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 subduction 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 [calc-alkaline basalt (CAB), intraplate alkaline (IA) basalt, high-alumina olivine 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 western Mexico but not the Cascades. Second is the occurrence of HAOT in the Cascades but not western Mexico. The presence of lamprophyric lavas in western Mexico, but not in the Cascades may be due to the presence of older metasomatized upper mantle beneath Mexico. Many explanations have been proposed for the origin of the IA- and HAOT-series including melting of depleted lithospheric mantle, both shallow and deep melting of asthenosphere, and melting of heterogeneous mantle. However, the most compelling and successful idea is that these series result from decompression melting of depleted and/or enriched asthenosphere. Models for the genesis of calc-alkaline basalt series are similar for both arcs - melting of a subduction modified mantle at depths near the base of the crust. Future studies must concentrate on unravelling the importance of source contamination (due to slab dehydration) and crustal contamination. (C) 2000 Elsevier Science B.V. All rights reserved.