U-Th-Pb and Rb-Sr systematics in 2700 × 10⁶-year old plutons from the southern Wind River Range, Wyoming

Abstract

To distinguish between episodic and continuous-diffusion loss mechanisms, and to compare the response of different parent-daughter systems to metamorphism, we have studied a series of well-exposed plutons, all about 2700 × 10⁶ yr old and showing different degrees of metamorphism. The Louis Lake granodiorite pluton in the southern Wind River Range, Wyoming was selected to represent granitic rocks only slightly affected by later disturbances (intrusion of Precambrian diabase dikes and gentle Laramide uparching). Rb-Sr data on total-rock samples form a linear array on the Sr-evolution diagram, indicating that the pluton was emplaeed 2630 ± 20 × 10⁶ yr ago (λ^(Rb⁸⁷) = 1.39 × 10⁻¹¹yr⁻¹) with an initial Sr⁸⁷Sr⁸⁶ ratio of 0.702 ± 0.001. Data on cogenetic zircon fractions form a linear array on a coupled Pb-U evolution diagram with a primary intercept of 2687 ± 15 × 10⁶ yr. The secondary intercept of 130 ±100 × 10⁶ yr is distinctly lower than the 480 to 660 × 10⁶ yr intercepts predicted by single-phase continuous-diffusion models. If the zircons reflect Laramide disturbance, this is not detected by the Rb-Sr data on mineral separates. Disturbance of these data is readily apparent only in rocks with high RbSr ratios and has yielded no apparent secondary ages younger than 2000 × 10⁶ yr.

Additional details