Bimodal volcanism of back-arc basin formation and in the products of both continental and oceanic hotspots (e.g. Yellowstone and the Canary Islands). Bimodal volcanism is normally explained as a result of partial melting of the crust, creating granitic magmas, during the emplacement of large volumes of relatively hot basaltic magma from a mantle source. The two magma types then form separate magma chambers giving rise to periodic eruption of both types of lava. Bimodal volcanism Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of
Archean felsic volcanic rocks from 62 to 78 wt% in rock. In terms of mineralogy, the felsic volcanic rocks are rich in feldspar and quartz. A typical mineral assemblage is quartz + feldspar (albite/oligoclase) + amphibole (chlorite) + micas (biotite and/or muscovite). The mineralogy seems similar with modern rhyolites and dacites. The volcanics are aphanitic, whereas some exhibits porphyritic texture that certain larger minerals (phenocrysts) are visible by eyes. Felsic volcanic rocks also include felsic tuff that was formed when tephra was consolidated. Tuff is composed of volcanic ash, glass shards and lithic fragments. Reported eutaxitic tuff from Superior Province, Canada (Figure 3), contains