Magmatic evolution of Romeral Volcano: insights from subduction segments and slab tear in the Colombian Andes

The current subduction of the Nazca plate below the Colombian continental margin includes two different segments of flat and normal subduction of the Nazca Plate, separated by a major slab tear at 5.5°N. These two different segments are also characterized by major differences in their volcanic character. Whereas the northern flat slab segments include mainly highly differentiated monogenetic volcanoes, with local peraluminous and adakite character. The southern segment consists of a series of polygenetic volcanoes and monogenetic volcanic fields are mainly andesites to dacites, with more restricted differentiated rhyolites. This magmatism shows high-K calc-alkaline signature, presents negative anomalies of Nb and Ti and enrichment in LREE and depleted in HREE which are typical subduction signatures in a convergent margin setting. Local adakite-like geochemical signatures have also been documented.  

The origin of the magmas in this segment has been attributed traditionally to processes such as mantle melting, slab fluid–induced metasomatism of the mantle wedge, subduction erosion, relamination, crustal assimilation and fractional crystallization. In contrast the role of the tear on the magmatic evolution, especially the potential effects on the incorporation of deep mantle reservoirs due to changes in mantle flow or melting of the lower crust remains poorly studied.

The Romeral Volcano located near the 5° N in the limit of the "Caldas Tear" represents one of the less studied volcanoes in the Colombian Andes, which could have a magmatic history that can allow to evaluate magma genesis in the transition between shallow and normal subductions.
In the field the volcanic structure is strongly dissected by glacial erosion evidenced by glacial striations and typical geoforms including U-shaped valleys.  New stratigraphic observations show that the volcano is made of series of porphyritic lava flows of andesite composition (pyroxene, amphibole and plagioclase) with trachytic textures and evidence of magma mingling, interspersed with block and ash deposits. New U-Pb geochronological constraints yield crystallization ages between 1.8 Ma and 1.6 Ma and εHf (0.1 to 5.3). Additionally, inherited zircons of Precambrian age (751.8 ± 6.2 Ma and 933.8 ± 6.4 Ma). 

These preliminary constraints suggest a significant crustal input and the presence of polybaric crystallization processes associated with the magmatic evolution. The origin of the crustal input from the upper or lower plate associated with tear evolution remains to be explored. We expect to incorporate whole rock and mineral geochemical constraints together with additional geochronological data to further understand the history of this volcano.