Basaltic ignimbrite-like rocks on Saikhan Volcano, northeastern Khangai, Mongolia: Mineralogic and geochemical evidence |
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| Authors: | E I Demonterova A V Ivanov N S Karmanov |
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| Institution: | 1. Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033, Russia
2. Geological Institute, Buryat Science Center, Siberian Branch, Russian Academy of Sciences, Ulan Ude, 670047, Russia
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| Abstract: | Rocks having a pseudofluidal ignimbrite texture have been found on Saikhan Volcano in northeastern Khangai, Mongolia. The
rocks have a typically nodular banded texture. The fiamme and the bands vary in width between a few millimeters to a few centimeters.
These rocks have the same bulk composition as trachybasalts 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 basaltic 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 crystallization of olivine and Ti-magnetite.
Much of the pyroxenes (∼20%) in the ignimbrite-like rocks show calculated pressures during their generation to have been in
the range of 6.5–14 kbars, while all pyroxenes in the ordinary lavas and dikes crystallized at pressures below 0.3 kbars.
It thus follows that the magmas that have produced the ignimbrite-like rocks began crystallizing in the subcrustal magma chamber
under fluid-saturated conditions, whence they were rapidly transported to the surface. |
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