Oxygen isotope study of source and alteration of granitic magmas in the western United States and the Superior Province, Canada

Oxygen isotope ratios of individual minerals can provide valuable information about a rock's complex igneous and metamorphic history. Zircons preserve the most reliable magmatic value of δ18O. Reliable δ 18O(magma) values allow comparisons with radiogenic isotope ratios and serve as a monitor of postmagmatic alteration. Igneous zircons from the Sturgeon Lake volcanic complex (Superior Province, Canada) have δ 18O values consistent with other Archean igneous rocks. Quartz phenocrysts and whole-rock powders indicate low temperature alteration representing metamorphism associated with igneous activity post-dating the majority of volcanic eruptions in the complex.; The analysis of minerals for both oxygen and U-Pb isotopes is important in order to document δ18O at an exact age. Besides zircon, titanite is an important mineral for geochronology and could be meaningful for δ18O studies provided the fractionation factor is well calibrated. The empirical calibration of the titanite fractionation factor results in a self-consistent value that differs from that calculated by the semiempirical increment method.; Igneous zircons from the Idaho Batholith indicate a relatively homogeneous magmatic source. Analyses of δ18O for co-existing garnet and zircon suggest the assimilation of a high δ18O rock into the magma after zircon crystallization, before the garnet crystallization. The western edge of the batholith intrudes both sides of a major crustal boundary between the North American craton and accreted terranes. Granitic magmas intruding accreted terranes have more primitive initial 87Sr/ 86Sr and δ18O relative to magmas intruding the craton. The Sr and O isotope ratios of rocks intruding the transition zone reflect the age and source of rocks entrained in the suture zone.; The influence of crustal rocks on granitic magmas is also observed in the Northern Great Basin (Utah, Nevada). The δ18O(zircon) of Jurassic, Cretaceous, and Tertiary granitic bodies in the Great Basin reflect the temporal variability in crustal thickness. Values of δ18 O(Zrc) show a discontinuity in the crustal composition of the Great Basin coinciding with radiogenic discontinuities. These studies show that zircon is an important primary mineral to monitor magmatic source and evolution in rocks that experienced multiple stages of development and metamorphism.