THE ANCESTRAL STAGE OF NEVADO DEL RUIZ VOLCANO, COLOMBIA: THE GEOCHEMICAL SIGNATURE OF ITS EFFUSIVE ACTIVITY

The Nevado del Ruiz Volcano (NRV: 4° 50-55’N, 75° 14-20’ W, 5,390 masl), one of Colombia’s most active volcanoes rises at the northernmost end of the Andean volcanic chain in the Cordillera Central, is made up of three main stages: (1) the “Ancestral Ruiz”: 1.8-1.0 Ma, (2) the “Older Ruiz” : 0.8-0.2 Ma, and (3) the “Present Ruiz” : <0,15 Ma.

A new interpretation of the Ancestral Ruiz stage is presented in this work based on photogeology, DEM’s, fieldwork and chemistry of the lavas. The effusive products of this stage covered an area of about 150 Km², and this activity was related with the former volcanic vents probably defined by some stratovolcanoes and domes. In this work, we also include as part of this Stage, a set of monogenetic vents located to the northwest of the NRV aligned parallel to the N75ºW Villa Maria –Termales fault system. These small vents controlled by the fault system were fed by lateral magma flow from the intermediate storage system, located 5-10 km below the position of the present NRV.

Whole-rock geochemical data of basaltic andesites, andesites, dacites and rhyodacites from the different vents share specific characteristics: SiO₂ = 53.69 -70.1 wt. %, relatively high MgO (1.1 - 8.34 wt.%) and medium K₂O (1.06 - 4.36 wt.%) contents, low FeO/MgO ratios (typically less than 1,9), low fractioned rare earth element patterns with low Y and heavy rare earth elements contents, Sr commonly between 500 and 1,000 ppm and Ba up to 1,600 ppm.

K/Rb ratios always show a negative correlation with SiO₂ and Rb suggesting assimilation-fractional crystallization (AFC) mechanism. Rb/Sr, Ba/Sr and Rb/Ba ratios display a positive correlation with SiO₂, typical of calcalkaline trends related with subduction zones. Only, the more acidic suites display the strongest adakitic character such as higher Sr, and lower HREE and Y contents.