The central Kenya peralkaline province:Insights into the evolution of peralkaline salic magmas

摘要

The central Kenya peralkaline province comprises five young (<1 Ma)volcanic complexes dominated by peralkaline trachytes and rhyolites.The geological and geochemical evolution of each complex is described and issues related to the development of peralkalinity in salic magmas are highlighted.The peralkaline trachytes may have formed by fractionation of basaltic magma via metaluminous trachyte and in turn generated pantellerite by the same mechanism.Comenditic rhyolites are thought to have formed by volatile-induced crustal anatexis and may themselves have been parental to pantelleritic melts by crystal fractionation.The rhyolites record very low temperatures of equilibration (<=700 deg C)at low f_(O_2)(<=FMQ).The development of compositional zonation within the magma reservoirs has been ubiquitous,involving up to tens of cubic km of magma at timescales of 10~3-10~4 years.Magma mixing has also been commonplace,sometimes between adjacent centres.Isotopic evidence relating to rates and timescales of pre-emptive residence times and crystal fractionation processes is summarized.