Quaternary tephra of Northern Central America

Silicic Plinian tephra units representing more than 30 Quaternary eruptions blanket Guatemala and El Salvador. They were erupted mainly from 5 principal sources, all of them calderas. Several of the eruptions were accompanied by ash flows. These eruptions also have the most extensive tephra deposits. The total volume of material erupted is equivalent to 300–500 km3 of dense rock. A major uncertainty is the volume of tephra scattered very far from the source. The volume of silicic magma erupted in the Quaternary is similar to the volumes of mafic lava produced at the volcanic front. The basaltic and andesitic cones of the volcanic front parallel the offshore Middle America trench and the active underthrust zone. The five caldera sources form a trend parallel to the volcanic front, on the side opposite the trench, where the older continental crust abuts the volcanic zone. The ages of silicic volcanism precede and overlap with the age of mafic volcanic front, which is largely younger than 50,000 years. All of the calderas have multiple eruptions which span at least many tens of thousands of years. Between the calderas the interfingering of ashes has allowed a network of relative ages to be established. We used a variety of techniques to characterize these units. They can be readily distinguished from units from many other provinces, but considerable effort is required to distiguish among the local units. Standard field and petrographic observations (stratigraphic data, thicknesses, grain size, lithic content, mineralogy) establish the critical framework which disallows most erroneous correlations. Geochemical analysis, particularly trace elements, provide a rapid means of ruling out many more possible corrections. Qualitative mineralogical analysis by electron microprobe of hornblende and Fe-Ti oxides was a very effective last resort for correlation