是腾冲地区最新的火山活动年龄。明确了大六冲火山晚更新世发生大规模溢流和爆炸喷发的时间约为0.55Ma，目前所见的主锥体及残留火山颈就是那时形成的。大六冲火山通道规模较大，可能是区内晚更新世火山喷发的主通道，区内上百平方千米范围内巨量的火山碎屑物可能正是经由大六冲火山通道喷出和降落堆积的，火山上可能有岩石崩塌和滑坡灾害。