Rubidium-strontium dating of the trondhjemite of Rio Brazos, New Mexico and of the Kroenke Granodiorite, Colorado

Abstract

The quartz-eye trondhjemite and associated hornblendite near Rio Brazos, Brazos Peak quadrangle, New Mexico, and the Kroenke Granodiorite of the Mount Harvard quadrangle and Sawatch Range, Colo., lie on an approximate 1,700-m.y. Rb-Sr isochron. Their initial 8 7 Sr/ 8 6 Sr ratio is about 0.7026. Six samples of the trondhjemite of Rio Brazos contain 8-43 ppm Rb and 57-194 ppm Sr, and six of the Kroenke intrusive, 53-99 ppm Rb and 314-595 ppm Sr. The Rio Brazos intrusive contains more SiO 2 but less Al 2 O 3 and K 2 O than does the Kroenke intrusive. The hornblendite contains much MgO and CaO but is low in alkalies; two samples show 2.5-3.2 ppm Rb and 288-431 ppm Sr. These rocks are of the same general age as the much more abundant, typically calc-alkaline Boulder Creek Granodiorite and associated intrusives of Colorado and northern New Mexico.