湖光岩玛珥湖火山灰的成分及其来源

COMPOSITION AND ORIGIN OF TEPHRA OF THE HUGUANGYAN MAAR LAKE

湖光岩玛珥湖沉积物中的火山灰保存了其原始形貌、原生的沉积和成分特征 ,是进行岩芯沉积物对比和定年的理想材料之一。除了对火山灰进行常规的定年外 ,通过精确测定火山灰中玻璃碎屑 (玻屑 )的成分 ,并与已知时代的火山喷发物成分对比确定火山灰的年龄已成为火山灰年代学研究的重要领域之一 ,因为它能够区分年龄差别在同位素定年误差之内的数层火山灰。本文利用电子探针分析了湖光岩玛珥湖岩芯火山灰中玻屑的成分 ,经与全球年龄已知火山喷发物的成分对比和分析 ,认为该火山灰的年龄为 740 0 0aB .P .,在此基础上 ,讨论了湖光岩玛珥湖岩芯 740 0 0aB .P .以来沉积速率的变化趋势。

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, they 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 ～ 65μm. 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 SiO 2 ranges from 77.6% to 78.8%,Al 2O 3 is from 12.4% to 12.8%,TFeO is between 0.71% and 0.92%,Na 2O is 2.10% to 3.14% and K 2O 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 shards 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 74?000aB.P. Youngest Toba Tuff (YTT), found in India Ocean, Sumutra, Malaysia, South China 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.