PRELIMINARY RESULTS OF AN INVESTIGATION ON POST-20 MA PLUTONS BETWEEN OPHIR AND TELLURIDE IN THE WESTERN SAN MOUNTAINS, COLORADO

In the western San Juan Mountains, major magmatic episodes are recorded from 80-65 Ma, 35-23 Ma, and 20-4 Ma. Late Oligocene to Pliocene dioritic to granitic plutons were emplaced at upper crustal levels during the transition from Laramide subduction to regional extension. Plutonic events in southwestern Colorado record a long-term shift from dominantly intermediate magmas in the Laramide to bimodal magmatism after 30 Ma. Widely distributed post-20 Ma felsic stocks and dikes are closely allied to mineralization and late Cenozoic uplift in the region. The ages and petrologic details of this generation of plutons are poorly constrained but are critical for understanding the spatial and temporal patterns of post-20 Ma magmatism and the role these magmas played in the geologic history.

In this study, three felsic stocks exposed near Ophir and Telluride were investigated to assess their compositions and ages. These plutons were formerly mapped as “rhyolite” to “dacite” on the basis of their textures. Our field surveys and petrographic analyses establish that they are monzonite or granite with distinctive 1-4 mm phenocrysts of quartz and feldspar. The plutons are hosted in Mesozoic to Cenozoic strata and were previously interpreted to be Oligocene to Pliocene on the basis of field relationships. Recent age constraints on similar rocks in the area suggest that these plutons are likely 15-4 Ma which will be confirmed with U-Pb zircon analyses. In this presentation, we offer preliminary results that contribute to greater understanding of latest Cenozoic plutons in the region and their significance in the geologic story of the Western San Juan Mountains.