A comparison of basaltic volcanism in the Cascades and western Mexico: compositional diversity in continental arcs

Many physical similarities between the Cascades and western Mexican subduct ion zones would lead one to believe that volcanism in these two arcs should be similar. Despite the fact that there are four basaltic lava series [cal c-alkaline basalt (CAB), intraplate alkaline (IA) basalt, high-alumina oliv ine tholeiite (HAOT) and high K (lamprophyric and relatively dry potassic)] represented in these arcs, there are significant differences which remain difficult to explain. First is the occurrence of lamprophyric lavas in west ern Mexico but not the Cascades. Second is the occurrence of HAOT in the Ca scades but not western Mexico. The presence of lamprophyric lavas in wester n Mexico, but not in the Cascades may be due to the presence of older metas omatized upper mantle beneath Mexico. Many explanations have been proposed for the origin of the IA- and HAOT-series including melting of depleted lit hospheric mantle, both shallow and deep melting of asthenosphere, and melti ng of heterogeneous mantle. However, the most compelling and successful ide a is that these series result from decompression melting of depleted and/or enriched asthenosphere. Models for the genesis of calc-alkaline basalt ser ies are similar for both arcs - melting of a subduction modified mantle at depths near the base of the crust. Future studies must concentrate on unrav elling the importance of source contamination (due to slab dehydration) and crustal contamination. (C) 2000 Elsevier Science B.V. All rights reserved.