| Abstract: | The regional variation of physical and geochemical characteristics of Central American volcanoes occurs in two fundamentally different patterns. The first pattern is symmetrical about Nicaragua. Crustal thickness, silica contents of mafic lavas and volcanic edifice heights are lowest in Nicaragua and increase smoothly toward Costa Rica to the south and Guatemala to the north. Magma density is maximum in Nicaragua and decreases smoothly outward. The regional variation in crustal thickness is just enough so that magma densities, calculated at appropriate Moho pressures, are the same at the base of the crust throughout the region. This is consistent with magma ponding at the base of the crust. The bulk compositions of Central American basalts show the same symmetrical variation. Suites of Nicaraguan basalts plotted in pseudo-ternary CMAS projections indicate large olivine and plagioclase primary-phase volumes. Toward Costa Rica and Guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller, which is consistent with fractionation at increasing depth.The second pattern is the segmentation of the volcanic front and the plate margin in general. The segmentation strongly affects the spacing and size of volcanic centers. At segment boundaries volcanic centers are generally small and unusually widely spaced. Toward segment interiors volcano spacing and size increase systematically. The LIL element contents of lavas strongly reflect this pattern. For lavas with similar silica contents the larger the volcano, the higher the LIL element contents. The relationships between segmentation, volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab. The relationship between volcano volume and LIL element content is qualitatively in agreement with an open-system fractionation model. |
