RESUMEN El Volcán Cerro Machín (VCM) se localiza en el flanco oriental de la Cordillera Central de Colombia a 17 km al occidente de la ciudad de Ibagué (Tolima). El VCM es un volcán Holocénico de composición dacítica, con evidencia de grandes erupciones (VEI 5). Estratigráficamente, en las paredes internas del cráter es posible observar una secuencia de depósitos de Corrientes de Densidad Piroclástica (CDPs) diluida con características de depósitos de oleadas basales. La presencia de líticos accidentales evidencia que durante la fragmentación fue involucrado el basamento. De esta manera es posible sugerir que la interacción agua/magma jugó un papel importante en la dinámica del último evento eruptivo, el cual constituye los depósitos que conforman el cráter. Análisis morfológicos evidencian un cráter de 2,4 km de diámetro con una abertura hacia el sector suroccidental, el cual alberga un domo de 490 m de altura que posiblemente durante su emplazamiento fue el causante de la destrucción del cráter. Aunque el VCM es sin duda un volcán poligenético (seis erupciones en los últimos 5000 años), las características sedimentológicas de los depósitos del cráter así como su morfología, sugieren una estructura monogenética de cono de toba. En conclusión, el VCM debe ser definido como un "volcán compuesto con morfología de cono de toba". Palabras clave: Corrientes de Densidad Piroclástica (CDPs); oleadas piroclásticas; geomorfología volcánica; volcán poligenético; cono de toba.

The Combia Volcanic Province (~11-5 Ma), is a volcaniclastic sequence located in northwest Colombia between the Central and Western cordilleras at 5-6° N latitude. Its source is associated with the volcanic activity of the magmatic arc produced by the subduction of the Nazca plate under the South American plate in the northern Andes. The distribution, composition, and chronostratigraphy of the province’s deposits are well-known, with the volcanic sequences characterized as compositionally bimodal. The older rocks (ca. 11-9 Ma) display tholeiitic affinity, whereas the younger (ca. 9-5 Ma) are mostly calc-alkaline, with some adakite-like signature recognized. While the magmatic system for the calc-alkaline magmas has been previously extensively studied, the processes that occurred during the magma stagnation and ascent are unknown for the tholeiitic magmas. This work bridges this gap by the study of tholeiitic lava flows outcropping at the center of the province, through petrography, mineral chemistry, whole-rock analysis, and geothermobarometry calculations of the crystallization conditions. Texturally, the rocks are porphyritic with plagioclase (An50-90) and clinopyroxene (augite and pigeonite) phenocrysts and microphenocrysts, embedded in a glassy and microcrystalline groundmass. Compositionally, the rocks vary from basaltic andesite to andesite (52.8-57.8 wt% SiO2), with relative enrichments of LILE to HFSE and REE to chondrite. Crystallization conditions, based on several plagioclase-melt and pyroxene-melt geothermobarometers, were estimated at T=1,095-1,153 °C and P=0.22-0.60 GPa for the plagioclase, T=1,046-1,131 °C and P=0.09-0.21 GPa for the augite, and T=867-1,039 °C and P=0.40-0.60 GPa for the pigeonite. These results suggest a relatively rapid magma ascent for the tholeiitic products as well as an evolution mostly through fractional crystallization. The LILE elements enrichment, the negative trend in the FeOt, TiO2 and CaO versus SiO2 content, together with some disequilibrium textures, are also evidence of crustal contamination and magma recharge. Thus it is proposed that the Combia Volcanic Province started as a simple magmatic system, where the tholeiitic products were generated by a relatively rapid magma ascent. Then, a more complex magmatic system linked to long-term magma stagnation, allowed melt evolution to form calc-alkaline magmas as previously defined.