LATE EOCENE INITIATION OF EXTENSION-RELATED HIGH-SILICA MAGMATISM AND MINERALIZATION IN THE SOUTHERN ROCKY MOUNTAINS

The onset of continental rifting is defined by the timing of basin formation, thermochronology of rift-flanking uplifts, and by evidence of lithospheric thinning. Bimodal magmatism has also been interpreted as synchronous with rifting, and in the southern Rocky Mountains the felsic end-member of magmatism includes fluorine-rich, high silica magmas and Climax-type porphyry mineral systems. However, significant uncertainty about their formation persists due to the overall lack of robust geochronological data. Accurately determining when these systems formed is important for understanding the start of regional extension and for mineral exploration models.

New geochemistry, high-precision U-Pb zircon geochronology and spatial modeling of magmatism demonstrate a major transition in the spatial pattern and composition of magmatism in the southern Rocky Mountains at ca. 37 Ma. This transition coincides with the onset of Climax-type mineralization in the region, as the largest deposits formed between 30 to 25 Ma, although regional Climax-type mineralization continued until at least 15 Ma. New ages of NE-trending rhyolite dikes, previously interpreted as early low-fluorine barren dikes of the Climax porphyry deposit, indicate that they intruded at 39 Ma, whereas N-S oriented dikes associated with small Climax-type prospects in the Sawatch Range formed at ca. 36 Ma. Stochastic point pattern models of the magmatism document a significant shift in the orientation of magmatism between 45 to 35 Ma at multiple spatial scales, including a counter-clockwise rotation at the smallest scale evaluated (<75 km). Local counter-clockwise rotation occurred within a large NW-SE oriented flare-up of magmatism between 40 to 35 Ma. Radiogenic isotope compositions shift through time within individual magma centers, suggesting changes in magma genesis during rift evolution, but preclude significant melting of ancient felsic crust. These observations are interpreted to indicate Late Eocene onset of extension-related mineralization following the end of the Laramide orogeny, before upper crustal formation of Rio Grande rift basins and flank uplifts. We hypothesize that intrusion of mantle-derived mafic magma formed hot zones in the mafic lower crust that led to pre- and syn-rift Climax-type mineralization.