Silali

Silali, the largest volcano in the northern Gregory Rift, forms a low-angle shield of 30x25 km extending the full width of the rift and rising 760 m above the rift floor. At the summit there is a spectacular elliptical caldera of 7.5x5 km with unbreached walls up to 340 m high. The northern, southern and eastern flanks of the volcano are cut by a volcanic rift zone 10 km wide and 30 km long which is associated with the eruption of large volumes of basalt lava. A very detailed account of the stratigraphy, structural evolution and geochronology of the volcano is given by Smith et al. (1995), of the mineral and rock chemistry and petrogenesis by Macdonald et al. (1995) and of the general geology and geothermal activity by Dunkley et al. (1993), who include with their report a detailed geological map on a scale of 1:50,000. The volcano is built predominantly of peralkaline trachyte and mildly alkaline to transitional basalt lavas, the former being some ten times more voluminous than the latter. Lavas of hawaiite, mugearite, phonolite and some mixed rocks are minor while pyroclastic rocks are locally significant (Smith et al., 1995). The total thickness of the strata is in excess of 700 m and has been divided into four groups for which the stratigraphy, major lithologies, thickness, age and style of activity are summarised in tabular form by Smith et al. (1995, Fig. 3B). Nearly half the total thickness of the strata is exposed in the caldera walls which has considerably facilitated geological interpretation. The earliest activity is probably represented by the Mission Basalt lavas, although these are undated, which were followed by trachyte lavas that built a lava shield. There were then two periods of trachytic pyroclastic activity separated by a period of eruption of intermediate lavas. Activity was then concentrated on the western flanks in the form of trachytic lavas and tuffs, including air-fall and surge deposits. A swarm of basalt dykes was then emplaced in the rift zone through which large volumes of basalt were erupted followed, in the summit area, by ejection of further basalt and trachyte from a major fissure at the same time as three large pit craters formed on the southern flanks, probably as a result of phreatomagmatic activity. The development of the summit crater was then followed by eruption of lavas of basalt and trachyte within the crater, growth of trachyte lava domes on the eastern flanks and finally eruption of minor basalt flows on the outer flanks. Most of the Silali basalts are phenocryst-poor or aphyric and consist of plagioclase, clinopyroxene, olivine and titanomagnetite with accessory alkali feldspar and in some samples analcime (Macdonald et al., 1995). Hawaiite, mugearite and benmoreite are scarce. The trachytes vary from two feldspar to dominant one feldspar types, the former comprising subhedral phenocrysts of andesine (An21-24), anorthoclase (An<15), augite, titanomagnetite, olivine and apatite and the latter alkali feldspar (Or35), clinopyroxene, titanomagnetite and fayalite. The trachytoid groundmass of the two feldspar rocks consists of alkali feldspar, opaque phases and clinopyroxene rimmed by a blue-brown amphibole, whereas the groundmass of the one feldspar trachytes contains riebeckite-arfvedsonite, alkali feldspar, titanomagnetite and, in some summit trachytes, quartz. A full range of whole rock analyses with trace elements, including REE, are given by Macdonald et al. (1995) who also present numerous Sr, Nd and Pb isotopic values and, using these data, discuss the petrogenesis in some detail. Rock analyses will also be found in the geochemical study of McCall and Hornung (1972).