The relation between monogenetic and polygenetic dacitic volcanism. Case study from Tapias dome (<95 ka) and Cerro Machín volcano (<50 ka), Colombia

The Tapias lava dome is located ~5 km SE of the explosive, Cerro Machín polygenetic volcano, both located on the Colombian Central Cordillera. The edifices have been interpreted as generated by the same magmatic system; however, the genetic and temporal relations between both edifices have not been established. In this work, the Tapias dome is studied using petrographic characterizations, mineral-, whole-rock- and isotopic chemistry, geothermobarometric calculations, and a K/Ar radiogenic age, and subsequently it is explored the relationship with the Cerro Machín. The rocks from Tapias dome are porphyritic with phenocrysts of plagioclase (An28-48), amphibole (tschermakite and magnesiohastingsite), biotite, quartz, and Fe-Ti oxides, embedded in a glassy groundmass. Petrographically, two types of amphibole crystals were recognized: type I, which is homogeneous, and type II, which displays different rims. The composition of Tapias is dacitic (SiO2: 66.59 wt.%) with calc-alkaline affinity. Our geothermobarometric calculations indicate that type I amphibole crystallized at 871-874°C, between 10.2 and 10.4 km depth. The core of the type II amphibole crystallized at 852-920°C, between 8.5 and 12.3 km depth, while its rim did it at higher temperatures (880-973°C), but at similar depths (9.1-11.8 km). The K/Ar radiogenic age obtained for the groundmass shows an emplacement age not older than 95 ± 0.05 ka. When comparing the results to the Cerro Machín, it is noted that the mineral and compositional characteristics are nearly identical. These results suggest that indeed both volcanoes share the same magmatic system and are fed by magma hosted between ~9 and ~12 km depth. Moreover, it is proposed that the vent location of both edifices, as well as the time gap between the eruption at Tapias and the beginning of the activity at the Cerro Machín (~50 ka), are the result of crustal deformation caused by the interaction between the Otú-Pericos and Ibagué faults.