Proterozoic geology of the Needle Mountains; a summary

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.