Genesis of rhyolitic melts in the upper crust: Fractionation and remobilization of an intermediate cumulate at Lake City caldera, Colorado, USA

Lake City Caldera (22.93 ± 0.02 Ma) is the youngest of 25 Tertiary calderas within the Southern Rocky Mountain Volcanic Field (SRMVF) and offers an opportunity to study the relationship between plutonic rocks and their volcanic equivalents. Extreme topographical relief of the area reveals the three-dimensional exposure of a complex, high-K calc-alkaline, magmatic system. Lake City caldera is comprised of two principal units: 1) a resurgent quartz syenite intrusion, and 2) the Sunshine Peak Tuff (Lower, Middle, and Upper). The Lower and Middle Sunshine Peak Tuff (LSPT and MSPT) are crystal-poor rhyolite, while the Upper Sunshine Peak Tuff (USPT) is a crystal-rich trachyte. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by melt extraction from a long-lived quartz syenitic magma reservoir at intermediate crystallinity (~50–70% crystals). A slight compositional zoning from the Lower Sunshine Peak Tuff (~76 wt% SiO2) to the Middle Sunshine Peak Tuff (~74 wt% SiO2) ignimbrites indicates that further modification by either mixing or additional crystal fractionation might have occurred following melt extraction from the mushy syenitic reservoir. Geochemical and textural analyses show that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the left-over syenite cumulate. Quartz syenite crystal size distributions of potassium feldspar show the characteristic shape of crystal accumulation, and both volcanic and plutonic units contain abundant glomerocrysts. The presence of mafic enclaves within both the syenite and USPT trachyte, coupled with large increases of Ba content in rims of the K-feldspars from the trachyte suggest that the injection of less evolved magma into the host reservoir caused the re-melting of the syenite cumulate mush, allowing for the eruption of the full SPT sequence. Titanium-in-quartz thermobarometry shows that the majority of grains in all units formed below ~800 °C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera.