FLUID OVERPRESSURE IN ANDESITE MELTS FROM CENTRAL SLOVAKIA - EVIDENCEFROM INCLUSIONS IN MINERALS

Unusual melt inclusions, containing a very dense hydrous solution in a ddition to glass, were found in plagioclase phenocrysts (An 68-36) of andesite from the Stiavnica caldera in Central Slovakia. The fluid com ponent of the inclusions is homogeneous or contains a gas phase. Accor ding to measurements in 65 inclusions, ice melting temperatures vary f rom -0.6 to -7.8 degrees C, which corresponds to a salinity of 0.9-11. 5 wt % equiv NaCl with an average value of 4.5 wt %. At T = 20 degrees C, the solution of this salinity has a density of 1.00-1.08 g/cm(3) w ith an average of 1.03 g/cm(3). The solution from melt inclusions with a two-phase fluid component homogenizes into a liquid phase at 140-28 5 degrees C, which gives 0.79-0.97 g/cm(3) for the density of the hydr ous fluid. The chemical composition of glass from the melt inclusions is characterized, according to 15 electron microprobe analyses, by hig h concentrations of SiO2 (72.6 wt %), K2O (6.4 wt %), and Cl (0.28 wt %), and low contents of TiO2, MgO, and CaO. The calculation of water c oncentration in the magma, including water in melt inclusions, yielded high values of 7.1 to 13.3 wt %. The fluid pressure was very high dur ing ma,oma crystallization-5.6 to 15-17 kbar at T = 800-900 degrees C. At the same time, the CO2 fluid inclusions found in the cores of the phenocrysts have a relatively low density (0.35 g/cm(3)), which corres ponds to a fluid pressure of not more than 0.9-1.1 kbar at 800-1000 de grees C. This value may be indicative of a magma chamber at a depth of 3-4 km. The occurrence of two different fluids in the same phenocryst s-early CO2 at 0.9-1.1 kbar and late H2O at 5.6-17 kbar-indicates that water was injected into a closed magma reservoir. Apparently, the sup er-high fluid pressure was the factor responsible for magma rise towar ds the earth's surface.