EXTENSIONAL TECTONICS AND THE DIVERSE PRIMITIVE VOLCANIC-ROCKS IN THEWESTERN MEXICAN VOLCANIC BELT

In the western portion of the Mexican Volcanic Belt, primitive volcani c racks are relatively common in comparison with most of the world's s ubduction-related area. These primitive rocks are conveniently grouped in three suites, which have erupted concurrently at least since the P liocene: (1) hy-normative calc-alkaline basalts, (2) ne- and hy-normat ive lamprophyres such as minettes, leucitites, spessartites, and kersa ntites, characterized by phlogopite and hornblende phenocrysts in the absence of plagioclase phenocrysts, and (3) ne- to hy-normative intrap late-type alkaline basalts. The abundance and diversity of primitive v olcanic rocks in the western Mexican Volcanic Belt appear to be primar ily related to Plio-Quaternary rifting within and along the margins of the Jalisco Block, which favored passage of primitive magmas to the s urface. The lamprophyres are interpreted to form mainly from melting o f phlogopite-bearing pyroxenite Veins generated in the mantle wedge as melts rising from the subducting slab react with peridotite. Such vei ns are probably formed by hybridization reactions in most subduction z ones, but the low-degree lamprophyre-forming melts rarely erupt to the surface; instead, they stagnate as crustal lamprophyric dikes. Althou gh uncommon, other examples of volcanic lamprophyres are known in acti ve arcs, from Baja California, Japan, and Papua New Guinea. With great er dilution of the vein component by melts from the peridotitic wallro ck, calc-alkaline basalts are generated. This process appears to domin ate in many arcs. Although abundances of incompatible elements are con siderably lower than for the lamprophyres, the two rock types share si milar patterns of relative enrichment of elements and Sr, Nd, and Pb i sotopic ratios. The ''essence'' of subduction-zone geochemistry, there fore, is most purely represented by the lamprophyres, with the calc-al kaline basalts being diluted relatives. The intraplate-type alkaline b asalts seem to reflect partial melting of convecting upper mantle that was compositionally unaffected by subduction, but advected into the r egion beneath the rifting continental lithosphere.