The detailed eruptive histories of contrasting volcanic fields in western Mexico: Implications for the origin and evolution of continental crust.

The detailed eruptive history of arc volcanic fields provides the fundamental data necessary to accurately evaluate long-term volumetric eruption rates and magma budgets on a global scale. In addition, the detailed age, volume, and compositional data from a single arc can provide new perspectives on first-order questions bearing on the growth and evolution of continental crust. The first part of this study presents extensive 40Ar/ 39Ar geochronology, geochemistry, and volumetric analyzes from the Tancitaro volcanic field, Michoacan, Mexico. A complete continuum of lava has erupted ranging from 51 to 63 wt% SiO2 with a notable absence of dacites and rhyolites over the last 1.1 Myr. Based upon the textures, phenocryst assemblages, and chemical compositions, the diversity of lavas does not reflect differentiation in upper crustal magma chambers. Instead, the differentiation of magmas erupted in the Tancitaro volcanic field appears to be primarily controlled by processes in the middle to lower crust, where most (but not all) mantle-derived basalts stall, partially to completely crystallize, release residual melts and/or fluids, and transfer heat to induce variable partial melting of the surrounding crust. Over millions of years the continental crust becomes stratified, with a mafic lower crust, an intermediate middle crust, and a felsic upper crust, through a zone refining process that does not require upper crustal magma chambers or preexisting felsic crust to make new continental crust. The second part of this study presents the eruptive history of the Mascota-San Sebastian, Tapalpa, and Ayutla volcanic fields within the Jalisco block in western Mexico. These volcanic fields have erupted calc-alkaline basaltic andesites and andesites in close spatial and temporal association with highly potassic minettes and absarokites. New 40Ar/39Ar geochronology documents an age progression of volcanism across the Jalisco Block over the last ∼7.4 Myr, continuing to the present. These data suggest that the potassic volcanism does not represent a coeval volcanic front but may instead be the result of hot asthenosphere replacing lithospheric mantle. This causes previously metasomatized mantle veins to partially melt, forming absarokites and minettes, which are emplaced into the lower crust and invoke lower crustal melting to produce the calcalkaline intermediate lavas. Future geophysical studies are needed in the Jalisco Block to determine the origin of the upwelling asthenosphere.Ph.D.Earth SciencesGeochemistryGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126788/2/3276259.pd