Chapter 8 Regional comparisons, petrochemistry and petrogenesis

Abstract

In a short review of the structure and volcanism of the southern part of the Gregory Rift, Dawson (1992) drew attention to differences in the contemporaneous volcanism in southern Kenya and northern Tanzania, the differences taking place abruptly at a latitude of c. 3°S. In Chapter 3 (Dawson 2008), differences were noted in the relationship of the volcanism to their setting relative to the buried cratonic margin and to the thermal structure of the crust and the upper mantle in the two areas. The composition of the mantle and its effect on the nature of the volcanicity are discussed in greater detail here. Table 8.1 summarizes the volcanic stratigraphy of southern Kenya and northern Tanzania, mainly from c. 8 Ma, i.e. the age of the oldest known activity in northern Tanzania at Essimingor. The period 8– c. 1.3 Ma (the time of the Older Extrusives) is dominated by central-type volcanoes characterized by effusion of basanites, basalts and minor trachytes, the last being reasonably interpreted as fractionates of the more voluminous basic rocks. There were some peralkaline volcanoes such as Essimingor, Sadiman and Mosonik, which, particularly in the case of Essimingor, the oldest of the northern Tanzania volcanoes, are similar to the peralkaline volcanoes (Mt Elgon, Napak, Yelele) that erupted prior to the onset of basaltic activity in Kenya (Baker et al. 1972). View this table: Table 8.1. Volcanic stratigraphy of S. Kenya and N. Tanzania In southern Kenya during this period, volcanic rocks were erupted from both major central volcanoes or from fissures and very minor volcanoes, the latter including the rift basalts and the Magadi flood trachytes. The Lengitoto trachytes (6.9–5.0 Ma) and the Kirikiti basalts (3.1–2.5 Ma) are exposed along the western flanks of the rift (Crossley 1979), whereas the faults along the eastern flank expose the Singaraini, Ol Keja Nero and …