Effusive and evolved monogenetic volcanoes: two newly identified (~800 ka) cases near Manizales City, Colombia

Abstract

AbstractTwo recently discovered volcanoes (Las Margaritas 1 and 2) located near the city of Manizales in central Colombia (northern Andes) show that monogenetic volcanoes can be both effusive and be fed by evolved compositions, unlike in most other monogenetic fields. This study presents the results of cartographic, petrographic, geochemical, and geochronological analyses. Mapping indicates that the volcanoes are purely effusive, where the first erupted a dome coulée and the second erupted a lava flow. K/Ar dating of the groundmass yielded emplacement ages of 0.77 ± 0.04 and 0.80 ± 0.05 Ma for each volcano. The rocks in both volcanoes contain plagioclase, amphibole, and Fe-Ti oxides as ubiquitous minerals, but only one volcano hosts biotite. The two volcanoes can also be differentiated by the presence of amphibole oxidation rims in one of them. Both volcanoes are andesitic in composition and have a calk-alkaline signature. Trace elements show light rare earth element (LREE) enrichment, and negative Th, Nb, Ta, and Ti anomalies. Overall, the results indicate equilibrium conditions that allowed not only phenocrysts and microphenocrysts to crystallize, but also convection and stagnation processes that allowed zonation and glomerocrysts to form. This, along with the chemical information, indicates subduction characteristics that can be explained by small magma batches breaking off from crustal reservoirs. These effusive monogenetic eruptions are thus associated with efficient degassing during ascent, while compositional evolution is related to relative long-term magma stagnation in the crust.