INSIGHT INTO THE COMPOSITION AND PROVENANCE OF VOLCANIC DETRITUS IN THE TELLURIDE CONGLOMERATE, SOUTHWESTERN COLORADO

Fluvial and debris flow deposits of the Telluride Conglomerate lie unconformably on Mesozoic and Paleozoic sedimentary rocks across the western San Juan Mountains. Previous detrital zircon age analyses on this unit established a minimum age of deposition at ~29 Ma. The relationship of this unit to caldera eruptions in the western San Juan Mountains from 28 to 26 Ma, however, remained unclear. The presence of dacitic to andesitic volcanic clasts in the Telluride Conglomerate near Telluride, Colorado offered an opportunity to further constrain the timing of deposition and the relationship of this mid-Cenozoic fluvial system to nearby caldera eruptions. In this investigation, we conducted research on deposits within the Telluride Conglomerate to constrain the source of the volcanic clasts by determining their composition, proportions, and age.

The volcanic clasts in the Telluride Conglomerate are highly altered dacitic to andesitic fragments that underwent extensive propylitic alteration prior to entrainment in streams and debris flows. Most of the clasts have remnant phenocrysts of quartz along with partially to completely altered feldspar, hornblende, and biotite. U-Pb zircon analyses on two of the volcanic clasts yield ages of 68.98 ± 0.86 Ma and 65.70 ± 0.75 Ma. The U-Pb data reveal that the clasts were derived from a previously unidentified, and now eroded, Laramide volcanic complex in the western San Juan Mountains. Though the exact source of the clasts is uncertain, given their ages and high degree of alteration, they may represent the vestiges of a volcanic system that originally developed above the Blowout stock at Ouray. The Laramide volcanic center that was the source of volcanic clasts in the Telluride Conglomerate could also be a potential source for ~65 Ma volcanic fragments found in fluvial deposits on the northern San Juan Basin. Our work also reveals that extensive erosion happened between deposition of the Telluride Conglomerate and the development of more modern stream systems in the western San Juan Mountains, in order to remove the Laramide volcanic complex.