Bridging basalts and rhyolites in the Yellowstone-Snake River Plain volcanic province: The elusive intermediate step

- Rhyolites of the bimodal Heise volcanic field (Yellowstone hotspot) were studied.
- Two units contain pyroxenes and plagioclases in equilibrium with intermediate melts.
- Melt inclusions in pyroxenes have 57-67% SiO2 and fit on liquid lines of descent.
- MI compositions support AFC as dominant petrogenetic mechanism for Heise rhyolites.
- Large intermediate cumulate bodies are expected after AFC in anorogenic settings.

Many magmatic provinces produce strongly bimodal volcanism with abundant mafic and silicic magmas yet a scarcity of intermediate (55-65 wt.% SiO2) compositions. In such bimodal settings, much debate revolves around whether the basaltic magmas act as heat sources to melt pre-existing crust, or whether they are the parents to the silicic magmas (a fractionation-dominated evolution). Until now, this lack of coeval intermediate compositions has commonly been used to support models involving large degrees of crustal melting. Detailed analysis of mineral cargoes of ignimbrites from the 6.6-4 Ma Heise volcanic field in the famously bimodal Yellowstone-Snake River Plain (YSRP) volcanic province has revealed the existence of intermediate liquids associated with main stage rhyolitic volcanism. Two closely spaced rhyolitic ignimbrites, the Wolverine Creek Tuff and the Conant Creek Tuff, contain pyroxene crystals with major and trace elemental compositions in equilibrium with intermediate melts prior to significant plagioclase fractionation. Hosted within these crystals are glassy melt inclusions that have compositions (57-67 wt.% SiO2) directly recording the intermediate liquids. The combined mineral and melt inclusion data provide the first evidence for the occurrence of intermediate melts, typically erased in the high temperature YSRP ignimbrites by crystal resorption or diffusive re-equilibration. The results suggest the existence of mostly unerupted mid-crustal reservoirs that drive magma compositions towards the erupted rhyolites via assimilation-fractional crystallisation (AFC).