TOWARDS AN EVOLUTION OF THE FUEGO VOLCANIC COMPLEX, GUATEMALA

The Fuego and Meseta volcanoes closely overlap and comprise the Fuego Volcanic Complex. Historic activity occurs exclusively at Fuego, the southern center, and consists of high-Al basalts. Meseta, the inactive northern center, is predominantly composed of basaltic andesites with minor basalt and andesite. A thick sequence of lava flows and dikes is exposed by a steep escarpment on the east flank of Meseta. The upper 75% of the sequence (41 lava flows) was sampled from three interfingering stratigraphic sections in 1981 and 1992. Temporal geochemical trends indicate an older period of edifice growth dominated by increasingly more silicic lavas, and a younger cycle where lavas become progressively more mafic up-section. This abrupt reversal in chemical trends is interpreted to signal a change in magmatic evolution from a crystal fractionation dominated style to one controlled mostly by magma mixing. Increased supply rates of mafic magma into the sub-volcanic magma chamber may be responsible for this change. Termination of Meseta eruptive activity has been tentatively linked to sector collapse and formation of the Meseta escarpment. Eruptive activity then migrated south forming the Fuego volcano where historic activity is similar in composition to lavas erupted during the younger eruptive episode at Meseta. A large debris avalanche deposit identified on the coastal plain south of Fuego appears likely to have originated at Meseta. To test this hypothesis, 24 lava block samples were collected along a longitudinal traverse of the Escuintla Debris Avalanche (EDA) in 1999 and 2003. Petrographic and geochemical comparisons between Meseta lavas and EDA lava blocks strongly suggests that the EDA did not originate from the scarp on Meseta volcano as previously proposed. The presence of amphibole-bearing dacitic rocks in the EDA requires a more evolved source than Meseta or the younger Fuego cone. Comparison to other nearby volcanoes (Agua and Acatenango), suggest a possible Acatenango source for the EDA, however, a suite of amphibole-bearing lava blocks collected in barrancas on Fuego's south flank in 2006 raise the possibility that an ancient Fuego cone collapsed to form the EDA.