Uncommon mafic rocks (MgO >10 wt.%) in the northernmost Andean volcanic chain (4° 25″ N): Implications for magma source and evolution

Abstract

Mafic volcanic rocks are typically formed by primitive magmas (MgO >8 wt.%) and are evidence of both a rapid magma ascent from the source and a negligible differentiation en route to the surface. Thus, mafic rocks on Earth are relatively close in composition to the primary magma. These rocks are common in intraplate and extensional tectonic environments, but not in subduction settings. At subduction zones, where magma differentiation is virtually inevitable, most magmas, if not all, evolve before reaching the surface. Hence it is difficult to approach primary compositions to understand, for instance, the percentage of partial melting in the mantle. This is the case in the northernmost Andean volcanic chain (~140 km) in Colombia. There, however, mafic rocks have been recently recognised, providing an opportunity to approach the composition of primitive magmas. The rocks are located at the south of the volcanic chain and are associated with the Quaternary Pijaos monogenetic volcanic field. The rocks are porphyritic, phenocryst-poor, and host two main mineral phases: olivine (Fo71-90) and clinopyroxene (Wo38-45, En45-54, Fs7-12), with plagioclase, pyroxene and Fe–Ti oxides as microlites. The products are basaltic to basaltic andesite, and have the typical sub-alkaline/high-K calc-alkaline chemical signature of subduction settings. Applied geothermobarometers indicate that olivine crystallised between 1325 and 1266 °C, while clinopyroxene between 1273 and 1117 °C. These values somehow constrain the partial melting temperatures not only below the field but also below the region. High-forsterite and high-CaO clinopyroxene evidenced in the rocks, also contribute to the comprehension of the magma evolution through deep fractional crystallisation in the complex magmatic plumbing system that dominates this region of the Andean volcanic chain.