Eruptive history, petrology, and petrogenesis of the Joe Lott Tuff Member of the Mount Belknap Volcanics, Marysvale volcanic field, west-central Utah

摘要: The Joe Lott Tuff Member of the Mount Belknap Volcanics is largest rhyolitic ash-flow tuff sheet in Marysvale volcanic field. It was erupted 19 m.y. ago, shortly after changeover from intermediate-composition calcalkalic volcanism to bimodal basalt-rhyolite volcanism. Eruption resulted formation Caldera whose pyroclastic intracaldera stratigraphy parallels that outflow facies. a composite changes laterally simple cooling unit near source four distinct units toward distal end. lowest these and most widespread; it 64 m thick contains basal vitrophyre. lower led initial collapse caldera. followed upward by 43 middle unit, 26 pink-colored which separated prominent airfall layer, 31 upper unit. an alkali rhyolite with 75.85-77.31 wt.% silica 8.06-9.32 K20+Na20; agpaitic index (Na20+ K20/A1203 ) 77-.98. about 1% phenocrysts quartz, sanidine, oligoclase, augite, apatite, zircon, sphene, biotite, oxidized Fe-Ti oxides. vitrophyre accessory allanite, chevkinite, magnesiohastingsite. main are chemically mineralogically zoned indicating magma chamber restratified prior each major eruption. Within two thickest units, mineralogy systematically upwards; Or content relative volume sanidine decreases An plagioclase increases. has bulk composition lies thermal trough minima Or-Ab-Q at 1 kb pH20. Microprobe analyses feldspar chemical modeling on experimental systems indicate preeruption temperatures were 750 X C temperature increased during eruption units. gradients apatite whole-rock data consistent mineral assemblages throughout cannot be explained crystal settling. fractionation appears closer fit model convection-driven thermogravitational diffusion.