Geochronology of Some Precambrian Rocks of the Southern Front Range, Colorado

Abstract

Rb-Sr analyses of whole-rock samples from Precambrian coarse-grained quartz monzonite and fine-grained granite exposed in the Blue Ridge and Royal Gorge areas in the southern part of the Front Range, Colorado, yield ages of 1,781 ± 66 and 1,722 ± 50 m.y., respectively [λ (Rb 87 ) = 1.39 × 10 −11 yr −1 ]. These ages correlate with the oldest recognized igneous and metamorphic events of the Front Range in Colorado. Analyses of minerals separated from two samples of the quartz monzonite define a metamorphic isochron with an age of 1,430 ± 50 m.y. Analyses of minerals from pegmatite in the Blue Ridge area indicate an age of 1,450 ± 100 m.y., with some geological uncertainty in interpretation of the data. These ages correlate with widespread 1,450-m.y.-old igneous and metamorphic activity recognized throughout the Front Range. Field relations indicate that metasedimentary rocks in the region have been intruded by the quartz monzonite and therefore are older than 1,781 ± 66 m.y. No evidence of igneous or metamorphic activity corresponding to the age of the Pikes Peak Granite (1,040 m.y.) was found.