Magmatic evolution of basaltic rear-arc monogenetic volcanism in southern Colombia

The Guamuez-Sibundoy Monogenetic Volcanic Field is located in the Southern Volcanic Segment of the Central Cordillera of Colombia. This field hosting 19 volcanoes has been defined as formed by scoria cones, maars and lava domes. This type of volcanism is part of the volcanic arc related to the interaction between the Nazca and South American plates. This interaction has led to a transtensional tectonic environment represented by pull-apart basins linked to the so-called Algeciras Fault System. This study integrates different techniques including field geology, petrography, mineral chemistry, whole-rock chemistry and geothermobarometric analysis in order to know the compositional characteristics and reconstruct the evolution of the magmas that gave rise to several volcanoes in the field: Guayapungo, Yaku, Mujundinoy Sur and Fuisanoy volcanoes. This study is the starting point to understand the possible future activity in the field. The eruptive products associated with these volcanoes correspond to concentrated and dilute pyroclastic density currents and lava flows, the latter predominating throughout the field. Petrographically, all the studied rocks are porphyritic, with olivine (Fo 71–90 ), clinopyroxene (Wo 33–48 , En 36–54 , Fe 7–16 ), orthopyroxene (Wo 3–5 , En 68–71 , Fe 24–28 ), plagioclase (An 26–82 ), amphibole (magnesiohastingsite and tschermakite) and Fe-Ti oxides as mineral phases embedded in a microcrystalline, cryptocrystalline and glassy groundmass. They present glomeroporphyritic texture in olivine and pyroxene, zoning in plagioclase, skeletal in olivine and pyroxene, and spinifex-like texture in pyroxene. Fluidal and serial textures are also common. Fe-Ti oxides are present as a minor phase in all volcanoes. Chemically, the volcanoes are basaltic and basaltic andesite in composition and display calc-alkaline affinity, typical of subduction environments. Geothermobarometric analyses indicate that the crystallisation conditions for olivine were 1188–1137 °C, for clinopyroxene 1199–1015 °C and 1.7–0.2 GPa and for plagioclase 1127–1077 °C and 06–0.2 GPa. The analyses indicate that the magmas share a common origin at the asthenosphere-lithosphere boundary and that fractional crystallisation was the most important magmatic evolution process during magma ascent. The crystallisation conditions of the mineral phases indicate olivine, followed by clinopyroxene, as the first to start forming at the mantle-crust boundary (64–7 km), while plagioclase crystallised only at cortical levels (24–7 km). Chemical data and crystallisation conditions indicate that the Guayapungo volcano ascended directly from the mantle and experienced a rapid ascent from 15 km to the surface. The lack of crystallisation, in addition to the presence of amphibole xenocrysts, indicates that the Yaku and Mujundinoy Sur magmas experienced mild assimilation. Crystallisation conditions and further evolution at Fuisanoy volcano indicate slow ascent. • Monogenetic volcanoes in southern Colombia originated from partial melting at the asthenosphere-lithosphere boundary. • The geochemical signature of the volcanic products is typically of subduction environments. • The magma evolution is linked to fractional crystallisation processes during ascent. • The magmas experienced short periods of stagnation and slight cortical assimilation.

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