This study addresses the role of mafic and intermediate- composition magmas in the genesis of rhyolites at the Tomochic volcanic center (TVC), where the stratigraphic section is the most complete and where the history of volcanism is best documented among known eruptive centers in the Sierra Madre Occidental. Recent field and chronologic work at the TVC has indicated that two distinct lithologies, porphyritic two-pyroxene andesite and crystal-poor basaltic andesite, were important components of the magma systems that fed large-volume rhyolite ash-flow eruptions. A genetic relationship between these lithologies and the rhyolites is allowed, but is poorly constrained, by available data. The principal goal of this study will be to evaluate that relationship further, via: (a)comparison and modeling of mineralogic, isotopic (Sr, Nd), and geochemical variations among all rock types, and (b) petrologic study of hybrid post-collapse lavas, to characterize the deep, mafic parts of the reservoir beneath the Las Varas caldera, for comparison with extruded basaltic andesite. Results will constrain model pertaining to the genesis of rhyolites, and of associated, more mafic magmas, in what is considered the world's largest "rhyolite-dominated"volcanic field. Results will also be important to models linking style of volcanism with tectonic regime, and to our understanding of processes associated with the evolution of large-volume subcaldera reservoirs.
