EMPLACEMENT AND PETROLOGICAL EVOLUTION OF THE ANDESITIC DOME OF GALERAS-VOLCANO, 1990-1992

Since the reactivation of Galeras volcano in 1988, the emplacement of a lava dome in the main crater of the volcano has been its most signif icant magmatic activity. Changes leading to the emplacement of the dom e were present as early as June-July 1990, when the trend of degassing for the major species (mainly SO2 and CO2) in the summit fumaroles ch anged drastically, and small vulcanian explosions become more frequent . Moreover, after August 1990, an inflationary trend as measured by an electronic tiltmeter was observed to increase over background levels, and the temperature of a newly formed fumarole (Besolima) increased f rom 130 to 738 degrees C within a month. The dome was seen for the fir st time in early October 1991 at the base of the main crater. The dome grew for about a month to a total volume of approximate to 4 x 10(5) m(3). During this time, the growth rate was calculated to be 12 x 10(3 ) m(3)/day or 0.14 m(3)/s. An explosive eruption on 16 July 1992 destr oyed most of the dome. The chemical composition of the dome ranges fro m 59.4 to 61.1 wt.% SiO2 (anhydrous). The presence of (1) large plagio clase phenocrysts with strongly inversely zoned rims and (2) small euh edral plagioclase crystals with normal zoning and Ca-rich cores indica tes that a magma mixing event occurred early in the history of dome em placement. Large amounts of crystal fractionation then took place. Abo ut 70% fractionation can explain the mineral suite and the major- and trace-element compositions of the glasses. The uppermost part of the d ome, degassing freely at the surface for ten months, was able to isola te the magmatic system, producing a highly evolved melt in the dome in terior. The July 1992 eruption tapped not only the uppermost part of t he dome but also a deeper H2O-saturated magma. Analyses from these sam ples show that most of the H2O was degassed while Cl and B contents we re higher than in the uppermost part of the dome. However, the F conte nt is lower in this deeper part of the conduit, while it increases in the matrix glass of the degassed upper part of the dome.