ABRUPT CHANGE IN MAGMA GENERATION PROCESSES ACROSS THE CENTRAL-AMERICAN ARC IN SOUTHEASTERN GUATEMALA - FLUX DOMINATED MELTING NEAR THE BASE OF THE WEDGE TO DECOMPRESSION MELTING NEAR THE TOP OF THE WEDGE

Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth ele ment concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La /Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinction s are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced co ntributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas e rupted within 20 km of the front, there is little apparent petrogeneti c continuity between front and behind-the-front magmas. What little ge ochemical continuity exists is in radiogenic isotopes: Nd-143/Nd-144 f alls across the are, Pb isotopic ratios (except Pb-206/Pb-204) rise ac ross the are, and Sr-87/Sr-86 rise across the are after an initial dis continuity within 20 km of the front. These continuous across-are chan ges in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once th e effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inher ent isotopic heterogeneity of the source, the mantle wedge. Geochemica l disconnection in southeastern Guatemala suggests that behind-the-fro nt magmas are produced by decompression melting near the top of the we dge, not by flux-dominated melting near the base of the wedge.