Petrogenetic insights provided by compositional transects across the Central American arc: Southeastern Guatemala and Honduras

Two detailed cross-are transects of the Central American subduction zone ha ve been completed across southeastern Guatemala and Honduras. The two trans ects display both contrasting and similar cross-are geochemical variations. Across southeastern Guatemala, ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Cs/La and Ba/Nb, are typically high at the volcanic front, low behind the front, and, most importantly, unchan ging once behind the front. Across Honduras, by contrast, such incompatible clement ratios often systematically decline from the elevated values chara cterizing the volcanic front. Across both transects, many incompatible elem ent contents and ratios of highly incompatible to moderately incompatible e lements, such as the La/Yb ratio, are higher behind the front. The similari ties between the two transects may be caused by (1) declining total slab in puts away from the volcanic front, (2) more contributions from subducting c arbonate sediments behind the volcanic front, (3) declining degrees of melt ing away from the volcanic front, (4) more decompression-induced melting aw ay from the volcanic front, and (5) more melting of enriched domains in the mantle wedge away from the volcanic front. The distinctions between the tw o transects are attributed to varying proximity to the North American-Carib bean transform boundary. Motions along this boundary have caused greater ex tension close behind the front in southeastern Guatemala, serving to abrupt ly cut off or reduce chemical transfer from the subducting Cocos plate. Beh ind the front in Honduras, by contrast, the chemical umbilical to the Cocos plate may be only progressively cut or, alternatively, may also be abruptl y cut, but at a much greater distance from the volcanic front. In both tran sects, however, certain ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Ba/La and Pb/Ce, remain considerably above mid-ocean ridge basalt or oceanic island basalt values some 100 km behind t he front. This common slab signature may represent "fossil" slab modificati on of the Central American mantle wedge.