Abstract: The Sihai Tephra Sheet(STS)was formed by the youngest eruption of Longgang volcanic area.The basaltic air fall deposit of this eruption composed the Jinlongdingzi scoria cone and the fallout tephra sheet east of the volcano,near Hongqilinchang in Huinan county and Sihailinchang in Jingyu county.This eruption is considered as Sub-Plinian eruption by using Walker G P L's method.It suggests that the strength of Jinlongdingzi volcano eruption was large.The cumulative frequency plots area shows a good coordination with its concentrated area,as well as a straight right-hand uprising tendency,which indicates a similar sorting process due to gravity during its transportation and setting down.The proximal samples have more coarse grains while the distal part becomes fine-rich as the distance from the vent increased.Although the samples from the other scoria cones are taken from near the vent,the enrichment tendency of the fine material becomes more gentle and have a bigger mass fraction during φ<0,which suggests that the eruption strength of the other volcanoes are obviously smaller than that of the STS.Comparing the distribution feature of the STS with that of Tianchi volcano one can see that the slopes of the Tianchi fallout changes less while the slopes of STS have an obvious variation.The averaged maximum diameter of the STS is smaller than that of Tianchi but the coarse particle proportion of the STS is bigger than that of Tianchi.The up-rise tendency of the fine particles from the STS is not as high as that the samples from the Tianchi volcano,which indicates that the explosivity of the STS is less than that of Tianchi.

Key words: Sihai Tephra Sheet, grain-size characteristics, Sub-Plinian eruption

摘要： 四海火山灰是龙岗火山群中的一次火山爆发形成的,这次火山爆发形成的玄武质空降堆积物分别组成金龙顶子火山渣锥和位于金龙顶子火山锥以东的、分布于辉南县红旗林场和靖宇县四海林场一带的低缓开阔的火山碎屑席。通过投点得知金龙顶子火山喷发类型为次布里尼式(Sub-Plinian)喷发,反映金龙顶子火山爆发强度很大。四海火山灰空降碎屑物7个样品的粒度累计频率曲线投点分布范围、集中区域均有较好的一致性,累计频率曲线表明碎屑物在空中搬运与沉降时都经过了类似的重力分选作用。近火口缘样品粗粒碎屑含量较高,随着与火口缘距离的增加,粗粒部分含量明显降低,细粒碎屑含量增加趋势明显。龙岗火山区内其它岩渣锥火山碎屑物粒度分布范围明显宽于四海火山灰粒度分布范围,累积频率曲线斜率较为一致。虽然样品距火山口距离均较近,但也出现了细粒富集程度变缓的现象,反映了龙岗火山区其它火山锥喷发强度明显小于四海火山。对比长白山天池火山碎屑物粒度分布特征发现,天池火山空降堆积物粒度分布斜率变化比较均匀,四海火山灰斜率有明显变化;四海火山灰最大粒度小于长白山天池火山空降堆积物,但是粗粒度碎屑物含量较高。细粒度碎屑物部分累计频率曲线上升趋势较缓,说明金龙顶子火山的喷发爆破性小于天池火山。

关键词: 四海火山灰, 粒度特征, 次布里尼式喷发