an understanding of the nature, spatial variability and evolution of the magmatic source region at subduction zones is one key toward an overall understanding of crust-mantle and lithosphere- asthenosphere interactions. Valuable insights into the magmatic source beneath the Central American subduction zone have been determined through an evaluation of systematic trace element and isotopic variations along the volcanic front (Feigenson and Carr, 1986; Walker, 1989; Walker and others, 1989; Carr and others 1989). These variations indicate that contributions to the magmatic source from the subducted Cocos plate decrease northward from Central Nicaragua. Preliminary geochemical data suggest that such "slab" contributions may also decrease across the arc. This project is a trace element and sr, Nd and o isotopic study of behind-the-front (or BVF basaltic lavas from southeastern Guatemala, which should possess a minimal geochemical inheritance from the subductd plate or slab. As crustal differentiation processes are also minimized behind the front, the geochemistry of BVF lavas will provide the clearest picture of perhaps the most vital element of the magmatic source, the mantle wedge. Comparison of trace element and isotopic data for Guatemalan volcanic front (or VF) and BVF basaltic lavas will permit an evaluation of source involvement in across-arc geochemical variations particularly and degree of involvement of the subcontinental lithosphere. Study of trace element and isotopic data on both V or BVF Basalts will therefore provide a comprehensive understanding of the nature and variations in the magmatic source for a single subduction zone.
