Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calcalkaline magmas. Both suites include relatively rare ''primitive'' characterized types, by high Mg, Cr, and Ni contents. Primitive hypersthene-normative basalts, parental to the calc-alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic-front calc-alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic-front locations also contain hydrous minerals. These nepheline-normative suites include basanites, phlogopite-bearing minettes, and other hornblende-bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB- normalized elemental plots, all lamprophyres show have patterns similar to the calc-alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements.

The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc-alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc-alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite-rich veins cutting asthenospheric periodtite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through-crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc-alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock. ¿ American Geophysical Union 1989