The Gregory Rift extends approximately 900-1000 km, between Turkana Lake in Kenya to the lakes of Manyara and Balangida in northern Tanzania. Geological activity within the Gregory Rift began approx. 35-30 Ma within the northern part, and continues currently within the southern part (carbonatite-nephelinite-phonolitic Oldoinyo Lengai active volcano). The geological time frames of volcanism development in the area of the modern Gregory Rift is examined in various publications and many geological, geochemical and mineralogical studies of various volcanic rocks have been described (e.g. Dawson, 2008).
The Crater Highlands area is located within the northern part of the Gregory Rift. This area is formed by large volcanoes and separate lava flows, the formation of which had taken place between 4.5 and 0.8 Ma (Mollel and Swisher, 2012). The volcanoes are distinguished into two types by the chemical composition of eruptive rocks. The first type - volcanoes of mainly basaltic and trachytic composition (Ngorongoro, Lemagrut and Oldeani), and the second type – volcanoes with predominately nephelinitic rocks (Sadiman and possibly Embakai). Separate lava fields that are spatially not connected with large volcanoes are located in the South-Western part of the upland. These lavas are known as Ogol lavas, which are the subject of our research.
Previous studies (Hay, 1987) postulate that the Ogol lava flows erupted from small separate channels (craters) or cones. These channels and cones stretch in a west to easterly direction for approx. 25 km, and are 0.5 to 1.5 km in diameter and from 50 to 150 m in height.
There is only one scientific publication, dedicated to the research of Ogol lava mineralogy and geochemistry – a PhD thesis by Mollel (2007). In this thesis only four rock samples have been studied (bulk chemical composition of rocks and chemical composition of major minerals). According to Mollel (2007) the Ogol lavas are basaltoid porphyritic rock with olivine, augite, biotite, alkali feldspar phenocrysts, as well as with plagioclase and nepheline microphenocrysts. By chemical composition the Ogol lavas are high-magnesian rocks (Mg# = 56-59) with high chromium content (514-863 ppm). In comparison to other volcanic rocks, known in the Crater Highlands, these are the most primitive (poorly differentiated) volcanic rocks.
The studies are performed on rock material, available in Saint Petersburg State University and Institute of Precambrian geology and geochronology collections. At the author's disposal there is a collection, consisting of 13 specimens, picked out from seven outcrops. At the moment we have studied seven specimens of Ogol lava, obtaining the following results:
1) The major (forsterite, plagioclase, diopside, magnetite), minor (apatite, amphibole, calcite) and accessory (pyrrhotine, chromite, baryte) minerals were determined in the rocks. Na and K silicates that are not diagnosed properly yet are also present in the groundmass.
2) Two mineral assemblages were observed in basalts: primary (magmatic) (forsterite, plagioclase, diopside, magnetite, apatite, amphibole, chromite) and secondary (hydrothermal) (baryte, calcite, as well as an undiagnosed Mg hydrous silicate that forms after forsterite).
3) Among the studied specimens at the very least two basalt varieties can be distinguished: a) olivine-pyroxene basalts and b) plagioclase basalts.
The new data acquired from the Ogol lavas basalt mineralogy will help to improve the understanding of the volcanism history of the Crater Highlands, and together with the future geochemical (including radigenic isotopes) data permits answering the question of whether the genetic relationship between the Ogol lavas and large volcanoes exist.