Resumen

BORRERO, C; TORO, L. M.; ALVARAN, M  y  CASTILLO, H. Geochemistry and tectonic controls of the effusive activity related with the ancestral Nevado del Ruiz volcano, Colombia. Geofís. Intl [online]. 2009, vol.48, n.1, pp.149-169. ISSN 2954-436X.

In this paper, we discuss the geochemical aspects and the tectonic setting of the effusive activity between 2.0 to 1.0 Ma (the Ancestral Ruiz stage) of Nevado del Ruiz Volcano (NRV: 4° 50-55'N, 75° 14-20' W, 5,390 masl), this volcano rises at the northernmost end of the Andean volcanic chain in the Cordillera Central of Colombia. Whole-rock geochemical data of andesitic to rhyodacitic samples from the different vents share specific characteristics: SiO₂ = 56 -70.1 wt. %, relatively high MgO (1.1 - 8.1 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. The more acidic suites display the strongest adakitic character such as higher Sr, and lower HREE and Y contents. The N75°W Villa Maria -Termales fault system was the controller of the magma displacement aside of the main N-S volcanic axis during the Ancestral Ruiz stage. This tectonic control is evidenced by the distribution of the small effusive eruptions in monogenetic vents aligned parallel to the fault system, which were fed by lateral magma flow from the intermediate storage system located 5-10 km below the position of the present Nevado del Ruiz Volcano.

Palabras llave : Medium-K magmatism; adakitic signature; Late Pliocene -Early Pleistocene lavas; Villa María-Termales fault system; Nevado del Ruiz volcano.