High-Mg andesites from the Gila Bend Mountains, southwestern Arizona: Evidence for hydrous melting of lithosphere during Miocene extension. The pre-Jurassic geotectonic evolution of the Coahuila terrane, northwestern Mexico: Grenville basement, a late Pal

This dissertation is two part and investigates igneous rocks using Sr-Nd-Pb isotope, trace element, and major element geochemistry to address tectonic hypotheses. The second study also uses U-Pb zircon geochronology. The first study deals with relatively primitive high-Mg andesites and basaltic andesites from the Gila Bend Mountains, southwestern Arizona. These lavas have high Mg;The second study uses zircon geochronology to establish the timing of a late Paleozoic Gondwanan volcanic arc south of the Ouachita suture in Coahuila, Mexico. This tectonism was Alleghanian. The arc developed on Grenville basement and was intruded by post-orogenic Triassic plutons. New U/Pb zircon ages of the volcanic arc correlate with the timing of events during the Ouachita that are documented north of the suture in the Marathon uplift, Texas. This data is does not support the previous tectonic origin for this arc which postulated a southwestern North American origin. A delamination model proposed for the southern Appalachians during the collision is not supported for the Coahuila portion of the suture. The geochemistry of the arc lavas and post-orogenic plutons show no signatures typical of delamination-related magmas or magmas generated in thick crust.