AN ENRICHED MANTLE SOURCE FOR POTASSIC BASANITES - EVIDENCE FROM KARISIMBI VOLCANO, VIRUNGA VOLCANIC PROVINCE, RWANDA

Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O greater-than-or-equal-to 1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n > 18 and Nb/Zr > 0.25. The Sr-87/Sr-86 and Nd-143/Nd-144 ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances > 4% there is no systematic variation of isotope ratios with fractionation. At < 4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a "primitive" K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both end-members are mantle-derived and isotopically distinct, with the PKB being characterised by Sr-87/Sr-86 up to 0.707 and Nd-143/Nd-144 as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties Pb-206/Pb-204 remains roughly constant at approximately 19.2 while Pb-207/Pb-204 and Pb-208/Pb-204 vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at approximately 1Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated Pb-208/Pb-204 and Pb-208*/Pb-206* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated Pb-207/Pb-204 ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (approximately 27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at approximately 1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was greater-than-or-equal-to 5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high Pb-208/Pb-204 and Pb-208*/Pb-206* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.