大同火山活动在区域沉积中的记录和阶段性历史研究

大同火山活动的阶段性历史是一个尚未被揭示清楚的地学问题。野外实地考察及室内磁化率、化学成分、粒度和古地磁的测定结果显示：古湖滨处水平层理发育的火山碎屑层存在4个不同倾角的倾斜;古湖近岸处的湖相沉积层中含有4层火山碎屑层;古湖深处的湖相沉积层中出现4个磁化率显著正异常的层位,且这些层位粒度都较粗;古湖岸上的黄土沉积中出现3个磁化率为峰值、Rb/Sr比值为谷值的层位;古湖深处湖相沉积剖面中存在B/M和Jaramillo磁极性界限。据此进一步分析后认为：① 湖相沉积剖面中的磁化率显著正异常层位、黄土沉积中的高磁化率—低Rb/Sr比值层位可能是混入了大量火山灰物质;它们是区域火山活跃期的记录。② 系列沉积剖面特征指示区域火山活动曾经历4个活跃期,每个活跃期期间有多次密集的火山喷发。③ 4个活跃期的年代大约为早于0.90 Ma BP（第一活跃期）、~0.47 Ma BP（第二活跃期）、~0.31 Ma BP（第三活跃期）和~0.09 Ma BP（第四活跃期）;黄土沉积中还记录了区域北部有一个发生于~0.19 Ma BP的活跃期。此外,区域湖相沉积中所记录的4个火山活跃期都出现在湖退时期,平静期出现在湖侵时期。

The sedimentary record of Datong volcano eruption and its active stages

Datong Volcano eruption is an important geologic phenomenon of the Quaternary in North China and has been attracting scientists' attention; however, its episodic activity has remained unclear. Here, we demonstrate some new findings to reveal the episodic activity of the volcano during the Quaternary. (1) Volcano clastic layers with different ages which deposited near the paleolakeside incline with dip angles of 30°, 21°, 9° and 4°, respectively. (2) There exist four fine volcano clastic layers in the grayish-green lacustrine sediment in the paleolake inshore. (3) There are four sediment layers in the center of the paleolake which display high magnetic susceptibility value, indicating that some volcanic ash may deposit in them. Besides, the grain size of the four lacustrine layers is much coarser than that of others. (4) There exist three sediment layers in the loess section in the land some away from the paleolake, which display high magnetic susceptibilily value and low Rb/Sr one, indicating that some volcanic ash may deposit in them. (5) Magnetic boundary of B/M and Jaramillo Positive Subzone were found in the lacustrine section in the center of the paleolake. Based on these findings, we deduced that (1) Datong volcano experienced four episodic activities during the Quaternary and there occur many times volcano eruptions during each episodic activity; (2) the beginning times of four episodic activities are older than 0.90 Ma BP, ~0.47 Ma BP, ~0.31 Ma BP and 0.09 Ma BP, respectively; (3) another episodic activity with the beginning time ~0.19 Ma BP was also recorded in regional loess sediment; (4) the occurrence of each of the episodic activities of the volcano is synchronous with that of each of the paleolake regressions, implying that there is some relationship between the volcano eruptions and the paleolake regressions.