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 having rare-earth element (REE) patterns more characteristic of synorogenic granites. The Eolus batholith also differs from most anorogenic batholiths by having undergone extensive fractionation, involving both partial-melt fractionation and hornblende-dominated fractional crystallization.

A consequence of this fractionation was progressive enrichment of uranium as the Eolus batholith evolved. In early products of differentiation, uranium is held entirely within relatively nonlabile accessory minerals such as zircon, sphene, apatite, and allanite. In late-stage differentiates, particularly the Trimble Granite, most uranium is found in scattered grains of more readily leachable high-uranium phases such as uraninite and uranothorite. These late-stage differentiates were source rocks for later mineralization.

It is believed that the Eolus batholith formed by melting of an underplated mantle wedge above subducted oceanic plate. Melting may have been initiated by low pressure in an extensional regime, caused by a transition in tectonic style from synorogenic ductile compression to postorogenic brittle extension. Melting was probably localized along preexisting zones of weakness, which would account for synchronous magmatism over a broad transcontinental belt. Differences in distance from a subduction zone, crustal thickness, depth of melting, and other factors may account for the chemical dissimilarities between the postorogenic Eolus batholith and other “anorogenic” batholiths of approximately the same age.