Abstract: The Changbaishan Tianchi volcano is the most dangerous volcano in China. The volcano has erupted for several times since Holocene,and especially the violent Plinian-type eruption at about 1000 years ago had produced volcanic fallouts that had been carried as far as to Japan,and ignimbrite deposit within the range of 50 km around the volcanic crater. The ignimbrite deposit can be divided into two types: the valley-pond ignimbrite and ignimbrite sheet. The valley-pond ignimbrites are distributed mainly in Fushilin (Pumice Forest) and the valleys of the Jingjiang,Heishihe and Yalujiang Rivers,having a thickness of 20~60m. The ignimbrite sheets are distributed mainly in Guangming,Xinghuo,Dongfanghong and Weidong Forestry Centers,having a thickness of less than 10m. The ignimbrite deposits consist mainly of volcanic ash and pumice,associated with small amount of crystal and lithic fragments. It was commonly accepted in previous works that the pumice from ignimbrite deposit is grayish white in color and belongs to comendite. However,our field investigation has revealed that in addition to grayish white pumice,a small amount of black,gray and other colored pumices are also widely distributed. The samples of pumices of various colors have been collected during field investigation,and their chemical composition has been analyzed in the lab for comparison. The results show that the grayish white and black pumices belong respectively to comendite and trachyte,while the gray pumice belongs to trachyte but closer to comendite end member. The pumices of various colors came from the same crustal magma chamber,and were the products of different evolution stages of magma within the chamber. The simultaneous extrusion of these pumices indicates the zonation of the magma chamber and the mingling of magmas of various compositions.

Key words: pumice, petrochemistry, mingling of magma, Changbaishan Tianchi volcano

摘要： 长白山天池火山大约1000年前的大喷发,形成了巨厚的火山碎屑流堆积层,其主要组成是浮岩与火山灰。以往的研究普遍认为其中的浮岩为灰白色,属流纹质。笔者在考察中发现了不少黑色及少量其它颜色的浮岩,系统地采集了各色样品作浮岩化学成分分析,结果表明,灰白色浮岩与黑色浮岩分别为流纹质和粗面质,灰色浮岩属于粗面质但靠近流纹质端元。它们都来源于地壳岩浆房,是岩浆房内不同分异演化阶段的产物,它们同时喷出说明岩浆房内具有分带性及不同性质岩浆的混合。

关键词: 天池火山, 浮岩, 岩石化学, 岩浆混合