Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4716466 | Lithos | 2012 | 12 Pages |
Young (< 1 Ma) volcanic complexes in the Kenya Rift Valley are used to outline recent progress in our understanding of how peralkaline silicic systems evolve. Such systems are all to some degree unique, varying in their structural development, the interplay of petrogenetic complexes, and the range of lithologies. Peralkaline silicic reservoirs vary greatly in size and form (even beneath volcanoes of roughly similar size) but all are basalt-driven, in that basaltic magma is the fundamental source of heat and volatiles in the magmatic system. Fractional crystallisation of basaltic magma is the dominant differentiation mechanism but important contributions are made by magma mixing, remobilisation of crystal mushes and feldspar resorption, exsolution of carbonate phases and various volatile-magma interactions. Peralkaline silicic magmas are water-rich (> 4 wt.%) and the rhyolitic varieties evolve to temperatures < 800 °C at oxygen fugacities largely within the range ∆FMQ 0 to − 1. They can be categorised as of cold-wet-reduced type. Low viscosities (< 104–106 Pa s) result in rapid growth of volcanic edifices, highly efficient crystal–melt separation and the ubiquitous development of compositionally zoned caps to reservoirs. No peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc–alkaline systems has yet been found.
► Evolution, nature of magma chambers, mechanisms of caldera collapse in peralkaline silicic systems. ► Complexity of petrogenetic processes. ► P–T–X conditions in pre-eruption chambers; recognition of a cold-wet-reduced category of silicic magmatism. ► Volcanological consequences of peralkalinity.