Isotopic and geochemical evidence for a heterogeneous mantle plume origin of the Virunga volcanics, Western rift, East African Rift system

摘要

Virunga volcanics in the western rift of the East African Rift system (EARS) show silica-undersaturated, ultra-alkaline, alkalic-mafic compositions. The two active Virunga volcanoes, Nyiragongo and Nyamuragira, are 15 km apart. Nyiragongo shows unusual compositions not seen globally and has the lowest recorded viscosity among terrestrial magmas while Nyamuragira is unusually effusive. These volcanoes occur along the fringes of a topographic uplift within the EARS. We analyzed major, trace elements and Nd-Sr-Pb isotopes in Nyiragongo and Nyamuragira lavas including samples from 2002, 2003 and 2004 eruptions. The youngest Nyiragongo lavas are ultrapotassic, ultrasodic and have low SiO_2 (36.6 wt%). Nyamuragira lavas are basalts, basanites and tephrites, distinct from the foiditic Nyiragongo lavas. Both volcanic products show enrichment in light rare earths, large ion lithophile and high field strength elements. High chondrite-normalized Dy/Yb suggests residual garnet in the source(s). Nyiragongo and Nyamuragira lavas show high Nb/U similar to oceanic basalts while Ce/Pb ratios are unusually high in Nyiragongo. 2002 and 2003 Nyiragongo lavas show superchondritic Zr/Hf suggesting carbonate metasomatism in their source. Both these volcanics show low K/Rb (231-356) ratios suggesting phlogopite-melting in their mantle source. Residual garnet and phlogopite suggests melt derivation from depths between 80 and 150 km. Nyiragongo lavas show bulk-earth like Nd-Sr isotopes. In Nd-Sr-Pb isotope space, both Nyiragongo and Nyamuragira show correlations similar to ocean island basalts with a strong affinity for EM II. Pb-isotopic variations in some parts of the 2002 Nyiragongo lava flow are caused by leaching of Pb by various degrees of fusion of granitic basement rocks found as xenoliths in the lava and within the volcanic cone. In a plot of ~(207)Pb/~(206)Pb versus ~(208)Pb/~(206)Pb, however, the least contaminated lavas plot conspicuously in the field of Type I kimberlites suggesting a sub-lithospheric mantle origin. Also, the reported MORE-like He-isotopic composition of the Nyiragongo lavas are distinctly higher than those of the sub-continental lithospheric mantle. In summary, the Nyiragongo lavas are bulk silicate earth-like in Nd and Sr-isotopes, Group I kimberlite-like in their Pb-isotopes, and their high Ce/Pb. low SiO_2 rule out continental lithospheric sources, particularly in conjunction with all the isotopic data. Our combined geochemical data with available experimental petrological data on peridotitic compositions suggest that Nyiragongo lavas formed at greater depths by low degree partial melting of a garnet, clinopyroxene, and phlogopite-bearing carbonated mantle, while the Nyamuragira lavas are products of larger degree partial melting at comparatively shallower mantle depths with a recycled crustal component. We argue that simultaneous volcanism in adjacent Nyiragongo and Nyamuragira, with magmas originating from different depths requires the presence of a heterogeneous mantle plume beneath the Tanzanian craton. This plume caused chemically distinctive volcanic provinces around the Tanzanian craton, in the Western and Kenya Rift.