PETROLOGY
CHEMISTRY
In order to create a more complete data set previous analyses of Fuego were also used. These analyses are mostly of rocks erupted in the 1970's (Rose et al., 1973, 1978; Martin, 1979). Analyses of lavas from the eruptions of 1932 (Deger, 1932), 1962 (Pushkar, 1966), and 1966 (Stoiber and Rose, 1970) were also used. Nearly all of these rocks are basaltic in composition. The older rocks of Fuego are generally more silicic than the recent eruption products. Fuego eruptives range between calc-alkaline basalts and andesites when Irvine and Baragar's (1971) classification is used.
(Chesner and Rose, 1984)
- Selected Harker variation diagrams for the Fuego rock suite. Circled fields indicate analyses of Fuego's historical eruptions. Triangles are weighted average parent compositions of the the 1974 eruption. Bars give a measure (±1 sigma) of the precision for each element (Chesner and Rose, 1984).
PETROGRAPHY
The 60 thin sections examined by Chesner and Rose (1984) include basalts, basaltic andesites, and andesites. In general, all eruptive types are porphyritic with complexly zoned, inclusion-rich plagioclase being the most abundant phenocryst phase (Anderson, 1984). Mafic minerals in Fuego rocks commonly occur in glomero-porphyritic clumps. In many instances olivines were noted to have thin clinopyroxene reaction rims. An inverse relation between abundance of orthopyroxene and olivine/clinopyroxene is obvious throughout the suite. Modal analyses of the 14 stratigraphic samples show this relation quite well. Magnetite always occurs as a phenocryst and groundmass phase. Apatite is the only common accessory mineral. A diversity of groundmass textures occur in Fuego's rocks. Most of these textures range from hypocrystalline to holocrystalline varieties. Mineral compositions for phases in the 1974 eruption of Fuego were: plagioclase, olivine, augite, and magnetite (Rose et al., 1978). These mineral compositions probably reflect those in older Fuego rocks because all of Fuego's rocks are similar in texture and bulk chemistry.
- Summary of petrography for different rock types in the Fuego suite
(Chesner and Rose, 1984).
PETROLOGIC SUMMARY OF 1974 ERUPTION
The largest of a series of 20 vulcanian eruptions of Fuego volcano between 1944 and 1976 occurred in October 1974 in four distinct 4-17-hour pulses over a 10-day period. The eruption produced more than 0.2 km³ of pyroclastic high-Al2O3 basalt (equivalent to >0.1 km³ of void-free magma), quenched at a temperature of about 1050°C. Early erupted magma was rich in plagioclase and poor in mafic phenocrysts, magmas erupted next in sequence were poor in all phenocryst phases, while the final magmas were rich in all phenocryst phases. These changes, and variations in 8 major and 21 minor elements can be explained by crystal/liquid fractionation of high-Al2O3 basalt. Crystals in Fuego's magma contain inclusions of glass with an average H2O content of about 3%, an amount that suggests that crystallization began at a depth of >5 km. Petrographic data, observed extrustion rates, glass inclusion analyses, regional geological features, and various geophysical considerations collectively suggest that crystallization adn differentiation occurred within a vertical, dike-like conduit during a period of a few days to several months. The Fuego parental basalt contained 5% MgO, 10% FeO and about 0.8% K2O, it began crystallization at 1030 ±50°C with plagioclase, olivine, and magnetite the principal phases. Plagioclase was apparently rafted upward by bubbles within the magma body. Tidal triggering of crystal growth, magma ascent and eruption are suggested.
(Rose et al., 1978)
