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.

The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile and Bakers Bridge Granites invaded the metavolcanic sequence between 1,680 and 1,700 Ma. The timing of deposition of the conglomerates, clean quartzites, and pelites of the Vallecito Conglomerate and the Uncompahgre Formation with respect to events in the metavolcanic sequence is poorly constrained, largely because shear zones lie along crucial contacts. Recent work suggests that much if not all of this clastic sedimentary sequence may be parautochthonous and was probably deposited sometime after 1,680 to 1,700 Ma. Despite uncertainties in timing of deposition, it is clear that a second event of deformation and metamorphism affected the region, producing polyphase features in both the metavolcanic sequence and the clastic sedimentary sequence, as well as a weak foliation in the Tenmile Granite. The Eolus Granite and related intrusions, ranging in age from 1,320 to 1,450 Ma, crosscut all structural features in the region and provide a minimum age limit for the latest deformation in the region. This younger event is recorded nowhere else in Colorado but may correlate with events in northern New Mexico.