Abstract

Rubidium-strontium dating of whole-rock and feldspar samples indicates that the major regional metamorphism in the northern Front Range occurred 1.75 ± 0.03b.y. ago (λ = 1.39 X 10−11/yr). Sedimentation and folding predate this metamorphism. Syntectonic and late tectonic igneous rocks give a concordant age of 1.71 ± 0.07b.y. Microcline and plagioclase from a concretion pegmatite yield an age of 1.76 ± 0.08 b.y. and a relatively high initial Sr87/Sr86 ratio consistent with their being locally derived during metamorphism.

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