MAGMATIC HISTORY OF THE GRIZZLY PEAK CALDERA, SAWATCH RANGE, COLORADO

The Eocene Grizzly Peak caldera is the northernmost caldera in the Colorado Mineral Belt (CMB) and is the source of the isotopically diverse and crystal-rich, ~600 km³ Grizzly Peak Tuff (GPT). The largely rhyolitic GPT, preserved only as intracaldera facies, hosts pumice fiamme with among the most crustal Sr and Nd isotopic compositions in the CMB; the most isotopically similar rocks are rift-related leucogranites and rhyolites associated with Mo mineralization (“Climax-type deposits”). The GPT is intruded by several generations of post-caldera magmatism, including a main stage of resurgent plutons that are cut by a suite of late-stage stocks and dikes associated with hydrothermal alteration and mineralization. However, there are few published data on the compositional diversity of the post-caldera magmatic units, their sources, or precisely when they were assembled.

To assess the evolution and longevity of the Grizzly Peak magmatic system, we present new geochemistry and U-Pb geochronology for the GPT and several generations of pre- and post-caldera intrusions. Chemical abrasion-thermal ionization mass spectrometry U-Pb zircon data for the GPT yield a Th-corrected ²⁰⁶Pb/²³⁸U weighted mean age of ~34.78 ± 0.03 Ma, in agreement with a published sanidine ⁴⁰Ar/³⁹Ar age recalculated using a modern ⁴⁰K decay constant and Fish Canyon sanidine standard age. A granitic xenolith hosted in a post-eruption dike yields a preliminary U-Pb zircon age of ~38 Ma. The xenolith has less evolved Sr and Nd isotope compositions than the GPT and main stage resurgent plutons, suggesting the GPT and main resurgent plutons were derived from a different source than previous magmatism in the area. Geochemical data from the late-stage stock and dike suite confirm it is bimodal, ranging from 55–60 and 68–78 wt% SiO₂. The most silicic dike analyzed has 46 ppm Nb, 23 ppm Zr, and a flat rare earth element pattern with a moderate negative Eu anomaly, characteristic of rift-related Climax-type porphyry Mo deposits. Additional geochronologic and isotopic data for the bimodal suite will permit assessment of the suite’s source(s) relative to the GPT, how long magmatic activity persisted in the caldera after the GPT eruption, and how magmatism in the Sawatch Range compares to Rio Grande rift-related magmatism to the east.