The Eolus batholith of the Needle Mountains, southwestern Colorado, contains two principal map units, the Eolus and Trimble Granites. The Eolus Granite has been dated at 1,460 Ma; the Trimble Granite formed about 1,350 Ma. Thus, the Eolus and Trimble Granites fall within the time range spanned by two well-documented anorogenic periods in North America (see Bickford and others, this volume).

Although the rocks of the Eolus batholith share many chemical traits with anorogenic, or A-type, granites, they differ by having a wider, and on the average lower, range of silica contents, by being calcalkaline rather than alkaline, and by...