Basaltic Ignimbrite-Like Rocks on Saikhan Volcano, Northeastern Khangai, Mongolia: Mineralogic and Geochemical Evidence

摘要

Rocks having a pseudofluidal ignimbrite texture have been found on Saikhan Volcano in northeast-ern Khangai, Mongolia. The rocks have a typically nodular banded texture. The fiamme and the bands vary inwidth between a few millimeters to a few centimeters. These rocks have the same bulk composition as trachy-basalts and do not differ from the ordinary trachybasalts found on this volcano in the form of dikes and lavas.The difference consists in the composition of glasses and minerals, as well as in the concentration of CO2(which is higher in the ignimbrite-like rocks). The glasses in the ignimbrite-like rocks show a trend from basal-tic trachyandesites to tephriphonolites and foidites, thus indicating the liquidus crystallization of clinopyroxene.The glasses in the lavas and dikes have a trachyte composition, indicating a residual origin following the crys-tallization of olivine and Ti-magnetite. Much of the pyroxenes (-20%) in the ignimbrite-like rocks show calcu-lated pressures during their generation to have been in the range of 6.5-14 kbars, while all pyroxenes in theordinary lavas and dikes crystallized at pressures below 0.3 kbars. It thus follows that the magmas that haveproduced the ignimbrite-like rocks began crystallizing in the subcrustal magma chamber under fluid-saturatedconditions, whence they were rapidly transported to the surface.