Author:Guo Zhengfu Liu Jiaqi Chu Guoqiang J.F .W. Negendank

Key words:maar lake, tephra, composition of glass shards. Huguangyan

Abstract:

Volcanic glass shards and pumice were recognized in core from the Huguangyan maar lake, Lei-Qiong volcanic field, southeast China. The tephra could not comprise ash layer in the sediments；however, the were dispersed in the same depth of the core, which is located in 27.60m beneath the top of the core C. The grains consist mainly of colorless glass shards and minor pumice.The average grain sizes of glass shards range from 8 ～ 65 μm. The glass shards display a curved thin—walled platy, cuspidate morphology and are optically isotopic. Electronic microprobe analyses of the interiors of the fresh glass shards show that Si02 ranges from 77.6％ to 78.8％, AL2O3 is from 12.4％ to 12.8％, TFeO is between 0.71％ and 0.92％ , Na2O is 2.10％ to 3.14％ and K2O is 4.7％ to 5.0％, which consist of high—silica rhyolite. The chemical composition of all shards is identical. This lack of significant compositional variation both within and between shards reflects a single eruptive origin. These compositions of glass shard are much different from those of volcanic rocks erupted in Lei Qiong volcanic field and are remarkably similar to those of distal fallout glass shards from the 74000aB.P.Youngest Toba Tu (YTT), found in India Ocean, Sumutra, Malaysia, South Cihna Sea.India subcontinent and Bay of Bengal. On this basis, we consider that all of the glass shards in the sediments of Huguangyan maar lake have plausibly originated from the YTT. The tephra occurrence in Huguangyan maar lake shows that the magma volume of the Toba eruption is much larger than those estimated before.

湖光岩玛珥湖火山灰的成分及其来源