Abstract: Gushantun,200km to Tianchi volcano,is located in the Longgang volcanic area of northeastern China. Cryptotephra were discovered in the sediment at 40～43cm depth,and the particular shape,such as bubbles,flat and sharp rim,suggests that the tephra shards are the result of direct sinking to the bottom rather than the one of secondary transportation. Electron microprobe analysis showed that the volcanic ash was derived from rhyolitic lavas which are different from the characteristics of the Longgang area volcanism in Quaternary.
The age of the bed at depth 40～43cm in Gushantun is 260～420cal a BP calibrated by using AMS ¹⁴C.This is very close to the Tianchi volcano eruptions which happened in 1702,1668 and 1597AD.Moreover,these three eruptions are all rhyolitic eruptions which have the similar characteristics with tephra shards detected in Gushantun. All of the information shows the Gushantun shards could come from Tianchi 1702,1668 or 1597AD eruptions. There are few records about these eruptions,however,they happened after the millennium eruption of Tianchi volcano(1199-1200AD)and have geochemical characteristics very close to that of the millennium eruption. In the future work,if the tephra shards of these three eruptions are discovered in the sediments,we can do more careful work as tephra subsampling and extracting in the underlying layers to look for the possible Tianchi tephra. This will be very helpful for searching tephra shards of the Tianchi millennium eruption in the research area. If more Tianchi tephra shards are discovered in Longgang volcanic area,the firm tephrochronology and tephrostratigraphy frame can be established to provide the time basis for stratigraphic comparison and trace the evolution of volcano eruption. Moreover,the use of multi dating methods could provide precise and higher resolution time scale for recording the climatope,which is very significant for studying the past global changes.

Key words: Gushantun, cryptotephra, electron microprobe analysis, tephrochronology

摘要： 孤山屯位于东北龙岗火山区,距离长白山天池火山200km,在沉积物深度40～43cm处发现少量火山灰颗粒,其形态不规则,呈多气泡状、薄板状、尖角状结构,属原生火山灰。电子探针分析结果显示其属于碱流质火山喷发,而不同于龙岗地区第四纪火山活动的特征。通过AMS ¹⁴C年龄所建立的年龄模式推断孤山屯沉积物深度40～43cm(即发现火山灰的层位)处的年龄应为260～420cal a BP, 这与史料记载的天池火山公元1702年、1668年和1597年的喷发年龄较接近,而且公元1702年、1668年和1597年主要为碱流质岩浆喷发,与孤山屯沉积物中发现的火山灰的地球化学特征也相吻合,进一步证明孤山屯沉积物深度40～43cm处的少量火山灰应来源于天池火山千年大喷发后的公元1702年、1668年和1597年喷发。关于天池火山公元1702年、1668年和1597年喷发的记录比较少,这3次小规模的喷发发生于天池火山千年大喷发之后,且其喷发物的特征与天池火山千年大喷发的特征比较接近,却亦有不同之处。因此,在对沉积物中火山灰的探寻研究工作中,若发现了来自公元1702年、1668年或1597年的火山灰颗粒,可对其下部地层做更为细致的分样以及火山灰提取工作,这对未来在研究区域寻找长白山天池火山千年大喷发的记录有很大的帮助。如果天池火山大喷发的喷发物在龙岗火山区更多的沉积物中发现,便可在该地区建立非常稳固的火山灰年代地层学框架,为地层对比和追踪火山演化历史提供时间依据,最重要的是多种测年方法的应用可为记录气候环境提供准确的高分辨率时间标尺,而这对于过去全球变化研究亦非常重要。

关键词: 孤山屯, 显微火山灰, 电子探针分析, 火山灰年代学