Overview and plumbing system implications of monogenetic volcanism in the northernmost Andes' volcanic province

Abstract

Monogenetic volcanic fields are commonly related to rifts and/or intraplate tectonic settings. However, they are also less commonly recognised in subduction zones, including both front and back-arc volcanoes (e.g. monogenetic volcanoes associated with the Llaima volcano in Chile or the San Agustín Volcanic Field in Colombia). Here, we describe monogenetic volcanic fields associated with subduction-related polygenetic volcanism in the northernmost part of the Andes Northern Volcanic Zone (NVZ) (2° S to 4°30′N). These fields are associated with the main axis of the Quaternary volcanoes. They are linked to the polygenetic San Diego – Cerro Machín Volcano Tectonic Province (~140 km long; SCVTP) in Colombia, the chain that hosts the iconic Nevado del Ruiz volcano. Presently, three monogenetic volcanic fields, with a typical calc-alkaline signature, have been identified on both sides of this province. From south to north, they are: 1) Pijaos Monogenetic Volcanic Field (PMVF), 2) Villamaría – Termales Monogenetic Volcanic Field (VTMVF), and 3) Samaná Monogenetic Volcanic Field (SMVF). PMVF is located ~25 km south of Cerro Machín volcano, the southernmost active volcano of the SCVTP. This field was formed by at least two eruptions with both effusive and explosive eruptive styles. Three cones and a maar are recognised. Previous work has defined the volcanoes as basaltic andesitic in composition, which we recognised as the most mafic expression (MgO 10–11 wt%) in the whole SCVTP. Its source is related to the same, but deeper magma that feeds the volcanoes in the SCVTP. Stratigraphic relationships show that the volcanoes are younger than the underlying alluvial and volcaniclastic Ibagué fan (<2.58 Ma). VTMVF is located in the northwestern region of the SCVTP (>5 km of the axis of the province). This field is made up of at least 14 volcanoes aligned with the active Villamaría – Termales fault system, as previous work has recognised. The volcanism has been mainly effusive, represented by lava domes and some lava flows. Since the 1980s, studies have framed these andesitic to dacitic volcanoes in the context of the history of the Nevado del Ruiz volcano. It is inferred that the magmatic source is a shallow magma reservoir underneath the SCVTP. Stratigraphic and field relationships have shown that the last eruption occurred <38 ka. SMVF is located ~50 km north of Romeral composite volcano, the northernmost active volcano from the SCVTP. Previous studies have revealed that this field comprises at least two volcanoes: A maar (~20 ka years old) and a pyroclastic cone (~33 ka years old). These studies defined the volcanic products as andesitic and dacitic in composition. It is inferred that this field is a result of the same magmatism as that of the SCVTP. Overall, it is clear that monogenetic volcanism is not atypical in the area. We thus propose a SCVTP magmatic plumbing system joining both the monogenetic and polygenetic volcanoes related to the subduction arc.