Volumetric and compositional evolution of El Negrillar, one of the largest monogenetic volcanic field in the Central Andes, Chile

PARRA, DANIELA; LOAIZA, CAMILA; LARREA, PATRICIA; SALINAS, SERGIO; GODOY, BENIGNO; LE ROUX, PETRUS; GROSSE, PABLO

Congreso; vVMSG 2021; 2021

El Negrillar is a monogenetic volcanic field located in the Central Volcanic Zone (CVZ) (latitude 14° to 28°S). This field is placed within The Altiplano-Puna Volcanic Complex (APVC; latitudes 21° -24°S) which was mainly formed by four high dacitic-to-rhyolitic magmatic pulses, and hosts basaltic-andesitic monogenetic volcanism erupted in the last 1 Ma (Godoy et al., 2019). The number and distribution of mafic centers along the arc front of the CVZ suggests that the rise of these magmas is structurally controlled on a lithospheric scale, which provides paths for rapid magmatic ascent (e.g. Van Alderwerelt et al., 2017).El Negrillar is located at the southern boundary of the APVC, made up of more than 25 eruptive centers distributed over ~168 km2. It is divided into three zones according to spatial distribution of centers, related to the Socompa volcanic debris avalanche deposit (DAD): 1) Northern El Negrillar (NEN) located to the NE of the DAD, 2) Central El Negrillar (CEN), adjacent to the eastern border of the DAD, and 3) Southern El Negrillar (SEN), to the SW of the DAD. The NEN contains nine volcanic vents, with one to five lava flows emitted; the CEN contains eight volcanic vents that emitted between two to seven lava flows each; and the SEN zone consists of nine volcanic vents, with one to six lava flows emitted respectively. All these flows are mainly characterized as blocky lava flow morphologies. The magma volumes emitted by the CEN and SEN were calculated using a 12 m resolution TanDEMX Digital Elevation Model with the ArcGIS(c) surface difference tool, after reconstructing the prevolcanic topography in these areas. In both sectors, we estimated a total volume of vents, and the volume of each recognized flow. Both zones show similar emitted volumes, with 2.49 km3 for the SEN and 2.25 km3 for the CEN, comprising a total emitted volume of ~4.74 km3. These preliminary calculations position El Negrillar field as one of the most voluminous monogenetic systems in the CVZ. According our preliminary geochemical data and previous data by Hoffmann (2011) the erupted magmas from the NEN are trachyandesites, andesites and dacites, the CEN range in composition from basaltic andesites to dacites with 57 - 65 wt.% SiO2, whereas the SEN are trachyandesites with 60 wt.% SiO2. Overall, these monogenetic volcanoes exhibit more mafic compositions than the evolved polygenetic volcanoes that characterize the Central Andes. These less-evolved magmatic products might provide information on deep magma generation processes, something not quite common within an area dominated by evolved compositions.