Primary and metamorphic Rb-Sr chronology in Central Colorado

Abstract

Rb-Sr analyses have been made on 15 whole rock samples of granitic to granodioritic Precambrian rocks from a region in central Colorado bounded roughly by Independence Pass, Gunnison, Monarch Pass, Leadville, and Red Cliff. Thirteen of these samples define a 1640±35 m.y. isochron with a primary Sr87/S86 ratio of 0.703±0.002. Analysis of separated minerals from four of these rocks indicates redistribution of strontium and possibly rubidium about 300 m.y. later. In two cases well-defined reequilibration isochrons are found. Fresh samples of K-feldspar, muscovite, and biotite from pegmatite dikes of the Quartz Creek pegmatite district, near Gunnison, Colorado, give Rb-Sr ages ranging from 1465 to 1380 m.y. These results are interpreted to imply that considerable emplacement of granitic to granodioritic rocks in this area occurred about 1650 m.y. ago. This was followed by intrusion of pegmatites about 200 m.y. later. A subsequent event or series of events about 1350 m.y. ago resulted in extensive redistribution, and in some cases, complete equilibration, of strontium between the mineral phases of these rocks. The results of this study suggest that mineral ages previously measured in central Colorado may largely reflect the 1350-m.y. event and that the primary age of much of the basement is 1650 m.y. or older.