The volcano-tectonic evolution of the Miocene Santa Lucía Volcano, Boaco district, Nicaragua

The present-day Santa Lucía caldera is an erosional relic of a Late Oligocene to Early Miocene stratoshield volcano located in the south-western part of the Chortis Block in Central Nicaragua. Six main lithological units were recognized: (Unit I) dacitic ignimbrite of Boaco type, which represents the basement of the Santa Lucía caldera; (Unit II) dacitic ignimbrite of Fonseca type, locally intercalated with epiclastic and dacitic lavas; (Unit III) “lower” andesite lavas; (Unit IV) blocky, lithic-rich pyroclastic flow deposits, (Unit V) “upper” andesite and basalt lavas, and (Unit VI) epiclastic rocks (lahar deposits). On the basis of field mapping, petrological and geochemical data, a new model for the evolution of the Santa Lucía Volcano is presented. The first stage consisted of a series of strong Sub-Plinian eruptions, which produced thick ignimbrite units. These events destroyed the pre-existing volcanic edifice. The second stage was dominated by large explosive eruptions producing mainly non-welded dacitic—andesitic ignimbrites. The next stage resulted in the formation of andesitic lava flows and minor tephra fall-out deposits, covered by voluminous basaltic lavas. Lahars probably triggered by volcanic and/or seismic events represent the final stage of volcanic activity. From the Miocene onwards, volcanic rocks became deeply weathered and locally eroded. Strongly weathered rocks are susceptible to landslides, and surficial modification by post-volcanic slope movements is observed on the slopes of the Santa Lucía Volcano. Identification of the lithological units and their comparison with wider Tertiary volcanic areas in Central Nicaragua will contribute to the knowledge of the regional volcanic stratigraphy and evolution.