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Journal of Petrology | Volume 39 | Number 6 | Pages 1223-1247 | 1998
© Oxford University Press 1998

The Generation of Potassic Lavas from the Eastern Virunga Province, Rwanda

N. W. Rogers1,*, D. James2, S. P. Kelley1 and M. De Mulder3,{dagger}

1 Department of Earth Sciences, The Open University Milton Keynes MK7 6AA, Uk
2 Department of Geology and Geophysics, University of Edinburgh Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK
3 Laboratorium Voor Aardkunde Krijgslaan 281, B-9000, Gent, Belgium

Received November 14, 1997; Revised typescript accepted January 12, 1998


  Abstract

Lavas from the eastern Virunga province, Rwanda, are dominated by K-hawaiites, K-basanites and latites. All lavas are shoshonitic with 1 < K2O/Na2O < 2 and strongly enriched in incompatible elements.87Sr/86Sr varies from 0.70586 in the K-basanites to 0.70990 in the latites, 143Nd/144Nd from 0.51254 to 0.51206, and Pb isotopes define sub-vertical trends on isotope diagrams (206Pb/204Pb 19.30-19.51, 207Pb/204Pb 15.69–15.93 and 208Pb/204Pb 40.28–41.5). 40Ar/39Ar ages of leucite and phlogopite separates suggest that the latites are between 100 and 200 ka and the K-basanites <100 ka. The latites are hybrid magmas produced by mixing between a K-basanite melt with a silicic melt from the deep crust. The low-silica K-basanites reflect interaction between a mafic K-basanitic melt with 143Nd/144Nd ~0.51204, 87Sr/86Sr ~0.707, and a nephelinite with 143Nd/144Nd ~0.51267 and 87Sr/86Sr ~0.7045. Both are derived from the mantle lithosphere with source ages of 1 Ga and 0.5 Ga, respectively, and the youngest ages correspond to the deepest magma sources. The magma production rate in the Virunga is low (~0.04 km3/yr), and reflects prolonged (10–15 My) heating of the lithosphere by the East African mantle plume.

KEY WORDS: Virunga; K-basanites; latites; mantle lithosphere; contamination

* Corresponding author.

{dagger} Present address: Ommegangstraat 69, 9080 Lochristi, Belgium.


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