Transition from small-volume ephemeral lava emission to explosive hydrovolcanism: The case of Cerro Tujle maar, northern Chile

Cerro Tujle is an isolated Quaternary maar located 21 km south-east of the Salar de Atacama basin. It is situated at 3,554 m a.s.l., on the top of the Cordon de Tujle ridge forming a north-south striking morphological element in the Central Volcanic Zone of the Andes, in northern Chile. The material erupted at Cerro Tujle lies over the Tucticaro Ignimbrite (3.2 +/- 0.3 Ma), covering the whole area. The origin of the magma source, as well as the evolution and control of the volcanic eruptive styles, were determined by fieldwork, stratigraphic, morphometric, textural (density and vesicularity), petrographic, and geochemical analyses. The Cerro Tujle maar features an elliptical crater (333 x 279 m wide in east-west and north-south directions, respectively) surrounded by lava flow and tephra deposits. The total Dense Rock Equivalent volume of the products was calculated in 1.53 x 10(5) m(3), and mainly correspond to andesites (58.62-59.80 wt% of SiO2). The lava flow can be categorized in at least two types: i) brown-red and green-to-gray andesites located on the north-northwestern flank of the crater, and ii) black andesites with felsic rock clasts distributed around the crater underlying the other lavas. In contrast, the pyroclastic deposit displays breccias of juvenile pyroclasts with xenoliths of rhyolitic composition. These felsic clasts suggest magma assimilation with a shallow magma reservoir during a relatively fast ascent of the magma. This is evidenced by disequilibrium textures in mineral levels, such as reaction rim, and skeletal and sieve textures of the phenocrysts. The field evidence and results of the different analyses suggest a change in the eruptive style from effusive to explosive. This transition is associated with a decrease of the magma flux and a withdrawal of magma below the groundwater table, which has produced a phreatomagmatic eruption by magma-water interaction.