VOLCANISM AND HYDROTHERMAL ACTIVITY: THE VIEW FROM GALUNGGUNG VOLCANO, INDONESIA
Within ~300 m of the granodiorite, contact metamorphism has produced assemblages containing tourmaline, pyroxene, garnet and biotite. At shallower depths, amphiboles, epidote, chlorite and illite dominate the secondary assemblages. These assemblages record the development of a high temperature liquid-dominated geothermal system generated by intrusion of the stock. The age of the granodiorite is constrained to the period between ~5910 + 76 and 4200 + 150 y BP by 14C dating of charcoal. At ~4200 y BP, flank collapse and the formation of the volcano's crater, Kawah Galunggung resulted in decompression and boiling of the early liquid reservoir. Amorphous silica, chalcedony and quartz precipitated during this event. Fluid inclusion salinities up to 24 wt. % NaCl-CaCl2 equivalent record the progressive boiling-off of the early liquid-dominated system and the formation of the vapor-dominated regime. As the fluid pressures declined, descending condensate deposited anhydrite and carbonates. Tourmaline, fluorite, apatite and native sulfur precipitated near the vapor chimney, reflecting interactions with magmatic gases. Chlorite and wairakite formed where condensate mixed with the deeper reservoir fluids. Scales of Na, K, and Fe chlorides document the dryout of the vapor-dominated zone.