POTASSIUM CONTENT OF LAVAS AND DEPTH TO THE SEISMIC ZONE IN CENTRAL-AMERICA

Recent volcanic rocks in Central America occur in three structural settings. The first is a narrow zone of volcanoes, the volcanic front, where large volumes of calc-alkaline rocks are erupted. Most volcanoes in this zone are basaltic to andesitic composite cones in lineaments parallel to the strike of the seismic zone and clearly related to it. The second structural setting is behind the volcanic front and above transverse structures where the strike or dip of the seismic zone abruptly changes. Here quartz- and olivine-normative basalts are common and nepheline-normative basalts are present. Occasional rhyolites make the distribution of lavas distinctly bimodal. Most volcanoes in this structural setting are small monogenetic shields and cinder cones forming clusters located 25-100 km or more landward from the volcanic front. The third structural setting is far behind the volcanic front. Here strongly undersaturated lavas are erupted from volcanoes having no relation to the structure of the inclined seismic zone. In Central America there is no evidence for an increase in potassium contents of lavas with the depth of the seismic zone for either the basaltic to andesitic lavas of the volcanic front or for the basaltic lavas behind the volcanic front and above transverse breaks in the arc. When both groups are considered together, the increase in potassium contents of lavas across the Central American arc is unrelated to the depth to the seismic zone. © 1979.