Rhyolite and trachyte formation at Lake City Caldera: Insight from quantitative textural and geochemical analyses

Lake City Caldera (22.93 +/- 0.02 Ma) is the youngest of 15 Tertiary calderas found within the Southern Rocky Mountain Volcanic Field (SRMVF). Extreme topographical relief of the area reveals the three dimensional exposure of a complex, high-k calc-alkaline, magmatic system, which provides us with the opportunity to study, in detail, how large silicic magmas evolve in the subsurface. The Lake City igneous system is comprised of two major units: a resurgent syenite intrusion and the Sunshine Peak Tuff (Lower, Middle, and Upper). Bulk-rock analyses indicate that the Lower and Middle Sunshine Peak tuff are crystal poor and rhyolitic in composition, while the Upper Sunshine Peak Tuff is crystal rich and trachytic in composition. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by fractional crystallization of the syenite magma reservoir once it reached ~40% crystallinity. Zoning of these ignimbrites LSPT (76 wt % SiO2) to MSPT (74 wt% SiO2) shows that further crystal fractionation occurred following melt extraction from the syenite magma. This is interpreted as occurring within a melt-rich rhyolite cap within the syenite magma reservoir. Geochemical and textural analysis shows that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the syenite cumulate. Crystal Size Distributions (CSDs) generated for both the syenite and trachyte show signs of textural coarsening, although trachytes exhibit these signatures to a more advanced degree. Syenite CSDs show characteristics of crystal accumulation, and both units are comprised mainly of glomerocryst networks. Titanium in quartz geothermometry gives temperature estimates for quartz formation across all units and shows that the majority of grains were formed between 735°C and 785°C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera. The presence of mafic enclaves within both the syenite and USPT trachyte coupled with reverse zoning noted in trachyte potassium feldspar grains alludes to mafic injection into the magma reservoir being responsible for both the re-melting of the syenite cumulate and the eruption mechanism for all three units of the Sunshine Peak Tuff. These models, for both rhyolite and trachyte production, are consistent with other models put forth for rhyolite production in the shallow crust and the origin of zoned ignimbrites.