Journal of Petrology, Volume 39, Issue 6: June 1 1998.

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

N. W. ROGERS¹, D. JAMES², S. P. KELLEY¹ AND M. DE MULDER³

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 ³LABORATORIUM VOOR AARDKUNDE, KRIJGSLAAN 281, B-9000, GENT, BELGIUM

Lavas from the eastern Virunga province, Rwanda, are dominated by K-hawaiites, K-basanites and latites. All lavas are shoshonitic with 1 < K_{2O/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.

Keywords: Virunga;K-basanites; latites; mantle lithosphere; contamination

Pages 1223-1247

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