Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-13
Presentation Time: 9:00 AM-6:00 PM


BROWNE, Brandon and KHOURY, Regina, Department of Geology, Humboldt State University, 1 Harpst Street, Arcata, CA 95521

The Golden Trout volcanic field in the southern Sierra Nevada is a small bimodal volcanic field with eight Pleistocene vents. Long Canyon Dome is the youngest rhyolite in the field and the only rhyolite that erupted at broadly the same time as basalts. Field relations confirm that the eruption initiated explosively with the deposition of a 12-m-thick pyroclastic stratigraphy with layers of jig-sawed vitrophyric rhyolite clasts in a matrix of ash and obsidian flakes interbedded with coarse-grained, blocky, clast-supported rhyolite pumice. The eruption then transitioned to the emplacement of crystal-rich rhyolite lava domes (~0.05 km3) within an unconsolidated tuff ring. Whole-rock compositions of juvenile material from the initial pyroclastic stratigraphy, tuff ring, and lava domes reveal a steady increase in the silica content from 73.9 to 77.1% SiO2 over time. Concentrations of K2O, Na2O, and Rb also increased as did the crystallinity of juvenile material from ~10% to ~25%, whereas Al2O3, FeO, TiO2, Sr, Y, and Zr decreased. Compositional heterogeneity of the magma also decreased during the eruption, as juvenile material erupted initially show more variation compared to the lava domes. Rhyolite mineralogy includes quartz, plagioclase (An10-An25), sanidine mantled by oligoclase rims, biotite, zircon, rutile, magnetite and ilmenite. Touching magnetite-ilmenite cores record pre-eruption temperatures of 667 to 788 C by QUILF. Magnetite-ilmenite rims record temperatures 10 to 80 degrees C warmer than corresponding cores. In contrast, pre-eruptive temperatures of quartz cores using TitaniQ range from 589 to 698 C. Quartz rims record a similar range of temperatures but some rims record temperatures as high as 910 C. On average, pre-eruptive temperatures recorded by quartz decrease from ~640 C in early pyroclasts to ~615 C in dome lavas. We hypothesize that this eruption was driven by the intrusion of a warm rhyodacite or low-silica rhyolite through a low-temperature, crystal-rich, and high-silica rhyolite mush prior to eruption. Warmer rhyolitic magma tunneled through the colder and more rigid rhyolite mush to erupt at the surface first, possibly through ice or snow. Continued tunneling of warmer rhyolite thawed portions of the older rhyolite, allowing it to ascend later and erupt last as a sequence of lava domes.