Bridging Basalts and Rhyolites in the Yellowstone-Snake River Plain Volcanic Province: the Elusive Intermediate Step

Many magmatic provinces produce strongly bimodal volcanism with abundant mafic and silicic magmas yet a scarcity of intermediate compositions (55-65 wt % SiO₂). 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 (fractionation dominated evolution). Commonly, this scarcity of intermediate compositions has been used to support models involving large degrees of crustal melting.

We present evidence of intermediate liquids associated with rhyolite petrogenesis in a famously bimodal province, the Yellowstone-Snake River Plain (YSRP) volcanic area in the western USA. The intermediate (57-67 wt % SiO₂) liquids with compositions representing liquid lines of descent are preserved as melt inclusions in pyroxene crystals from two rhyolitic ignimbrites erupted from the 6.6-4.5 Ma Heise volcanic field in eastern Idaho. The host pyroxenes also yield major and trace element compositions in equilibrium with intermediate melts prior to significant plagioclase fractionation.

The occurrence of such intermediate melts, most likely typically erased in the high temperature rhyolitic ignimbrites of the YSRP by diffusive re-equilibration, supports the importance of assimilation-fractional crystallisation (AFC) as a primary petrogenetic process. The crystal fractionation driving magma compositions towards the erupted rhyolites requires that unerupted cumulate reservoirs are left behind in the mid-crust – a conclusion supported by earlier seismic studies.