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

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.