The Proterozoic ancestry of the Colorado Mineral Belt : ca. 1.4 Ga shear zone system in central Colorado

The Paleocene to Oligocene magmatism and mineralization that are the Phanerozoic expressions of the Colorado Mineral Belt developed along a NE-trending system of subvertical mylonites and ultramylonites that formed in the Mesoproterozoic and that, in turn, overprinted higher-temperature Paleoproterozoic high strain zones of similar orientation. In this thesis, I distinguish the Colorado Mineral Belt itself from a Proterozoic ‘Colorado Mineral Belt shear zone system’ that includes the Homestake, Gore Range, St. Louis Lake, and Idaho Springs-Ralston shear zone segments. In situ electron microprobe monazite dating of mylonites of the Colorado Mineral Belt shear zone system, coupled with field studies of relative timing of shearing and pluton emplacement, demonstrate a ca. 100 Ma history of recurrent shearing. This history involves movement at 1.45 Ga along Idaho Springs-Ralston shear zones and possibly along Homestake shear zone, synchronous with emplacement of the Mt. Evans pluton. At 1.42 Ga, movement took place along St. Louis Lake and again along Idaho Springs-Ralston shear zones, synchronous with emplacement of the Silver Plume pluton. At 1.38 Ga, movement took place along Homestake and Idaho Springs-Ralston shear zones, synchronous with emplacement of the St. Kevin pluton, and post-1.38 Ga movements reactivated Homestake, St. Louis Lake, and Idaho Springs-Ralston shear zones. In each shear zone segment, kilometer-wide Mesoproterozoic mylonite zones consist of multiple, parallel, one to ten meter-wide mylonite strands, which overprint higher-temperature Paleoproterozoic high-strain domains that are several kilometers wide. Monazite dating of the higher temperature high-strain domains indicates pulses of Paleoproterozoic deformation that occurred at 1.71-1.69 Ga, 1.67 Ga, 1.65, and 1.62 Ga. Thus, the tectonic fingerprint of the Colorado Mineral Belt shear zone system includes two ~100 Ma long orogenic periods in the Proterozoic, each with important pulses of deformation that occurred every fifteen to twenty million years. This shear zone system may be analogous to modern-day intracontinental zones of weakness like the Tien Shan, which record both original assembly of tectonic blocks and reactivation of intracontinental weaknesses during later plate convergence at a distant margin.GeologyMastersUniversity of New Mexico. Dept. of Earth and Planetary SciencesKarlstrom, KarlSelverstone, JaneWilliams, Joh