(Table 1) Geochemistry of glass shards at DSDP Leg 84 Holes

Three Leg 84 sites provided a good record of explosive volcanism onshore (in Central America). Ash layers and many ashy pods are interbedded in Recent to Oligocene strata. Evidence of the main periods of activity was noted in Recent to upper Pleistocene, Pliocene-Pleistocene, lower Pliocene to upper Miocene, lower Miocene, and upper Oligocene. Noticeable traces of older volcanism were found in upper Eocene strata. The chemical analyses of glass shards show a dacitic to rhyolitic composition with a low to moderate calc-alkalinity. A preliminary distinction of samples in three geochemical groups according to their K2O/SiO2 contents is done to test a magmatic evolution. Comparisons are made with Leg 67 and on-land data.