Andesitic (SiO2: ~60 wt%) monogenetic volcanism in the northern Colombian Andes: Crystallisation history of three Quaternary volcanoes

Abstract

The Quaternary Villamaría-Termales Monogenetic Volcanic Field (VTMVF) is one of the monogenetic fields located within the subduction related, volcanic chain in the northern Colombian Andes. The dominantly andesitic composition of the products from this field provides an opportunity to study the crystallisation history of monogenetic volcanic products, associated with long term crustal stagnation before eruption. This work focuses on the Victoria, Gallinazo and Tesorito monogenetic effusive volcanoes, using a combination of petrography, mineral chemistry and whole-rock chemical analysis, in addition to geothermobarometers, to estimate the crystallisation conditions of the main mineral phases and infer the depths at which such phases crystallised. We also present two K/Ar radiometric ages for Victoria and Gallinazo volcanoes in order to add to the known age (1.2 Ma) of the Tesorito volcano. This helps us to timeframe this volcanism within the ~140 km volcanic chain where it is located. The rocks are porphyritic with a glassy and microcrystalline groundmass, and display glomeroporphyritic and disequilibrium textures such as textures sieve in plagioclase and reaction rims in pyroxene. The main mineral phases forming the rocks are plagioclase (An22–67), orthopyroxene (Wo02 En65–80 Fs18–33), clinopyroxene (Wo39–45 En39–49 Fs07–17), amphibole (magnesiohastingsite, magnesihornblende, edenite and tschermakite), and Fe-Ti oxides Usp37–23 Mag63–77 and Ilm79–62 Hem21–39. Olivine (Fo71–87) only appears in the Victoria volcano. The volcanoes are subalkaline andesites with calc-alkaline affinity. Estimation of the crystallisation conditions (i.e. temperature and pressure) for in-equilibrium mineral phases, indicate the depths at which they crystallised. Olivine crystallised at 1198–1191 °C and ~1.4 GPa, orthopyroxene-clinopyroxene coexisted at 963–880 °C and 0.78–0.47 GPa, plagioclase formed at 928–908 °C and 0.76–0.17 GPa, amphibole crystallised at 998–877 °C and 0.79–0.16 GPa, and Fe-Ti oxides crystallised at 937–904 °C. This suggests that crystallisation starts at depths of ~53 km and it continues until the surface. The obtained radiometric ages for the Gallinazo (1.92 ± 0.06 Ma) and Victoria (1.97 ± 0.06 Ma) volcanoes indicate that they are ~0.7 Ma older than the nearby Tesorito volcano.