The first part of this dissertation deals with the timing of Mesozoic igneous activity in southern Cordillera of North America and its tectonic implications. A representative section in Santa Rita Mountains is dated using the zircon U-Th-Pb isotopic method. The oldest unit, the lower member of the Mt. Wrightson Formation, is concordantly dated at 210 ± 3 Ma. Initial basaltic andesite to andesite volcanism was followed by deposition of redbeds and associated volcanic rocks that are dated at 200 Ma. Felsic volcanism and eolian sand deposition may have spanned from 190 to 170 Ma. The Piper Gulch Granodiorite, representing the earliest Mesozoic intrusive equivalent, gives concordant dates of 188 ± 2 Ma. A second cycle of andesite and rhyolitic volcanism and sedimentation is dated at 151 ± 5 Ma using the whole-rock Rb-Sr isotopic method. The Hovatter Volcanics in the Little Harquahala Mountains, southwestern Arizona is dated at 165 Ma. Whole-rock Rb-Sr isotopic method on the same rocks gives a coherent reset isochron of 70 ± 3 Ma (Appendix III). A new stratigraphic correlation is proposed based on the dating data. Tectonic models proposed by previous workers to account for what seemed to be the lack of Triassic volcanic rocks are not necessary. This part of the Cordillera was an uplifted arc terrane during the Early Mesozoic (Appendix II) and may have provided volcanic detritus to the Late Triassic Chinle Formation in the Colorado Plateau. The second part of the dissertation deals with magma evolution and crust modification during arc magmatism. Rocks in southeastern Arizona have ƭ(N)(d) values of -3.4 to -6.4, while rocks to the west have ƭ(N)(d) values ranging from -8.5 to -9.2. An ƭ(N)(d) value of +2 for a Jurassic basalt indicates the presence of depleted mantle under the arc. Using lower crust and mantle end-members, 20 to 40% mantle input is estimated. This seems to argue for continuous growth model of the continental crust. Combined REE and isotopic data indicate that assimilation of lower crust by mantle melts followed by fractional crystallization took place. Detailed study indicates that the lower crust along sites of arc magmatism gets progressively hybridized by the mantle, becoming more mantle-like with time.