The Petrology of Moroto Mountain, Eastern Uganda, and the Origin of Nephelinites |
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| Authors: | VARNE R |
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| Institution: | Department of Geology, The University Manchester, 13 |
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| Abstract: | Moroto Mountain is one of the Tertiary alkaline volcanoes ofeastern Uganda. Two differentiated series of alkaline undersaturatedrocks occur in the volcano: a nephelinite series which is commonto all the volcanoes of the province, and an alkaline olivinebasalt series which occurs only at Moroto. It is proposed that the nephelinite series derives from theincongruent melting of pargasite. Since pargasite occurs inhydrated ultrabasic rocks, it is also suggested that the twoseries are formed from the partial melting of hydrated peridotite:the nephelinites from the homblendic portions and the alkalineolivine basalts from the less volatile-rich portions. The two series of the Moroto volcano typify the two major associationsof East African alkaline vulcanicity and, spatially, the volcanolies between regions each characterized by one of these twomajor associations. It may be that the volcano lies above azone of transition in the upper mantle between a relativelyanhydrous, volatile-poor portion and a hydrated, volatile-richportion where hornblendites are developed. |
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