Regional and local variations in geochemistry and tectonics along and across Central America

Three tectono-magmatic systems create volcanism in Central America; the volcanic front, a weak secondary front and the back-arc. Most magma output occurs at the volcanic front along narrow lines of historically active composite volcanoes in strikeslip/extensional settings. Lavas have strong slab signature, are water-rich and plagioclase and pyroxene phyric. A sporadic secondary front occurs about 30 km behind the main front. Lavas have moderate slab signature and are plagioclase and pyroxene phyric. Most secondary front centers are at least moderately eroded but some Holocene activity occurred in southern Honduras. These two magma systems likely originate from slab flux that lowers the melting point in the mantle wedge. Back-arc volcanism occurs in clusters of cinder cones and small shields in strongly extensional settings, such as the Ipala graben. Lavas have moderate to near zero slab signatures. Typical lavas are nearly aphyric with rare olivine phenocrysts. There has been no historic activity but moderate Holocene activity is certain given the youthful morphology of many southeast Guatemalan and Salvadoran cones and lavas. Decompression melting is likely in these strongly extensional back arc regions. Typical volcanic front lavas are calc-alkaline basalts through rhyolites. The mafic lavas of this group have low HFS elements and are locally called low-Ti lavas. In contrast, most back arc lavas and some distinct volcanic front lavas have moderate to no depletion in HFS elements and higher TiO2. They are locally called high-Ti lavas, even though most have only 1.0 to 1.6 wt. % TiO2. At the Nicaraguan volcanic front most volcanic centers erupt both high-Ti and low-Ti lavas. In Nicaragua the high-Ti lavas are highly depleted in K and LREE, indicating a previous episode of melting. Along the rest of Central America there are occasional mafic, incompatible and HFS element rich lavas mixed in with the normal calc-alkaline suite. In central Costa Rica some of these lavas are alkaline. Wherever high-Ti and low-Ti lavas coexist, it is impossible to derive one from the other by shallow AFC differentiation. Because of the common presence of two magma series at the same volcanic center and even the same vent, it is clear that composite cones can erupt all nearby magmas. It is possible that some high-Ti lavas have migrated from the back-arc to erupt at the front. Mixtures of high-Ti and low-Ti magmas are common, especially in western Nicaragua and therefore it is difficult to separate them. The best places to find the widest range of magma types are small volcanic centers with multiple distinct vents. Large centers are more extensively mixed, probably because of large, long-lived magma chambers. There is extensive geochemical zoning both along and across the arc. These chemical variations arise from changes in the mantle, changes in the strength of the slab signature and variation the type of slab signature, primarily the extent of the hemipelagic