ABSTRACT

The Precambrian Virginia Dale ring dike complex, centered on the Colorado-Wyoming border, consists of four ring zones and a central stock. Three of the ring zones and the central stock are granitic; the fourth is a composite of gabbro, diorite, and older metamorphic rocks. Forty-seven samples from the ring dike complex and surrounding igneous rocks were analyzed for major elements, Rb, Sr, Zr, Ba, Cu, and Zn, by X-ray fluorescence and atomic absorption. Chemical variations suggest that the complex was formed during two magmatic events, probably separated by a short period of time: (1) an earlier event involving the gabbro-diorite association and the Trail Creek granite; and (2) a later event involving the Cap Rock and Log Cabin (central core) lithologies, which are apparently related by crystal fractionation. In that portion of the ring dike associated with the younger event, ages seem generally to decrease toward the center of the complex while the degree of fractionation increases inward, suggesting a “classic” ring dike sequence. Although there is abundant field evidence for magma mixing/assimilation, these processes had no apparent major effects on chemical trends in the major lithologic units of the ring dike complex.