Implications of 40Ar/39Ar Geochronology for Fluid Transfer and Magma Generation Across Southeastern Guatemala

Behind-the-front , or back-arc, monogenetic volcanism extends some 110 km across southeastern Guatemala providing an important window into fluid transfer and melting in the Central American subduction zone. New 40Ar/39Ar incremental heating measurements on basaltic lavas from cinder cones in southeastern Guatemala place important temporal constraints on this across-arc volcanism. Behind-the-front (BVF) volcanism has been notably long-lived: from 1070 ± 22 ka to 137 ± 25 ka. The morphologies of some undated cinder cones suggest that BVF volcanism has probably extended into the Holocene. Most morphological parameters for the dated cinder cones show no systematic changes with age, suggesting that cone degradation is rapid in the tropical climate of Guatemala, quickly approaching a morphological equilibrium stabilized by the thick vegetative cover. The volumes of cinder cones, however, have consistently increased towards the present. In additon, Ar/Ar ages become younger towards Pacaya volcano on the volcanic front, indicating a southwestward migration of eruptive activity with time. Some incompatible element concentrations and ratios, such as Ba, La, and La/Y, decrease with time suggesting increasing amounts of melting in the mantle wedge, consistent with the production of larger and larger cinder cones. Increasing melting, however, does not appear to be directly related to fluid transfer from the slab as the Ba/La, Ba/Th and U/Th ratios of basalts erupted from the dated cinder cones have also declined over time. Thus, the new age constraints suggest the increasing influence of decompression, versus flux, melting in the BVF region, an influence that has also spread into Pacaya volcano on the volcanic front (Cameron et al., 2002).