The mid-cenozoic ignimbrites of western Mexico constitute the largest rhyolite province in the world, and definitive evidence regarding their genesis must be recorded in the deep crust and upper mantle beneath the region. In previous petrogenetic studies of voluminous ignimbrites from Mexico and elsewhere, it has not been possible to quantitatively evaluate the relative contributions of crustal and mantle components for the following reasons: (1) The deep crust is heterogeneous, and almost invariable, its age and composition beneath the volcanic field can only be inferred. (2) The sub-crustal isotopic compositions of continental arc basaltic magma are rarely known because any magma that passed through the continental crust may acquire a crustal isotopic signature. The La Olivina area of chihuahua, Mexico provides an unusual opportunity to elucidate the origin and evolution of calc-alkalic rocks, including voluminous ignimbrites, by drawing on data from crustal and mantle xenoliths. The primary goals of this research are as follows: (1) Determine the igneous and metamorphic history of the deep crust beneath central chihuahua by dating zircons from granulite facies xenoliths using the SHRIMP ion microprobe and conventional techniques. The SHRIMP analyses will be made by Roberta Rudnick at the Australian National University. (2) Completely characterize the deep crustal xenoliths and volcanic rocks from the region by major element, isotopic (Sr, Nd, Pb), and geothermic-geobaric studies. (3) Quantitatively test petrogenetic models relating the volcanic rocks to specific deep crustal lithologies. These models are uniquely constrained by data from mantle metapyroxenite xenoliths that record the isotopic compositions of "uncontaminated" continental arc basaltic magmas. The final result of this research should be an exceptional documentation of the impact of voluminous silicic magmatism on the deep crust.
