THE SECONDARY FRONT IN SOUTHEASTERN GUATEMALA: FLUX VERSUS DECOMPRESSION MELTING

Some modern subduction zones have a contemporaneous secondary volcanic belt situated parallel to and landward of the volcanic front (VF). The secondary front (SF) characteristically has fewer volcanoes and lower volumes of volcanic material than the VF. The presence and position of the SF likely results from the complex interplay between melt generation, slab dehydration, and mantle wedge convection. The Central American arc has a distinct SF concentrated largely in southeastern Guatemala. Whereas the VF consists of complex stratovolcanoes with multiple vents, dissected composite cones form the SF, except for a small caldera called Retana. New geochemical data for the SF volcanoes in southeastern Guatemala provide insight on the melt generation processes occurring in this curious arc setting.

Lavas from the SF central volcanoes exhibit a continuous spectrum of compositions from basalt to rhyolite, though basaltic andesite and andesite represent the most dominant lithologies. Key trace element ratios that effectively measure the magnitude of the slab input show enrichments over MORB-OIB levels, but generally fall below the maximum values recorded at the VF stratovolcanoes. U/La and Ba/Th show somewhat progressive declines with distance across the arc. A negative correlative on a Ba/La versus La/Yb diagram suggests that slab inputs control degree of melting for the SF lavas favoring a flux melting process. On a plot of Ba/Th versus U/La, the SF lavas exhibit a negative array that ranges from carbonate sediment to hemipelagic sediment signatures. Thus, the SF central volcanoes resemble the VF stratovolcanoes both in terms of eruptive style and the spectrum of compositions erupted. The crude decline in slab inputs with distance behind the front suggests that melting is closely tied to the subducting Cocos plate and might reflect the drying out of the subducted lithosphere on its passage to the upper mantle.