THE GENERATION OF POTASSIC LAVAS FROM THE EASTERN VIRUNGA PROVINCE, RWANDA

Lavas from the eastern Virunga province, Rwanda, are dominated by K-ha waiites, K-basanites and latites. All lavas are shoshonitic with 1 < K 2O/Na2O < 2 and strongly enriched in incompatible elements. Sr-87/Sr-8 6 varies from 0.70586 in the K-basanites to 0.70990 in the latites, Nd -143/(144)d from 0.51254 to 0.51205 and Pb isotopes define sub-vertica l trends on isotope diagrams (Pb-206/Pb-204 19.30-19.51, Pb-207/Pb-204 15.69-15.93 and Pb-208/Pb-204 40.28-41.5). Ar-40/Ar-39 ages of leucit e and phlogopite separates suggest that the latites are between 100 an d 200 ka and the K-basanites <100 ka. The latites are hybrid magmas pr oduced by mixing between a K-basanite melt with a silicic melt from de ep crust. The low-silica K-basanites reflect interaction between a maf ic K-basanitic melt with Nd-143/Nd-144 similar to 0.51204, Sr-87/Sr-86 similar to 0.707, and a nephelinite with Nd-143/Nd-144 similar to 0.5 1267 and Sr-87/Sr-86 similar to 0.7045. Both are derived from the mant le lithosphere with source ages of 1 Ga and 0.5 Ga, respectively, and the youngest ages correspond to the deepest magma sources. The magma p roduction rate in the Virunga is low (similar to 0.04 km(3)/yr), and r eflects production (10-15 My) heating of the lithosphere by the East A frican mantle plume.