Whole-Rock Rb-Sr Age of Metasedimentary Rocks Below the Stillwater Complex, Montana

Abstract

Eight metasedimentary rocks selected from a suite collected below the Stillwater Complex in the Beartooth Mountains, Montana, yield a whole-rock Rb-Sr age of 2730 ± 150 m.y. (λ = 1.39 × 10⁻¹¹/yr). The samples represent rocks located 0.4 to 6 miles from the lower contact of the Complex, and the nearer samples are definitely within the contact metamorphic aureole of the Complex. The age may be the time of sedimentation, or the time of isotopic homogenization resulting from the intrusion of the Stillwater Complex.