Abstract:

Through the facies analysis of volcanic deposits in the field, the microscopic identification and the rock-forming mineral chemistry, it can be proved that the comenditic lava at Late Qixiangzhan Stage in Tianchi volcano, Changbaishan area, is formed by the overflow but not the clastogenic lava. The top and bottom of the lava flow is the obsidian comendite of surface facies, and the middle part belongs to the comenditic interior facies with the typical plastic deformation and flow bending as well as the alternation between the dense thick layers and the crumbly thin layers. The rocks from the lava flow have porphyritic and glomeroporphyritic textures and from surface to interior facies, the vitreous texture, spherulite texture and felsitic texture in the matrix can be observed in turn, because of the difference of cooling rate. The phenocrysts, sodian sanidine, hedenbergite, fayalite and a little quartz, appear in all the facies of lava flow, and yet arfvedsonite, ferrorichterite and ilmenite occur as the microphenocrysts in the matix of interior facies. In comparison with the peralkaline pantellerite from Pantelleria volcano, Italy, the oversaturated comendites at Late Qixiangzhan Stage are rich in Si, Al, K, Mg, Ca and P, and poor in Fe, Na, Mn and Ti, and belongs to peraluminous alkaline series.

Key words: Tianchi volcano, Late Qixiangzhan Stage, comenditic lava, spherulite texture