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Ansrnlcr In order to provide a precise age for the Red Mountain pluton, the youngest of the syenitic bodies of the Laramie Anorthosite Complex, Wyoming, zircons were extracted and analyzed for U-Pb isotopes. Zircon grains were selected and individually analyzed in order to minimize potential complications of inherited components. No grains with inherited cores were identified, either by microscopic examination or from the isotopic data. On a concordia diagram, the U-Pb data form a slightly discordant array. A best-fit line intersects concordia at 1439!! Ma and 102 + 64 Ma (950/o confidence level). The upper intercept is interpreted as the crystallization age of the Red Mountain pluton and provides a younger age limit for the rest of the Laramie Anorthosite Complex. INrnooucrroN Petrogenetic studies ofPrecambrian rocks rely increasingly upon precise age determinations. When the crystallization age of an intrusion is well known, calculated initial isotopic ratios of Sr, Nd, and/or Pb may provide important constraints upon its origin and evolution. U-Pb dating ofzircons often yields very precise ages that provide a chronological framework and the basis for such petrogenetic modeling. Although analyses of grain-size and/ or magnetically separated zircon fractions frequently provide very precise concordia-intercept ages, in some cases population studies may not always be the most successful approach. For example, if several generations of zircons are present, discordia constructed from mixed populations may yield meaningless concordia-intercept ages. An alternative approach is the analysis of single zircon crystals. Although analytically more demanding, the single-zircon approach has the advantage that grains with distinct morphological properties or grains with inherited cores will be analyzed separately, and the age of the different times of zircon growth may be established. The goal of this study is to provide a precise age for the Red Mountain pluton, the youngest of the syenitic plutons associated with the Laramie Anorthosite Complex. The Laramie Anorthosite Complex is the subject of ongoing collaborative field, petrologic, and geochemical studies (e.g., Fuhrman et al., 1988; Kolker and Lindsley, 1989; Anderson et al., 1987, 1988). Recently, Nd and Sr isotopic studies have been initiated (see Geist et al., I 990), which require precise age data for the calculation ofinitial isotopic compositions. Prior to the present contribution, no age determinations of the Laramie Anorthosite Complex have been published other than as summarized in thesis and abstract form (Subbarayudu, 1975; Subbara