Geochemistry and petrology of the Galeras Volcanic Complex, Colombia

The Galeras Volcanic Complex (GVC) has erupted lavas and pyroclastic flows, ranging from basaltic andesites to dacites, during the last million years. The chemical composition of the volcanic products has been fairly constant, characterized by calc-alkaline, high-silica, medium-potassium andesites. Variations from basaltic andesites to dacites have occurred during individual stages, but overall, composition and petrographic characteristics are quite constant. The Guaca cinder cone is the exception, being more alkaline, rather than calc-alkaline. The trace-element and REE contents of the GVC samples suggest that sediments or fluids from the subducted sediments are involved in the generation of the magmas at the source region. Although high Sr and Ba contents are also present in the upper continental crust, the high B levels are consistent with the contribution of sediment-related elements. Small degrees of partial melting of a hydrated mantle wedge are consistent with the observed distribution of REE, plus the addition of highly soluble elements from the subducted sediments. Crystal fractionation at depths of about 36 km (approximately 12 kbar) of a hydrated tholeiitic basalt melt can produce andesites and basaltic andesites having a phase assemblage of amphibole, plagioclase, clinopyroxene, magnetite, ilmenite and apatite. The magma then re-equilibrates at higher levels, reaching levels as shallow as 10 km (about 2.5 to 3 kbar) and crystallizing orthopyroxene and large amounts of plagioclase. At these shallow levels, amphibole is no longer stable and re-equilibrates to oxides, pyroxene and plagioclase. The melts become saturated with respect to most volatiles by depressurization and crystallization. In the GVC, we observe that it is not necessary to have large volumes of highly differentiated magma to cause caldera-forming eruptions.