U-Th-Pb Chronology of Zircons from the St. Kevin Granite, Northern Sawatch Range, Colorado

Three samples of zircon from the St. Kevin Granite, northern Sawatch Range, Colorado, were analyzed for uranium, thorium, and lead content and for lead isotopic composition; the concentrated HNO₃ leaches of the zircons were similarly analyzed. The concordia age on the zircons was interpreted to be 1420 ± 40 m.y., an age in good agreement with a Rb-Sr whole-rock isochron age of 1470 m.y. (λ⁸⁷Rb = 1.39 × 10⁻¹¹ yr⁻¹). The concordia age of the leaches was found to be greater by about 5 percent than that of the zircons, perhaps reflecting loss of intermediate daughters in the ²³⁸U decay chain over an extended period of time. Geologic evidence indicates that the St. Kevin Granite formed in large part by local melting of crustal rocks similar to the present wall rocks. If so, xenocrysts of zircon may be present in the granite. Isotopic evidence that the zircons were derived from older rocks is not convincing; however, possible evidence for a xenocrystic component is found in the feet that Pb-Pb ages of two nearly concordant zircons differ by 1.9 percent, an amount that exceeds analytical uncertainty. In addition, the zircon sample that has the greatest Pb-Pb age (1440 m.y.) has a ²⁰⁸Pb/²³²Th age of 1615 m.y. ²⁰⁸Pb/²³²Th ages greater than Pb-Pb ages are unusual and may suggest a complicated history for the sample. This sample is not the one suspected as the most likely to contain xenocrysts of zircon because xenoliths are not abundant at the sample locality. The zircons, if they are xenocrysts, apparently were almost entirely recrystallized or else lost nearly all their lead to the melt by diffusion.