Texture and mineralogy of tuffs and tuffisites at Ruri Volcano in western Kenya: a carbonatite, melilitite, mantle-debris trio

Ruri is located in the Kavirondo rift, near Homa in western Kenya It is a twin volcano with an ijolite-sovite core in its northern half, coupled with a carbonatite-melilitite pyroclastic centre in the southern half. The petrography and mineral chemistry of pyroclastic rocks from the collection of Ruri material at The Natural History Museum, London, have been studied in order to learn more of the nature of the volcanic activity and the erupted magmas. Four types of tuffs have been recognised at South Ruri, on the basis of rock structure and texture, the nature of fragmental materials and matrix relationships: 1 heterolithic tuff: a mixture of accidental fragments with no discrete physical juvenile component; 2- pelletal tuft: heterolithic tuff with a carbonatitic matrix, with a minor juvenile component of rounded melilititic lapilli cored by mafic HP xenocrysts; 3-lapilli tuff: formed mainly of juvenile lapilli, with concentric melilitite layers around a kernel of clinopyroxene or Cr-spinel, immersed in a carbonatite ash-matrix; 4-lapilli-ash tuff: mostly microporphyritic carbonatite lapilli in a micritic carb onatite matrix (ie. extrusive carbonatite). Types 1 to 3 are interpreted as conduit and/or vent facies and type 4 is a surge deposit. The juvenile components of the deposits are of melilititic-carbonatitic type. Their proportion increases with eruptive sequence, indicating a progressively shallower level of magma fragmentation in the conduit. The juvenile silicate material is a feldspar-free association of melilite (now altered) and olivine, with foids, although these are also highly altered. Clinopyroxene, phlogopiteand amphibole are slightly fresher. Some Cr-rich clinopyroxene, phlogopite, olivine and chromite have strain features and compositions indicating they are from disaggregated mantle xenoliths. The melilititic melts were co-eruptive with igneous carbonate at Ruri They may represent a small volume, near-primary magma that erupted directly from the mantle, whereas the subvolcanic rocks at Ruri are more differentiated, as are the carbonatites and nephelinites of the nearby Kisingiri stratovolcano. Ruri adds another example to the general pattern that the most primitive magma compositions are erupted at small centres adjacent to the large alkaline/carbonatitic volcanoes. O ther examples are Deeti, close to Kerimasi and the Monticchio Lakes at Vulture. At the liquidus temperature and low pressure of melilitite stability, carbonatite is not a miscible phase and may have erupted as mechanically separated, fine sprays of droplets and ash fragments that now form the matrix of the tuffs, as well as discrete porphyritic carbonatite lapilli.