COMPARATIVE U-TH-PB SYSTEMATICS IN 2.7X109YR PLUTONS OF DIFFERENT GEOLOGIC HISTORIES

A comparative study was made of the Pb-U-Th systematics on cogenetic zircons from three plutons with different geological histories but the same age (2.69 × 109 yr). The first of these is in the Superior Province, Canada and is believed to be a virginal body. Assuming an episodic model, a secondary age of 455 ± 305105 m.y. is inferred. The second body is from the Grenville front and is known to have suffered an episode at 920 m.y. However, a secondary age of 350 ± 50 m.y. is inferred. A third body in the Wind River Mountains, Wyoming, has no known metamorphic history, but was uparched in Laramide time. After Naylor et al. (1968) a secondary age of 130 ±80100 m.y. is inferred,. We conclude that unless independently documented, the secondary ages inferred by the episodic model are in general invalid. The zircon systematics appear to be dominated by the occurrence of highly discordant and almost concordant phases. The discordant phase responding to diffusion loss which may be in the non linear" region. © 1969."