Monogenetic, behind-the-front volcanism in southeastern Guatemala and western El Salvador: 40Ar/39Ar ages and tectonic implications

Eighteen lavas erupted from cinder cones behind the volcanic front in southeastern Guatemala and western El Salvador have been dated using the 40Ar/39Ar method on groundmass separates. Plateau ages range between 1069.5 ± 22.1and 37 ± 12.0 ka. The majority of dated lavas are younger than 600 ka. Importantly, cinder cone lavas become progressively younger toward the volcanic front/trench. In addition, the lavas become systematically depleted in Cs, Ba, rare earth elements, Cs/Zr, La/Zr, and δ18O with time. The baseline values of these same geochemical parameters increase with distance from the volcanic front. The cinder cone lavas are closely associated with rifting south of the North American–Caribbean plate boundary. The trenchward migration of behind-the-front volcanism may be linked to a westward migration of this rifting. Existing evidence suggests there was an important change in the face of volcanism behind the volcanic front in southeastern Guatemala: from a small number of large polygenetic volcanoes to abundant smaller monogenetic cones. The temporal/spatial changes in the geochemistry of cinder cone lavas is attributed to decreasing contamination by Paleozoic basement rocks, both with time and with diminishing distances to the volcanic front.