Hanang And Balangida

The Hanang volcano is the most southerly manifestation of Neogene volcanism associated with the East African Rift. It rises 1700 m above the plain, but is deeply eroded and has a breached crater. The volcano consists principally of nephelinite agglomerates and younger nephelinite flows. A final explosive phase of crater formation produced calcareous tuffs and agglomerates which are widespread. The agglomerates comprise mainly nephelinite blocks in a calcareous matrix, but near the summit xenoliths of ijolite, melteigite and olivine monchiquite are abundant. Microphenocrysts of nepheline and aegirine-augite are present in the nephelinites in a matrix of these two minerals plus accessory melanite, titanite, apatite and opaque minerals with zeolites in vesicles. Monchiquite, olivine analcimitite and olivine-rich melanephelinite also occur. Nephelinite also forms dykes in the summit area. Calcareous tuffs, which are widely distributed to the south and west, consist of crystals of aegirine-augite, K-feldspar, plagioclase, quartz, phlogopite, nepheline, olivine and opaques in a calcareous matrix. Agglomerates, comprising blocks of granite, dolerite and nephelinite, are found around most of the craters, only one cone being associated with a lava flow of monchiquite. Explosion craters and collapsed vents extend for 15 km to the north and east of Hanang and 25 km to the south and southwest. One of the volcanic features to the southeast of Hanang (Labait) is a breached cone with a flow of katungite. The flow contains numerous xenoliths of upper mantle peridotite (Dawson, 1997; Lee and Rudnick, 199). Some indicate a high thermal anomaly in the mantle below this southerly part of the rift valley (Dawson et al., 1997), and others are metasomatised and include glassy pockets and veins containing precipitated phases (Dawson, 1999). Lake Balangida lies between rift escarpments northwest of Hanang and carbonatite flows and thin sovite dykes and carbonatite agglomerates outcrop along the escarpments on either side of the lake. Three carbonatite lava flows have been identified east of Lake Balangida on the lower northwestern slopes of Hanang (M.S. Garson, pers. comm., 1988). The flows vary from 0.75 to 6 m in thickness and are all similar. They contain numerous xenoliths of fenitized granodiorite in a calcite matrix often crowded with stratiform magnetite crystals, variable amounts of biotite, locally abundant pyrochlore, sodic amphibole and aegirine. What is believed to be a continuation of the lowermost flow has been identified on the northern side of the lake. Thin sovite dykes occur on the lower northern slopes of the mountain.