Volcanic history of the Conchagua Peninsula (eastern El Salvador)

International audienceNew results of detailed geological mapping, K-Ar dating and geochemical study of the Conchagua Peninsula in eastern El Salvador are presented. Volcanism in the area was controlled by intersection of three tectonic structures, the trench-parallel Central Graben, perpendicular Comayagua Graben, and the Guayape Fault Zone. The age of the volcanic activity spans from Miocene to Quaternary, however, the volcano itself is extinct. The basement is built of the welded rhyolitic Playitas ignimbrite, which extends as far as to the Island of Zacatillo. The pyroclastic rocks of La Unión unit (mean K-Ar age: 13.3 ± 3.7 Ma) display signs of mingling between basaltic and dacitic magmas (banded pumice, deposits containing both mafic scoria and felsic pumice fragments), and this is interpreted as a result of eruptions triggered by injection of a basaltic magma into a dacitic magma chamber. Lavas and pyroclastic flow deposits of the subsequent Pozo unit are poorly exposed and strongly altered. Following effusive activity hereby defined as Pilón Lavas was dominated by andesite and basaltic andesite lavas. Pleistocene volcanic activity is represented by the Pre-Conchagua edifice (1.6 ± 0.6 Ma), Cerro Montoso, El Bable and Juana-Pancha. Regarding the trace element composition, some lavas of the Pre-Conchagua - Juana-Pancha are distinct from common volcanic front products (lower Zr/Nb, Th/Nb, Ba/Nb), resembling the lavas of Tegucigalpa volcanic field to the north, which is located behind the volcanic arc in the Comayagua Graben. Behind-arc extensional tectonics could have facilitated the magma genesis via decompression melting of the mantle wedge. The current Conchagua Volcano consists of two cones, Ocotal and Banderas, built by repeated Strombolian eruptions associated with effusions of basaltic lavas. The uppermost unit consists of a white tuff preserved in the sedimentary fill of several tectonic valleys west of Conchagua. The white tuff was interpreted as distal fall-out of the Tierra Blanca Joven eruption of the Ilopango Caldera